Abstract: The thermal conductivity of graphene is in the range of 3000–5000 W m−1 K−1, showing great potential in high thermal conductivity devices. However, the thermal conductivity of graphene-reinforced polymer is typically lower than 10 W m−1 K−1, which is far from theoretical expectations. To understand the mechanisms of heat transfer in graphene-reinforced polymers, this work investigated the effect of graphene addition on the thermal conductive performance of polyetheretherketone (PEEK) matrix. The study examined the number of layers, deflection angles, and interlayer distances using molecular dynamics (MD) simulations. The results showed that the improvement of thermal conductivity of PEEK nanocomposite was not only related to the content of graphene but also to the angle between the benzene ring in the molecular chain of PEEK and the transfer direction of heat flow. Increasing the number of graphene layers is more beneficial to the enhancement of thermal conductivity. In particular, the enhancement of thermal conductivity is most significant when the number of graphene layers is the same, and the interlayer distance is less than the truncation radius. read less

Abstract: The development of carbon-based reverse osmosis membranes for water desalination is hindered by challenges in achieving a high pore density and controlling the pore size. We use molecular dynamics simulations to demonstrate that graphene foam membranes with a high pore density provide the possibility to tune the pore size by applying mechanical strain. As the pore size is found to be effectively reduced by a structural transformation under strain, graphene foam membranes are able to combine perfect salt rejection with unprecedented water permeability. read less

Abstract: The penetration of atomic hydrogen through defect-free graphene was generally predicted to have a barrier of at least several eV, which is much higher than the 1 eV barrier measured for hydrogen-gas permeation through pristine graphene membranes. Herein, our density functional theory calculations show that ripples, which are ubiquitous in atomically thin crystals and mostly overlooked in the previous simulations, can significantly reduce the barriers for all steps constituting the mechanism of hydrogen-gas permeation through graphene membranes, including dissociation of hydrogen molecules, reconstruction of the dissociated hydrogen atoms and their flipping across graphene. Especially, the flipping barrier of hydrogen atoms from a cluster configuration is found to decrease rapidly down to<1 eV with increasing ripples' curvature. The estimated hydrogen permeation rates by fully considering the distribution of ripples with all realistic curvatures and the major reaction steps that occurred on them are quite close to the experimental measurements. Our work provides insights into the fundamental understanding of hydrogen-gas permeation through graphene membranes and emphasizes the importance of nanoscale non-flatness (ripples) in explaining many surface and transport phenomena (for example, functionalization, corrosion and separation) in graphene and other two-dimensional materials. read less

Abstract: By using MD simulation method, this study shows the influences of cooling rates on the solidifying temperature of water inside a single-wall-carbon-nanotube under different ambient pressures when cooling the systems from 300 K down to 200 K. Our results showed that the more rapid cooling rate of the systems creates more disruptive and dramatic phase transitions. Moreover, we also found that the lower of pressures correlates to the more dramatic phase transitions of water, regardless of cooling rate. This study generally provides more insight into water behavior in the SWCNT with variations in ambient conditions. read less

Abstract: 3D graphene assemblies are proposed as solutions to meet the goal toward efficient utilization of 2D graphene sheets, showing excellent performances in applications such as mechanical support, energy storage, and electrochemical catalysis. However, given the diversity and complexity of possible graphene 3D structures, there does not yet exist a systematic approach that can generate target 3D shapes and also, evaluate their performance. Here high‐throughput data generation is combined with artificial intelligence approaches to realize rapid structure formation and property quantification of 3D graphene foams with mathematically controlled topologies, driven by molecular dynamics simulations. More than 4000 different foam structures are created, which feature diverse topologies that contain potential pathways for small molecules and auxetic structures with negative Poisson's ratio. Empowered by machine learning (ML) algorithms including graph neural networks, not only global properties such as elastic moduli, but also local behaviors such as atomic stress can be predicted and optimized based on their atomic structure, bypassing expensive atomistic simulations. The key findings of the research reported in this paper include a high‐throughput virtual framework of generating diverse 3D graphene assemblies with mechanical performances quantification, and highly efficient methods of evaluating physical properties based on ML. read less

Abstract: Phase change materials (PCM) have had a significant role as thermal energy transfer fluids and nanofluids and as media for thermal energy storage. Molecular dynamics (MD) simulations, can play a significant role in addressing several thermo-physical problems of PCMs at the atomic scale by providing profound insights and new information. In this paper, the reviewed research is classified into five groups: pure PCM, mixed PCM, PCM containing nanofillers, nano encapsulated PCM, and PCM in nanoporous media. A summary of the equilibrium and non-equilibrium MD simulations of PCMs and their results is presented as well. The primary results of the simulated systems are demonstrated to be efficient in manufacturing phase change materials with better thermal energy storage features. The goals of these studies are to achieve higher thermal conductivity, higher thermal capacity, and lower density change, determine the melting point, and understand the molecular behaviors of PCM composites. A molecular dynamics-based grouping (PCM simulation table) was presented that is very useful for the future roadmap of PCM simulation. In the end, the PCFF force field is presented in detail and a case problem is studied for more clarity. The results show that simulating the PCMs with a similar strategy could be performed systematically. Results of investigations of thermal conductivity enhancement showed that this characteristic can be increased at the nano-scale by the orientation of PCM molecules. read less

Abstract: The sliding motion of gold slabs adsorbed on a graphite substrate is simulated using molecular dynamics. The central quantity of interest is the mean lateral force, that is, the kinetic friction rather than the maximum lateral forces, which correlates with the static friction. For most setups, we find Stokesian damping to resist sliding. However, velocity-insensitive (Coulomb) friction is observed for finite-width slabs sliding parallel to the armchair direction if the bottom-most layer of the three graphite layers is kept at zero stress rather than at zero displacement. Although the resulting kinetic friction remains much below the noise produced by the erratic fluctuations of (conservative) forces typical for structurally lubric contacts, the nature of the instabilities leading to Coulomb friction could be characterized as quasi-discontinuous dynamics of the Moiré patterns formed by the normal displacements near a propagating contact line. It appears that the interaction of graphite with the second gold layer is responsible for the symmetry break occurring at the interface when a contact line moves parallel to the armchair rather than to the zigzag direction. read less

Abstract: . read less

Abstract: Piezoelectric effect has proved itself to be a promising energy conversion mechanism that can convert mechanical energy into electricity. Here, we propose an indirect thermoelectric conversion mechanism based on a combination of the thermophoresis and piezoelectric effects. We first analyze this thermally driven mechanism using a simplified theoretical model and then numerically analyze a molecular dynamics (MD) simulation of a hybrid system constructed of a single-layer MoS2 nanoribbon and a concentric carbon nanotube. We show that the thermophoresis-induced piezoelectric output voltage can reach 3.5 V, and this value can be tuned using a temperature difference. The output voltage obtained using this mechanism is significantly higher than that obtained by heating piezoelectric materials directly. Given the generality of the thermophoresis effect in Van der Waals structures, this mechanism has potential applications in the conversion of thermal energy into electrical energy at the nanoscale level. read less

Abstract: The negative Poisson’s ratio (NPR) is a novel property of materials, which enhances the mechanical feature and creates a wide range of application prospects in lots of fields, such as aerospace, electronics, medicine, etc. Fundamental understanding on the mechanism underlying NPR plays an important role in designing advanced mechanical functional materials. However, with different methods used, the origin of NPR is found different and conflicting with each other, for instance, in the representative graphene. In this study, based on machine learning technique, we constructed a moment tensor potential for molecular dynamics (MD) simulations of graphene. By analyzing the evolution of key geometries, the increase of bond angle is found to be responsible for the NPR of graphene instead of bond length. The results on the origin of NPR are well consistent with the start-of-art first-principles, which amend the results from MD simulations using classic empirical potentials. Our study facilitates the understanding on the origin of NPR of graphene and paves the way to improve the accuracy of MD simulations being comparable to first-principle calculations. Our study would also promote the applications of machine learning interatomic potentials in multiscale simulations of functional materials. read less

Abstract: Mechanical properties of porous graphene can be effectively tuned by tailoring the nanopore arrangement. Knowledge of the relationship between the porous structure and overall mechanical properties is thus essential for the wide potential applications, and the existing challenge is to efficiently predict and design the mechanical properties of porous graphene due to the diverse nanopore arrangements. In this work, we report on how the SISSO (Sure Independence Screening and Sparsifying Operator) algorithm can be applied to build a bridge between the mechanical properties of porous graphene and the uniform nanopore array. We first construct a database using the strength and work of fracture calculated by large-scale molecular dynamics simulations. Then the SISSO algorithm is adopted to train a predictive model and automatically derive the optimal fitting formulae which explicitly describe the nonlinear structure–property relationships. These expressions not only enable the direct and accurate prediction of targeted properties, but also serve as a convenient and portable tool for inverse design of the porous structure. Compared with other forecasting methods including several popular machine learning algorithms, the SISSO algorithm shows its advantages in both accuracy and convenience. read less

Abstract: Carbon nanotube based multi-terminal junction configurations are of great interest because of the potential aerospace and electronic applications. Multi-terminal carbon nanotube junction has more than one carbon nanotube meeting at a point to create a 2D or 3D structure. Accurate atomistic models of such junctions are essential for characterizing their thermal, mechanical and electronic properties via computational studies. In this work, computational methodologies that uses innovative Computer-Aided Design (CAD) based optimization strategies and remeshing techniques are presented for generating such topologically reliable and accurate models of complex multi-terminal junctions (called 3-, 4-, and 6-junctions). This is followed by the prediction of structure-property relationship via study of thermal conductivity and mechanical strength using molecular dynamics simulations. We observed high degradation in the thermal and mechanical properties of the junctions compared to pristine structures which is attributed to high concentration of non-hexagonal defects in the junction. Junctions with fewer defects have better thermal transport capabilities and higher mechanical strengths, suggesting that controlling the number of defects can significantly improve inherent features of the nanostructures. read less

Abstract: The present paper aims at evaluating non-classical continuum parameters for each class of armchair and zigzag single-walled CNTs focusing on the scale effect in their flexural behavior observed in molecular dynamics (MD) simulations. Through a non-linear optimization approach, the bending rigidities obtained from atomistic simulations are compared to those derived from non-classical continua. For MD simulations, a novel method ensuring pure bending is introduced and for continuum modeling, micropolar, constrained micropolar, and modified couple stress theories are employed. The results reveal that adopted non-classical theories, notably micropolar theory, provide reasonable outcomes with an obvious failure of classical Cauchy theory. read less

Abstract: Hydrogenated small fullerenes (Cn, n<60) are of interest as potential astrochemical species, and as intermediates in hydrogen catalysed fullerene growth. However computational identification of key stable species is difficult due to the vast combinatorial space of structures. In this study we explore routes to predict stable hydrogenated small fullerenes. We show that neither local fullerene geometry nor local electronic structure analysis are able to correctly predict subsequent low energy hydrogenation sites, and indeed sequential stable addition searches also sometimes fail to identify most stable hydrogenated fullerene isomers. Of the empirical and semi-empirical methods tested, GFN2-xTB consistently gives highly accurate energy correlation (r>0.99) to full DFT-LDA calculations at a fraction of the computational cost. This allows identification of the most stable hydrogenated fullerenes up to 4H for four fullerenes, namely two isomers of C28 and C40, via “brute force” systematic testing of all symmetry inequivalent combinations. The approach shows promise for wider systematic studies of smaller hydrogenated fullerenes. read less

Abstract: Implantation and subsequent behaviour of heavy noble gases (Ar, Kr, and Xe) in few-layer graphene sheets and in nanodiamonds are studied both using computational methods and experimentally using X-ray absorption spectroscopy. X-ray absorption spectroscopy provides substantial support for Xe-vacancy (Xe-V) defects as main sites for Xe in nanodiamonds. It is shown that noble gases in thin graphene stacks distort the layers, forming bulges. The energy of an ion placed in between flat graphene sheets is notably lower than that in domains with high curvature. However, if the ion is trapped in the curved domain, considerable additional energy is required to displace it. This phenomenon is likely responsible for strong binding of noble gases implanted into disordered carbonaceous phase in meteorites (the Q-component). read less

Abstract: Notably known for its extraordinary thermal and mechanical properties, graphene is a favorable building block in various cutting-edge technologies such as flexible electronics and supercapacitors. However, the almost inevitable existence of defects severely compromises the properties of graphene, and defect prediction is a difficult, yet important, task. Emerging machine learning approaches offer opportunities to predict target properties such as defect distribution by exploiting readily available data, without incurring much experimental cost. Most previous machine learning techniques require the size of training data and predicted material systems of interest to be identical. This limits their broader application, because in practice a newly encountered material system may have a different size compared with the previously observed ones. In this paper, we develop a transferable learning approach for graphene defect prediction, which can be used on graphene with various sizes or shapes not seen in the training data. The proposed approach employs logistic regression and utilizes data on local vibrational energy distributions of small graphene from molecular dynamics simulations, in the hopes that vibrational energy distributions can reflect local structural anomalies. The results show that our machine learning model, trained only with data on smaller graphene, can achieve up to 80% prediction accuracy of defects in larger graphene under different practical metrics. The present research sheds light on scalable graphene defect prediction and opens doors for data-driven defect detection for a broad range of two-dimensional materials. read less

Abstract: Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene sheets (GSs), have been adopted as resonators in vibration-based nanomechanical sensors because of their extremely high stiffness and small size. Diamond nanothreads (DNTs) are a new class of one-dimensional carbon nanomaterials with extraordinary physical and chemical properties. Their structures are similar to that of diamond in that they possess sp3-bonds formed by a covalent interaction between multiple benzene molecules. In this study, we focus on investigating the mechanical properties and vibration behaviors of DNTs with and without lattice defects and examine the influence of density and configuration of lattice defects on the two them in detail, using the molecular dynamics method and a continuum mechanics approach. We find that Young’s modulus and the natural frequency can be controlled by alternating the density of the lattice defects. Furthermore, we investigate and explore the use of DNTs as resonators in nanosensors. It is shown that applying an additional extremely small mass or strain to all types of DNTs significantly changes their resonance frequencies. The results show that, similar to CNTs and GSs, DNTs have potential application as resonators in nano-mass and nano-strain sensors. In particular, the vibration behaviors of DNT resonators can be controlled by alternating the density of the lattice defects to achieve the best sensitivities. read less

Abstract: Lubricants are widely used in macroscopic mechanical systems to reduce friction and wear. However, on the microscopic scale, it is not clear to what extent lubricants are beneficial. Therefore, in this study, we consider two diamond solid-state gears at the nanoscale immersed in different lubricant molecules and perform classical MD simulations to investigate the rotational transmission of motion. We find that lubricants can help to synchronize the rotational transmission between gears regardless of the molecular species and the center-of-mass distance. Moreover, the influence of the angular velocity of the driving gear is investigated and shown to be related to the bond formation process between gears. read less

Abstract: The silicon/carbon nanotube (core/shell) nanocomposite electrode model is one of the most promising solutions to the problem of electrode pulverization in lithium-ion batteries. The purpose of this study is to analyze the mechanical behaviors of silicon/carbon nanotube nanocomposites via molecular dynamics computations. Fracture behaviors of the silicon/carbon nanotube nanocomposites subjected to tension were compared with those of pure silicon nanowires. Effective Young’s modulus values of the silicon/carbon nanotube nanocomposites were obtained from the stress and strain responses and compared with the asymptotic solution of continuum mechanics. The size effect on the failure behaviors of the silicon/carbon nanotube nanocomposites with a fixed longitudinal aspect ratio was further explored, where the carbon nanotube shell was found to influence the brittle-to-ductile transition behavior of silicon nanowires. We show that the mechanical reliability of brittle silicon nanowires can be significantly improved by encapsulating them with carbon nanotubes because the carbon nanotube shell demonstrates high load-bearing capacity under tension. read less

Abstract: In consideration of the presented optical-thermally excited resonant mass detection scheme, molecular dynamics calculations are performed to investigate the thermal actuation and resonant mass sensing mechanism. The simulation results indicate that an extremely high temperature exists in a 6% central area of the graphene sheet exposed to the exciting laser. Therefore, constraining the laser driving power and enlarging the laser spot radius are essential to weaken the overheating in the middle of the graphene sheet, thus avoiding being burned through. Moreover, molecular dynamics calculations demonstrate a mass sensitivity of 214 kHz/zg for the graphene resonator with a pre-stress of 1 GPa. However, the adsorbed mass would degrade the resonant quality factor from 236 to 193. In comparison, the sensitivity and quality factor could rise by 1.3 and 4 times, respectively, for the graphene sheet with a pre-stress of 5 GPa, thus revealing the availability of enlarging pre-stress for better mass sensing performance. read less

Abstract: This study investigated the thermal transport behaviors of branched carbon nanotubes (CNTs) with cross and T-junctions through non-equilibrium molecular dynamics (NEMD) simulations. A hot region was created at the end of one branch, whereas cold regions were created at the ends of all other branches. The effects on thermal flow due to branch length, topological defects at junctions, and temperature were studied. The NEMD simulations at room temperature indicated that heat transfer tended to move sideways rather than straight in branched CNTs with cross-junctions, despite all branches being identical in chirality and length. However, straight heat transfer was preferred in branched CNTs with T-junctions, irrespective of the atomic configuration of the junction. As branches became longer, the heat current inside approached the values obtained through conventional prediction based on diffusive thermal transport. Moreover, directional thermal transport behaviors became prominent at a low temperature (50 K), which implied that ballistic phonon transport contributed greatly to directional thermal transport. Finally, the collective atomic velocity cross-correlation spectra between branches were used to analyze phonon transport mechanisms for different junctions. Our findings deeply elucidate the thermal transport mechanisms of branched CNTs, which aid in thermal management applications. read less

Abstract: The structure of amorphous materials has been debated since the 1930s as a binary question: amorphous materials are either Zachariasen continuous random networks (Z-CRNs) or Z-CRNs containing crystallites. It was recently demonstrated, however, that amorphous diamond can be synthesized in either form. Here we address the question of the structure of single-atom-thick amorphous monolayers. We reanalyze the results of prior simulations for amorphous graphene and report kinetic Monte Carlo simulations based on alternative algorithms. We find that crystallite-containing Z-CRN is the favored structure of elemental amorphous graphene, as recently fabricated, whereas the most likely structure of binary monolayer amorphous BN is altogether different than either of the two long-debated options: it is a compositionally disordered "pseudo-CRN" comprising a mix of B-N and noncanonical B-B and N-N bonds and containing "pseudocrystallites", namely, honeycomb regions made of noncanonical hexagons. Implications for other nonelemental 2D and bulk amorphous materials are discussed. read less

Abstract: We investigate the concept of molecular-sized outward-swinging gate, which allows for entropy decrease in an isolated system. The theoretical analysis, the Monte Carlo simulation, and the direct solution of governing equations all suggest that under the condition of local nonchaoticity, the probability of particle crossing is asymmetric. It is demonstrated by an experiment on a nanoporous membrane one-sidedly surface-grafted with bendable organic chains. Remarkably, through the membrane, gas spontaneously and repeatedly flows from the low-pressure side to the high-pressure side. While this phenomenon seems counterintuitive, it is compatible with the principle of maximum entropy. The locally nonchaotic gate interrupts the probability distribution of the local microstates, and imposes additional constraints on the global microstates, so that entropy reaches a nonequilibrium maximum. Such a mechanism is fundamentally different from Maxwell's demon and Feynman's ratchet, and is consistent with microscopic reversibility. It implies that useful work may be produced in a cycle from a single thermal reservoir. A generalized form of the second law of thermodynamics is proposed. read less

Abstract: The effect of the size of nickel nanoparticles on the fabrication of a Ni–graphene composite by hydrostatic pressure at 0 K followed by annealing at 1000 and 2000 K is studied by molecular dynamics simulation. Crumpled graphene, consisting of crumpled graphene flakes interconnected by van der Waals forces is chosen as the matrix for the composite and filled with nickel nanoparticles composed of 21 and 47 atoms. It is found that the main factors that affect composite fabrication are nanoparticle size, the orientation of the structural units, and temperature of the fabrication process. The best stress–strain behavior is achieved for the Ni/graphene composite with Ni47 nanoparticle after annealing at 2000 K. However, all of the composites obtained had strength property anisotropy due to the inhomogeneous distribution of pores in the material volume. read less

Abstract: Three-dimensional (3D) aggregation of graphene is dramatically weak and brittle due primarily to the prevailing interlayer van der Waals interaction. In this report, motivated by the recent success in synthesis of monolayer amorphous carbon (MAC) sheets, we demonstrate that outstanding strength and large plastic-like strain can be achieved in layered 3D MAC composites. Both surface roughening and the ultracompliant nature of MACs count for the high strength and gradual failure in 3D MAC. Such properties are not seen when intact graphene or multiple stacked MACs are used as building blocks for 3D composites. This work demonstrates a counterintuitive mechanism that surface roughening due to initial defects and low rigidity may help to realize superb mechanical properties in 3D aggregation of monolayer carbon. read less

Abstract: Enhancement of polymer thermal conductivity using nanographene fillers and clarification of its molecular-scale mechanisms are of great concern in the development of advanced thermal management materials. In the present study, molecular dynamics simulation was employed to theoretically show that the in-plane aspect ratio of a graphene filler can have a significant impact on the effective thermal conductivity of paraffin/graphene composites. Our simulation included multiple graphene fillers aggregated in a paraffin matrix. The effective thermal conductivity of a paraffin/graphene composite, described as a second-rank tensor in the framework of equilibrium molecular dynamics simulation, was calculated for two types of graphene fillers with the same surface area but in-plane aspect ratios of 1 and 10. The filler with the higher aspect ratio was found to exhibit a much higher thermal conductivity enhancement than the one with the lower aspect ratio. This is because a high in-plane aspect ratio strongly restricts the orientation of fillers when they aggregate and, consequently, highly ordered agglomerates are formed. On decomposing the effective thermal conductivity tensor into various molecular-scale contributions, it was identified that the thermal conductivity enhancement is due to the increased amount of heat transfer inside the graphene filler, particularly along the longer in-plane axis. The present result indicates a possibility of designing the heat conduction characteristics of a nanocomposite by customizing the filler shapes so as to control the aggregation structure of the fillers. read less

Abstract: The emergent cooling technologies based on the caloric effect provide a green alternative to the conventional vapor-compression one which brings about the serious environment problem. However, the existing caloric materials are much inferior to their traditional counterparts in cooling performance. Here we report the colossal barocaloric effect in liquid-solid-transition materials, i.e. n-alkanes. Their excellent cooling performance is superior to those for existing caloric materials and comparable to those of traditional refrigerants. Theoretical calculations suggest the liquid state n-alkanes has huge configuration entropy characterized by large dispersion of bond lengths. Appling pressure significantly reduces the configuration entropy and eventually induces the liquid-solid-transition, leading to the colossal barocaloric effect. This work provides promising refrigerants for caloric cooling technology, and opens a new avenue for exploring colossal barocaloric materials. read less

Abstract: Graphene or other 2D materials are often used as agents to reinforce engineering structures because they possess extremely high mechanical strength and structural flexibility. This is however not cost effective and the reported enhancement is often limited although the mechanical properties of graphene is often several orders higher than cements or concretes. Defective graphene is mechanically weaker than pristine graphene but stronger than engineering structures, moreover, it is cheaper because the synthesis condition is low. In this work we perform systematic molecular dynamics simulations to evaluate the effect of porous graphene (PG), a type of defective graphene, on reinforcing mechanical properties of silicon dioxide (SiO2) which is the key components of engineering structures. Our results show that PG is mechanically weaker than pristine graphene but stronger than SiO2, therefore, with certain amount of PG encapsulation into SiO2, the mechanical properties can be improved under tensile, shear and compressive loadings, although not as significant as the effective of pristine graphene. The modification mechanism is found to depend both on the intrinsic mechanical properties of GP and the interface induced surface stress redistribution in SiO2. The effects of defect concentration, volume fraction, loading methods and interface roughness are found to be influential on the reinforcing effect. Our findings are expected to offer new strategies for rational design of low-cost but high-strength engineering composite structures. read less

Abstract: Understanding the structural behavior of graphene flake, which is the structural unit of bulk crumpled graphene, is of high importance, especially when it is in contact with the other types of atoms. In the present work, crumpled graphene is considered as storage media for two types of nanoclusters—nickel and hydrogen. Crumpled graphene consists of crumpled graphene flakes bonded by weak van der Waals forces and can be considered an excellent container for different atoms. Molecular dynamics simulation is used to study the behavior of the graphene flake filled with the nickel nanocluster or hydrogen molecules. The simulation results reveal that graphene flake can be considered a perfect container for metal nanocluster since graphene can easily cover it. Hydrogen molecules can be stored on graphene flake at 77 K, however, the amount of hydrogen is low. Thus, additional treatment is required to increase the amount of stored hydrogen. Remarkably, the size dependence of the structural behavior of the graphene flake filled with both nickel and hydrogen atoms is found. The size of the filling cluster should be chosen in comparison with the specific surface area of graphene flake. read less

Abstract: The carbon deposition and permeation on nickel surfaces were investigated from thermodynamic and kinetic aspects by using density functional theory (DFT), ab initio atomic thermodynamics, and class... read less

Abstract: Coiled carbon nanotubes (CCNTs) have increasingly become a vital factor in the new generation of nanodevices and energy-absorbing materials due to their outstanding properties. Here, the multiobjec... read less

Abstract: Carbon nanotubes (CNTs) have been widely used as the motor and rotor in a rotational transmission nanosystem (RTnS), whose function is to transfer the input rotational frequency of the motor into the output frequency of the rotor through motor-rotor interactions. A wide range of techniques has been explored to achieve a CNT-based RTnS with a stable and adjustable transmission. In this work, a CNT-based rotor is partly immersed into a water box and the associated water-rotor interaction leads to effective manipulation of the transmission efficiency of RTnS. Molecular dynamics simulations are performed on this new RTnS to investigate the dynamic response of the rotor and the local flow field near the water-rotor interface. Various parameters, including ambient temperature, tubes’ radii, and volume fractions of water in the box (V f) are examined for their effects on the rotational transmission efficiency. This study offers useful guidelines for the design of stable RTnS with controllable transmission efficiency. read less

Abstract: The molecular structure and dynamics of water differ considerably at various interfaces. We compare the interfacial water structure–property relationship on three different carbon substrates, namely, amorphous carbon, compressed expanded natural graphite, and pure graphite by utilizing atomistic molecular dynamics simulations. The effect of different substrates on the structural and dynamical properties of water can readily be observed. The density distributions parallel and normal to the substrates show oblate droplet structures. The normal to the substrate water distribution shows a strong hydration layer at the interface that does not vary with substrates. However, the disparity in the structure and dynamics on three different substrates shows that the surface morphologies of the substrates are critical for determining nanoscale water properties. Furthermore, it is observed that the formation of an interfacial water layer or the hydration layer is a direct consequence of both water “confinement” at the nanoscale and “attraction” between water molecules and the carbon substrates. read less

Abstract: Micro and nano devices generally have the characteristics of high performance and compact size, so their own heat transfer and heat dissipation problems are becoming more and more serious. Therefore, it is necessary to clarify the heat transport mechanism in the micro–nano structure by analyzing the heat transport properties of nanomaterials, and then control the thermal conductivity of nanodevices. We have investigated the lattice heat transfer of egg-tray graphene using non-equilibrium molecular dynamics simulations. Three structures (I, II and III) are studied according to the number of hexagons as 10, 16, and 56 respectively. The increases of lattice thermal conductivity with an increase of length in sub-microns implies the large mean free path of phonons in egg-tray graphene, similar as that of graphene. The large-size-limit thermal conductivity is 43, 45, and 60 W m−1 K−1 for I, II, and III respectively, much smaller than that of graphene (393 W m−1 K−1) in our model. The thermal conductivity decreases with an increase of strain, as well as temperature. The heat transfer performance of structure-II is sensitive to both phonon modes and phonon quantities in compression, while in tension it is determined only by the phonon modes. Our results may be useful in thermal conductivity engineering and heat transfer management in egg-tray graphene. read less

Abstract: To apply carbon nanotubes (CNTs) as reinforcing agents in next-generation composites, it is essential to improve their nominal strength. However, since it is difficult to completely remove the defects, the synthesis guideline for improving nominal strength is still unclear, i.e., the effective strength and the number of nanotube layers required to improve the nominal strength has been undermined. In this study, molecular dynamics simulations were used to elucidate the effects of vacancies on the mechanical properties of CNTs. Additionally, the relationships between the number of layers and effective and nominal strengths of CNTs were discussed theoretically. The presence of extensive vacancies provides a possible explanation for the low nominal strengths obtained in previous experimental measurements of CNTs. This study indicates that the nominal strength can be increased from the experimentally obtained values of 10 GPa to approximately 20 GPa by using six to nine nanotube layers, even if the increase in effective strength of each layer is small. This has advantages over double-walled CNTs, because the effective strength of such CNTs must be approximately 60 GPa to achieve a nominal strength of 20 GPa. read less

Abstract: Efficient application of carbon nanotubes (CNTs) in nano-devices and nano-materials requires comprehensive understanding of their mechanical properties. As observations suggest size dependent behaviour, non-classical theories preserving the memory of body’s internal structure via additional material parameters offer great potential when a continuum modelling is to be preferred. In the present study, micropolar theory of elasticity is adopted due to its peculiar character allowing for incorporation of scale effects through additional kinematic descriptors and work-conjugated stress measures. An optimisation approach is presented to provide unified material parameters for two specific class of single-walled carbon nanotubes (e.g., armchair and zigzag) by minimizing the difference between the apparent shear modulus obtained from molecular dynamics (MD) simulation and micropolar beam model considering both solid and tubular cross-sections. The results clearly reveal that micropolar theory is more suitable compared to internally constraint couple stress theory, due to the essentiality of having skew-symmetric stress and strain measures, as well as to the classical local theory (Cauchy of Grade 1), which cannot accounts for scale effects. To the best of authors’ knowledge, this is the first time that unified material parameters of CNTs are derived through a combined MD-micropolar continuum theory. read less

Abstract: Edge mode could disturb the ultra-subtle mass detection for graphene resonators. Herein, classical molecular dynamics simulations are performed to investigate the effect of edge mode on mass sensing for a doubly clamped strained graphene resonator. Compared with the fundamental mode, the localized vibration of edge mode shows a lower frequency with a constant frequency gap of 32.6 GHz, despite the mutable inner stress ranging from 10 to 50 GPa. Furthermore, the resonant frequency of edge mode is found to be insensitive to centrally located adsorbed mass, while the frequency of the fundamental mode decreases linearly with increasing adsorbates. Thus, a mass determination method using the difference of these two modes is proposed to reduce interferences for robust mass measurement. Moreover, molecular dynamics simulations demonstrate that a stronger prestress or a higher width–length ratio of about 0.8 could increase the low-quality factor induced by edge mode, thus improving the performance in mass sensing for graphene resonators. read less

Abstract: Truncated carbon nanocones (CNCs) can be taken as energy suppliers because of their special structures. In this paper, we demonstrate the stability of truncated CNCs under compression and the escape behavior of a fullerene catapulted from a compressed CNC by molecular dynamics simulations and theoretical models. The strain energy of a CNC and cohesive energy between a fullerene and the CNC (due to their van der Waals interactions) dominate the stability and catapulting capability of the cone, which strongly depend on geometrical parameters (apex angle, top radius and height) of each CNC and axial distances between them. In particular, the additional transverse vibration of buckled CNCs after released plays a significant role in their catapulting abilities and efficiencies. Finally, finite element method and experiments are further performed to validate the escape mechanism. This study should be of great importance to providing a theoretical support for designing novel nanodevices in mico/nanoelectromechanical systems. read less

Abstract: The extreme thinness of graphene combined with its tensile strength made it a material appealing for discussing and even making complex cut-kirigami or folded-only origami. In the case of origami, its stability is mainly defined by the positive energy of the single- or double-fold curvature deformation counterbalanced by the energy reduction due to favorable van der Waals contacts. These opposite sign contributions also have notably different scaling with the size L of the construction, the contacts contributing in proportion to area ~ L2, single folds as ~ L, and highly strained double-fold corners as only ~ L0 = const. Computational analysis with realistic atomistic-elastic representation of graphene allows one to quantify these energy contributions and to establish the length scale, where a single fold is favored (7 nm < L < 21 nm) or a double fold becomes sustainable (L > 21 nm), defining the size of the smallest possible complex origami designs as L ≫ 21 nm. The flexibility and foldability of graphene are some of its attractive properties inspiring the designs of origami structures with potential use in flexible electronics and electromechanical nanodevices. The aesthetics, precision, and ease of folding and stability, however, have limitations at the nanoscale. Here, by means of large-scale atomistic calculations and continuum models, it is quantified how the dimensions determine the relative robustness of the elementary folds of graphene (a single fold and a double-folded graphene forming a single order-four vertex), thereby mapping the spatial resolution limits and providing important guidance for graphene nano-origami realizations. read less

Abstract: ABSTRACT Thermal transport issues are one of the major concerns of scientists as the size decreases. To this end, the thermal conductivity of perfect and defective single-walled carbon nanotubes (SWCNTs) functionalized with carbene, which is an important functional group in nanodevices, is investigated. A series of molecular dynamics (MD) simulations are performed and the thermal conductivity is determined by the approach introduced by Muller-Plathe. The results demonstrate that functionalization decreases the thermal conductivity. Also, the thermal conductivity of functionalized SWCNT declines as the weight percentage of functional group increases. Additionally, it is shown that increasing the weight percentage decreases the sensitivity of thermal conductivity. According to the obtained results, simulation temperature does not affect the thermal conductivity of functionalized SWCNTs considerably compared to pure ones. Finally, it is revealed that the presence of vacancy defect reduces the thermal conductivity of functionalized SWCNTs. read less

Abstract: Newly synthesized nanocars have shown great potential to transport molecular payloads. Since wheels of nanocars dominate their motion, the study of the wheels helps us to design a suitable surface for them. We investigated C60 thermal diffusion on the hexagonal boron-nitride (h-BN) monolayer as the wheel of nanocars. We calculated C60 potential energy variation during the translational and rotational motions at different points on the substrate. The study of the energy barriers and diffusion coefficients of the molecule at different temperatures indicated three noticeable changes in the C60 motion regime. C60 starts to slide on the surface at 30 K-40 K, slides freely on the boron-nitride monolayer at 100 K-150 K, and shows rolling motions at temperatures higher than 500 K. The anomaly parameter of the motion reveals that C60 has a diffusive motion on the boron-nitride substrate at low temperatures and experiences superdiffusion with Levy flight motions at higher temperatures. A comparison of the fullerene motion on the boron-nitride and graphene surfaces demonstrated that the analogous structure of the graphene and hexagonal boron-nitride led to similar characteristics such as anomaly parameters and the temperatures at which the motion regime changes. The results of this study empower us to predict that fullerene prefers to move on boron-nitride sections on a hybrid substrate composed of graphene and boron-nitride. This property can be utilized to design pathways or regions on a surface to steer or trap the C60 or other molecular machines, which is a step toward directional transportation at the molecular scale. read less

Abstract: Edge effects have significant implications in friction at the nanoscale. Despite recent progress, a detailed understanding of the relationship between nanoscale friction and contact edges is still sorely lacking. Here, using molecular dynamics simulations, we investigate the intrinsic effect of the edge size on the nanoscale friction between graphene layers in the incommensurate case based on the model of graphene flakes on a supported graphene substrate. An original rectangular graphene sheet is cut and divided into two independent parts, namely, the inside and outside zones, according to a certain path with a hexagonal boundary. The friction of the inside and the outside flakes placed on a substrate is calculated. The results interestingly reveal that the sum of the friction forces on the inside and outside of flakes, termed the “equivalent friction force”, is substantially greater than that of the original rectangular graphene sheet because the additional edge friction of the former two systems is more than that of the latter system. More importantly, the equivalent friction force is linearly proportional to the edge size due to the larger cropped edge size having more edge friction. This work demonstrates the intrinsic dependence of friction on the contact edge size of incommensurate graphene layers. read less

Abstract: Recently, laser-assisted chemical vapor deposition has been used to synthesize a free-standing, continuous, and stable monolayer amorphous carbon (MAC). MAC is a pure carbon structure composed of randomly distributed five, six, seven, and eight atom rings, which is different from that of disordered graphene. More recently, amorphous MAC-based nanotubes (a-CNT) and nanoscrolls (a-CNS) were proposed. In this work, we have investigated (through fully atomistic reactive molecular dynamics simulations) the mechanical properties and sublimation points of pristine and a-CNT and a-CNS. The results showed that a-CNT and a-CNS have distinct elastic properties and fracture patterns compared to those of their pristine analogs. Both a-CNT and a-CNS presented a non-elastic regime before their total rupture, whereas the CNT and CNS underwent a direct conversion to fractured forms after a critical strain threshold. The critical strain values for the fracture of the a-CNT and a-CNS are about 30% and 25%, respectively, and they are lower than those of the corresponding CNT and CNS cases. Although less resilient to tension, the amorphous tubular structures have similar thermal stability in relation to the pristine cases with sublimation points of 5500 K, 6300 K, 5100 K, and 5900 K for a-CNT, CNT, a-CNS, and CNS, respectively. An interesting result is that the nanostructure behavior is substantially different depending on whether it is a nanotube or a nanoscroll, thus indicating that the topology plays an important role in defining its elastic properties. read less

Abstract: Our extensive molecular dynamics simulations reveal a significant screening effect of monolayer graphene and hexagonal boron nitride (h-BN) on surface deicing of substrates with different degrees of hydrophilicity, including superhydrophilic (SHP) and superhydrophobic (SHB) substrates. Compared with bare surfaces, graphene and h-BN reduce the interfacial shear strength and the normal detaching strength of ice on an SHP substrate but increase the shear and detaching strengths on hydrophobic and SHB substrates. However, the shear and detaching strengths of ice become approximately unified on all of the surfaces, when interface ice layers melt into liquid water, demonstrating the screening capability from graphene and h-BN that weakens the influence of substrates on ice adhesion. Graphene and h-BN coatings suppress ice premelting on the SHP surface and change the dielectric constant of interface ice or water. This work could deepen our understanding of the role of van der Waals crystals in deicing coating. read less

Abstract: Understanding the relationship between the microstructures and overall properties is one of the basic concerns for material design and applications. As a ubiquitous structural configuration in nature, the folded morphology is also widely observed in graphene-based nanomaterials, namely grafold. Recently, a self-folded graphene film (SF-GF) material has been successfully fabricated by the assembly of grafolds and exhibits promising applications in thermal management. However, the dependence of thermal properties of SF-GF on the structural features of grafold has remained unclear. We here develop a theoretical model to describe the thermal transport behavior in SF-GF. Our model demonstrates the relationship between the fold length of grafolds and thermal properties of SF-GF. It serves as an efficient and portable tool to predict the temperature profile and thermal conductivity of SF-GF with good validations by large-scale molecular dynamics simulations. Using this model, we further study the evolution of thermal conductivity of SF-GF with the unfolding deformation during the stretch. Moreover, the effect of geometrical irregularity of grafolds is uncovered. The model developed in this work not only provides practical guidelines for the manipulation and design of thermal properties of SF-GF, but also benefits the understanding of thermal transport behaviors in other two-dimensional nanomaterials with folded structures. read less

Abstract: Coherent and incoherent long-range atomic motions accompany singlet exciton fission in pentacene. Singlet exciton fission (SEF) is a key process for developing efficient optoelectronic devices. An aspect rarely probed directly, yet with tremendous impact on SEF properties, is the nuclear structure and dynamics involved in this process. Here, we directly observe the nuclear dynamics accompanying the SEF process in single crystal pentacene using femtosecond electron diffraction. The data reveal coherent atomic motions at 1 THz, incoherent motions, and an anisotropic lattice distortion representing the polaronic character of the triplet excitons. Combining molecular dynamics simulations, time-dependent density-functional theory, and experimental structure factor analysis, the coherent motions are identified as collective sliding motions of the pentacene molecules along their long axis. Such motions modify the excitonic coupling between adjacent molecules. Our findings reveal that long-range motions play a decisive part in the electronic decoupling of the electronically correlated triplet pairs and shed light on why SEF occurs on ultrafast time scales. read less

Abstract: Molecular dynamics simulations are used to understand the torsional buckling of pristine and irradiated carbon nanotube (CNT) bundles. Irradiation-induced inter-tube defects are shown to significantly increase the critical buckling torque and critical buckling angle, while slightly increasing the torsional stiffness. In contrast, intra-tube defects are found to degrade the torsional properties. Such competing interactions cause irradiation enhancement to occur in large bundles where significant inter-tube bonding can occur. However, the irradiation enhancement effect becomes weak for very large bundles in which enhanced inter-tube interactions already exist in unirradiated bundles. In pristine CNT bundles of all sizes under torsional loading, CNTs can slip via the weakly interacting van der Waals force, whereas in the irradiated bundles, the inter-tube defects prevent slipping. The study further shows that the formation of one-dimensional carbon chain defects contributes to enhanced friction under slipping. read less

Abstract: Various properties of water are affected by confinement as the space-filling of the water molecules is very different from bulk water. In our study, we challenged the creation of a stable system in which water molecules are permanently locked in nanodimensional graphene traps. For that purpose, we developed a technique, nitrocellulose-assisted transfer of graphene grown by chemical vapor deposition, which enables capturing of the water molecules below an atomically thin graphene membrane structured into a net of regular wrinkles with a lateral dimension of about 4 nm. After successfully confining water molecules below a graphene monolayer, we employed cryogenic Raman spectroscopy to monitor the phase changes of the confined water as a function of the temperature. In our experiment system, the graphene monolayer structured into a net of fine wrinkles plays a dual role: (i) it enables water confinement and (ii) serves as an extremely sensitive probe for phase transitions involving water via graphene-based spectroscopic monitoring of the underlying water structure. Experimental findings were supported with classical and path integral molecular dynamics simulations carried out on our experimental system. Results of simulations show that surface premelting of the ice confined within the wrinkles starts at ∼200 K and the melting process is complete at ∼240 K, which is far below the melting temperature of bulk water ice. The processes correspond to changes in the doping and strain in the graphene tracked by Raman spectroscopy. We conclude that water can be confined between graphene structured into nanowrinkles and silica substrate and its phase transitions can be tracked via Raman spectral feature of the encapsulating graphene. Our study also demonstrated that peculiar behavior of liquids under spatial confinement can be inspected via the optical response of atomically thin graphene sensors. read less

Abstract: ABSTRACT The possibility of strain engineering of linear motion property of the double walled carbon nanotube thermal actuator is explored in this paper. Systematical study of axial strain effect on linear motion property of the carbon nanotube thermal actuator is conducted using molecular dynamics, and real-time controlling method for double walled carbon nanotube thermal actuator by applying axial strains is proposed as well. It is found that the axial strain has great influences on the linear motion behaviour of the thermal actuator, implying that the strain effect has potential to control the property of nano actuator. The strain effect and its mechanism are revealed by investigating the variation of thermal driving force and intertube distance as the axial strain is applied. The intertube interaction affected by axial strain is analyzed using the intertube stiffness constant drawn from the analytical model. The valuable conclusions for strain engineering of nanoscale actuator are obtained. This work suggests the great potential of strain effect to be real-time control method to nanoelectromechanical system and provides new ideas for the application of carbon nanotube thermal actuator. read less

Abstract: The motion of two-dimensional (2D) materials on the liquid surface can be controlled by a pre-set temperature gradient. We propose a conceptual design of driving a graphene sheet on the water surface with a temperature gradient and demonstrate that both the velocity and orientation of the motion can be controlled by carefully assigning the magnitude and direction of the gradient of the liquid temperature. The driving force and friction force during the movement of the graphene sheet are derived theoretically by considering the temperature-dependent surface tension of water and the partial slip boundary condition between water and graphene. With this theoretical model, we predict the velocity and direction of the motion of graphene. Comprehensive molecular dynamics (MD) simulations are implemented to validate the theoretical predictions and the results agree well with the theoretical predictions. The motion and assembly of multiple graphene sheets are demonstrated to illustrate the potential application of the temperature gradient of the liquid surface in the manufacturing of low-dimensional materials into architected superstructures. read less

Abstract: Tailoring the composite interfaces with graphene enabled effective utilization of the nanofillers. The nanofiller reinforcing effect in nanocomposites is often far below the theoretically predicted values, largely because of the poor interfacial interaction between the nanofillers and matrix. Here, we report that graphene-wrapped B4C nanowires (B4C-NWs@graphene) empowered exceptional dispersion of nanowires in matrix and superlative nanowire-matrix bonding. The 0.2 volume % B4C-NWs@graphene reinforced epoxy composite exhibited simultaneous enhancements in strength (144.2 MPa), elastic modulus (3.5 GPa), and ductility (15%). Tailoring the composite interfaces with graphene enabled effective utilization of the nanofillers, resulting in two times increase in load transfer efficiency. Molecular dynamics simulations unlocked the shear mixing graphene/nanowire self-assembly mechanism. This low-cost yet effective technique presents unprecedented opportunities for improving nanocomposite interfaces, enabling high load transfer efficiency, and opens up a new path for developing strong and tough nanocomposites. read less

Abstract: Thermoelectric and plasmonic properties of systems comprising small golden nanoparticles (NPs) linked by narrow conductive polymer bridges are studied using the original hybrid quantum‐classical model. The bridges are considered here to be either conjugated polyacetylene, polypyrrole, or polythiophene chain molecules terminated by thiol groups. The parameters required for the model are obtained using density functional theory and density functional tight‐binding simulations. Charge‐transfer plasmons in the considered dumbbell structures are found to possess frequency in the infrared region for all considered molecular linkers. The appearance of plasmon vibrations and the existence of charge flow through the conductive molecule, with manifestation of quantum properties, are confirmed using frequency‐dependent polarizability calculations implemented in the coupled perturbed Kohn–Sham method. To study the thermoelectric properties of the 1D periodical systems, a universal equation for the Seebeck coefficient is derived. The phonon part of the thermal conductivity for the periodical –NP–S–C8H8 – system is calculated by the classical molecular dynamics. The thermoelectric figure of merit ZT is calculated by considering the electrical quantum conductivity of the systems in the ballistic regime. It is shown that for Au309 nanoparticles connected by polyacetylene, polypyrrole, or polythiophene chains at T = 300 K, the ZT value is {0.08;0.45;0.40}, respectively. read less

Abstract: The effects of the intercalated ion concentration on the cross-plane thermal conductivity and the thermal boundary conductance in the graphite/lithiated graphite interface are investigated from molecular dynamics simulations. At low ion concentration, the cross-plane thermal conductivity of the lithiated graphite is lower than that of the pristine graphite. However, as the intercalated ion concentration increases, the cross-plane thermal conductivity increases rapidly, even exceeding that of the pristine graphite at high ion concentration. By analyzing the variations of the cross-plane elastic constants and phonon dispersion relation with the intercalated ion concentration, it is found that the intercalated ions significantly increase the phonon irradiation heat flux along the cross-plane direction. Our study further shows that the variation of the intercalated ion concentrations can also modulate the thermal boundary conductance in the graphite/lithiated graphite interface. The non-equilibrium molecular dynamics simulations show that the thermal boundary conductance between graphite and lithiated graphite decreases as the lithiation level increases, which would worsen the thermal performance of Li-ion batteries. A one-dimensional atomic chain model is proposed to elaborate on how the effective spring stiffness of material influences the interfacial transmission of phonons with different frequencies. This work provides a quantitative calculation of the cross-plane thermal conductivity and thermal boundary conductance in intercalated graphite samples and is also extremely important for the thermal management and structural design of lithium-ion batteries. read less

Abstract: Compared to nanoscale friction of translational motion, the mechanisms of rotational friction have received less attention. Such motion becomes an important issue for the miniaturization of mechanical machineries which often involve rotating gears. In this study, molecular dynamics simulations are performed to explore rotational friction for solid-state gears rotating on top of different substrates. In each case, viscous damping of the rotational motion is observed and found to be induced by the pure van-der-Waals interaction between gear and substrate. The influence of different gear sizes and various substrate materials is investigated. Furthermore, the rigidities of the gear and the substrate are found to give rise to different dissipation channels. Finally, it is shown that the dominant contribution to the dissipation is related to the excitation of low-frequency surface-phonons in the substrate. read less

Abstract: Owing to a finite and single-atom-thick two-dimensional structure, graphene nanostructures such as nanoribbons possess outstanding physical properties and unique size-dependent characteristics due to nanoscale defects, especially for mechanical properties. Graphene nanostructures characteristically exhibit strong nonlinearity in deformation and the defect brings about an extremely localized singular stress field of only a few nanometers, which might lead to unique fracture properties. Fundamental understanding of their fracture properties and criteria is, however, seriously underdeveloped and limited to the level of continuum mechanics and linear elasticity. Here, we demonstrate the breakdown of continuum-based fracture criteria for graphene nanoribbons due to the strong nonlinearity and discreteness of atoms emerging with decreasing size and identify the critical sizes for these conventional criteria. We further propose an energy-based criterion considering atomic discrete nature, and show that it can successfully describe the fracture beyond the critical sizes. The complete clarification of fracture criterion for nonlinear graphene with nanoscale singularity contributes not only to the reliable design of graphene-based nanodevices but also to the elucidation of the extreme dimensional limit in fracture mechanics. read less

Abstract: Graphene, as a typical two-dimensional material, is popular in the design of nanodevices. The interlayer relative sliding of graphene sheets can significantly affect the effective bending stiffness of the few-layered graphene. For restricting the relative sliding, we adopted the atomic shot peening method to bond the graphene sheets together by ballistic C60 fullerenes from its two surfaces. Collision effects are evaluated via molecular dynamics simulations. Results obtained indicate that the fullerenes’ incident velocity has an interval, in which the graphene sheet can be bonded after collision while no atoms on the fullerenes escaping from the graphene ribbon after collision. The limits of the interval increase with the layer number. Within a few picoseconds of collision, a stable carbon network is produced at an impacted area. The graphene sheets are bonded via the network and cannot slide relatively anymore. Conclusions are drawn to show the way of potential applications of the method in manufacturing a new graphene-based two-dimensional material that has a high out-of-plane bending stiffness. read less

Abstract: Two-dimensional (2D) transition-metal dichalcogenides (TMDs) hold great potential for many important device applications, such as field effect transistors and sensors, which require a robust control of defect type, density, and distribution. However, how to control the defect type, density, and distribution in these materials is still a challenge. In this study, we explore the kinetics and dynamics of four types of grain boundaries (GBs) in monolayer MoS2, which are composed of S-polar dislocation (S5|7), Mo-polar dislocation (Mo5|7), dislocation-double S vacancy complex (S4|6), and dislocation-double S interstitial complex (S6|8), respectively. Our study shows that these four GBs in monolayer MoS2 exhibit a great disparity in their migration behavior. More specifically, the S4|6 and S6|8 GBs possess a much higher migration mobility than the S5|7 and Mo5|7 GBs under the same thermal fluctuations or temperature gradient. Interestingly, the S4|6 and S6|8 GBs follow an abnormal relationship with temperature, due to the change in defect configurations with temperature. Our study further shows that the remarkably high mobilities of the S4|6 and Mo6|8 GBs may enable the reactions of GBs, leading to the annihilation and reduction of defect density. In addition, the movement of GBs in MoS2 under a temperature gradient field can cause defect redistribution, which in turn changes the thermal conductivity. The present study not only deepens our understanding of the dynamic evolution of GBs in TMDs, but also presents new opportunities to engineer GBs for novel electronic applications. read less

Abstract: In this research, the thermal transport behavior of the branched carbon nanotube (CNT) with T-junction was investigated using non-equilibrium molecular dynamics simulation. Both symmetric and asymmetric temperature-controlled simulations were imposed to evaluate how the heat flowed inside the branched CNT with three branches of equal length and same chirality. The branch length and strain effects on the heat flow were examined. In addition, the simulated heat flow was compared with the prediction made by conventional thermal circuit calculation based on diffusive phonon transport. The heat was observed to flow straight rather than sideway inside the branched CNT with T-junction under the asymmetric temperature setup; this finding contradicts the conventional thermal circuit calculation. There are two possible explanations for this phenomenon. One is ballistic phonon transport and the other is phonons have different interactions or scattering with the defective atomic configurations at the T-junction. Moreover, the tensile strain could tune the heat flow, a finding that might be useful in thermal management applications. read less

Abstract: Thermal metamaterials can effectively manipulate heat flux to achieve different thermal management functions, such as thermal cloak, concentrator, and rotator. To date, most of these metamaterials are based on macroscopic compound structures, such as metal/polymer. Herein, the concept of thermal metamaterials is extended to two‐dimensional (2D) graphene‐based systems because of their fast response speeds, in contrast to traditional three‐dimensional metamaterials. Three thermal metamaterials with heterogeneous thermal parameters are constructed using nano‐holed graphene, and some extraordinary thermal phenomena, such as effective shielding, accumulation and rotation of heat flux, are observed due to the significant anisotropic thermal conductivity of these 2D systems. Moreover, these thermal phenomena are insensitive to external disturbances. For designing thermal metamaterials, this study provides a novel approach, which can be applied to other 2D thermal functional materials. read less

Abstract: With the rapid development of electronic materials and technologies, the working frequencies of electronic components and devices have been greatly improved and the volume of electronic products has been shrinking. The integration density has increased significantly, which puts forward higher requirements for thermal management. One of the keys to the heat dissipation of electronic components is to transfer the heat rapidly to the radiator through the heat conducting medium. Therefore, the development of high conductive materials has become a research hotspot of high-density integrated devices and systems. Due to their excellent heat transfer properties, carbon nanomaterials such as carbon nanotube and graphene have attracted extensive attention. The thermal conductivities of carbon nanotube and graphene have obvious anisotropy, which limited their applications to some extent. In this paper, three-dimensional composite structures composed of graphene sheets and carbon nanotubes are considered. The heat transfer processes are simulated by molecular dynamics method and the heat transfer characteristics of van der Waals interaction and chemical bond structures are analyzed. The effects of heat flow and nanotube layout on the thermal properties of three-dimensional composite structures are discussed. read less

Abstract: Amorphous carbons are disordered carbons with densities of circa 1.9–3.1 g/cc and a mixture of sp2 and sp3 hybridization. Using molecular dynamics simulations, we simulate diffusion in amorphous carbons at different densities and temperatures to investigate the transition between amorphous carbon and the liquid state. Arrhenius plots of the self-diffusion coefficient clearly demonstrate that there is a glass transition rather than a melting point. We consider five common carbon potentials (Tersoff, REBO-II, AIREBO, ReaxFF and EDIP) and all exhibit a glass transition. Although the glass-transition temperature (Tg) is not significantly affected by density, the choice of potential can vary Tg by up to 40%. Our results suggest that amorphous carbon should be interpreted as a glass rather than a solid. read less

Abstract: We find stiffness properties of van der Waals heterostructures made of graphene and carbon nanotubes (CNTs) using molecular dynamics (MD) simulations. The variation of the stiffness tensor is studied at different temperatures for structures with changing CNT density, and orientations. Orthotropic structures with unidirectional CNTs have high stiffness ∼300–500 GPa, in one direction. Structures with transverse isotropy show stiffness of ∼150–350 GPa along two mutually perpendicular directions. The orthotropic structures also show greater stiffness with increasing CNT density, while the others are insensitive to it. In general,the stiffness constant values are decrease as temperatures increase. The results can aid in determining useful heterostructure configuration for a particular application and in finding overall stiffness of devices having additional components. read less

Abstract: Small organic molecules are thought to provide building blocks for the formation of complex interstellar polycyclic aromatic hydrocarbons (PAHs). However, the underlying chemical mechanisms remain unclear, particularly concerning the role of interstellar dust. Using molecular dynamics, we simulate the chemical reaction between dehydrogenated benzene molecules in the gas phase or on the surface of an onion-like carbon nanoparticle (NP). The reaction leads to the formation of PAHs of complex structures. The size of the formed molecules is found to roughly increase with increasing temperature up to 800 K, and to be correlated with the level of dehydrogenation. Morphology analysis features the formation of large rings that contain up to 32 carbon atom at high temperature. Density functional theory (DFT) calculations are performed to search the fundamental energetic reaction pathways. The DFT results quantitatively confirm the correlation between the reactivity and the dehydrogenation level, and the formation of stable C-8 rings. Moreover, the nanostructures formed on the NP surface point to a possible layer-by-layer formation mechanism for interstellar fullerene and carbon onions. read less

Abstract: ABSTRACT Exploring the thermal transport of graphene is significant for the application of its thermal properties. However, it is still a challenge to regulate the thermal conductivity of graphene interface. We study the interfacial thermal transport mechanism of the bilayer graphene by utilizing the molecular dynamics simulations. During the simulation, the interfacial thermal conductivity is regulated and controlled by lattice matching and tailoring. The lattice mismatched bilayer graphene model, combining the straining and torsion, can increase the interfacial thermal resistance (ITR) about 3.7 times. The variation trend of the ITR is explained by utilizing the vibrational spectra and the overlap factor. Besides, the thermal conductivity is proportional to the overlapping area. Our results show that the tailoring models can regularly control the thermal conductivity in a wide range by twisting the angle between upper and lower layers. These findings can provide a guideline for thermoelectric management and device design of thermal switch. read less

Abstract: Cove-edged graphene nanoribbons (CGNR) are a class of nanoribbons with asymmetric edges composed of alternating hexagons and have remarkable electronic properties. Although CGNRs have attractive size-dependent electronic properties their mechanical properties have not been well understood. In practical applications, the mechanical properties such as tensile strength, ductility and fracture toughness play an important role, especially during device fabrication and operation. This work aims to fill a gap in the understanding of the mechanical behaviour of CGNRs by studying the edge and size effects on the mechanical response by using molecular dynamic simulations. Pristine graphene structures are rarely found in applications. Therefore, this study also examines the effects of topological defects on the mechanical behaviour of CGNR. Ductility and fracture patterns of CGNR with divacancy and topological defects are studied. The results reveal that the CGNR become stronger and slightly more ductile as the width increases in contrast to normal zigzag GNR. Furthermore, the mechanical response of defective CGNRs show complex dependency on the defect configuration and distribution, while the direction of the fracture propagation has a complex dependency on the defect configuration and position. The results also confirm the possibility of topological design of graphene to tailor properties through the manipulation of defect types, orientation, and density and defect networks. read less

Abstract: Wetting state transition regulated by surface roughness has increasing importance for its wide applications. Molecular dynamics simulations have been performed to study the wetting state transition induced by surface roughness in the gallium-carbon system. There is a transition from the Wenzel state to the Cassie state when the roughness is changed. When the surface roughness is more than 1.8, the gallium droplet is in a Cassie state, but when it is less than 1.6, it is in the Wenzel state. The substrate composed of irregular pillars has a similar effect on the wetting state transition. Besides, distinctive variations occur in the interface tension, the mean-squared displacement, the wetted surface and the interaction energy as the wetting state changes, which are further explained by the proposed model. This study would provide significant guidance for designing superhydrophobic surfaces in the future. read less

Abstract: Thermal transport in graphene is strongly influenced by strain. We investigate the influence of biaxial tensile strain on the thermal conductivity of zigzag and armchair graphene (AG and ZG) using non-equilibrium molecular dynamics simulations (NEMD). We observe that the thermal conductivity is significantly reduced under strain with a maximum reduction obtained at equi-biaxial strain. It is interesting to note that the high lateral to longitudinal strain ratios reduce the negative impact of strain on the thermal conductivity of AG and ZG. The in-plane acoustic modes are found to be the major heat carriers in unstrained graphene but are severely softened due to strain, and hence, their contribution to the conductivity drops down significantly. Strain alleviates the out-of-plane fluctuations in graphene and the group velocity of the out-of-plane acoustic mode (ZA) increases due to the linearisation of its dispersion relation. These factors result in the dominance of ZA mode in the thermal transport of strained graphene. Significant increase in the size dependence of the thermal conductivity of strained graphene is observed, which is attributed to the long-wavelength ZA phonons. The discrepancies between the results of BTE studies and NEMD are also discussed. This study suggests that biaxial strain can be an effective method to tune the thermal transport in graphene. Our findings can lead to better phonon engineering of graphene for various nanoscale applications. read less

Abstract: Bulky sp2-carbon Schwarzites with negative Gaussian curvature are promising structures for practical applications due to their unique properties such as high surface area, large porosity, and stability against graphitization. Herein, a comprehensive study on the tension, compression and shear mechanical characteristics of seven triply periodic carbon Schwarzite foams with distinct topologies is performed using reactive molecular dynamics (MD) simulations. All carbon Schwarzites exhibit unique thermal and mechanical properties that are markedly dictated by the topology. One of the structures presents a negative thermal expansion coefficient. Under uniaxial tension, the temperature is able to play a positive or negative role in the tensile stiffness, and there is no apparent positive relationship between tensile strength and mass density. Subjected to compression and shear loads, carbon Schwarzites can fail due to brittle fracture, and uniform and stepwise structural instabilities. Both compression- and tension-negative Poisson's ratios are revealed to originate from a curvature-flattening deformation mechanism. Analysis of the crush force efficiency, the stroke efficiency and the energy-absorption demonstrates that carbon Schwarzites are effective energy-absorbers. This study provides a fundamental understanding of the relationship between the topology and mechanical properties of carbon Schwarzites for designing 3D graphitic nanostructures with good mechanical performances. read less

Abstract: Octadecane is an alkane that is used to store thermal energy at ambient temperature as a phase change material. A molecular dynamics study was conducted to investigate the effects of adding graphene and a boron nitride nanosheet on the thermal and structural properties of octadecane paraffin. The PCFF force field for paraffin, AIREBO potential for graphene, Tersoff potential for the boron nitride nanosheet, and Lennard-Jones potential for the van der Waals interaction between the nanoparticles and n-alkanes were used. Equilibrium and nonequilibrium molecular dynamics simulations were used to study the nano-enhanced phase change material properties. Results showed that the nanocomposite had a lower density change, more heat capacity (except at 300 K), more thermal conductivity, and a lower diffusion coefficient in comparison with pure paraffin. Additionally, the nanocomposite had a higher melting point, higher phonon density of state and radial distribution function peaks. read less

Abstract: Nanoindentation simulations are performed for a Ni(111) bi-crystal, in which the grain boundary is coated by a graphene layer. We study both a weak and a strong interface, realized by a 30∘ and a 60∘ twist boundary, respectively, and compare our results for the composite also with those of an elemental Ni bi-crystal. We find hardening of the elemental Ni when a strong, i.e., low-energy, grain boundary is introduced, and softening for a weak grain boundary. For the strong grain boundary, the interface barrier strength felt by dislocations upon passing the interface is responsible for the hardening; for the weak grain boundary, confinement of the dislocations results in the weakening. For the Ni-graphene composite, we find in all cases a weakening influence that is caused by the graphene blocking the passage of dislocations and absorbing them. In addition, interface failure occurs when the indenter reaches the graphene, again weakening the composite structure. read less

Abstract: Carbon nanotube complexes are known for their miraculous mechanical and electronic properties that are crucial for nano-electromechanical systems (MEMS). In this study, through molecular dynamics simulations we found for the first time that the electric field and temperature can be used to co-regulate a reversible change of cross-sectional configuration of single-wall carbon nanotubes (SWCNTs). We showed that the electric field can help induce the collapse of an SWCNT when it contains a water droplet, while the increase of temperature can quickly recover its configuration. This controllable bistability of SWCNTs is promising for the design of nanodevices such as electromechanical switches in NEMS. read less

Abstract: In this paper, we present the results of a computational study of the electrical and photovoltaic properties of a perspective composite material; that is, layered composite films of covalently bonded graphene and single-walled carbon nanotubes (SWCNTs). The purpose of the study is to identify the topological patterns in controlling the electrical and photovoltaic properties of mono- and bilayer graphene/CNT composite films with a covalent bonding of a nanotube and graphene sheet, using in silico methods. This in silico study was carried out for the super-cells of mono- and bilayer graphene/CNT composite films with the CNTs (10,0) and (12,0) at distances between the nanotubes of 10 and 12 hexagons. This found that the type of conductivity of the nanotubes does not fundamentally affect the patterns of current flow in the graphene/CNT composite films. This control of the diameter of the nanotubes and the distance between them allows us to control the profile of the absorption spectrum of the electromagnetic waves in the range of 20–2000 nm. The control of the distance between the SWCNTs allows one to control the absorption intensity without a significant peak shift. This revealed that there is no obvious dependence of the integrated photocurrent on the distance between the nanotubes, and the photocurrent varies between 3%–4%. read less

Abstract: We present an adaptive mesoscale model for carbon nanotube (CNT) systems. In our model, CNTs are represented as a chain of nodes connected by tensile and torsion springs to describe stretching and ... read less

Abstract: ABSTRACT In this paper, a nanoscale rotary system, composed of graphene substrate and triple-walled nanotube driven by defect effect, is proposed. Its rotational properties, as well as the influence of temperature, defect location and chiral combination of carbon nanotube, are systematically investigated using molecular dynamic simulations. It is found that the rotation of the nanotubes is driven by the defects placed on the graphene with a stable rotation frequency, and that the source of energy for the rotary system is originated from the unbalanced atomic vibration on graphene near carbon nanotubes. The results show that the system temperature, the location of the defect and the chiral pair of the carbon nanotubes have effects on the rotor rotation frequency. Based on the analysis of molecular dynamic simulation results, the mechanism of this rotation is studied. The application of nano-rotation system, composed of graphene and triple-walled carbon nanotubes in nanomachines, is prospected. read less

Abstract: The broad incorporation of microscopic methods is yielding a wealth of information on atomic and mesoscale dynamics of individual atoms, molecules, and particles on surfaces and in open volumes. Analysis of such data necessitates statistical frameworks to convert observed dynamic behaviors to effective properties of materials. Here we develop a method for stochastic reconstruction of effective acting potentials from observed trajectories. Using the Silicon vacancy defect in graphene as a model, we develop a statistical framework to reconstruct the free energy landscape from calculated atomic displacements. read less

Abstract: Origami offers a distinct approach for designing and engineering new material structures and properties. The folding and stacking of atomically thin van der Waals (vdW) materials, for example, can lead to intriguing new physical properties including bandgap tuning, Van Hove singularity, and superconductivity. On the other hand, achieving well‐controlled folding of vdW materials with high spatial precision has been extremely challenging and difficult to scale toward large areas. Here, a deterministic technique is reported to fold vdW materials at a defined position and direction using microfluidic forces. Electron beam lithography (EBL) is utilized to define the folding area, which allows precise control of the folding geometry, direction, and position beyond 100 nm resolution. Using this technique, single‐atomic‐layer vdW materials or their heterostructures can be folded without the need for any external supporting layers in the final folded structure. In addition, arrays of patterns can be folded across a large area using this technique and electronic devices that can reconfigure device functionalities through folding are also demonstrated. Such scalable formation of folded vdW material structures with high precision can lead to the creation of new atomic‐scale materials and superlattices as well as opening the door to realizing foldable and reconfigurable electronics. read less

Abstract: Recently (C.-T. Toh et al., Nature 577, 199 (2020)), the first synthesis of free-standing monolayer amorphous carbon (MAC) was achieved. MAC is a pure carbon structure composed of five, six, seven and eight atom rings randomly distributed. MAC proved to be surprisingly stable and highly fracture resistant. Its electronic properties are similar to boron nitride. In this work, we have investigated the mechanical properties and thermal stability of MAC models using fully-atomistic reactive molecular dynamics simulations. For comparison purposes, the results are contrasted against pristine graphene (PG) models of similar dimensions. Our results show that MAC and PG exhibit distinct mechanical behavior and fracture dynamics patterns. While PG after a critical strain threshold goes directly from elastic to brittle regimes, MAC shows different elastic stages between these two regimes. Remarkably, MAC is thermally stable up to 3600 K, which is close to the PG melting point. These exceptional physical properties make MAC-based materials promising candidates for new technologies, such as flexible electronics. read less

Abstract: The structure and morphology of the reinforcing material play an important role in the vibration damping characteristics of polymer composites. In this work, multiwalled carbon nanotubes (MWCNTs) with different structures and morphologies are incorporated into a polymer matrix. The vibration damping characteristics of the nanocomposites, in Oberst beam configuration, are studied using a free vibration test in cantilever mode. Inner tube oscillation is established as the vibration damping mechanism by correlating the extent of the loss factor obtained from the two nanocomposites with the dissimilarities in the structure and morphology of the two varieties of MWCNTs. Inner tube oscillation is simulated using molecular dynamics (MD). Since the open-ended double walled CNT (DWCNT) models used in earlier studies over predict the damping, we propose a capped DWCNT model. This can simulate the atomic interactions at the end caps of the tube. This study indicates that the contributions to the observed damping have their origins in the interaction between atoms that constitute the inner and outer tubes rather than the inter-tube frictional energy loss. read less

Abstract: Double elastic shock waves are rarely observed in two-dimensional (2D) materials and normally unexpected for elastically isotropic 2D crystals such as graphene. With large-scale molecular dynamics simulations, we show that in single-crystal graphene shock-loaded along nonzigzag and nonarmchair directions, double elastic shock waves (quasilongitudinal and quasitransverse) can emerge. Quantitative acoustic wave equation analysis reveals that shock-induced symmetry reduction in lattice, as well as in elastic stiffness tensor, gives rise to the normally unexpected quasitransverse wave following the quasilongitudinal wave.Double elastic shock waves are rarely observed in two-dimensional (2D) materials and normally unexpected for elastically isotropic 2D crystals such as graphene. With large-scale molecular dynamics simulations, we show that in single-crystal graphene shock-loaded along nonzigzag and nonarmchair directions, double elastic shock waves (quasilongitudinal and quasitransverse) can emerge. Quantitative acoustic wave equation analysis reveals that shock-induced symmetry reduction in lattice, as well as in elastic stiffness tensor, gives rise to the normally unexpected quasitransverse wave following the quasilongitudinal wave. read less

Abstract: ABSTRACT Recent work suggests that layered solids deform through buckling of basal planes. When isolated locally, as in graphite, these buckles, termed ripplocations, behave superficially similar to dislocations, but have no Burgers vectors. Through atomistic simulations, we demonstrate the easy transitions of ripplocations in graphite between many closely-spaced energy states, even at low temperatures. Between 60 and 350 K, their migration barrier is estimated at 32 meV, independent of segment length. Ripplocations spontaneously migrate towards vacancies and away from compressive stresses. These results shed more light on this new micromechanism and potentially explain experimental observations that evade sufficient description through dislocation-based models. GRAPHICAL ABSTRACT IMPACT STATEMENT These results shed more light on this new micromechanism and the high mobility and vacancy interactions of ripplocations potentially explain experimental observations that evade sufficient description through dislocation-based model. read less

Abstract: ABSTRACT To account for certain essential features of material such as dispersive behaviour and optical branches in dispersion curves, a fundamental departure from classical elasticity to polar theories is required. Among the polar theories, micromorphic elasticity of appropriate grades and anisotropy is capable of capturing these physical phenomena completely. In the mathematical framework of micromorphic elasticity, in addition to the traditional elastic constants, some additional constants are introduced in the pertinent governing equations of motion. A precise evaluation of the numerical values of the aforementioned elastic constants in the realm of the experimentations poses serious difficulties. Thus this paper aims to provide a remedy as how to determine the micromorphic elastic constants theoretically in terms of the atomic force constants and lattice parameters of the crystalline solid with general anisotropy. In this treatment capture of the discrete nature of matter becomes an essential factor. To this end, the discrete lattice dynamics equations of a crystal are related to the pertinent anisotropic micromorphic equations of motion. This approach allows incorporating the symmetry groups of the crystals within lattice dynamics equations conveniently. For the illustration of the current theoretical developments, the micromorphic elastic constants of diamond and silicon crystals are computed in conjunction with ab initio density functional perturbation theory (DFPT). Moreover, the longitudinal and transverse optical and acoustic branches pertinent to [100] and [110] directions are presented. The accuracy of the results is verified by comparing the dispersion curves derived from the micromorphic theory, those of available experiments, and those directly obtained from DFPT calculations. read less

Abstract: In this paper, we perform molecular dynamics simulations to propose a novel bio-inspired nanopumping mechanism that is achieved through the rotation of graphene nanoribbons. Due to the rotation and interaction with water, the graphene nanoribbons undergo morphological transformation. It is shown that with appropriate geometrical and spatial parameters, the resulting morphology is twisted ribbon, which is efficient in pumping of water through a channel. This mimics the propulsive behavior of bacterial flagella through continual rotation at the base and causing morphology of the geometry into twisted ribbons, thus driving flow. It was observed that the maximum flux rate decreases upon reaching the optimal configuration even with increasing rotational speed and graphene width. This is due to the development of cavitation near the region of the nanoribbon with tip velocities approaching the speed of sound in water. The simulation shows promising results where the flux rate of the driven flow outperforms various nanopump configurations that have been reported in recent literature by more than one order. read less

Abstract: Triply Periodic Minimal Surfaces (TPMS) possess locally minimized surface area under the constraint of periodic boundary conditions. Different families of surfaces were obtained with different topologies satisfying such conditions. Examples of such families include Primitive (P), Gyroid (G) and Diamond (D) surfaces. From a purely mathematical subject, TPMS have been recently found in materials science as optimal geometries for structural applications. Proposed by Mackay and Terrones in 1991, schwarzites are 3D crystalline porous carbon nanocrystals exhibiting a TPMS-like surface topology. Although their complex topology poses serious limitations on their synthesis with conventional nanoscale fabrication methods, such as Chemical Vapour Deposition (CVD), schwarzites can be fabricated by Additive Manufacturing (AM) techniques, such as 3D Printing. In this work, we used an optimized atomic model of a schwarzite structure from the D family (D8bal) to generate a surface mesh that was subsequently used for 3D-printing through Fused Deposition Modelling (FDM). This D schwarzite was 3D-printed with thermoplastic PolyLactic Acid (PLA) polymer filaments. Mechanical properties under uniaxial compression were investigated for both the atomic model and the 3D-printed one. Fully atomistic Molecular Dynamics (MD) simulations were also carried out to investigate the uniaxial compression behavior of the D8bal atomic model. Mechanical testings were performed on the 3D-printed schwarzite where the deformation mechanisms were found to be similar to those observed in MD simulations. These results are suggestive of a scale-independent mechanical behavior that is dominated by structural topology. read less

Abstract: Freestanding indentation is a widely used method to characterise the elastic properties of two-dimensional (2D) materials. However, many controversies and confusion remain in this field due to the lack of appropriate theoretical models in describing the indentation responses of 2D materials. Taking the multilayer gallium telluride (GaTe) as an example, in this paper we conduct a series of experiments and simulations to achieve a comprehensive understanding of its freestanding indentation behaviours. Specifically, the freestanding indentation experiments show that the elastic properties of the present multilayer GaTe with a relatively large thickness can only be extracted from the bending stage in the indentation process rather than the stretching stage widely utilised in the previous studies on thin 2D materials, since the stretching stage of thick 2D materials is inevitably accompanied with severe plastic deformations. In combination with existing continuum mechanical models and finite element simulations, an extremely small Young’s modulus of multilayer GaTe is obtained from the nanoindentation experiments, which is two orders of magnitude smaller than the value obtained from first principles calculations. Our molecular dynamics (MD) simulations reveal that this small Young’s modulus can be attributed to the significant elastic softening in the multilayer GaTe with increasing thickness and decreasing length. It is further revealed in MD simulations that this size-induced elastic softening originates from the synergistic effects of interlayer compression and interlayer shearing in the multilayer GaTe, both of which, however, are ignored in the existing indentation models. To consider these effects of interlayer interactions in the theoretical modelling of the freestanding indentation of multilayer GaTe, we propose here novel multiple-beam and multiple-plate models, which are found to agree well with MD results without any additional parameters fitting and thus can be treated as more precise theoretical models in characterising the freestanding indentation behaviours of 2D materials. read less

Abstract: In this work, we study the peeling of a cylindrical shell attached to a smooth rigid substrate and subjected to a vertical force. A generalized peeling model based on the energy-variational approach is presented, and its numerical solutions characterize the cross-section profile and peeling force. The interfacial interactions are represented by the Lennard-Jones potential. Molecular dynamics simulations are performed for the peeling system with single-walled carbon nanotubes and gold substrates, and simulation results show good agreement with the theoretical predictions. We show that there are three stages (stable peeling stage, line-contact stage, and pull-off stage) in the entire peeling process. A spring-like behavior is observed in the stable peeling stage. With the peeling displacement increasing, the second stage has a marked feature of line contact and the peeling force arrives at a peak pull-off force. Furthermore, we show that the pull-off force strongly depends on the flexural stiffness of cylindrical shell and two Lennard-Jones parameters, but is independent of the initial radius of cylindrical shell. Our findings may help to reveal the interactions between thin-walled nanotubes and substrates. read less

Abstract: An accurate prediction of the mechanical behavior of long carbyne chains depends on the suitable modeling of bond alternation in these chains. While first-principles methods are a good approach, less computationally demanding empirical potentials are preferable for large carbyne-containing systems. AIREBO and Reax empirical potentials have extensively and successfully been used for simulating the mechanical behavior of graphene and carbon nanotubes. However, it remains unclear if these potentials can be directly applied in the accurate mechanical modeling of carbon nanostructures with sp hybridization, without re-parameterization. Here, a new force-field for carbyne, designated as C13 potential, that takes bond alternation into account, is presented. This new empirical potential was parameterized from ab initio calculations. Molecular dynamics (MD) simulations using the developed force-field are then conducted to determine the mechanical properties of carbyne chains under tensile loading, namely to assess their dependence on chain length and temperature. The bending stiffness of carbyne and its persistence length are also calculated. The results obtained are validated through comparison with results available in the literature. Lastly, the C13 potential is employed to model, for the first time, the tensile and the compressive behaviors of the hybrid system composed of carbon nanotubes infilled with carbyne chains. read less

Abstract: The extraordinary properties of graphene have made it an elite candidate for a broad range of emerging applications since its discovery. However, the introduction of structural defects during graphene production often compromises the theoretically predicted performance of graphene-based technologies to a great extent. In this study, a counterintuitive defect enlargement strategy to recover from defect-induced mechanical degradation is explored, of which the realization may lead to an enhanced operating efficiency and manufacturing feasibility. Our molecular-dynamics simulation results show that the enlargement of a preexisting defect to an elliptical shape can potentially recover from the mechanical degradation that the very defect has caused. For a defective graphene sheet having a failure strain of 48% of the pristine graphene sheet, enlarging the defect can enhance the failure strain up to 80% of the pristine graphene sheet. The mechanism of degradation recovery lies in a reduced change in curvature during deformation, which is further solidified by theoretical quantification and stress-field analysis. This theory can also predict and pinpoint the location of the initiation of the fracture—where the curvature changes most significantly during the deformation. In addition, the influence of an elliptical defect on the mechanical properties of a graphene sheet is systematically studied, which is not well understood today. Finally, the degradation recovery potential of defect of various sizes is examined, showing that the initial defect that can create the highest degree of geometric asymmetry has the best potential for degradation recovery. This study investigates the recovery from defect-induced mechanical degradation and the influence of elliptical defects on the mechanical properties of a graphene sheet, which widens our understanding of the possibility of fine-tuning mechanical properties via defect engineering and has the potential to improve materials for emerging technologies such as supercapacitor devices. read less

Abstract: The cross-sectional shape of the nanotube is a key factor governing fundamental mechanical properties of the nanotube and the nanotube forest. In contrast to most circular nanotubes, in the present work, we demonstrate that the holey phenine nanotubes have polygonal cross sections with diameter-dependent number of sides. The non-circular cross section is attributed to the high twistability of the continuous C–C chains in the phenine nanotube. Consequently, the phenine nanotube forest has a square lattice structure rather than the regular hexagonal lattice of the carbon nanotube forest, resulting in a smooth buckling process under biaxial compression. The buckling pattern of the phenine nanotube forest is highly ordered with the orientation determined by the initial dislocation that frequently appears in the phenine nanotube forest. read less

Abstract: Based on the experimentally observed templating effects in CNTs containing carbon fibers, new types of inter-connected annular graphite structures are proposed and designed in order to significantly improve the cross-plane thermal conductivity of graphite. The calculations of the thermal conductivity of the newly designed structures were carried out by combining macroscopic continuous equations and microscopic molecular dynamic (MD) simulations. First, MD simulation was used to examine the influence of bending curvature on the in-plane thermal conductivity of a graphene sheet along and perpendicular to the rolling direction. Next, various types of annular graphite structures with single and inter-connected double/triple multi-layered graphite sheets were designed. Finally, finite element analysis was used to calculate the effective out of plane thermal conductivities of these structural models. The cross-plane thermal conductivity of a common graphite film is 2-3 orders of magnitude lower than its in-plane thermal conductivity, which strongly restricts its heat dissipation ability. However, the formation of annular graphite structures and the inter-connections of the outer layers lead to a dramatic improvement of effective out of plane thermal conductivity from 2.3 W m-1 K-1 to 799.8 W m-1 K-1 in this work, which is superior to common metal materials, especially considering the relatively lower density of carbon materials. These results would be valuable for designing and fabricating highly thermally conductive carbon materials for heat dissipation and temperature management. read less

Abstract: Lightweight materials with high ballistic impact resistance and load-bearing capabilities are regarded as a holy grail in materials design. Nature builds these complementary properties into materials using soft organic materials with optimized, complex geometries. Here, the compressive deformation and ballistic impact properties of three different 3D printed polymer structures, named tubulanes, are reported, which are the architectural analogues of cross-linked carbon nanotubes. The results show that macroscopic tubulanes are remarkable high load-bearing, hypervelocity impact-resistant lightweight structures. They exhibit a lamellar deformation mechanism, arising from the tubulane ordered pore structure, manifested across multiple length scales from nano to macro dimensions. This approach of using complex geometries inspired by atomic and nanoscale models to generate macroscale printed structures allows innovative morphological engineering of materials with tunable mechanical responses. read less

Abstract: Water molecules at solid surfaces typically arrange in layers. The physical origin of the hydration layers is usually explained by two different reasons: (1) the attraction between the surface and water and (2) the water confinement due to the surface. While the attraction is specific to the particular solid, the confinement is a general property of surfaces; a differentiation between the two effects is, therefore, critical for research on interactions at aqueous interfaces. Here, we investigate the graphite-water interface, which is a widely used model system where the solid-water attraction is often considered to be negligible. Similar to previous studies, we observe hydration layers using three-dimensional atomic force microscopy at the graphite-water interface. We explain why the confinement could cause the formation of hydration layers even in the absence of attraction between surface and water by employing Monte Carlo simulations. Using additional molecular dynamics simulations, we continue to show that at ambient conditions, however, the confinement alone does not cause the formation of layers at the graphite-water interface. We thereby demonstrate that there is a significant graphite-water attraction. read less

Abstract: There is a great deal of attention given to spiral carbon-based nanostructures (SCBNs) because of their unique mechanical, thermal and electrical properties along with fascinating morphology. Dispersing SCBNs inside a polymer matrix leads to extraordinary properties of nanocomposites in diverse fields. However, the role of the interfacial mechanical properties of these nanocomposites remains unknown. Here, using molecular dynamics simulations, the characteristics of interfacial load transfer of SCBN-polyethylene nanocomposites are explored. Considering the geometric characteristics of SCBNs, new insight into the separation behavior of nanoparticles in normal and sliding modes is addressed. Interestingly, the results show that the maximum force and the separation energy of the SCBNs are much larger than those of graphene because of interlocking of the coils and polymer. The heavy influence of changes in the geometric characteristics of SCBNs on the separation behavior is observed. Pullout tests reveal that the influence of parameters such as the length and number of polyethylene chains, temperature, and functionalization of the SCBNs on the interfacial mechanical properties is also significant. This study sheds new light in understanding the crucial effect of the interaction of SCBNs with polymer chains on the interfacial mechanical properties, which can lead to better performance of nanocomposites. read less

Abstract: We report results of a systematic computational study on the mechanical response of graphene nanomeshes (GNMs) to uniaxial tensile straining based on molecular-dynamics simulations of dynamic deformation tests according to a reliable bond-order interatomic potential. We examine the effects on the GNM mechanical behavior under straining along different directions of the nanomesh pore morphology and pore edge passivation by testing GNMs with elliptical pores of various aspect ratios and different extents of edge passivation through termination with H atoms of under-coordinated edge C atoms. We establish the dependences of the ultimate tensile strength, fracture strain, and toughness of the GNMs on the nanomesh porosity, derive scaling laws for GNM strength-density relations, and find the GNMs' mechanical response to uniaxial straining to be anisotropic for pore morphologies deviating from circular pores. We also find that the GNM tensile strength decays exponentially with increasing GNM porosity and that pore edge termination with H atoms causes a reduction in the GNMs' elastic stiffening, strength, deformability, and toughness; this hydrogen embrittlement effect is more pronounced at a high level of pore edge passivation that renders the edge C atoms sp3-hybridized. The underlying mechanisms of crack initiation and propagation and nanomesh failure for the various types of GNMs examined also are characterized in atomistic detail. Overall, even highly porous GNMs remain particularly strong and deformable and, therefore, constitute very promising 2D mechanical metamaterials.We report results of a systematic computational study on the mechanical response of graphene nanomeshes (GNMs) to uniaxial tensile straining based on molecular-dynamics simulations of dynamic deformation tests according to a reliable bond-order interatomic potential. We examine the effects on the GNM mechanical behavior under straining along different directions of the nanomesh pore morphology and pore edge passivation by testing GNMs with elliptical pores of various aspect ratios and different extents of edge passivation through termination with H atoms of under-coordinated edge C atoms. We establish the dependences of the ultimate tensile strength, fracture strain, and toughness of the GNMs on the nanomesh porosity, derive scaling laws for GNM strength-density relations, and find the GNMs' mechanical response to uniaxial straining to be anisotropic for pore morphologies deviating from circular pores. We also find that the GNM tensile strength decays exponentially with increasing GNM porosity and that po... read less

Abstract: Nanoimprinting behaviors of copper substrates and double-walled carbon nanotubes with interwall sp 3 bonds are investigated using molecular dynamics simulations. A high-frequency mechanical vibration with various amplitudes is applied on the carbon nanotube (CNT) mold and copper substrate in different directions. Results show that exciting mechanical resonances both on the CNT and substrate drastically decrease the maximum imprint force and interfacial friction up to 50% under certain amplitudes. Meanwhile, it is demonstrated that defects occur in the {111} plane in the copper substrate during nanoimprinting. For different CNT array densities, a higher grafting density needs more imprint force to transfer patterns. The maximum imprint force for a large range of CNT array densities can be reduced by vibrational perturbations, while reduction rates depend on the CNT grafting density. This work sheds deep insights into the nanoimprint process at the atomic level, suggesting that vibration perturbation is an effective approach for improving the nanoimprinting accuracy and preventing the fracture of nanopatterns. read less

Abstract: Manipulating and coupling molecule gears is the first step towards realizing molecular-scale mechanical machines. Here, we theoretically investigate the behavior of such gears using molecular dynamics simulations. Within a nearly rigid-body approximation we reduce the dynamics of the gears to the rotational motion around the orientation vector. This allows us to study their behavior based on a few collective variables. Specifically, for a single hexa (4-tert-butylphenyl) benzene molecule we show that the rotational-angle dynamics corresponds to the one of a Brownian rotor. For two such coupled gears, we extract the effective interaction potential and find that it is strongly dependent on the center of mass distance. Finally, we study the collective motion of a train of gears. We demonstrate the existence of three different regimes depending on the magnitude of the driving-torque of the first gear: underdriving, driving and overdriving, which correspond, respectively, to no collective rotation, collective rotation and only single gear rotation. This behavior can be understood in terms of a simplified interaction potential. read less

Abstract: Nanotubes are prone to collapsing under compression due to the competition between the bending stiffness of the walls and the van der Waals interactions. The different radial morphologies during collapse may affect the electrical properties of nanotube, which may find promising potential applications in strain engineering. In this paper, the finite-deformation model is introduced to determine the radial morphologies, energy barrier and radial deformability of a nanotube under compression, in which the adhesion interactions are analytically obtained. The analytical solutions of the radial morphologies during compression are consistent with the molecular dynamics simulations results, indicating the effectiveness of the finite-deformation model. The analytical results reveal that both the energy barrier and the radial deformability show a decreasing tendency with the increase of the nanotube diameter. read less

Abstract: In a transmission system, the rotational speed of the output can be adjusted by the system. In this study, we introduce a three-stage rotational transmission nanosystem model that uses carbon nanotubes with excellent mechanical properties to fabricate coaxially distributed nanomotors with three nanobearings. Driven by a gigahertz nanomotor at 300 K, the nanobearings are changed to adjust the output rotational frequency. In view of the differences in chirality and radius of the carbon nanotubes, 17 transmission models are established and tested by molecular dynamics simulation. The rotor’s rotational transmission ratio curves show the dynamic response of the transmission system. A better down-converting transmission system can be obtained when the radius of the rotating component is between 0.58nm and 0.88nm or the difference in radii between the rotating components is greater than 0.2 nm. From the results obtained by molecular dynamics simulation, some key points are demonstrated for future deceleration of the rotary nanomotor.In a transmission system, the rotational speed of the output can be adjusted by the system. In this study, we introduce a three-stage rotational transmission nanosystem model that uses carbon nanotubes with excellent mechanical properties to fabricate coaxially distributed nanomotors with three nanobearings. Driven by a gigahertz nanomotor at 300 K, the nanobearings are changed to adjust the output rotational frequency. In view of the differences in chirality and radius of the carbon nanotubes, 17 transmission models are established and tested by molecular dynamics simulation. The rotor’s rotational transmission ratio curves show the dynamic response of the transmission system. A better down-converting transmission system can be obtained when the radius of the rotating component is between 0.58nm and 0.88nm or the difference in radii between the rotating components is greater than 0.2 nm. From the results obtained by molecular dynamics simulation, some key points are demonstrated for future deceleration o... read less

Abstract: Nanoscale mechanical resonator-based nanoelectromechanical systems have been reported with ultrahigh sensitivity, which are normally acquired from an ultravacuum environment at cryostat temperature. To facilitate their practical applications for gas sensing or bio-detection, it is critical to understand how the fluid (gas or liquid) environment will impact the resonance behaviors of the nanoresonator. This work reports a first-time comprehensive investigation on the influence of the N2 gaseous environment on the resonance properties of carbon nanotube (CNT)-based mechanical resonator, through a combination of grand canonical Monte Carlo and large-scale molecular dynamics simulations. It is shown that the gaseous environment exerts a significant effect on the resonance properties of the CNT resonator through a dynamic desorption and readsorption process. Under the temperature of 100 K and the pressure of 1 bar, the displacement amplitude of the CNT resonator is found to experience a sharp reduction of abou... read less

Abstract: We present the results of investigation of the nanopore filling of planar layered and bulk pillared graphene (PGR) as well as films and 3D samples of glass-like porous carbon (GLC) with potassium atoms. The patterns of charge transfer, electronic structure, and shift of the Fermi level during the filling of nanopores with potassium atoms are established. It is found that the greatest charge transfer from potassium atoms to the carbon framework is observed in PGR with a density of 1.1-1.4 g cm-3 (that is, with a nanopore volume of 1300-1800 nm3) regardless of the framework topology. The maximum charge transfer occurs already when the mass fraction of potassium is 12 wt%. At the same potassium concentration, a maximum shift of the Fermi level to zero by ∼3 eV occurs in a bilayer PGR film with a density of 1.4 g cm-3. Thus, our work shows for the first time that the electronic properties of nanoporous materials doped with alkaline earth metals (in particular, potassium) can be controlled by varying the volume of doped nanopores, i.e. by controlling the density of the nanoporous material. We first demonstrated that the potassium doping of PGR would be more effective than potassium doping of GLC. It is established that 2D samples of PGR and GLC completely reproduce the electronic properties of the bulk samples and even surpass them in some parameters. To carry out research, we developed a new method for nanopore filling with dopant atoms based on both the randomness of the nanopore filling and the energy advantage of this process. This method allows us to reliably determine the maximum possible mass fraction (wt%) of dopant atoms of any porous material. read less

Abstract: To design a rotor with recoverable deformation for conversion between rotation and translation in a nanodevice, an internally hydrogenated deformable part (HDP) was introduced in the carbon nanotube-based rotor. Initially, under van der Waals (vdW) force, the hydrogenated areas on the HDP curved toward the rotating axis. When a rotational frequency was exerted on the rotor, the hydrogenated parts on the HDP were separated under strong centrifugal force. Translational motion of the free edge of the rotor was generated synchronously during deformation of the HDP. Once removing the input rotation, the rotor would stop rotating by friction from the stators, and the HDP shrank back by strong vdW force but weakening centrifugal force. Hence, the nanoconvertor has recoverability, which was verified by molecular dynamics simulations with considering the effects of hydrogenation schemes and input rotational frequency at room temperature. Conclusions were drawn for a design of a nanodevice based on the present rotation-translation nanoconvertor model. read less

Abstract: Graphene nanobubbles consist of a substance that is trapped between graphene sheets and atomically flat substrates. This substance is an example of confinement in which both the bulk and surface interactions and the tension of the graphene determine the mechanical and thermodynamic properties of the system. The van der Waals pressure build up due to the graphene-substrate attraction and surface influence facilitates the advanced condensation of trapped substances. Different phases of the trapped substance are assumed to be found inside the graphene nanobubbles depending on their radii. Smaller radii are attributed to the crystal and liquid phases, and larger radii correspond to the gas phase. In this study, graphene nanobubbles filled with ethane on a graphite substrate are investigated. The choice of trapped substance is inspired by typical experiments in which graphene nanobubbles are obtained with a mixture of hydrocarbons inside. We apply a multiscale model based on both molecular dynamics simulations and a continuum 1D model to obtain the shape of the bubble, stress distribution and phase state of the trapped substance. Calculations are performed for a set of temperatures below and above the critical temperature of ethane. A liquid-gas phase transition below the critical temperature leads to a 'forbidden range' of radii, in which no stable bubbles exist. read less

Abstract: The nanofriction of graphene is critical for its broad applications as a lubricant and in flexible electronics. Herein, using a Au substrate as an example, we have investigated the effect of the grain boundary on the nanofriction of graphene by means of molecular dynamics simulations. We have systematically examined the coupling effects of the grain boundary with different mechanical pressures, velocities, temperatures, contact areas, and relative rotation angles on nanofriction. It is revealed that grain boundaries could reduce the friction between graphene and the gold substrate with a small deformation of the latter. Large lateral forces were observed under severe deformation around the grain boundary. The fluctuation of lateral forces was bigger on surfaces with grain boundaries than that on single-crystal surfaces. Friction forces induced by the armchair grain boundaries was smaller than those by the zigzag grain boundaries. read less

Abstract: The mechanical properties of graphene-Cu nanolayered (GCuNL) composites under bend loading are investigated via an energy-based analytical model and molecular dynamics (MD) simulations. For an anisotropic material, if it has a weak strength in a certain direction, improving the mechanical properties along this direction is normally difficult for its composites. Here, we find that the flexibility of GCuNL composites can be improved considerably by graphene interfaces, despite graphene's small bending stiffness. The graphene interfaces can delocalize slip bands in the inner Cu layers of GCuNL composites, and impede local nucleation of dislocations, thus greatly increasing the yield and failure bend angles. As the thickness decreases, the flexibility of GCuNL nanofilms increases. However, the GCuNL nanofilms are thermodynamically unstable due to interface instability when the repeat layer spacing is less than 2 nm. The energy-based analytical model for large deformation can accurately characterize the bending response of GCuNL nanofilms. read less

Abstract: It was discovered that a sudden jump of the output torque moment from a rotation transmission nanosystem made from carbon nanotubes (CNTs) occurred when decreasing the system temperature. In the nanosystem from coaxial-layout CNTs, the motor with specified rotational frequency (ωM) can drive the inner tube (rotor) to rotate in the outer tubes. When the axial gap between the motor and the rotor was fixed, the friction between their neighbor edges was stronger at a lower temperature. Especially at temperatures below 100 K, the friction-induced driving torque increases with ωM. When the rotor was subjected to an external resistant torque moment (Mr), it could not rotate opposite to the motor even if it deformed heavily. Combining molecular dynamics simulations with the bi-sectioning algorithm, the critical value of Mr was obtained. Under the critical torque moment, the rotor stopped rotating. Accordingly, a transmission nanosystem can be designed to provide a strong torque moment via interface friction at low temperature. read less

Abstract: Ultra-low friction can be achieved with 2D materials, particularly graphene and MoS2. The nanotribological properties of these different 2D materials have been measured in previous atomic force microscope (AFM) experiments sequentially, precluding immediate and direct comparison of their frictional behavior. Here, friction is characterized at the nanoscale using AFM experiments with the same tip sliding over graphene, MoS2, and a graphene/MoS2 heterostructure in a single measurement, repeated hundreds of times, and also measured with a slowly varying normal force. The same material systems are simulated using molecular dynamics (MD) and analyzed using density-functional theory (DFT) calculations. In both experiments and MD simulations, graphene consistently exhibits lower friction than the MoS2 monolayer and the heterostructure. In some cases, friction on the heterostructure is lower than that on the MoS2 monolayer. Quasi-static MD simulations and DFT calculations show that the origin of the friction contrast is the difference in energy barriers for a tip sliding across each of the three surfaces. read less

Abstract: Herein, a peripherally clamped stretched square monolayer graphene sheet with a side length of 10 nm was demonstrated as a resonator for atomic-scale mass sensing via molecular dynamics (MD) simulation. Then, a novel method of mass determination using the first three resonant modes (mode11, mode21 and mode22) was developed to avoid the disturbance of stress fluctuation in graphene. MD simulation results indicate that improving the prestress in stretched graphene increases the sensitivity significantly. Unfortunately, it is difficult to determine the mass accurately by the stress-reliant fundamental frequency shift. However, the absorbed mass in the middle of graphene sheets decreases the resonant frequency of mode11 dramatically while having negligible effect on that of mode21 and mode22, which implies that the latter two frequency modes are appropriate for compensating the stress-induced frequency shift of mode11. Hence, the absorbed mass, with a resolution of 3.3 × 10−22 g, is found using the frequency ratio of mode11 to mode21 or mode22, despite the unstable prestress ranging from 32 GPa to 47 GPa. This stress insensitivity contributes to the applicability of the graphene-based resonant mass sensor in real applications. read less

Abstract: In this study, extensive molecular dynamics simulations were carried out to investigate failure processes along different symmetric tilt grain boundaries (STGB) of bicrystal graphene sheet. Two different types of STGBs graphene mainly zigzag and arm-chair types were investigated. The dependence of fracture strength, strain as well as Young’s moduli on different STGBs were examined. The results clearly show that pristine graphene has the highest values of fracture strength and strain to fracture. Furthermore, bicrystal graphene with zigzag-oriented grain boundaries have improved mechanical properties in comparison to those with arm-chair oriented grain boundaries. Fracture behavior was investigated by applying mode I loadings to the outer boundary of bicrystalline graphene sheet with several misorientation angles. The critical stress intensity factors (SIFs) are calculated as a function of displacement were determined by using crack-tip opening displacements (CTOD) at the incipient bond breaking. The atomistic results show that the crack propagation along armchair-orientation grain boundaries are faster than that of zigzag-orientation grain boundaries of bicrystal graphene. read less

Abstract: Despite a wide range of current and potential applications, one primary concern of brittle materials is their sudden and swift collapse. This failure phenomenon exhibits an inability of the materials to sustain tension stresses in a predictable and reliable manner. However, advances in the field of fracture mechanics, especially at the nanoscale, have contributed to the understanding of the material response and failure nature to predict most of the potential dangers. In the following contribution, a comprehensive review is carried out on molecular dynamics (MD) simulations of brittle fracture, wherein the method provides new data and exciting insights into fracture mechanism that cannot be obtained easily from theories or experiments on other scales. In the present review, an abstract introduction to MD simulations, advantages, current limitations and their applications to a range of brittle fracture problems are presented. Additionally, a brief discussion highlights the theoretical background of the macroscopic techniques, such as Griffith’s criterion, crack tip opening displacement, J-integral and other criteria that can be linked to the fracture mechanical properties at the nanoscale. The main focus of the review is on the recent advances in fracture analysis of highly brittle materials, such as carbon nanotubes, graphene, silicon carbide, amorphous silica, calcium carbonate and silica aerogel at the nanoscale. These materials are presented here due to their extraordinary mechanical properties and a wide scope of applications. The underlying review grants a more extensive unravelling of the fracture behaviour and mechanical properties at the nanoscale of brittle materials. read less

Abstract: The study of the long-term evolution of slow chemical reactions is challenging because quantum-based reactive molecular dynamics simulation times are typically limited to hundreds of picoseconds. Here, the extended Lagrangian Born-Oppenheimer molecular dynamics formalism is used in conjunction with parallel replica dynamics to obtain an accurate tool to describe the long-term chemical dynamics of shock-compressed benzene. Langevin dynamics has been employed at different temperatures to calculate the first reaction times in liquid benzene at pressures and temperatures consistent with its unreacted Hugoniot. Our coupled engine runs for times on the order of nanoseconds (one to two orders of magnitude longer than traditional techniques) and is capable of detecting reactions that are characterized by rates significantly slower than we could study before. At lower pressures and temperatures, we mainly observe Diels-Alder metastable reactions, whereas at higher pressures and temperatures we observe stable polymerization reactions. read less

Abstract: ABSTRACT Recent findings of atomic-scale modelling studies are reviewed on graphene derivatives and metamaterials fabricated through chemical functionalization and/or defect engineering of graphene sheets. Results of molecular-statics and molecular-dynamics simulations according to a reliable bond-order potential, as well as first-principles density functional theory calculations are reviewed that have established useful structure-properties relations in two-dimensional materials, such as graphene nanomeshes (GNMs), electron-irradiated graphene, and interlayer-bonded twisted bilayer graphene. Quantitative relationships are established for the elastic moduli, mechanical properties, and thermal conductivity of GNMs as a function of the nanomesh porosity and the mechanical response of GNMs to uniaxial tensile straining is explored over the range of nanomesh porosities. The dependence of structural, mechanical, and thermal transport properties of electron-irradiated graphene sheets on the density of irradiation-induced defects is reviewed, highlighting an amorphization transition accompanied by a brittle-to-ductile transition and a transition in thermal transport mechanism beyond a critical defect concentration. The tunability of the electronic band structure, mechanical properties, and structural response to mechanical loading of graphene-diamond nanocomposite superstructures consisting of nanodiamond superlattices in interlayer-bonded twisted bilayer graphene also is demonstrated by precise control of the density and distribution of covalent interlayer C–C bonds. read less

Abstract: An ultra-light icosahedral fullerene structure is described having 12 pentagonal faces at the vertices of the regular icosahedron and 20 triangular faces, which are expanded by hexagonal faces that consist of carbon atoms. When its size exceeds a critical value, the density of the icosahedron becomes lower than that of air. Since the hollow structure deforms under atmospheric pressure, we proposed the addition of helium atoms as a means to obtaining a floatable icosahedral fullerene with pressure-resistant character. According to these requirements, the radius of the midpoint ball of the edge is 493.33 μm; the total density of the helium-filled structure is 0.18 kg m-3; and the buoyancy per cubic meter in the air is 10.88 N. Theoretical calculation and simulation are combined to explore a new approach to ultra-light materials. read less

Abstract: Metamaterials, rationally designed multiscale composite systems, have attracted extensive interest because of their potential for a broad range of applications due to their unique properties such as negative Poisson's ratio, exceptional mechanical performance, tunable photonic and phononic properties, structural reconfiguration, etc. Though they are dominated by an auxetic structure, the constituents of metamaterials also play an indispensable role in determining their unprecedented properties. In this vein, 2D materials such as graphene, silicene, and phosphorene with superior structural tunability are ideal candidates for constituents of metamaterials. However, the nanostructure–property relationship and composition–property relationship of these 2D material-based metamaterials remain largely unexplored. Mechanical anisotropy inherited from the 2D material constituents, for example, may substantially impact the physical stability and robustness of the corresponding metamaterial systems. Herein, classical molecular dynamics simulations are performed using a generic coarse-grained model to explore the deformation mechanism of these 2D material-based metamaterials with sinusoidally curved ligaments and the effect of mechanical anisotropy on mechanical properties, especially the negative Poisson's ratio. The results indicate that deformation under axial tensile load can be divided into two stages: bending-dominated and stretching-dominated, in which the rotation of junctions in the former stage results in auxetic behavior of the proposed metamaterials. In addition, the auxetic behavior depends heavily on both the amplitude/wavelength ratio of the sinusoidal ligament and the stiffness ratio between axial and transverse directions. The magnitude of negative Poisson's ratio increases from 0 to 0.625, with an associated increase of the amplitude/wavelength ratio from 0 to 0.225, and fluctuates at around 0.625, in good agreement with the literature, with amplitude/wavelength ratios greater than 0.225. More interestingly, the magnitude of negative Poisson's ratio increases from 0.47 to 0.87 with the increase of the stiffness ratio from 0.125 to 8, in good agreement with additional all-atom molecular dynamics simulations for phosphorene and molybdenum disulfide. Overall, these research findings shed light on the deformation mechanism of auxetic metamaterials, providing useful guidelines for designing auxetic 2D lattice structures made of 2D materials that can display a tunable negative Poisson's ratio. read less

Abstract: We investigate the effects of fullerene functionalization on the thermal transport properties of graphene monolayers via atomistic simulations. Our systematic molecular dynamics simulations reveal that the thermal conductivity of pristine graphene can be lowered by more than an order of magnitude at room temperature (and as much as by ∼93% as compared to the thermal conductivity of pristine graphene) via the introduction of covalently bonded fullerenes on the surface of the graphene sheets. We demonstrate large tunability in the thermal conductivity by the inclusion of covalently bonded fullerene molecules at different periodic inclusions, and we attribute the large reduction in thermal conductivities to a combination of resonant phonon localization effects, leading to band anticrossings and vibrational scattering at the sp3 bonded carbon atoms. The torsional force exerted by the fullerene molecules on the graphene sheets and the number of covalent bonds formed between the two carbon allotropes is shown to significantly affect the heat flow across the hybrid structures, while the size of the fullerene molecules is shown to have a negligible effect on their thermal properties. Moreover, we show that even for a large surface coverage, the mechanical properties of these novel materials are uncompromised. Taken together, our work reveals a unique way to manipulate vibrational thermal transport without the introduction of lattice defects, which could potentially lead to high thermoelectric efficiencies in these materials.We investigate the effects of fullerene functionalization on the thermal transport properties of graphene monolayers via atomistic simulations. Our systematic molecular dynamics simulations reveal that the thermal conductivity of pristine graphene can be lowered by more than an order of magnitude at room temperature (and as much as by ∼93% as compared to the thermal conductivity of pristine graphene) via the introduction of covalently bonded fullerenes on the surface of the graphene sheets. We demonstrate large tunability in the thermal conductivity by the inclusion of covalently bonded fullerene molecules at different periodic inclusions, and we attribute the large reduction in thermal conductivities to a combination of resonant phonon localization effects, leading to band anticrossings and vibrational scattering at the sp3 bonded carbon atoms. The torsional force exerted by the fullerene molecules on the graphene sheets and the number of covalent bonds formed between the two carbon allotropes is shown... read less

Abstract: Using molecular dynamics (MD) simulations, the frictional properties of the interface between graphene nanoflake and single crystalline diamond substrate have been investigated. The equilibrium distance between the graphene nanoflake and the diamond substrate has been evaluated at different temperatures. This study considered the effects of temperature and relative sliding angle between graphene and diamond. The equilibrium distance between graphene and the diamond substrate was between 3.34 Å at 0 K and 3.42 Å at 600 K, and it was close to the interlayer distance of graphite which was 3.35 Å. The friction force between graphene nanoflakes and the diamond substrate exhibited periodic stick-slip motion which is similar to the friction force within a graphene–Au interface. The friction coefficient of the graphene–single crystalline diamond interface was between 0.0042 and 0.0244, depending on the sliding direction and the temperature. Generally, the friction coefficient was lowest when a graphene flake was sliding along its armchair direction and the highest when it was sliding along its zigzag direction. The friction coefficient increased by up to 20% when the temperature rose from 300 K to 600 K, hence a contribution from temperature cannot be neglected. The findings in this study validate the super-lubricity between graphene and diamond and will shed light on understanding the mechanical behavior of graphene nanodevices when using single crystalline diamond as the substrate. read less

Abstract: During high temperature pyrolysis of polymer thin films, nanocrystalline graphene with a high defect density, active edges and various nanostructures is formed. The catalyst-free synthesis is based on the temperature assisted transformation of a polymer precursor. The processing conditions have a strong influence on the final thin film properties. However, the precise elemental processes that govern the polymer pyrolysis at high temperatures are unknown. By means of time resolved in situ transmission electron microscopy investigations we reveal that the reactivity of defects and unsaturated edges plays an integral role in the structural dynamics. Both mobile and stationary structures with varying size, shape and dynamics have been observed. During high temperature experiments, small graphene fragments (nanoflakes) are highly unstable and tend to lose atoms or small groups of atoms, while adjacent larger domains grow by addition of atoms, indicating an Ostwald-like ripening in these 2D materials, besides the mechanism of lateral merging of nanoflakes with edges. These processes are also observed in low-dose experiments with negligible electron beam influence. Based on energy barrier calculations, we propose several inherent temperature-driven mechanisms of atom rearrangement, partially involving catalyzing unsaturated sites. Our results show that the fundamentally different high temperature behavior and stability of nanocrystalline graphene in contrast to pristine graphene is caused by its reactive nature. The detailed analysis of the observed dynamics provides a pioneering overview of the relevant processes during ncg heating. read less

Abstract: The deformation and fracture behaviour of one-atom-thick mechanically exfoliated graphene has been studied in detail. Monolayer graphene flakes with different lengths, widths and shapes were successfully prepared by mechanical exfoliation and deposited onto poly(methyl methacrylate) (PMMA) beams. The fracture behaviour of the monolayer graphene was followed by deforming the PMMA beams. Through in situ Raman mapping at different strain levels, the distributions of strain over the graphene flakes were determined from the shift of the graphene Raman 2D band. The failure mechanisms of the exfoliated graphene were either by flake fracture or failure of the graphene/polymer interface. The fracture of the flakes was observed from the formation of cracks identified from the appearance of lines of zero strain in the strain contour maps. It was found that the strength of the monolayer graphene flakes decreased with increasing flake width. The strength dropped to less than ∼10 GPa for large flakes, thought to be due to the presence of defects. It is shown that a pair of topological defects in monolayer graphene will form a pseudo crack and the effect of such defects upon the strength of monolayer graphene has been modelled using molecular mechanical simulations. read less

Abstract: Graphene nanobubbles (GNBs) are formed when a substance is trapped between a graphene sheet (a 2D crystal) and an atomically flat substrate. The physical state of the substance inside GNBs can vary from the gas phase to crystal clusters. In this paper, we present a theoretical description of the gas–liquid phase transition of argon inside GNBs. The energy minimization concept is used to calculate the equilibrium properties of the bubble at constant temperature for a given mass of captured substance. We consider the total energy as a sum of the elastic energy of the graphene sheet, the bulk energy of the inner substance and the energy of adhesion between this substance, the substrate and graphene. The developed model allows us to reveal a correlation between the size of the bubble and the physical state of the substance inside it. A special case of a GNB that consists of argon trapped between a graphene sheet and a graphite substrate is considered. We predict the ‘forbidden range’ of radii, within which no stable GNBs exist, that separates bubble sizes with liquid argon inside from bubble sizes with gaseous argon. The height-to-radius ratio of the bubble is found to be constant for radii greater than 200 nm, which is consistent with experimental observations. The proposed model can be extended to various types of trapped substances and 2D crystals. read less

Abstract: It is essential to understand the size scaling effects on the mechanical properties of graphene networks to realize the potential mechanical applications of graphene assemblies. Here, a “highly dense‐yet‐nanoporous graphene monolith (HPGM)” is used as a model material of graphene networks to investigate the dependence of mechanical properties on the intrinsic interplanar interactions and the extrinsic specimen size effects. The interactions between graphene sheets could be enhanced by heat treatment and the plastic HPGM is transformed into a highly elastic network. A strong size effect is revealed by in situ compression of micro‐ and nanopillars inside electron microscopes. Both the modulus and strength are drastically increased as the specimen size reduces to ≈100 nm, because of the reduced weak links in a small volume. Molecular dynamics simulations reveal the deformation mechanism involving slip‐stick sliding, bending, buckling of graphene sheets, collapsing, and densification of graphene cells. In addition, a size‐dependent brittle‐to‐ductile transition of the HPGM nanopillars is discovered and understood by the competition between volumetric deformation energy and critical dilation energy. read less

Abstract: A rotary nanomotor is an essential component of a nanomachine. In the present study, a rotary nanomotor from wedged diamonds and triple-walled nanotubes was proposed with the consideration of boundary effect. The outer tubes and mid-tubes were used as nanobearing to constrain the inner tube. Several wedges of the diamond were placed near the inner tube for driving the inner tube to rotate. At a temperature lower than 300 K, the inner tube as the rotor had a stable rotational frequency (SRF). It is shown that both the rotational direction and the value of SRF of the rotor depended on the temperature and thickness of the diamond wedges. The dependence was investigated via theoretical analysis of the molecular dynamics simulation results. For example, when each diamond wedge had one pair of tip atoms (unsaturated), the rotational direction of the rotor at 100 K was opposite to that at 300 K. At 500 K, the rotating rotor may stop suddenly due to breakage of the diamond needles. Some conclusions are drawn for potential application of such a nanomotor in a nanomachine. read less

Abstract: The contact conductance of single, double, and triple thermal contacts of single-walled carbon nanotubes (SWCNTs) was investigated using molecular dynamics simulations. Our results showed that the effect of the thermal contact number on the contact conductance was not as strong as previously reported. The percentages of contact conductance of double and triple thermal contacts were about 72% and 67%, respectively, compared to that of a single thermal contact. Moreover, we found that the contact conductance of the double and triple thermal contacts was associated with the SWCNT length and the positional relationship of the thermal contacts. read less

Abstract: Molecular dynamics simulations of armchair graphene nanoribbons and zigzag graphene nanoribbons with different sizes were performed at room temperature. Double vacancy defects were introduced in each graphene nanoribbon at its center or at its edge. The effect of defect on the mechanical behavior was studied by comparing the stress–strain response and the fracture toughness of each pair of pristine and defective graphene nanoribbon. Results show that the effect of vacancies in zigzag graphene nanoribbon is more profound than in armchair graphene nanoribbon. Also, the effect of double vacancy defect on the ultimate failure stress is greater in zigzag graphene nanoribbons than in armchair graphene nanoribbon due to bond orientation with respect to loading direction. Strength reduction can be as high as 17.5% in armchair graphene nanoribbon with no significant difference between single and double vacancies, while for zigzag graphene nanoribbon, the strength reduction is up to 30% for single vacancy and 43% for double vacancy defects. It is observed that for zigzag graphene nanoribbon with double vacancy at the edge, the direction of the failure plane is oriented at ±30° with respect to the loading direction while it is always perpendicular to the direction of loading in armchair graphene nanoribbon. Results have been verified through studying the fracture toughness parameters in each case as well. read less

Abstract: In this work, we investigate pressure‐driven water flows in graphene‐coated copper nanochannels through molecular dynamics simulations. It is found that the flow rate in bare copper nanochannel can be significantly enhanced by a factor of 45 when the nanochannel is coated with monolayer graphene. The enhancement factor for the flow rate reaches about 90 when the nanochannel is modified with 3 or more graphene layers. The dipole relaxation time and the hydrogen bond lifetime of interfacial water molecules show that the graphene coating promotes the mobility of water molecules at the interface. The distribution of the potential of mean force and the free energy barriers also confirm that graphene coating reduces the flow resistance and 3 layers of graphene can fully screen the surface effects. The results in this work provide important information for the design of graphene‐based nanofluidic systems for flow enhancement. read less

Abstract: Because of their high thermal conductivity, graphene nanoribbons (GNRs) can be employed as fillers to enhance the thermal transfer properties of composite materials, such as polymer-based ones. However, when the filler loading is higher than the geometric percolation threshold, the interfacial thermal resistance between adjacent GNRs may significantly limit the overall thermal transfer through a network of fillers. In this article, reverse non-equilibrium molecular dynamics is used to investigate the impact of the relative orientation (i.e., horizontal and vertical overlap, interplanar spacing and angular displacement) of couples of GNRs on their interfacial thermal resistance. Based on the simulation results, we propose an empirical correlation between the thermal resistance at the interface of adjacent GNRs and their main geometrical parameters, namely the normalized projected overlap and average interplanar spacing. The reported correlation can be beneficial for speeding up bottom-up approaches to the multiscale analysis of the thermal properties of composite materials, particularly when thermally conductive fillers create percolating pathways. read less

Abstract: Understanding the details of thermal transport in graphdiyne and its nanostructures would help to broaden their applications. On the basis of the molecular dynamics simulations and spectrally decomposed heat current analysis, we show that the high-frequency phonons in graphdiyne can be strongly hindered in nanoribbons because of the boundary scattering. The isotropic transport in graphdiyne can be switched to anisotropic along the armchair and zigzag directions. Adding side chains onto the nanoribbon edges further reduces the thermal conductivity (TC) along both armchair and zigzag directions thanks to the reduction of heat current carried by low-frequency modes, a mechanism that arises from the phonon resonances. The uniaxial tensile strain plays a different role in the TC of graphdiyne, armchair nanoribbons, and zigzag nanoribbons. Tensile strain causes the thermal conductivities of graphdiyne, and armchair nanoribbons increase first and then get reduced, whereas for zigzag nanoribbons, the TC decreases with strain first and reaches to a plateau. The different low-frequency phonon response on strain is the main reason for the different TC behavior. For graphdiyne and armchair nanoribbons, the low-frequency heat current is enhanced gradually first and then get reduced with the increase of strain, while that of zigzag nanoribbons decreases with strain and then increases slightly. The current studies could help us understand the phonon transport in graphdiyne and its nanoribbons, which is useful for their TC engineering. read less

Abstract: Negative friction coefficients, where friction is reduced upon increasing normal load, are predicted for superlubric graphite-hexagonal boron nitride heterojunctions. The origin of this counterintuitive behavior lies in the load-induced suppression of the moiré superstructure out-of-plane distortions leading to a less dissipative interfacial dynamics. Thermally induced enhancement of the out-of-plane fluctuations leads to an unusual increase of friction with temperature. The highlighted frictional mechanism is of a general nature and is expected to appear in many layered material heterojunctions. read less

Abstract: Understanding the behavior of water molecule transport through artificial nano-channels is essential in designing novel nanofluidic devices that could be used especially in nanofiltration processes. In this study, using nonequilibrium molecular dynamics (MD) simulations, we simulated the water flow through different graphene-based channels to investigate the influences of some key factors such as the channel thickness and applied pressure on the water flow. It was demonstrated that the water flow was enhanced by increasing the applied pressure and channel thickness. Our results indicated that a third order polynomial curve could describe the variation of the water flow as a function of the channel thickness and the applied pressure. In addition, we improved the hydrodynamics equation used to consider the water flow through nano-channels, by adding two terms to describe the slip effect and the entrance/exit effect, in which the first term increased the water flow rate, while the second term reduced it. This study may be helpful in designing high-performance graphene-based membranes with some practical applications such as desalination. read less

Abstract: In this paper, a molecular dynamics (MD) simulation model of carbon-fiber/pyrolytic-carbon (Cf/PyC) interphase in carbon/carbon (C/C) composites manufactured by the chemical vapor phase infiltration (CVI) process was established based on microscopic observation results. By using the MD simulation method, the mechanical properties of the Cf/PyC interphase under tangential shear and a normal tensile load were studied, respectively. Meanwhile, the deformation and failure mechanisms of the interphase were investigated with different sizes of the average length L¯a of fiber surface sheets. The empirical formula of the interfacial modulus and strength with the change of L¯a was obtained as well. The shear properties of the isotropic pyrolysis carbon (IPyC) matrix were also presented by MD simulation. Finally, the mechanical properties obtained by the MD simulation were substituted into the cohesive force model, and a fiber ejection test of the C/C composite was simulated by the finite element analysis (FEA) method. The simulation results were in good agreement with the experimental ones. The MD simulation results show that the shear performance of the Cf/PyC interphase is relatively higher when L¯a is small due to the effects of non-in-plane shear, the barrier between crystals, and long sheet folding. On the other hand, the size of L¯a has no obvious influence on the interfacial normal tensile mechanical properties. read less

Abstract: Structural superlubricity, a nearly frictionless state between two contact solid surfaces, has attracted rapidly increasing attention during the past few years. Yet a key problem that limits its promising applications is the high anisotropy of friction which always leads to its failure. Here we study the friction of a graphene flake sliding on top of a graphene substrate using molecular dynamics simulation. The results show that by applying strain on the substrate, biaxial stretching is better than uniaxial stretching in terms of reducing interlayer friction. Importantly, we find that robust superlubricity can be achieved via both biaxial and uniaxial stretching, namely for stretching above a critical strain which has been achieved experimentally, the friction is no longer dependent on the relative orientation mainly due to the complete lattice mismatch. The underlying mechanism is revealed to be the Moiré pattern formed. These findings provide a viable approach for the realization of robust superlubricity through strain engineering. read less

Abstract: Crack propagation in graphene monolayer under tear loading is investigated via an energy-based analytical model and molecular dynamics (MD) simulations. The classical mechanics-based model describes steady-state crack propagation velocity as a function of applied stress, lateral dimension and loading geometry, as well as the critical stress and critical size for initiating steady crack propagation. MD simulations reveal that cracks propagate along the zigzag direction but yield different "fracture surface" roughnesses for different loading geometries. MD simulations and the predictions of the analytical model are in excellent agreement. Our findings lead to an improved fundamental understanding of the mode-III crack of monolayer graphene necessary for the design and fabrication of graphene-based devices. read less

Abstract: Vapor‐compression dominates the market for refrigeration devices due to low cost and relatively high efficiency. However, the most efficient vapor refrigerants are either ozone depleting or global warming substances. Solid‐state cooling is a young field of research with promising results toward the development of new, efficient, and environment friendly technology for a new generation of refrigeration devices. One of these methods is based on the so‐called elastocaloric effect (ECE), which consists of a temperature variation of a system in response to the application of adiabatic stresses. Although most of the literature describes the study of ECE solid‐state cooling based on materials undergoing phase‐transitions, a study recently predicted that carbon nanotubes (CNTs) present ECE as large as 30 K for 3% of strain. This motivates research toward the development of nanorefrigerators. As nobody knows the efficiency of such an ECE‐based CNT nanorefrigerator, here, significantly high coefficient of performance values of 4.1 and 6.5, and extracted heat per weight as large as 40 J g−1 are reported for a zigzag CNT nanorefrigerator operating in an Otto‐like thermodynamic cycle. This efficiency is shown to overcome that of some other ECE materials. read less

Abstract: The mechanical behavior of nanocomposites consisting of highly ordered nanoporous nickel (HONN) and its carbon nanotube (CNT)-reinforced composites (CNHONNs) subjected to a high temperature of 900 K is investigated via molecular dynamics (MD) simulations. The study indicates that, out-of-plane mechanical properties of the HONNs are generally superior to its in-plane mechanical properties. Whereas the CNT shows a significant strengthening effect on the out-of-plane mechanical properties of the CNHONN composites. Compared to pure HONNs, through the addition of CNTs from 1.28 wt‰ to 5.22 wt‰, the weight of the composite can be reduced by 5.83‰ to 2.33% while the tensile modulus, tensile strength, compressive modulus and compressive strength can be increased by 2.2% to 8.8%, 1% to 5.1%, 3.6% to 10.2% and 4.9% to 10.7%, respectively. The energy absorption capacity can also be improved due to the existence of CNTs. Furthermore, the MD simulations provide further insights into the deformation mechanism at the atomic scale, including fracture in tension, pore collapse in compression and local changes in lattice structures due to stacking faults. read less

Abstract: M. W. Barsoum,1,* X. Zhao,2 S. Shanazarov,1 A. Romanchuk,1 S. Koumlis,2 S. J. Pagano,2 L. Lamberson,2,† and G. J. Tucker3,‡ 1Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA 2Department of Mechanical Engineering, Drexel University, Philadelphia, Pennsylvania 19104, USA 3Department of Mechanical Engineering, Colorado School of Mines, Golden, Colorado 80401, USA read less

Abstract: With an ultralarge surface-to-volume ratio, a recently synthesized three-dimensional graphene structure, namely, carbon honeycomb, promises important engineering applications. Herein, we have investigated, via molecular dynamics simulations, its mechanical properties, which are inevitable for its integrity and desirable for any feasible implementations. The uniaxial tension and nanoindentation behaviors are numerically examined. Stress–strain curves manifest a transformation of covalent bonds of hinge atoms when they are stretched in the channel direction. The load–displacement curve in nanoindentation simulation implies the hardness and Young’s modulus to be 50.9 GPa and 461±9 GPa, respectively. Our results might be useful for material and device design for carbon honeycomb-based systems. read less

Abstract: Graphene is an ideal material in the reinforcement of metal-matrix composites owing to its outstanding mechanical and physical properties. Herein, we have investigated the surface enhancement of iron via a computational nanoindentation process using molecular dynamics simulations. The findings of our study show that graphene can enhance the critical yield strength, hardness and elastic modulus of the composite to different degrees with the change of the number of graphene layers. In the six tested models, the composite with trilayer graphene on the surface produces the strongest reinforcement, with an increased magnitude of 432.1% and 169.5% in the hardness and elastic modulus, respectively, compared with pure iron. Furthermore, it is revealed that high temperature could weaken the elastic bearing capacity of the graphene, resulting in a decrease on the elastic mechanical properties of the graphene/Fe composite. read less

Abstract: Controlling water molecular motion at the nanoscale is critical for many important applications, such as water splitting to produce hydrogen and oxygen, biological and chemical cell reactions, nanofluidics, drug delivery, water treatment, etc. In this paper, we propose a new nanoscale device based on carbon nanotubes (CNTs) with a stiffness gradient to create a spontaneous directional motion of water molecules, and perform molecular dynamics simulations to analyze its transport characteristics. We find that the (6, 6) CNT possesses an optimal water transport rate. In the thinner CNTs, the water molecules are strongly confined by the CNT wall, resulting in a higher friction force; while in the thicker CNTs, the driving force is lower, and the water molecules tend to form ring-like configurations, resulting in a slower motion. For the (6, 6) CNT, water molecules tend to favor a chain-like configuration, through which the molecules are able to move synergistically along the stiffness gradient, and the transportation efficiency increases with the stiffness gradient but decreases with temperature. Both energetic and kinetic analyses are performed to elucidate this fascinating directional motion. Our work demonstrates a new strategy for controlling water molecular motion at the nanoscale without resorting to any active driving source, such as electric field, temperature or pressure difference. read less

Abstract: We report on a series of controlled computational experiments based on nonequilibrium molecular dynamics and show that at the nanoscale, the thermal rectification is determined by the thermal boundary resistance, i.e., the thermal resistance of the interface, and cannot be explained without it. In the graphene–bilayer graphene system that we study, the sign of the thermal rectification is opposite to the value predicted from bulk-derived models, i.e., phonons preferentially flow in the opposite direction. This behavior derives from the temperature dependence of the thermal boundary resistance and from the fact that the latter, at the nanoscale, has large relative weight compared to the total thermal resistance. These results outline the importance of properly accounting for the active role of the interface.We report on a series of controlled computational experiments based on nonequilibrium molecular dynamics and show that at the nanoscale, the thermal rectification is determined by the thermal boundary resistance, i.e., the thermal resistance of the interface, and cannot be explained without it. In the graphene–bilayer graphene system that we study, the sign of the thermal rectification is opposite to the value predicted from bulk-derived models, i.e., phonons preferentially flow in the opposite direction. This behavior derives from the temperature dependence of the thermal boundary resistance and from the fact that the latter, at the nanoscale, has large relative weight compared to the total thermal resistance. These results outline the importance of properly accounting for the active role of the interface. read less

Abstract: Graphene combined with fractal structures would probably be a promising candidate design of an antenna for a wireless communication system. However, the thermal transport properties of fractal graphene, which would influence the properties of wireless communication systems, are unclear. In this paper, the thermal transport properties of graphene with a Sierpinski fractal structure were investigated via the reverse non-equilibrium molecular dynamics simulation method. Simulation results indicated that the thermal conductivity of graphene with fractal defects decreased from 157.62 to 19.60 (W m−1 K−1) as the fractal level increased. Furthermore, visual display and statistical results of fractal graphene atomic heat flux revealed that with fractal levels increasing, the real heat flux paths twisted, and the angle distributions of atomic heat flux vectors enlarged from about (−30°, 30°) to about (−45°, 45°). In fact, the fractal structures decreased the real heat flow areas and extended the real heat flux paths, and enhanced the phonon scattering in the defect edges of the fractal graphene. Analyses of fractal graphene thermal transport characters in our work indicated that the heat transfer properties of fractal graphene dropped greatly as fractal levels increased, which would provide effective guidance to the design of antennae based on fractal graphene. read less

Abstract: A comprehensive molecular dynamics study is conducted to investigate the elastic field at an atomic inhomogeneity in graphene in the form of a circular hole or a circular boron-nitride inclusion. In addition, the effect on the stress field due to the interaction between an inhomogeneity and a crack is investigated. The results confirm that consideration must be given to the mechanical properties of the resulting system when atomic defects and inclusions are introduced to graphene to tailor optical and electronic properties. read less

Abstract: We study the structure and dynamics of water confined in a nanoporous graphitic carbon structure using molecular dynamics (MD) simulations. The carbon structure is generated by a reactive MD simulation of oxidation of a silicon carbide nanoparticle. We embed water molecules in the nanopores and study structural and dynamical properties of nanoconfined water as a function of temperature. MD simulation results indicate the presence of high-density water (HDW) and low-density water (LDW). Radial distribution functions (RDF) and spatial density functions (SDF) indicate that the second solvation shell of the HDW is broken. We calculate the self-diffusion coefficient of confined water molecules as a function of temperature and find a significant decrease in the diffusion of water molecules around T = 190K. The cage correlation function c(t) of confined water molecules at T = 200K exhibits stretched exponential decay, c(t) = exp(-(t/τ)β), with β = 0.43, which matches exactly with the theoretical prediction β = 3/7 . Furthermore, the self-intermediate scattering function at T = 200 K indicates differences in small-scale and large-scale dynamics of water molecules. read less

Abstract: A new carbon allotrope, namely popgraphene, has been recently demonstrated to possess high potentials for nanodevice applications. Here, the fracture of defective popgraphene was studied using molecular dynamics simulations and continuum modeling. Three scenarios of defects were considered, including an individual point defect, distributed point defects, and nanocracks. It was found that the fracture stress of popgraphene with an individual point defect was governed by both the geometry of the defect and the critical bond where fracture initiates. Moreover, the fracture stress of popgraphene with distributed point defects was discovered to be inversely proportional to the defect density, showing a nice linear trend. Furthermore, for popgraphene with a nanocrack, it failed in a brittle fashion and exhibited a negligible lattice trapping effect. Griffith criterion was subsequently employed with the consideration of crack deflection to accurately predict the dependence of fracture stress on crack size. The present study lays a mechanistic foundation for nanoscale applications of popgraphene and offers a better understanding of the roles of defects in fracture of low-dimensional materials. read less

Abstract: By bending a straight carbon nanotube and bonding both ends of the nanotube, a nanoring (or nano-wheel) is produced. The nanoring system can be driven to rotate by fixed outer nanotubes at room temperature. When placing some atoms at the edge of each outer tube (the stator here) with inwardly radial deviation (IRD), the IRD atoms will repulse the nanoring in their thermally vibration-induced collision and drive the nanoring to rotate when the repulsion due to IRD and the friction with stators induce a non-zero moment about the axis of rotational symmetry of the ring. As such, the nanoring can act as a wheel in a nanovehicle. When the repulsion is balanced with the intertubular friction, a stable rotational frequency (SRF) of the rotor is achieved. The results from the molecular dynamics simulation demonstrate that the nanowheel can work at extremely low temperature and its rotational speed can be adjusted by tuning temperature. read less

Abstract: The consequence of this result on the fracture behavior of graphene, including the crack initiation, crack propagation, blunting, fracture strength and energy release rate is the main topic of this paper. We proposed three mechanisms by which crack growth can occur in such blunted regions and also performed simulations on three different specimens of graphene sheets to demonstrate elastic blunting. For the study of fracture strength of graphene with different crack tip radius, different crack initiation behaviors are revealed, and it is demonstrated that the blunting effect of tip edges plays an important role in the fracture crack initiation and propagation of graphene. The characterized crack tip radius from 1.642 to 2.843 Å is observed, which can be used to estimate the fracture strength due to blunting at crack tip. The results of this work are deemed to be of importance from the perspective of modeling the fracture behavior of graphene sheet and in predicting its ultimate failure, post-blunting in fracture mechanism. read less

Abstract: How carbon nanotubes (CNTs) interact with substrates is fundamental for understanding their physical properties. In existing theoretical and modeling studies, the substrates are considered to be rigid with semi-infinite thickness. In this work, the effects of finite substrate thickness and elasticity are analyzed theoretically and numerically for free boundary conditions. Based on the energy-variational approach, considering the interfacial van der Waals interactions and bending strain energies stored in CNTs and substrates, the governing equations and boundary conditions are derived analytically. The theoretical predictions are in reasonable agreement with the results of molecular dynamics simulations. When the substrate is sufficiently thick, the results of the present theoretical model are entirely consistent with previous models for the infinite-thickness substrate. However, for relatively thin substrates, the effect of substrate thickness is significant due to the geometric large deformation. Three stable adhesive states (initial non-adhesive, partially adhesive, and fully wrapping states) can be achieved, dependent on the substrate thickness, the number of CNT walls, and the interfacial adhesion work. The stability of adhesive configurations is explored by analyzing the energy variations corresponding to the adhesive deformation. We show that there exist several modes of energy variations, depending on the adhesion work and the substrate-CNT bending stiffness ratio, which exhibit linear and nonlinear influences, respectively. Our results could serve as guidelines to design CNT-on-substrate systems.How carbon nanotubes (CNTs) interact with substrates is fundamental for understanding their physical properties. In existing theoretical and modeling studies, the substrates are considered to be rigid with semi-infinite thickness. In this work, the effects of finite substrate thickness and elasticity are analyzed theoretically and numerically for free boundary conditions. Based on the energy-variational approach, considering the interfacial van der Waals interactions and bending strain energies stored in CNTs and substrates, the governing equations and boundary conditions are derived analytically. The theoretical predictions are in reasonable agreement with the results of molecular dynamics simulations. When the substrate is sufficiently thick, the results of the present theoretical model are entirely consistent with previous models for the infinite-thickness substrate. However, for relatively thin substrates, the effect of substrate thickness is significant due to the geometric large deformation. Three s... read less

Abstract: Molecular dynamics (MD) simulation has been employed to study the wetting transitions of liquid gallium droplet on the graphene surfaces, which are decorated with three types of carbon nanopillars, and to explore the effect of the surface roughness and morphology on the wettability of liquid Ga. The simulation results showed that, at the beginning, the Ga film looks like an upside-down dish on the rough surface, different from that on the smooth graphene surface, and its size is crucial to the final state of liquid. Ga droplets exhibit a Cassie–Baxter (CB) state, a Wenzel state, a Mixed Wetting state, and a dewetting state on the patterned surfaces by changing distribution and the morphology of nanopillars. Top morphology of nanopillars has a direct impact on the wetting transition of liquid Ga. There are three transition states for the two types of carbon nanotube (CNT) substrates and two for the carbon nanocone (CNC) one. Furthermore, we have found that the substrates show high or low adhesion to the Ga droplet with the variation of their roughness and top morphology. With the roughness decreasing, the adhesion energy of the substrate decreases. With the same roughness, the CNC/graphene surface has the lowest adhesion energy, followed by CNT/graphene and capped CNT/graphene surfaces. Our findings provide not only valid support to previous works but also reveal new theories on the wetting model of the metal droplet on the rough substrates. read less

Abstract: We report results for the electronic structure of irradiated and irradiation-induced amorphized graphene based on first-principles density functional theory calculations, using models of irradiated graphene sheets that were initially relaxed structurally according to molecular-dynamics simulations. We find that localized states appear at the Fermi level upon irradiation damage and the corresponding local density of states increases with increasing defect density. Electronic structure calculations show that band flattening occurs due to electron localization in the vicinity of irradiation-induced defects and reduces the charge carrier mobility. This band flattening effect becomes stronger with increasing defect density due to a greater degree of carrier localization at irradiation-induced carbon dangling bonds. Passivating these dangling bonds with hydrogen atoms delocalizes the charge density, reduces the density of states at the Fermi level, and increases the band dispersion. Hydrogen passivation also has the additional effect of quenching any localized magnetic moments at dangling bonds. Our studies show that defect engineering of graphene—even at a gross level without atomic-scale precision—can be employed to tune its properties for additional electronic functionality. read less

Abstract: Gas filtration by means of membranes is becoming increasingly important for industrial processes due to its low cost. In particular, membranes can be applied to separate methane in natural gas from pollutants such as hydrogen sulfide and carbon dioxide. The recent advent of nanoporous graphene as material for membranes helped to overcome the current problems of polymeric membranes, namely the permeability/selectivity tradeoff. However, the factors that determine gas filtration through nanoporous graphene are not completely clear yet. In this work, we show that pore size, shape and functionalization severely affect the selectivity of the membrane toward CO 2 and H 2 S with respect to CH 4 . We identified that the critical diameter of circular pore for the separation of contaminants from methane with graphene membranes is 5.90 Å. An elliptical pore is discovered to select gas species having similar sizes on the basis of their shape. The more elongated CO 2 is allowed to pass though the pore while the more spherical H 2 S and CH 4 are rejected. Finally, the gas-membrane interactions are found to decisively affect the filtration performances. Functionalization with hydroxyl groups led to a higher permeability of the gas species with polar bonds while keeping an excellent selectivity. read less

Abstract: We consider graphene disclination networks (DNs) — periodic distributions of disclination defects. Disclinations manifest themselves as 4-, 5-, 7- or 8-member carbon rings in otherwise 6-member ring ideal 2D graphene crystal lattice. Limiting cases of graphene-like 2D carbon lattices without 6-member motives, i.e., pseudographenes, are also studied. The geometry and energy of disclinated 2D carbon configurations are analyzed with the help of molecular dynamics (MD) simulation technique. A comparison of the obtained MD results with analytical calculations within the framework of the theory of defects of elastic continuum is presented.We consider graphene disclination networks (DNs) — periodic distributions of disclination defects. Disclinations manifest themselves as 4-, 5-, 7- or 8-member carbon rings in otherwise 6-member ring ideal 2D graphene crystal lattice. Limiting cases of graphene-like 2D carbon lattices without 6-member motives, i.e., pseudographenes, are also studied. The geometry and energy of disclinated 2D carbon configurations are analyzed with the help of molecular dynamics (MD) simulation technique. A comparison of the obtained MD results with analytical calculations within the framework of the theory of defects of elastic continuum is presented. read less

Abstract: AFM experiments and molecular dynamics simulation of rectangular graphdiyne films are performed in this paper. The force-deflection curves are obtained, and the elastic modulus is calculated as 218.5 GPa and 482.615 GPa, respectively. The simulated maximum stress and pre-tension of graphdiyne film are 33.088 GPa and 0.551 GPa, respectively. It is observed that the graphdiyne film fractured in the central point once the indentation depth over the critical depth. Also, the obviously elastic behaviour has found during the loading-unloading-reloading process. The deformation mechanisms and fractured behaviour of the graphdiyne film are discussed in detail during the loading process. Moreover, the effects of various factors including loading speed and indenter radii of the graphdiyne film by the MD simulation are discussed. read less

Abstract: Graphene is one of the most important nanomaterials. The twisted bilayer graphene shows superior electronic properties compared to graphene. Here, we demonstrate via molecular dynamics simulations that twisted bilayer graphene possesses outstanding mechanical properties. We find that the mechanical strain rate and the presence of cracks have negligible effects on the linear elastic properties, but not the nonlinear mechanical properties, including fracture toughness. The “two-peak” pattern in the stress-strain curves of the bilayer composites of defective and pristine graphene indicates a sequential failure of the two layers. Our study provides a safe-guide for the design and applications of multilayer grapheme-based nanoelectronic devices. read less

Abstract: Diamondene, a carbon nanomaterial containing both sp2 and sp3 carbon atoms, is obtained by compressing two or more layers of graphene. By curving rectangular diamondene and matching the unsaturated C-C bonds on the two unbent edges, a nanotube is built. We build two diamondene nanotubes (DNTs) with different radii and test their strengths under uniaxial tension. From the stress-strain curves, we discover that DNTs exhibit softening followed by hardening. The mechanism is as follows: the bond lengths and bond angles impart different stiffnesses to the tube at different axial strains. Molecular dynamics simulations demonstrate that the feature of the softening-hardening process is independent of either the tube radii or the system temperature. The critical strain for the tensile strength of a DNT becomes lower at a higher temperature. This is caused by thermal vibration of the atoms in the tubes. At the same temperature, for a DNT with a larger radius, the value of critical strain is higher. These properties will be beneficial for the potential applications of DNTs in nanodevices. read less

Abstract: Significance Nanocarbons can be characterized by their curvature—that is, positively curved fullerenes, zero-curved graphene, and negatively curved schwarzites. Schwartzites are fascinating materials but have not been synthesized yet, although disordered materials with local properties similar to schwarzites (“random schwarzites”) have been isolated. A promising synthetic method allows for the interior surfaces of zeolites to be templated with sp2 carbon, but theoretical study of these zeolite-templated carbons (ZTCs) has been limited because of their noncrystalline structures. In this work, we develop an improved molecular description of ZTCs, show that they are equivalent to schwartzites, and thus make the experimental discovery of schwarzites ex post facto. Our topological characterization of ZTCs lends insights into how template choice allows for the tunability of ordered microporous carbons. Zeolite-templated carbons (ZTCs) comprise a relatively recent material class synthesized via the chemical vapor deposition of a carbon-containing precursor on a zeolite template, followed by the removal of the template. We have developed a theoretical framework to generate a ZTC model from any given zeolite structure, which we show can successfully predict the structure of known ZTCs. We use our method to generate a library of ZTCs from all known zeolites, to establish criteria for which zeolites can produce experimentally accessible ZTCs, and to identify over 10 ZTCs that have never before been synthesized. We show that ZTCs partition space into two disjoint labyrinths that can be described by a pair of interpenetrating nets. Since such a pair of nets also describes a triply periodic minimal surface (TPMS), our results establish the relationship between ZTCs and schwarzites—carbon materials with negative Gaussian curvature that resemble TPMSs—linking the research topics and demonstrating that schwarzites should no longer be thought of as purely hypothetical materials. read less

Abstract: As a new class of two-dimensional (2D) materials, 2D covalent organic frameworks (COFs) are proven to possess remarkable electronic and magnetic properties. However, their mechanical behaviours remain almost unexplored. In this work, taking the recently synthesised dimethylmethylene-bridged triphenylamine (DTPA) sheet as an example, we investigate the mechanical behaviours of 2D COFs based on molecular dynamics simulations together with density functional theory calculations. A novel phase transformation is observed in DTPA sheets when a relatively large in-plane compressive strain is applied to them. Specifically, the crystal structures of the transformed phases are topographically different when the compressive loading is applied in different directions. The compression-induced phase transformation in DTPA sheets is attributed to the buckling of their kagome lattice structures and is found to have significant impacts on their material properties. After the phase transformation, Young's modulus, band gap and thermal conductivity of DTPA sheets are greatly reduced and become strongly anisotropic. Moreover, a large in-plane negative Poisson's ratio is found in the transformed phases of DTPA sheets. It is expected that the results of the compression-induced phase transformation and its influence on the material properties observed in the present DTPA sheets can be further extended to other 2D COFs, since most 2D COFs are found to possess a similar kagome lattice structure. read less

Abstract: In this paper, mechanical characteristics of the aluminum layer coated with graphene are investigated by performing numerical tensile experiments through classical molecular dynamics simulations. Based on the results of the simulations, it is shown that coating with graphene enhances the Young’s modulus of aluminum by 88% while changing the tensile behavior of aluminum with hardening–softening mechanisms and significantly increased toughness. Furthermore, the effect of loading rate is examined and a transformation to an amorphous phase is observed in the coated aluminum structure as the loading rate is increased. Even though the dominant component of the coated hybrid structure is the aluminum core in the elastic region, the graphene layer shows its effects majorly in the plastic region by a 60% increase in the ultimate tensile strength. High loading rates at room temperature cause the structure transforms to an amorphous phase, as expected. Thus, effects of loading rate and temperature on amorphization are investigated by performing the same simulations at different strain rates and temperatures (i.e., 0, 300, and 600 K). read less

Abstract: Carbon nanotubes (CNTs) are highly promising for strength reinforcement in polymer nanocomposites, but conflicting interfacial properties have been reported by single nanotube pull-out experiments. Here, we report the interfacial load transfer mechanisms during pull-out of CNTs from PMMA matrices, using massively- parallel molecular dynamics simulations. We show that the pull-out forces associated with non-bonded interactions between CNT and PMMA are generally small, and are weakly-dependent on the embedment length of the nanotube. These pull-out forces do not significantly increase with the presence of Stone Wales or vacancy defects along the nanotube. In contrast, low-density distribution of cross-links along the CNT-PMMA interface increases the pull-out forces by an order of magnitude. At each cross-linked site, mechanical unfolding and pull-out of single or pair polymer chain(s) attached to the individual cross-link bonds result in substantial interfacial strengthening and toughening, while contributing to interfacial slip between CNT and PMMA. Our interfacial shear-slip model shows that the interfacial loads are evenly-distributed among the finite number of cross-link bonds at low cross-link densities or for nanotubes with short embedment lengths. At higher cross-link densities or for nanotubes with longer embedment lengths, a no-slip zone now develops where shear-lag effects become important. Implications of these results, in the context of recent nanotube pull-out experiments, are discussed. read less

Abstract: Styrene–ethylene–butylene–styrene (SEBS) composite films containing well-dispersed and highly aligned hexagonal boron nitride (hBN) platelets were achieved by a ball milling process followed by hot-pressing treatment. An ultrahigh in-plane thermal conductivity of 45 W m−1 K−1 was achievable in the SEBS composite film with 95 wt% hBN. The corresponding out-of-plane thermal conductivity was also as high as 4.4 W m−1 K−1. The hBN/SEBS composite film was further used to cool a CPU connected to a computer, resulting in a decrease by about 4 °C in the stable temperature. Percolation thresholds over 40 wt% and 60 wt% in the hBN/SEBS composites were obtained in the in-plane and out-of-plane directions, respectively. This phenomenon has rarely been reported in polymer composites. Molecular dynamics simulations were also conducted to support this percolation threshold. The linear coefficients of the thermal expansion value of the hBN/SEBS composite with 95 wt% hBN was as low as 16 ppm K−1. This was a significant decrease compared to that of pure SEBS (149 ppm K−1). The proposed strategy provides valuable advice about the heat-transfer mechanism in polymer composites containing oriented two-dimensional materials. read less

Abstract: Transport of saltwater through pristine and positively charged single-layer graphene nanoporous membranes is investigated using molecular dynamics simulations. Pressure-driven flows are induced by motion of specular reflecting boundaries at feed and permeate sides with constant speed. Unlike previous studies in the literature, this method induces a desired flow rate and calculates the resulting pressure difference in the reservoirs. Due to the hexagonal structure of graphene, the hydraulic diameters of nano-pores are used to correlate flow rate and pressure drop data. Simulations are performed for three different pore sizes and flow rates for the pristine and charged membrane cases. In order to create better statistical averages for salt rejection rates, ten different initial conditions of Na+ and Cl- distribution in the feed side are used for each simulation case. Using data from 180 distinct simulation cases and utilizing the Buckingham Pi theorem, we develop a functional relationship between the volumetric flow rate, pressure drop, pore diameter, and the dynamic viscosity of saltwater. A linear relationship between the volumetric flow rate and pressure drop is observed. For the same flow rate and pore size, charged membranes exhibit larger pressure drops. Graphene membranes with 9.90 Å pore diameter results in 100% salt rejection with 163.2 l/h cm2 water flux, requiring a pressure drop of 35.02 MPa. read less

Abstract: Bending rigidity plays an important role in graphene from mechanical behavior to magnetic and electrical properties. However, it is still in a theoretical debate whether the bending rigidity of graphene increase or decrease with increasing temperature. The liquid membranes renormalization theory is always used to calculate the bending modulus of 2D membrane (graphene) at different temperatures. Although this theory has been successfully used to describe the mechanical behavior of liquid membranes like cell membrane, we point out some possible unsuitable places when it is used to evaluate the temperature effect on the bending rigidity of graphene. The energy difference between the notional planar and pure bending graphene is related to the definition of the bending rigidity directly. Based on this energy variation analysis, we demonstrate that the bending rigidity of graphene increases with increasing temperature. Moreover, we reveal the mechanism is that the configurational entropy plays a crucial role in the variation of the free energy of graphene with increasing temperature. Our approach also paves a way to investigate the temperature effect on the bending rigidity of other 2D materials.Bending rigidity plays an important role in graphene from mechanical behavior to magnetic and electrical properties. However, it is still in a theoretical debate whether the bending rigidity of graphene increase or decrease with increasing temperature. The liquid membranes renormalization theory is always used to calculate the bending modulus of 2D membrane (graphene) at different temperatures. Although this theory has been successfully used to describe the mechanical behavior of liquid membranes like cell membrane, we point out some possible unsuitable places when it is used to evaluate the temperature effect on the bending rigidity of graphene. The energy difference between the notional planar and pure bending graphene is related to the definition of the bending rigidity directly. Based on this energy variation analysis, we demonstrate that the bending rigidity of graphene increases with increasing temperature. Moreover, we reveal the mechanism is that the configurational entropy plays a crucial role in... read less

Abstract: A cross-over in the interfacial strength, with increase in the separation rate, is observed between graphite-cis-1,4-polyisoprene and amorphous silica-cis-1,4-polyisoprene interfaces. Molecular dynamics simulations are used to compare the traction-separation characteristics of the two interfaces in the opening mode of separation at various separation rates and temperatures above the glass transition temperature of cis-1,4-polyisoprene. It was observed that various parameters governing the interface strength, such as strength modulus (ratio of peak traction to the separation at peak traction), peak traction, and the work of adhesion are higher for the silica substrated interface at very low separation rates. However, at higher rates, the graphite substrated interface showed higher values for the strength parameters. The reasons for this interface strength cross-over are explained using the potential energy, mobility, entanglement strength, tensile stiffness, and densities of the polymer over both substrates and the interface cohesive binding energy. Based on these observations, it is concluded that silica filled rubber nanocomposites are suitable for normal automobile tire applications; however, graphite fillers may be more suitable for resisting very large impact loads.A cross-over in the interfacial strength, with increase in the separation rate, is observed between graphite-cis-1,4-polyisoprene and amorphous silica-cis-1,4-polyisoprene interfaces. Molecular dynamics simulations are used to compare the traction-separation characteristics of the two interfaces in the opening mode of separation at various separation rates and temperatures above the glass transition temperature of cis-1,4-polyisoprene. It was observed that various parameters governing the interface strength, such as strength modulus (ratio of peak traction to the separation at peak traction), peak traction, and the work of adhesion are higher for the silica substrated interface at very low separation rates. However, at higher rates, the graphite substrated interface showed higher values for the strength parameters. The reasons for this interface strength cross-over are explained using the potential energy, mobility, entanglement strength, tensile stiffness, and densities of the polymer over both substrate... read less

Abstract: We demonstrate snake-like motion of graphene nanoribbons atop graphene and hexagonal boron nitride ( h-BN) substrates using fully atomistic nonequilibrium molecular dynamics simulations. The sliding dynamics of the edge-pulled nanoribbons is found to be determined by the interplay between in-plane ribbon elasticity and interfacial lattice mismatch. This results in an unusual dependence of the friction-force on the ribbon's length, exhibiting an initial linear rise that levels-off above a junction-dependent threshold value dictated by the pre-slip stress distribution within the slider. As part of this letter, we present the LAMMPS implementation of the registry-dependent interlayer potentials for graphene, h-BN, and their heterojunctions that were used herein, which provides enhanced performance and accuracy. read less

Abstract: ABSTRACT Surface characteristics of graphene have an important impact on its performance. Substantial attention has been devoted to studying the static wetting behaviour of a graphene-coated substrate with little attention to the dynamic wetting behaviour. The impact of contact line forces (CLFs) on the droplet-spreading process has not been revealed completely yet. A series of molecular dynamics (MD) simulation is performed to investigate the spreading process of the water droplet on the graphene-coated substrate in this research. The increase of interaction potential parameter between substrate and water droplet makes the spreading radius getting bigger and the final static contact angle smaller. Apart from that, the higher hydrophilicity of underlying substrate can lead to the greater averaged forces of atoms near contact line. CLFs correlate well with the variation of kinetic energy of water molecules located in the contact line region. Surface tensions of water droplets on graphene-coated substrates are also examined. The liquid-vapour and solid-vapour surface tensions are constant. An increase in the surface tension of liquid-solid lead to the increase of balanced contact angles of water on the substrate. The results are useful for understanding the effect of CLFs on the dissipation of kinetic energy of water molecules. read less

Abstract: Graphene, viz., the one-atom-thick sheet of carbon, exhibits outstanding mechanical properties, but defects, which are inevitable at the time of synthesis, may strongly affect these properties. In this study, the effects of two types of Stone-Thrower-Wales (namely, STW-1 and STW-2) defects on the mechanical properties of graphene sheets at different temperatures and strain rates were investigated on the basis of molecular dynamics simulations. The authors also investigated the effect of the strain rate and defect concentration on the failure morphology of STW-1 and STW-2 defected graphene sheets. It was observed that, irrespective of the strain rate, the fracture strengths of STW-1 and STW-2 defected graphene sheets are identical in the zigzag and armchair directions, respectively, at low temperatures. It was also observed that the fracture strengths of graphene sheets with STW-1 defects in the armchair direction and STW-2 defects in the zigzag direction decrease drastically at higher temperatures and also at lower strain rates. On the other hand, it was noticed that the fracture strengths of graphene sheets with STW-1 defects in the zigzag direction and STW-2 defects in the armchair direction decrease gradually with an increase in the temperature and a decrease in the strain rate. It was also predicted that the failure morphology of graphene sheets with STW-1 defects in the zigzag direction and STW-2 defects in the armchair direction depends on the defect concentration and the strain rate. read less

Abstract: A systematic study of axial strain effects on friction in carbon nanotube bearings is conducted in this paper. The relationships between friction and axial strains are determined by implementing molecular dynamics simulations. It is found that the dependence of friction on velocity and temperature is altered by axial strains. The mechanism of strain effects is revealed through numerical and theoretical analyses. Based on phonon computations, axial strain effects tune friction by adjusting the distribution of the phonon frequency density, which affects the transfer efficiency of orderly kinetic energy into disorderly thermal energy. The findings in this work advance the understanding of friction in carbon nanotubes and suggest the great potential of axial strain effects on tuning friction in nanodevice applications. read less

Abstract: Bond transition of sp2 to sp3 in carbon nanotube can be realized through bending operation at buckling location, which affects the electronic, mechanical and thermal properties of buckled carbon nanotube. In this work, thermal properties of buckled tri-walled carbon nanotube with sp3 bonds are explored using molecular dynamics. Our results reveal that interfacial thermal conductance at buckling location is sensitive to the bending angle, which decreases exponentially with increasing bending angle until 90 degree because of increasing the number of interlayer sp3 bonds. When the bending angle is beyond 90 degree, there are sp3 bonds formed on the outer-tube walls which provide new paths for heat transfer. The insight of mechanism of thermal properties is analyzed by determining atomic micro-heat flux scattering. Our findings provide a flexible and applicable method to design thermal management device. This unusual phenomenon is explained by the micro-heat flux migration and stress distributions. read less

Abstract: The competition between the out-of-plane rigidity and the van der Waals interaction leads to the scrolled and folded structural configurations of graphene. These configuration changes, as compared with the initially planar geometry, significantly affect the electronic, optical and mechanical properties of graphene, promising exciting applications in graphene-nanoelectronics. We propose a finite-deformation theoretical model, in which no presumed assumptions on the geometries of deformed configurations are required. Both the predicted deformed profiles and the critical conditions show great agreements with molecular dynamics simulations results when compared with existing studies with simple geometrical assumptions. Moreover, MD simulations are performed to explore the morphology transitions between different configurations. It is observed that the folded configuration is energetically favorable for a short graphene sheet, while a long graphene sheet tends to scroll. Of particular interest, we observe the morphology transition from a Fermat scroll to the Archimedean scroll for the bi-scrolled graphene. These findings are useful for understanding the stability of graphene and may provide guidance to the design of programmable graphene-nanoelectronics. read less

Abstract: The inverse Stone-Wales defect is a typical defect in graphene, which causes local bumps and local deformation in graphene sheets. Our molecular dynamics simulations show that the spontaneous rolling up of graphene sheets can be induced by orderly distributed inverse Stone-Wales defect bumps, when defective graphene is cut into small strips. This spontaneous process is mainly dominated by the defect density and tailored graphene size. When the tailored length is longer than the upper threshold length, graphene sews up as a curly one-dimensional structure: heart-shaped nanotube. For medium length graphene (the length is in between the lower threshold value and upper threshold value), the results reveal that graphene finally curls into a completely or incompletely stitched nanotube similar to a cylindrical shell. This spontaneous process is produced by a high-frequency damped vibration accompanied by elastic and viscoelastic deformation in defective graphene. Thus, the properties of vibration are further investigated for graphene that has the tailored length shorter than the lower threshold length. This kind of graphene gradually forms a curved nanoribbon rather than a nanotube. It is also found that the bending rigidity of defective graphene is larger than that of pristine graphene. read less

Abstract: Using non-equilibrium molecular dynamic simulations, we investigate the effect of adsorbates on nanoscopic friction. We find that the interplay between different channels of energy dissipation at the frictional interface may lead to non-monotonic dependence of the friction force on the adsorbate surface coverage and to strongly nonlinear variation of friction with normal load (non-Amontons' behavior). Our simulations suggest that the key parameter controlling the variation of friction force with the normal load, surface coverage and temperature is the time-averaged number of adsorbates confined between the tip and the substrate. Three different regimes of temperature dependence of friction in the presence of adsorbates are predicted. Our findings point on new ways to control friction on contaminated surfaces. read less

Abstract: Graphene has received great attention due to its fascinating thermal properties. The inevitable defects in graphene, such as single vacancy, doping, and functional group, greatly affect the thermal conductivity. The sole effect of these defects on the thermal conductivity has been widely studied, while the mechanisms of the coupling effects are still open. We studied the combined effect of defects with N-doping, the -CH3 group, and single vacancy on the thermal conductivity of multi-layer graphene at various temperatures using equilibrium molecular dynamics with the Green-Kubo theory. The Taguchi orthogonal algorithm is used to evaluate the sensitivity of N-doping, the -CH3 group, and single vacancy. Sole factor analysis shows that the effect of single vacancy on thermal conductivity is always the strongest at 300 K, 700 K, and 1500 K. However, for the graphene with three defects, the single vacancy defect only plays a significant role in the thermal conductivity modification at 300 K and 700 K, while the -CH3 group dominates the thermal conductivity reduction at 1500 K. The phonon dispersion is calculated using a spectral energy density approach to explain such a temperature dependence. The combined effect of the three defects further decreases the thermal conductivity compared to any sole defect at both 300 K and 700 K. The weaker single vacancy effect is due to the stronger Umklapp scattering at 1500 K, at which the combined effect seriously covers almost all the energy gaps in the phonon dispersion relation, significantly reducing the phonon lifetimes. Therefore, the temperature dependence only appears on the multi-layer graphene with combined defects.Graphene has received great attention due to its fascinating thermal properties. The inevitable defects in graphene, such as single vacancy, doping, and functional group, greatly affect the thermal conductivity. The sole effect of these defects on the thermal conductivity has been widely studied, while the mechanisms of the coupling effects are still open. We studied the combined effect of defects with N-doping, the -CH3 group, and single vacancy on the thermal conductivity of multi-layer graphene at various temperatures using equilibrium molecular dynamics with the Green-Kubo theory. The Taguchi orthogonal algorithm is used to evaluate the sensitivity of N-doping, the -CH3 group, and single vacancy. Sole factor analysis shows that the effect of single vacancy on thermal conductivity is always the strongest at 300 K, 700 K, and 1500 K. However, for the graphene with three defects, the single vacancy defect only plays a significant role in the thermal conductivity modification at 300 K and 700 K, while the... read less

Abstract: Recent advances in graphene and other two-dimensional (2D) material synthesis and characterization have led to their use in emerging technologies, including flexible electronics. However, a major challenge is electrical contact stability, especially under mechanical straining or dynamic loading, which can be important for 2D material use in microelectromechanical systems. In this letter, we investigate the stability of dynamic electrical contacts at a graphene/metal interface using atomic force microscopy (AFM), under static conditions with variable normal loads and under sliding conditions with variable speeds. Our results demonstrate that contact resistance depends on the nature of the graphene support, specifically whether the graphene is free-standing or supported by a substrate, as well as on the contact load and sliding velocity. The results of the dynamic AFM experiments are corroborated by simulations, which show that the presence of a stiff substrate, increased load, and reduced sliding velocity lead to a more stable low-resistance contact. read less

Abstract: By applying a constant vertical upward velocity to graphene which was on silicon substrate, the adhesion behavior of graphene was observed. The effects of silicon substrate size, graphene layer number and exfoliation velocity on adhesion properties of graphene were studied. The influence of silicon substrate size on the exfoliation process can be ignored when it was 40 Ålarger than graphene. The minimum velocity to exfoliate monolayer graphene was 4.3 Å/ps, and the maximum adhesive force was 25.04 nN. For two layer graphene, velocity was applied on the top layer, 5.2 Å/ps and 12 Å/ps were the critical velocities. When the velocity was no more than 5.2 Å/ps, the top layer graphene cannot be exfoliated. The top layer graphene was moved upward, meanwhile driving the second layer upward because of interlayer interaction between layers, as the velocity was in the range of 5.2 Å/ps~12 Å/ps. While the velocity increased greater than 12 Å/ps, the top layer was broken through the barrier of substrate and the second layer, then exfoliated alone. The velocity was extremely high to exfoliate graphene, and the adhesion energy was 299.81 mJ/m2, thus the adhesion between graphene and silicon was very strong. read less

Abstract: Conventional methods to induce strain in 2D materials can hardly catch up with the sharp increase in requirements to design specific strain forms, such as the pseudomagnetic field proposed in graphene, funnel effect of excitons in MoS2 , and also the inverse funnel effect reported in black phosphorus. Therefore, a long-standing challenge in 2D materials strain engineering is to find a feasible scheme that can be used to design given strain forms. In this article, combining the ability of experimentally synthetizing in-plane heterostructures and elegant Eshelby inclusion theory, the possibility of designing strain fields in 2D materials to manipulate physical properties, which is called internal stress assisted strain engineering, is theoretically demonstrated. Particularly, through changing the inclusion's size, the stress or strain gradient can be controlled precisely, which is never achieved. By taking advantage of it, the pseudomagnetic field as well as the funnel effect can be accurately designed, which opens an avenue to practical applications for strain engineering in 2D materials. read less

Abstract: The influence of vibration on friction at the nanoscale was studied via molecular dynamics (MD) simulations. The results show that average friction increases in a high-frequency range. This can be attributed to the vibration of the tip following vibration excitation, which results in peaks of repulsive interaction between tip and substrate and leads to higher friction. However, when the frequency is lower than a certain value, friction decreases. This is because vibration excitation results not in an obvious vibration of the tip but in a slightly larger interface distance, which leads to a decrease in friction. read less

Abstract: Topological defects (e.g. pentagons, heptagons and pentagon-heptagon pairs) have been widely observed in large scale graphene and have been recognized to play important roles in tailoring the mechanical and physical properties of two-dimensional materials in general. Thanks to intensive studies over the past few years, optimizing properties of graphene through topological design has become a new and promising direction of research. In this chapter, we review some of the recent advances in experimental, computational and theoretical studies on the effects of topological defects on mechanical and physical properties of graphene and applications of topologically designed graphene. The discussions cover out-of-plane effects, inverse problems of designing distributions of topological defects that make a graphene sheet conform to a targeted three-dimensional surface, grain boundary engineering for graphene strength, curved graphene for toughness enhancement and applications in engineering energy materials, multifunctional materials and interactions with biological systems. Despite the rapid developments in experiments and simulations, our understanding on the relations between topological defects and mechanical and physical properties of graphene and other 2D materials is still in its infancy. The intention here is to draw the attention of the research community to some of the open questions in this field. read less

Abstract: A nano continuous variable transmission (nano-CVT) system is proposed by means of carbon nanotubes (CNTs). The dynamic behavior of the CNT-based nanosystem is assessed using molecular dynamics simulations. The system contains a rotary CNT-motor and a CNT-bearing. The tube axes of the nanomotor and the rotor in the bearing are laid in parallel, and the distance between them is known as the eccentricity of the rotor with a diameter of d. By changing the eccentricity (e) of the rotor from 0 to d, some interesting rotation transmission phenomena are discovered, whose procedures can be used to design various nanodevices. This might include the failure of rotation transmission—i.e. the rotor has no rotation—when e ≥ d at an extremely low temperature, or when the edges of the two tubes are orthogonal at their intersections in any condition. This hints that the state of the nanosystem can be used as an on/off switch or breaker. For a system with e = d and a high temperature, the rotor rotates in the reverse direction of the motor. This means that the output signal (rotation) is the reverse of the input signal. When changing the eccentricity from 0 to d continuously, the output signal gradually decreases from a positive value to a negative value; as a result a nano-CVT system is obtained. read less

Abstract: The influence of the orientations and concentrations of the Stone–Wales (SW) defects on the thermal conductivity of zigzag and armchair graphene nanoribbons (GNRs) is explored using the reverse non-equilibrium molecular dynamics method. The results show that the thermal conductivity of GNRs with two different chirality cases reaches the minimum in the range of 0.1–0.7% defect concentration. Beyond a critical value of the SW defect concentration, the thermal conductivity increases with the increase in SW concentration for both zigzag and armchair GNRs. It is shown that at high concentrations of the SW defects, the thermal conductivity of zigzag GNRs with Type II defects is larger than the GNRs with Type I defects. Finally, the dependence of the SW defect concentration and orientation on the power spectra overlaps have also been explored. read less

Abstract: Tribological and structural properties of water monolayers confined at interfaces between graphene and Cu substrates at cryogenic and room temperatures are extensively studied using molecular dynamics simulations and first-principles calculations. The frictions caused by the sliding of graphene sheets and increasing temperature will reduce the interfacial density of water molecules and lead to structural transformations of water monolayers and direct contacts of graphene with the underlying Cu substrates. Such changes in water structures give rise to higher friction forces and shear strengths at the graphene/Cu interfaces. Depending on the water coverage density and temperature, the motions of graphene on monolayer water covered Cu exhibit stick-slip and continuous slipping behaviors. The strong association of friction characteristics with structural transformations of water molecules could be used to unveil interfacial information of graphene on water adsorbed metal surfaces. read less

Abstract: Heterostructures composed of dissimilar two-dimensional nanomaterials can have nontrivial physical and mechanical properties which are potentially useful in many applications. Interestingly, in some cases, it is possible to create heterostructures composed of weakly and strongly stretched domains with the same chemical composition, as has been demonstrated for some polymer chains, DNA, and intermetallic nanowires supporting this effect of two-phase stretching. These materials, at relatively strong tension forces, split into domains with smaller and larger tensile strains. Within this region, average strain increases at constant tensile force due to the growth of the domain with the larger strain, at the expense of the domain with smaller strain. Here, the two-phase stretching phenomenon is described for graphene nanoribbons with the help of molecular dynamics simulations. This unprecedented feature of graphene that is revealed in our study is related to the peculiarities of nucleation and the motion of the domain walls separating the domains of different elastic strain. It turns out that the loading–unloading curves exhibit a hysteresis-like behavior due to the energy dissipation during the domain wall nucleation and motion. Here, we put forward the idea of implementing graphene nanoribbons as elastic dampers, efficiently converting mechanical strain energy into heat during cyclic loading–unloading through elastic extension where domains with larger and smaller strains coexist. Furthermore, in the regime of two-phase stretching, graphene nanoribbon is a heterostructure for which the fraction of domains with larger and smaller strain, and consequently its physical and mechanical properties, can be tuned in a controllable manner by applying elastic strain and/or heat. read less

Abstract: We use molecular dynamics simulation to calculate the thermal conductivities of (5, 5) carbon nanotube superlattices (CNTSLs) and defective carbon nanotubes (DCNTs), where CNTSLs and DCNTs have the same size. It is found that the thermal conductivity of DCNT is lower than that of CNTSL at the same concentration of Stone–Wales (SW) defects. We perform the analysis of heat current autocorrelation functions and observe the phonon coherent resonance in CNTSLs, but do not observe the same effect in DCNTs. The phonon vibrational eigen-mode analysis reveals that all modes of phonons are strongly localized by SW defects. The degree of localization of CNTSLs is lower than that of DCNTs, because the phonon coherent resonance results in the phonon tunneling effect in the longitudinal phonon mode. The results are helpful in understanding and tuning the thermal conductivity of carbon nanotubes by defect engineering. read less

Abstract: In the present work, we use atomic force microscopy nanomanipulation of 2D-material standing folds to investigate their mechanical deformation. Using graphene, h-BN and talc nanoscale wrinkles as testbeds, universal force–strain pathways are clearly uncovered and well-accounted for by an analytical model. Such universality further enables the investigation of each fold bending stiffness κ as a function of its characteristic height h0. We observe a more than tenfold increase of κ as h0 increases in the 10–100 nm range, with power-law behaviors of κ versus h0 with exponents larger than unity for the three materials. This implies anomalous scaling of the mechanical responses of nano-objects made from these materials. read less

Abstract: Carbon nanotubes (CNTs) can undergo collapse from the ordinary cylindrical configurations to bilayer ribbons when adhered on substrates. In this study, the collapsed adhesion of CNTs on the silicon substrates is investigated using both classical molecular dynamics (MD) simulations and continuum analysis. The governing equations and transversality conditions are derived based on the minimum potential energy principle and the energy-variational method, considering both the van der Waals interactions between CNTs and substrates and those inside CNTs. Closed-form solutions for the collapsed configuration are obtained which show good agreement with the results of MD simulations. The stability of adhesive configurations is investigated by analyzing the energy states. It is found that the adhesive states of single-walled CNTs (SWCNTs) (n, n) on the silicon substrates can be categorized by two critical radii, 0.716 and 0.892 nm. For SWCNTs with radius larger than 0.892 nm, they would fully collapse on the silicon substrates. For SWCNTs with radius less than 0.716 nm, the initial cylindrical configuration is energetically favorable. For SWCNTs with radius between two critical radii, the radially deformed state is metastable. The non-contact ends of all collapsed SWCNTs are identical with the same arc length of 2.38 nm. Finally, the role of number of walls on the adhesive configuration is investigated quantitatively. For multi-walled CNTs with the number of walls exceeding a certain value, the cylindrical configuration is stable due to the increasing bending stiffness. The present study can be useful for the design of CNT-based nanodevices. read less

Abstract: We perform molecular dynamics simulations to investigate the irreversibility of crumpled graphene obtained by hydrostatic or biaxial compression. Our results show that there is a critical degree of crumpling, above which the crumpling is irreversible after the external force is removed. The critical degree of irreversible crumpling is closely related to the self-adhesion phenomenon of graphene, which leads to a step-like jump or decrease in the adhesion energy. We find the critical degree of crumpling is about 0.5 or 0.55 for hydrostatic or biaxial compression, which matches analytic predictions based on a competition between adhesive and bending energies in folded graphene. read less

Abstract: Graphene-based materials exhibit intriguing phononic and thermal properties. In this paper, we have investigated the heat conductance in graphene sheets under shear-strain-induced wrinkling deformation, using equilibrium molecular dynamics simulations. A significant orientation dependence of the thermal conductivity of graphene wrinkles (GWs) is observed. The directional dependence of the thermal conductivity of GWs stems from the anisotropy of phonon group velocities as revealed by the G-band broadening of the phonon density of states (DOS), the anisotropy of thermal resistance as evidenced by the G-band peak mismatch of the phonon DOS, and the anisotropy of phonon relaxation times as a direct result of the double-exponential-fitting of the heat current autocorrelation function. By analyzing the relative contributions of different lattice vibrations to the heat flux, we have shown that the contributions of different lattice vibrations to the heat flux of GWs are sensitive to the heat flux direction, which further indicates the orientation-dependent thermal conductivity of GWs. Moreover, we have found that, in the strain range of 0-0.1, the anisotropy ratio of GWs increases monotonously with increasing shear strain. This is induced by the change in the number of wrinkles, which is more influential in the direction perpendicular to the wrinkle texture. The findings elucidated here emphasize the utility of wrinkle engineering for manipulation of nanoscale heat transport, which offers opportunities for the development of thermal channeling devices. read less

Abstract: A potentially new, single-atom thick semiconducting 2D-graphene-like material, called Anisotropic-cyclicgraphene , has been generated by the two stage searching strategy linking molecular and ab initio approach. The candidate was derived from the evolutionary-based algorithm and molecular simulations was then profoundly analysed using first-principles density functional theory from the structural, mechanical, phonon, and electronic properties point of view. The proposed polymorph of graphene (rP16-P1m1) is mechanically, dynamically, and thermally stable and can achieve semiconducting with a direct band gap of 0.829 eV. read less

Abstract: Graphene nanopores are utilized in various notable applications such as water desalination, molecular separation, and DNA sequencing. However, the creation of stable nanopores is still challenging due to the self-healing nature of graphene. In this study, using molecular dynamics simulations we explore the drilling of nanopores through graphene by bombardment with Si-nanoparticles. This enables the Si-passivation along the nanopore rim, which is known as an efficient way to stabilize graphene nanopores. The interplay between graphene and projectile causes the anomalous behaviors such as local maxima depending on particle size. The observations are thoroughly analyzed with interaction energy and shape changes. read less

Abstract: Using the non-equilibrium molecular dynamics method, the thermal properties of two dimensional nanomaterials are investigated by considering graphene and graphyne nanosheets with circular boundaries. The thermal transport efficiency of graphene and graphyne under heat flux from the inner boundary to outer boundary is revealed to be tunable by applying in-plane torsion at the inner boundary, and the tunable range of thermal conductivity for graphyne could be up to 37% (15% for graphene). With the increase of rotation angle, the thermal conductivities of both graphene and graphyne are found to increase at small rotation angles and then decrease after the occurrence of wrinkle deformation. The maximum thermal conductivity appears at the onset of wrinkling which depends on the lattice structure and stiffness of the nanosheets. By systematically investigating the morphological characteristics and the phonon spectra under different torsion angles, the tunable thermal conductivities of both graphene and graphyne are found to be controlled by three factors including surface smoothness, stress concentration and lattice instability. The increase of thermal conductivity with small torsion angles is caused by the suppressed surface fluctuation which decreases the phonon scattering, while the wrinkling and lattice instability occurring under large torsion angles accounts for the deterioration of thermal conductivity. Since the fluctuation of graphyne is efficiently compressed at smaller torsion angles compared to graphene, the maximum thermal conductivity of graphyne appears earlier than graphene. Such correlation between out-of-plane deformation and in-plane thermal conductivity provides new insights into the thermal management of two dimensional nanomaterials. read less

Abstract: Herein, we report a self-organized core/shell (CS) structure consisting of an Al core and a Pb shell in the liquid Pb alloys at a constant temperature. This is contrary to a believed opinion that the thermal gradient acts as the only driving force for the formation of this kind of structure. The results show that its forming ability greatly depends on the composition and temperature. Importantly, when the alloys are placed in the confined space, a structure of Al–Pb–Al sandwich construction and a completely different CS structure composed of a Pb core and an Al shell are obtained; this suggests a new strategy for controlling the phase-separated structures at the nanoscale. More interestingly, an abnormal phenomenon in the low Pb alloys with the amorphous thin Pb shell and the crystal Al core is observed after solidification, which does not occur in the high Pb alloys that just possess a secondary wrapped structure that never appears in the liquid state. The result of this study may help to shed light on understanding the formation of this core/shell structure and controlling it at an atomic view that have a potential application in the fabrication of these structural materials through nanotechnology. read less

Abstract: Atomistic simulations were used to study conductance across the interface between a nanoscale gold probe and a graphite surface with a step edge. Conductance on the graphite terrace was observed to increase with load and be approximately proportional to contact area calculated from the positions of atoms in the interface. The relationship between area and conductance was further explored by varying the position of the contact relative to the location of the graphite step edge. These simulations reproduced a previously-reported current dip at step edges measured experimentally and the trend was explained by changes in both contact area and the distribution of distances between atoms in the interface. The novel approach reported here provides a foundation for future studies of the fundamental relationships between conductance, load and surface topography at the atomic scale. read less

Abstract: The dynamics of a water nano-droplet on a flexible graphene sheet, in the presence of constant and alternative electric fields with various amplitudes and frequencies, was considered using a molecular dynamics method. It was found that because the water molecules respond to electric field, the nano-droplet elongates in the field direction for a field amplitude larger than 0.08 V Å-1, which is stronger than the predicted value from the Young-Laplace equation. This difference can be described by considering the van der Waals attractions between the droplet molecules and the substrate, which can be calculated by modifying the Young-Laplace equation. Furthermore, under the influence of an alternating field over the GHz frequency range, it was shown that the droplet shape will not change above a threshold frequency, which depends on the relaxation time of the water dipole. read less

Abstract: Radiochromic films change color upon exposures to radiation doses as a result of solid-state polymerization (SSP). Commercially available radiochromic films are primarily designed for, and have become widely used in, clinical X-ray dosimetry. However, many intriguing properties of radiochromic films are not yet fully understood. The present work aimed at developing a theoretical model at both atomic and macroscopic scales to provide a platform for future works to understand these intriguing properties. Despite the fact that radiochromic films were primarily designed for clinical X-ray dosimetry, dose-response curves for the Gafchromic EBT3 film obtained for ultraviolet (UV) radiation were employed to develop our model in order to avoid complications of ionization, non-uniform energy deposition, as well as dispersed doses caused by secondary electrons set in motion by the indirectly ionizing X-ray photons, which might introduce added uncertainties to the model and overshadow the basic SSP processes. The active layer in the EBT3 film consisted of diacetylene (DA) pentacosa-10,12-diynoate monomers, which were modelled using molecular dynamics (MD). The degrees of SSP in the atomic scale upon different UV exposures were obtained to determine the absorption coefficients of the active layer, which were then input into the finite element method (FEM). The classical steady-state Helmholtz equation was engaged to model the reflection from the active layer using the FEM technique. The multifrontal massively parallel sparse direct solver (MUMPS) was employed to solve the present numerical problem. Very good agreement between experimentally and theoretically obtained coloration in terms of net reflective optical density was achieved for different UV exposures. In particular, for UV exposures larger than ~40 J/cm2, the reflected light intensity decreased at a lower rate when compared to other UV exposure values, which was explained by the densely cross-linked structure under near-complete polymerization, and thus the lower efficiency for further bond formation between DA monomer strands. read less

Abstract: The interlayer shear resistance plays an important role in graphene related applications, and different mechanisms have been proposed to enhance its interlayer load capacity. In this work, we performed molecular dynamics (MD) simulations and theoretical analysis to study interlayer shear behaviors of three dimensional graphene-carbon (3D-GC) nanotube networks. The shear mechanical properties of carbon nanotubes (CNTs) crosslink with different diameters are obtained which is one order of magnitude larger than that of other types of crosslinks. Under shear loading, 3D-GC exhibits two failure modes, i.e., fracture of graphene sheet and failure of CNT crosslink, determined by the diameter of CNT crosslink, crosslink density, and length of 3D-GC. A modified tension-shear chain model is proposed to predict the shear mechanical properties and failure mode of 3D-GC, which agrees well with MD simulation results. The results presented in this work may provide useful insights for future development of high-performan... read less

Abstract: A string of fullerenes is used for generating a nanotube by self-assembly of a black phosphorus (BP) nanoribbon at a temperature of 8 K. Among the fullerenes in the string, there are at least two fixed fullerenes placed along the edge of the BP ribbon for keeping its configuration stability during winding. By way of molecular dynamics simulations, it is found that successful generation of a BP nanotube depends on the bending stiffness of the ribbon and the attraction between the fullerenes and the ribbon. When the attraction is strong enough, the two edges (along the zigzag direction) of the BP ribbon will be able to bond covalently to form a nanotube. By the molecular dynamics approach, the maximum width of the BP ribbon capable of forming a nanotube with a perfect length is investigated in three typical models. The maximum width of the BP ribbon becomes larger with the string containing more fullerenes. This finding reveals a way to control the width of the BP ribbon which forms a nanotube. It provides guidance for fabricating a BP nanotube with a specified length, the same as to the width of the ribbon. read less

Abstract: Molecular dynamics (MD) simulations are performed to explore the layering structure and liquid–liquid transition of liquid water confined between two graphene sheets with a varied distance at different pressures. Both the size of nanoslit and pressure could cause the layering and liquid–liquid transition of the confined water. With increase of pressure and the nanoslit's size, the confined water could have a more obvious layering. In addition, the neighboring water molecules firstly form chain structure, then will transform into square structure, and finally become triangle with increase of pressure. These results throw light on layering and liquid–liquid transition of water confined between two graphene sheets. read less

Abstract: By using equilibrium molecular dynamics simulations and lattice dynamic calculations, we investigate the thermal transport properties of graphyne nanotubes (GNTs). The results demonstrate that the GNTs exhibit relatively low thermal conductivity, which is approximately 10–25% of that in a carbon nanotube. Detailed lattice dynamic analyses indicate that the presence of sp carbon atoms can induce local resonance modes in the low frequency region, which results in the remarkable decrease of thermal conductivity. Meanwhile, the high-frequency phonon lifetimes also drop because of the vibrational mismatch between sp and sp2 carbon atoms (enhanced phonon scattering). Our results show the local resonance of phonons is an important mechanism for the reduction of thermal conductivity in low-dimensional nanostructures. read less

Abstract: Using molecular dynamics simulations, we find an in-plane negative Poisson's ratio intrinsically existing in the graphene-based three-dimensional (3D) carbon foams (CFs) when they are compressed uniaxially. Our study shows that the negative Poisson's ratio in the present CFs is attributed to their unique molecular structures and triggered by the buckling of the CF structures. This mechanism makes the negative Poisson's ratio of CFs strongly depend on their cell length, which offers us an efficient means to tune the negative Poisson's ratio in nanomaterials. Moreover, as the buckling modes of CFs are topographically different when they are compressed in different directions, their negative Poisson's ratio is found to be strongly anisotropic, which is in contrast to the isotropic positive Poisson's ratio observed in CFs prior to buckling. The discovery of the intrinsic negative Poisson's ratio in 3D CFs will significantly expand the family of auxetic nanomaterials. Meanwhile, the mechanism of nano-auxetics proposed here may open up a door to manufacture new auxetic materials on the nanoscale. read less

Abstract: Among efforts made to improve thermoelectric efficiency, the use of structurally modified graphene nanomaterials as thermoelectric matter are one of the promising strategies owing to their fascinating physical and electrical properties, and these materials are anticipated to be less thermally conductive than regular graphene structures, as a result of an additional phonon scattering introduced at the modified surfaces. In this study, we explore the thermal conductivity behaviors of strain-induced rippled graphene sheets by varying the ripple amplitude, periodicity, and dimensions of the structure. We introduce a technique which enables creation of a graphene sheet with evenly distributed ripples in molecular dynamics simulation, and the Green-Kubo linear response theory is used to calculate the thermal conductivity of the structures of interest. The results reveal the reduction of thermal conductivity with the greater degree of strain, the smaller system dimension, and the shorter ripple wavelength, which, in turn, could lead to the thermoelectric efficiency enhancement. This work has significance in that it presents the capability of generating repeated and controllable patterns in molecular dynamics, and so, it enables the atomic-level transport study in the regularly patterned two-dimensional surface or in any structures with a specified degree of strain.Among efforts made to improve thermoelectric efficiency, the use of structurally modified graphene nanomaterials as thermoelectric matter are one of the promising strategies owing to their fascinating physical and electrical properties, and these materials are anticipated to be less thermally conductive than regular graphene structures, as a result of an additional phonon scattering introduced at the modified surfaces. In this study, we explore the thermal conductivity behaviors of strain-induced rippled graphene sheets by varying the ripple amplitude, periodicity, and dimensions of the structure. We introduce a technique which enables creation of a graphene sheet with evenly distributed ripples in molecular dynamics simulation, and the Green-Kubo linear response theory is used to calculate the thermal conductivity of the structures of interest. The results reveal the reduction of thermal conductivity with the greater degree of strain, the smaller system dimension, and the shorter ripple wavelength, which... read less

Abstract: Graphene is emerging as a versatile material with a diverse field of applications. Synthesis techniques for graphene introduce several topological defects such as vacancies, dislocations and Stone-Thrower-Wales (STW) defects. Among them STW defects are generated without deleting any atom from the lattice position, but are introduced by rotating single C-C bonds. In this article, molecular dynamics based simulations have been performed to study the effect of STW defects on the fracture toughness of pristine graphene as well as graphene with crack edges passivated with hydrogen atoms. STW defects help in generating out of plane displacement in conjunction with redistribution of stress around the crack edges that can be used to improve the fracture toughness of brittle graphene. An overall improvement in the fracture toughness of pristine graphene as well as graphene containing hydrogen at the crack edges was predicted in this work. read less

Abstract: Three dimensional graphene-carbon nanotube networks (3D-GC) have attracted great interests due to their superior thermal, optical, and hydrogen storage properties. In our work, the in-plane mechanical properties of nanoporous 3D-GC with different diameters of the joint carbon nanotube (CNT) and porosity have been studied. During in-plane tension, the fracture of 3D-GC first initiates at the heptagonal defects of the junctions between graphene sheets and CNTs where large tensile residual stress is observed. The in-plane tensile strength of 3D-GC decreases with the increasing of CNT parameter and porosity, and the tensile modulus is mainly determined by the porosity. Although the fracture strain decreases with the CNT diameter, it increases with the porosity. Compared to the nanoporous graphene, 3D-GC has larger in-plane tensile strength and fracture strain due to the additional support of CNTs. However, the in-plane tensile modulus of 3D-GC is usually smaller than that of the nanoporous graphene due to the... read less

Abstract: Due to lack of the third dimension in 3D bulk materials, the crack tip in graphene locates on several atoms implying that its fracture behavior can be closely associated with its lattice structure, i.e., the bond length and angle. As the bond length reflects the discrete nature of the atomic structure, theoretical discussion is focused on the concomitant size effect at the nanoscale with few or no reports about the influence of the bond angle. Through the comparisons between theoretical calculations and experimental data, here it is first demonstrated that the bond angle is essential for understanding the fracture behavior in graphene, serving as an intrinsic notch reducing the stress singularity near the crack tip (the intrinsic notch effect), leading to the breakdown of the Griffith criterion in graphene. The work provides a framework for the studying of the brittle fracture in 2D materials, which gives rise to the more reliable device design based on 2D materials. More importantly, the significance of the intrinsic notch effect is profound and far-reaching, paving the way to a more comprehensive and deep understanding of the mechanical properties in nano as well as nanostructured materials. read less

Abstract: Graphene is considered as an ideal material candidate for next‐generation electronic devices due to its high carrier mobility while the associated thermal management has become a critical barrier. Designing graphene whose thermal transport properties can be tuned through external fields is highly desired. Here, an auxetic graphene (AG) and a contractile graphene (CG) are created and a conceptual design of thermal controllable graphene heterostructures is demonstrated by tailoring them together. Using computational simulations, it is shown that the thermal conductivity of graphene heterostructures can be regulated by patterning AG and CG unit cells with different interface properties under a uniaxial tensile strain. Analyses of both mechanical deformation and vibrational spectra indicate that the thermal transport properties of graphene heterostructures are highly dependent on their mechanical stress distribution, and also rely on the interfaces that are parallel with the directions of mechanical loadings. Theoretical models that integrate the contributions of mechanical loading and patterned‐interfaces are developed to quantitatively describe the thermal conductivity of graphene heterostructures. Good agreement of thermal conductivity between theoretical predictions and extensive simulations is obtained. These designs and findings are expected to pave a new route to seek interface‐mediated stretchable thermal electronics with mechanically controllable performance. read less

Abstract: Many factors can have a significant influence on the output power of a thermally driven rotary nanomotor made of carbon nanotubes (CNTs). Making use of a computational molecular dynamics approach, we evaluate for the first time the output power of a nanomotor, considering some of the main factors including temperature, the diameter of the rotor and the number of IRD atoms (N) on the stator. When applying extra-resistant torque to the rotor to let the stable value of the rotational frequency of the rotor fluctuate near zero, the value of the resistant torque can be considered as the output power of the rotor. The effects of these factors on the output power of a motor are roughly predicted via a fitting approach. Using stepwise regression analysis, we discover that N has the greatest influence on the output power. The second and the third main factors that affect the output power of a nanomotor are the diameter of the rotor, and the interaction between N and the diameter, respectively. To improve the output power of a nanomotor, one can place more IRD atoms in the system and/or employ CNTs with larger diameters. read less

Abstract: The surface wettability effect on fluid transport in nanoscale slit pores is quantitatively accessed by using non-equilibrium molecular dynamics (NEMD) simulation incorporating with density functional theory (DFT). In particular, the slip lengths of benzene steady flows under various wetting conditions are computed with NEMD simulations and a quasi-general expression is given, while the structural properties are investigated with DFT. By taking into account the inhomogeneity of fluid density inside pore, we find that the conventional flux enhancement rate is associated with both the molecule slipping and geometrical confinement, and it becomes drastically high in solvophobic pores especially when the pore size is of several fluid diameters. In good agreement with experimental results, we further show that the wettability effect competes with pore size effect in determining the flux after pore inner surface modification, and a high flux can be achieved when the deposited layer is solvophobic yet thin. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1704–1714, 2017 read less

Abstract: Nanotube synthesizing from black phosphorus (BP) is still challenging in laboratory. Fabricating a BP nanotube by self-assembling of a BP nanoribbon seems promising. To estimate the feasibility of such fabrication method, this study performs numerical experiments of self-assembling a jammed BP ribbon on a fixed carbon nanotube using molecular dynamics simulation. The study is based on the following two facts: The phosphorus–phosphorus (P–P) bond is weaker than the bond of carbon–carbon (C–C) and the van der Waals interaction among nonbonding phosphorus atoms is stronger than that between phosphorus atoms and carbon atoms. The results show that when a longer BP ribbon is jammed by a shorter BP ribbon the self-assembling result depends on the relative positions of carbon nanotube (CNT) and the two BP ribbons. Only when the shorter BP ribbon is on the outside of the longer ribbon can the longer BP ribbon be wound on the CNT to form an ideal BP nanotube. The finding is helpful for practical applications of BP... read less

Abstract: In this study, we investigated the interwall sliding behaviours of double-wall carbon nanotubes (DWCNTs) using molecular dynamics (MD) simulations, focusing on the effects of different structural defects including the vacancy, adsorbed atom (Adatom) and Stone-Wales (SW) defects. The simulation results showed that structural defects, especially the Adatom ones, caused large fluctuations and decreased the overall pull-out force. Stick-slip motions were observed in the interwall sliding processes of DWCNTs containing multiple structural defects. Among three types of structural defects, the Adatom defects most significantly weaken the interwall load transferring capability and degrade the interface shear strength (IFSS). This work provides useful information for promoting DWCNTs’ applications in Micro/Nano Electro-Mechanical Systems (M/NEMS). read less

Abstract: The properties of graphene can be chemically altered by changing its local binding configurations. In the present work, we investigate fundamentals of chemisorption of atomic hydrogen on graphene and its influence on mechanical properties of as-hydrogenated graphene by means of molecular dynamics simulations. Our simulation results indicate that there are diversiform hydrogen-graphene configurations formed in the chemisorption process. Especially, energetically favorable hydrogen pairs result in less even no atomic distortion of graphene than sp3 hybridization. The hydrogenation-induced deterioration of mechanical properties of graphene shows a strong dependence on its chirality. The evolution of bond structures in uniaxial tension along armchair direction is more sensitive to local failure of graphene than zigzag direction, leading to a more pronounced decrease in both fracture stress and fracture strain. It is indicated that the chemisorption of hydrogen on graphene can be strongly affected by operating temperature primarily due to the temperature dependent graphene morphology. These findings advance our understanding of chemical vapor deposition of graphene synthesis and hydrogenation of graphene. read less

Abstract: Heat transfer across thermal interface material, such as graphene-polymer composite, is a critical issue for microelectronics thermal management. To improve its thermal performance, we use chemical functionalization on the graphene with hydrocarbon chains in this work. Molecular dynamics simulations are used to identify the thermal conductivity of monolayer graphene and graphene-polymer nanocomposites with and without grafted hydrocarbon chain. The influence of functionalization with hydrocarbon chains on the interfacial thermal conductance of graphene-polyethylene nanocomposites was investigated using a non-equilibrium molecular dynamics (NEMD) simulation. We also study the effects of the graft density (number of hydrocarbon chain) on the thermal conductivity of graphene and the nanocomposite. read less

Abstract: An approach utilizing low energy N ion beam irradiation is applied in joining two monolayer graphene flakes. Raman spectrometry and atomic force microscopy show the joining signal under 40 eV and 1 × 1014 cm−2 N ion irradiation. Molecular dynamics simulations demonstrate that the joining phenomenon is attributed to the punch-down effect and the subsequent chemical bond generation between the two sheets. The generated chemical bonds are made up of inserted ions (embedded joining) and knocked-out carbon atoms (saturation joining). The electronic transport properties of the joint are also calculated for its applications. read less

Abstract: In this work, we perform molecular dynamics (MD) simulations to study the effect of rippling on the Poisson's ratio of graphene. Due to the atomic scale thickness of graphene, out-of-plane ripples are generated in free standing graphene with topological defects (e.g. heptagons and pentagons) to release the in-plane deformation energy. Through MD simulations, we have found that the Poisson's ratio of rippled graphene decreases upon increasing its aspect ratio η (amplitude over wavelength). For the rippled graphene sheet η = 0.188, a negative Poisson's ratio of -0.38 is observed for a tensile strain up to 8%, while the Poisson's ratio for η = 0.066 is almost zero. During uniaxial tension, the ripples gradually become flat, thus the Poisson's ratio of rippled graphene is determined by the competing factors of the intrinsic positive Poisson's ratio of graphene and the negative Poisson's ratio due to the de-wrinkling effect. Besides, the rippled graphene exhibits excellent fracture strength and toughness. With the combination of its auxetic and excellent mechanical properties, rippled graphene may possess potential for application in nano-devices and nanomaterials. read less

Abstract: In recent years, carbon-nanotube (CNT)-based gigahertz oscillators have been widely used in numerous areas of practical engineering such as high-speed digital, analog circuits, and memory cells. One of the major challenges to practical applications of the gigahertz oscillator is generating a stable oscillation process from the gigahertz oscillators and then maintaining the stable process for a specified period of time. To address this challenge, an oscillator from a triple-walled CNT-based rotary system is proposed and analyzed numerically in this paper, using a molecular dynamics approach. In this system, the outer tube is fixed partly as a stator. The middle tube, with a constant rotation, is named Rotor 2 and runs in the stator. The inner tube acts as Rotor 1, which can rotate freely in Rotor 2. Due to the friction between the two rotors when they have relative motion, the rotational frequency of Rotor 1 increases continuously and tends to converge with that of Rotor 2. During rotation, the oscillation of Rotor 1 may be excited owing to both a strong end barrier at Rotor 2 and thermal vibration of atoms in the tubes. From the discussion on the effects of length of Rotor 1, temperature, and input rotational frequency of Rotor 2 on the dynamic response of Rotor 1, an effective way to control the oscillation of Rotor 1 is found. Being much longer than Rotor 2, Rotor 1 will have perfect oscillation, i.e., with both stable (or nearly constant) period and amplitude—especially at relatively low temperature. This discovery can be taken as a useful guidance for the design of an oscillator from CNTs. read less

Abstract: Graphite surfaces can be manipulated by several methods to create graphene structures of different shapes and sizes. Scanning tunneling microscopy (STM) can be used to create these structures either through mechanical contact between the tip and the surface or through electro-exfoliation. In the latter, the mechanisms involved in the process of exfoliation at an applied voltage are not fully understood. Here, we show how a graphite surface can be locally exfoliated in a systematic manner by applying an electrostatic force with a STM tip at the edge of a terrace, forming triangular flakes several nanometers in length. We demonstrate, through experiments and simulations, how these flakes are created by a two-step process: first a voltage ramp must be applied at the edge of the terrace, and then the tip must be scanned perpendicular to the edge. Ab initio electrostatic calculations reveal that the presence of charges on the graphite surface weakens the interaction between layers allowing for exfoliation at voltages in the same range as those used experimentally. Molecular dynamics simulations show that a force applied locally on the edge of a step produces triangular flakes such as those observed under STM. Our results provide new insights into surface modification that can be extended to other layered materials. read less

Abstract: A single polyethylene chain was reported to have a high metal-like thermal conductivity (TC), which stands in sharp contrast to the thermally insulating feature of common bulk polyethylene materials. This work numerically investigates the impact of torsion and stretching on the TC of polyethylene strands by using equilibrium molecular dynamics simulations. The simulation results show that torsion slightly reduces the TC of a single polyethylene chain. In contrast, the heat conduction of polyethylene strands could be slightly enhanced under torsional loading with a specific torsional angle. Particularly, an apparent improvement of TC of polyethylene strands is achieved by combining torsion and stretching functions. It is found that the TC of torsional polyethylene strands is sensitive to torsional patterns. Our study proposes a specific torsional pattern of polyethylene strands that significantly enhances the heat conduction of the original counterpart. This study will play an essential role in guiding the... read less

Abstract: Molecular dynamics simulations are performed to investigate the laser-induced graphitization of amorphous carbon (a-C) films at different densities for heat-assisted magnetic recording. Physical insights into the graphitization caused by laser heating are provided by analyzing the virial pressure, bond angle distribution, radial distribution function, pair distribution function, and atomic strains during the laser heating and cooling process. The effects of density and laser irradiation times on the graphitization of a-C films are investigated. The laser-induced ultrafast nonthermal phase transformation occurs in the heating process. The sp3–sp2 transformation is mainly contributed by the atoms in sp3–sp3 bonds both in the heating and cooling process. Less energy is required for those atoms to overcome the energy barrier. The atomic shear strains and volume strains can accelerate the sp3–sp2 transformation. For all a-C films at different densities, the rate of sp2-hybridized atoms increases rapidly in the first laser irradiation process. read less

Abstract: Control of both the regularity of a material ensemble and nanoscale architecture provides unique opportunities to develop novel thermoelectric applications based on 2D materials. As an example, the authors explore the electronic and thermal properties of functionalized graphene nanoribbons (GNRs) in the single‐sheet and helical architectures using multiscale simulations. The results suggest that appropriate functionalization enables precise tuning of the doping density in a planar donor/acceptor GNR ensemble without the need to introduce an explicit dopant, which is critical to the optimization of power factor. In addition, the self‐interaction between turns of a GNR may induce long‐range disorder along the helical axis, which suppresses the thermal contribution from phonons with long wavelengths, leading to anomalous length independent phonon thermal transport in the quasi‐1D system. read less

Abstract: We present the first data on emission of C60- stimulated by single impacts of 50 keV C602+ on the self-assembled molecular layer of C60 deposited on free standing 2 layer graphene. The yield, Y, of C60- emitted in the transmission direction is 1.7%. To characterize the ejection and ionization of molecules, we have measured the emission of C60- from the surface of bulk C60 (Y = 3.7%) and from a single layer of C60 deposited on bulk pyrolytic graphite (Y = 3.3%). To gain insight into the mechanism(s) of ejection, molecular dynamic simulations were performed. The scenario of the energy deposition and ejection of molecules is different for the case of graphene due to the confined volume of projectile-analyte interaction. In the case of 50 keV C602+ impacts on graphene plus C60, the C atoms of the projectile collide with those of the target. The knocked-on atoms take on a part of the kinetic energy of the projectile atoms. Another part of the kinetic energy is deposited into the rim around the impact site. The ejection of molecules from the rim is a result of collective movement of the molecules and graphene membrane, where the membrane movement provides the impulse for ejection. The efficient emission of the intact molecular ions implies an effective ionization probability of intact C60. The proposed mechanism of ionization involves the tunneling of electrons from the vibrationally exited area around the hole to the ejecta. read less

Abstract: Ballistic impact induces multiaxial loading on Kevlar® and polyethylene fibers used in protective armor systems. The influence of multiaxial loading on fiber failure is not well understood. Experiments show reduction in the tensile strength of these fibers after axial and transverse compression. In this paper, we use molecular dynamics (MD) simulations to explain and develop a fundamental understanding of this experimental observation since the property reduction mechanism evolves from the atomistic level. An all-atom MD method is used where bonded and non-bonded atomic interactions are described through a state-of-the-art reactive force field. Monotonic tension simulations in three principal directions of the models are conducted to determine the anisotropic elastic and strength properties. Then the models are subjected to multi-axial loads—axial compression, followed by axial tension and transverse compression, followed by axial tension. MD simulation results indicate that pre-compression distorts the crystal structure, inducing preloading of the covalent bonds and resulting in lower tensile properties. read less

Abstract: In this paper, we develop a new modified embedded atom method (MEAM) potential that includes the bond order (MEAM-BO) to describe the energetics of unsaturated hydrocarbons (double and triple carbon bonds) and also develop improved parameters for saturated hydrocarbons from those of our previous work. Such quantities like bond lengths, bond angles, and atomization energies at 0 K, dimer molecule interactions, rotational barriers, and the pressure-volume-temperature relationships of dense systems of small molecules give a comparable or more accurate property relative to experimental and first-principles data than the classical reactive force fields REBO and ReaxFF. Our extension of the MEAM potential for unsaturated hydrocarbons (MEAM-BO) is a step toward developing more reliable and accurate polymer simulations with their associated structure-property relationships, such as reactive multicomponent (organic/metal) systems, polymer-metal interfaces, and nanocomposites. When the constants for the BO are zero, MEAM-BO reduces to the original MEAM potential. As such, this MEAM-BO potential describing the interaction of organic materials with metals within the same MEAM formalism is a significant advancement for computational materials science. read less

Abstract: The epitaxially grown alkane layers on graphene are prepared by a simple drop-casting method and greatly reduce the environmentally driven doping and charge impurities in graphene. Multiscale simulation studies show that this enhancement of charge homogeneity in graphene originates from the lifting of graphene from the SiO2 surface toward the well-ordered and rigid alkane self-assembled layers. read less

Abstract: Localization of atomic defect-induced electronic transport through a single graphene layer is calculated using a full-valence electronic structure description as a function of the defect density and taking into account the atomic-scale deformations of the layer. The elementary electronic destructive interferences leading to Anderson localization are analyzed. The low-voltage current intensity decreases with increasing length and defect density, with a calculated localization length ζ = 3.5 nm for a defect density of 5%. The difference from the experimental defect density of 0.5% required for an oxide surface-supported graphene to obtain the same ζ is discussed, pointing out how interactions of the graphene supporting surface and surface chemical modifications also control electronic transport localization. read less

Abstract: We studied the behavior of H2 and CH4 flowing through various pore geometries of nanoporous graphene using molecular dynamics method. Ten different geometries of pore-18, with different eccentricities, were prepared. It was found that the gas permeance and adsorption layer were heavily influenced by the eccentricity of the pores. On further investigation, it was also found that the jaggedness of the pore geometry played a role as well. It was also noted that at specific eccentricities, pore-18 exhibited hydrogen selective behavior, which was found to extend to pore-12, -14, -16, -20, -24, and -30 as well. Furthermore, it was shown that the H2 permeance of these pores can reach 9 times the value of that of pore-10 (which was previously found to be the only selective pore). Hence, these pores show H2 selectivity with high H2 yields. Thus, this study demonstrates the exciting possibility of creating highly efficient H2 separators by pore geometry variation. Recent experimental studies, which involve an atom-... read less

Abstract: Pillared 3D carbon architectures, with the graphene layers and carbon nanotubes connected by topological junctions, have been produced and observed, as reported recently. However, the atomistic details of such junctions are hard to discern in microscopy and remain presently unclear. The simplest junction contains six heptagons in the transition region between the nanotube and graphene. Although these junctions make the pillared architectures possible, they are susceptible to failure when the whole structure undergoes mechanical or thermal stress. In this work we consider “nanochimneys”, a variety of special junctions with cones in between the nanotube and graphene parts. We explore the structures of the nanochimneys (NCs) and determine their underlying topological requirements. We also study the thermal conductance of these pillared architectures and show that NCs conduct heat better than regular simple junctions. read less

Abstract: The mechanics of a lightweight three-dimensional graphene assembly, quantified by computational simulation and 3D printing, are discussed. Recent advances in three-dimensional (3D) graphene assembly have shown how we can make solid porous materials that are lighter than air. It is plausible that these solid materials can be mechanically strong enough for applications under extreme conditions, such as being a substitute for helium in filling up an unpowered flight balloon. However, knowledge of the elastic modulus and strength of the porous graphene assembly as functions of its structure has not been available, preventing evaluation of its feasibility. We combine bottom-up computational modeling with experiments based on 3D-printed models to investigate the mechanics of porous 3D graphene materials, resulting in new designs of carbon materials. Our study reveals that although the 3D graphene assembly has an exceptionally high strength at relatively high density (given the fact that it has a density of 4.6% that of mild steel and is 10 times as strong as mild steel), its mechanical properties decrease with density much faster than those of polymer foams. Our results provide critical densities below which the 3D graphene assembly starts to lose its mechanical advantage over most polymeric cellular materials. read less

Abstract: Characteristics of nanocrystalline materials are known substantially dependent on the microstructure such as grain size, crystal orientation, and grain boundary. Thus it is desired to have systematic characterization methods on the various nanomaterials with complex geometries, especially in low dimensional nature. One of the interested nanomaterials would be a pure two-dimensional material, graphene, with superior mechanical, thermal, and electrical properties. In this study, mechanical properties of “polycrystalline” graphene were numerically investigated by molecular dynamics simulations. Subdomains with various sizes would be generated in the polycrystalline graphene during the fabrication such as chemical vapor deposition process. The atomic models of polycrystalline graphene were generated using Voronoi tessellation method. Stress strain curves for tensile deformation were obtained for various grain sizes (5~40 nm) and their mechanical properties were determined. It was found that, as the grain size increases, Young`s modulus increases showing the reverse Hall-Petch effect. However, the fracture strain decreases in the same region, while the ultimate tensile strength (UTS) rather shows slight increasing behavior. We found that the polycrystalline graphene shows the reverse Hall-Petch effect over the simulated domain of grain size (< 40 nm). read less

Abstract: One of the intriguing characteristics of honeycomb lattices is the appearance of a pseudomagnetic field as a result of mechanical deformation. In the case of graphene, the Landau quantization resulting from this pseudomagnetic field has been measured using scanning tunneling microscopy. Here we show that a signature of the pseudomagnetic field is a local sublattice symmetry breaking observable as a redistribution of the local density of states. This can be interpreted as a polarization of graphene's pseudospin due to a strain induced pseudomagnetic field, in analogy to the alignment of a real spin in a magnetic field. We reveal this sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunneling microscope. The tip locally lifts the graphene membrane from a SiO2 support, as visible by an increased slope of the I(z) curves. The amount of lifting is consistent with molecular dynamics calculations, which reveal a deformed graphene area under the tip in the shape of a Gaussian. The pseudomagnetic field induced by the deformation becomes visible as a sublattice symmetry breaking which scales with the lifting height of the strained deformation and therefore with the pseudomagnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models, revealing fields of 1000 T. These results might be the starting point for an effective THz valley filter, as a basic element of valleytronics. read less

Abstract: Vertical integration of 2D materials has recently appeared as an effective method for the design of novel nano-scale devices. Using non-equilibrium molecular dynamics simulations, we study the interfacial thermal transport property of graphene/phosphorene heterostructures where phosphorene is sandwiched in between graphene. Various modulation techniques are thoroughly explored. We found that the interfacial thermal conductance at the interface of graphene and phosphorene can be enhanced significantly by using vacancy defects, hydrogenation and cross-plane compressive strain. By contrast, the reduction in the interfacial thermal conductance can be achieved by using cross-plane tensile strain. Our results provide important guidelines for manipulating the thermal transport in graphene/phosphorene based-nano-devices. read less

Abstract: The fracture toughness of single-crystal graphene and bi-crystal graphene with different misorientation angles is investigated by molecular dynamics simulation. We find that the fracture toughness fluctuates when a crack propagates across the grain boundary. It indicates that the grain boundary affects the fracture toughness during the fracture process. The affected region on the graphene is limited to a small zone around the grain boundary. However, for the complete tearing-failure case, fracture toughness of bi-crystal graphene is approximate to that of single-crystal graphene, which implies that the fracture toughness is not very sensitive to the grain boundary. For comparison, the tensile fracture simulations of the single-crystal graphene and bi-crystal graphene are carried out. The results show that the grain boundaries block the crack propagation and affect fracture toughness significantly in bi-crystal graphene under tensile force. Furthermore, we analyze the fracture of a single C–C bond at the crack tip of single-crystal graphene under tearing load from the atomic view. We find that the fracture toughness of the single C–C bond occupies about half of the fracture toughness for the complete failure of the total single-crystal graphene, and the other half energy distributes in the rest of the graphene. read less

Abstract: D-loops, a new type of structural defect in carbon fibers, are presented, which have highly detrimental effect on their mechanical properties and can define a new fundamental upper limit to their strength. These defects form exclusively during polyacrylonitrile carbonization, act as stress concentrators in the graphitic basal plane, and cannot be removed by local annealing. read less

Abstract: By employing a series of MD simulations, buckling behaviour of penta-graphene and functionalised penta-graphene having different hydrogen (H) coverage is presented in this study. To this end, the buckling onset strain is determined for different systems. The results reveal that the new allotrope is slightly stiffer than graphene. Moreover, the effect of H adatoms in the range 0–56% on buckling behaviour is investigated. Finally it is shown that the H coverage has an influence on stability, and ripple-type destortion of the penta-graphene under compression. read less

Abstract: We report a systematic analysis on the effects of hydrogenation on the mechanical behavior of irradiated single-layer graphene sheets, including irradiation-induced amorphous graphene, based on molecular-dynamics simulations of uniaxial tensile straining tests and using an experimentally validated model of electron-irradiated graphene. We find that hydrogenation has a significant effect on the tensile strength of the irradiated sheets only if it changes the hybridization of the hydrogenated carbon atoms to sp3, causing a reduction in the strength of irradiation-induced amorphous graphene by ∼10 GPa. Hydrogenation also causes a substantial decrease in the failure strain of the defective sheets, regardless of the hybridization of the hydrogenated carbon atoms, and in their fracture toughness, which decreases with increasing hydrogenation for a given irradiation dose. We characterize in detail the fracture mechanisms of the hydrogenated irradiated graphene sheets and elucidate the role of hydrogen and the ex... read less

Abstract: The technique of laminating one or several layers of graphene on a substrate and making a bridge of small dimensions and then measuring the changes in the electrical properties of the circuit obtained from this connection has raised the hopes of miniaturizing electronic devices even further. Due to the importance of this subject and the need to mechanically design such systems before signal-taking, in this paper, the effects of geometry, temperature and mechanical deformations on the mechanical properties of graphene nanodiodes and nanotransistors have been studied by employing the molecular dynamics method. A tensile test has been used as a suitable means of measuring the mechanical properties of suspended graphene sheets and graphene nanodiodes, and a virtual indentation test has been employed for zigzag and armchair transistors. After validating the method used by comparing it to former works, the most important finding for the diode case reveals that by increasing the rectification angle from zero to 90°, the modulus of elasticity drops from 1.092 to 0.79 TPa for certain dimensions; which is a reduction of 27.65%. In the graphene transistor, the modulus of elasticity increases with the increase of transistor width. Nevertheless, in stark contrast to armchair transistors, the difference between the mechanical properties of zigzag transistors and suspended graphene becomes greater as the transistor length increases. In general, the square transistor enjoys the highest modulus of elasticity and, thus, the best stiffness; while such square graphene sheets are ruptured at lower strains and stresses, compared to other graphene transistor dimensions. read less

Abstract: Suspended graphene is difficult to image by scanning probe microscopy due to the inherent van-der-Waals and dielectric forces exerted by the tip which are not counteracted by a substrate. Here, we report scanning tunneling microscopy data of suspended monolayer graphene in constant-current mode revealing a surprising honeycomb structure with amplitude of 50$-$200 pm and lattice constant of 10-40 nm. The apparent lattice constant is reduced by increasing the tunneling current $I$, but does not depend systematically on tunneling voltage $V$ or scan speed $v_{\rm scan}$. The honeycomb lattice of the rippling is aligned with the atomic structure observed on supported areas, while no atomic corrugation is found on suspended areas down to the resolution of about $3-4$ pm. We rule out that the honeycomb structure is induced by the feedback loop using a changing $v_{\rm scan}$, that it is a simple enlargement effect of the atomic resolution as well as models predicting frozen phonons or standing phonon waves induced by the tunneling current. Albeit we currently do not have a convincing explanation for the observed effect, we expect that our intriguing results will inspire further research related to suspended graphene. read less

Abstract: Molecular dynamics simulations are used to investigate lithium (Li) transport rates in symmetric tilt graphite grain boundaries (GBs). Experiments have quantified highly varied diffusion rates of Li in graphite and recent computational work exposed similar differences in Li intercalation rates into GBs from a free surface. This work extends findings of intercalation studies to provide explanations for various rates and uses said differences to predict bulk GB transport behavior. Various structural properties are presented for graphite GBs, including bond angles, bond order, and free volume analysis which is rarely presented in covalent systems. The importance of free volume connectivity is also discussed as is Li structure in GBs. read less

Abstract: Recent research progress shows that graphene exhibits distinct adhesion and friction behaviors. In the paper, the static and dynamic analyses of a diamond tip sliding on suspended graphene surface are conducted via theoretical and numerical research methods, and the adhesion and friction properties between them are investigated. The analytical expression of interaction potential between a diamond tip and graphene surface is derived based on the interatomic pairwise potential, and then, the lateral and normal interaction forces are calculated. The equilibrium heights and adhesion energy of the diamond tip are calculated on three particular sites of graphene surface. The influence of vertical distance between the tip and graphene surface is studied on the maximum static frictional force and initial velocity of tip. What is more, the influence of scanning velocity and damping are also analyzed on the frictional force and dynamic behaviors of the scanning tip, and the “stick-slip” phenomenon is observed and d... read less

Abstract: In this work, we use MD simulation to model the graphitization process of a-C film by laser heating in heat-assisted magnetic recording. The amorphous carbon is heated to 800K and cooled down to room temperature within nanosecond. During the laser heating and cooling process, the rate of the sp2-hybridized atoms rate keeps increasing from 16% to 68%. The trend of sp2 hybridized atoms rate is analogous with the pressure of the system. The pressure is an essential ingredient in the graphitization of amorphous carbon. read less

Abstract: Recently, the domains of low-dimensional (low-D) materials and disordered materials have been brought together by the demonstration of several new low-D, disordered systems. The thermal transport properties of these systems are not well-understood. Using amorphous graphene and glassy diamond nanothreads as prototype systems, we establish how structural disorder affects vibrational energy transport in low-dimensional, but disordered, materials. Modal localization analysis, molecular dynamics simulations, and a generalized model together demonstrate that the thermal transport properties of these materials exhibit both similarities and differences from disordered 3D materials. In analogy with 3D, the low-D disordered systems exhibit both propagating and diffusive vibrational modes. In contrast to 3D, however, the diffuson contribution to thermal transport in these low-D systems is shown to be negligible, which may be a result of inherent differences in the nature of random walks in lower dimensions. Despite the lack of diffusons, the suppression of thermal conductivity due to disorder in low-D systems is shown to be mild or comparable to 3D. The mild suppression originates from the presence of low-frequency vibrational modes that maintain a well-defined polarization and help preserve the thermal conductivity in the presence of disorder. read less

Abstract: In this work we have assessed the ability of a recently proposed three-dimensional integral equation approach to describe the explicit spatial distribution of molecular hydrogen confined in a crystal formed by short-capped nanotubes of C50 H10. To that aim we have resorted to extensive molecular simulation calculations whose results have been compared with our three-dimensional integral equation approximation. We have first tested the ability of a single C50 H10 nanocage for the encapsulation of H2 by means of molecular dynamics simulations, in particular using targeted molecular dynamics to estimate the binding Gibbs energy of a host hydrogen molecule inside the nanocage. Then, we have investigated the adsorption isotherm of the nanocage crystal using grand canonical Monte Carlo simulations in order to evaluate the maximum load of molecular hydrogen. For a packing close to the maximum load explicit hydrogen density maps and density profiles have been determined using molecular dynamics simulations and the three-dimensional Ornstein–Zernike equation with a hypernetted chain closure. In these conditions of extremely tight confinement the theoretical approach has shown to be able to reproduce the three-dimensional structure of the adsorbed fluid with accuracy down to the finest details. read less

Abstract: Nanoindentation has been widely used to determine the mechanical properties of pristine and polycrystalline graphene in experiments. To investigate the effects of grain size on the mechanical properties of polycrystalline graphene, we use molecular dynamics simulations to study the nanoindentation response of suspended polycrystalline graphene layer. The force-displacement behavior from the simulations enables the interpretation of Young’s modulus and breaking strength of the 2D material, which are exhibited to strongly depend on the grain size. Introduction Pristine graphene with perfectly hexagonal carbon rings exhibits extraordinary mechanical properties. However, large-scale monolayers of graphene grown by chemical vapor deposition (CVD) usually contain topological defects and grain boundaries, which might strongly influence the apparent properties of the 2D material. There have been increasing experimental and simulation studies on the stiffness and strength of polycrystalline graphene. For example, atomic force microscopy (AFM) has been employed to measure the mechanical properties of monolayer-graphene in a nanoindentation setup, leading to the interpretation of 2D elastic modulus and breaking strength, respectively [1,2]. Both pristine and polycrystalline graphene (with varying grain sizes) have been measured by nanoindentation in AFM, and it was shown that the stiffness and failure load of poly-graphene are comparable with those of pristine graphene, despite the existence of grain boundaries [2]. In contrast, some other studies showed that grain boundaries severely reduce the elastic modulus and fracture strength of graphene [3,4]. For nanoindentation technique, the experimental measuremens and simulation predictions on the modulus and strength of polycrystalline graphene were found to be scattered widely [5-8], which were also different from those by uniaxial tension [9,10]. This work aims to explore the role of grain size in influencing the mechanical properities of polycrystalline graphene. Molecular dynamics (MD) simulations are employed to perform the nanoindentation test in mimicking the experimental measurements in AFM. Indentation of Polycrystalline Graphene: Atomistic Simulations In experiments, polycrystalline graphene is usually generated by chemical vapor deposition, in which the growth of graphene monolayers starts from multiple nucleation sites simultaneously. As such, the orientations of individual grains rely on the local nucleation sites and are randomly distributed [11]. To account for such growth, we perform computational procedure to generate a certain number N of randomly placed seeds within a square region, and use Voronoi tessellation method to divide the area into N polygons. Each polygon is filled by a pristine graphene fragment with a random orientation angle θi, which is represented by the angle between the armchair direction and the horizontal direction. Owing to the hexagonal honeycomb structure of graphene, the angle θi falls into the range between –π/6 and π/6. The atoms near grain boundary junctions that are closer than 1.4 Å to any 2nd Annual International Conference on Advanced Material Engineering (AME 2016) © 2016. The authors Published by Atlantis Press 215 neighboring grains are removed. This treatment is based on the observation that the system with grain boundary atoms closer than this value is unstable. In this way, the coordinates of all carbon atoms in polycrystalline graphene are generated within a simulation cell (selected as 20 nm by 20 nm). Fig. 1(a) shows the topological structure of a polycrystalline graphene sample with the mean grain size of 2 nm, which contains 127 grains in total. A circular region with radius R is selected from the sample for nanoindentation simulations, in which the atoms along the periphery are fixed as boundary condition (Fig. 1(b)). We confirm that the distribution of misorientation angle from this sample preparation resembles a random distribution, meaning that there is no preferred orientation for the grains (Fig. 1(c)). The distribution of grain size is Gaussian with the average value of 2 nm, as shown in Fig. 1(d). The polycrystalline graphene samples with varying grain sizes can be readily produced by changing the number N of nucleation sites within the given area. Fig. 1 (a) The topological structure of polycrystalline graphene with the average grain size of 2 nm. The simulation cell (20 nm by 20 nm) contains 127 grains that are randomly oriented. Colors in the figure are used to distinguish neighboring grains. (b) Nanoindentation simulation of a suspended polycrystalline graphene layer. The atoms along the periphery (in black) are fixed during the simulation, and the color bar represents the energy of the atoms. The distributions of (c) the misorientation angle and (d) the grain size of the sample in (a). Next, we use molecular dynamics simulations to study the nanoindentation behavior of such polycrystalline graphene. A circular membrane of polycrystalline graphene (Radius: R = 10 nm) is perpendicularly indented by a hemispherical diamond tip (Radius: r = 1 nm) at the membrane center, as shown in Fig. 1(b). Our nanoindentation simulations were performed at constant atom number (N), volume (V) and temperature (T) (i.e., NVT ensemble) by using the large-scale atomic/molecular massively parallel simulator (LAMMPS). The system temperature was maintained at 1 K by employing Nos’e-Hoover thermostat, and the initial velocities of atoms in each direction were extracted from Gaussian distribution for the given temperature. The adaptive intermolecular reactive empirical bond order (AIREBO) potential [12] was used for carbon-carbon interaction in the simulations, with a switch function parameter 2.0 Å for the breakage of C-C bonds [13]. For the atomic-scale interaction between the atoms from the indenter tip and graphene layer, respectively, Lennard-Jones potential was used to describe the van der Walls type interaction, namely, read less

Abstract: The mechanical response of single and multiple graphene sheets under uniaxial compressive loads was studied with molecular dynamics (MD) simulations, using different semi-empirical force fields at different boundary conditions or constrains. Compressive stress–strain curves were obtained and the critical stress/strain values were derived. The MD results are compared to the linear elasticity continuum theory for loaded slabs. Concerning the length dependence of critical values, qualitatively similar behavior is observed between the theory and numerical simulations for single layer graphenes, as the critical stress/strain for buckling was found to scale to the inverse squared length. However discrepancies were noted for multilayer graphenes, where the critical buckling stress also decreased with increasing length, though at a slower rate than expected from elastic buckling analysis. read less

Abstract: Graphynes are the allotrope of graphene. In this work, extensive molecular dynamics simulations are performed on four different graphynes ( α-, β-, γ-, and 6,6,12-graphynes) to explore their mechanical properties (shear modulus, shear strength, and bending rigidity) under shearing and bending. While the shearing properties are anisotropic, the bending rigidity is almost independent of the chirality of graphynes. We also find that the shear modulus and shear fracture strength of graphynes decrease with increasing temperature. The effect of the percentage of the acetylenic linkages on the shear mechanical properties and bending rigidity is investigated. It is shown that the fracture shear strengths and bending rigidities of the four types of graphynes decrease, while the fracture shear strain increases, with increasing percentages of the acetylenic linkages. Significant wrinkling is observed in graphyne under shear strain. The influence of the temperatures and percentages of the acetylenic linkages on the r... read less

Abstract: As the simplest two-dimensional (2D) polymer, graphene has immensely high intrinsic strength and elastic stiffness but has limited toughness due to brittle fracture. We use atomistic simulations to explore a new class of graphene/polyethylene hybrid 2D polymer, "graphylene", that exhibits ductile fracture mechanisms and has a higher fracture toughness and flaw tolerance than graphene. A specific configuration of this 2D polymer hybrid, denoted "GrE-2" for the two-carbon-long ethylene chains connecting benzene rings in the inherent framework, is prioritized for study. MD simulations of crack propagation show that the energy release rate to propagate a crack in GrE-2 is twice that of graphene. We also demonstrate that GrE-2 exhibits delocalized failure and other energy-dissipating fracture mechanisms such as crack branching and bridging. These results demonstrate that 2D polymers can be uniquely tailored to achieve a balance of fracture toughness with mechanical stiffness and strength. read less

Abstract: The grain boundary (GB) energy is a quantity of fundamental importance for understanding several key properties of graphene. Here we present a comprehensive theoretical and numerical study of the entire space of symmetric and asymmetric graphene GBs. We have simulated over 79 000 graphene GBs to explore the configuration space of GBs in graphene. We use a generalized Read–Shockley theory and the Frank–Bilby relation to develop analytical expressions for the GB energy as a function of the misorientation angle and the line angle, and elucidate the salient structural features of the low energy GB configurations. read less

Abstract: Enriching near-zigzag single-walled carbons, which have a small tube-catalyst interface, by a “tandem plate” CVD method. Chemical vapor deposition (CVD) growth is regarded as the most promising method for realizing structure-specific single-walled carbon nanotube (SWNT) growth. In the past 20 years, many efforts dedicated to chirality-selective SWNT growth using various strategies have been reported. However, normal CVD growth under constant conditions could not fully optimize the chirality because the randomly formed cap structure allows the nucleation of all types of SWNTs and the chirality of an SWNT is unlikely to be changed during the following elongation process. We report a new CVD process that allows temperature to be periodically changed to vary SWNT chirality multiple times during elongation to build up the energetically preferred SWNT-catalyst interface. With this strategy, SWNTs with small helix angles (less than 10°), which are predicted to have lower interfacial formation energy than others, are enriched up to ~72%. Kinetic analysis of the process suggests a multiple redistribution feature whereby a large chiral angle SWNT tends to reach the near-zigzag chirality step by step with a small chiral angle change at each step, and hence, we named this method “tandem plate CVD.” This method opens a door to synthesizing chirality-selective SWNTs by rational catalyst design. read less

Abstract: To describe and analyse mechanical structures due to external loads, equation of motion must be supplemented with appropriate boundary conditions. When nanomechanical structures (especially layered systems) are considered, satisfying the boundaries is more challenging. It may be essential to develop a specific methodology for any system in such scale and configuration. This work introduces a general approach for boundary conditions of monolayer graphene sheets as the most important part of future sensors and resonators in nanoelectromechanical systems. Comparing with the experiments, it has been demonstrated that the graphene sheets on the adhesive surfaces should be assumed as a hinged edge condition and not as a clamped one. As a result, inaccuracy of the commonly used rigid base as a clamped condition is proven due to comparison with a reported experiment. It is suggested that a flexible substrate can be replaced to have better accordance. A flexible silicon base with equilibrium distance equal to s = 2.5 A and depth of potential well equal to e = 0.02 eV is obtained as the best substrate for a square graphene sheet under doubly clamped edge conditions. read less

Abstract: A new kind of three-dimensional carbon allotrope, termed carbon honeycomb (CHC), has recently been synthesized [PRL 116, 055501 (2016)]. Based on the experimental results, a family of graphene networks has been constructed, and their electronic and phonon properties are studied by various theoretical approaches. All networks are porous metals with two types of electron transport channels along the honeycomb axis and they are isolated from each other: one type of channel originates from the orbital interactions of the carbon zigzag chains and is topologically protected, while the other type of channel is from the straight lines of the carbon atoms that link the zigzag chains and is topologically trivial. The velocity of the electrons can reach ∼10(6) m s(-1). Phonon transport in these allotropes is strongly anisotropic, and the thermal conductivities can be very low when compared with graphite by at least a factor of 15. Our calculations further indicate that these porous carbon networks possess high storage capacity for gaseous atoms and molecules in agreement with the experiments. read less

Abstract: Molecular dynamics simulations have been performed to investigate the coalescence of two spreading droplets on different nano carbon substrates. Simulation results show that the drop formation behavior is influenced by the surface structures of carbon substrates and geometrical shapes of drops. In the case of pillared graphene substrates, the unexpected detachment of Au drops is observed due to the dewettability. Moreover, the increase of minimum height of the bridge connecting drops exhibits power-law behavior, revealing that the bridge evolves with self-similar dynamics, which is further illustrated by the scaling profiles. Our results provide an available method to tune the coalescence of metallic liquid drops with alteration of surfaces or initial shapes, which has important implications in the industrial application such as ink-jet printing and metallurgy. read less

Abstract: A one atom-thick sheet of carbon exhibits outstanding elastic moduli and tensile strength in its pristine form but structural defects which are inevitable in graphene due to its production techniques can alter its structural properties. These defects in graphene are introduced either during the production process or deliberately by us to tailor its properties. This article discusses the performance enhancement of graphene by introducing pentagon–heptagon–heptagon–pentagon (5–7–7–5) defects. The effect of geometrical parameters such as the nearest neighbour distance and angular orientation between 5–7–7–5 defects on the mechanical properties and failure morphology of graphene was investigated in the frame of molecular dynamics. The mechanical properties and failure morphology of graphene was predicted to be the function of geometrical parameters between 5–7–7–5 defects. It has been predicted from the current study that the brittle behaviour of graphene can be modified to ductile with well controlled distribution of 5–7–7–5 defects. Also it has been predicted that the mechanical properties of graphene can be altered by proper distribution of 5–7–7–5 defects. read less

Abstract: A controllable nanoscale rotating actuator system consisting of a double carbon nanotube and graphene driven by a temperature gradient is proposed, and its rotating dynamics performance and driving mechanism are investigated through molecular dynamics simulations. The outer tube exhibits stable pure rotation with certain orientation under temperature gradient and the steady rotational speed rises as the temperature gradient increases. It reveals that the driving torque is caused by the difference of atomic van der Waals potentials due to the temperature gradient and geometrical features of carbon nanotube. A theoretical model for driving torque is established based on lattice dynamics theory and its predicted results agree well with molecular dynamics simulations. Further discussion is taken according to the theoretical model. The work in this study would be a guide for design and application of controllable nanoscale rotating devices based on carbon nanotubes and graphene. read less

Abstract: In recent years carbon nanotubes and other carbon nanostructures such as graphene sheets have attracted a lot of attention due to their unique mechanical, thermal and electrical properties. These structures can be used in desalination of sea water, removal of hazardous substances from water tanks, gases separation, and so on. The nanoporous single layer graphene membranes are very efficient for desalinating water due to their very low thickness. In this method, water-flow thorough the membrane and salt rejection strongly depend on the applied pressure and size of nanopores that are created in graphene membrane. In this study, the mechanism of passing water and salt ions through nanoporous single-layer graphene membrane are simulated using classical molecular dynamics. We examined the effects of applied pressure and size of nanopores on desalination performance of NPG membrane. Unlike previous researches, we considered the flexibility of the membrane. The results show that by increasing the applied pressure and diameter of the nanopores, water-flow through membrane increases, meanwhile salt rejection decreases. read less

Abstract: Molecular dynamics (MD) simulations are performed to study the freezing process of Al-Si melts on heterogeneous Si substrates in detail. We highlight the inherent nanostructure of both the Si primary phase and the Al-Si binary phase. It is found for the first time that the primary Si phase displays a "pyramidal configuration" when the Al-Si melt congeals. Experimental measurements could also verify our simulation results. It can be found that the binary Al-Si phase turns into a "Si-Al-Si sandwich construction" during solidification, regardless of freezing on a single substrate or in the restricted space between substrates. This peculiar phenomenon results from the combined effects of the van der Waals potential well and the interatomic interaction between Al and Si. Furthermore, it is also able to control the thickness of the Si atomic shell of the "sandwich construction", resulting in the silicene-like unilaminar Si nanostructure. Our findings provide novel strategies to fabricate desired shaped nanostructures by means of nanocasting in Al-Si melts at the nanoscale. read less

Abstract: Understanding phonon scattering by topological defects in graphene is of particular interest for thermal management in graphene-based devices. We present a study that quantifies the roles of the different mechanisms governing defect phonon scattering by comparing the effects of ten different defect structures using molecular dynamics. Our results show that phonon scattering is mainly influenced by mass density difference, with general trends governed by the defect formation energy and typical softening behaviors in the phonon density of state. The phonon scattering cross-section is found to be far larger than that geometrically occupied by the defects. We also show that the lattice thermal conductivity can be reduced by a factor of up to ~30 in the presence of the grain boundaries formed by these defects. read less

Abstract: In this work we determine and discuss free-energy barriers associated with the detachment of metal (gold) nanoparticles covered by an organic shell from carbon nanotubes functionalized by hydrazide segments. At neutral pH, both compounds can form hydrazone bonds which in turn lead to the chemically corked form of the nanotube. At slightly acidic pH, the hydrazone bonds undergo hydrolysis, leading to chemically unbonded nanotube and gold nanoparticles. We found that at this state the dispersion interactions between the nanotube and gold nanoparticles are still very strong and spontaneous detachment of gold nanoparticles does not occur. Therefore, the uncorked state of the nanotube cannot be realized at normal conditions. The presence of guest molecules (cisplatin) in the inner cavity of the nanotube affects the energetic balance of the system, and spontaneous uncorking can occur with some small activation barrier. However, the uncorking is in this case related to the shift of the nanoparticle from the nano... read less

Abstract: Two novel two-dimensional (2D) carbon allotropes named C(y) and C(z) with large meshes are predicted based on first-principles calculations. Their formation energies are lower than that of graphdiyne, which was recently synthesized in an experiment. Molecular dynamics simulations indicate that C(y) and C(z) are stable even when the temperature is over 1000 K. The calculated Poisson's ratios of C(y) and C(z) show their anisotropic mechanical properties. The electronic structure calculations indicate that C(y) is a metal, while C(z) behaves as a semiconductor. Moreover, C(z) shows conductive anisotropy suggesting its potential in nanoelectronic devices. Meanwhile, their well-defined mesh structures are suitable for molecular sieves. read less

Abstract: Here we report a unique method to locally determine the mechanical response of individual covalent junctions between carbon nanotubes (CNTs), in various configurations such as "X", "Y", and "Λ"-like. The setup is based on in situ indentation using a picoindenter integrated within a scanning electron microscope. This allows for precise mapping between junction geometry and mechanical behavior and uncovers geometry-regulated junction stiffening. Molecular dynamics simulations reveal that the dominant contribution to the nanoindentation response is due to the CNT walls stretching at the junction. Targeted synthesis of desired junction geometries can therefore provide a "structural alphabet" for construction of macroscopic CNT networks with tunable mechanical response. read less

Abstract: Understanding radiation effects on the mechanical properties of SiC composites is important to their application in advanced reactor designs. By means of molecular dynamics simulations, we found that due to strong interface bonding between the graphene layers and SiC, the sliding friction of SiC fibers is largely determined by the frictional behavior between graphene layers. Upon sliding, carbon displacements between graphene layers can act as seed atoms to induce the formation of single carbon atomic chains (SCACs) by pulling carbon atoms from the neighboring graphene planes. The formation, growth, and breaking of SCACs determine the frictional response to irradiation. read less

Abstract: Graphene sheet including single vacancy, double vacancy and Stone-Wales with armchair and zigzag structure was simulated using molecular dynamics simulation. The effect of defects on shear’s modulus, shear strength and fracture  strain was investigated. Results showed that these shear properties reduce when the degrees of all kinds of defects increase. The dangling bond in SV and DV defected graphene leads to decrease its mechanical properties especially shear strength and fracture strain where the role of weak interatomic bonds are important. The vacancies in DV defected graphene are also next to each other and slide over each other under shear deformation results to less shear strength than that of SV defected graphene. Results can be useful in tuning the mechanical properties of graphene-based materials that is a key-role parameter in designing and fabrication of nanomechanical systems. However, the maximum and minimum reduction occurs for single vacancy and Stone-Wales defects, respectively. It was also found that distinction between shear properties of zigzag and armchair structures is preserved in defected graphene. read less

Abstract: The twisting response of armchair graphene nanoribbons with tilt grain boundaries is theoretically and numerically investigated. It is found that the critical instability twist rate of graphene nanoribbons with grain boundaries is generally about 10% higher than that of common armchair graphene nanoribbons when the width of nanoribbons is less than 4.0 nm. Our analytical analysis indicates that the strengthening effect is resulted from the rotation of the compressed direction, deflection of grain boundaries, and the reflexing of the creased angle in nanoribbons: the rotation of the compressed direction induced by grain boundaries improves the buckling strength of nanoribbons due to the chirality-dependent buckling in graphene; the deflection of grain boundaries leads to a nonzero strain in the axle wire of nanoribbons, which eventually decreases the compressed stress; grain boundaries induce a spontaneous creased angle in nanoribbons, which is reflexed under twist loading and impedes the propagation of instability in nanoribbons. Furthermore, we found and demonstrated that grain boundaries changed the transport properties of twisted graphene nanoribbons. It is expected that our findings would improve the fundamental understanding of the strain-engineering of graphene nanoribbons used in nanodevices. read less

Abstract: By employing both molecular dynamics (MD) simulations and ab initio calculations based on the density functional theory (DFT), we studied the efficiency of doping graphene with low energy Si ions implantation. Mainly two types of substitutional doping configurations resulting from Si ion implantation were found in graphene, namely perfect Si substitution at monovacancy (Si@MV), and Si interstitial defect at divacancy site (Si@DV). High efficiency for Si substitutions was obtained within a wide energy range varied between 30–150 eV. At the optimum energy of 70 eV, up to 59% of the incident Si ions would be incorporated in graphene by Si@MV. Moreover, the experimental doping efficiency should be higher than the above value of 59% because Si adatom on graphene surface can be eventually turned into a substitution atom via annihilating with a vacancy defect produced in the collision process. Such high doping efficiency makes ion implantation a powerful tool to dope graphene with Si and similar elements. Our results provide a theoretical clue for the property engineering of graphene by using ion irradiation technique, in particular for doping graphene with heavy ions. read less

Abstract: In this work we perform molecular dynamics simulations to investigate the thermodynamic mechanisms of amorphous polystyrene ablated by a femtosecond laser pulse. The utilized width and fluence of the laser pulse are 10 fs and 1012 W/cm2, respectively. Furthermore, the effect of chain length on the microscopic material removal mechanisms and related macroscopic evolutions of system quantities is studied. Simulations results indicate that the ultra-short laser pulse irradiation introduces significant instabilities into the target material because of the lattice perturbation, which leads to the relaxation of lattice destabilizations after the pulse through evaporation from the top surface and expansion within the bulk. It is found that the competition between the two mechanisms of evaporation and expansion is strongly influenced by the chain length. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42713. read less

Abstract: Carbon nanotube (CNT)/copper (Cu) composite material is proposed to replace Cu-based through-silicon vias (TSVs) in micro-electronic packages. The proposed material is believed to offer extraordinary mechanical and electrical properties and the presence of CNTs in Cu is believed to overcome issues associated with miniaturization of Cu interconnects, such as electromigration. This study introduces a multi-scale modeling of the proposed TSV in order to evaluate its mechanical integrity under mechanical and thermo-mechanical loading conditions. Molecular dynamics (MD) simulation was used to determine CNT/Cu interface adhesion properties. A cohesive zone model (CZM) was found to be most appropriate to model the interface adhesion, and CZM parameters at the nanoscale were determined using MD simulation. CZM parameters were then used in the finite element analysis in order to understand the mechanical and thermo-mechanical behavior of composite TSV at micro-scale. From the results, CNT/Cu separation does not take place prior to plastic deformation of Cu in bending, and separation does not take place when standard thermal cycling is applied. Further investigation is recommended in order to alleviate the increased plastic deformation in Cu at the CNT/Cu interface in both loading conditions. read less

Abstract: The elastic modulus of multilayer graphene is found to be more robust to damage created by high-energy α-particle irradiation as compared to monolayer graphene. Theoretical analysis indicates that irradiation of multilayer graphene generates interlayer links that potentially increase the stiffness of the multilayer by passivating local defects. read less

Abstract: We present the first data from individual C60 impacting one to four layer graphene at 25 and 50 keV. Negative secondary ions and electrons emitted in transmission were recorded separately from each impact. The yields for C(n)(-) clusters are above 10% for n ≤ 4, they oscillate with electron affinities and decrease exponentially with n. The result can be explained with the aid of MD simulation as a post-collision process where sufficient vibrational energy is accumulated around the rim of the impact hole for sputtering of carbon clusters. The ionization probability can be estimated by comparing experimental yields of C(n)(-) with those of C(n)(0) from MD simulation, where it increases exponentially with n. The ionization probability can be approximated with ejecta from a thermally excited (3700 K) rim damped by cluster fragmentation and electron detachment. The experimental electron probability distributions are Poisson-like. On average, three electrons of thermal energies are emitted per impact. The thermal excitation model invoked for C(n)(-) emission can also explain the emission of electrons. The interaction of C60 with graphene is fundamentally different from impacts on 3D targets. A key characteristic is the high degree of ionization of the ejecta. read less

Abstract: Molecular dynamics simulations are performed to study the nanoindentation models of monolayer suspended graphene and graphyne. Fullerenes are selected as indenters. Our results show that Young's modulus of monolayer-thick graphyne is almost half of that of graphene, which is estimated to be 0.50 TPa. The mechanical properties of graphene and graphyne are different in the presence of strain. A pre-tension has an important effect on the mechanical properties of a membrane. Both the pre-tension and Young's modulus plots demonstrate index behavior. The toughness of graphyne is stronger than that of graphene due to Young's modulus magnitude. Young's moduli of graphene and graphyne are almost independent of the size ratio of indenter to membrane. read less

Abstract: A carbon nanoscroll (CNS) can be formed easily by rolling a graphene sheet around a carbon nanotube (CNT) [Zhang and Li, 2010, APL, 97, 081909]. When the CNS is driven by the rotary CNT to rotate at a high speed, the attractive interaction within the CNS or between the CNS and CNT is crippled by the centrifugal force on the CNS. The unwinding of CNS is triggered when the kinetic energy increment approaches to the variation of interaction energy of the system during CNS formation. Numerical experiments also indicate that the unwinding of CNS happens earlier when the CNT has a higher rotational speed or the system is at a higher temperature. read less

Abstract: Due to the difficulty of performing uniaxial tensile testing, the strengths of graphene and its grain boundaries have been measured in experiments by nanoindentation testing. From a series of molecular dynamics simulations, we find that the strength measured in uniaxial simulation and the strength estimated from the nanoindentation fracture force can differ significantly. Fracture in tensile loading occurs simultaneously with the onset of crack nucleation near 5-7 defects, while the graphene sheets often sustain the indentation loads after the crack initiation because the sharply concentrated stress near the tip does not give rise to enough driving force for further crack propagation. Due to the concentrated stress, strength estimation is sensitive to the indenter tip position along the grain boundaries. Also, it approaches the strength of pristine graphene if the tip is located slightly away from the grain boundary line. Our findings reveal the limitations of nanoindentation testing in quantifying the strength of graphene, and show that the loading-mode-specific failure mechanism must be taken into account in designing reliable devices from graphene and other technologically important 2D materials. read less

Abstract: Recent experimental reports on the production of nanopolycrystalline diamond with outstanding mechanical stiffness highlight the importance of theoretical investigation of this unusual effect. Here, we provide the comprehensive theoretical investigation of such material. We traced the evolution of the nanopolycrystalline diamond stiffness characterized by bulk modulus with the grain size increasing up to 10 nm and found samples with bulk modulus higher than that of diamond. We studied nature of such specific behavior and proposed a mechanism of stiffening in nanopolycrystalline diamond which could explain reference experimental results. read less

Abstract: Non-equilibrium molecular dynamic simulations reveal that the thermal conductivity of ultrathin carbon nanotube (CNT)(2, 1) is significantly suppressed upon hydrogenation. The addition of hydrogen atoms to two-coordinated carbon atoms lowers the participation ratios of phonon modes, thus indicating that the spatial distribution of phonons becomes localized. Furthermore, the phonon lifetimes are remarkably shortened in hydrogenated CNT(2, 1) (HCNT(2, 1)) compared with those of bare CNT(2, 1). The lowered participation ratios and lifetimes of phonon modes are responsible for the significant reduction of thermal conductivity in HCNT(2, 1). Our study is also helpful for understanding the weakened thermal transport abilities in carbon polymers, namely, the cross links formed between individual polymer chains will hinder the thermal conduction along polymers, even though the single straight carbon polymer has a high and divergent thermal conductivity. read less

Abstract: This article reports the latest developments of our theoretical studies of gas cluster bombardment of model macromolecular samples using molecular dynamics simulations. Here, we perform a detailed comparison of the effects of the sample molecular weight, the Ar cluster incidence angle (45° vs 0°), and the cluster nature (CH4 vs Ar) on the soft sputtering of polymeric samples. The results of Ar cluster-induced sputtering and fragmentation at 45° incidence for molecular targets with three different molecular weights (282, 1388, and 14002 amu) indicate a pronounced influence of that parameter beyond 1000 amu, which is explained by the extra energy needed to form fragments from longer chains and to overcome mechanical entanglement. An excellent agreement is found between the computed statistics of sputtering and the available experimental data for similar molecular weights. The variance of the sputtering and polymer fragmentation results with changing beam parameters is explained via the microscopic analysis ... read less

Abstract: The interfacial mechanical properties of carbon nanotube (CNT)-reinforced silicon nanocomposites are investigated by using molecular dynamics simulation method. The hybrid potential that includes Tersoff_2 potential for Si–Si in the silicon matrix, AIREBO potential for C–C in the CNTs and the Lennard-Jones (LJ) potential for Si–C in the interface is used in the nanocomposite system. The effects of such parameters as the CNT chirality, the CNT diameter and the CNT embedded length, the defects (Vacancy defects, Stone–Wales defects), the size of model, the temperature, the bonding strength and the cut-off distance of the interfacial LJ potential of nanocomposites on the pull force and the average interfacial shear strength (ISS) are investigated and discussed. The results show that the toughness and the maximum tensile strength have been increased significantly by adding the CNTs into the Si matrix. Also by increasing the LJ bonding strength and the cut-off distance of the LJ potential, the pull force and the ISS are increased significantly. The CNT chirality, the CNT diameter and the CNT embedded length have a great influence on the pull force and the ISS, while the effects of temperature, the defects and the size of model are very slight. read less

Abstract: We identify the inhibition effect of a non-permeating gas component on gases permeating through the nanoporous graphene membranes and reveal its mechanisms from molecular dynamics insights. The membrane separation process involves the gas mixtures of CH4/H2 and CH4/N2 with different partial pressures of the non-permeating gas component (CH4). The results show that the permeance of the H2 and N2 molecules decreases sharply in the presence of the CH4 molecules. The permeance of the N2 molecules can be reduced to as much as 64.5%. The adsorption of the CH4 molecules on the graphene surface weakens the surface adsorption of the H2 and N2 molecules due to a competitive mechanism, accordingly reducing the permeability of the H2 and N2 molecules. For the N2 molecules with stronger adsorption ability, the reduction of the permeance is greater. On the other hand, the CH4 molecules near the nanopore have a blocking effect, which further inhibits the permeation of the H2 and N2 molecules. In addition, we predict the selectivity of the nanopore by using density functional theory calculations. This work can provide valuable guidance for the application of nanoporous graphene membranes in the separation of the gas mixtures consisting of permeating and non-permeating components with different adsorption abilities. read less

Abstract: The behavior of water droplets located on graphene in the presence of various external electric fields (E-fields) is investigated using classical molecular dynamics (MD) simulations. We explore the effect of E-field on mass density distribution, water polarization as well as hydrogen bonds (H-bonds) to gain insight into the wetting properties of water droplets on graphene and their interfacial structure under uniform E-fields. The MD simulation results reveal that the equilibrium water droplets present a hemispherical, a conical and an ordered cylindrical shape with the increase of external E-field intensity. Accompanied by the shape variation of water droplets, the dipole orientation of water molecules experiences a remarkable change from a disordered state to an ordered state because of the polarization of water molecules induced by static E-field. The distinct two peaks in mass density and H-bond distribution profiles demonstrate that water has a layering structure in the interfacial region, which sensitively depends on the strong E-field (>0.8 V nm(-1)). In addition, when the external E-field is parallel to the substrate, the E-field would make the contact angle of the water droplets become small and increase its wettability. Our findings provide the possibility to control the structure and wetting properties of water on graphene by tuning the direction and intensity of external E-field which is of importance for relevant industrial processes on the solid surface. read less

Abstract: Using non-equilibrium molecular dynamics simulations, we investigate the thermal conductivity variation of graphene with different hydrogen doping coverage and doping orientation. It shows that the thermal conductivity of graphene decreases with increasing hydrogen doping coverage. The decreasing rate, however, depends on the doping orientation. Based on the kinetic theory of lattice thermal transport, we study the effect of doping coverage and orientation on the phonon density of states, phonon dispersion relation, phonon relaxation time, and the specific heat. While hydrogen doping has little effect on the specific heat, it decreases the phonon group velocity and increases phonon-phonon scattering in graphene. The phonon group velocity reduction is only due to the increment of doping coverage and is independent of doping orientation. A larger angle between the doping stripe orientation and the heat flux direction leads to smaller relaxation times, i.e., stronger phonon-phonon scattering, resulting in a ... read less

Abstract: Experiments and simulations demonstrating reversible and repeatable crumpling of graphene warrant a detailed understanding of the underlying mechanisms of graphene crumple formation, especially for design of tailored nanostructures. To systematically study the formation of crumples in graphene, we use a simple molecular dynamics model, and perform a series of simulations to characterize the finite number of deformation regimes of graphene on substrate after compression. We formulate a quantitative measure of predicting these deformations based on observed results of the simulations and distinguish graphene crumpling considered in this study from others. In our study, graphene is placed on a model substrate while controlling and varying the interfacial energy between graphene and substrate and the substrate roughness through a set of particles embedded in the substrate. We find that a critical value of interfacial adhesion energy marks a transition point that separates two deformation regimes of graphene on substrate under uniaxial compression. The interface between graphene and substrate plays a major role in the formation of crumples, and we show that the choice of substrate can help in designing desired topologies in graphene. read less

Abstract: A motion transmission system made from coaxial carbon nanotubes (CNTs) is introduced. In the system, the motor is built from a single-walled carbon nanotube (SWCNT) and the converter is made from triple-walled carbon nanotubes (TWCNTs). The outer shell acts as a stator with two fixed tube ends. The inner tube (rotor 1) and the middle tube (rotor 2) can move freely in the stator. When the axial gaps between the motor and the TWCNTs are small enough and the motor has a relatively high rotational speed, the two rotors have either stable rotation or oscillation, which can be considered as output signals. To investigate the effects of such factors as the length of rotor 2, the rotational speed of the motor, and the environmental temperature on the dynamic response of the two rotors, numerical simulations using molecular dynamics (MD) are presented on a device model having a (5, 5) motor and a (5, 5)/(10, 10)/(1, 15) converter. Numerical results show that the two inner tubes can act as both rotor(s) and oscillator, simultaneously if the middle tube is longer than the inner tube. In particular, we find a new phenomenon, mode conversion of the rotation of rotor 1 by changing the environmental temperature. Briefly, rotor 1 rotates synchronously with the high-speed motor at a higher temperature or with rotor 2 at a lower temperature. The effect of radii difference among the three tubes in the bearing are also discussed by replacing the middle tube (10, 10) with different zigzag tubes. read less

Abstract: Carbon nanotubes and graphene are promising materials for thermal management applications due to their high thermal conductivities. However, their thermal properties are anisotropic, and the radial or cross-plane direction thermal conductivity is low. A 3D Carbon nanotube (CNT)-graphene structure has previously been proposed to address this limitation, and direct molecular dynamics simulations have been used to predict the associated thermal conductivity. In this work, by recognizing that thermal resistance primarily comes from CNT-graphene junctions, a simple network model of thermal transport in pillared graphene structure is developed. Using non-equilibrium molecular dynamics, the resistance across an individual CNT-graphene junction with sp2 covalent bonds is found to be around 6×10−11 m2K/W, which is significantly lower than typical values reported for planar interfaces between dissimilar materials. In contrast, the resistance across a van der Waals junction is about 4×10−8 m2K/W. Interestingly, when... read less

Abstract: A thermally driven nanotube nanomotor provides linear mass transportation controlled by a temperature gradient. However, the underlying mechanism is still unclear, as the mass transportation velocity in experiment is much lower than that resulting from simulations. Considering that defects are common in fabricated nanotubes, we use molecular dynamics simulations to show that the mass transportation would be considerably impeded by defects. The outer tube of a double-walled carbon nanotube transports along the coaxial inner tube subject to a temperature gradient. While encountering the defects in the inner tube, the outer tube might be bounced back or trapped at some specific sites due to the potential barriers or wells induced by the defects. The stagnation phenomenon provides a probable picture to understand the low transportation velocity at the microscopic level. We also show that a similar stagnation phenomenon holds in mass transportation of a fullerene encapsulated in a defective carbon nanotube. Ou... read less

Abstract: A rotational transmission system from coaxial carbon nanotubes (CNTs) is investigated using a computational molecular dynamics approach. The system consists of a motor from a single-walled carbon nanotube and a bearing from a double-walled carbon nanotube. The motor has a high fixed rotational frequency and the two ends of the outer tube in the bearing are fixed. The inner tube in the bearing works as a rotor. Because of the interlayer friction in the bearing, configurations of the joint between the adjacent ends of motor and rotor have significant effects on rotational transmission properties. Four factors are considered in simulation, i.e., the bonding types of atoms (sp1 and sp2) on the ends of motor and rotor, the difference between motor and rotor radii, the rotational speed of motor, and the environmental temperature. It is found that the synchronous transmission happens if the sp1 atoms on the jointed ends of motor and rotor are bonded each other and become new sp2 atoms. Therefore, the lower diffe... read less

Abstract: Well-known effects of mechanical stiffness degradation under the influence of point defects in macroscopic solids can be controversially reversed in the case of low-dimensional materials. Using atomistic simulation, we showed here that a single-layered graphene film can be sufficiently stiffened by monovacancy defects at a tiny concentration. Our results correspond well with recent experimental data and suggest that the effect of mechanical stiffness augmentation is mainly originated from specific bonds distribution in the surrounded monovacancy defects regions. We showed that such unusual mechanical response is the feature of presence of specifically monovacancies, whereas other types of point defects such as divacancy, 555-777 and Stone-Wales defects, lead to the ordinary degradation of the graphene mechanical stiffness. read less

Abstract: The surfaces of layered materials such as graphite exhibit step edges that affect friction. Step edges can be exposed, where the step occurs at the outmost layer, or buried, where the step is underneath another layer of material. Here, we study friction at exposed and buried step edges on graphite using an atomic force microscope (AFM) and complementary molecular dynamics simulations of the AFM tip apex. Exposed and buried steps exhibit distinct friction behavior, and the friction on either step is affected by the direction of sliding, i.e., moving up or down the step, and the bluntness of the tip. These trends are analyzing in terms of the trajectory of the AFM tip as it moves over the step, which is a convolution of the topography of the surface and the tip shape. read less

Abstract: Molecular dynamics (MD) simulations are performed to systematically study the structural evolution of a Si melt confined in nanoscale space. The freezing Si structure at 300 K is stratification which is composed of a stable crystalline shell and a metastable glassy core. Due to the spatial restriction effect, the confined structure consists of higher-coordinated clusters compared to the bulk Si. It is revealed that the statistical average of the ordered shell and the disordered core gives rise to the split of the second peak of the pair distribution function curves of the Si melt. Moreover, increasing the cavity size is detrimental to the stability of the layered configuration of the confined melt and increasing the cooling rate mainly influences the arrangement of Si atoms adjacent to the SWCNT wall. Interestingly, we also find that the cylindric cavity is more beneficial than the square one in inducing the formation of long-range crystalline order in nanoscale space. read less

Abstract: The radial stability and configuration transition of carbon nanotubes (CNTs) with enclosed cores have been studied in this paper by using atomistic simulations. We found that an abnormal transition of CNTs from open to collapse can be regulated by enclosing deformable and rigid cores. The energy barrier for the configuration transition can be reduced by nearly one order of magnitude due to the presence of these cores, i.e., from ∼0.3 eV/A to ∼0.03 eV/A. These findings may provide guidance for the design of controllable CNT-based carrier systems for the delivery of drug, gene and fluid. read less

Abstract: As the electronic industry moves toward higher power consumption, integrated functions and minimized geometry, one of the important challenges is the dramatically increasing power density. Thus, efficient thermal management has become a critical requirement for the design of modern electronic packages. A promising approach for this challenge is to find a high performance thermal interface materials (TIMs) made of a material with extremely high thermal conductivity. In this work, an innovative carbon-based nanomaterial, carbon nanoscroll (CNS) will be presented to yield extremely high thermal conductivity and have great potential as the component of TIMs in electronic packages. A CNS can be regarded as a monolayer graphene rolling up in a spiral form with a structure similar to a multi-walled carbon nanotube (CNT). Unlike the closed CNTs, CNS is topologically open-ended, like a Swiss roll. Using molecular dynamics (MD) simulations, the thermal conductivity of CNSs is investigated to be comparable to graphene, i.e. 3000- 5000 Wm-1K-1. Various factors that impact the thermal transport behavior of CNSs are investigated extensively. The MD simulation results show that the thermal conductivity of CNS is sensitive to the number of CNS walls, temperature, defects and functionalization. When the number of walls increases from 1 to 3, the thermal conductivity of CNSs is reduced by ~8.9%. With environmental temperature rising from 300 K to 400K, the thermal conductivity of CNSs decreases by ~16.5%. When the CNSs have single vacancy defects or functionalized hydrogen, their thermal conductivity decreases gradually with the higher densities of defects or functionalization. The results reveal that the vertical aligned CNSs can be superior to vertical aligned CNTs in serving as the thermal interface materials in electronic packages, due to their higher thermal conductivity. CNSs can also be used as superior thermally conductive fillers in polymeric TIMs. Using effective medium theory, the thermal conductivity of polymeric TIMs composited of epoxy resin matrix and CNS fillers is calculated. It is found that polymeric TIMs with epoxy resin matrix and 10% volume fraction of CNS fillers yield an effective thermal conductivity of ~79 Wm-1K-1, which is one magnitude higher than the commonly used TIMs in current electronic packaging industry. The present work reveals new insights about the extremely high thermal conductivity of CNS and its great potential in improving the thermal management of electronic packages. read less

Abstract: Molecular dynamics (MD) simulations were used to model amplitude modulation atomic force microscopy (AM-AFM). In this novel simulation, the model AFM tip responds to both tip–substrate interactions and to a sinusoidal excitation signal. The amplitude and phase shift of the tip oscillation observed in the simulation and their variation with tip–sample distance were found to be consistent with previously reported trends from experiments and theory. These simulation results were also fit to an expression enabling estimation of the energy dissipation, which was found to be smaller than that in a corresponding experiment. The difference was analyzed in terms of the effects of tip size and substrate thickness. Development of this model is the first step toward using MD to gain insight into the atomic-scale phenomena that occur during an AM-AFM measurement. read less

Abstract: Hydrogen, of which the application is limited due to the difficulties in finding the ideal storage material, has been considered alternative for petroleum as the main energy source. With its large surface area and other extraordinary physical properties, graphene has been the focus of many researchers as the promising candidate for hydrogen storage and transportation. In this work, the hydrogen storage characteristics of graphene have been investigated by MD simulations. We found that, under the temperature of 70 K and the pressure of 1 MPa, the hydrogen uptake percentage can be as high as 54%. And the majority of the hydrogen atoms are absorbed during the initial 100 – 200 ps of the simulation. Moreover, the hydrogen storage properties of graphene with different environment temperatures have been studied. We found that with increasing temperature, the hydrogen uptake percentage towards the end of the simulation decreases. Furthermore, the number of layers of the graphene sheet also exerts influence of the hydrogen absorption capability of the sample. We conclude that the more graphene sheets are being used, the less hydrogen atoms are being absorbed by the sample. Our work provides insight into optimizing the environmental temperature and thickness of the graphene sheet when designing novel energy storage devices, especially hydrogen storage devices. read less

Abstract: We numerically demonstrate the spontaneous formation of various 3D carbon nanostructures, like multi-tube carbon nanotubes, nanopyramids, nanocubes, artificially rippled graphene, and other exotic nanomaterials, starting from graphene nanoribbons and inducing controllably engineered defects consisting of carbon adatoms or inverse Stone–Wales defects. The evolution of the initial defected planar structures towards the final 3D nanoarchitectures is obtained through molecular dynamics simulations, using different force fields to ensure the reproducibility of the derived results. The presented carbon nanostructures of different shapes, sizes, and morphologies, can be used in applications ranging from storage of hydrogen or other molecules, enhanced chemical reactions or catalysis in confined compartments, to drug delivery nanodevices and biosensors. read less

Abstract: Strain engineering of the thermal conductivity of graphene is highly desirable for various nanoscale thermal devices. Previous investigations have been focused mainly on the uniform strain applied uniaxially or biaxially. In this work we investigated, by non-equilibrium molecular dynamics simulations, the thermal transport behavior of graphene nanoribbons under local, nonuniform strain. A capped carbon nanotube (CNT) is used as a representative tip to indent the graphene, which creates a local stress field similar to those induced by nanoindentation or molecular adsorption. The relationship among structural deformations, phonon transport, and stress field was analyzed, and the effects of indentation depth and tip-surface interaction strength were discussed. More than 50% reduction of thermal conductance can be observed for a 20 nm × 5 nm graphene nanoribbon upon indentation. Our study revealed that the thermal transport of graphene responds flexibly and sensitively to the local strain, which can be exploited for new functional nanodevices across various disciplines such as position sensing or molecular sensing. Thermal sensors based on graphene can then be constructed. read less

Abstract: The purpose of this article is to provide a systematic evaluation to examine characteristics of the thermal conductivity and thermal rectification of H-terminated graphene nanoribbons (HGNRs) with Lpristine/LH-terminated = 1. The results show that HGNR thermal conductivities increase in both directions across the entire temperature range tested. Simultaneously, thermal rectification of HGNRs at various temperatures is detected. We found that with increasing temperature the thermal rectification has a gradual decreasing tendency. Furthermore, the overlap of power spectra was calculated to elucidate the underlying mechanism of thermal rectification. This work indicates a possible route to achieve thermal rectification for 2D materials by hydrogenation engineering. read less

Abstract: Molecular dynamics (MD) simulations are carried out to study the coalescence of identical adjacent and nonadjacent water films on graphene (G), vertically or horizontally stacked carbon nanotube arrays (VCNTA and HCNTA respectively). We highlight the key importance of carbon-based substrates in the growth of the liquid bridge connecting the two water films. This simulation provides reliable evidence to confirm a linear increase of the liquid bridge height, which is sensitive to the surface properties and the geometric structure. In the case of nonadjacent water films, the meniscus liquid bridge occurs solely on the VCNTA, which is attributed to the spreading of water films driven by the capillary force. Our results provide an available method to tune the coalescence of adjacent or nonadjacent films with alteration of topographically patterned surfaces, which has important implications in the design of condensation, ink-jet printing and drop manipulation on a substrate. read less

Abstract: The present work combines experimental and theoretical studies of the collision between keV ion projectiles and clusters of pyrene, one of the simplest polycyclic aromatic hydrocarbons (PAHs). Intracluster growth processes induced by ion collisions lead to the formation of a wide range of new molecules with masses larger than that of the pyrene molecule. The efficiency of these processes is found to strongly depend on the mass and velocity of the incoming projectile. Classical molecular dynamics simulations of the entire collision process-from the ion impact (nuclear scattering) to the formation of new molecular species-reproduce the essential features of the measured molecular growth process and also yield estimates of the related absolute cross sections. More elaborate density functional tight binding calculations yield the same growth products as the classical simulations. The present results could be relevant to understand the physical chemistry of the PAH-rich upper atmosphere of Saturn's moon Titan. read less

Abstract: Shock response of a basic defect type in graphene, the Stone–Wales defect (SWD), is investigated with molecular dynamics simulations. Shock compression is applied to embedded SWDs along the armchair and zigzag directions. Upon shock loading, C–C bonds tend to rotate to an orientation perpendicular to the shock direction. SWD’s shock response shows pronounced anisotropy because of the structural anisotropies in both graphene and SWD with respect to the loading direction, and overall SWD shows stronger resistance to deformation for the armchair-direction loading. For the zigzag-direction loading, slip nucleates in SWD via formation of two pentagons by compressing two meta- or other-position atoms together during a rotation of central C–C bond and grows by means of alternating formation of two pentagons and a twisted hexagon. For the armchair-direction loading, healing, generation, and pentagon–heptagon pair separation of SWD occur via a 90° rotation of C–C bond, whereas at high shock strengths, slip may nuc... read less

Abstract: In a thermally driven rotary motor made from double-walled carbon nanotubes, the rotor (inner tube) can be actuated to rotate within the stator (outer tube) when the environmental temperature is high enough. A sudden stoppage of the rotor can occur when the inner tube has been actuated to rotate at a stable high speed. To find the mechanisms of such sudden stoppages, eight motor models with the same rotor but different stators are built and simulated in the canonical NVT ensembles. Numerical results demonstrate that the sudden stoppage of the rotor occurs when the difference between radii is near 0.34 nm at a high environmental temperature. A smaller difference between radii does not imply easier activation of the sudden rotor stoppage. During rotation, the positions and electron density distribution of atoms at the ends of the motor show that a sp1 bonded atom on the rotor is attracted by the sp1 atom with the biggest deviation of radial position on the stator, after which they become two sp2 atoms. The strong bond interaction between the two atoms leads to the loss of rotational speed of the rotor within 1 ps. Hence, the sudden stoppage is attributed to two factors: the deviation of radial position of atoms at the stator’s ends and the drastic thermal vibration of atoms on the rotor in rotation. For a stable motor, sudden stoppage could be avoided by reducing deviation of the radial position of atoms at the stator’s ends. A nanobrake can be, thus, achieved by adjusting a sp1 atom at the ends of stator to stop the rotation of rotor quickly. read less

Abstract: Laboratoire d’Energetique Moleculaire et Macroscopique, CNRS UPR 288, Ecole Centrale Paris,Grande Voie des Vignes, 92295 Châtenay-Malabry, France(Received 15 December 2014; revised manuscript received 22 January 2015; published 25 February 2015)The carbon nanotubes’ resilience to mechanical deformation is a potentially important feature forimparting tunable properties at the nanoscale. Using nonequilibrium molecular dynamics and empiricalinteratomic potentials, we examine the thermal conductivity variations with bending in the thermaltransport regime where both ballistic and diffusive effects coexist. These simulations are enabled by therealistic atomic-scale descriptions of uniformly curved and buckled nanotube morphologies obtained byimposing objective boundary conditions. We uncover a contrasting behavior. At shorter lengths, thephonon propagation is affected significantly by the occurrence of localized structural buckling. As thenanotube length becomes comparable with the phonon mean free path, heat transport becomes insensitiveto the buckling deformations. Our result settles the controversy around the differences between the currentexperimental and molecular-dynamics measurements of the thermal transport in bent nanotubes. read less

Abstract: We report, based on its variation in electronic transport to coupled tension and shear deformation, a highly sensitive graphene-based strain sensor consisting of an armchair graphene nanoribbon (AGNR) between metallic contacts. As the nominal strain at any direction increases from 2.5 to 10%, the conductance decreases, particularly when the system changes from the electrically neutral region. At finite bias voltage, both the raw conductance and the relative proportion of the conductance depend smoothly on the gate voltage with negligible fluctuations, which is in contrast to that of pristine graphene. Specifically, when the nominal strain is 10% and the angle varies from 0 ° ?> to 90 ° ?> , the relative proportion of the conductance changes from 60 to ∼90%. read less

Abstract: There are prevailing concerns with the critical dimensions when conventional theories break down. Here we find that the Griffith criterion remains valid for cracks down to 10 nm but overestimates the strength of shorter cracks. We observe the preferred crack extension along the zigzag edge in graphene, and explain this phenomenon by local strength-based failure rather than energy-based Griffith criterion. These results provide a mechanistic basis for reliable applications of graphene in miniaturized devices and nanocomposites. read less

Abstract: Mechanical failure of an ideal crystal is dictated either by an elastic instability or a soft-mode instability. Previous interpretations of nanoindentation experiments on suspended graphene sheets,1,2 however, indicate an anomaly: the inferred strain in the graphene sheet directly beneath the diamond indenter at the measured failure load is anomalously large compared to the fracture strains predicted by both soft-mode and acoustic analyses. Through multiscale modeling combining the results of continuum, atomistic, and quantum calculations, and analysis of experiments, we identify a strain-shielding effect initiated by mechanochemical interactions at the graphene-indenter interface as the operative mechanism responsible for this anomaly. Transmission electron micrographs and a molecular model of the diamond indenter's tip suggest that the tip surface contains facets comprising crystallographic {111} and {100} planes. Ab initio and molecular dynamics (MD) simulations confirm that a covalent bond (weld) formation between graphene and the crystallographic {111} and {100} facets on the indenter's surface can be induced by compressive contact stresses of the order achieved in nanoindentation tests. Finite element analysis (FEA) and MD simulations of nanoindentation reveal that the shear stiction provided by the induced covalent bonding restricts relative slip of the graphene sheet at its contact with the indenter, thus initiating a local strain-shielding effect. As a result, subsequent to stress-induced bonding at the graphene-indenter interface, the spatial variation of continuing incremental strain is substantially redistributed, locally shielding the region directly beneath the indenter by limiting the buildup of strain while imparting deformation to the surrounding regions. The extent of strain shielding is governed by the strength of the shear stiction, which depends upon the level of hydrogen saturation at the indenter's surface. We show that at intermediate levels of hydrogen saturation the strain-shielding effect can enable the graphene to support experimentally determined fracture loads and displacements without prematurely reaching locally limiting states of stress and deformation. read less

Abstract: A molecular dynamics study is undertaken to estimate the mechanical responses of crumpled graphene subjected to hydrostatic compression and uniaxial compression, respectively. The crumpled graphene is found to be a non-Hookean medium showing a non-linear stress–strain relation even for small strain and this is explained by structural changes that start to occur when a small stress is applied. Analysis of the unloading curves suggests that the elasticity limit is achieved at smaller densities under hydrostatic compression rather than under uniaxial compression. Corners of double-folded graphene flakes are formed under hydrostatic compression, while uniaxial compression results mainly in formation of single folds with less damage of the graphene lattice. read less

Abstract: We designed a water pumping system based on double-walled carbon nanotube. In this system, the inner tube was fixed as the water channel, while the exterior was moved similarly to the piston motion along the axial direction to induce pumping force. Molecular dynamics simulations confirmed that the water flux is sensitive to the motion velocity of the outer tube so that giant and controllable unidirectional water flow can be achieved in this system by varying the velocity. The enhancement of the pumping ability mainly results from the carbon–water van der Waals driving forces of the exterior tube and the osmosis pressure of the water reservoir. This design may open a new way for water pumping in the field of nanodevices. read less

Abstract: Graphene can be made auxetic through the introduction of vacancy defects. This results in the thinnest negative Poisson's ratio material at ambient conditions known so far, an effect achieved via a nanoscale de-wrinkling mechanism that mimics the behavior at the macroscale exhibited by a crumpled sheet of paper when stretched. read less

Abstract: The process of graphene cleaning of a copper film by bombarding it with Ar13 clusters is investigated by the molecular dynamics method. The kinetic energies of the clusters are 5, 10, 20, and 30 eV and the incident angles are θ = 90°, 75°, 60°, 45°, and 0°. It is obtained that the cluster energy should be in the interval 20 eV–30 eV for effective graphene cleaning. There is no cleaning effect at vertical incidence (θ = 0°) of Ar13 clusters. The bombardments at 45° and 90° incident angles are the most effective on a moderate and large amount of deposited copper, respectively. read less

Abstract: We theoretically and numerically investigate the growth of buckling-driven wrinkles in graphene monolayers. It is found that the growth of buckling-driven wrinkles in a graphene monolayer is remarkably chirality- and size-dependent. In small sizes, the flexural response of a graphene sheet cannot be accurately described by the classical Euler regime, and the non-continuum effect leads to zigzag-along-preferred buckling. With the increase of size, the width/length ratio α of the compressed region plays an important role in the growth of buckling-driven wrinkles. When α < 0.5, the oblique buckling happens in armchair-along compression; when 0.5 < α < 1.0, the effect of edge warp leads to zigzag-along-preferred buckling. When 1.0 < α < 3.0, the potential energy density difference due to chiral bending stiffness leads to armchair-along-preferred buckling. When α > 3.0, the non-continuum effect and chiral bending stiffness can both be neglected, and the buckling in a graphene monolayer is isotropic. The chirality-along-preferred transition of compressed buckling in a graphene monolayer leads to an improved fundamental understanding of the dynamics mechanism of graphene-based nanodevices, especially for the nanodevices with high frequency response. read less

Abstract: A stable rotational transmission system is designed with a single-walled carbon nanotube (SWCNT)-based motor and double-walled carbon nanotubes (DWCNTs)-based bearing. The system response is investigated using molecular dynamics (MD) simulation. It is found that the rotating motor can actuate the rotation of the inner tube in bearing because of the attraction between the two adjacent coaxial ends of motor and rotor (the inner tube in bearing). To have a stable nanostructure, each carbon atom on the adjacent ends of motor and rotor is bonded with a hydrogen atom. To obtain a stable high-speed rotational transmission system, both an armchair and a zigzag model are used in MD simulation. In each model, the motor with different diameters and rotational speeds is employed to examine the rotational transmission of corresponding DWCNTs. It is demonstrated that the long range van der Waals interaction between the adjacent ends of motor and rotor leads to a stable configuration of the adjacent ends, and further leads to a stable rotation of rotor when driven by a high-speed motor. As compared with the armchair model, the rotor in the zigzag model could reach a stable rotation mode much easier. read less

Abstract: We study the mechanical behavior and fracture of graphene nanomeshes (GNMs) consisting of hexagonal lattices of unpassivated circular pores based on molecular-dynamics simulations of uniaxial tensile deformation tests. We analyze the GNMs' mechanical response as a function of their porosity for porosities up to 80%. We find that the fracture strain exhibits a minimum at a porosity ∼15%, which marks the onset of a transition in the mechanical behavior of the nanomeshes; beyond this critical porosity, the GNM ductility increases and the toughness remains practically constant with increasing porosity. The mechanism of crack initiation and propagation is characterized in both cases of mechanical response. read less

Abstract: Graphyne, a novel carbon allotrope, is a two-dimensional lattice of sp2+sp1 hybridization-type carbon atoms, similar to graphene. The initiation and development of wrinkles in single-layer graphynes (α-, β-, γ-, and 6, 6, 12-graphyne) subjected to in-plane circular shearing are investigated. In comparison with graphene, wrinkle pattern and profile characterization in relation to wave number, wavelength and amplitude of graphynes are extensively explored using classic molecular-dynamics (MD) simulations. Unlike graphene, the wave numbers of graphynes increase with increasing rotational angles; the wavelengths reduce correspondingly. The amplitudes show an increasing trend, with some local drops when the rotational angles increase. The drops occur as the positions of the wave numbers increase. Graphynes have superior fracture properties to graphene, despite the densities of graphynes being far lower. The fracture rotational angles depend on the percentages of acetylenic linkages in the graphyne structures: the more acetylenic linkages, the larger the fracture rotational angles. Meanwhile, acetylenic linkages also affect the bond length strains of the graphynes during the wrinkling process. The influences of the temperature on the fracture rotational angles are also examined to obtain further insights into the mechanical properties of such kinds of carbon allotropes. The achieved results can be used as guidelines for the wrinkling control and potential applications of graphynes. read less

Abstract: Rotation of the inner tube in a double-walled carbon nanotube (DWCNT) system with a fixed outer tube is investigated and found to be inducible by a relatively high uniform temperature (say, 300 K). We also found the mechanism of a gradientless temperature-driven rotating motor lies in the inner tube losing its geometric symmetry in a high-temperature field. This mechanism can be taken as a guide for designing a motor from such a bi-tube system. Using a computational molecular dynamics (CMD) approach and the adaptive intermolecular reactive empirical bond order (AIREBO) potential, the dynamic behavior of a bi-tube system subjected to uniformly distributed temperature is studied. In particular, the effects of environmental temperature, boundary conditions of the outer tube, and intertube gap on the dynamic behavior of the bi-tube system are investigated. Numerical examples show that a bi-tube system with the inner tube having 0.335 nm of interlayer gap produces the highest rotational speed. read less

Abstract: Among the most fascinating nanostructure morphologies are spirals, hybrids of somewhat obscure topology and dimensionality with technologically attractive properties. Here, we investigate mechanical and electromechanical properties of graphene spirals upon elongation by using density-functional tight-binding, continuum elasticity theory, and classical force field molecular dynamics. It turns out that electronic properties are governed by interlayer interactions as opposed to strain effects. The structural behavior is governed by van der Waals interaction: in its absence spirals unfold with equidistant layer spacings, ripple formation at spiral perimeter, and steadily increasing axial force; in its presence, on the contrary, spirals unfold via smooth local peeling, complex geometries, and nearly constant axial force. These electromechanical trends ought to provide useful guidelines not only for additional theoretical investigations but also for forthcoming experiments on graphene spirals. read less

Abstract: Defects are unavoidable during synthesizing and fabrication of graphene based nanoelecromechanical systems. This paper presents a comprehensive molecular dynamics simulation study on the mechanical properties of finite graphene with vacancy defects. We characterize the strength and stiffness of graphene using the concept of surface stress in three-dimensional crystals. Temperature and strain rate dependent atomistic model is also presented to evaluate the strength of defective graphene. Free edges have a significant impact on the stiffness; the strength, however, is less affected. The vacancies exceedingly degrade the strength and the stiffness of graphene. These findings provide a remarkable insight into the strength and the stiffness of defective graphene, which is critical in designing experimental and instrumental applications. read less

Abstract: Suspended graphene exhibits distinct behavior in which nanoscale friction first increases and then decreases with load; this is in contrast to the monotonic increase of friction with load exhibited by most materials, including graphene supported by a substrate. In this work, these friction trends are reproduced for the first time using molecular dynamics simulations of a nanoscale probe scanning on suspended and supported graphene. The atomic-scale detail available in the simulations is used to correlate friction trends to the presence and size of a wrinkle on the graphene surface in front of the probe. The simulations also provide information about how frictional load dependence is affected by the size of the graphene, the size of the probe, and the strength of the interaction between graphene and probe. read less

Abstract: We present the molecular dynamics study of benzene molecules confined into the single wall carbon nanotube. The local structure and orientational ordering of benzene molecules are investigated. It is found that the molecules mostly group in the middle distance from the axis of the tube to the wall. The molecules located in the vicinity of the wall demonstrate some deviation from planar shape. There is a tilted orientational ordering of the molecules which depends on the location of the molecule. It is shown that the diffusion coefficient of the benzene molecules is very small at the conditions we report here. © 2015 Wiley Periodicals, Inc. read less

Abstract: The thermal conductances of the carbon nanotube (CNT) junctions that would be found in a CNT aerogel are predicted using molecular dynamics simulations. At a temperature of 300 K, the thermal conductance of a perpendicular junction converges to 40 pW/K as the CNT lengths approach 100 nm. The key geometric parameter affecting the thermal conductance is the angle formed by the two CNTs. At pressures above 1 bar, the presence of a surrounding gas leads to an effective increase in the junction thermal conductance by providing a parallel path for energy flow. read less

Abstract: Using molecular-dynamics simulations, we study the thermal transport properties of graphene nanomeshes (GNMs) as a function of material density, pore morphology, pore edge passivation, and the lattice arrangement of the nanomesh pores. Relations for the density dependence of the GNMs' thermal conductivity are established. For GNMs with circular pores, we find that the thermal conductivity is an exponential function of the GNM's neck width with a very weak dependence on the pore lattice structure and pore edge passivation. For GNMs with elliptical pores, the thermal conductivity becomes anisotropic and this anisotropy becomes stronger with decreasing GNM density. read less

Abstract: A multi-scale theoretical model is presented that is the first to offer quantitative agreement with experimental measurements of self-retraction and oscillation of bilayer graphene. The model integrates density-functional theory calculations of the energetics driving flake retraction and molecular-dynamics simulations capturing the dynamic response of laterally-offset rough surfaces. We demonstrate that nanoscale roughness explains self-retraction motion and propose a recipe for tuning that motion by controlling friction. read less

Abstract: By using molecular dynamics simulations, the mechanical properties and failure mechanisms of a graphene sheet containing bi-grain-boundaries were examined. The results reveal that both temperature and density of defects play central roles in the mechanical characteristics of graphene containing bi-grain-boundaries. By increasing the temperature, the tensile strength and fracture strain significantly decrease. The graphene containing high density defects is much stronger than that containing lower density defects. The dependence of Young's modulus on temperature is also investigated. The results also show that the failure processes of graphene sheets containing bi-grain-boundaries are dominated by brittle cracking. read less

Abstract: The interfacial nanoroughness of liquid plays an important role in the reliability of liquid lenses, capillary waves, and mass transfer in biological cells [Grilli et al., Opt. Express 16, 8084 (2008), Wang et al., IEEE Photon. Technol. Lett. 18, 2650 (2006), and T. Fukuma et al., 92, 3603 (2007)]. However, the nanoroughness of liquid is hard to visualize or measure due to the instability and dynamics of the liquid-gas interface. In this study, we blanket a liquid water surface with monolayer graphene to project the nanoroughness of the liquid surface. Monolayer graphene can project the surface roughness because of the extremely high flexibility attributed to its one atomic thickness. The interface of graphene and water is successfully mimicked by the molecular dynamics method. The nanoroughness of graphene and water is defined based on density distribution. The correlation among the roughness of graphene and water is developed within a certain temperature range (298-390 K). The results show that the roughness of water surface is successfully transferred to graphene surface. Surface tension is also calculated with a simple water slab. The rise of temperature increased the roughness and decreased the surface tension. Finally, the relationship between graphene roughness and surface tension is fitted with a second-order polynomial equation. read less

Abstract: Graphene has rapidly become one of the most popular materials for technological applications and a test material for new condensed matter ideas. This paper reviews the mechanical properties of graphene and effects related to them that have recently been discovered experimentally or predicted theoretically or by simulation. The topics discussed are of key importance for graphene's use in integrated electronics, thermal materials, and electromechanical devices and include the following: graphene transformation into other hybridization forms; stability to stretching and compression; ion-beam-induced structural modifications; how defects and graphene edges affect the electronic properties and thermal stability of graphene and related composites. read less

Abstract: This work deals with molecular dynamics simulations of Congo red (CR) interaction with carbon nanotubes (CNT). We studied several combinations of systems parameters in order to assess how the nanotube diameter and Congo red density affect the structure and stability of CNT–CR conjugates at various pH conditions. We found that, at the considered conditions, the CR binds strongly to the CNT surfaces and the CNT–CR conjugates are thermodynamically stable according to the determined values of the free energies. Adsorption on wider nanotubes is stronger than on the narrow ones and larger densities of CR on the CNT surfaces lead to weakening of binding energy per single CR molecule. Changes of pH, that is varying concentration of protonated and deprotonated forms of CR, lead to significant changes in binding energies as well as to qualitative changes of the structure of the adsorbed CR. It was found that at pH > 5.5 the CR molecules readily occupy inner cavities of the nanotubes. Upon lowering pH the occupation of the inner space of CNTs is strongly reduced and the preferred configuration is formation of a densely packed CR layer on the sidewalls of the CNT. This effect can potentially be utilized in pH controlled corking/uncorking of carbon nanotubes in water solutions. read less

Abstract: Graphyne is an allotrope of graphene. The mechanical properties of graphynes (α-, β-, γ- and 6,6,12-graphynes) under uniaxial tension deformation at different temperatures and strain rates are studied using molecular dynamics simulations. It is found that graphynes are more sensitive to temperature changes than graphene in terms of fracture strength and Young's modulus. The temperature sensitivity of the different graphynes is proportionally related to the percentage of acetylenic linkages in their structures, with the α-graphyne (having 100% of acetylenic linkages) being most sensitive to temperature. For the same graphyne, temperature exerts a more pronounced effect on the Young's modulus than fracture strength, which is different from that of graphene. The mechanical properties of graphynes are also sensitive to strain rate, in particular at higher temperatures. read less

Abstract: By employing molecular dynamics simulations, the evolution of deformation of a monolayer graphene sheet under a central transverse loading are investigated. Dependence of mechanical responses on the symmetry (shape) of the loading domain, on the size of the graphene sheet, and on temperature, is determined. It is found that the symmetry of the loading domain plays a central role in fracture strength and strain. By increasing the size of the graphene sheet or increasing temperature, the tensile strength and fracture strain decrease. The results have demonstrated that the breaking force and breaking displacement are sensitive to both temperature and the symmetry of the loading domain. In addition, we find that the intrinsic strength of graphene under a central load is much smaller than that of graphene under a uniaxial load. By examining the deformation processes, two failure mechanisms are identified namely, brittle bond breaking and plastic relaxation. In the second mechanism, the Stone-Wales transformation occurs. read less

Abstract: : The proposed work was focused on understanding the capabilities of polymeric materials to form interfacial structures around carbon nanotubes and other nano-carbon materials. The proposed effort led to the development of a new processing route for dispersing nano-carbons in dilute polymer solutions. This dispersion process involved steps of sonication, shearing, and crystallization. The specific combination of these processes resulted in the formation of polymer interfacial growth (i.e., interphase structures) on the nano-carbon surfaces. The interphase formed consisted of either extended-chain or folded-chain polymer crystals depending on the processing route used. This processing approach for the dispersion of nano-carbons and formation of polymer interphase was implemented into fiber processing procedures. In general, these studies showed that the inclusion of interphase structures in the composite fibers led to dramatic increases in the mechanical properties. Beyond mechanical enhancement, the composite fiber morphology was also examined to understand the fundamental links between the processing route use and the resultant structure-property relationship. read less

Abstract: Nanoporous graphene (NPG) shows tremendous promise as an ultra-permeable membrane for water desalination thanks to its atomic thickness and precise sieving properties. However, a significant gap exists in the literature between the ideal conditions assumed for NPG desalination and the physical environment inherent to reverse osmosis (RO) systems. In particular, the water permeability of NPG has been calculated previously based on very high pressures (1000-2000 bars). Does NPG maintain its ultrahigh water permeability under real-world RO pressures (<100 bars)? Here, we answer this question by drawing results from molecular dynamics simulations. Our results indicate that NPG maintains its ultrahigh permeability even at low pressures, allowing a permeate water flux of 6.1 × 10−15 l/h bar per pore [Corrected], or equivalently 1041 ± 20 l/m(2)-h-bar assuming a nanopore density of 1.7 × 10(13) cm(-2). read less

Abstract: The recent capability of synthesizing large-scale crumpled graphene-related 2D materials has motivated intensive efforts to boost its promising applications in electronics, energy storage, composites and biomedicine. As deformation of graphene-related 2D materials can strongly affect their properties and the performance of graphene-based devices and materials, it is highly desirable to attain subtle control of reversible wrinkling and crumpling of graphene. Graphyne, a 2D lattice of sp(2)- and sp(1)-hybridized carbons similar to graphene, has remained unexplored with respect to its crumpling behavior. Here we employ molecular dynamics simulation to explore the behavior of graphynes under geometric confinement across various temperatures, sizes, and crumpling rates and compare them to graphene under the same conditions, with a focus on the mechanical stabilizing mechanisms and properties of the crumpled structures. The lower density of graphynes creates less deformation-induced bending energy than graphene; as such the graphynes exhibit a markedly increased propensity for stable crumpling. It is also shown that the crumpled 2D carbon materials demonstrate the hardness and bulk modulus of an equivalent magnitude with crumpled graphene, with the most important behavior-determining factor being the number of linking sp(1)-hybridized carbons in the material. Our results show that irrespective of the initial geometry and crumpling rate, the final structures present intriguing and useful properties which can be incorporated into crumpled graphene structures. read less

Abstract: A combination of computational and experimental methods was implemented to understand and confirm that conformational changes of a polymer [specifically polyacrylonitrile (PAN)] vary with the dispersion quality and confinement between single-wall carbon nanotubes (SWNT) in the composite fibers. A shear-flow gel-spinning approach was utilized to produce PAN-based composite fibers with high concentration (i.e., loading of 10 wt %) of SWNT. Dispersion qualities of SWNT ranging from low to high were identified in the fibers, and their effects on the structural morphologies and mechanical properties of the composites were examined. These results show that, as the SWNT dispersion quality in terms of distribution in the fiber and exfoliation increases, PAN conformations were confined to the extended-chain form. Full atomistic computational results show that the surface interaction energy between isolated PAN and SWNT was not preferred, leading to the self-agglomeration of PAN. However, confinement of the polymer chains between SWNT bundles or individual tubes (i.e., molecular crowding) resulted in large increases in the PAN-SWNT interaction energy. In other words, the crowding of polymer chains by the SWNT at high concentrations can promote extended-chain conformational development during fiber spinning. This was also evidenced experimentally by the observance of significantly improved PAN orientation and crystallization in the composite. Ultimately this work provides fundamental insight toward the specific structural changes capable at the polymer/nanotube interface which are important toward improvement of the effective contribution of the SWNT to the mechanical performance of the composite. read less

Abstract: In this study, analytical expressions are introduced to provide a better understanding of carbon nanotubes (CNTs) curvature on the overall behavior of nanocomposites. The curviness of CNT is modeled as the wave geometries, and the transformed physical characteristics are applied to micromechanical framework. Since five independent elastic constants of CNTs are essential to derive the waviness effect, atomistic molecular statics simulations with varying nanotube radii are conducted. Influences of CNT curviness on the effective stiffness of the nanocomposites are analyzed, noting that the curvature effect is significantly influential on the effective stiffness of the nanocomposites, and it may improve or reduce the reinforcing effect depending on the orientation of CNTs. In addition, the predictions are compared with experimental data of the CNT-reinforced nanocomposites to assess the reliability of the proposed method. The developed constitutive model is expected to be used to determine the volume concentration of the reinforcing CNTs and mechanical responses of CNT-reinforced composites under various CNT curvature, radius, and orientation conditions. read less

Abstract: We present results of a reverse non-equilibrium molecular dynamics study of thermal transport in single-walled carbon nanotube (SWCNT)-graphene junctions comprised of carbon-carbon (C-C) bonds with either sp2 or mixed sp2/sp3 hybridization. In both cases, a finite interfacial thermal resistance is observed at the SWCNT-graphene junctions for thermal transport in the out-of-plane direction. The interfacial thermal resistance at the junctions is attributed to the combined effects of scattering of the phonons at the SWCNT-graphene junctions due to the presence of distorted sp2 bonds in the junction region and the change in dimensionality of the medium along the phonon transport path as the phonons propagate from SWCNT pillars (quasi-1D) to graphene sheet (2D) and then again to SWCNTs. Moreover, the thermal resistance is found to depend on the C-C bond hybridization at the intramolecular junctions with mixed sp2/sp3 hybridization showing a higher interfacial resistance when compared to pure sp2 bonding. Therm... read less

Abstract: In the growth of polycrystalline graphene via chemical vapor deposition, grain boundaries (GBs) were shown to be energetically favorable sites for hydrogenation. Thus, it is of both scientific interest and technological significance to understand how hydrogenation on GBs affects the mechanical properties of polycrystalline graphene. Here we perform molecular dynamics simulations to investigate the mechanical properties of hydrogenated polycrystalline graphene. Our simulations reveal that the fracture strength of hydrogenated polycrystalline graphene is significantly reduced by the combined weakening effect of bond prestraining in highly defective GBs and sp3 hybridization of hydrogenated GB atoms. In addition, this reduction in fracture strength due to GB hydrogenation is observed in polycrystalline graphene samples of different grain sizes ranging from 2.5 to 10 nm. Our findings show that the loss of mechanical strength due to GB hydrogenation must be taken into account in the application of polycrystall... read less

Abstract: We use molecular dynamics method to study the strengthening effect of graphene-metal nanolayered composites under shock loading. The graphene interfaces have the advantages of both strong and weak interfacial features simultaneously, which solves a strengthening paradox of interfacial structures. On one hand, the weak bending stiffness of graphene leads to interlayer reflections and weakening the shock wave. On the other hand, the strong in-plane sp2-bonded structures constrain the dislocations and heal the material. The elastic recovery due to graphene interfacial constraints plays an important role in the strengthening effect, and the shock strength can be enhanced by decreasing the interlayer distance. This interface with strong/weak duality should lead to an improved fundamental understanding on the dynamic mechanism of composites with interfacial structures. read less

Abstract: Wrinkles in graphene with desirable morphology have practical significance for electronic applications. Here we carry out a systematic molecular dynamics study of the wrinkling instability of graphene on substrate-supported nanoparticles (NPs). At a large NP dispersion distance, a monolayer graphene adheres to the substrate and bulges out locally to wrap around individual NPs, forming isolated dome-shaped protrusions. At a small NP dispersion distance, tunneling wrinkles form in graphene to bridge the NP-induced protrusions. A critical NP dispersion distance for the onset of tunneling wrinkle instability of graphene is determined as a function of the NP size. The prediction from the modeling study agrees well with recent experimental observations. Results from the present study offer further insights into the formation of desirable wrinkles in graphene deposited on a substrate with engineered protrusions and, thus, can potentially enable novel design of graphene-based electronics. read less

Abstract: This study investigates heat dissipation at carbon nanotube (CNT) junctions supported on silicon dioxide substrate using molecular dynamics simulations. The temperature rise in a CNT (~top CNT) not making direct contact with the oxide substrate but only supported by other CNTs (~bottom CNT) is observed to be hundreds of degree higher compared to the CNTs well-contacted with the substrate at similar power densities. The analysis of spectral temperature decay of CNT-oxide system shows very fast intra-tube energy transfer in a CNT from high frequency band to intermediate frequency bands. The low frequency phonon band (0-5 THz) of top CNT shows two-stage energy relaxation which results from the efficient coupling of low frequency phonons in the CNT-oxide system and the blocking of direct transport of high and intermediate frequency phonons of top CNT to the oxide substrate by bottom CNT. read less

Abstract: Using non-equilibrium molecular dynamics method, we investigate thermal rectification (TR) in hybrid pristine carbon nanotube (PCNT) and hydrogenated carbon nanotube (HCNT) structures. The interface thermal resistance of the junction is dependent on the direction of thermal transport, leading to TR. We show that by selecting nanotubes of smaller diameters, and/or increasing the hydrogen coverage of HCNT, the TR can be amplified. The observed TR does not decrease by increasing the system length, which presents PCNT/HCNT system as a promising thermal rectifier at room temperature. read less

Abstract: The oscillatory behavior of a double-walled carbon nanotubes with a rotating inner tube is investigated using molecular dynamics simulation. In the simulation, one end of the outer tube is assumed to be fixed and the other is free. Without any prepullout of the rotating inner tube, it is interesting to observe that self-excited oscillation can be triggered by nonequilibrium attraction of the ends of two tubes. The oscillation amplitude increases until it reaches its maximum with decrease of the rotating speed of the inner tube. The oscillation of a bitube is sensitive to the gap between two walls. Numerical results also indicate that the zigzag/zigzag commensurate model with a larger gap of >0.335 nm can act as a terahertz oscillator, and the armchair/zigzag incommensurate model plays the role of a high amplitude oscillator with the frequency of 1 GHz. An oblique chiral model with a smaller gap of <0.335 nm is unsuitable for the oscillator because of the steep damping of oscillation. read less

Abstract: We present investigations of pull-out tests on CNTs embedded in palladium by means of molecular dynamics (MD) and compare our results of maximum pull-out forces with values of nano scale in situ pull-out tests inside a scanning electron microscope (SEM). Our MD model allows the investigation of crucial influencing parameters on the interface behaviour, like CNT diameter, intrinsic CNT defects and functional groups. For the experiments we prepared simple specimens using silicon substrates and wafer level compliant technologies. We realised the nano scale experiment with a nanomanipulation system supporting an AFM cantilever with known stiffness as a force sensing element inside a SEM. Greyscale correlation has been used to evaluate the cantilever deflection. From simulations derived maximum pull-out forces are approximately 17 nN and depend on the existence of intrinsic defects or functional groups and weakly on temperature. Experimentally obtained maximum pull-out forces with values between 16-29 nN are in good agreement with the computational predictions. Our results are of significant interest for the design and a failure-mechanistic treatment of future mechanical sensors with integrated single-walled CNTs showing high piezoresistive gauge factor or other nano scale systems incorporating CNT-metal interfaces. read less

Abstract: Molecular dynamics simulation has been performed to study melting behavior of aluminum nanowires (NWs) encapsulated in armchair single-walled carbon nanotubes (SWCNTs). Results show that intriguing phenomena may appear when the diameters of the inner NWs exceed a threshold value that melting occurs first at the inner layers, followed by the diffusion of the inner atoms toward the surface layers. Two melting temperatures can be obtained while the first one is even lower than that of the free-standing one in some cases and the second one is slightly higher than the bulk. The lengths of the tubes also have a great effect on the melting behavior of the inner NWs. This abnormal melting behavior is might due to the van der Waals potential well in SWCNTs. read less

Abstract: The malleable nature of atomically thin graphene makes it a potential candidate material for nanoscale origami, a promising bottom-up nanomanufacturing approach to fabricating nanobuilding blocks of desirable shapes. The success of graphene origami hinges upon precise and facile control of graphene morphology, which still remains as a significant challenge. Inspired by recent progresses on functionalization and patterning of graphene, we demonstrate hydrogenation-assisted graphene origami (HAGO), a feasible and robust approach to enabling the formation of unconventional carbon nanostructures, through systematic molecular dynamics simulations. A unique and desirable feature of HAGO-enabled nanostructures is the programmable tunability of their morphology via an external electric field. In particular, we demonstrate reversible opening and closing of a HAGO-enabled graphene nanocage, a mechanism that is crucial to achieve molecular mass uptake, storage, and release. HAGO holds promise to enable an array of carbon nanostructures of desirable functionalities by design. As an example, we demonstrate HAGO-enabled high-density hydrogen storage with a weighted percentage exceeding the ultimate goal of US Department of Energy. read less

Abstract: The influence of vacancy defects on the electronic and phonon properties of graphene is studied with the models based on a unit cell of 180 carbon atoms and of 1, 2, 3, 6, and 24 vacancies. Ordered, with one defect per unit cell, and non‐ordered (randomly arranged) vacancies are calculated with first‐principle and molecular dynamics methods. Randomly oriented vacancies lead to a creation of characteristic V1(5‐9) defects and the amorphization of graphene with different rings. Electronic and phonon densities of states are analyzed. The switching of the electrical conductivity from metal to semiconductor type is observed when increasing the defect sizes from a single vacancy to large clusters. The characteristic phonon modes are found in all these cases for their future experimental identification. Novel types of devices are proposed via doping of defective graphene. read less

Abstract: A theoretical model is developed to investigate the mechanical behavior of closely packed carbon nanoscrolls (CNSs), the so-called CNS crystals, subjected to uniaxial lateral compression/ decompression. Molecular dynamics simulations are performed to verify the model predictions. It is shown that the compression behavior of a CNS crystal can exhibit strong hysteresis that may be tuned by an applied electric field. The present study demonstrates the potential of CNSs for applications in energy-absorbing materials as well as nanodevices, such as artificial muscles, where reversible and controllable volumetric deformations are desired. read less

Abstract: Thermophoresis has been emerging as a novel technique for manipulating nanoscale particles. Materials with good thermal conductivity and low surface friction, such as graphene, are best suited to serve as a platform for solid-solid transportations or manipulations. Here we employ nonequilibrium molecular dynamics simulations to explore the feasibility of utilizing a thermal gradient on a large graphene substrate to control the motion of a small graphene nanoflake on it. Attempts to systematically investigate the mechanism of graphene-graphene transportation have centered on the fundamental driving mechanism of the motion and the quantitative effect of significant parameters such as temperature gradient and geometry of graphene on the motion of the nanoflake. Simulation results have demonstrated that temperature gradient plays the pivotal role in the evolution of the motion of the nanoflake on the graphene surface. Also, the geometry of nanoflakes has presented an intriguing signature on the motion of the nanoflake, which shows the nanoflakes with a circular shape move slower but rotate faster than other shapes with the identical area. It reveals that edge effects can stabilize the angular motion of thermophoretically driven particles. An interesting relation between the effective initial driving force and temperature gradient has been quantitatively captured by employing the steered molecular dynamics. These findings will provide fundamental insights into the motion of nanodevices on a solid surface due to thermophoresis, and will offer the novel view for manipulating nanoscale particles on a solid surface in techniques such as cell separation, water purification, and chemical extraction. read less

Abstract: Molecular dynamics simulation and the two-temperature method are carried out to model the effects of substrate roughness as well as electron–phonon coupling on thickness-dependent friction on graphene. It is found that substrate roughness can significantly enhance friction of graphene, which is orders of magnitude larger than that on smooth substrate due to puckering effect. Additionally, the adhesive force between graphene and substrate plays opposite roles for smooth and rough substrates. While on a smooth substrate, a larger adhesion hinders the wrinkle formation in graphene, therefore suppressing friction, on a rough substrate, adhesion helps induce atomic roughness in graphene and leads to friction enhancement. We also incorporate electron–phonon coupling into the atomistic modelling through a two-temperature method, and discover that its effect on friction is very small compared to that of roughness. read less

Abstract: Defects are generally believed to deteriorate the superlative performance of graphene-based devices but may also be useful when carefully engineered to tailor the local properties and achieve new functionalities. Central to most defect-associated applications is the defect coverage and arrangement. In this work, we investigate, by molecular dynamics simulations, the mechanical properties and fracture dynamics of graphene sheets with randomly distributed vacancies or Stone–Wales defects under tensile deformations over a wide defect coverage range. With defects presented, an sp–sp2 bonding network and an sp–sp2–sp3 bonding network are observed in vacancy-defected and Stone–Wales-defected graphene, respectively. The ultimate strength degrades gradually with increasing defect coverage and saturates in the high-ratio regime, whereas the fracture strain presents an unusual descending–saturating–improving trend. In the dense vacancy defect situation, the fracture becomes more plastic and super-ductility is observed. Further fracture dynamics analysis reveals that the crack trapping by sp–sp2 and sp–sp2–sp3 rings and the crack-tip blunting account for the ductile fracture, whereas geometric rearrangement on the entire sheet for vacancy defects and geometric rearrangement on the specific defect sites for Stone–Wales defects account for their distinctive rules of the evolution of the fracture strain. read less

Abstract: Graphene scrolls have been widely investigated for applications in electronics, sensors, energy storage, etc. However, graphene scrolls with tens of micrometers in length and with other materials in their cavities have not been obtained. Here nanowire templated semihollow bicontinuous graphene scroll architecture is designed and constructed through "oriented assembly" and "self-scroll" strategy. These obtained nanowire templated graphene scrolls can achieve over 30 μm in length with interior cavities between the nanowire and scroll. It is demonstrated through experiments and molecular dynamic simulations that the semihollow bicontinuous structure construction processes depend on the systemic energy, the curvature of nanowires, and the reaction time. Lithium batteries based on V3O7 nanowire templated graphene scrolls (VGSs) exhibit an optimal performance with specific capacity of 321 mAh/g at 100 mA/g and 87.3% capacity retention after 400 cycles at 2000 mA/g. The VGS also shows a high conductivity of 1056 S/m and high capacity of 162 mAh/g at a large density of 3000 mA/g with only 5 wt % graphene added which are 27 and 4.5 times as high as those of V3O7 nanowires, respectively. A supercapacitor made of MnO2 nanowire templated graphene scrolls (MGSs) also shows a high capacity of 317 F/g at 1A/g, which is over 1.5 times than that of MnO2 nanowires without graphene scrolls. These excellent energy storage capacities and cycling performance are attributed to the unique structure of the nanowire templated graphene scroll, which provides continuous electron and ion transfer channels and space for free volume expansion of nanowires during cycling. This strategy and understanding can be used to synthesize other nanowire templated graphene scroll architectures, which can be extended to other fabrication processes and fields. read less

Abstract: Molecular dynamics (MD) simulations have been performed for 10 keV C60 bombardment of an octane molecular solid at normal incidence. The results are analyzed using the steady-state statistical sputtering model (SS-SSM) to understand the nature of molecular motions and to predict a depth profile of a δ-layer. The octane system has sputtering yield of ~150 nm(3) of which 85% is in intact molecules and 15% is fragmented species. The main displacement mechanism is along the crater edge. Displacements between layers beneath the impact point are difficult because the nonspherically shaped octane molecule needs a relatively large volume to move into and the molecule needs to be aligned properly for the displacement. Since interlayer mixing is difficult, the predicted depth profile is dominated by the rms roughness and the large information depth because of the large sputtering yield. read less

Abstract: Molecular dynamics simulation is performed to analyze the deformation mechanism of graphene nanoribbons. When the load is applied along the zigzag orientation, tensile stress yields brittle fracture and compressive stress results in lattice shearing and hexagonal-to-orthorhombic phase transformation. Along the armchair direction, tensile stress produces lattice shearing and phase transformation, but compressive stress leads to a large bonding force. The phase transformation induced by lattice shearing is reversible for 17% and 30% strain in compressive loading along the zigzag direction and tensile loading along the armchair direction. The energy dissipation is less than 10% and resulting pseudo-elasticity enhances the ductility. read less

Abstract: The peeling process and average peeling force of a graphene (GE) sheet on a corrugated surface are investigated using molecular dynamics simulation. It is found that the peeling behaviour varies with the substrate surface roughness and the peeling angle. Three kinds of typically peeling behaviours include (a) GE sheet directly passing the valley of the substrate roughness; (b) bouncing off from the substrate; and (c) continuously peeling off similarly to that on a flat substrate. As a result, the average peeling force is strongly dependent of the peeling behaviours. Furthermore, some interesting phenomena are caught, such as partial detaching and partial sliding of GE sheet in the valley of the substrate roughness, which are mainly due to the effects of pre-tension in GE sheet and the reduction of friction resistance. The results in this paper should be useful for the design of nano-film/substrate systems. read less

Abstract: Experiments and simulations both suggest that the pressure experienced by an adsorbed phase confined within a carbon nanoporous material can be several orders of magnitude larger than the bulk phase pressure in equilibrium with the system. To investigate this pressure enhancement, we report a molecular-simulation study of the pressure tensor of argon confined in slit-shaped nanopores with walls of various models, including carbon and silica materials. We show that the pressure is strongly enhanced by confinement, arising from the effect of strongly attractive wall forces; confinement within purely repulsive walls does not lead to such enhanced pressures. Simulations with both the Lennard-Jones and Barker-Fisher-Watts intermolecular potentials for argon-argon interactions give rise to similar results. We also show that an increase in the wall roughness significantly decreases the in-pore pressure due to its influence on the structure of the adsorbate. Finally, we demonstrate that the pressures calculated from the mechanical (direct pressure tensor calculations) and the thermodynamic (volume perturbation method) routes yield almost identical results, suggesting that both methods can be used to calculate the local pressure tensor components in the case of these planar geometries. read less

Abstract: The propensity of carbon nanotubes (CNTs) to self-organize into continuous networks of bundles has direct implications for thermal transport properties of CNT network materials and defines the importance of clear understanding of the mechanisms and scaling laws governing the heat transfer within the primary building blocks of the network structures—close-packed bundles of CNTs. A comprehensive study of the thermal conductivity of CNT bundles is performed with a combination of non-equilibrium molecular dynamics (MD) simulations of heat transfer between adjacent CNTs and the intrinsic conductivity of CNTs in a bundle with a theoretical analysis that reveals the connections between the structure and thermal transport properties of CNT bundles. The results of MD simulations of heat transfer in CNT bundles consisting of up to 7 CNTs suggest that, contrary to the widespread notion of strongly reduced conductivity of CNTs in bundles, van der Waals interactions between defect-free well-aligned CNTs in a bundle have negligible effect on the intrinsic conductivity of the CNTs. The simulations of inter-tube heat conduction performed for partially overlapping parallel CNTs indicate that the conductance through the overlap region is proportional to the length of the overlap for CNTs and CNT-CNT overlaps longer than several tens of nm. Based on the predictions of the MD simulations, a mesoscopic-level model is developed and applied for theoretical analysis and numerical modeling of heat transfer in bundles consisting of CNTs with infinitely large and finite intrinsic thermal conductivities. The general scaling laws predicting the quadratic dependence of the bundle conductivity on the length of individual CNTs in the case when the thermal transport is controlled by the inter-tube conductance and the independence of the CNT length in another limiting case when the intrinsic conductivity of CNTs plays the dominant role are derived. An application of the scaling laws to bundles of single-walled (10,10) CNTs reveals that the transition from inter-tube-conductance-dominated to intrinsic-conductivity-dominated thermal transport in CNT bundles occurs in a practically important range of CNT length from ∼20 nm to ∼4 μm. read less

Abstract: The models of graphene nanoribbons (GNRs) with angles 0°, 30°, 60°, 90° and 120° were constructed to investigate the thermal conduction by using the reverse non-equilibrium molecular dynamics method. A substantially negative correlation between the thermal conductivity and the bent angle shows a nonlinear decline from 0° to 90°. It also shows that there is a little increase from 90° to 120° due to the edge effect. To weaken the edge effect, the nitrogen doping method is adopted to recompose the bent GNRs. The results show that it is effective for the thermal management, and a strict monotonous relationship between the thermal conductivity and bent angle can be obtained. In the meantime, an interesting phenomenon is observed that the GNRs with edge modification by N-doping can get a much better thermal conduction than original GNRs without edge modification. We can understand the physical mechanism by phonon analysis for these GNRs. The investigation implies that we can change the thermal conductivity of GNRs by design by N-doping. read less

Abstract: Thermalization in nonlinear systems is a central concept in statistical mechanics and has been extensively studied theoretically since the seminal work of Fermi, Pasta, and Ulam. Using molecular dynamics and continuum modeling of a ring-down setup, we show that thermalization due to nonlinear mode coupling intrinsically limits the quality factor of nanomechanical graphene drums and turns them into potential test beds for Fermi-Pasta-Ulam physics. We find the thermalization rate Γ to be independent of radius and scaling as Γ∼T*/εpre2, where T* and εpre are effective resonator temperature and prestrain. read less

Abstract: Graphyne, a new type of carbon allotropes, has attracted considerable attention in recent years. Using molecular dynamics simulations, we investigate the mechanical properties of four different graphynes (α-, β-, γ-, and 6,6,12-graphynes) functionalized with hydrogen. The simulations results show that hydrogenation can greatly deteriorate the mechanical properties of the graphynes. For the different graphynes with 100% H-coverage, the reduction in fracture stress depends on the percentage of acetylenic linkages in the graphyne structures: The more the acetylenic linkages, the larger the reduction. For the same graphyne, the reduction in fracture stress depends on the hydrogenation location, distribution, and coverage. Hydrogenation on the acetylenic linkages causes a larger reduction in fracture stress than that on the hexagonal rings. A line hydrogenation perpendicular to the tensile direction leads to a larger reduction in fracture stress than that when the line hydrogenation is parallel to the tensile direction. For random hydrogenation, the fracture stress and Young's modulus decrease rapidly at low H-coverage (<10%), and then level off with increasing coverage. The reduction in the mechanical properties due to hydrogenation is found to be related to the formation of weakened out-of-plane C-C bonds, which leads to earlier breaking of those bonds and subsequent fracture of the graphynes. The present study not only offers an in-depth understanding in the mechanical properties of hydrogenated graphynes and their fracture mechanisms but it also presents an important database for the design and practical applications of hydrogenated graphynes. read less

Abstract: Nanocarbon-based photovoltaics offer a promising new architecture for the next generation of solar cells. We demonstrate that a key factor determining the efficiency of single-walled carbon nanotube (SWCNT)/fullerene devices is the chirality of the SWCNT. This is shown via current density vs voltage measurements of nanocarbon devices prepared with (9,7), (7,6) and (6,5) SWCNTs, as well as density-functional theory (DFT) density of states calculations of C60 adsorbed onto the corresponding SWCNTs. The trends in efficiency are rationalized in terms of the relative energies of the fullerene and SWCNT conduction band energy levels. read less

Abstract: Transverse wave propagation in single-layer graphene sheet (SLGS) is studied via molecular dynamics simulation, continuum, and non-continuum analysis. We found that the propagation of transverse waves with frequency over 3 THz is remarkably chirality-dependent. Furthermore, the wave propagation in zigzag direction remains undistorted only when the frequency is below 16 THz, while this threshold is 10 THz in the armchair direction. The minimum permissible wavelength is proposed to explain the frequency limitation due to non-continuity. Our findings lead to an improved fundamental understanding on the vibration of graphene-based nanodevices and have potential applications in design and fabrication of nanoelectromechanical systems. read less

Abstract: Different types of defects can be introduced into graphene during material synthesis, and significantly influence the properties of graphene. In this work, we investigated the effects of structural defects, edge functionalisation and reconstruction on the fracture strength and morphology of graphene by molecular dynamics simulations. The minimum energy path analysis was conducted to investigate the formation of Stone-Wales defects. We also employed out-of-plane perturbation and energy minimization principle to study the possible morphology of graphene nanoribbons with edge-termination. Our numerical results show that the fracture strength of graphene is dependent on defects and environmental temperature. However, pre-existing defects may be healed, resulting in strength recovery. Edge functionalization can induce compressive stress and ripples in the edge areas of graphene nanoribbons. On the other hand, edge reconstruction contributed to the tensile stress and curved shape in the graphene nanoribbons. read less

Abstract: The mechanism of dissipation operative at the nanoscale remains poorly understood for most cases. In this work, using molecular dynamics simulations, we show that the unstable out-of-plane mode leads to the absorption of energy from the in-plane motion in graphene. The in-plane vibration modulates the potential energy profile for the out-of-plane modes. For the fundamental out-of-plane mode in the loading direction, the minimum of the potential energy shifts because of in-plane compressive strain. The structure takes a finite amount of time to relax to the new potential energy configuration. A hysteresis in the out-of-plane dynamics is observed when the time period of in-plane excitation becomes comparable to the time required for this relaxation. Increasing the stiffness of the out-of-plane modes by giving an initial tensile strain leads to a considerable decrease in dissipation rate. read less

Abstract: Discrete breathers (DBs) in graphane (fully hydrogenated graphene) are investigated using molecular dynamics simulations. It is found that the DB can be excited by applying an out-of-plane displacement on a single hydrogen atom of graphane. The vibration frequency of the DB lies either within the gap of the phonon spectrum of graphane or beyond its upper spectrum bound. Both soft and hard types of anharmonicity of the DB, which have not been found in the same system, are observed in graphane. The study shows that the DB is robust and its lifetime is affected by various factors including its anharmonicity type, its amplitude and frequency, and the force on the hydrogen atom that forms it, whose competition results in a complex mechanism for the lifetime determination. The investigation of the maximum kinetic energy of DBs reveals that they may function to activate or accelerate dehydrogenation of hydrogenated graphene at high temperatures. read less

Abstract: (Received 24 January 2013; revised manuscript received 14 June 2013; published 3 July 2013)Atomic force microscopy experiments and molecular dynamics simulations show that friction between ananoscale tip and atomically stepped surfaces of graphite is influenced by the environment. The presence of asmall amount of water increases friction at atomic steps, but does not strongly influence friction on flat terraces.DOI: 10.1103/PhysRevB.88.035409 PACS number(s): 68 read less

Abstract: The open topology of a carbon nanoscroll (CNS) inspires potential applications such as high capacity hydrogen storage. Enthusiasm for this promising application aside, one crucial problem that remains largely unexplored is how to shuttle the hydrogen molecules adsorbed inside CNSs. Using molecular dynamics simulations, we demonstrate two effective transportation mechanisms of hydrogen molecules enabled by the torsional buckling instability of a CNS and the surface energy induced radial shrinkage of a CNS. As these two mechanisms essentially rely on the nonbonded interactions between the hydrogen molecules and the CNS, it is expected that similar mechanisms could be applicable to the transportation of molecular mass of other types, such as water molecules, deoxyribonucleic acids (DNAs), fullerenes, and nanoparticles. read less

Abstract: Molecular dynamics simulations are performed to investigate the effect of surface energy on equilibrium configurations and self-collapse of carbon nanotube bundles. It is shown that large and reversible volumetric deformation of such bundles can be achieved by tuning the surface energy of the system through an applied electric field. The dependence of the bundle volume on surface energy, bundle radius, and nanotube radius is discussed via a dimensional analysis and determined quantitatively using the simulation results. The study demonstrates potential of carbon nanotubes for applications in nanodevices where large, reversible, and controllable volumetric deformations are desired. read less

Abstract: The wettability of defective graphene and its relation with the interfacial property of water have not been well tackled. By conducting molecular dynamics simulations on the interactions between water and graphene with some vacancy defects, we show that the contact angle is more sensitive to the Stone–Wales (SW) and single-vacancy (SV) defects than the double-vacancy (DV) defects. Density profiles indicate that the graphene sheet induces the interfacial water to change from a liquid into an ordered three-layer structure, which restricts the self-diffusion of water molecules seriously. Moreover, the effect of defects on the ordered liquid layer has also been discussed. Our results are helpful for controlling the wettability of graphene and developing the applications of graphene in nanodevices. read less

Abstract: Combining molecular dynamics and quantum transport simulations, we study the degradation of mobility in graphene nanoribbons caused by substrate-induced ripples. First, the atom coordinates of large-scale structures are relaxed such that surface properties are consistent with those of graphene on a substrate. Then, the electron current and low-field mobility of the resulting non-flat nanoribbons are calculated within the Non-equilibrium Green's Function formalism in the coherent transport limit. An accurate tight-binding basis coupling the σ- and π-bands of graphene is used for this purpose. It is found that the presence of ripples decreases the mobility of graphene nanoribbons on SiO2 below 3000 cm2/Vs, which is comparable to experimentally reported values. read less

Abstract: Molecular dynamics simulations were performed to investigate how the orientation of grain boundary (GB) affects the tensile mechanics of polycrystalline graphene, where two opposite GB groups, i.e., armchair (AC) and zigzag (ZZ)-oriented tilted GBs were considered for the anisotropic study. We found very close mechanical similarities between the two groups in misorientation angle effect and critical bond length effect to determine the tensile strength. Mono-atomic carbon chains (MACCs) were commonly generated at tensile failure in both groups, as bridged between fractured sections, yielding the considerably higher population density and achievable length (4.51 nm−2 and 1.47 nm, maximally) compared to pristine graphene. Notably, we found that polycrystalline graphene exhibited distinctly different behaviors in this MACC production depending on GB orientation, being 1.2–3.0 times denser and 1.6–5.0 times longer for ZZ-oriented GBs. Atomic stress analyses indicated that all key reactions emerging before tensile failure would not be affected by the GB orientation of polycrystalline graphene since the reactions only occurred along GBs, explaining why anisotropic mechanical GB response has not been observed so far, in contrast to the MACC dynamics occurring after tensile failure. read less

Abstract: Based on molecular dynamics simulations, the formation possibility of molecular junctions between two tailored graphene sheets under ultrafast laser irradiation was investigated. It was found that single layer graphene sheet can survive under significantly high intensity of laser beam. The fluctuations of graphene sheets in the plane and out of plane under laser irradiation provided the “driving power” for the possible joining process. The relative position of two graphene sheets can influence the joining difficulties and this influence was found to be attributed to the dangling bonds of edges. The saturation of dangling bonds can prompt the self-assembly of carbon networks in the joining area and lead to the connection of two graphene sheets. [doi:10.2320/matertrans.MD201213] read less

Abstract: Adhesive contacts between graphene sheets and corrugated surfaces are investigated. It is found that the final configuration between the graphene sheet and the substrate depends not only on the surface roughness of the substrate, but also on the length of graphene. A continuous transition, rather than a recent observation of ‘snap-through’ transition, is exhibited in our study. For a graphene sheet with a fixed length, it is easy to fully conform to the substrate of small roughness. Otherwise, the graphene sheet will remain flat on top of the corrugated substrate due to the unsatisfied bending energy or partially conform to the substrate due to the resistance of large interface friction. In order to reduce the effect of interface friction on the adhesive configuration, a new method, i.e. tilting the graphene sheet with a proper angle, is proposed. The tilting angle will significantly influence the final conformation of the adhesive interface. Some interesting types of behaviour are observed, such as rolling graphene, a double layer of graphene and fully adhesive contact, which is physically determined by the competition of thermal fluctuation and interfacial van der Waals interaction. read less

Abstract: Some findings in heterogeneous nucleation that the structural features of a growing crystal are usually inherited from the heterogeneous nucleus, although attracting more and more attention, are not yet well understood. Here we report numerical simulations of copper nucleation on bending graphene (BG) to explore the microscopic details of how the curved surface influences the freezing structure of the liquid metal. The simulation result clearly shows that copper atoms become layered at the solid-liquid interface in a "C"-shaped pattern resembling the BG. This kind of shape control decays with increasing distance from the wall and the outmost layers transform into twin crystal composed of two fcc wedges. It is found that the final structures have striking correlations with the curvature radius, central angle and arc length of the BG. Our study would provide an opportunity for comprehensive and satisfactory understanding of the heterogeneous nucleation on curved surfaces. read less

Abstract: In this paper we focus on the studies of graphene wrinkling, from its formation to collapse, and its dependence on aspect ratio and temperature using molecule dynamics simulation. Based on our results, the first wrinkle is not formed on the edge but in the interior of graphene. The fluctuations of edge slack warps drive the wrinkling evolution in graphene which is distinguished from the bifurcation in continuum film. There are several obvious stages in wrinkling progress, including incubation, infancy, youth, maturity and gerontism periods which are identified by the atomic displacement difference due to the occurrences of new wrinkles. The wrinkling progress is over when the C-C bonds in highly stretched corners are broken which contributes to the wrinkling collapse. The critical wrinkling strain, the wrinkling pattern and extent depend on the aspect ratio of graphene, the wrinkling level and collapsed strains do not. Only the collapsed strain is sensitive to the temperature, the other wrinkling parameters are independent of the temperature. Our results would benefit the understanding of the physics of graphene wrinkling and the design of nanomechanical devices by tuning the wrinkles. read less

Abstract: Cluster secondary ion mass spectrometry is now widely used for the characterization of nanostructures. In order to gain a better understanding of the physics of keV cluster bombardment of surfaces and nanoparticles (NPs), the effects of the atomic masses of the projectile and of the target on the energy deposition and induced sputtering have been studied by means of molecular dynamics simulations. 10 keV C60 was used as a model projectile and impacts on both a flat polymer surface and a metal NP were analyzed. In the first case, the mass of the impinging carbon atoms was artificially varied and, in the second case, the mass of the NP atoms was varied. The results can be rationalized on the basis of the different atomic mass ratios of the projectile and target. In general, the emission is at its maximum, when the projectile and target have the same atomic masses. In the case of the supported NP, the emission of the underlying organic material increases as the atomic mass of the NP decreases. However, it is always less than that calculated for the bare organic surface, irrespective of the mass ratio. The results obtained with C60 impacts on the flat polymer are also compared to simulations of C60 and monoatomic Ga impacts on the NP. read less

Abstract: We investigate the nucleation of carbon and hydrogen atoms in the gas phase to form large carbon chains, clusters and cages by reactive molecular dynamics simulations. We study how temperature, particle density, presence of hydrogen, and carbon inflow affect the nucleation of molecular moieties with different characteristics. read less

Abstract: We present a computational analysis of the morphology and adhesion energy of graphene on the surface of amorphous silica (a-SiO2). The a-SiO2 model surfaces obtained from the continuous random network model-based Metropolis Monte Carlo approach show Gaussian-like height distributions with an average standard deviation of 2.91 ± 0.56 A, in good agreement with existing experimental measurements (1.68–3.7 A). Our calculations clearly demonstrate that the optimal adhesion between graphene and a-SiO2 occurs when the graphene sheet is slightly less corrugated than the underlying a-SiO2 surface. From morphology analysis based on fast Fourier transform, we find that graphene may not conform well to the relatively small jagged features of the a-SiO2 surface with wave lengths of smaller than 2 nm, although it generally exhibits high-fidelity conformation to a-SiO2 topographic features. For 18 independent samples, on average the van der Waals interaction at the graphene/a-SiO2 interface is predicted to vary from Evd... read less

Abstract: CNT/metal interfaces under mechanical loads are investigated using molecular dynamics by simulating pull-out tests of single walled carbon nanotubes (CNTs) emdedded in single crystal gold lattices. As a result of our simulations we present obtained force-displacement data. We investigated the influence of two different Lennard Jones (LJ) coefficients pairs, two CNT types and three lattice directions of the gold matrix with respect to the embedding direction. Additionally we incorporated structural defects into our model and report on their influence. The change of the CNT type leads to a change in the maximum pull-out force. Here we attribute this to the change in CNT diameter, where a bigger diameter entails an increased maximum pull-out force. Changing the LJ coefficient pair has a strong impact on the maximum pull-out forces, where a higher bonding energy results in a higher maximum pull-out force. Defects also show a positive effect on the maximum pull-out force. The presented results have impact on bonding strength of CNT/metal interfaces. read less

Abstract: Different types of defects can be introduced into graphene during material synthesis, and significantly influence the properties of graphene. In this work, we investigated the effects of structural defects, edge functionalisation and reconstruction on the fracture strength and morphology of graphene by molecular dynamics simulations. The minimum energy path analysis was conducted to investigate the formation of Stone-Wales defects. We also employed out-of-plane perturbation and energy minimization principle to study the possible morphology of graphene nanoribbons with edge-termination. Our numerical results show that the fracture strength of graphene is dependent on defects and environmental temperature. However, pre-existing defects may be healed, resulting in strength recovery. Edge functionalization can induce compressive stress and ripples in the edge areas of graphene nanoribbons. On the other hand, edge reconstruction contributed to the tensile stress and curved shape in the graphene nanoribbons. read less

Abstract: Molecular dynamics computer simulations are used to elucidate the cross-linking processes induced by 0.6-50 keV C60 and Arn cluster bombardment in a C60 fullerite solid sample. The obtained results indicate the presence of a "chemical effect" when C 60 projectile is used. Namely, the bombarding C60 delivers additional, highly reactive, radicals which interact with the atoms of the fullerite sample, increasing the efficiency of the cross-linking process. The omission of those interactions in the analysis makes the C60 very similar to the case of the Ar18 bombardment. For Arn cluster bombardment, the initial energy per atom in the projectile is the parameter which has the predominant influence on the cross-linking process. Furthermore, a relationship between the energy thresholds for fragmentation of the target molecules and cross-linking initiation and the size of the Ar clusters is observed. Both of these thresholds decrease with increasing size of the projectile. © 2013 American Chemical Society. read less

Abstract: Hydrogenation of graphene leads to local bond distortion of each hydrogenated carbon atom. Therefore, programmable hydrogenation of graphene can open up new pathways to controlling the morphology of graphene and therefore enable the exploration of graphene-based unconventional nanomaterials. Using molecular dynamics simulations, we show that single-sided hydrogenation can cause the scrolling of graphene. If a proper size of the graphene is hydrogenated on one side, the graphene can completely scroll up into a carbon nanoscroll (CNS) that remains stable at room temperature. We perform extensive simulations to delineate a diagram in which three types of scrolling behaviour of partially single-sided hydrogenated graphene are identified in the parameter space spanned by the hydrogenation size and the graphene size. Such a diagram can serve as quantitative guidelines that shed important light on a feasible solution to address the challenge of fabricating high-quality CNSs, whose open topology holds promise to enable novel nanodevices. read less

Abstract: This study focuses on the microscopic modeling of 0-25 keV Bi 1-3-5 and C60 cluster impacts on three different targets (Au crystal, adsorbed Au nanoparticle, and organic solid), using molecular dynamics simulations, and on the comparison of the calculated quantities with recent experimental results, reported in the literature or obtained in our laboratory. The sputtering statistics are reported, showing nonlinearity of the sputtering yields with the number of cluster atoms at the same incident velocity for Bi1-5 bombardment. They are compared to experiments (especially for the organic target), and the microscopic explanation of the observations is analyzed. The results show that the respective behaviors and performances of the different projectiles are strongly dependent on the target, with clusters of heavy Bi atoms being more efficient at sputtering gold and, conversely, fullerene clusters inducing the largest sputtering yields of the organic material (mass matching). For organic targets, some important and novel conclusions of this work are the following: (i) The increase of the sputtering yield when going from Bi atoms to Bi clusters is insufficient to explain the much larger increase of characteristic ion yields, suggesting a projectile-dependent ionization probability. (ii) The extent of molecular fragmentation follows the order of Bi > Bi3 > Bi5 > C60, that is, softer emission with larger clusters. (iii) Even 5-10 keV Bi atoms create collective molecular motions and craters in the polymeric solid, though the collision cascades are rather dilute. Finally, a second series of simulations performed at low energies predict that 0.1-1 keV Bin clusters should not provide better results for sputtering and depth profiling than isoenergetic single atoms. © 2013 American Chemical Society. read less

Abstract: In this work molecular dynamics simulations are carried out to investigate the defect-mediated self-assembly of graphene paper from several layers of graphene sheets with vacancy defects. Tensile and shear deformations are applied to the obtained architectures to investigate both the in-plane and the out-of-plane mechanical properties. The effect of incipient defect coverage is analyzed and super-ductility is observed in the high defect density situation. While the stiffness and strength decrease with the increasing of incipient defect coverage under in-plane deformations, they increase under out-of-plane deformations, which can be attributed to the enhanced defect-induced interlayer cross-linking. Effects of crack-like flaws are also investigated to demonstrate the robustness of this structure. Our results demonstrate that defects, which are sometimes unavoidable and undesirable, can be engineered in a favorable way to provide a new approach for graphene-based self-assembly of vertically aligned architectures with mechanical robustness and high strength. read less

Abstract: Using planewave pseudopotential density functional theory and classical molecular dynamics simulations, we investigate the transport of noble gases through a family of two-dimensional hydrocarbon polymer membranes that generalize the “porous graphene” (PG) material synthesized by Bieri et al. by insertion of (E)-stilbene (ES) groups. We find that density functional theory overestimates the barrier height and empirical dispersion corrections underestimate the barrier height, compared to reference MP2/cc-pVTZ calculations on PG. The barrier height for noble gas transport is greatly reduced from PG to PG-ES1, but additional increases in the size of the pore in PG-ES2 and PG-ES3 lead to an attractive potential well instead of a repulsive barrier. Using the computed potential energy surfaces, we compute pressure- and temperature-driven tunneling probabilities of He isotopes, and refit an improved classical force-field. Using classical molecular dynamics simulations, we find that PG-ES1 has an He permeance of 6... read less

Abstract: Argon clusters are increasingly used for organic mass spectrometry and 3D imaging, but their interaction with complex samples is not well understood yet. In this study, the interaction of kiloelectronvolt argon cluster projectiles (Ar60–2000) with an organic crystal of polyethylene (PE) covered with gold nanoparticles (Au-NPs) is investigated by means of molecular dynamics simulations. The results are compared as a function of the impact point and also to previous simulations involving C60 and Au400 projectiles. According to the impact point, fragmentation of the polymer or the NP increases monotonically with the energy per atom in the projectile; i.e., the emission of fragments decreases monotonically with increasing cluster size at a given total energy. For impacts on the Au-NP, however, the sputtering yield of organic material exhibits a nonmonotonic dependence on the Ar cluster size, with a maximum for similar projectile and Au-NP sizes. At constant energy per atom, the emission of gold is additionall... read less

Abstract: Molecular dynamics computer simulations are used to investigate the ejection process of molecules from a benzene crystal bombarded with keV large Ar clusters. The effect of projectile size, the kinetic energy and the impact angle on the sputtering efficiency is investigated. The results show that although the sputtering yield depends on all projectile parameters, this dependence can be greatly simplified if the sputtering yield per projectile nucleon is expressed as a function of projectile kinetic energy per nucleon. A different dependence of the total sputtering yield on the impact angle has been observed for small and large projectiles. This effect is attributed to a ‘washing out’ mechanism. For large projectiles most of the organic molecules are ejected by gentle collective action of argon atoms. Proper selection of the projectile parameters allows for achieving conditions where only intact molecules are emitted. Copyright © 2012 John Wiley & Sons, Ltd. read less

Abstract: The thermal conductivity of gas‐permeated single‐walled carbon nanotube (SWCNT) aerogel (8 kg m−3 density, 0.0061 volume fraction) is measured experimentally and modeled using mesoscale and atomistic simulations. Despite the high thermal conductivity of isolated SWCNTs, the thermal conductivity of the evacuated aerogel is 0.025 ± 0.010 W m−1 K−1 at a temperature of 300 K. This very low value is a result of the high porosity and the low interface thermal conductance at the tube–tube junctions (estimated as 12 pW K−1). Thermal conductivity measurements and analysis of the gas‐permeated aerogel (H2, He, Ne, N2, and Ar) show that gas molecules transport energy over length scales hundreds of times larger than the diameters of the pores in the aerogel. It is hypothesized that inefficient energy exchange between gas molecules and SWCNTs gives the permeating molecules a memory of their prior collisions. Low gas‐SWCNT accommodation coefficients predicted by molecular dynamics simulations support this hypothesis. Amplified energy transport length scales resulting from low gas accommodation are a general feature of CNT‐based nanoporous materials. read less

Abstract: We report results from a molecular simulation study of aqueous rubidium bromide solutions confined in slit-shaped carbon pores 6.5–16 Å in width, which encompasses the range typically found in nanoporous carbon electrode materials. For each slit-pore model, the structure of the solvation shells surrounding the Rb+ and Br− ions in 0.1, 0.5 and 1.0 M solutions was examined using the ion–water radial distribution functions. The impact of pore geometry on solvation structure of ions was also investigated using a disordered carbon model that morphologically resembles real nanoporous carbon electrodes. Monte Carlo simulations were used to determine the propensity of ions to reside in pores of specific sizes in the model carbon, allowing the solvation structure of Rb+ and Br− ions to be analysed as a function of pore size. We find that a dramatic drop in the solvation number occurs in pore sizes below 10 Å for slit-shaped pores, while more complex geometries see a steady decrease in solvation number as pore size is decreased. Our results suggest that, when compared to the disordered carbon model, the slit-pore model may not provide qualitatively accurate predictions regarding the structural properties of electrolytes confined in complex electrode materials. read less

Abstract: Irradiation effects in polyethylene and cellulose were examined using molecular dynamics simulations. The governing reactions in both materials were chain scissioning and generation of small hydrocarbon and peroxy radicals. Recombination of chain fragments and cross-linking between polymer chains were found to occur less frequently. Crystalline cellulose was found to be more resistant to radiation damage than crystalline polyethylene. Statistics on radical formation are presented and the dynamics of the formation of radiation damage discussed. read less

Abstract: In current study we perform molecular dynamics simulations of nanoscratching of amorphous polystyrene. The AIREBO potential is utilized to describe intermolecular and intramolecular interactions in simulated polystyrene system. In addition, the effect of chain length on the deformation behavior of polystyrene chains, force variation, and increment of temperature and potential energy is studied. Our simulations results demonstrate the permanent deformation of polystyrene during nanoscratching is governed by chain sliding and deformation of chain itself, i.e.. change in chain structure and rotation of phenyl group. There is strong chain length dependence of the mobility of the chain, which in turn affects deformation behavior of polystyrene and machining force. The influence of chain mobility on temperature and potential energy variations is also discussed. read less

Abstract: A method has been developed to obtain uniform thin layers of the supramolecule cryptophane A (CrypA) on quartz crystals to produce high-sensitivity gravimetric methane gas sensors. Molecular mechanics and ab initio simulations were used for the first time to calculate the binding energy of methane to CrypA. The results justify the use of CrypA to selectively sense methane amongst a mixture of gases. CrypA molecules were synthesized by an improved 2-step trimerization method and characterized by 1H-NMR spectroscopy. The response of these sensors to different concentrations of methane was studied. The detection limit of 3ppm (i.e., 0.003%) with a sensitivity of 80mHz/ppm show significant improvement over the prior art. read less

Abstract: We present accurate calculations of friction in graphene films in configurations simulating the presence of an anchoring substrate. We find that a slider induces both out-of-plane and shear deformations, which increase with the thickness of the supported film. We elucidate the new frictional mechanism connected to shear layer motions, which is minimal for systems with the smallest number of layers. read less

Abstract: We consider nanometer-sized fluid annuli (rings) deposited on a solid substrate and ask whether these rings break up into droplets due to the instability of Rayleigh-Plateau-type modified by the presence of the substrate, or collapse to a central drop due to the presence of azimuthal curvature. The analysis is carried out by a combination of atomistic molecular dynamics simulations and a continuum model based on a long-wave limit of Navier-Stokes equations. We find consistent results between the two approaches, and demonstrate characteristic dimension regimes which dictate the assembly dynamics. read less

Abstract: Formed by rolling up a monolayer graphene into a spiral structure, a carbon nanoscroll (CNS) is topologically open and has two free edges along its axial direction, distinct from a multi-walled carbon nanotube (MWCNT). Through systematic molecular mechanics simulations, we show that the unique structure of a CNS produces distinct features of its buckling instability under axial compression, twisting, and bending from those of a MWCNT. The results should be instrumental in future structural design of CNS-based applications. As an example, we demonstrate molecular mass transport through a CNS enabled by its torsional buckling instability. The understanding of reversible buckling instability of CNSs could potentially enable the design of novel nano-devices. read less

Abstract: Molecular dynamics simulations have been used to investigate the structures of Lennard-Jones (LJ) nanowires (NWs) encapsulated by carbon nanotubes (CNTs). We found that, when the radius of CNTs is small, the structures of NWs are quite simple, {\it i.e,} only multishell NWs are formed. For CNTs with larger radius, the structure of NWs becomes much richer. In addition to crystal and multishell NWs, three new kinds of NWs are found, {\it i.e,} a hybrid NW with a crystal core coated by a few shells, a crystal NW with droplet-like pits on the side of NW, and a multishell NW with droplet-like pits on the side of NW. The 'phase' diagram, which describes the structure change with the number of LJ atoms and the interaction between NWs and CNTs, is also obtained. read less

Abstract: The electronic and magnetic properties of bilayer graphene (BLG) depend on the stacking order between the two layers. We introduce a new conceptual structure of "slip corona" on BLG, which is a transition region between A-A stacking close to a nanopore composed of bilayer edges (BLEs) and A-B stacking far away. For an extremely small nanopore (diameter D(pore) < ~5 nm), both atomistic simulations and a continuum model reach consistent descriptions on the shape and size of this "corona" (diameter ~50 nm), which is much larger than the width of the typical dislocation core (~1 nm) in 3D metals or the nanopore itself, due to the weak van der Waals interactions and low interlayer shear resistance between two adjacent layers of graphene. The continuum model also suggests that the width of this "corona" from the BLE to the A-B stacking area would increase as D(pore) increases and converge to ~40 nm when D(pore) is more than ~80 nm. This large stacking transition region provides a new avenue for tailoring BLG properties. read less

Abstract: In this work we determined the mechanical properties (Young's modulus, Poisson's ratio, and shear modulus) of 400 single-walled carbon nanotubes of radii from 2.1; ((0, 5) nanotube) to 17.3 A ((0, 45) nanotube). All nanotubes were simulated with AIREBO forcefield. It turns out that zigzag nanotubes are mechanically more resistant than armchair nanotubes. read less

Abstract: We show from a series of molecular dynamics simulations that the tensile fracture behavior of a nanocrystalline graphene (nc-graphene) nanostrip can become insensitive to a pre-existing flaw (e.g., a hole or a notch) below a critical length scale in the sense that there exists no stress concentration near the flaw, the ultimate failure does not necessarily initiate at the flaw, and the normalized strength of the strip is independent of the size of the flaw. This study is a first direct atomistic simulation of flaw insensitive fracture in high-strength nanoscale materials and provides significant insights into the deformation and failure mechanisms of nc-graphene. read less

Abstract: Hypervelocity impact properties of two different graphene armor systems are investigated using molecular dynamics simulations. One system is the so-called spaced armor which consists of a number of graphene plates spaced certain distance apart. Its response under normal impact of a spherical projectile is studied, focusing on the effect of the number of graphene monolayers per plate (denoted by n ) on the penetration resistance of the armor. We find that under normal impact by a spherical projectile the penetration resistance increases with decreasing number of monolayers per plate ( n ), and the best penetration resistance is achieved in the system with one graphene layer for each plate. Note that the monolayers in all the simulated multilayer graphene plates were AB-stacked. The second system being studied is the laminated copper/graphene composites with the graphene layers inside copper, on impact or back surface, or on both the impact and back surfaces. The simulation results show that under normal impact by a spherical projectile the laminated copper/graphene composite has much higher penetration resistance than the monolithic copper plate. The best efficiency is achieved when the graphene layers are on both the impact and back surfaces. read less

Abstract: Stress arising from structural or thermal misfit impacts the reliability of graphene-related devices. The deformation behaviour of graphene nanoribbons (GNRs) with Stone–Wales defects under stress studied by molecular dynamics shows that nearly all the SW defects annihilate via inverse rotation of C–C bonds. The fracture stress of defective GNRs is comparable to that of perfect ones and similar to mechanical annealing observed from bulk metals. It is a competition between bond rotation and fracture and depends on the strain rate and temperature. At a lower strain rate, such as 10−5 ps−1, the rotation velocity of C–C bonds of 4.2 Å ps−1 is three orders of magnitude larger than the velocity of the collective movement of atoms (1.2 × 10−3 Å ps−1). There is enough time for the C–C bond rotation to respond to the external load, but it becomes more difficult at higher strain rates. This stress-induced SW defect annihilation can be enhanced at higher temperatures because of enhanced exchange of atomic momentum and energy. The results reveal the dominant influence of SW defects on the mechanical properties of two-dimensional materials. read less

Abstract: The irradiation effects in graphene supported by SiO2 substrate including defect production and implantation efficiency are investigated using the molecular dynamics (MD) method with empirical potentials. We show that the irradiation damage in supported graphene comes from two aspects: the direct damage induced by the incident ions and the indirect damage resulting from backscattered particles and sputtered atoms from the substrate. In contrast with the damage in suspended graphene, we find that the indirect damage is dominant in supported graphene at high energies. As a result, enhanced irradiation damage in supported graphene is observed when the incident energy is above 5 keV for Ar and 3 keV for Si. The direct damage probability at all energies, even the total damage probability at low energies, in supported graphene is always much lower than that in suspended graphene because of the higher threshold displacement energy of carbon atoms. In addition, we demonstrate the striking finding that it is possible to dope graphene with sputtered atoms from the substrate and the implantation probability is considerable at optimal energies. Our results indicate that the substrate is an important factor in the process of ion-irradiation-assisted engineering of the properties of graphene. read less

Abstract: The great majority of investigations of thermal transport in carbon nanotubes (CNTs) in the open literature focus on low heat fluxes, that is, in the regime of validity of the Fourier heat conduction law. In this paper, by performing nonequilibrium molecular dynamics simulations we investigated thermal transport in a single-walled CNT bridging two Si slabs under constant high heat flux. An anomalous wave-like kinetic energy profile was observed, and a previously unexplored, wave-dominated energy transport mechanism is identified for high heat fluxes in CNTs, originated from excited low frequency transverse acoustic waves. The transported energy, in terms of a one-dimensional low frequency mechanical wave, is quantified as a function of the total heat flux applied and is compared to the energy transported by traditional Fourier heat conduction. The results show that the low frequency wave actually overtakes traditional Fourier heat conduction and efficiently transports the energy at high heat flux. Our findings reveal an important new mechanism for high heat flux energy transport in low-dimensional nanostructures, such as one-dimensional (1-D) nanotubes and nanowires, which could be very relevant to high heat flux dissipation such as in micro/nanoelectronics applications. read less

Abstract: Using molecular dynamics simulation with empirical potentials, we show that energetic cluster ion beam is a powerful tool to drill nanopores in graphene, which have been proved to possess the potential applications in nanopore-based single-molecule detection and analysis such as DNA sequencing. Two types of clusters are considered, and different cluster size and incident energies are used to simulate the impact events. Our results demonstrate that by choosing suitable cluster species and controlling its energy, a nanopore with expected size and quality could be created in a graphene sheet. Furthermore, suspended carbon chains could be formed at the edge of the nanopore via adjusting the ion energy, which provided a feasible way to decorate the nanopore with chemical methods such as adsorption of large molecules or foreign atoms for biosensing applications. read less

Abstract: We report results of a systematic molecular-dynamics study on the vacancy-induced amorphization of single-layer graphene. An inserted vacancy concentration between 5% and 10% marks the onset of the amorphization transition. The computed amorphized configurations are in agreement with recent experimental observations. We find that the transition becomes less abrupt with vacancy concentration as the temperature increases and determine the surface roughness of the defective graphene as a function of vacancy concentration. We also find that the electronic density of states of vacancy-amorphized graphene is characterized by introduction of localized states near the Fermi level of perfect single-layer graphene. read less

Abstract: Equilibrium molecular dynamics (EMD) simulations and experimental data show that the thermal contact resistance (TCR) between carbon nanotube (CNT) and azide-functionalized polymer with C-N bond is significantly decreased compared to that with Van der Waals force interaction. EMD simulations indicate that C-N covalent bond between CNT and polymer is the most efficient way to reduce TCR, and we measured the lowest thermal interface resistance of Si/CNT/Polymer/Cu thermal interface material as 1.40 mm2 KW−1 with CNTs of 10 μm length. These results provide useful information for future designs of thermal glue for carbon-based materials with better thermal conduction. read less

Abstract: We discuss the relative complexity and computational cost of several popular many-body empirical potentials, developed by the materials science community over the past 30 years. The inclusion of more detailed many-body effects has come at a computational cost, but the cost still scales linearly with the number of atoms modeled. This is enabling very large molecular dynamics simulations with unprecedented atomic-scale fidelity to physical and chemical phenomena. The cost and scalability of the potentials, run in serial and parallel, are benchmarked in the LAMMPS molecular dynamics code. Several recent large calculations performed with these potentials are highlighted to illustrate what is now possible on current supercomputers. We conclude with a brief mention of high-performance computing architecture trends and the research issues they raise for continued potential development and use. read less

Abstract: Precise positioning and packing of nanoscale building blocks is essential for the fabrication of many nanoelectro-mechanical devices. Carrying out such manipulations at the nanoscale still remains a challenge. Here we propose the use of graphone domain arrays embedded in a graphene sheet as a template to precisely position and pack molecules. Our atomistic simulations show that a graphone domain is able to adopt well-defined three-dimensional geometries, which in turn create ‘energy wells’ to trap molecules by means of physisorption. Using the C60 molecule as a model block, the stable trapping conditions are identified. The present work presents a novel route to position and pack molecules for nanoengineering applications. read less

Abstract: We utilize classical molecular dynamics to study the quality (Q)-factors of monolayer CVD-grown graphene nanoresonators. In particular, we focus on the effects of intrinsic grain boundaries of different orientations, which result from the CVD growth process, on the Q-factors. For a range of misorientation angles that are consistent with those seen experimentally in CVD-grown graphene, i.e. 0° to ∼20°, we find that the Q-factors for graphene with intrinsic grain boundaries are 1-2 orders of magnitude smaller than that of pristine monolayer graphene. We find that the Q-factor degradation is strongly influenced by both the symmetry and structure of the 5-7 defect pairs that occur at the grain boundary. Because of this, we also demonstrate that the Q-factors of CVD-grown graphene can be significantly elevated, and approach that of pristine graphene, through application of modest (1%) tensile strain. read less

Abstract: Nowadays we are living in the information age with the fast development of computational technologies and modern facilities. Larger data sets are produced by experiments and computer simulations. In contrast to conventional scientific approaches where simple models are built to fit the data, automated procedures are urged to obtain insights into the core messages carried by the large volume of data. read less

Abstract: We investigate the thermal transport properties of carbon isotope doped graphene using nonequilibrium molecular dynamics simulations. We find that the interfacial thermal resistance between graphene and the isotope atoms causes severe reduction in thermal conductivity of the doped graphene. Furthermore, we find that thermal rectification occurs in the interface. Tensile strain leads to an increase in the interfacial thermal resistance and thermal rectification, while increasing temperature decreases these parameters. We calculate the phonon spectra and find that the thermal rectification is associated with the overlap areas in the phonon spectra. read less

Abstract: Using the valence force field model of Perebeinos and Tersoff (2009 Phys. Rev. B 79 241409(R)), different energy modes of suspended graphene subjected to tensile or compressive strain are studied. By carrying out Monte Carlo simulations it is found that: (i) only for small strains (|ε| ⪅ 0.02) is the total energy symmetrical in the strain, while it behaves completely differently beyond this threshold; (ii) the important energy contributions in stretching experiments are stretching, angle bending, an out-of-plane term, and a term that provides repulsion against π–π misalignment; (iii) in compressing experiments the two latter terms increase rapidly, and beyond the buckling transition stretching and bending energies are found to be constant; (iv) from stretching–compressing simulations we calculated the Young’s modulus at room temperature 350 ± 3.15 N m−1 , which is in good agreement with experimental results (340 ± 50 N m−1 ) and with ab initio results (322–353) N m−1 ; (v) molar heat capacity is estimated to be 24.64 J mol−1 K−1 which is comparable with the Dulong–Petit value, i.e. 24.94 J mol−1 K−1, and is almost independent of the strain; (vi) nonlinear scaling properties are obtained from height–height correlations at finite temperature; (vii) the used valence force field model results in a temperature independent bending modulus for graphene, and (viii) the Grüneisen parameter is estimated to be 0.64. read less

Abstract: Carbon nanotubes are promising candidates for field-emitters. It has been shown that the presence of various gases can enhance or degrade the performance of nanotube emitters. Small hydrocarbons are of particular interest because of their ability to enhance the emission properties. The authors report a simulation study of field-emission from a carbon nanotube exposed to methane in various configurations with an emphasis on calculating the emission current. The Hartree–Fock theory combined with a Green’s functions approach was used for the simulations. It was observed that the change in the emission current strongly depends on the particular arrangement of the methane molecules on the nanotube. read less

Abstract: Numerous theoretical and experimental studies on various species of natural composites, such as nacre in abalone shells, collagen fibrils in tendon, and spider silk fibers, have been pursued to provide insight into the synthesis of novel bioinspired high-performance composites. However, a direct link between the mechanical properties of the constituents and the various geometric features and hierarchies remains to be fully established. In this paper, we explore a common denominator leading to the outstanding balance between strength and toughness in natural composite materials. We present an analytical model to link the mechanical properties of constituents, their geometric arrangement, and the chemistries used in their lateral interactions. Key critical overlap length scales between adjacent reinforcement constituents, which directly control strength and toughness of composite materials, emerge from the analysis. When these length scales are computed for three natural materials-nacre, collagen molecules, and spider silk fibers-very good agreement is found as compared with experimental measurements. The model was then used to interpret load transfer capabilities in synthetic carbon-based materials through parametrization of in situ SEM shear experiments on overlapping multiwall carbon nanotubes. read less

Abstract: When one-dimensional single atom chain was placed at the center of single wall carbon nano-tube (SWCNT) along the axis, and using the molecular dynamics simulation, the mechanical strength of a single-walled could be obtained in terms of the stress-strain curve. At this temperature of 0K, the ultimate stress of a SWCNT with one-dimensional single atom chain was higher than that of a SWCNT without this chain structure, and the result was reverse at 300K. With regard to the rupture of SWCNT, it can be observed that the SWCNT without single atom chain yielded more easily than the decorated SWCNT. The rupture of SWCNT was initiated at the location of distortion, which was introduced by thermal fluctuation and the interference of metal atoms. With the temperature rising, the SWCNT with or without atom Ni will be more likely to fracture. At last, for investigating the influence of increasing atom chains on the mechanical properties of the SWCNT, multi atom chains structure was also used in implementing the same simulation. read less

Abstract: Torsion-induced mechanical couplings of single-walled carbon nanotubes (SWCNTs) are studied by using molecular dynamics simulations. We show that these mechanical couplings are strongly dependent on the chirality of SWCNTs. In particular, the structural difference between armchair and zigzag nanotubes can remarkably influence the Poynting effect [J. H. Poynting, Proc. R. Soc. Lond. A 82, 546 (1909)], i.e., torsion-induced axial strain response. For SWCNTs with large aspect ratios and small chiral angles, an intriguing torsion-induced bending effect is observed. This effect results from the release of torsion-induced axial stress and may probably affect the torsional oscillation behavior of nanoelectromechanical systems based on SWCNTs. read less

Abstract: We investigate the phenomenon of actuation of relative linear motion in double-walled carbon nanotubes (DWNTs) resulting from a temperature gradient. Molecular dynamics simulations of DWNTs with short outer tube reveal that the outer tube is driven towards the cold end of the long inner tube. It is also found that the terminal velocity of the sleeve roughly depends linearly on the applied thermal gradient. We calculate the inter-tube interaction energy surface which is revealed to have a gradient depending upon the applied thermal gradient. Consequently, it is proposed that the origin of the thermophoretic motion of the outer tube may be attributed partially to the existence of such an energy gradient. A simple analytical model is presented accounting for the gradient in energy profile as well as the effect of biased thermal noise. It is shown that the proposed model predicts the dynamical behaviour of the long-time performance reasonably well. read less

Abstract: The effect of free edges of a monoatomic graphene sheet leads to excess edge energy due to the reconstruction of dangling bonds. Molecular static calculations show, that individual carbon atoms near the edge are displaced out of plane for relaxed nanoribbons [1]. In this work we are considering the effect of excess edge energy for almost circular graphene patches. To tackle this problem in the framework of continuum mechanics we are modelling the edge effect with a non‐Euclidean plate model. A linear stability analysis of the flat configuration leads to the stability boundary in the parameter plane. (© 2011 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: For this paper, we carried out molecular dynamics simulation to calculate the bonding energy of the metal-SWNT interface. Three kinds of metal, namely iron, nickel and gold, were studied. The results show that the iron-SWNT interface has the strongest bonding energy, and then nickel and gold. To confirm these results, tensile loading tests were also performed to study the breaking force of the metal-SWNT interface. The force needed to debond the metal-SWNT interface is at the order of nano-newton. The more adhesion energy the interface has, the bigger force that must be loaded to break the joint. read less

Abstract: The use of cluster beams in secondary ion mass spectrometry enables molecular depth profiling, a technique that is essential to many fields. The success of the technique often hinges upon the chemical nature of the substrate, the kinetic energy and incident angle of the primary cluster ion beam, and the sample temperature. It has been shown experimentally that the quality of depth profiles can be improved with cobombardment by a C60 cluster beam and a low-energy argon (Ar) beam. We present molecular dynamics simulations to elucidate the mechanistic reasons for the improved molecular depth profiles with an aim of understanding whether this cobombardment approach is generally applicable. We conclude that the low-energy Ar beam breaks up the surface topology created by the C60 beam, increasing the sputtering yield and reducing the buildup of chemical damage. The simulations also suggest that an equivalent result could be achieved without the Ar cobombardment by optimizing the conditions of the C60 beam. read less

Abstract: The thermal conductivity of hybrid graphene-graphane nanoribbons (GGNRs) have been investigated using nonequilibrium molecular dynamics simulations. The interface between graphene and graphane leads to a Kapitza resistance with strongly dependence on the imposed heat flux direction. We introduce GGNRs as promising thermal rectifiers at room temperature. By calculating phonon spectra, underlying mechanisms were investigated. read less

Abstract: Efforts to modulate the electronic properties of atomically thin crystalline nanoribbons requires precise control over their morphology. Here, we perform atomistic simulations on freestanding graphene nanoribbons (GNRs) to first identify the minimal shapes as a function of ribbon width, and then develop a core-edge framework based on classical plate theory to explore the effect of size and ribbon elasticity in more general systems. The elastic edge-edge interactions are central to stabilization of the flat phase in ultra-narrow ribbons, and their bifurcation to twisted and bent shapes at critical widths that vary inversely with edge stress. In the case of compressive edge stress, we uncover hitherto ignored saddle shapes that are energetically indistinguishable with twisted shapes in the vicinity of the bifurcation yet dominate the morphological space with increasing width. At much larger widths with negligible edge-edge interactions, rippling instabilities set in, i.e. edge ripples and midline dimples for compressive and tensile edge stresses, respectively. Simulations of tapering GNRs reveal the dynamics of these shape transitions. Our results capture the interplay between geometry and mechanics that sets the morphology of crystalline nanoribbons and also highlight the utility of the core-edge framework in developing a unified understanding of the interplay. read less

Abstract: The morphology of graphene sheets overlying a substrate plays an important role in controlling the electronic behaviour of graphene-based electronic devices. Atomistic simulations and energetic analysis are performed to study the morphology of graphene edges on a model substrate. We find that the van der Waals interactions are able to significantly reduce the amplitude, penetration depth and wavelength of edge ripples for a single-layer graphene sheet, but suppression of the ripples is not observed in multilayer graphene sheets with unsaturated edges. We explain this difference using an analytical model that accounts for the energetics of rippling and van der Waals interactions. read less

Abstract: Thermostat algorithms in a molecular dynamics simulation maintain an average temperature of a system by regulating the atomic velocities rather than the internal degrees of freedom. Herein, we present a "phonostat" algorithm that can regulate the total energy in a given internal degree of freedom. In this algorithm, the modal energies are computed at each time step using a mode-tracking scheme and then the system is driven by an external driving force of desired frequency and amplitude. The rate and amount of energy exchange between the phonostat and the system is controlled by two distinct damping parameters. Two different schemes for controlling the external driving force amplitude are also presented. In order to test our algorithm, the method is applied initially to a simple anharmonic oscillator for which the role of various phonostat parameters can be carefully tested. We then apply the phonostat to a more realistic (10,0) carbon nanotube system and show how such an approach can be used to regulate energy of highly anharmonic modes. read less

Abstract: We report the thickness dependence of intrinsic friction in few-layer graphenes, adopting molecular dynamics simulations. The friction force drops dramatically with decreasing number of layers and finally approaches zero with two or three layers. The results, which are robust over a wide range of temperature, shear velocity, and pressure are quantitatively explained by a theoretical model with regard to lateral stiffness, slip length, and maximum lateral force, which could provide a new conceptual framework for understanding stick–slip friction. The results reveal the crucial role of the dimensional effect in nanoscale friction, and could be helpful in the design of graphene-based nanodevices. read less

Abstract: The interfaces between graphene and graphane play an important role in developing graphene-graphane-based electronic devices. We identify three most common types of graphane-graphene interface, and perform atomistic simulations to determine three key parameters that characterize the interface structural properties: mismatch strain, tilt angle, and interfacial stress. Further through atomistic simulations, we find that the composite sheets can develop complex morphologies, such as ripples, warps and wedges. These morphologies can be accurately reproduced by our finite-element modeling with interfacial properties explicitly included. We further show that mismatch in the lattice parameters between graphene and graphane is the dominant factor that causes the complex morphologies. Our work provides a quantitative framework for analyzing and designing graphene-graphane composite sheet architectures, and further for exploring their electronic properties. read less

Abstract: This paper derives a methodology to enable spatial and temporal control of thermally inhomogeneous molecular dynamics (MD) simulations. The primary goal is to perform non-equilibrium MD of thermal transport analogous to continuum solutions of heat flow which have complex initial and boundary conditions, moving MD beyond quasi-equilibrium simulations using periodic boundary conditions. In our paradigm, the entire spatial domain is filled with atoms and overlaid with a finite element (FE) mesh. The representation of continuous variables on this mesh allows fixed temperature and fixed heat flux boundary conditions to be applied, non-equilibrium initial conditions to be imposed and source terms to be added to the atomistic system. In effect, the FE mesh defines a large length scale over which atomic quantities can be locally averaged to derive continuous fields. Unlike coupling methods which require a surrogate model of thermal transport like Fourier's law, in this work the FE grid is only employed for its projection, averaging and interpolation properties. Inherent in this approach is the assumption that MD observables of interest, e.g. temperature, can be mapped to a continuous representation in a non-equilibrium setting. This assumption is taken advantage of to derive a single, unified set of control forces based on Gaussian isokinetic thermostats to regulate the temperature and heat flux locally in the MD. Example problems are used to illustrate potential applications. In addition to the physical results, data relevant to understanding the numerical effects of the method on these systems are also presented. read less

Abstract: The grand canonical Monte Carlo method is used to simulate the adsorption isotherms of water molecules on different types of model soot particles. These soot models are constructed by first removing atoms from onion-fullerene structures in order to create randomly distributed pores inside the soot, and then performing molecular dynamics simulations, based on the reactive adaptive intermolecular reactive empirical bond order (AIREBO) description of the interaction between carbon atoms, to optimize the resulting structures. The obtained results clearly show that the main driving force of water adsorption on soot is the possibility of the formation of new water-water hydrogen bonds with the already adsorbed water molecules. The shape of the calculated water adsorption isotherms at 298 K strongly depends on the possible confinement of the water molecules in pores of the carbonaceous structure. We found that there are two important factors influencing the adsorption ability of soot. The first of these factors, dominating at low pressures, is the ability of the soot of accommodating the first adsorbed water molecules at strongly hydrophilic sites. The second factor concerns the size and shape of the pores, which should be such that the hydrogen bonding network of the water molecules filling them should be optimal. This second factor determines the adsorption properties at higher pressures. read less

Abstract: We investigate the variation in fracture strength of graphene with temperature, strain rate, and crack length using molecular dynamics (MD) simulations, kinetic analysis of fracture with a nonlinear elastic relation, and the quantized fracture mechanics theory. Young’s modulus does not vary significantly with temperature until about 1200 K, beyond which the material becomes softer. Temperature plays a more important role in determining the fracture strength of graphene. Our studies suggest that graphene can be a strong material even, when subjected to variations in temperature, strain rate, and cracks. read less

Abstract: Recent microscopy experiments have revealed novel reconstructions of the commonly observed zigzag and armchair edges in graphene. We show that tensile edge stresses at these reconstructed edges lead to large-scale curling of graphene sheets into cylindrical surfaces, in contrast to the warping instabilities predicted for unreconstructed edges. Using atomic-scale simulations and large deformation plate models, we have derived scaling laws for the curvature and strain of the curled sheets in terms of the edge stress, shape, and the bending and stretching moduli. For graphene nanoribbons, we show that tensile edge stress leads to periodic ripples, whose morphologies are distinct from those observed due to thermal fluctuations or thermally generated mismatch strains. Since the electronic properties of graphene can be altered by both curvatures and strain, our work provides a route for potentially fabricating nanoelectronic devices such as sensors or switches that can detect stresses induced by dopants at the edges. read less

Abstract: Atomistic simulations were utilized to develop fundamental insights regarding the elongation process starting from ultranarrow graphene nanoribbons (GNRs) and resulting in monatomic carbon chains (MACCs). There are three key findings. First, we demonstrate that complete, elongated, and stable MACCs with fracture strains exceeding 100% can be formed from both ultranarrow armchair and zigzag GNRs. Second, we demonstrate that the deformation processes leading to the MACCs have strong chirality dependence. Specifically, armchair GNRs first form DNA-like chains, then develop into monatomic chains by passing through an intermediate configuration in which monatomic chain sections are separated by two-atom attachments. In contrast, zigzag GNRs form rope-ladder-like chains through a process in which the carbon hexagons are first elongated into rectangles; these rectangles eventually coalesce into monatomic chains through a novel triangle–pentagon deformation structure under further tensile deformation. Finally, we show that the width of GNRs plays an important role in the formation of MACCs, and that the ultranarrow GNRs facilitate the formation of full MACCs. The present work should be of considerable interest due to the experimentally demonstrated feasibility of using narrow GNRs to fabricate novel nanoelectronic components based upon monatomic chains of carbon atoms. read less

Abstract: The principal objective of this work is to implement a new material development paradigm using atomistic simulations to guide the molecular design of materials. Traditional empirical macroscopic material development studies omit the fundamental insight needed to understand material behavior at the atomic and molecular levels where material response begins. The new paradigm relies heavily on a tight integration between simulation and experimental efforts to design and process new materials with nanometer-scale precision. Exploiting nanotechnology requires atomic-molecular-level material design and the ability to process these materials with atomic-molecular-level precision. Processing materials with nanoscale precision poses formidable theoretical, computational, and experimental challenges to developing advanced materials. High performance computers and advanced physics-based simulations can complement experimental efforts to design, test, synthesize, and analyze novel materials and innovative structural designs. This method can be applied to a wide range of material designs. As a proof of concept, we began our work on the design of novel carbon nanotube-based materials. The mechanical properties of carbon nanotubes such as low-density, high-stiffness, and exceptional strength make them ideal candidates for reinforcement material in a wide range of high performance composites. Molecular dynamics simulations are used to predict the tensile response of fibers composed of aligned carbon nanotubes with intermolecular bonds of interstitial carbon atoms. The effects of bond density and carbon nanotube length distribution on fiber strength and stiffness are investigated. Results indicate that including cross link atoms between the carbon nanotubes in the strands significantly increases the load transfer between the carbon nanotubes and prevents them from slipping. This increases the elastic modulus and yield strength of the fibers by an order-of-magnitude. Carbon nanotube-based materials appear poised to affect civil and military engineering significantly over the next two decades by providing materials with an order-of- magnitude improvement in strength-to-weight and stiffness-to-weight ratios over existing materials. read less

Abstract: Molecular dynamics simulations have been performed to study the mechanical properties of methyl (CH3) functionalized graphene. It is found that the mechanical properties of functionalized graphene greatly depend on the location, distribution and coverage of CH3 radicals on graphene. Surface functionalization exhibits a much stronger influence on the mechanical properties than edge functionalization. For patterned functionalization on graphene surfaces, the radicals arranged in lines perpendicular to the tensile direction lead to larger strength deterioration than those parallel to the tensile direction. For random functionalization, the elastic modulus of graphene decreases gradually with increasing CH3 coverage, while both the strength and fracture strain show a sharp drop at low coverage. When CH3 coverage reaches saturation, the elastic modulus, strength and fracture strain of graphene drop by as much as 18%, 43% and 47%, respectively. read less

Abstract: Thermocompression bonding of carbon nanotubes (CNTs) to metallic substrates is studied using molecular dynamics. The interaction of the CNT and the metal cluster at high temperature is investigated first. For the diffusion bonding process, the effects of temperature and external pressure are examined. In addition, we apply the tensile loading to examine the mechanical properties and the failure modes during the debonding process. The results show that formation of coalescence structure between the CNT and the metal cluster provides a nanoscale metal surface to facilitate diffusion bonding. Both high temperature and high pressure will enhance the bonding. In addition, the debonding position of the samples under the tensile loading depends on the competition of CNT-metal and metal-metal interface strength. For samples bonded under high temperature and high pressure, the debonding first occurs at the CNT-metal interface. While for samples bonded under low temperature and low pressure, the interdiffusion is n... read less

Abstract: An energetic model is proposed to describe the edge elastic properties of defect-free single-layer graphene sheets. Simulations with the adaptive intermolecular reactive empirical bond order potential are used to extract the edge stress and edge moduli for different edges structures, namely, zigzag and armchair edges, zigzag and armchair edges terminated with hydrogen, and reconstructed zigzag and armchair edges. It is found that the properties of graphene are sensitively dependent on the edge structures; armchair and zigzag edges with and without hydrogen termination are in compression, while reconstructed edges are in tension. read less

Abstract: How carbon nanotubes behave in an external electric field? What will be the relation between the intensity of the electric field and the tube's deformation? What are the geometry effects on the response of carbon nanotubes to electric fields? To answer these questions, we have developed a new combined computational technique to study electrostatic field induced deformations of carbon nanotubes. In this work, we find that the deflection angle of cantilevered semiconducting single-walled carbon nanotubes is proportional to the square of the electric field strength, and the tubes can be most bent when the field angle ranges from 45 to 60 degrees. Furthermore, the deflection angle is also found to be proportional to the aspect ratio L/R. Our results provide a good qualitative agreement with those of one previous experimental study. read less

Abstract: Simulations of amorphous carbon were performed at densities ranging from 2.0 to 3.0 g cm−3 with a reactive bond-order potential. The fraction of sp3 bonding increases with increasing density, as is observed experimentally, but with generally too much sp2 content. Ring size distributions are calculated, with a number of large rings observed. It is suggested that structural quantities that are more directly related to physical properties—such as void volumes and coordination numbers—are more useful than ring size distributions in characterizing the structure of amorphous carbon. Void fractions and void volume distributions are calculated, indicating that a percolating void network exists at 2.0 g cm−3, large, non-percolating voids exist at intermediate density, and no voids are found larger than atomic volumes at 3.0 g cm−3. read less

Abstract: Towards the development of a useful mechanism for hydrogen storage, we have studied the hydrogenation of single-walled carbon nanotubes with atomic hydrogen using core-level photoelectron spectroscopy and x-ray absorption spectroscopy. We find that atomic hydrogen creates C-H bonds with the carbon atoms in the nanotube walls, and such C-H bonds can be completely broken by heating to 600 degrees C. We demonstrate approximately 65 +/- 15 at % hydrogenation of carbon atoms in the single-walled carbon nanotubes, which is equivalent to 5.1 +/- 1.2 wt % hydrogen capacity. We also show that the hydrogenation is a reversible process. read less

Abstract: Selective ALD of Ru on Si-based materials with simultaneous ALD inhibition on the amorphous carbon surface enabled by remote H plasma. read less

Abstract: A nano rotation-translation convertor with a deformable rotor is presented, and the dynamic responses of the system are investigated considering the coupling among the van der Waals (vdW), centrifugal and frictional forces. When an input rotational frequency (ω) is applied at one end of the rotor, the other end exhibits a translational motion, which is an output of the system and depends on both the geometry of the system and the forces applied on the deformable part (DP) of the rotor. When centrifugal force is stronger than vdW force, the DP deforms by accompanying the translation of the rotor. It is found that the translational displacement is stable and controllable on the condition that ω is in an interval. If ω exceeds an allowable value, the rotor exhibits unstable eccentric rotation. The system may collapse with the rotor escaping from the stators due to the strong centrifugal force in eccentric rotation. In a practical design, the interval of ω can be found for a system with controllable output translation. read less

Abstract: Stick-slip motion is the most well-known phenomenon in nanotribology. Maier et al. previously studied the dependence of slip time on contact geometry. In their paper, they were able to identify the intermediate state during slip motion. However, detailed study of this intermediate state is difficult due to the fast dynamics. The advantage of molecular dynamics (MD) simulation is that it can provide detailed information and direct visualization of the tribological phenomena on a time scale of a few nanoseconds. In this paper, we investigate the detailed mechanism of stick-slip motion in nanoscale. MD simulation precisely mimics friction force microscopy experiments. In MD simulations, a crystalline Si tip slides on a graphene surface, and the tip size is varied. The simulation results provide evidence of the intermediate state during slip motion and reveal the hierarchical structure of the stick-slip motion in nanoscale. Detailed relations among stick-slip motion, contact geometry, and energy state are also analyzed. read less

Abstract: In the field of electronics, due to its excellent mechanical and electrical properties, graphene has become the most promising material for the production of next generation thin and flexible graphene-based electronic components. In this work, we present an assessment of the thermal stability and dynamics of asymmetric grain boundaries in graphene for different misorientation angles at finite temperature and up to extremely high temperatures. In particular, we have focused on configurations with misorientation angle of 16.1◦, 30◦ and 38.2◦. In contrast to pristine defect-free graphene, which has no band-gap and therefore is of limited use for semiconductor-based electronics, it has been shown theoretically that line defects in graphene might insert transport gaps, opening up the possibility of device applications based on the structural engineering of graphene boundaries. read less

Abstract: The effect of temperature on the tensile behavior of the armchair (6, 6) single-walled carbon nanotubes with a Ni-coating (SWCNT-Ni) was investigated using molecular dynamics (MD) methods. The mechanical properties of SWCNT-Ni and SWCNT were calculated and analyzed at different temperatures in the range from 220 K to 1200 K. From the MD results, temperature was determined to be the crucial factor affecting the mechanical properties of SWCNT-Ni and SWCNT. After coating nickel atoms onto the surface of a SWCNT, the Young's modulus, tensile strength, and tensile failure strain of SWCNT were greatly reduced with temperature rising, indicating that the nickel atoms on the surface of SWCNT degrade its mechanical properties. However, at high temperature, the Young's modulus of both the SWCNT and the SWCNT-Ni exhibited significantly greater temperature sensitivity than at low temperatures, as the mechanical properties of SWCNT-Ni were primarily dominated by temperature and C-Ni interactions. During these stretching processes at different temperatures, the nickel atoms on the surface of SWCNT-Ni could obtain the amount of energy sufficient to break the C-C bonds as the temperature increases. read less

Abstract: The effect of length difference on oscillation and rotation of the inner tube in a double-walled carbon nanotube (DWNT) is investigated. In the analysis, the inner tubes of the DWNTs are assumed to be of the same length but the length of the outer tubes may be different from that of inner tubes. Further, the outer tube is entirely fixed after adequate relaxation at 300 K and the inner tube is unconstrained and can move freely in the outer tube. In the simulations, each bi-tube system initially has a symmetric cross-section, and the inner tube oscillates while rotating along its own axis. The dynamic behavior studied includes oscillation and rotation of the inner tubes. The oscillation decreases along with an increase in the length difference. A greater length difference between the two tubes in a bi-tube system leads to a smaller amplitude of oscillation of mass center of the inner tube (MCIT) and a higher self-rotating speed. Moreover, the inner tube in a bi-tube with different chirality of the two tubes may have different self-rotational directions. read less

Abstract: Grain boundaries (GBs) in graphene are stable strings of pentagon‐heptagon dislocations. The GBs have been believed to favor an alignment of dislocations, but increasing number of experiments reveal diversely sinuous GB structures whose origins have long been elusive. Based on dislocation theory and first‐principles calculations, an extensive analysis of the graphene GBs is conducted and it is revealed that the sinuous GB structures, albeit being longer than the straight forms, can be energetically optimal once the global GB line cannot bisect the tilt angle. The unusually favorable sinuous GBs can actually decompose into a series of well‐defined bisector segments that effectively relieve the in‐plane stress of edge dislocations, and the established atomic structures closely resemble recent experimental images of typical GBs. In contrast to previously used models, the sinuous GBs show improved mechanical properties and are distinguished by a sizable electronic transport gap, which may open potential applications of polycrystalline graphene in functional devices. read less

Abstract: In the present study, reverse nonequilibrium molecular dynamics is employed to study thermal resistance across interfaces comprising dimensionally mismatched junctions of single layer graphene floors with (6,6) single-walled carbon nanotube (SWCNT) pillars in 3D carbon nanomaterials. Results obtained from unit cell analysis indicate the presence of notable interfacial thermal resistance in the out-of-plane direction (along the longitudinal axis of the SWCNTs) but negligible resistance in the in-plane direction along the graphene floor. The interfacial thermal resistance in the out-of-plane direction is understood to be due to the change in dimensionality as well as phonon spectra mismatch as the phonons propagate from SWCNTs to the graphene sheet and then back again to the SWCNTs. The thermal conductivity of the unit cells was observed to increase nearly linearly with an increase in cell size, that is, pillar height as well as interpillar distance, and approaches a plateau as the pillar height and the interpillar distance approach the critical lengths for ballistic thermal transport in SWCNT and single layer graphene. The results indicate that the thermal transport characteristics of these SWCNT-graphene hybrid structures can be tuned by controlling the SWCNT-graphene junction characteristics as well as the unit cell dimensions. read less

Abstract: The effects of wavenumber and chirality on the axial compressive behavior and properties of wavy carbon nanotubes (CNTs) with multiple Stone-Wales defects are investigated using molecular mechanics simulations with the adaptive intermolecular reactive empirical bond-order potential. The wavy CNTs are assumed to be point-symmetric with respect to their axial centers. It is found that the wavy CNT models, respectively, exhibit a buckling point and long wavelength buckling mode regardless of the wave numbers and chiralities examined. It is also found that the wavy CNTs have nearly the same buckling stresses as their pristine straight counterparts. read less

Abstract: Two-dimensional flat carbon sheets, commonly seen in graphite, are a much sought after material with interesting electronic and mechanical characteristics. It is expected to be used in the next generation microchips in lieu of silicon and also expected to first enter the commercial market in the form of conducting plastics and composites. Due to the increase in the number of such applications along with business opportunities, producing or isolating this layer of graphite cost-effectively is an urgent challenge to be addressed. Widespread efforts in this direction are focusing more on chemical methods to separate (from bulk graphite) or deposit (using epitaxial methods) this two-dimensional layer; such chemical methods not only are environmentally unfriendly, but also produce poor yield and can result in graphene layers with undesirable functional groups attached. One of the early methods, and still sometimes used today for research, of separating graphene from bulk graphite is by mechanical cleavage using a scotch tape. This method is known to produce high quality of graphene layers. However, this method is not reliable for mass manufacturing of graphene; the full potential for such mechanical methods remain to be explored. In this study, a novel method to synthesize carbon nano-sheets specially few layers of graphene from bulk graphite by mechanical exfoliation is presented. The method involves the use of an ultra-sharp single crystal diamond wedge to cleave highly or- read less

Abstract: Nanocomposites of silicon nanoparticles (Si NPs) dispersed in between graphene layers emerge as potential anode materials of high-charge capacity for lithium-ion batteries. A key design requirement is to keep Si NPs dispersed without aggregation. Experimental design of the Si NP dispersion in graphene layers has remained largely empirical. Through extensive molecular dynamics simulations, we determine a critical NP dispersion distance as the function of NP size, below which Si NPs in between graphene layers evolve to bundle together. These results offer crucial and quantitative guidance for designing NP-graphene nanocomposite anode materials with high charge capacity. read less

Abstract: In this paper we present our results of simulating a pull-out test of single walled carbon nanotubes (SWCNT) out of a single crystal gold lattice by means of molecular dynamics. We compare the obtained force-displacement data of the pullout test to results of simulated uniaxial tensile strain tests of SWCNTs. In doing so, we make a theoretical estimation about the quality of the clamping of SWCNTs in a gold crystal. We investigated the influence of chirality of SWCNTs and of the system temperature. Dependent on SWCNT chirality two different pull-out behaviours can be described. Zigzag nanotubes show stronger pull-out resistance than chiral or armchair nanotubes. Our results indicate a minor influence of embedding length of the SWCNT in the gold matrix on pull-out forces. The system temperature has only little effect on the maximum pull-out forces. The presented results have impact on design criteria of SWCNT-metal interfaces. read less

Abstract: This paper investigates the size effects in the dynamic torsional response of single-walled carbon nanotubes (SWCNTs) by developing a modified nonlocal continuum shell model. The purpose is to facilitate the design of devices based on CNT torsion by providing a simple, accurate, and efficient continuum model that can predict the frequency of torsional vibrations and the propagation speed of torsional waves. To this end, dispersion relations of torsional waves are obtained from the proposed nonlocal model and compared to classical models. It is seen that the classical and nonlocal models predict nondispersive and dispersive behavior, respectively. Molecular dynamics simulations of torsional vibrations of (6, 6) and (10, 10) SWCNTs are also performed, the results of which are compared with the classical and nonlocal models and used to extract consistent values of the nonlocal elasticity constant. The superiority and accuracy of the nonlocal elasticity model in predicting the size-dependent dynamic torsional response of SWCNTs are demonstrated. read less

Abstract: Molecular dynamics computer simulations are used to probe the development of the surface morphology and the processes that determine the depth resolution in depth profiling experiments performed by secondary ion and neutral mass spectrometry (SIMS/SNMS). The Ag(111) surface is irradiated by an impact of 20‐keV Au3, C60 and Ar872 clusters that represent a broad range of cluster projectiles used in SIMS/SNMS experiments. Improvements in the simulation protocol including automation and optimal sample shape allow for at least 1000 consecutive impacts for each set of initial conditions. This novel approach allows to shrink the gap between single‐impact simulations and real experiments in which numerous impacts are used. Copyright © 2010 John Wiley & Sons, Ltd. read less

Abstract: Using nonequilibrium molecular dynamics (NEMD) based direct method and spectral energy density (SED) method, we calculate the size-dependent thermal conductivities (TCs) of single layer graphene (SLG), AB-stacked bilayer graphene (AB-BLG), and 21.78° twisted BLG (tBLG) in a robust and consistent manner. Our NEMD analysis reveals discrepancies in high TC reported for graphene systems in some of the earlier studies. Similarly, some of the previous SED based studies were done with unreliable SED Φ′ approach. We conduct size-dependent analysis of the graphene systems by the SED method for the first time and report that bulk TCs for SLG and tBLG systems are nearly the same when calculated by either the direct or the SED method. Contrary to studies that claim that phonon group velocities of AB-BLG and tBLG samples do not change, we find that although average group velocities in SLG and AB-BLG are almost the same, they are around 30% higher when compared to tBLG samples with different twist angles. On the other hand, average phonon lifetimes are almost similar for AB-BLG and 21.78° tBLG samples but around 43% lower than the average phonon lifetime of SLG. Together these trends suggest the reason behind the decreasing order of TCs across three systems. We also systematically study the basic phonon mode contributions to TCs and their properties and find that the high-symmetry modes contribute the most in all three systems. read less

Abstract: The creation of multilayer graphene (Gr), while preserving the brilliant properties of monolayer Gr derived from its unique band structure, can expand the application field of Gr to the macroscale. However, the energy-favorable AB stacking structure in the multilayer Gr induces a strong interlayer interaction and alters the band structure. Consequently, the intrinsic properties of each monolayer are degraded. In this work, we insert carbon nanotubes (CNTs) as nanospacers to modulate the microstructure of multilayer stacking Gr. Nanospacers can increase the interlayer distance and reduce the interlayer interaction. The Gr/CNT stacking structure is experimentally fabricated using a dry transfer method in a layer-by-layer manner. Raman spectroscopy verifies the reduction in the interlayer interaction within the stacking structure. Atomic force microscopy shows an increase in the interlayer distance, which can explain the weakening of the interlayer interactions. The microstructure of the stacked Gr and CNTs is studied by molecular dynamics simulation to systematically investigate the effect of CNT insertion. We found that the distribution distance, size, and arrangement of the CNT can modulate the interlayer distance. These results will help us to understand and improve the properties of the composite systems consisting of Gr and CNTs. read less

Abstract: In this paper, we conducted molecular dynamics simulations to investigate the mechanical properties of double-layer and monolayer irida graphene (IG) structures and the influence of cracks on them. IG, a new two-dimensional material comprising fused rings of 3-6-8 carbon atoms, exhibits exceptional electrical and thermal conductivity, alongside robust structural stability. We found the fracture stress of the irida graphene structure on graphene sheet exceeds that of the structure comprising solely irida graphene. Additionally, the fracture stress of bilayer graphene significantly surpasses that of bilayer irida graphene. We performed crack analysis in both IG and graphene and observed that perpendicular cracks aligned with the tensile direction result in decreased fracture stress as the crack length increases. Moreover, we found that larger angles in relation to the tensile direction lead to reduced fracture stress. Across all structures, 75° demonstrated the lowest stress and strain. These results offer valuable implications for utilizing bilayer and monolayer IG in the development of advanced nanoscale electronic devices. read less

Abstract: We report results on the mechanical and structural response to shear deformation of nanodiamond superstructures in interlayer-bonded twisted bilayer graphene (IB-TBG) and interlayer-bonded graphene bilayers with randomly distributed individual interlayer C–C bonds (RD-IBGs) based on molecular-dynamics simulations. We find that IB-TBG nanodiamond superstructures subjected to shear deformation undergo a brittle-to-ductile transition (BDT) with increasing interlayer bond density (nanodiamond fraction). However, RD-IBG bilayer sheets upon shear deformation consistently undergo brittle failure without exhibiting a BDT. We identify, explain, and characterize in atomic-level detail the different failure mechanisms of the above bilayer structures. We also report the dependence of the mechanical properties, such as shear strength, crack initiation strain, toughness, and shear modulus, of these graphene bilayer sheets on their interlayer bond density and find that these properties differ significantly between IB-TBG nanodiamond superstructures and RD-IBG sheets. Our findings show that the mechanical properties of interlayer-bonded bilayer graphene sheets, including their ductility and the type of failure they undergo under shear deformation, can be systematically tailored by controlling interlayer bond density and distribution. These findings contribute significantly to our understanding of these 2D graphene-based materials as mechanical metamaterials. read less

Abstract: Introduction: We perform molecular dynamics (MD) simulations of nanoscopic liquid water drops on a graphite substrate mimicking the carbon-rich pore surface in the presence of CH4/CO2 mixtures at temperatures in the range 300 K–473 K.Methods: The surface tension in MD simulation is calculated via virial expression, and the water droplet contact angle is obtained through a cylindric binning procedure.Results: Our results for the interfacial tension between water and methane as a function of pressure and for the interfacial tension between water and CH4/CO2 mixtures as a function of their composition agree well with the experimental and computational literature.Discussion: The modified Young’s equation has been proven to bridge the macroscopic contact angle and microscopic contact with the experimental literature. The water droplet on both the artificially textured surface and randomly generated surface exhibits a transition between the Wenzel and Cassie–Baxter states with increased roughness height, indicating that surface roughness enhances the hydrophobicity of the solid surface. read less

Abstract: ABSTRACT This present study investigates the mechanical and deformation behaviour of pristine and carbon nanotube (CNT)-reinforced AlCoCrFeNi high-entropy alloys (HEAs) using molecular dynamics (MD) simulations. The results reveal that an increase in the atomic fraction of Al in pristine AlCoCrFeNi HEAs leads to reduced mechanical behaviour. The mechanical behaviour of the pristine AlCoCrFeNi HEAs notably improves following CNT reinforcement, particularly when using CNT with higher chirality. As the chirality of the CNT increases from (6,6) to (15,15), Young's modulus, yield stress, and toughness of the (15,15) CNT-Al20CoCrFeNi HEA enhance by 17.34%, 29.44%, and 44.44% compared to the (6,6) CNT – Al20CoCrFeNi HEA. HEAs with lower Al fractions experience more substantial stress drops due to rapid structural changes. CNT reinforcement, particularly with higher chirality, decelerates this structural transformation, enhancing yield strength greatly. The analysis of the dislocation evolution revealed that the CNT-reinforced HEA exhibits higher dislocation density compared to the pristine HEA, indicating strain hardening from CNT reinforcement. Furthermore, examination of atomic shear strain reveals confined deformation along shear bands in CNT-reinforced HEAs, leading to the deformation and eventual fracture of CNTs. This study provides valuable insights for enhancing the mechanical behaviour of CNT-reinforced AlCoCrFeNi HEAs, aiding in their design and development. read less

Abstract: Developing solutions to keep up with the needs of increased power output of contemporary microprocessors is an ongoing challenge in the electronics industry. As such, thermal interface materials, which act as a filler to smooth out the contact imperfections between heat source and heat sink have been an important area of research. Two‒dimensional (2D) materials may be a solution to having a material that has high thermal conductivity, flexibility, and a long service life. Although highly thermally conductive, the electrical conductivity graphene makes it unsuitable for use directly adjacent to the active layer in electronics. Hexagonal boron nitride (hBN) has attracted attention for use as an insulating layer due to its structural similarity to graphene with a lattice mismatch of only 1.8%. In this research, equilibrium molecular dynamics (EMD) via the Green‒Kubo (GK) method is used to calculate the thermal conductivity of a hexagonal boron nitride/graphene (hBN/Gr) heterostructure. It is thought that replacing the secondary hBN layer would increase the thermal conductivity of the structure. read less

Abstract: Molecular dynamics simulations are carried out for calculating the surface loss probabilities of neutral species from an argon–methane plasma. These probabilities are the sum of the sticking and surface recombination probabilities. This study considers both the formation of reactive and nonreactive volatile species for evaluating recombination probabilities. Results show that stable species are reflected when hydrocarbon film starts growing on the surface. CH3 is mainly lost by surface recombination leading to the formation of volatile products while very little contributes to film growth. C2H has surface loss probability in agreement with the literature. While C2H loss is usually attributed to sticking on the surface, our results show that its main loss process is due to surface recombination. read less

Abstract: The purpose of this work is to investigate the contribution of in-plane and out-of-plane phonon modes to interface thermal conductivities (ITC) of the Cu/graphene/Cu interface through nonequilibrium molecular dynamics simulations. The proportions of the ITC of the in-plane and out-of-plane phonon modes in the pristine ITC are 1.1% and 99.3%, respectively. Defect engineering can change the coupling strength between in-plane and out-of-plane phonon modes. There is a strong coupling between the in-plane and the out-of-plane phonon mode when the defect concentration is lower than 3%. Phonon coupling has been transformed into weak interaction when the defect concentration is higher than 3%. The high defect concentration can suppress the coupling between in-plane and out-of-plane phonon modes. The results of the phonon density of states show that the out-of-plane phonons are mainly concentrated at low frequencies, and the in-plane phonons are mainly concentrated at high frequencies. This work helps to understand the mechanism of heat transfer of the graphene-based interface and provides theoretical guidance for the application of graphene-based interface nanodevices. read less

Abstract: Although chemical vapor deposition (CVD) has emerged as an important method for producing large-scale and relatively high-quality graphene, CVD-grown graphene inherently contains grain boundaries (GBs), which degrade its mechanical properties. To compensate for these characteristics, various studies have been conducted to maintain the mechanically superior properties by controlling the density of defects and GBs. In this study, the mechanical properties of triple junction (TJ)-free polycrystalline graphene, which is expected to exhibit excellent properties, were investigated through molecular dynamics simulations because TJ is well-known as a crack nucleation site due to stress concentration. We adopted the phase-field crystal method to model CVD-grown graphene-containing TJ-free polycrystalline materials. From a series of numerical simulations, we found that the fracture strength increases as the density of the GB increases. This trend is consistent with that presented in a previous experimental study measured by nanoindentation. It was determined that the variation in the fracture strength is related to the discontinuous density of 5–7 pairs, which act as stress-concentration sites. Additionally, we observed that the fracture strength was higher than that of polycrystalline graphene with TJ. We believe that these results have a higher mechanical advantage compared to the low strength of TJs shown in previous studies and will be important for future structural reliability-based graphene applications. read less

Abstract: Phenine nanotubes (PNTs) have recently been synthesized as a promising new one-dimensional material for high-performance electronics. The periodically distributed vacancy defects in PNTs result in novel semiconducting properties, but may also compromise their mechanical properties. However, the role of these defects in modifying the structural and mechanical properties is not yet well understood. To address this, we conducted systematic molecular dynamics simulations investigating the structural evolution and mechanical responses of PNTs under various conditions. Our results demonstrated that the twisting of linear carbon chains in both armchair and zigzag PNTs led to interesting structural transitions, which were sensitive to chiralities and diameters. Additionally, when subjected to tensile and compressive loading, PNTs’ cross-sectional geometry and untwisting of linear carbon chains resulted in distinct mechanical properties compared to carbon nanotubes. Our findings provide comprehensive insights into the fundamental properties of these new structures while uncovering a new mechanism for modifying the mechanical properties of one-dimensional nanostructures through the twisting–untwisting of linear carbon chains. read less

Abstract: In this work, the method of fabrication of the composites based on a graphene network and metal nanoparticles (Ni and Al) by deformation-heat treatment is considered by molecular dynamics simulation. A graphene network filled with Al or Ni nanoparticles are considered to study the fabrication of the composites and their mechanical properties. The diameter of Ni and Al nanoparticle is 6.2 and 7.5 Å, while in the final composite state, the ratio of metal and carbon atoms in the graphene/Al and graphene/Ni systems are 8 and 7.7 at.%, respectively. It is shown that hydrostatic compression is an effective way to obtain graphene/metal composites. In the compressed structure (composite state) no pores are found since compression is conducted to the maximum possible density. It is found that the graphene/Ni composite has better mechanical properties in comparison with the graphene/Al composite. The enhanced mechanical properties of the graphene/Ni composite are explained by the formation of a graphene network of high strength with metal nanoparticles uniformly distributed over the graphene pores. It has been established, that in the graphene/Al composite, both during hydrostatic and uniaxial tension, due to its low cohesion energy with graphene, metal nanoparticles begin to coagulate, which is more energetically favorable. The larger Al nanoparticles that appear in the structure are the weak places of the composite, where the fracture of the composite occurs most easily. read less

Abstract: A diamane, a hydroginated diamond layer, is an ultrathin film with specific ph ysical an d mechanical properties. Molecular dynamics simulation is used to analyze the equilibrium states of the diamane and calculate its mechanical properties at different conditions. The methodology for the calculation of elastic constants of two-dimensional material by molecular dynamics is developed and applied to the calculation of stiffness and compliance constants of diamane. Two morphologies of diamane (D-AA and D-AB) are considered, but it is shown that morphology does not affect the stiffness and compliance constants. It is found that the size of the simulation box does not affect the values of stiffness and compliance constants of diamane. It is shown that the values of elastic constants are affected by the presence of hydrogen in the diamane structure. The elastic constants are almost not affected by the diamane morphology: constants for AA and AB diamane are very close. The proposed methodology can be further applied to various two-dimensional structures for the calculation of elastic constants. read less

Abstract: Neglected for a long time in molecular simulations of fluid adsorption and transport in microporous carbons, adsorption-induced deformations of the matrix have recently been shown to have important effects on both sorption isotherms and diffusion coefficients. Here we investigate in detail the behavior of a recently proposed 3D-connected mature kerogen model, as a generic model of aromatic microporous carbon with atomic H/C ∼ 0.5, in both chemical and mechanical equilibrium with argon at 243 K over an extended pressure range. We show that under these conditions the material exhibits some viscoelasticity, and simulations of hundreds of nanoseconds are required to accurately determine the equilibrium volumes and sorption loadings. We also show that neglecting matrix internal deformations and swelling can lead to underestimations of the loading by up to 19% (swelling only) and 28% (swelling and internal deformations). The volume of the matrix is shown to increase up to about 8% at the largest pressure considered (210 MPa), which induces an increase of about 33% of both pore volume and specific surface area via the creation of additional pores, yet does not significantly change the normalized pore size distribution. Volume swelling is also rationalized by using a well-known linearized microporomechanical model. Finally, we show that self-diffusivity decreases with applied pressure, following an almost perfectly linear evolution with the free volume. Quantitatively, neglecting swelling and internal deformations tends to reduce the computed self-diffusivities. read less

Abstract: This paper examines the flow structure and heat transfer characteristics of a reactive variable viscosity polyalphaolefin (PAO)-based nanolubricant containing titanium dioxide (TiO2) nanoparticles in a microchannel. The nonlinear model equations are obtained and numerically solved via the shooting method with Runge–Kutta–Fehlberg integration scheme. Pertinent results depicting the effects of emerging thermophysical parameters on the reactive lubricant velocity, temperature, skin friction, Nusselt number and thermal stability criteria are presented graphically and discussed. It is found that the Nusselt number and thermal stability of the flow process improve with exothermic chemical kinetics, Biot number, and nanoparticles volume fraction but lessen with a rise in viscous dissipation and activation energy. read less

Abstract: Grain boundaries and pores commonly manifest in graphene sheets during experimental preparation. Additionally, pores have been intentionally incorporated into graphene to fulfill specific functions for various applications. However, how does the simultaneous presence of pores and grain boundaries impact the mechanical properties of graphene? This paper establishes uniaxial tension models of single-layer graphene-containing pores and three types of experimentally observed. The effect of interaction between pores and grain boundaries on the fracture strength of graphene was studied respectively for three types of grain boundaries by employing molecular dynamics simulations and considering factors such as pore size, the distance between pores and grain boundaries, and loading angle. A competitive mechanism between the intrinsic strength of pristine graphene with grain boundaries (referred to as pristine GGBs), which varies with the loading angle and the fracture strength of graphene sheets with pores that changes with the size of the pores, governs the fracture strength and failure modes of GGBs with pores. When the former exceeds the latter, the fracture strength of GGBs with pores primarily depends on the size of the pores, and fractures occur at the edges of the pores. Conversely, when the former is lower, the fracture strength of GGBs with pores relies on the loading angle and the distance between pores and grain boundaries, leading to grain boundary rupture. If the two strengths are comparable, the failure modes are influenced by the distance between pores and grain boundaries as well as the loading angle. The findings further elucidate the impact of coexisting grain boundaries and pores on the fracture behavior of graphene, providing valuable guidance for the precise design of graphene-based devices in the future. read less

Abstract: In the present work, the thermal conductivity and thermal expansion coefficients of a new morphology of Ni/graphene composites are studied by molecular dynamics. The matrix of the considered composite is crumpled graphene, which is composed of crumpled graphene flakes of 2–4 nm size connected by van der Waals force. Pores of the crumpled graphene matrix were filled with small Ni nanoparticles. Three composite structures with different sizes of Ni nanoparticles (or different Ni content—8, 16, and 24 at.% Ni) were considered. The thermal conductivity of Ni/graphene composite was associated with the formation of a crumpled graphene structure (with a high density of wrinkles) during the composite fabrication and with the formation of a contact boundary between the Ni and graphene network. It was found that, the greater the Ni content in the composite, the higher the thermal conductivity. For example, at 300 K, λ = 40 W/(mK) for 8 at.% Ni, λ = 50 W/(mK) for 16 at.% Ni, and λ = 60 W/(mK) for 24 at.% Ni. However, it was shown that thermal conductivity slightly depends on the temperature in a range between 100 and 600 K. The increase in the thermal expansion coefficient from 5 × 10−6 K−1, with an increase in the Ni content, to 8 × 10−6 K−1 is explained by the fact that pure Ni has high thermal conductivity. The results obtained on thermal properties combined with the high mechanical properties of Ni/graphene composites allow us to predict its application for the fabrication of new flexible electronics, supercapacitors, and Li-ion batteries. read less

Abstract: Molecular dynamics computer simulations are employed to investigate processes leading to particle ejection from single-wall carbon nanotubes bombarded by keV C60 projectiles. The effect of the primary kinetic energy, the incidence angle, and the nanotube diameter on the ejection process is studied. Armchair nanotubes with diameters of 3.26, 5.4, and 8.2 nm are tested. C60 projectiles bombard these targets with kinetic energy between 3 and 50 keV and the angle of incidence ranging between 0° and 75°. The particle ejection yield is a result of the interplay between the amount of kinetic energy available for breaking interatomic bonds, the size of the bombarded area, and the size and form of projectiles hitting this area. Much of the initial kinetic energy is dissipated in the nanotubes as waves, especially for low-energy impacts. Computer simulations are used to find the optimal conditions leading to the gentle ejection of unfragmented organic molecules adsorbed on nanotube substrates. This knowledge may be helpful in the potential application of nanotube substrates in secondary ion mass spectrometry or secondary neutral mass spectrometry. read less

Abstract: Molecular dynamics simulation is used to study and compare the mechanical properties obtained from compression and tension numerical tests of multilayered graphene with an increased interlayer distance. The multilayer graphene with an interlayer distance two-times larger than in graphite is studied first under biaxial compression and then under uniaxial tension along three different axes. The mechanical properties, e.g., the tensile strength and ductility as well as the deformation characteristics due to graphene layer stacking, are studied. The results show that the mechanical properties along different directions are significantly distinguished. Two competitive mechanisms are found both for the compression and tension of multilayer graphene—the crumpling of graphene layers increases the stresses, while the sliding of graphene layers through the surface-to-surface connection lowers it. Multilayer graphene after biaxial compression can sustain high tensile stresses combined with high plasticity. The main outcome of the study of such complex architecture is an important step towards the design of advanced carbon nanomaterials with improved mechanical properties. read less

Abstract: We report results from a systematic analysis of thermal transport in 2D diamond superstructures in interlayer-bonded twisted bilayer graphene (IB-TBG) based on molecular-dynamics simulations. We find that the introduction of interlayer C–C bonds in these bilayer structures causes an abrupt drop in the thermal conductivity of pristine, non-interlayer-bonded bilayer graphene, while further increase in the interlayer C–C bond density (2D diamond fraction) leads to a monotonic increase in the thermal conductivity of the resulting superstructures with increasing 2D diamond fraction toward the high thermal conductivity of 2D diamond (diamane). We also find that a similar trend is exhibited in the thermal conductivity of interlayer-bonded graphene bilayers with randomly distributed individual interlayer C–C bonds (RD-IBGs) as a function of interlayer C–C bond density, but with the thermal conductivity of the IB-TBG 2D diamond superstructures consistently exceeding that of RD-IBGs at a given interlayer bond density. We analyze the simulation results employing effective medium and percolation theories and explain the predicted thermal conductivity dependence on interlayer bond density on the basis of lattice distortions induced in the bilayer structures as a result of interlayer bonding. Our findings demonstrate that the thermal conductivity of IB-TBG 2D diamond superstructures and RD-IBGs can be precisely tuned by controlling interlayer C–C bond density and have important implications for the thermal management applications of interlayer-bonded few-layer graphene derivatives. read less

Abstract: Achieving high-resolution images using dynamic atomic force microscopy (AFM) requires understanding how chemical and structural features of the surface affect image contrast. This understanding is particularly challenging when imaging samples in water. An initial step is to determine how well-characterized surface features interact with the AFM tip in wet environments. Here, we use molecular dynamics simulations of a model AFM tip apex oscillating in water above self-assembled monolayers (SAMs) with different chain lengths and functional groups. The amplitude response of the tip is characterized across a range of vertical distances and amplitude set points. Then relative image contrast is quantified as the difference of the amplitude response of the tip when it is positioned directly above a SAM functional group vs positioned between two functional groups. Differences in contrast between SAMs with different lengths and functional groups are explained in terms of the vertical deflection of the SAMs due to interactions with the tip and water during dynamic imaging. The knowledge gained from simulations of these simple model systems may ultimately be used to guide selection of imaging parameters for more complex surfaces. read less

Abstract: Directional transport of droplets is crucial for industrial applications and chemical engineering processes, which has demonstrated considerable promise in several fields, such as microelectromechanical systems and sensor devices. Nevertheless, controlled directional transport of nanodroplets in a 2D nanochannel has yet to be studied. In this work, we report an approach to achieving a self-driven behavior of a nanodroplet in a 2D nanochannel via a strain gradient. Meanwhile, the effect on the movement speed of the nanodroplet of different channel parameters is studied, including the magnitude of the strain gradient, interlayer distance, and interlayer angle of the nanochannel. Furthermore, how the nanochannel materials affect spontaneous movement is also explored. These simulation results are highly expected to shed new light on the study of the directional transport of a nanodroplet and open a new avenue for research on heat dissipation in microelectromechanical systems. read less

Abstract: Recently, many researchers in the semiconductor industry have attempted to fabricate copper with carbon nanotubes for developing efficient semiconductor systems. In this work, tensile tests of a carbon-nanotube-reinforced copper specimen were conducted using the molecular statics method. The copper substrate utilized in the tensile tests had an edge half-crack, with the carbon nanotube located on the opposite side of the copper substrate. Subsequently, the effects of carbon nanotube radius were investigated. The mechanical properties of the copper/carbon nanotube composite were measured based on the simulation results, which indicated that the atomic behavior of the composite system exhibited the blocking phenomenon of crack propagation under tension. The fracture toughness of the composite system was measured using the Griffith criterion and two-specimen method, while the crack growth resistance curve of the system was obtained by varying the crack length. This study demonstrated that the mechanical reliability of copper can be improved by fabricating it with carbon nanotubes. read less

Abstract: The manufacturing of high-modulus, high-strength fibers is of paramount importance for real-world, high-end applications. In this respect, carbon nanotubes represent the ideal candidates for realizing such fibers. However, their remarkable mechanical performance is difficult to bring up to the macroscale, due to the low load transfer within the fiber. A strategy to increase such load transfer is the introduction of chemical linkers connecting the units, which can be obtained, for example, using carbon ion-beam irradiation. In this work, we investigate, via molecular dynamics simulations, the mechanical properties of twisted nanotube bundles in which the linkers are composed of interstitial single carbon atoms. We find a significant interplay between the twist and the percentage of linkers. Finally, we evaluate the suitability of two different force fields for the description of these systems: the dihedral-angle-corrected registry-dependent potential, which we couple for non-bonded interaction with either the AIREBO potential or the screened potential ReboScr2. We show that both of these potentials show some shortcomings in the investigation of the mechanical properties of bundles with carbon linkers. read less

Abstract: Hydrogel is a three‐dimensional cross‐linked hydrophilic network that can imbibe a large amount of water inside its structure (up to 99% of its dry weight). Due to their unique characteristics of biocompatibility and flexibility, it has found applications in diversified fields, including tissue engineering, drug delivery, biosensors, and agriculture. Even though hydrogels are widely used in the biomedical field, their lower mechanical strength still limits their application to its full potential. Hydrogels can be reinforced with organic, inorganic, and metal‐based nanofillers to improve their mechanical strength. Due to improved computational power, computational‐based techniques are emerging as a leading characterization technique for nanocomposites and hydrogels. In nanomaterials, atomistic description governs the mechanical strength and thermal behavior that realized atomistic level simulations as an appropriate approach to capture the deformation governing mechanism. Among atomistic simulations, the molecular dynamics (MD)‐based approach is emerging as a prospective technique for simulating neat and nanocomposite‐based hydrogels' mechanical and thermal behavior. The success and accuracy of MD simulation entirely depend on the force field. This review article will compile the force field employed by the research community to capture the atomistic interactions in different nanocomposite‐based hydrogels. This article will comprehensively review the progress made in the atomistic approach to study neat and nanocomposite‐based hydrogels' properties. The authors have enlightened the challenges and limitations associated with the atomistic modeling of hydrogels. read less

Abstract: Interatomic interaction potentials are compared using a molecular dynamics modeling method to choose the simplest, but most effective, model to describe the interaction of copper nanoparticles and graphene flakes. Three potentials are considered: (1) the bond-order potential; (2) a hybrid embedded-atom-method and Morse potential; and (3) the Morse potential. The interaction is investigated for crumpled graphene filled with copper nanoparticles to determine the possibility of obtaining a composite and the mechanical properties of this material. It is observed that not all potentials can be applied to describe the graphene–copper interaction in such a system. The bond-order potential potential takes into account various characteristics of the bond (for example, the angle of rotation and bond lengths); its application increases the simulation time and results in a strong interconnection between a metal nanoparticle and a graphene flake. The hybrid embedded-atom-method/Morse potential and the Morse potential show different results and lower bonding between graphene and copper. All the potentials enable a composite structure to be obtained; however, the resulting mechanical properties, such as strength, are different. read less

Abstract: Interaction of β-D-glucopyranuronic acid (GlcA), N-acetyl-β-D-glucosamine (GlcNAc), N-acetyl-β-D-galactosamine (GalNAc) and two natural decameric glycosaminoglycans, hyaluronic acid (HA) and Chondroitin (Ch) with carboxylated carbon nanotubes, were studied using molecular dynamics simulations in a condensed phase. The force field used for carbohydrates was the GLYCAM-06j version, while functionalized carbon nanotubes (fCNT) were described using version two of the general amber force field. We found a series of significant differences in carbohydrate-fCNT adsorption strength depending on the monosaccharide molecule and protonation state of surface carboxyl groups. GlcNAc and GalNAc reveal a strong adsorption on fCNT with deprotonated carboxyl groups, and a slightly weaker adsorption on the fCNT with protonated carboxyl groups. On the contrary, GlcA weakly adsorbs on fCNT. The change in protonation state of surface carboxyl groups leads to the reversal orientation of GlcNAc and GalNAc in reference to the fCNT surface, while GlcA is not sensitive to that factor. Adsorption of decameric oligomers on the surface of fCNT weakens with the increasing number of monosaccharide units. Chondroitin adsorbs weaker than hyaluronic acid and incorporation of four Ch molecules leads to partial detachment of them from the fCNT surface. The glycan–fCNT interactions are strong enough to alter the conformation of carbohydrate backbone; the corresponding conformational changes act toward a more intensive contact of glycan with the fCNT surface. Structural and energetic features of the adsorption process suggest the CH-π interaction-driven mechanism. read less

Abstract: Diamane superlattice generated by the interlayer bonding of twisted bilayer graphene (IB-TBG) has attracted much attention thanks to its excellent properties inherited from bulk diamond, as well as the versatile modulation of physical and mechanical properties, which may open up novel electronic applications. In this work, we have systematically studied the in-plane and interlayer mechanical behaviors of IB-TBG through molecular dynamics simulations and theoretical analysis by considering different structural parameters, such as the twisted angle, stack pattern, and interlayer bonding density. It is found that interlayer bonding density plays a crucial role in determining the in-plane and interlayer shear mechanical properties of IB-TBG. Both the in-plane tensile modulus and strength follow the same linear attenuation relationship with interlayer bonding density for different twisted angles and stacked patterns, while the interlayer shear modulus increases with interlayer bonding density following the same power law, and the critical shear strain of failure linearly decreases with interlayer bonding density. Furthermore, two failure modes are observed under shear deformation, i.e., the failure of interlayer bonding (mode I) and fracture of graphene sheets (mode G). Then, theoretical prediction is carried out by considering the balance of in-plane tension and interlayer shear, which can identify the two failure modes well. The results presented herein yield useful insights for designing and tuning the mechanical properties of IB-TBG. read less

Abstract: This research paper studies the fracture and mechanical properties of rippled graphene containing dislocation dipoles. The atomistic simulation is performed to study the deformation behavior of pristine and defective wrinkled graphene. Graphene wrinkling considerably decreases the ultimate tensile strength of graphene with and without defects but increases the fracture strain. For graphene with the dislocation dipoles, temperature increase slightly affects mechanical properties, in contrast to graphene and graphene with Stone–Wales defect. The extremely similar slopes of the stress-strain curves for graphene with the dislocation dipoles with different arms imply that the distance between dislocations in the dipole does not have noticeable effects on the elastic modulus and strength of graphene. Defects in graphene can also affect its wrinkling; for example, preventing wrinkle formation. read less

Abstract: The interfacial thermal resistance of the nanocontact system of carbon nanotubes and nickel crystals was investigated using molecular dynamics. It was found that with the increase in temperature, the interface thermal resistance gradually increased. In addition, the interfacial thermal resistance also increases gradually with the increase of the contact distance. The ballistic transport of phonons is proposed to be the main reason for the interfacial thermal resistance in this case. read less

Abstract: In the present work, the effects on water transport due to the orientation of the layer in the multilayered porous graphene and the different patterns formed when the layer is oriented to some degrees are studied for both circular and non-circular pore configurations. Interestingly, the five-layered graphene membrane with a layer separation of 3.5 Å used in this study shows that the water transport through multilayered porous graphene can be augmented by introducing an angle to certain layers of the multilayered membrane system. read less

Abstract: The thermal conductivity of a single polymer chain, which is an important factor in the rational design of polymer-based thermal management materials, is strongly affected by the strain state of the chain. In the present study, using non-equilibrium molecular dynamics simulations, the thermal conductivity of a single polyethylene chain, representing a typical polymer chain, was calculated as a function of strain. To investigate the effect of different modeling of covalent bonds, the results were compared for reactive and non-reactive potential models, the AIREBO and NERD potentials, respectively. When the strain ε was as small as ε < −0.03, i.e., under slight compression, the thermal conductivity values were similar regardless of the potential model and increased with increasing strain. However, the two potential models showed qualitatively different behaviors for larger strains up to ε < 0.15: the thermal conductivity calculated by the non-reactive potential continually grows with increasing strain, whereas that by the reactive potential model is saturated. The analysis of internal stress and vibrational density of states suggested that the saturation behavior is due to the weakening of the covalent bond force as the C–C bond elongates, and thus, the result of the reactive model is likely more realistic. However, for ε > 0.1, the reactive potential also produced unphysical results due to the effect of the switching function, describing the formation and breaking of covalent bonds. The present results indicate that careful selection of the potential model and deformation range is necessary when investigating the properties of polymers under tensile strain. read less

Abstract: γ-graphdiyne (γ-GDY) is a new two-dimensional carbon allotrope that has received increasing attention in scientific and engineering fields. The mechanical properties of γ-GDY should be thoroughly understood for realizing their practical applications. Although γ-GDY is synthesized and employed mainly in their bilayer or multilayer forms, previous theoretical studies mainly focused on the single-layer form. To evaluate the characteristics of the multilayer form, the mechanical properties of the bilayer γ-GDY (γ-BGDY) were tested under uniaxial tension using the molecular dynamics simulations. The stress–strain relation of γ-BGDY is highly temperature-dependent and exhibits a brittle-to-ductile transition with increasing temperature. When the temperature is below the critical brittle-to-ductile transition temperature, γ-BGDY cracks in a brittle manner and the fracture strain decreases with increasing temperature. Otherwise, it exhibits ductile characteristics and the fracture strain increases with temperature. Such a temperature-dependent brittle-to-ductile transition is attributed to the interlayer cooperative deformation mechanism, in which the co-rearrangement of neighboring layers is dominated by thermal vibrations of carbon atoms in diacetylenic chains. Furthermore, the brittle-to-ductile transition behavior of γ-BGDY is independent of loading direction and loading rate. The ultimate stress and Young’s modulus decrease at higher temperatures. These results are beneficial for the design of advanced γ-GDY-based devices. read less

Abstract: It has been shown that the nanoplastic particles present in graphene membranes have a high tendency to cause fouling in them due to the high affinity between graphene and nanoplastic molecules. This poses a significant challenge for the use of graphene membranes for desalination. In this paper, we introduce a double-layer graphene slit membrane as a viable solution to significantly reduce fouling caused by the presence of nanoplastic particles in graphene membranes. The molecular dynamics (MD) simulations performed in this work show that when fouling occurs in a single-layer membrane, the presence of nanoplastics reduces the average permeability by close to 40%, from 1877 LMBH to 1148 LMBH, with a large standard deviation of 26% between runs. With the addition of the secondary membrane, the average permeability increases by 17%, with a significantly reduced standard deviation of 7%. These suggest that the secondary layer acts as a sacrificial shield, attracting the nanoplastic contaminants and preventing them from coming into close proximity with the primary membrane, thus preventing fouling at the primary rejection layer. Furthermore, due to the affinity of the nanoplastic particles with the secondary graphene membrane, this membrane design points toward an effective and efficient way of extracting nanoplastic particles for further analysis or processing. read less

Abstract: Sliding‐induced friction behavior of a single‐asperity silica probe against few‐layer (FL) graphene and bulk graphite is measured in the presence of n‐hexadecane using an atomic force microscope (AFM). The load‐dependent nanoscale friction measurements display friction hysteresis, i.e., higher friction forces during unloading of the contact than loading, at a given normal load. However, unlike hysteresis in friction of graphene measured in ambient, several unique trends are observed when the contact is immersed in n‐hexadecane. First, the friction hysteresis is measured up to a transition load and beyond which is found negligible; second, a similar behavior is observed on bulk graphite; and third, a friction strengthening of the contact persisted up to several nanometers. Quasi‐static force‐separation curves identify up to four layers of n‐hexadecane solvation layers on graphitic surfaces. Molecular dynamic simulations illustrate that the solvated n‐hexadecane molecules within the contact carry the probe load and determine the generated contact area, affecting friction hysteresis. Further, during AFM probe sliding, instead of a pucker, a molecular pile‐up of n‐hexadecane, in front of the tip is observed. These findings provide new perspectives on understanding of the dissipation mechanisms of graphene that predominantly are surrounded by structured liquid molecules. read less

Abstract: Rocket kerosene plays an important role in the regenerative cooling process of rocket thrust chambers. Its thermal conductivity determines the cooling efficiency and the tendency to coke in rocket kerosene engines. In this paper, graphene nanoplatelets (GNPs) were introduced into rocket kerosene to improve its thermal conductivity. Molecular dynamics simulation was used to investigate the thermal conductivity of the composite system and its underlying thermal conductivity mechanism. Firstly, by studying the effect of the mass fraction of GNPs, it was found that, when the graphene mass fraction increases from 1.14% to 6.49%, the thermal conductivity of the composite system increases from 4.26% to 17.83%, which can be explained by the percolation theory. Secondly, the influence of the size of GNPs on the thermal conductivity of the composite system was studied. Basically, the thermal conductivity was found to increase by increasing the aspect ratio of GNPs, indicating that GNPs with a higher aspect ratio are more conducive to improving the thermal conductivity of rocket kerosene. By carefully analyzing the effect of the size of GNPs on thermal conductivity, it was concluded that the thermal conduction enhancement by adding GNPs is determined by the combined effect of the percolation theory and the Brownian motion. The results of the temperature effect study showed that the ratio of thermal conductivity to rocket kerosene increased from 1.16 to 1.26 and from 1.07 to 1.11 for the composite systems, with graphene sizes of 41.18 Å × 64.00 Å and 24.14 Å × 17.22 Å in the temperature range of 293 K to 343 K, respectively. It is further proved that the Brownian motion of GNPs has a non-negligible effect on the thermal conductivity of the composite system. This work provides microscopic insights into the thermal conduction mechanism of GNPs in nanofluids and will offer practical guidance for improving the thermal conductivity of rocket kerosene. read less

Abstract: According to the motion style, a nanomotor can be classified into linear nanomotor and rotary nanomotor. Nanomotors, as the core components of nanomachine, have broad research prospects and applications. Here, a molecular dynamics method is used to simulate the linear nanomotor on a stretched carbon nanotube substrate. The results show that the nanomotor speed is well controlled by the temperature gradient, the axial strain of the substrate and the nanomotor size. When the nanomotor moves stably on the substrate carbon nanotube with a temperature difference of 200 K at both ends, the time required for the nanomotor to travel the same distance on the substrate carbon nanotube with 15% strain is about 62% longer than that without strain. The mechanism for the nanomotor movement and speed control is attributed to the thermophoretic force acting on the nanomotor. Specifically, the thermophoretic force increases with increasing substrate temperature gradient and decreases with increasing substrate strain. These results provide a novel method for controlling the speed of a nanomotor and inform nanomotor design and manufacture, as well as presenting a deeper understanding of the mechanism and movement law of the nanomotor. read less

Abstract: As deformation and defects are inevitable during the manufacture and service of graphene resonators, comprehensive molecular dynamic (MD) simulations are performed to investigate the vibrational properties of the defective single-layer graphene sheets (SLGSs) during tension. Perfect SLGSs, SLGSs with single vacancy, SLGSs with low-concentration vacancies, and SLGSs with high-concentration vacancies are considered, respectively. The frequencies of the perfect and defective SLGSs at different stretching stages are investigated in detail. The effects of different external forces are also taken into account to study the vibration properties of the defective SLGSs. Results show that the perfect and defective SLGSs both successively perform four stages, i.e., the elastic stage, the yield stage, the hardening stage, and the fracture stage during stretching, and the elastic properties of the SLGSs are insensitive to the vacancy defects, while the ultimate strain is noticeably reduced by the vacancies. The single vacancy has no effect on the vibration properties of SLGS, while the frequency decreases with the increasing vacancy concentration for SLGS at the elastic stage. The frequency of yielded SLGS with a certain vacancy concentration is almost constant even with a varying external force. read less

Abstract: In this paper, the mechanical properties of Carbon nanotubes (CNTs) and Boron Nitride nanotubes (BNNTs) are studied systematically by using molecular dynamics simulations. CNTs are considered semi-metallic, whereas the BNNTs, of the large band gap, are considered to be insulators, regarding the difference in the electrical properties of CNTs and BNNTs; comparing the mechanical properties of both nanotubes offers great scientific significance for their prospective applications. The simulations were carried out with the help of a Large-scale atomic/molecular massively parallel simulator (LAMMPS) and were based on the Airebo and Tersoffs force fields for C-C interaction in CNTs and B-N interaction in BNNTs, respectively. Failure behavior of armchair and zigzag CNTs and BNNTs under tensile and compressive loading has been predicted and observed that for both the nanotubes the armchair nanotubes showed higher tensile and compressive strength as compared to zigzag nanotubes. The maximum tensile and compressive strength for CNTs is 205 GPa and 35.62 GPa respectively and for BNNTs are 159 GPa and 24.81 GPa respectively. CNTs are identified as axially stronger and stiffer than BNNTs for the same diameter under identical loading conditions. read less

Abstract: Solid‐state cooling exploits the thermal response of caloric materials when subjected to external physical fields and represents a promising alternative to conventional refrigeration technologies. However, existing caloric materials are often limited by relatively small caloric response and hysteresis issues rooted in first‐order structural transitions. Here, colossal and reversible elastocaloric effects near room temperature in superelastic graphene architectures are predicted by thermodynamic analysis and atomistic calculations. The estimated adiabatic temperature change can reach a value of 155 K under a compressive stress of 0.7 GPa in a wide temperature range of around 300 K, yielding outstanding elastocaloric strength and efficiency. More unique is that both cooling and warming can be realized in the materials by applying tensile and compressive strains to host conventional and inverse elastocaloric effects, respectively, with almost no fatigue behavior and hysteresis effect. Such unprecedented elastocaloric performance results from a strain‐induced large change in configurational entropy in the graphene architectures. These effects are potentially extendable to other superelastic nanomaterials and hence suggest new material settings for developing high‐performance solid‐state refrigerants. read less

Abstract: ABSTRACT The present paper is devoted to the study of diffusion of carbon nanotubes in water by molecular dynamics method. Two nanotube models were used, namely, 1D rigid rod and 3D (6, 6) armchair. The nanotube diameter was 0.818 nm, and their length ranged from 5.25 to 32.2 nm. Both translational and rotational diffusion coefficients were calculated. Besides, longitudinal and transverse diffusion was studied, and the corresponding diffusion coefficients were determined. The first of them was much larger than the second, and the difference reached two times. The average diffusion coefficients are relatively well described by the analytical dependences for rigid cylinders. The diffusion coefficients were calculated using Green–Kubo formula and Einstein relation. Relaxation of autocorrelation functions of nanotube velocity and angular velocity was systematically discussed for all cases. It is shown that this relaxation has two stages and the first stage is an exponential. The corresponding relaxation times were estimated. read less

Abstract: Though graphene is the strongest material in nature, its intrinsic brittleness hinders its applications where flexibility is the key figure of merits. In this work, we report the enhanced flexibility of graphene under nanoindentation by using kirigami technique. Based on molecular dynamics simulations, we find that graphene kirigami designed at the optimal cut parameter can sustain more than 45% larger out-of-plane deformation than its pristine counterpart while the maximum impact load is reduced by 20% due to the flexible cut edges. This trade-off between flexibility and strength in a graphene kirigami can be overcome by adding a pristine graphene as a supporting substrate. This double-layer structure consisting of one graphene kirigami and one pristine graphene can stand the maximum impact load three times larger than the single-layer graphene kirigami but its maximum indentation depth is merely 8% smaller. Our simulation results provide useful insights into the failure mechanism of the graphene kirigami under nanoindentation and useful guidelines to enhancing the flexibility of graphene for its applications as protection materials. read less

Abstract: In this study, some features of molecular dynamics simulation for evaluating the mechanical properties of a Ni/graphene composite and analyzing the effect of incremental and dynamic tensile loading on its deformation are discussed. A new structural type of the composites is considered: graphene network (matrix) with metal nanoparticles inside. Two important factors affecting the process of uniaxial tension are studied: tension strain rate (5 ×10−3 ps−1 and 5 ×10−4 ps−1) and simulation temperature (0 and 300 K). The results show that the strain rate affects the ultimate tensile strength under tension: the lower the strain rate, the lower the critical values of strain. Tension at room temperature results in lower ultimate tensile strength in comparison with simulation at a temperature close to 0 K, at which ultimate tensile strength is closer to theoretical strength. Both simulation techniques (dynamic and incremental) can be effectively used for such a study and result in almost similar behavior. Fabrication technique plays a key role in the formation of the composite with low anisotropy. In the present work, uniaxial tension along three directions shows a big difference in the composite strength. It is shown that the ultimate tensile strength of the Ni/graphene composite is close to that of pure crumpled graphene, while the ductility of crumpled graphene with metal nanoparticles inside is two times higher. The obtained results shed the light on the simulation methodology which should be used for the study of the deformation behavior of carbon/metal nanostructures. read less

Abstract: The effect of stress on irradiation responses of highly oriented pyrolytic graphite (HOPG) was studied by combing molecular dynamics (MD) simulation, proton irradiation, and Raman characterization. MD simulations of carbon knock-on at energies < 60 eV were used to obtain average threshold displacement energies (E¯d) as a function of strain ranging from 0 to 10%. Simulations at a higher irradiation energy of 2–5 keV were used to study the effect of strain on damage cascade evolution. With increasing tensile strain, E¯d was reduced from 35 eV at 0% strain to 31 eV at 10% strain. The strain-reduced E¯d led to a higher damage peak and more surviving defects (up to 1 ps). Furthermore, high strains induced local cleavage around the cavities, as one additional mechanism of damage enhancement. Experimentally, HOPG film was folded, and the folded region with the maximum tensile stress was irradiated by a 2 MeV proton beam. Raman characterization showed significantly enhanced D to G modes in comparison to the stress-free irradiation. Based on the strain dependence of E¯d and the Kinchin–Pease model, a formula for displacement estimation under different tensile strains is proposed. The stress effects need to be considered in graphite applications in a reactor’s harsh environment where both neutron damage and stress are present. read less

Abstract: Two-dimensional (2D) materials possess great potential for flexible devices, ascribing to their outstanding electrical, optical, and mechanical properties. However, their mechanical deformation property and fracture mechanism, which are inescapable in many applications like flexible optoelectronics, are still unclear or not thoroughly investigated due methodology limitations. In light of this, such mechanical properties and mechanisms are explored on example of gallium telluride (GaTe), a promising optoelectronic candidate with an ultrahigh photo-responsibility and a high plasticity within 2D family. Considering the driving force insufficient in atomic force microscopy (AFM)-based nanoindentation method, here the mechanical properties of both substrate-supported and suspended GaTe multilayers were systematically investigated through full-scale Berkovich-tip nanoindentation, micro-Raman spectroscopy, AFM, and scanning electron microscopy. An unusual concurrence of multiple pop-in and load-drop events in loading curve was observed. By further correlating to molecular dynamics calculations, this concurrence was unveiled originating from the interlayer sliding mediated layers-by-layers fracture mechanism within GaTe multilayers. The van der Waals force between GaTe multilayers and substrates was revealed much stronger than that between GaTe interlayers, resulting in the easy sliding and fracture of multilayers within GaTe. This work provides new insights into the deformation and fracture mechanisms of GaTe and other similar 2D multilayers in flexible applications. read less

Abstract: In this work, randomly three-dimensional graphene foam (RGF) is synthesized from the chemical reduction of graphene oxide solution in the self-assembly method. RGF/epoxy composite is then obtained by the RTM method. Tensile testing highlights the effect of RGF drying percentage on the composite and shows a 138% and 48% increase in the Young modulus and tensile strength compared to epoxy. Also, by considering representative unit cell (RUC), a multi-scale numerical method using finite element analysis based on Carrera Unified Formulation (CUF), and molecular dynamics (MD), are performed to obtain the mechanical properties of the RGF/epoxy composite material. read less