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 main objective of this work was to study the effects of carbon nanotubes (CNTs) on the strength and electrical properties of cement mortar. Molecular dynamic simulations (MDSs) were carried out to determine the mechanical and electrical properties of a cementitious composite and its associated mechanisms. To model the atomic structure of a calcium silicate hydrate (C-S-H) gel, tobermorite 11Å was chosen. Single-walled carbon nanotubes (SWCNTs) embedded in a tobermorite structure were tested numerically. In particular, it was concluded that a piezoelectric effect can be effectively simulated by varying the concentration levels of carbon nanotubes. The deformation characteristics were analyzed by subjecting a sample to an electrical field of 250 MV/m in the z-direction in a simulation box. The results indicated a progressively stronger converse piezoelectric response with an increasing proportion of carbon nanotubes. Additionally, it was observed that the piezoelectric constant in the z-direction, denoted by d33, also increased correspondingly, thereby validating the potential for generating an electrical current during sample deformation. An innovative experiment was developed for the electrical characterization of a cementitious composite of carbon nanotubes. The results showed that the electrostatic current measurements exhibited a higher electric sensitivity for samples with a higher concentration of CNTs. read less

Abstract: This research addresses the need for a multiscale model for the determination of the thermophysical properties of nanofiller-enhanced thermoset polymer composites. Specifically, we analyzed the thermophysical properties of an epoxy resin containing bisphenol-A diglyceryl ether (DGEBA) as an epoxy monomer and dicyandiamide (DICY) and diethylene triamine (DETA) as cross-linking agents. The cross-linking process occurs at the atomistic scale through the formation of bonds among the reactive particles within the epoxy and hardener molecules. To derive the interatomic coarse-grained potential for the mesoscopic model and match the density of the material studied through atomic simulations, we employed the iterative Boltzmann inversion method. The newly developed coarse-grained molecular dynamics model effectively reproduces various thermophysical properties of the DGEBA-DICY-DETA resin system. Furthermore, we simulated nanocomposites made of the considered epoxy additivated with graphene nanofillers at the mesoscopic level and verified them against continuum approaches. Our results demonstrate that a moderate amount of nanofillers (up to 2 wt.%) increases the elastic modulus and thermal conductivity of the epoxy resin while decreasing the Poisson’s ratio. For the first time, we present a coarse-grained model of DGEBA-DICY-DETA/graphene materials, which can facilitate the design and development of composites with tunable thermophysical properties for a potentially wide range of applications, e.g., automotive, aerospace, biomedical, or energy ones. read less

Abstract: Recent advancements in the field of two-dimensional (2D) materials have led to the discovery of a wide range of 2D materials with intriguing properties. Atomistic-scale simulation methods have played a key role in these discoveries. In this review, we provide an overview of the recent progress in ReaxFF force field developments and applications in modeling the following layered and nonlayered 2D materials: graphene, transition metal dichalcogenides, MXenes, hexagonal boron nitrides, groups III-, IV- and V-elemental materials, as well as the mixed dimensional van der Waals heterostructures. We further discuss knowledge gaps and challenges associated with synthesis and characterization of 2D materials. We close this review with an outlook addressing the challenges as well as plans regarding ReaxFF development and possible large-scale simulations, which should be helpful to guide experimental studies in a discovery of new materials and devices. read less

Abstract: Nanoporous materials show a promising combination of mechanical properties in terms of their relative density; while there are numerous studies based on metallic nanoporous materials, here we focus on amorphous carbon with a bicontinuous nanoporous structure as an alternative to control the mechanical properties for the function of filament composition.Using atomistic simulations, we study the mechanical response of nanoporous amorphous carbon with 50% porosity, with sp3 content ranging from 10% to 50%. Our results show an unusually high strength between 10 and 20 GPa as a function of the %sp3 content. We present an analytical analysis derived from the Gibson–Ashby model for porous solids, and from the He and Thorpe theory for covalent solids to describe Young’s modulus and yield strength scaling laws extremely well, revealing also that the high strength is mainly due to the presence of sp3 bonding. Alternatively, we also find two distinct fracture modes: for low %sp3 samples, we observe a ductile-type behavior, while high %sp3 leads to brittle-type behavior due to high high shear strain clusters driving the carbon bond breaking that finally promotes the filament fracture. All in all, nanoporous amorphous carbon with bicontinuous structure is presented as a lightweight material with a tunable elasto-plastic response in terms of porosity and sp3 bonding, resulting in a material with a broad range of possible combinations of mechanical properties. 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: Semi-empirical quantum models such as Density Functional Tight Binding (DFTB) are attractive methods for obtaining quantum simulation data at longer time and length scales than possible with standard approaches. However, application of these models can require lengthy effort due to the lack of a systematic approach for their development. In this work, we discuss the use of the Chebyshev Interaction Model for Efficient Simulation (ChIMES) to create rapidly parameterized DFTB models, which exhibit strong transferability due to the inclusion of many-body interactions that might otherwise be inaccurate. We apply our modeling approach to silicon polymorphs and review previous work on titanium hydride. We also review the creation of a general purpose DFTB/ChIMES model for organic molecules and compounds that approaches hybrid functional and coupled cluster accuracy with two orders of magnitude fewer parameters than similar neural network approaches. In all cases, DFTB/ChIMES yields similar accuracy to the underlying quantum method with orders of magnitude improvement in computational cost. Our developments provide a way to create computationally efficient and highly accurate simulations over varying extreme thermodynamic conditions, where physical and chemical properties can be difficult to interrogate directly, and there is historically a significant reliance on theoretical approaches for interpretation and validation of experimental results. read less

Abstract: We present an atomic cluster expansion (ACE) for carbon that improves over available classical and machine learning potentials. The ACE is parametrized from an exhaustive set of important carbon structures over extended volume and energy ranges, computed using density functional theory (DFT). Rigorous validation reveals that ACE accurately predicts a broad range of properties of both crystalline and amorphous carbon phases while being several orders of magnitude more computationally efficient than available machine learning models. We demonstrate the predictive power of ACE on three distinct applications: brittle crack propagation in diamond, the evolution of amorphous carbon structures at different densities and quench rates, and the nucleation and growth of fullerene clusters under high-pressure and high-temperature conditions. read less

Abstract: Disordered elemental semiconductors, most notably a-C and a-Si, are ubiquitous in a myriad of different applications. These exploit their unique mechanical and electronic properties. In the past couple of decades, density functional theory (DFT) and other quantum mechanics-based computational simulation techniques have been successful at delivering a detailed understanding of the atomic and electronic structure of crystalline semiconductors. Unfortunately, the complex structure of disordered semiconductors sets the time and length scales required for DFT simulation of these materials out of reach. In recent years, machine learning (ML) approaches to atomistic modeling have been developed that provide an accurate approximation of the DFT potential energy surface for a small fraction of the computational time. These ML approaches have now reached maturity and are starting to deliver the first conclusive insights into some of the missing details surrounding the intricate atomic structure of disordered semiconductors. In this Topical Review we give a brief introduction to ML atomistic modeling and its application to amorphous semiconductors. We then take a look at how ML simulations have been used to improve our current understanding of the atomic structure of a-C and a-Si. read less

Abstract: Significance In crystalline solids, atoms are arranged in periodic patterns on regular lattices. Amorphous solids, instead, lack long-range order; namely, a regular array of atoms beyond first or second nearest neighbors is absent. The lack of periodicity influences many properties of amorphous materials, including the coupling of electronic and nuclear motion. Here we study amorphous carbon, a system composed of a relatively light atom. We show that to understand its electronic properties, a quantum mechanical treatment of electron–nuclear coupling is essential, and we illustrate a simulation framework based on first principles to do so. We also discuss the role of specific defect states in the disordered network in determining the physical properties of amorphous carbon. read less

Abstract: The fullerene family, whose most popular members are the spherical C60 and C70 molecules, has recently added a new member, the cube-shaped carbon molecule C8 called a cubene. A molecular crystal based on fullerenes is called fullerite. In this work, based on relaxational molecular dynamics, two fullerites based on cubenes are described for the first time, one of which belongs to the cubic system, and the other to the triclinic system. Potential energy per atom, elastic constants, and mechanical stress components are calculated as functions of lattice strain. It has been established that the cubic cubene crystal is metastable, while the triclinic crystal is presumably the crystalline phase in the ground state (the potential energies per atom for these two structures are −0.0452 and −0.0480 eV, respectively).The cubic phase has a lower density than the monoclinic one (volumes per cubene are 101 and 97.7 Å3). The elastic constants for the monoclinic phase are approximately 4% higher than those for the cubic phase. The presented results are the first step in studying the physical and mechanical properties of C8 fullerite, which may have potential for hydrogen storage and other applications. In the future, the influence of temperature on the properties of cubenes will be analyzed. read less

Abstract: The paper examines the compressibility of media with nano-inhomogeneities using the example of an aluminum melt and C60 fullerenes immersed in it. The results of molecular dynamics simulations indicate a significant effect of the interface on the effective compressibility of a heterogeneous medium. It is found that the application of the rule of mixture for the Al/C60 system results in an incorrect qualitative picture of the dependence of compressibility on the concentration of fullerenes. To explain this effect, an analytical model is proposed that takes into account the reduction in distances between atoms of different components during compression. The model makes it possible to estimate the effective mechanical characteristics of a liquid with nano-inhomogeneities within the framework of the mechanical approach, and correctly predicts the nature of the change in the dependence of compressibility on concentration. read less

Abstract: Carbon fiber-reinforced polymer composites are used in various applications, and the interface of fibers and polymer is critical to the composites’ structural properties. We have investigated the impact of introducing different carbon nanotube loadings to the surfaces of carbon fibers and characterized the interfacial properties by molecular dynamics simulations. The carbon fiber (CF) surface structure was explicitly modeled to replicate the graphite crystallites’ interior consisting of turbostratic interconnected graphene multilayers. Then, single-walled carbon nanotubes and polypropylene chains were packed with the modeled CFs to construct a nano-engineered “fuzzy” CF composite. The mechanical properties of the CF models were calculated through uniaxial tensile simulations. Finally, the strength to peel the polypropylene from the nano-engineered CFs and interfacial energy were calculated. The interfacial strength and energy results indicate that a higher concentration of single-walled carbon nanotubes improves the interfacial properties. read less

Abstract: Alongside the evolution of density functional theory into a new era led by the dispersion‐corrected hybrid density functional theory approaches, formulation of a new generation of intermolecular potentials has also taken the center stage. An ideal potential formulation should desirably possess simplicity of functional forms, physically meaningful parameters, separability of various terms into atomic‐level contributions, computational tractability, ability to capture non‐additivity of interactions, transferability across different chemical species, and crucially, chemical fidelity in terms of reproducing the benchmark data. The Lennard‐Jones potential, one of the popular intermolecular pair potentials for performing large‐scale simulations fails to capture the intricate features of molecular interactions. Woven around the central theme of anisotropy in the nature of intermolecular interactions, herein, we describe various landmark contributions in the quest for chemical fidelity of empirical potential formulations that include (i) incorporation of the anisotropic nature of exchange‐repulsion and dispersion contributions, (ii) multipolar description of the dispersion terms, (iii) damping functions to provide an accurate description of the asymptotes, and (iv) transferability of intermolecular interaction terms. We illustrate the nuances of intermolecular force field development in the context of modeling the non‐covalent interactions governing the (i) binding of atoms and molecules with carbon nanostructures, (ii) molecular aggregates of polycyclic aromatic hydrocarbons, and (iii) interlayer interactions in layered nanostructures. We exemplify the hierarchy of empirical potentials by depicting them on the various rungs of the Jacob's ladder equivalent of density functional theory for the intermolecular force fields. Finally, we discuss some possible futuristic directions in intermolecular force field development. read less

Abstract: The tribochemistry and transfer film formation at the metal/polymer interface plays an essential role in surface protection, wear reduction, and lubrication. Although the topic has been studied for decades, challenges persist in clarifying the nanoscale mechanism and dynamic evolution of tribochemical reactions. To investigate the tribochemistry between iron and polytetrafluoroethylene (PTFE) in ambient and cryogenic environments, we have trained and expanded a ReaxFF reactive force field to describe iron-oxygen-water-PTFE systems (C/H/O/F/Fe). Using ReaxFF molecular dynamics simulations, we find that mechanical shearing of single asperity induced the degradation of PTFE molecules and radicals, showing subsequent oxidation and hydroxylation reactions of the radicals initiated by C-C bond cleavage, in agreement with previous experimental observations. Furthermore, we studied mechanisms of interfacial tribochemical reactions and formation of transfer films. We found that tribochemical wear and Fe-C and Fe-F bonding networks are important mechanisms for anchoring molecular chains to form a transfer film on the iron countersurface. Hydroxyl groups can dehydrogenate to form short and strong chelation bonds with the Fe2O3 countersurface. A friction-induced oriented molecular layer plays a key role in reducing friction, which is responsible for the excellent lubrication property. By varying temperatures in the range of 10-300 K, we found a nonmonotonic change in friction with a maxima at 100 K. At cryogenic temperatures, the molecular mobility was obviously suppressed, while the chain rigidity was enhanced, resulting in the less oriented interface and brittle-like shear interface, which is responsible for nonmonotonic friction. This work elucidates mechanisms of tribochemical reactions and transfer film formation between iron and PTFE at the atomistic level, facilitating design and development of self-lubricating materials, especially under harsh conditions. read less

Abstract: Machine learning (ML) has gained extensive attention in recent years due to its powerful data analysis capabilities. It has been successfully applied to many fields and helped the researchers to achieve several major theoretical and applied breakthroughs. Some of the notable applications in the field of computational nanotechnology are ML potentials, property prediction, and material discovery. This review summarizes the state-of-the-art research progress in these three fields. ML potentials bridge the efficiency versus accuracy gap between density functional calculations and classical molecular dynamics. For property predictions, ML provides a robust method that eliminates the need for repetitive calculations for different simulation setups. Material design and drug discovery assisted by ML greatly reduce the capital and time investment by orders of magnitude. In this perspective, several common ML potentials and ML models are first introduced. Using these state-of-the-art models, developments in property predictions and material discovery are overviewed. Finally, this paper was concluded with an outlook on future directions of data-driven research activities in computational nanotechnology. read less

Abstract: Soft graphitizable carbon-based multifunctional nanomaterials have found versatile applications ranging from energy storage to quantum computing. In contrast, their hard-carbon analogues have been poorly investigated from both fundamental and application-oriented perspectives. The predominant challenges have been (a) the lack of approaches to fabricate porous hard-carbons and (b) their thermally nongraphitizable nature, leading to inaccessibility for several potential applications. In this direction, we present design principles for fabrication of porous hard-carbon-based nanostructured carbon florets (NCFs) with a highly accessible surface area (∼936 m2/g), rivalling their soft-carbon counterparts. Subjecting such thermally stable hard-carbons to a synergistic combination of an electric field and Joule heating drives their transformation to free-standing macroscopic monoliths composed of onion-like carbons (OLCMs). This represents the first such structural transformation observed in sp2-based hard-carbon NCFs to sp2-networked OLCMs. Micro-Raman spectroscopy establishes the simultaneous increase in the intensity of D-, 2D-, and D + G-bands at 1341, 2712, and 2936 cm–1 and is correlated to the reorganization in the disordered graphitic domains of NCFs to curved concentric nested spheres in OLCMs. This therefore completely precludes the formation of a nanodiamond core that has been consistently observed in all previously reported OLCs. The Joule heating-driven formation of OLCMs is accompanied by ∼5700% enhancement in electrical conductivity that is brought about by the fusion of outermost graphitic shells of OLCs to result in monolithic OLC structures (OLCMs). The porous and inter-networked OLCMs exhibit an excellent adsorption-based capture of volatile organic compounds such as toluene at high efficiencies (∼99%) over a concentration range (0.22–1.86 ppm) that is relevant for direct applications such as smoke filters in cigarettes. read less

Abstract: ABSTRACT We present a comprehensive review of methods and applications of molecular simulations of interfacial systems. We give a detailed overview of the main techniques and major challenges in the following aspects of solid and fluid surfaces: adsorption at solid surfaces, interfacial transport and surface-to-bulk partitioning. We summarise methods to estimate macroscopic properties interfaces (adsorption isotherms, surface tension and contact angle) and ways to extract quantitative information about fluctuating liquid surfaces. We demonstrate the usage of these methods on recent applications from the fields of atmospheric science, material science and biophysics. The two main goals of this review are: (i) to provide guidance in practical questions, such as choosing software, force field, level of theory, system geometry, and finding the appropriate selective surface analysis methods based on the type of the interface and the nature of the physical problem to be studied; and (ii) to highlight domains where molecular simulations can bring about substantial advances in our understanding in important questions of applied science as a function of future method development and adaptation for applied fields. read less

Abstract: Two-dimensional (2D) materials exhibit a wide range of optical, electronic, and quantum properties divergent from their bulk counterparts. To realize scalable 2D materials, metal–organic chemical v... read less

Abstract: Disordered graphene networks (DGNs) can be regarded as the three-dimensional (3D) assembly of graphene-like fragments at the nanoscale, in which some intrinsic topological features are usually hidden in these formless fragments without clear understanding. Although some high-resolution structural patterns have been observed in pyrolytic carbons and flash graphene experimentally, it is still hard to characterize the topology and texture of DGNs considering continuous 3D connectivity. Toward this end, starting from the annealing process, we herein performed molecular dynamics simulations to investigate the formation and topological structure of DGNs. Three typical stages are found during the formation of DGNs, that is, the formation of polyaromatic fragments, formation of a disordered framework, and further graphitization. The topology of the obtained DGNs was then investigated, including topological defects, stacking behavior, and global curvature. Several typical in-plane and out-of-plane topological defects are found to connect the 3D network of graphene-like layers. The computed X-ray diffraction and angular defects demonstrate that a high-density DGN tends to form a randomly stacked structure with more connections, while a low-density DGN exhibits more bowl-shaped layers and a less distorted curvature. At low annealing temperatures, the local curvature of DGNs is highly distorted, and the structure seems to lack graphitization compared to high-temperature ones. read less

Abstract: We handshake statistical mechanics with continuum mechanics to develop a methodology for consistent evaluation of the continuum scale properties of two-dimensional materials. The methodology is tested on pristine graphene. Our scope is kept limited to elastic modulus, E, which has been reported to vary between 0.912 TPa and 7 TPa, Poisson’s ratio, ν, which has been reported to vary from being negative to a value as large as 0.46, and effective thickness, q, whose value varies between 0.75 Å and 3.41 Å. Such a large scatter arises due to inconsistent evaluation of these properties and making assumptions that may not be valid at atomistic scales. Our methodology combines three separate methods: uniaxial tension, equibiaxial tension, and flexural out-of-plane free vibrations of simply supported sheets, which, when used in tandem in molecular dynamics, can provide consistent values of E, ν and q. The only assumption made in the present study is the validity of the continuum scale thin plate vibration equation to represent the free vibrations of a graphene sheet. Our results suggest that—(i) graphene is auxetic in nature, (ii) E decreases with increasing size and temperature, and (iii) the effective thickness q increases with increasing size and temperature. Further, a robustness study of the computed mechanical properties shows consistent results, with differences varying between 1.4% and 6%. read less

Abstract: To achieve a realistic model of a carbon nanotube (CNT) membrane, a good understanding of the effects associated with CNT deformations is a key issue. In this study, using molecular dynamics simulation, argon flow through elliptical CNTs is studied. Two armchair CNTs (6, 6) and (10, 10) were considered. The results demonstrated non-uniform dependency of the flow rate to eccentricity of the tube, leading to an unexpectedly increased flow rate in some cases. The effects of tube size, temperature, and pressure gradient are investigated, and longitudinal variations of the interatomic potential and average axial velocity in different segments of the cross section are presented to justify the abnormal behavior of the flow rate with eccentricity. The results showed a significant deviation from the macroscale expectations and approved elliptical deformation as a non-negligible change in the overall flow rate, which should be considered in predictive models of CNT membranes. read less

Abstract: In this paper, nanoscale mechanical properties and failure behavior of graphene with Stone-Wales defect concentration were investigated using molecular dynamics simulations with the latest ReaxFFC-2013 potential that can accurately capture bond breakages of graphitic compounds. The choice of interatomic potential plays an essential role in capturing the deformation mechanism accurately. Stable configuration of two-dimensional graphene experiences out-of-plane deformation leading to ripples and wrinkles in graphene. It is observed that the mechanical properties such as Young’s modulus, ultimate tensile strength, and the fracture strain are dependent on the out-of-plane deformation, temperature, defect concentration, defect orientation, defect layout and loading configuration. It is observed that the post transient phase non-homogenous ripples and wrinkles influence the mechanical properties at low and high defect concentrations, respectively. read less

Abstract: The desalination performance (water permeability and salt rejection) of both uncharged and charged multi‐walled carbon nanotubes (MWCNTs) is computationally assessed by means of pressure‐driven molecular dynamics simulations. It is shown that the performance of these materials surpass that of the widely used polyamide reverse osmosis membranes and are even better than 2D materials such as nanoporous graphene or boron nitride. The molecular origin of the fast water transport through MWCNT materials is ascribed to a synergic effect between the existence of a single water layer and low friction between water molecules and the carbon nanotube surface. Furthermore, for charged MWCNTs it is highlighted that the electrical charges on the nanotube surface result in a strong anchoring of ions and water molecules. This leads to clogging of the annular region between nanotubes and the generation of a force, which makes water transport through the central channel of the MWCNT faster. read less

Abstract: The mechanism of cellulose pyrolysis with CaO is studied using a reactive force field in molecular dynamics simulations (ReaxFF MD). Through the analysis of the changes in the products and bonds ge... read less

Abstract: This study aims to gain an understanding of the mechanical response of nanocomposite systems at a molecular level and uses a carbon nanotube (CNT) and polyethylene (PE) nanocomposite as its example. The CNT and PE systems are modeled using molecular dynamics (MD) method and validated for small strain values. Tensile loading behavior of the nanocomposite is documented at various CNT mass fractions. The Young’s moduli of the several CNT mass fraction samples are evaluated at 2% to 4% strain and compared among them. The results from the nanocomposites, as compared to the mixing rule predictions, show a negligible gain in the tensile strength while reducing the ultimate strength of the nanocomposite.This study aims to gain an understanding of the mechanical response of nanocomposite systems at a molecular level and uses a carbon nanotube (CNT) and polyethylene (PE) nanocomposite as its example. The CNT and PE systems are modeled using molecular dynamics (MD) method and validated for small strain values. Tensile loading behavior of the nanocomposite is documented at various CNT mass fractions. The Young’s moduli of the several CNT mass fraction samples are evaluated at 2% to 4% strain and compared among them. The results from the nanocomposites, as compared to the mixing rule predictions, show a negligible gain in the tensile strength while reducing the ultimate strength of the nanocomposite. read less

Abstract: We provide a new reactive force field (ReaxFF) for simulations of silicon oxycarbide (SiCO) ceramics and of their syntheses from inorganic polymer precursors. The validity of the force field is extensively tested against experimental and computational thermochemical data. Its performance in simulation at elevated temperatures is gauged by the results of comprehensive ab initio molecular dynamics simulations. We apply the force field to the formation of amorphous SiCO in a simulated polymer pyrolysis. Modeling results are in good agreement with experimental observations and allow new insights into the formation of graphene segregations embedded in an amorphous oxycarbide matrix. The new ReaxFF for Si–C–O–H compounds enables large-scale and long-time atomistic simulations with unprecedented fidelity. read less

Abstract: We present an analytical bond-order potential for the Fe–O system, capable of reproducing the basic properties of wüstite as well as the energetics of oxygen impurities in -iron. The potential predicts binding energies of various small oxygen-vacancy clusters in -iron in good agreement with density functional theory results, and is therefore suitable for simulations of oxygen-based defects in iron. We apply the potential in simulations of the stability and structure of Fe/FeO interfaces and FeO precipitates in iron, and observe that the shape of FeO precipitates can change due to formation of well-defined Fe/FeO interfaces. The interface with crystalline Fe also ensures that the precipitates never become fully amorphous, no matter how small they are. read less

Abstract: Together with the second generation REBO reactive potential, replica-exchange molecular dynamics simulations coupled with systematic quenching were used to generate a broad set of isomers for neutral C$_n$ clusters with $n=24$, 42, and 60. All the minima were sorted in energy and analyzed using order parameters to monitor the evolution of their structural and chemical properties. The structural diversity measured by the fluctuations in these various indicators is found to increase significantly with energy, the number of carbon rings, especially 6-membered, exhibiting a monotonic decrease in favor of low-coordinated chains and branched structures. A systematic statistical analysis between the various parameters indicates that energetic stability is mainly driven by the amount of sp$^2$ hybridization, more than any geometrical parameter. The astrophysical relevance of these results is discussed in the light of the recent detection of C$_{60}$ and C$_{60}^+$ fullerenes in the interstellar medium. read less

Abstract: We developed the ReaxFF force field parameters for Ge/O/H interactions, specifically targeted for the applications of Ge/GeO2 interfaces and O-diffusion in bulk Ge. The original training set, taken... read less

Abstract: The thermodynamic coefficients of a free standing infinite graphene monolayer are calculated using the quasi-classical unsymmetrized self-consistent field method (USF). The basic nonlinear integral equations of this theory are solved numerically in the strong anharmonic approximation. The isothermal and adiabatic elastic bulk moduli, the isochoric and isobaric heat capacities, the thermal expansion, thermal pressure coefficient, and the macroscopic Gruneisen parameter are calculated in terms of the derivatives of a specifically chosen interatomic potential function for different values of stress and for temperatures ranging from below room temperature up to the point of loss of thermodynamic stability. The nearest-neighbor distances vary from ≳1.4 Å to ≲1.8 Å for zero stress. Under stress, these distances decrease. At room temperature the molar heat capacities are ∼5.0 Jmol−1K−1. The elasticity moduli vary from 15.0 eV Å − 2 up to zero at the temperature of loss of stability and are increased by stress. The thermal expansion coefficient has a strong dependence on the temperature and is negative for temperatures lower than ∼340 K. For high temperatures it monotonically increases and decreases with stress. The macroscopic Gruneisen parameter has a strong nonlinear dependence with temperature and is estimated in about 3.0 ÷ 3.7 ; at ∼340 K its value decreases to ∼1.0 K and for even lower temperature it shows a peak and deep structure similar to what has been earlier reported for fullerene C60. read less

Abstract: Molecular simulation is a powerful computational tool for a broad range of applications including the examination of materials properties and accelerating drug discovery. At the heart of molecular simulation is the analytic potential energy function. These functions span the range of complexity from very simple functions used to model generic phenomena to complex functions designed to model chemical reactions. The complexity of the mathematical function impacts the computational speed and is typically linked to the accuracy of the results obtained from simulations that utilize the function. One approach to improving accuracy is to simply add more parameters and additional complexity to the analytic function. This approach is typically used in non-reactive force fields where the functional form is not derived from quantum mechanical principles. The form of other types of potentials, such as the bond-order potentials, is based on quantum mechanics and has led to varying levels of accuracy and transferability. When selecting a potential energy function for use in molecular simulations, the accuracy, transferability, and computational speed must all be considered. In this focused review, some of the more commonly used potential energy functions for molecular simulations are reviewed with an eye toward presenting their general forms, strengths, and weaknesses.Molecular simulation is a powerful computational tool for a broad range of applications including the examination of materials properties and accelerating drug discovery. At the heart of molecular simulation is the analytic potential energy function. These functions span the range of complexity from very simple functions used to model generic phenomena to complex functions designed to model chemical reactions. The complexity of the mathematical function impacts the computational speed and is typically linked to the accuracy of the results obtained from simulations that utilize the function. One approach to improving accuracy is to simply add more parameters and additional complexity to the analytic function. This approach is typically used in non-reactive force fields where the functional form is not derived from quantum mechanical principles. The form of other types of potentials, such as the bond-order potentials, is based on quantum mechanics and has led to varying levels of accuracy and transferabilit... read less

Abstract: A new parametrization of the anisotropic interlayer potential for hexagonal boron nitride (h-BN ILP) is presented. The force-field is benchmarked against density functional theory calculations of several dimer systems within the Heyd-Scuseria-Ernzerhof hybrid density functional approximation, corrected for many-body dispersion effects. The latter, more advanced method for treating dispersion, is known to produce binding energies nearly twice as small as those obtained with pairwise correction schemes, used for an earlier ILP parametrization. The new parametrization yields good agreement with the reference calculations to within ∼1 and ∼0.5 meV/atom for binding and sliding energies, respectively. For completeness, we present a complementary parameter set for homogeneous graphitic systems. Together with our previously suggested ILP parametrization for the heterogeneous graphene/h-BN junction, this provides a powerful tool for consistent simulation of the structural, mechanical, tribological, and heat transp... read less

Abstract: ReaxFF is an efficient member of reactive molecular dynamics approaches to model chemical reactions for different chemical environments. Here it is applied to the structure formation process of twin polymerization, a newly developed method to obtain nanostructured functional materials. To achieve this, a site dependent atom type (SDAT) generalization of the classical ReaxFF approach is presented, which employs more then one atom type per chemical element. The efficacy of this SDAT-ReaxFF approach is demonstrated for two different cases: a benzene–benzyl reaction as well as for the twin polymerization. read less

Abstract: Interatomic potentials, which describe interactions between elements of nanosystems, are crucial in theoretical study of their physical properties. We focus on two well known empirical potentials, i.e. Tersoff's and Brenner's potentials, and compare their performance in calculation of thermal transport in carbon nanotubes. In this way, we study the temperature and diameter dependence of thermal conductivity of single walled armchair carbon nanotube by using the mentioned interatomic potentials. We take advantage of direct non-equilibrium molecular dynamics simulation, which well resembles the experimental set up for thermal conductivity measurement. The results show that increasing the temperature increases the conductivity in contrast with diameter growth which decreases the thermal conductivity. It is important to note that both interatomic potentials describe the system behavior very well, however they lead to different conductivity values. It is found that the difference between the performance of studied potentials can be seen more obviously in longer tubes. We also observe a peak in thermal conductivity by increasing system temperature. System is deformed at T≈1000 K, when Tersoff's potential is employed for description of interactions. While its instability occurs at higher temperature (T≈1600 K), when we try to simulate system by Brenner's potential. read less

Abstract: The major objective of this work is to present results of a classical molecular dynamics study to investigate the effect of changing the cut-off distance in the empirical potential on the stress–strain relation and also the temperature dependent Young’s modulus of pristine and defective hexagonal boron nitride. As the temperature increases, the computed Young’s modulus shows a significant decrease along both the armchair and zigzag directions. The computed Young’s modulus shows a trend in keeping with the structural anisotropy of h-BN. The variation of Young’s modulus with system size is elucidated. The observed mechanical strength of h-BN is significantly affected by the vacancy and Stone–Wales type defects. The computed room temperature Young’s modulus of pristine h-BN is 755 GPa and 769 GPa respectively along the armchair and zigzag directions. The decrease of Young’s modulus with increase in temperature has been analyzed and the results show that the system with zigzag edge shows a higher value of Young’s modulus in comparison to that with armchair edge. As the temperature increases, the computed stiffness decreases and the system with zigzag edge possesses a higher value of stiffness as compared to the armchair counterpart and this behaviour is consistent with the variation of Young’s modulus. The defect analysis shows that presence of vacancy type defects leads to a higher Young’s modulus, in the studied range with different percentage of defect concentration, in comparison with Stone–Wales defect. The variations in the peak position of the computed radial distribution function reveals the changes in the structural features of systems with zigzag and armchair edges in the presence of applied stress. read less

Abstract: For centuries, cutting and folding papers with special patterns have been used to build beautiful, flexible and complex three-dimensional structures. Inspired by the old idea of kirigami (paper cutting), and the outstanding properties of graphene, recently graphene kirigami structures were fabricated to enhance the stretchability of graphene. However, the possibility of further tuning the electronic and thermal transport along the 2D kirigami structures has remained original to investigate. We therefore performed extensive atomistic simulations to explore the electronic, heat and load transfer along various graphene kirigami structures. The mechanical response and thermal transport were explored using classical molecular dynamics simulations. We then used a real-space Kubo-Greenwood formalism to investigate the charge transport characteristics in graphene kirigami. Our results reveal that graphene kirigami structures present highly anisotropic thermal and electrical transport. Interestingly, we show the possibility of tuning the thermal conductivity of graphene by four orders of magnitude. Moreover, we discuss the engineering of kirigami patterns to further enhance their stretchability by more than 10 times as compared with pristine graphene. Our study not only provides a general understanding concerning the engineering of electronic, thermal and mechanical response of graphene, but more importantly can also be useful to guide future studies with respect to the synthesis of other 2D material kirigami structures, to reach highly flexible and stretchable nanostructures with finely tunable electronic and thermal properties. read less

Abstract: Stanene, a low thermal conductivity two-dimensional (2D) sheet composed of group-IV element Sn, is a prototype material with novel properties such as 2D topological insulating behavior and near-room-temperature quantum Hall effects. Monolayer graphene, on the other hand, possesses unusual thermal properties, but has a zero bandgap. By stacking stanene and graphene monolayers vertically into a hetero-bilayer, an indirect bandgap can be obtained, making the hetero-bilayer a good candidate for special applications. In this work, the in-plane thermal conductivity (κ) and out-of-plane interfacial thermal resistance (R) in the hetero-bilayer are systematically investigated using non-equilibrium molecular dynamics and transient pump-probe methods. Effects of dimension, system temperature and van der Waals coupling strength on the thermal properties are examined. The predicted in-plane thermal conductivity of the graphene/stanene hetero-bilayer is 311.1 W m-1 K-1, higher than most 2D materials such as phosphorene, hexagonal boron nitride (h-BN), MoS2 and MoSe2. Phonon power spectra are recorded for graphene and stanene individually to help the explanation of their κ difference. The inter-layer thermal resistance between graphene and stanene hetero-bilayers is predicted to be 2.13 × 10-7 K m2 W-1, which is on the same order of magnitude as several other 2D bilayer structures. read less

Abstract: Transferable ReaxFF-lg models of nitromethane that predict a variety of material properties over a wide range of thermodynamic states are obtained by screening a library of ∼6600 potentials that were previously optimized through the Multiple Objective Evolutionary Strategies (MOES) approach using a training set that included information for other energetic materials composed of carbon, hydrogen, nitrogen, and oxygen. Models that best match experimental nitromethane lattice constants at 4.2 K and 1 atm are evaluated for transferability to high-pressure states at room temperature and are shown to better predict various liquid- and solid-phase structural, thermodynamic, and transport properties as compared to the existing ReaxFF and ReaxFF-lg parametrizations. Although demonstrated for an energetic material, the library of ReaxFF-lg models is supplied to the scientific community to enable new research explorations of complex reactive phenomena in a variety of materials research applications. read less

Abstract: A detailed insight of key reactive events related to oxidation and pyrolysis of hydrocarbon fuels further enhances our understanding of combustion chemistry. Though comprehensive kinetic models are available for smaller hydrocarbons (typically C3 or lower), developing and validating reaction mechanisms for larger hydrocarbons is a daunting task, due to the complexity of their reaction networks. The ReaxFF method provides an attractive computational method to obtain reaction kinetics for complex fuel and fuel mixtures, providing an accuracy approaching ab-initio-based methods but with a significantly lower computational expense. The development of the first ReaxFF combustion force field by Chenoweth et al. (CHO-2008 parameter set) in 2008 has opened new avenues for researchers to investigate combustion chemistry from the atomistic level. In this article, we seek to address two issues with the CHO-2008 ReaxFF description. While the CHO-2008 description has achieved significant popularity for studying large hydrocarbon combustion, it fails to accurately describe the chemistry of small hydrocarbon oxidation, especially conversion of CO2 from CO, which is highly relevant to syngas combustion. Additionally, the CHO-2008 description was obtained faster than expected H abstraction by O2 from hydrocarbons, thus underestimating the oxidation initiation temperature. In this study, we seek to systemically improve the CHO-2008 description and validate it for these cases. Additionally, our aim was to retain the accuracy of the 2008 description for larger hydrocarbons and provide similar quality results. Thus, we expanded the ReaxFF CHO-2008 DFT-based training set by including reactions and transition state structures relevant to the syngas and oxidation initiation pathways and retrained the parameters. To validate the quality of our force field, we performed high-temperature NVT-MD simulations to study oxidation and pyrolysis of four different hydrocarbon fuels, namely, syngas, methane, JP-10, and n-butylbenzene. Results obtained from syngas and methane oxidation simulation indicated that our redeveloped parameters (named as the CHO-2016 parameter set) has significantly improved the C1 chemistry predicted by ReaxFF and has solved the low-temperature oxidation initiation problem. Moreover, Arrhenius parameters of JP-10 decomposition and initiation mechanism pathways of n-butylbenzene pyrolysis obtained using the CHO-2016 parameter set are also in good agreement with both experimental and CHO-2008 simulation results. This demonstrated the transferability of the CHO-2016 description for a wide range of hydrocarbon chemistry. read less

Abstract: We simulate the reaction process of 1,3,5-triamino-2,4,6-trinitrobenzene in wide temperature and pressure ranges by molecular dynamics and evaluate the intermediate molecules, chemical reaction rates, and Hugoniot relations. Based on them, the leading shock wave, fast reaction zone, Chapman-Jouguet state, and slow reaction zone under detonation are investigated by different theoretical methods. A complete structure of the detonation wave is obtained. The calculated detonation velocity, detonation pressure, detonation products, and the length of the reaction zone are in agreement with the experiments and others' calculations. We find that some intermediate molecules play an important role in determining the reaction path of explosives but just remain a little after detonation, such as H2 and NH3. read less

Abstract: Interfacial thermal conductance plays a vital role in defining the thermal properties of nanostructured materials in which heat transfer is predominantly phonon mediated. In this work, the thermal contact resistance (R) of a linear heterojunction within a hybrid graphene/hexagonal boron nitride (h-BN) sheet is characterized using non-equilibrium molecular dynamics (NEMD) simulations. The effects of system dimension, heat flux direction, temperature and tensile strain on the predicted R values are investigated. The spatiotemporal evolution of thermal energies from the graphene to the h-BN sheet reveals that the main energy carrier in graphene is the flexural phonon (ZA) mode, which also has the most energy transmissions across the interface. The calculated R decreases monotonically from 5.2 × 10(-10) to 2.2 × 10(-10) K m(2) W(-1) with system lengths ranging from 20 to 100 nm. For a 40 nm length hybrid system, the calculated R decreases by 42% from 4.1 × 10(-10) to 2.4 × 10(-10) K m(2) W(-1) when the system temperature increases from 200 K to 600 K. The study of the strain effect shows that the thermal contact resistance R between h-BN and graphene sheets increases with the tensile strain. Detailed phonon density of states (PDOS) is computed to understand the thermal resistance results. read less

Abstract: We present a computational tool, eReaxFF, for simulating explicit electrons within the framework of the standard ReaxFF reactive force field method. We treat electrons explicitly in a pseudoclassical manner that enables simulation several orders of magnitude faster than quantum chemistry (QC) methods, while retaining the ReaxFF transferability. We delineate here the fundamental concepts of the eReaxFF method and the integration of the Atom-condensed Kohn-Sham DFT approximated to second order (ACKS2) charge calculation scheme into the eReaxFF. We trained our force field to capture electron affinities (EA) of various species. As a proof-of-principle, we performed a set of molecular dynamics (MD) simulations with an explicit electron model for representative hydrocarbon radicals. We establish a good qualitative agreement of EAs of various species with experimental data, and MD simulations with eReaxFF agree well with the corresponding Ehrenfest dynamics simulations. The standard ReaxFF parameters available in the literature are transferrable to the eReaxFF method. The computationally economic eReaxFF method will be a useful tool for studying large-scale chemical and physical systems with explicit electrons as an alternative to computationally demanding QC methods. read less

Abstract: ABSTRACT In this paper, the mechanical properties of graphene oxide are obtained using the molecular dynamics analysis, including the ultimate stress, Young modulus, shear modulus and elastic constants, and the results are compared with those of pristine graphene. It is observed that the increase of oxide agents (–O) and (–OH) leads to the increase of C–C bond length at each hexagonal lattice and as a result, alter the mechanical properties of the graphene sheet. It is shown that the elasticity modulus and ultimate tensile strength of graphene oxides (–O) and (–OH) decrease significantly causing the failure behavior of graphene sheet changes from the brittle to ductile. The results of shear loading tests illustrate that the increase of oxide agents (–O/–OH) results in the decrease of ultimate shear stress and shear module of the graphene sheet. It is shown that the increase of oxide agents in the graphene sheet leads to decrease of the elastic constants, in which the reduction of elastic properties in the armchair direction is more significant than the zigzag direction. Moreover, the graphene sheet with oxide agents (–O) and (–O/–OH) presents an anisotropic behavior. read less

Abstract: Graphene is an elementary unit for various carbon based nanostructures. The recent technological developments have made it possible to manufacture hybrid and sandwich structures with graphene. In order to model these nanostructures in atomistic scale, a compatible interatomic potential is required to successfully model these nanostructures. In this article, an interatomic potential with modified cut-off function for Tersoff potential was proposed to avoid overestimation and also to predict the realistic mechanical behavior of single sheet of graphene. In order to validate the modified form of cut-off function for Tersoff potential, simulations were performed with different set of temperatures and strain rates, and results were made to compare with available experimental data and molecular dynamics simulation results obtained with the help of other empirical interatomic potentials. read less

Abstract: A golden opportunity for graphene Reducing friction can limit wear and improve the energy efficiency of mechanical devices. Graphene is a promising lubricant because the friction between sheets is minuscule under certain circumstances. Kawai et al. show that the same ultra-low frictional properties extend to other surfaces. They find ultralow friction when dragging graphene nanoribbons across a gold surface using an atomic force microscope. This discovery sets up the potential for developing nanographene frictionless coatings. Science, this issue p. 957 Experiments reveal ultralow friction when graphene nanoribbons slide across an oriented gold surface. The state of vanishing friction known as superlubricity has important applications for energy saving and increasing the lifetime of devices. Superlubricity, as detected with atomic force microscopy, appears when sliding large graphite flakes or gold nanoclusters across surfaces, for example. However, the origin of the behavior is poorly understood because of the lack of a controllable nanocontact. We demonstrated the superlubricity of graphene nanoribbons when sliding on gold with a joint experimental and computational approach. The atomically well-defined contact allows us to trace the origin of superlubricity, unraveling the role played by ribbon size and elasticity, as well as by surface reconstruction. Our results pave the way to the scale-up of superlubricity and thus to the realization of frictionless coatings. read less

Abstract: Molecular dynamics simulation is used to determine the mechanical properties of gold coating from indentation. The results show that the depressed surface generation process should be attributed to three key factors: first, the atom inclined to go to its original crystal position under the molecular interactions; second, the attractive force exerted by the indenter at the unloading stage; third, the self-organization effect induced by surface/subsurface atoms, and the final depressed surface is generated through the strong nonlinear coupling effect of these three basic factors. There are little anisotropy phenomenon in the elastic deformation stage while obvious anisotropy phenomenon is observed in the plastic deformation process. The maximum value of contact pressure is located at the edge of contact area and vanished outward from the contact circle. The magnitudes of the maximum principal stresses appear to shift outward, away from the center of contact area. read less

Abstract: We performed reactive force field molecular dynamics simulation to observe the hydrolysis reactions between water molecules and locally strained SiO2 geometry. We optimized the force field from J. Fogarty et al. 2010, to more accurately describe the hydroxylation reaction barrier for strained and nonstrained Si—O structures, which are about 20 and 30 kcal/mol, respectively. After optimization, energy barrier for the hydroxylation shows a good agreement with DFT data. The observation of silanol formation at the high-strain region of a silica nanorod also supports the concept that the adsorption of water molecule: hydroxyl formation favors the geometry with higher strain energy. In addition, we found three distinct hydroxylation paths—H3O+ formation reaction from the adsorbed water, proton donation from H3O+, and the direct dissociation of the adsorbed water molecule. Because water molecules and their hydrogen bond network behave differently with respect to temperature ranges, silanol formation is also affe... read less

Abstract: Due to their exceptional mechanical properties, thermal conductivity and a wide band gap (5-6 eV), boron nitride nanotubes and nanosheets have promising applications in the field of engineering and biomedical science. Accurate modeling of failure or fracture in a nanomaterial inherently involves coupling of atomic domains of cracks and voids as well as a deformation mechanism originating from grain boundaries. This review highlights the recent progress made in the atomistic modeling of boron nitride nanofillers. Continuous improvements in computational power have made it possible to study the structural properties of these nanofillers at the atomistic scale. read less

Abstract: A new three-dimensional (3D) printing process designated as shockwave-induced freeform technique (SWIFT) is explored for fabricating microparts from nanopowders. SWIFT consists of generating shockwaves using a laser beam, applying these shocks to pressure sinter nanoparticles at room temperature, and creating structures and devices by the traditional layer-by-layer formation. Shockwave cold compaction of nanoscale powders has the capability to overcome limitations, such as shrinkage, porosity, rough surface, and wide tolerance, normally encountered in hot sintering processes, such as selective laser sintering. In this study, the window of operating parameters and the underlying physics of SWIFT were investigated using a high-energy Q-switched Nd: YAG laser and nanodiamond (ND) powders. Results indicate the potential of SWIFT for fabricating high-performance diamond microtools with high aspect ratios, smooth surfaces, and sharp edges. The drawback is that the SWIFT process does not work for micro-sized powders. read less

Abstract: This review summarizes state-of-the-art progress in the molecular dynamics (MD) simulation of the novel thermal properties of graphene. The novel thermal properties of graphene, which include anisotropic thermal conductivity, decoupled phonon thermal transport, thermal rectification and tunable interfacial thermal conductance, have attracted enormous interest in the development of next-generation nano-devices. Molecular dynamics simulation is one of the main approaches in numerical simulation of the novel thermal properties of graphene. In this paper, the widely used potentials of MD for modeling the novel thermal properties of graphene are described first. Then MD simulations of anisotropic thermal conductivity, decoupled phonon thermal transport, thermal rectification and tunable interfacial thermal conductance are discussed. Finally, the paper concludes with highlights on both the current status and future directions of the MD simulation of the novel thermal properties of graphene. read less

Abstract: Graphene has served as the model 2D system for over a decade, and the effects of grain boundaries (GBs) on its electrical and mechanical properties are very well investigated. However, no direct measurement of the correlation between thermal transport and graphene GBs has been reported. Here, we report a simultaneous comparison of thermal transport in supported single crystalline graphene to thermal transport across an individual graphene GB. Our experiments show that thermal conductance (per unit area) through an isolated GB can be up to an order of magnitude lower than the theoretically anticipated values. Our measurements are supported by Boltzmann transport modeling which uncovers a new bimodal phonon scattering phenomenon initiated by the GB structure. In this novel scattering mechanism, boundary roughness scattering dominates the phonon transport in low-mismatch GBs, while for higher mismatch angles there is an additional resistance caused by the formation of a disordered region at the GB. Nonequilibrium molecular dynamics simulations verify that the amount of disorder in the GB region is the determining factor in impeding thermal transport across GBs. read less

Abstract: Simulations of molecular dynamics (MD) were performed to investigate the bending behavior of a carbon nanotube without and with fluid (argon atoms) inside. The unique deformed shapes and energy variations were observed for interactions between internal argon atoms and a carbon nanotube with various argon atom velocities and system temperatures. The mechanical properties of the nanotube were observed using the fluid structure interaction (FSI) methods for analyzing forces between a nanotube's solid wall and the internal fluid. Simulation results showed that the kinks obstructed the internal flow. On the other hand, it was confirmed that the generation of double kinks and the energy curve depend on the bending shape. read less

Abstract: We propose parametrizing the Stillinger–Weber potential for covalent materials starting from the valence force-field model. All geometrical parameters in the Stillinger–Weber potential are determined analytically according to the equilibrium condition for each individual potential term, while the energy parameters are derived from the valence force-field model. This parametrization approach transfers the accuracy of the valence force field model to the Stillinger–Weber potential. Furthermore, the resulting Stilliinger–Weber potential supports stable molecular dynamics simulations, as each potential term is at an energy-minimum state separately at the equilibrium configuration. We employ this procedure to parametrize Stillinger–Weber potentials for single-layer MoS2 and black phosphorous. The obtained Stillinger–Weber potentials predict an accurate phonon spectrum and mechanical behaviors. We also provide input scripts of these Stillinger–Weber potentials used by publicly available simulation packages including GULP and LAMMPS. read less

Abstract: Due to rapidly increasing power densities in nanoelectronics, efficient heat removal has become one of the most critical issues in thermal management and nanocircuit design. In this study, we report a surface nanoengineering design that can reduce the interfacial thermal resistance between graphene and copper substrate by 17%. Contrary to the conventional view that a rough surface tends to give higher thermal contact resistances, we find that by engraving the copper substrate with nanopillared patterns, an optimized hybrid structure can effectively facilitate the thermal transport across the graphene-copper interface. This counterintuitive behavior is due to the enhanced phonon interactions with the optimal nanopillared pattern. For pliable 2D materials like graphene, the structures can be easily bent to fit the surface formations of the substrate. The suspended areas of graphene are pulled towards the substrate via an attractive interatomic force, causing high local pressures (∼2.9 MPa) on the top region of nanopillars. The high local pressures can greatly enhance the thermal energy coupling between graphene and copper, thereby lowering the thermal contact resistances. Our study provides a practical way to manipulate the thermal contact resistance between graphene and copper for the improvement of nano-device performance through engineering optimal nanoscale contact. read less

Abstract: In recently synthesized two-dimensional superlattices of graphene and boron nitride, the atomic structure of the interface is complicated by a 2% lattice mismatch between the two materials. Using atomistic and continuum analysis, we show that the mismatch results in a competition between two strain-relieving mechanisms: misfit dislocations and rippling. For flat superlattices, beyond a critical pitch the interface is decorated by strain-relieving misfit dislocations. For superlattices that can deform out-of-plane, optimal ripple wavelengths emerge. read less

Abstract: High temperature annealing is the only method known to date that allows the complete repair of a defective lattice of graphenes derived from graphite oxide, but most of the relevant aspects of such restoration processes are poorly understood. Here, we investigate both experimentally (scanning probe microscopy) and theoretically (molecular dynamics simulations) the thermal evolution of individual graphene oxide sheets, which is rationalized on the basis of the generation and the dynamics of atomic vacancies in the carbon lattice. For unreduced and mildly reduced graphene oxide sheets, the amount of generated vacancies was so large that they disintegrated at 1773-2073 K. By contrast, highly reduced sheets survived annealing and their structure could be completely restored at 2073 K. For the latter, a minor atomic-sized defect with six-fold symmetry was observed and ascribed to a stable cluster of nitrogen dopants. The thermal behavior of the sheets was significantly altered when they were supported on a vacancy-decorated graphite substrate, as well as for the overlapped/stacked sheets. In these cases, a net transfer of carbon atoms between neighboring sheets via atomic vacancies takes place, affording an additional healing process. Direct evidence of sheet coalescence with the step edge of the graphite substrate was also gathered from experiments and theory. read less

Abstract: ReaxFF (van Duin, A.C.T.; Dasgupta, S.; Lorant, F.; Goddard, W.A. J. Phys. Chem. A, 2001, 105, 9396-9409) reactive potentials are parametrized for cyclotrimethylene trinitramine (RDX) and 1,1-diamino-2,2-dinitroethene (FOX-7) in a novel application combining data envelopment analysis and a modern self-adaptive evolutionary algorithm to optimize multiple objectives simultaneously and map the entire family of solutions. In order to correct the poor crystallographic parameters predicted by ReaxFF using its base parametrization (Strachan, A.; van Duin, A. C. T.; Chakraborty, D.; Dasgupta S.; Goddard, W. A. Phys. Rev. Lett., 2003, 91, 098301), we augmented the existing training set data used for parametrization with additional (SAPT)DFT calculations of RDX and FOX-7 dimer interactions. By adjusting a small subset of the ReaxFF parameters that govern long-range interactions, the evolutionary algorithm approach converges on a family of solutions that best describe crystallographic parameters through simultaneous optimization of the objective functions. Molecular dynamics calculations of RDX and FOX-7 are conducted to assess the quality of the force fields, resulting in parametrizations that improve the overall prediction of the crystal structures. read less

Abstract: In our study, the Ni/YSZ ReaxFF reactive force field was developed by combining the YSZ and Ni/C/H descriptions. ReaxFF reactive molecular dynamics (RMD) were applied to model chemical reactions, diffusion, and other physicochemical processes at the fuel/Ni/YSZ interface. The ReaxFF RMD simulations were performed on the H2/Ni/YSZ and C4H10/Ni/YSZ triple-phase boundary (TPB) systems at 1250 and 2000 K, respectively. The simulations indicate amorphization of the Ni surface, partial decohesion (delamination) at the interface, and coking, which have indeed all been observed experimentally. They also allowed us to derive the mechanism of the butane conversion at the Ni/YSZ interface. Many steps of this mechanism are similar to the pyrolysis of butane. The products obtained in our simulations are the same as those in experiment, which indicates that the developed ReaxFF potential properly describes complex physicochemical processes, such as the oxide-ion diffusion, fuel conversion, water formation reaction, coking, and delamination, occurring at the TPB and can be recommended for further computational studies of the fuel/electrode/electrolyte interfaces in a SOFC. read less

Abstract: The molecular dynamics simulation code ls1 mardyn is presented. It is a highly scalable code, optimized for massively parallel execution on supercomputing architectures and currently holds the world record for the largest molecular simulation with over four trillion particles. It enables the application of pair potentials to length and time scales that were previously out of scope for molecular dynamics simulation. With an efficient dynamic load balancing scheme, it delivers high scalability even for challenging heterogeneous configurations. Presently, multicenter rigid potential models based on Lennard-Jones sites, point charges, and higher-order polarities are supported. Due to its modular design, ls1 mardyn can be extended to new physical models, methods, and algorithms, allowing future users to tailor it to suit their respective needs. Possible applications include scenarios with complex geometries, such as fluids at interfaces, as well as nonequilibrium molecular dynamics simulation of heat and mass transfer. read less

Abstract: We study the shock-induced hot spot formation mechanism of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine-based explosives by molecular dynamics, compare different kinds of desensitizers and different shock velocities. A set of programs is written to calculate the physical picture of shock loading. Based on the simulations and analyses, the hot spots are found at the interface and are heated by plastic work in three ways: the interface intrinsic dissipation, the pore collapse, and the coating layer deformation. The work/heat transition rate is proved to be increasing with a loading speed. read less

Abstract: We developed a multiscale approach to explore the effective thermal conductivity of polycrystalline graphene sheets. By performing equilibrium molecular dynamics (EMD) simulations, the grain size effect on the thermal conductivity of ultra-fine grained polycrystalline graphene sheets is investigated. Our results reveal that the ultra-fine grained graphene structures have thermal conductivity one order of magnitude smaller than that of pristine graphene. Based on the information provided by the EMD simulations, we constructed finite element models of polycrystalline graphene sheets to probe the thermal conductivity of samples with larger grain sizes. Using the developed multiscale approach, we also investigated the effects of grain size distribution and thermal conductivity of grains on the effective thermal conductivity of polycrystalline graphene. The proposed multiscale approach on the basis of molecular dynamics and finite element methods could be used to evaluate the effective thermal conductivity of polycrystalline graphene and other 2D structures. read less

Abstract: This article reviews recent advances in the development of reactive empirical force fields or potentials. In particular, we compare two widely used reactive potentials with variable-charge schemes that are desirable for multicomponent or multifunctional systems: the ReaxFF (reactive force field) and charge-optimized many-body (COMB) potentials. Several applications of these approaches in atomistic simulations that involve metal-based heterogeneous systems are also discussed. read less

Abstract: In this chapter, we will present a contemporary review of the hitherto numerical characterization of nanowires (NWs). The bulk of the research reported in the literatures concern metallic NWs including Al, Cu, Au, Ag, Ni, and their alloys NWs. Research has also been reported for the investigation of some nonmetallic NWs, such as ZnO, GaN, SiC, SiO2. A plenty of researches have been conducted regarding the numerical investigation of NWs. Issues analyzed include structural changes under different loading situations, the formation and propagation of dislocations, and the effect of the magnitude of applied loading on deformation mechanics. Efforts have also been made to correlate simulation results with experimental measurements. However, direct comparisons are difficult since most simulations are carried out under conditions of extremely high strain/loading rates and small simulation samples due to computational limitations. Despite of the immense numerical studies of NWs, a significant work still lies ahead in terms of problem formulation, interpretation of results, identification and delineation of deformation mechanisms, and constitutive characterization of behavior. In this chapter, we present an introduction of the commonly adopted experimental and numerical approaches in studies of the deformation of NWs in Section 1. An overview of findings concerning perfect NWs under different loading situations, such as tension, compression, torsion, and bending are presented in Section 2. In Section 3, we will detail some recent results from the authors’ own work with an emphasis on the study of influences from different pre-existing defect on NWs. Some thoughts on future directions of the computational mechanics of NWs together with Conclusions will be given in the last section. read less

Abstract: Quantum information technologies require networks of interacting defect bits. Color centers, especially the nitrogen vacancy (NV−) center in diamond, represent one promising avenue, toward the realisation of such devices. The most successful technique for creating NV− in diamond is ion implantation followed by annealing. Previous experiments have shown that shallow nitrogen implantation (<10 keV) results in NV− centers with a yield of 0.01%–0.1%. We investigate the influence of channeling effects during shallow implantation and statistical diffusion of vacancies using molecular dynamics and Monte Carlo simulation techniques. Energy barriers for the diffusion process were calculated using density functional theory. Our simulations show that 25% of the implanted nitrogens form a NV center, which is in good agreement with our experimental findings. read less

Abstract: We investigate the effect of strain and isotopic disorder on thermal transport in suspended graphene by equilibrium molecular dynamics simulations. We show that the thermal conductivity of unstrained graphene, calculated from the fluctuations of the heat current at equilibrium, is finite and converges with size at finite temperature. In contrast, the thermal conductivity of strained graphene diverges logarithmically with the size of the models, when strain exceeds a relatively large threshold value of 2%. An analysis of phonon populations and lifetimes explains the divergence of the thermal conductivity as a consequence of changes in the occupation of low-frequency out-of-plane phonons and an increase in their lifetimes due to strain. read less

Abstract: Space vehicles often encounter very high temperature and harsh oxidative environments. To ensure proper thermal protection, layers composed of SiC and EPDM polymer are placed on the outer surface of the space vehicle. The O2 and H2O molecules are able to oxidize the SiC network, creating SiO2-type structures that may form a protective layer, while also pyrolyzing and burning the EPDM polymer, causing ablation. Reactive molecular dynamics simulations nicely complement experiment, as they provide direct observation and information to calculate physical parameters such as transport diffusivities and reaction constants. In this study, rate models were developed and molecular dynamics simulated trajectories were used to extract Arrhenius parameters that describe the initial stages of transport and kinetics of SiC oxidation by O2 and H2O and the combustion and pyrolysis of EPDM. The simulations showed that O2 was able to oxidize SiC more efficiently than H2O, with the transport activation barrier of O2 in the r... read less

Abstract: The complexity of the molecular recognition and assembly of biotic-abiotic interfaces on a scale of 1 to 1000 nm can be understood more effectively using simulation tools along with laboratory instrumentation. We discuss the current capabilities and limitations of atomistic force fields and explain a strategy to obtain dependable parameters for inorganic compounds that has been developed and tested over the past decade. Parameter developments include several silicates, aluminates, metals, oxides, sulfates, and apatites that are summarized in what we call the INTERFACE force field. The INTERFACE force field operates as an extension of common harmonic force fields (PCFF, COMPASS, CHARMM, AMBER, GROMACS, and OPLS-AA) by employing the same functional form and combination rules to enable simulations of inorganic-organic and inorganic-biomolecular interfaces. The parametrization builds on an in-depth understanding of physical-chemical properties on the atomic scale to assign each parameter, especially atomic charges and van der Waals constants, as well as on the validation of macroscale physical-chemical properties for each compound in comparison to measurements. The approach eliminates large discrepancies between computed and measured bulk and surface properties of up to 2 orders of magnitude using other parametrization protocols and increases the transferability of the parameters by introducing thermodynamic consistency. As a result, a wide range of properties can be computed in quantitative agreement with experiment, including densities, surface energies, solid-water interface tensions, anisotropies of interfacial energies of different crystal facets, adsorption energies of biomolecules, and thermal and mechanical properties. Applications include insight into the assembly of inorganic-organic multiphase materials, the recognition of inorganic facets by biomolecules, growth and shape preferences of nanocrystals and nanoparticles, as well as thermal transitions and nanomechanics. Limitations and opportunities for further development are also described. read less

Abstract: Typical empirical bond-order potentials are short ranged and give ductile instead of brittle behavior for materials such as crystalline silicon or diamond. Screening functions can be used to increase the range of these potentials. We outline a general procedure to combine screening functions with bond-order potentials that does not require to refit any of the potential's properties. We use this approach to modify Tersoff's [Phys. Rev. B 39, 5566 (1989)], Erhart & Albe's [Phys. Rev. B 71, 35211 (2005)] and Kumagai et al.'s [Comp. Mater. Sci. 39, 457 (2007)] Si, C and Si-C potentials. The resulting potential formulations correctly reproduce brittle materials response, and give an improved description of amorphous phases. read less

Abstract: A new bond-order potential for modeling systems containing silicon, carbon, and hydrogen, such as organosilicon molecules (CxSiyHz), solid silicon, solid carbon, and alloys of silicon and carbon, is presented. This reactive potential utilizes the formalism of the second-generation reactive empirical bond-order potential (REBO) [Brenner et al. J. Phys.: Condens. Matter 2002, 14, 783] for hydrocarbons and the REBO parameters for silicon [Schall, Gao, Harrison. Phys. Rev. B 2008, 77, 115209]. Modifications to the hydrocarbon REBO potential were made to improve the description of three-atom type systems. The widespread use of Brenner’s REBO potential, its ability to model a wide range of hydrocarbon materials, and the existence of parameters for several atom types are some of the motivating factors for obtaining this Si–C–H (2B-SiCH) parametrization. The usefulness and flexibility of this potential is demonstrated by examining the properties of organosilicon molecules, the bulk, surface, and defect properties... read less

Abstract: Understanding major mechanisms affecting material strength such as grain size, grain orientation and dislocation mechanism from atomistic viewpoint can empower scientists and engineers with the capability to produce vastly strengthened materials. Computational studies can offer the possibility of carrying out simulations of material properties at both larger length scales and longer times than direct atomistic calculations. The study has conducted theoretical modeling and experimental testing to investigate nanoscale mechanisms related to material strength and interfacial performance. Various computational algorithms in nanomechanics including energy minimization, molecular dynamics and hybrid approaches that mix atomistic and continuum methods to bridge the length and time scales have been used to thoroughly study the deformation and strengthening mechanisms. Our study has also performed experiments including depth-sensing indentation technique and in-situ pico-indentation to characterize the nanomechanisms related to material strength and tribological performance. In this project, we have developed the innovative mutil-scale algorithms in the area of nanomechanics. These approaches were used to studies the defect effect on the mechanical properties of thin film, mechanical properties of nanotubes, and tribological phenomena at nanoscale interfaces. read less

Abstract: Understanding the thermodynamics of metal hydrides is crucial in order to employ them for reversible hydrogen storage. The use of a supporting nanoporous matrix for embedding the hydride can improve the kinetics of metal hydride reactions and even change the overall reaction path. In this study, density functional theory calculations were performed to understand the size effect of MgH2 particles and changes in the physical and chemical properties of these nano-objects embedded in an amorphous carbon matrix. A stable amorphous carbon structure was successfully generated with two different approaches and further used as a template to construct the scaffold for nanophases of MgH2. Using five different structure models, we have studied the physical and chemical changes of the nano-MgH2 in this carbon scaffold. In addition, from small-angle neutron scattering studies, we could demonstrate experimentally that it is possible to incorporate such ultrasmall objects into the carbon scaffolds. read less

Abstract: A variable charge reactive empirical potential for carbon-based materials, hydrocarbons, organometallics, and their interfaces is developed within the framework of charge optimized many-body (COMB) potentials. The resulting potential contains improved expressions for the bond order and self-energy, which gives a flexible, robust, and integrated treatment of different bond types in multicomponent and multifunctional systems. It furthermore captures the dissociation and formation of the chemical bonds and appropriately and dynamically determines the associated charge transfer, thus providing a powerful method to simulate the complex chemistry of many-atom systems in changing environments. The resulting COMB potential is used in a classical molecular dynamics simulation of the room temperature, low energy deposition of ethyl radicals on the Cu (111) surface (a system with ∼5000 atoms) to demonstrate its capabilities at describing organic-metal interactions in a dynamically changing environment. read less

Abstract: We use molecular dynamics simulations to explore the lattice thermal transport in free-standing and supported single-wall carbon-nanotube (SWCNT) in comparison to that in graphene nanoribbon and graphene sheet. For free-standing SWCNT, the lattice thermal conductivity increases with diameter and approaches that of graphene, partly due to the curvature. Supported SWCNT thermal conductivity is reduced by 34%-41% compared to the free-standing case, which is less than that in supported graphene. Also, it shows an evident chirality dependence by varying about 10%, which we attribute to chirality-dependent interfacial phonon scattering. read less

Abstract: Following Gurtin and many others, the critical energy release rate is commonly identified as an ill-defined surface energy. The primary objectives of this paper are to clarify the definition of this surface energy and the role of the entropy inequality in the discussion of critical conditions. In view of an increasing emphasis on ab initio computations, a secondary objective is to show how the critical energy release rate and the compatibility constraint 1 can be used to solve a problem for which we have experimental data, using only ab initio estimates of surface tension and bond potential, both of which are increasingly available. read less

Abstract: Molecular dynamic simulations are used to model the vibrational bending modes of graphene ribbons of several sizes to calculate frequencies of the ribbons and determine the relationship between the size of the ribbon and their corresponding resonance frequencies. These ribbons can be utilized to fabricate several types of vibronic devices such as NEMS, sensors, terahertz generators, and devices for encoding, transferring, and processing information. The interaction of a graphene vibronic device with water and isopropyl alcohol molecules demonstrates that this device can be used as a very sensitive vibronic molecular sensor that is able to distinguish the chemical nature of the sensed molecule. The electrical properties of the graphene vibronic devices are also calculated for two cases, armchair and zigzag border. The zigzag border demonstrated in this work has the potential to generate THz electrical signals. read less

Abstract: Using reactive force-field (ReaxFF) and molecular dynamics simulation, we study atomistic scale chloride ion adsorption and transport through copper oxide thin films under aqueous conditions. The surface condition of passive oxide film plays a key role in chloride ion adsorption and facilitates initial adsorption when surface corrosion resistance is low. Using implemented surface defects, the structural evolution of the copper oxide film from thinning to breakdown is investigated. In addition to chemical thinning of passive film, extended defects in the metal substrate are observed, at high concentration of adsorbed chloride ions. The initial stage of breakdown is associated with rapid depletion of adjacent chloride ions, which creates a locally deficient environment of chloride ions in the solution. The dissolved copper cations gain higher charge upon interaction with chloride ions. Owing to the increased Coulomb interactions resulted from dissolved copper ions and locally low density of chloride ions, far-field chloride ions would diffuse into the local corrosion sites, thereby promoting further corrosion. read less

Abstract: In this work, we present the parametrization of Ca-O/H interactions within the reactive force field ReaxFF, and its application to study the hydration of calcium oxide surface. The force field has been fitted using density functional theory calculations on gas phase calcium-water clusters, calcium oxide bulk and surface properties, calcium hydroxide, bcc and fcc Ca, and proton transfer reactions in the presence of calcium. Then, the reactive force field has been used to study the hydration of the calcium oxide {001} surface with different water contents. Calcium oxide is used as a catalyzer in many applications such as CO(2) sequestration and biodiesel production, and the degree of surface hydroxylation is a key factor in its catalytic performance. The results show that the water dissociates very fast on CaO {001} bare surfaces without any defect or vacancy. The surface structure is maintained up to a certain amount of water, after which the surface undergoes a structural rearrangement, becoming a disordered calcium hydroxyl layer. This transformation is the most probable reason for the CaO catalytic activity decrease. read less

Abstract: Computational techniques are widely used to explore the structure and properties of nanomaterials. This review surveys the application of both quantum mechanical and force field based atomistic simulation methods to nanoparticles, with a particular focus on the methodologies available and the ways in which they can be utilised to study structure, phase stability and morphology. The main focus of this article is on partially ionic materials, from binary semiconductors through to mineral nanoparticles, with more detailed considered of three examples, namely titania, zinc sulphide and calcium carbonate. read less

Abstract: The dynamics of water confined to mesoporous regions in minerals such as swelling clays and zeolites is fundamental to a wide range of resource management issues impacting many processes on a global scale, including radioactive waste containment, desalination, and enhanced oil recovery. Large-scale atomic models of freely diffusing multilayer smectite particles at low hydration confined in a silicalite cage are used to investigate water dynamics in the composite environment with the ReaxFF reactive force field over a temperature range of 300-647 K. The reactive capability of the force field enabled a range of relevant surface chemistry to emerge, including acid/base equilibria in the interlayer calcium hydrates and silanol formation on the edges of the clay and inner surface of the zeolite housing. After annealing, the resulting clay models exhibit both mono- and bilayer hydration structures. Clay surface hydration redistributed markedly and yielded to silicalite water loading. We find that the absolute rates and temperature dependence of water dynamics compare well to neutron scattering data and pulse field gradient measures from relevant samples of Ca-montmorillonite and silicalite, respectively. Within an atomistic, reactive context, our results distinguish water dynamics in the interlayer Ca(OH)(2)·nH(2)O environment from water flowing over the clay surface, and from water diffusing within silicalite. We find that the diffusion of water when complexed to Ca hydrates is considerably slower than freely diffusing water over the clay surface, and the reduced mobility is well described by a difference in the Arrhenius pre-exponential factor rather than a change in activation energy. read less

Abstract: The thermoelectric properties of cubic zincblend silicon carbide nanowires (SiCNWs) with nitrogen impurities and vacancies along [111] direction are theoretically studied by means of atomistic simulations. It is found that the thermoelectric figure of merit ZT of SiCNWs can be sig nificantly enhanced by doping N impurities together with making Si vacancies. Aiming at obtaining a large ZT, we study possible energetically stable configurations, and disclose that, when N dopants locate at the c enter, a small number of Si vacancies at corners are most favored for n-type nanowires, while a large number of Si vacancies spreading into the flat edge sites are most favored for p-type nanowires. For the SiCNW with a diameter of 1.1 nm and a length of 4.6 nm, the ZT value for the n-type is shown capable of reaching 1.78 at 900K. The conditions to get higher ZT values for longer SiCNWs are also addressed. PACS numbers: 73.63.-b, 62.23.Hj, 61.46.Km read less

Abstract: In this paper, we examine the lattice thermal conductivity and dominant phonon scattering mechanisms of graphene. The interatomic interactions are modeled using the Tersoff interatomic potential and perturbation theory is applied to calculate the transition probabilities for three-phonon scattering. The matrix elements of the perturbing Hamiltonian are calculated using the anharmonic interatomic force constants obtained from the interatomic potential as well. The linearized Boltzmann transport equation is applied to compute the thermal conductivity of graphene for a wide range of parameters giving spectral and polarization-resolved information. The complete spectral detail of selection rules, important phonon scattering pathways, and phonon relaxation times in graphene are provided. We also highlight the specific scattering processes that are important in Raman spectroscopy-based measurements of graphene thermal conductivity, and provide a plausible explanation for the observed dependence on laser spot size. read less

Abstract: The growth mechanism and chirality formation of a single-walled carbon nanotube (SWNT) on a surface-bound nickel nanocluster are investigated by hybrid reactive molecular dynamics/force-biased Monte Carlo simulations. The validity of the interatomic potential used, the so-called ReaxFF potential, for simulating catalytic SWNT growth is demonstrated. The SWNT growth process was found to be in agreement with previous studies and observed to proceed through a number of distinct steps, viz., the dissolution of carbon in the metallic particle, the surface segregation of carbon with the formation of aggregated carbon clusters on the surface, the formation of graphitic islands that grow into SWNT caps, and finally continued growth of the SWNT. Moreover, it is clearly illustrated in the present study that during the growth process, the carbon network is continuously restructured by a metal-mediated process, thereby healing many topological defects. It is also found that a cap can nucleate and disappear again, which was not observed in previous simulations. Encapsulation of the nanoparticle is observed to be prevented by the carbon network migrating as a whole over the cluster surface. Finally, for the first time, the chirality of the growing SWNT cap is observed to change from (11,0) over (9,3) to (7,7). It is demonstrated that this change in chirality is due to the metal-mediated restructuring process. read less

Abstract: Soon after the discovery of carbon nanotubes, it was realized that the theoretically predicted mechanical properties of these interesting structures could make them ideal for a wealth of technological applications. A number of computer simulation methods applied to their modeling, has led over the past decade to an improved but by no means complete understanding of the mechanics of carbon nanotubes. Tersoff potential has been widely used but it has since been modified many times. The latest is the second-generation reactive empirical bond order potential by Brenner and co workers, which is being used in this work for manipulating these tiny structures. We outline the computational approaches that have been taken. The elastic moduli of armchair, zigzag and chiral nanotubes have been computed. We generate the coordinates of carbon nanotubes of different chirality’s and size. Each and every structure thus generated is allowed to relax till we obtain minima of energy. We then apply the requisite compressions, elongations and twists to the structures and compute the elastic moduli. Young’s modulus is found to be dependent on tube radius for thinner tubes and attains a constant value of the order 1TPa. Our results of Poisson’s ratio and shear modulus are also encouraging and compare well with other theoretical and experimental work. read less

Abstract: We have parameterized a reactive force field (ReaxFF) for lithium–aluminum silicates using density functional theory (DFT) calculations of structural properties of a number of bulk phase oxides, silicates and aluminates, as well as of several representative clusters. The force field parameters optimized in this study were found to predict lattice parameters and heats of formation of selected condensed phases in excellent agreement with previous DFT calculations and with experiments. We have used the newly developed force field to study the eucryptite phases in terms of their thermodynamic stability and their elastic properties. We have found that (a) these ReaxFF parameters predict the correct order of stability of the three crystalline polymorphs of eucryptite, α, β and γ, and (b) that upon indentation, a new phase appears at applied pressures ⩾7 GPa. The high-pressure phase obtained upon indentation is amorphous, as illustrated by the radial distribution functions calculated for different pairs of elements. In terms of elastic properties analysis, we have determined the elements of the stiffness tensor for α- and β-eucryptite at the level of ReaxFF, and discussed the elastic anisotropy of these two polymorphs. Polycrystalline average properties of these eucryptite phases are also reported to serve as ReaxFF predictions of their elastic moduli (in the case of α-eucryptite), or as tests against values known from experiments or DFT calculations (β-eucrypite). The ReaxFF potential reported here can also describe well single-species systems (e.g. Li-metal, Al-metal and condensed phases of silicon), which makes it suitable for investigating structure and properties of suboxides, atomic-scale mechanisms responsible for phase transformations, as well as oxidation–reduction reactions. read less

Abstract: Defects in graphene, a recently discovered one-atom-thick material with exceptional characteristics, may considerably alter its properties and have negative effects on the operation of graphene-based electronic devices. Defects, when deliberately created by ion and especially electron irradiation with a high spatial resolution, may also have a beneficial effect on the target. Thus the complete understanding of the energetics and dynamics of defects in graphene is required for engineering the properties of graphene-based materials and devices. In this Chapter we give an overview of the recent progress in the understanding of the role of defects in these materials. We briefly dwell on the experimental data on native and irradiation-induced defects in graphene, and give detailed account of recent simulation results for point and line defects in graphene. We also discussed at length the mechanisms of defect formation under ion and electron irradiation as revealed by atomistic computer simulations. read less

Abstract: We perform a theoretical investigation on the thermal conductivity of single-walled boron nitride nanotubes (SWBNT) using the kinetic theory. By fitting to the phonon spectrum of the boron nitride sheet, we develop an efficient and stable Tersoff-derived interatomic potential which is suitable for the study of heat transport in $sp2$ structures. We work out the selection rules for the three-phonon process with the help of the helical quantum numbers $(\ensuremath{\kappa},n)$ attributed to the symmetry group (line group) of the SWBNT. Our calculation shows that the thermal conductivity ${\ensuremath{\kappa}}_{\mathrm{ph}}$ diverges with length as ${\ensuremath{\kappa}}_{\mathrm{ph}}\ensuremath{\propto}{L}^{\ensuremath{\beta}}$ with exponentially decaying $\ensuremath{\beta}(T)\ensuremath{\propto}{e}^{\ensuremath{-}T/{T}_{c}}$, which results from the competition between boundary scattering and three-phonon scattering for flexure modes. We find that the two flexure modes of the SWBNT make dominant contribution to the thermal conductivity, because their zero frequency locates at $\ensuremath{\kappa}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\alpha}$, where $\ensuremath{\alpha}$ is the rotational angle of the screw symmetry in SWBNT. read less

Abstract: In this paper, we employed the classical molecular dynamics simulations using Tersoff’s potential to calculate the elastic constants of the silicon carbide (SiC) crystal at high temperature. The elastic constants of the SiC crystal were calculated based on the stressstrain characteristics, which were drawn by the simulation using LAMMPS software. At the same time, the elastic constants of the SiC ceramics were measured at different temperatures by impulse excitation testing (IET) method. Based on the simulated stress-strain results, the SiC crystal showed the elastic deformation characteristics at the low temperature region, while a slight plastic deformation behavior was observed at high strain over 1,000℃ temperature. The elastic constants of the SiC crystal were changed from about 475 ㎬ to 425 ㎬ by increasing the temperature from RT to 1,250℃. When compared to the experimental values of the SiC ceramics, the simulation results, which are unable to obtain by experiments, are found to be very useful to predict the stress-strain behaviors and the elastic constant of the ceramics at high temperature. read less

Abstract: We present the ReaxFF reactive force field methodology for modeling a gold–oxygen binary system. The force field parameters were fitted against a data set including equations of state, heats of formation and binding energies derived from DFT calculations. The trained force field was then used to study the diffusion properties of oxygen on a gold surface. The diffusion study shows that oxygen atoms have a relatively low mobility on the gold surface. We also present a prospective application of this force field by performing molecular dynamics simulations studying the effect of oxidation level on contact strength of a cold welded joint. The results indicate that low levels of oxidation can significantly impact the joint cohesive energy. read less

Abstract: We numerically analyzed unique near-zero friction regime of the C60 molecular bearings, graphite/C60/graphite interface, for the lateral scan along the [123̄0] direction under the relatively low loading condition. Here the C60 molecule slides, facing its six-membered ring nearly parallel to both the upper and lower graphene sheets. The sinusoidal motion of the C60 molecule along the carbon bond is continuous and reversible during the forward and backward scans. As a result, the hysteresis loop of the lateral force curve nearly disappears, which leads to a mean frictional force of nearly zero, (FL)≃ 0. The mechanism of this conservative motion is clarified by comparing the structural optimization of the C60 molecular bearing system with the direct calculation of the local minimum position located on the total potential energy surface Vtotal. The energy barrier between the neighboring minimum positions always exists, which prevents the C60 molecule from taking stick-slip motion. read less

Abstract: Metal-catalyzed growth mechanisms of carbon nanotubes (CNTs) were studied by hybrid molecular dynamics-Monte Carlo simulations using a recently developed ReaxFF reactive force field. Using this novel approach, including relaxation effects, a CNT with definable chirality is obtained, and a step-by-step atomistic description of the nucleation process is presented. Both root and tip growth mechanisms are observed. The importance of the relaxation of the network is highlighted by the observed healing of defects. read less

Abstract: In order to understand the underlying mechanisms of inelastic material behavior and nonlinear surface interactions, which can be observed on macroscale as damping, softening, fracture, delamination, frictional contact etc., it is necessary to examine the molecular scale. Force fields can be applied to simulate the rearrangement of chemical and physical bonds. However, a simulation of the atomic interactions is very costly so that classical molecular dynamics (MD) is restricted to structures containing a low number of atoms such as carbon nanotubes. The objective of this paper is to show how MD simulations can be integrated into the finite element method (FEM) which is used to simulate engineering structures such as an aircraft panel or a vehicle chassis. A new type of finite element is required for force fields that include multi-body potentials. These elements take into account not only bond stretch but also bending, torsion and inversion without using rotational degrees of freedom. Since natural lengths and angles are implemented as intrinsic material parameters, the developed molecular dynamic finite element method (MDFEM) starts with a conformational analysis. By means of carbon nanotubes and elastomeric material it is demonstrated that this pre-step is needed to find an equilibrium configuration before the structure can be deformed in a succeeding loading step. read less

Abstract: This review addresses the field of nanoscience as viewed through the lens of the scientific career of Peter Eklund, thus with a special focus on nanocarbons and nanowires. Peter brought to his research an intense focus, imagination, tenacity, breadth and ingenuity rarely seen in modern science. His goal was to capture the essential physics of natural phenomena. This attitude also guides our writing: we focus on basic principles, without sacrificing accuracy, while hoping to convey an enthusiasm for the science commensurate with Peter’s. The term ‘colloquial review’ is intended to capture this style of presentation. The diverse phenomena of condensed matter physics involve electrons, phonons and the structures within which excitations reside. The ‘nano’ regime presents particularly interesting and challenging science. Finite size effects play a key role, exemplified by the discrete electronic and phonon spectra of C60 and other fullerenes. The beauty of such molecules (as well as nanotubes and graphene) is reflected by the theoretical principles that govern their behavior. As to the challenge, ‘nano’ requires special care in materials preparation and treatment, since the surface-to-volume ratio is so high; they also often present difficulties of acquiring an experimental signal, since the samples can be quite small. All of the atoms participate in the various phenomena, without any genuinely ‘bulk’ properties. Peter was a master of overcoming such challenges. The primary activity of Eklund’s research was to measure and understand the vibrations of atoms in carbon materials. Raman spectroscopy was very dear to Peter. He published several papers on the theory of phonons (Eklund et al 1995a Carbon 33 959–72, Eklund et al 1995b Thin Solid Films 257 211–32, Eklund et al 1992 J. Phys. Chem. Solids 53 1391–413, Dresselhaus and Eklund 2000 Adv. Phys. 49 705–814) and many more papers on measuring phonons (Pimenta et al 1998b Phys. Rev. B 58 16016–9, Rao et al 1997a Nature 338 257–9, Rao et al 1997b Phys. Rev. B 55 4766–73, Rao et al 1997c Science 275 187–91, Rao et al 1998 Thin Solid Films 331 141–7). His careful sample treatment and detailed Raman analysis contributed greatly to the elucidation of photochemical polymerization of solid C60 (Rao et al 1993b Science 259 955–7). He developed Raman spectroscopy as a standard tool for gauging the diameter of a single-walled carbon nanotube (Bandow et al 1998 Phys. Rev. Lett. 80 3779–82), distinguishing metallic versus semiconducting single-walled carbon nanotubes, (Pimenta et al 1998a J. Mater. Res. 13 2396–404) and measuring the number of graphene layers in a peeled flake of graphite (Gupta et al 2006 Nano Lett. 6 2667–73). For these and other ground breaking contributions to carbon science he received the Graffin Lecture award from the American Carbon Society in 2005, and the Japan Carbon Prize in 2008. As a material, graphite has come full circle. The 1970s renaissance in the science of graphite intercalation compounds paved the way for a later explosion in nanocarbon research by illuminating many beautiful fundamental phenomena, subsequently rediscovered in other forms of nanocarbon. In 1985, Smalley, Kroto, Curl, Heath and O’Brien discovered carbon cage molecules called fullerenes in the soot ablated from a rotating graphite target (Kroto et al 1985 Nature 318 162–3). At that time, Peter’s research was focused mainly on the oxide-based high-temperature superconductors. He switched to fullerene research soon after the discovery that an electric arc can prepare fullerenes in bulk quantities (Haufler et al 1990 J. Phys. Chem. 94 8634–6). Later fullerene research spawned nanotubes, and nanotubes spawned a newly exploding research effort on single-layer graphene. Graphene has hence evolved from an oversimplified model of graphite (Wallace 1947 Phys. Rev. 71 622–34) to a new member of the nanocarbon family exhibiting extraordinary electronic properties. Eklund’s career spans this 35-year odyssey. read less

Abstract: Recent progress of studies on the interaction of keV atomic and fullerene ions during grazing scattering from solid surfaces is reported. We focus on electron capture processes for two systems that have been investigated in detail during the last years, the neutralization of He and C60 ions at metal surfaces. Both species can be considered as model systems for Auger neutralization and for resonant neutralization of a large molecule, respectively. Basic experimental concepts and results obtained for scattering from an Al(100) surface are discussed. read less

Abstract: The dynamics of water inside nanochannels is of great importance for biological activities as well as for the design of molecular sensors, devices, and machines, particularly for sea water desalination. When confined in specially sized nanochannels, water molecules form a single-file structure with concerted dipole orientations, which collectively flip between the directions along and against the nanotube axis. In this paper, by using molecular dynamics simulations, we observed a net flux along the dipole-orientation without any application of an external electric field or external pressure difference during the time period of the particular concerted dipole orientations of the molecules along or against the nanotube axis. We found that this unique special-directional water transportation resulted from the asymmetric potential of water-water interaction along the nanochannel, which originated from the concerted dipole orientation of the water molecules that breaks the symmetry of water orientation distribution along the channel within a finite time period. This finding suggests a new mechanism for achieving high-flux water transportation, which may be useful for nanotechnology and biological applications. read less

Abstract: This work is devoted to the study of model systems for the interaction of atoms, molecules, and their ions with solid surfaces. The thesis consists of three parts. In the first part, He atoms and ions with keV energies are scattered under grazing angles of incidence from Al(111), Al(100), and Al(110) surfaces. Fractions of surviving ions and normal energy gains of He+ ions prior to neutralization, derived from shifts of angular distributions for incident atoms and ions, are compared to results from three-dimensional Monte Carlo simulations based on theoretically calculated Auger neutralization rates and He ground-state energy shifts. From the good agreement of experimental data with simulations, a detailed microscopic understanding for a model system of ion-surface interactions is concluded. The studies are extended to noble gas atoms and surfaces with a more complex electronic structure as well as the Auger ionization process, for which a comparison to simulations based on first ab-initio calculations is presented. In the second set of experiments, the formation of doubly excited states of He atoms during collisions of He2+ ions with energies between 60 eV and 1 keV with Ni(110) and Fe(110) surfaces is studied via Auger electron spectroscopy. The electron spectra from autoionization of doubly excited states of 2`2`′ configurations show a pronounced dependence on the coverage of the target surface with adsorbates. Thermal desorption and dissolution of surface contaminations into the bulk at elevated temperatures provide an alternative interpretation of recent work where the local electron spin polarization of Ni(110) and Fe(110) surfaces was deduced from changes in the electron spectra as function of target temperature. In the third part, angular distributions, fragmentation, and charge fractions are studied for grazing scattering of C60 fullerene ions with keV energies from Al(100), Be(0001), and LiF(100) surfaces. At low energies for the motion along the surface normal, the fullerenes are scattered nearly elastically, whereas for larger normal energies, the energy loss is substantial with pronounced differences for metal and insulator surfaces. From a comparison with classical trajectory simulations, a strong perturbation of the elastic properties of the fullerene by a nearby metal surface is concluded. Shifts of angular distributions for incident C60 and C 2+ 60 projectiles for the metal surfaces are in quantitative accord with a classical over-the-barrier model and provide the first information on distances of electron transfer for positively charged fullerenes in front of metal surfaces. For the LiF(100) surface, pronounced kinematically induced internal excitations due to interactions with the periodic electric field at the surface are observed. read less

Abstract: A new parametrization for the previous empirical interatomic potential for indium antimonite is presented. This alternative parametrization is designed to correct the energetic sequence of structures. The effective empirical interatomic potential proposed consists of two and three body interactions which has the same functional form of the interatomic potential proposed by Vashishta et. al. to study other semiconductors (Branicio et al., 2003; Ebbsjo et al., 2000; Shimojo et al., 2000; Vashishta et al., 2008). Molecular dynamics simulations (MD) are performed to study high pressure phases of InSb up to 70 GPa and its thermal conductivity as a function of temperature. The rock-salt to cesium chloride, expected to occur at high pressures, is observed with the proposed interatomic potential. read less

Abstract: The ON–OFF state transition of the water transport induced by the structural bending of a carbon nanotube is studied by molecule dynamics simulation. The water permeation through a bent carbon nanotube shows excellent gating property with a threshold bending angle of about 14.6°. We also investigate the water density distribution inside the nanochannel to illustrate the mechanism. read less

Abstract: Many calculations require a simple classical model for the interactions between sp 2 -bonded carbon atoms, as in graphene or carbon nanotubes. Here we present a valence force model to describe these interactions. The calculated phonon spectrum of graphene and the nanotube breathing-mode energy agree well with experimental measurements and with ab initio calculations. The model does not assume an underlying lattice, so it can also be directly applied to distorted structures. The characteristics and limitations of the model are discussed. read less

Abstract: In the present paper we developed a model for calculating the energy of single-wall carbon nanotubes of arbitrary chirality. This model, which we call as the liquid surface model, predicts the energy of a nanotube with relative error less than 1% once its chirality and the total number of atoms are known. The parameters of the liquid surface model and its potential applications are discussed. The model has been suggested for open end and capped nanotubes. The influence of the catalytic nanoparticle, atop which nanotubes grow, on the nanotube stability is also discussed. The suggested model gives an important insight in the energetics and stability of nanotubes of different chirality and might be important for the understanding of nanotube growth process. For the computations we use empirical Brenner and Tersoff potentials and discuss their applicability to the study of carbon nanotubes. From the calculated energies we determine the elastic properties of the single-wall carbon nanotubes (Young modulus, curvature constant) and perform a comparison with available experimental measurements and earlier theoretical predictions. read less

Abstract: We investigate the phonon normal modes in hydrogen-terminated graphene nanoribbons GNRs using the second-generation reactive empirical bond order REBOII potential and density-functional theory calculations. We show that specific modes, absent in pristine graphene and localized at the GNR edges, are intrinsic signatures of the vibrational density of states of the GNRs. Three particular modes are described in details: a transverse phonon mode related to armchair GNRs, a hydrogen out-of-plane mode present in both armchair and zigzag GNRs, and the Raman radial-breathing-like mode. The good agreement between the frequencies of selected edge modes obtained using REBOII and first-principles methods shows the reliability of this empirical potential for the calculation and the assignment of phonon modes in carbon nanostructures where carbon atoms present a sp 2 hybridization. read less

Abstract: Existing interatomic potentials for the iron-carbon system suffer from qualitative flaws in describing even the simplest of defects. In contrast to more accurate first-principles calculations, all previous potentials show strong bonding of carbon to overcoordinated defects (e.g., self-interstitials, dislocation cores) and a failure to accurately reproduce the energetics of carbon-vacancy complexes. Thus any results from their application in molecular dynamics to more complex environments are unreliable. The problem arises from a fundamental error in potential design--the failure to describe short-ranged covalent bonding of the carbon p electrons. We describe a resolution to the problem and present an empirical potential based on insights from density-functional theory, showing covalent-type bonding for carbon. The potential correctly describes the interaction of carbon and iron across a wide range of defect environments. It has the embedded atom method form and hence appropriate for billion atom molecular-dynamics simulations. read less

Abstract: With the concourse of a variety of experimental techniques (neutron diffraction, x-ray photoelectron spectroscopy, $^{13}\text{C}$ nuclear magnetic resonance, electron microscopy, and Raman spectroscopy) and a combination of reverse Monte Carlo, molecular dynamics, and Monte Carlo simulations, we propose a model for the microscopic structure of a sample of carbon nanospheres obtained from chlorination of cobaltocene. The sample, which exhibits a high porosity, is shown to be formed by a series of interconnected sheets of graphene. Despite the large degree $(\ensuremath{\approx}80%)$ of $s{p}^{2}$ hybridization shown by carbon atoms, there is a non-negligible amount of $s{p}^{3}$-bonded carbons, some of them acting as links between graphene sheets. The transmission electron microscopy images simulated from the microscopic structure, which is extracted from the neutron diffraction data by a mixed reverse Monte Carlo\char21{}molecular dynamics/Monte Carlo procedure, agree remarkably well with the experimental results. read less

Abstract: We have parametrized a reactive force field for NaH, ReaxFF(NaH), against a training set of ab initio derived data. To ascertain that ReaxFF(NaH) is properly parametrized, a comparison between ab initio heats of formation of small representative NaH clusters with ReaxFF(NaH) was done. The results and trend of ReaxFF(NaH) are found to be consistent with ab initio values. Further validation includes comparing the equations of state of condensed phases of Na and NaH as calculated from ab initio and ReaxFF(NaH). There is a good match between the two results, showing that ReaxFF(NaH) is correctly parametrized by the ab initio training set. ReaxFF(NaH) has been used to study the dynamics of hydrogen desorption in NaH particles. We find that ReaxFF(NaH) properly describes the surface molecular hydrogen charge transfer during the abstraction process. Results on heat of desorption versus cluster size shows that there is a strong dependence on the heat of desorption on the particle size, which implies that nanostructuring enhances desorption process. To gain more insight into the structural transformations of NaH during thermal decomposition, we performed a heating run in a molecular dynamics simulation. These runs exhibit a series of drops in potential energy, associated with cluster fragmentation and desorption of molecular hydrogen. This is consistent with experimental evidence that NaH dissociates at its melting point into smaller fragments. read less

Abstract: Defects are common in fabricated nanochannels. In this paper, water permeation across a single-walled carbon nanotube with defects was studied using molecular dynamics simulations. It is found that the impact on water permeation is negligible when the density of the defects is small, while a significant reduction in water permeation is observed when the density of the defects is high. These findings should be helpful in both understanding water permeation across nanochannels and designing efficient artificial nanochannel. read less

Abstract: Structural stabilities of different types of carbon nanogears have been tested against temperature by means of a molecular dynamics procedure. Effects of periodic boundary conditions were also examined. It has been found that although the two types of nanogears (armchair and zigzag CNT yielding) investigated look similar in configuration, when tested against high temperatures, bond breakings and deformations occur at different regions. read less

Abstract: We present molecular models for 3 saccharose based carbons of different densities obtained using a Reverse Monte Carlo (RMC) protocol which incorporates an energy constraint. The radial distribution functions of the simulated models are in good agreement with experiment. Moreover, 3 and 4 member carbon rings, reported in the literature for many modeling studies of carbon, are absent or extremely rare in our final structural models. These small member rings are high energy structures and are believed to be an artifact of the usual RMC method. The presence of the energy penalty term in our simulation protocol penalizes the formation of these structures. Using a ring connectivity analysis method that we developed, we find that these atomistic models of carbons are made up of defective graphene segments twisted in a complex way. These graphene segments are largely made up of 6 carbon member rings, but also contain some 5 and 7 carbon member rings. We also found that in addition to the graphene segments there are some carbon chains which do not belong to any graphene segments. To characterize our models, we calculated the geometric pore size distribution and also simulated the adsorption of argon at 77.4 K in the models using GCMC simulations. The adsorption isotherm obtained for all three models are representative of microporous carbons. read less

Abstract: In this work we show that a reparametrized Tersoff potential accurately reproduces the bond length variations observed in ternary Ga1−xInxAs mixed crystals. The reparametrization is based on accurate first-principles electronic structure calculations. Previous parametrizations of the Tersoff potential for GaAs and InAs structures, although they accurately reproduce the properties of the zinc-blende GaAs and InAs crystals, are shown to be unable to reproduce the bond length variations in these mixed crystals. In addition to correcting the bond length inconsistencies, the new set of parameters is also shown to yield the elastic constants of GaAs and InAs that agree fairly well with measurements and to reproduce accurately their respective melting temperature. read less

Abstract: Empirical potentials based upon two and three body interactions were applied to the Li+–C system, assuming the Li+ ions to be distributed inside high-symmetry, single walled carbon nanotubes of different chirality. Structural optimizations for various assemblages were conducted using evolutionary and genetic algorithms, where differential evolution and particle swarm optimization techniques worked satisfactorily. The results were compared with the outcome of some rigorous molecular dynamics simulations. The potential for using the carbon nanotubes in the negative electrode of lithium ion batteries was also critically examined. read less

Abstract: In this paper a new method is introduced for carbon nanotubes modeling. It combines features of Molecular Mechanics and Finite Element Analysis. This method is based on the development of a new finite element, whose internal energy is determined by the semi-empirical Brenner molecular potential model; all quantities are calculated analytically in order to gain more accuracy. The method is validated through comparisons to results provided by other researchers and are obtained either by experimental procedures or theoretical predictions. The bending and shearing of CNTs is also simulated.© 2006 ASME read less

Abstract: In this study, the nucleation mechanism of carbon nanocones is investigated using molecular dynamics (MD) simulations and structural analyses and is compared with that of carbon nanotubes. It is shown that the structural stability of carbon nanocones is sensitive to the cone apex angle. Specifically, an increase in the conical angle results in a moderate improvement in the structural stability of the nanocone as a result of a lower strain energy in the capped mantle. The simulation results also show that the melting temperature of the nanocone increases with increasing conical angle. Furthermore, it is observed that a metastable tube-like structure is formed in carbon nanocones with a lower conical angle at temperatures ranging from 2400 to 3600 K. Finally, the numerical simulations reveal that the mechanical properties of carbon nanocones under nanoindentation are strongly dependent on the conical angle. For carbon nanocones with a large conical angle, the high deformation-promoted reactivity and reversible mechanical response have been performed due to highly symmetrical networks. read less

Abstract: We report the results of molecular dynamics simulations of melting and premelting of single-walled carbon nanotubes (SWNTs). We found that the traditional critical Lindemann parameter for the melting of bulk crystals is not valid for SWNTs. Using the much smaller critical Lindemann parameter for the melting of nanoparticles as a criterion, we show that the melting temperature of perfect SWNTs is around 4800 K. We further show that Stone–Wales defects in a SWNT significantly reduce the melting temperature of atoms around the defects, resulting in the premelting of SWNTs at 2600 K. read less

Abstract: In order to obtain mechanical properties of CNT/Polymer nano-composites, molecular dynamics simulation is performed. Overall system was modeled as a flexible unit cell in which carbon nanotubes are embedded into a polyethylene matrix for N T ensemble simulation. COMPASS force field was chosen to describe inter and intra molecular potential and bulk effect was achieved via periodic boundary conditions. In CNT-polymer interface, only Lennard-Jones non-bond potential was considered. Using Parrinello-Rahman fluctuation method, mechanical properties of orthotropic nano-composites under various temperatures were successfully obtained. Also, we investigated thermal behavior of the short CNT reinforced nanocomposites system with predicting glass transition temperature. read less

Abstract: This thin-film deposition study of tetrahedral amorphous carbon shows that including infrequent processes on the millisecond scale substantially improves the accuracy of molecular dynamics simulations. Elevated temperature between energetic impacts is used to activate processes which are typically ignored. In agreement with experiment, the simulations show an abrupt transition in which diamondlike carbon transforms into vertically oriented graphitic sheets. The simulations also highlight the importance of infrequent events in combination with energetic impact. In the absence of the latter, the transition temperature is significantly higher, in good correlation with experiment. read less

Abstract: Interfaces in materials may be grain boundaries between like crystals or phase boundaries between unlike crystals. Experimental approaches for the determination of the atomic structures of the interfaces are reviewed with emphasis on high-resolution electron microscopy (HREM). It will be shown that information on orientation relationship between the adjacent grains, the translation state and atomic relaxations can be elaborated with high precision. In a case study, the structures of one specific grain boundary in Al2O3 will be discussed in detail. Such experimental studies have provided a mass of structural information in recent years. read less

Abstract: An analytical nanomechanics model is developed for predicting the elastic self-consistent properties of single-walled carbon nanotube (SWCNT) as composite reinforcement. The molecular structural mechanics is employed to determine the in-plane stiffness and strength of continuous nanotubes in the axial direction of the tube. The effect of tube diameter of the SWCNT on the in-plane stiffness and strength is presented and discussed. The nonlinear stress-strain relationships for defect-free nanotubes have been predicted, which gives an engineering approximation on the ultimate strength and strain to failure of nanotubes. Elastic properties of nanotube composites are further predicted based on a composite micro-mechanics model, using the obtained mechanical properties of nanotubes, volume fraction, and typical polymer matrix properties. Results on the mechanical properties of nanocomposites show that the Young's moduli and strengths of carbon nanotube composites are sensitive to both fiber volume fraction and the tube diameter. read less

Abstract: The atomic and electronic properties of dislocations in III–N semiconductor layers, especially GaN, are presented. The atomic structure of the edge threading dislocation is now well established with three different cores (8 or full core, 5/7 or open core, and 4-atom ring). The use of atomistic simulations has confirmed these atomic structures and has given a good understanding of the electronic structure of the screw dislocation. Partial dislocations which are mostly confined in the area close to the substrate are now also being investigated. It is becoming clear that the electrical activity of all these defects is dependent on the layer quality, which is governed by the growth conditions. read less

Abstract: A new finite element technique for calculating the equilibrium configuration of atomic structures called the Consistent Atomic-scale Finite Element (CAFE) is introduced. Unlike traditional approaches for linking the atomic structure to its equivalent continuum, this method directly connects the atomic degrees of freedom to a reduced set of finite element degrees of freedom without passing through an intermediate homogenized continuum. As a result, there is no need to introduce stress and strain measures at the atomic level. The Tersoff-Brenner interatomic potential is used to calculate the consistent tangent stiffness matrix of the structure. In this finite element formulation, all local and non-local interactions between carbon atoms are taken into account using overlapping finite elements. In addition, a consistent hierarchical finite element modeling technique is developed for adaptively coarsening and refining the mesh over different parts of the model. This process is consistent with the underlying atomic structure and, by refining the mesh, molecular mechanic results will be recovered. This method is valid across the scales and can be used to concurrently model atomistic and continuum phenomena. Using this technique, vibration frequencies of carbon nanostructures such as graphene sheet and carbon nanotube are studied and results are compared with those using equivalent homogenized continuum. Range of the applicability of the continuum approach to the vibration of these nanostructures is discussed. Furthermore, nonlinear mechanics of radial deformation of a single-walled carbon nanotube and inversion of a carbon nanocone are studied. read less

Abstract: We present LCBOPII, an improvement of the long-range carbon bond-order potential (LCBOP) by Los and Fasolino [Phys. Rev. B 68, 024107 (2003)]. LCBOPII contains a coordination dependent medium range term for bond distances between 1.7 and $4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, meant to reproduce the dissociation energy curves for single, double, and triple bonds and improve the reactive properties as well as the description of the liquid and of low coordinated phases. Other features of LCBOPII are a coordination dependent angular correlation, a correction for antibonding states, and a conjugation dependent torsional interaction based on ab initio calculations of the torsional barriers for a set of molecular configurations. We present results for the geometry and energetics of the graphite-to-diamond transformation and of the vacancy in diamond and graphite as well as the prediction of the energy barrier of the 5-77-5 defect in graphite and graphene for which ab initio results are available only for unsuitably small samples. In the accompanying paper (Ghiringhelli et al., Phys. Rev. B 72, 214103 (2005) we use LCBOPII to evaluate several properties, including the equation of state, of liquid carbon. read less

Abstract: There are significant efforts to develop continuum theories based on atomistic models. These atomistic-based continuum theories are limited to zero temperature (T=0 K). We have developed a finite-temperature continuum theory based on interatomic potentials. The effect of finite temperature is accounted for via the local harmonic approximation, which relates the entropy to the vibration frequencies of the system, and the latter are determined from the interatomic potential. The focus of this theory is to establish the continuum constitutive model in terms of the interatomic potential and temperature. We have studied the temperature dependence of specific heat and coefficient of thermal expansion of graphene and diamond, and have found good agreements with the experimental data without any parameter fitting. We have also studied the temperature dependence of Young's modulus and bifurcation strain of single-wall carbon nanotube. read less

Abstract: Quantum dots made of individual metallic carbon nanotubes are theoretically studied under the influence of a magnetic field applied in the axial direction. After assessing the mechanical stability of the heterostructure by Monte Carlo simulations, the dependence of the electronic properties on the size of the nanotube quantum dot and applied magnetic field has been investigated within the Peierls approximation in a tight-binding model. The transport gaps induced by the magnetic field are found to be different from those of the perfect constituent tubes. Due to the presence of topological defects, some physical properties exhibit a lack of periodicity in the magnetic flux. The spin coupling to the magnetic field is also incorporated via a Zeeman term in the Hamiltonian; we have found huge differences between the up and down local densities of states which may be explored for future applications of carbon nanotube quantum dots as spintronic devices. Finally, the temperature dependence of the magnetic properties has also been addressed. We have found a diamagnetic response very similar to that of perfect tubes. read less

Abstract: Systematical molecular statics and dynamics simulations are performed on the interlayer friction and energy dissipation of biwalled carbon nanotubes of different chirality and size, with and without defects. The interlayer friction force of perfect bitube systems is strongly dependent on commensuration and independent of the tube length in incommensurate systems at very low temperature. However, the existence of defects can ruin the perfect-geometry controlled interlayer interaction and lead to a sharp increase in friction and energy dissipation rate. The oscillating energy dissipation rate increases monotonically even in an incommensurate bitube system with increasing tube length and defect density. The coupled effects of system registration, size, and defects are demonstrated, which can explain how an ultrasmooth nano-bitube system leads to a rougher longer tube system and provides a new mechanism for multiscale tribology. Simulations on the influence of attachments and terminal conditions at the end of the tubes show that H terminations lead to a higher rate of energy dissipation in a bitube oscillator than capped and freely open cores in the systems. The findings of these important effects can provide a fundamental understanding with which to create novel nanosystems from multiwalled carbon nanotubes. read less

Abstract: We review recent theoretical work on the manipulation of single molecules with scanning probes, in particular the scanning tunnelling microscope (STM). The aim of theories and simulations is to account for the processes, ideally at a quantitative level, that permit the controlled manipulation of matter at the atomic scale in adsorbed molecular systems. In order to achieve this, simulations rely on total energy and electronic structure calculations where a trade-off is made between the size of the system and the accuracy of the calculation. This first stage of the calculation yields the basic quantities used for the second stage: the evaluation of the coupled electron–nuclear dynamics. This second stage is a formidable task and many approximations are involved. In this review, we will present some of the customary approximations regarding the theoretical study of mechanical and inelastic manipulations. Mechanical manipulations use the interaction between the acting probe (usually a metallic tip) and the targeted adsorbate. We review recent results in the field of adsorbate mechanical manipulations and explain how manipulations can be effected by using the interaction between the probe’s tip and certain molecular groups of complex chemisorbed molecular systems. On the other hand, inelastic manipulations use the tunnelling current to convey energy with sub-ångström precision. This current can excite localized vibrations that can induce measurable variations of the tunnelling conductance, hence providing a means of detecting single-molecule vibrations. This current can also inject energy in a few reaction coordinates. Recently, the possibility of vibrational selective manipulations of NH3/Cu(100) has been experimentally demonstrated. The theory presented here addresses the actual pathways accessed when the molecule is excited by the tunnelling current from an STM. read less

Abstract: We report an extensive set of results for the radial breathing modes (RBM) of infinite and finite length single walled carbon nanotubes using the second generation reactive empirical bond order potential (REBO) developed by Brenner et al. As expected, the frequency, ν of the RBM is inversely proportional to the nanotube radius, R 0, We find two different linear fits to the data, one for zigzag tubes (3.25 THz nm) and one for armchair tubes For finite tubes, the RBM rapidly approaches the infinite length value for nanotubes greater than 5 nm in length. read less

Abstract: Carbon nanotubes (NTs) exhibit a wealth of fascinating structural, mechanical, and electronic properties. As an ideal one-dimensional structure, their properties are highly anisotropic. For example, mechanically the NTs are extraordinarily hard in the axial direction but soft in the radial direction; the compressibility anisotropy in the two directions exceeds orders of magnitude. As the NTs are virtually incompressible in the axial direction, much attention has been paid to their structural and mechanical behavior in the radial direction. 1‐8 The radial deformation (i.e., the change of cross-section shape) of single-walled carbon nanotubes (SWNTs) in turn influences their mechanical 1‐5 and electronic properties. 7‐9 The hardness, as one of the most important parameters characterizing the mechanical properties of SWNTS, has been intensively studied. 10‐12 However, so far, most studies have focused only on the ground-state hardness of SWNTs at ambient conditions. In this Paper, we investigate the mechanical properties, especially the hardness, of a single SWNT under hydrostatic pressure, using constant-pressure molecular-dynamics (MD) simulations and linear elastic analysis. We discover a pressure-induced hard-to-soft transition at which the radial modulus of the SWNT decreases by as much as two orders of magnitude. We show that this mechanical (hardness) transition is caused by a pressureinduced structural (shape) transition of SWNT characterized by a transformation of its cross section from a circular to elliptical shape. The critical transition pressure decreases with increasing tube radius. We use a constant-pressure MD method especially suited read less

Abstract: In this paper, the macroscopic elastic properties of carbon nanotube reinforced composites are evaluated through analysing the elastic deformation of a representative volume element (RVE) under various loading conditions. This RVE contains three components, i.e. a carbon nanotube, a transition layer between the nanotube and polymer matrix and an outer polymer matrix body. First, based on the force field theory of molecular mechanics and computational structural mechanics, an equivalent beam model is constructed to model the carbon nanotube effectively. The explicit relationships between the material properties of the equivalent beam element and the force constants have been set-up. Second, to describe the interaction between the nanotube and the outer polymer matrix at the level of atoms, the molecular mechanics and molecular dynamics computations have been performed to obtain the thickness and material properties of the transition layer. Moreover, an efficient three-dimensional eight-noded brick finite element is employed to model the transition layer and the outer polymer matrix. The macroscopic behaviours of the RVE can then be evaluated through the traditional finite element method. In the numerical simulations, the influences of various important factors, such as the stiffness of transition layer and geometry of RVE, on the final macroscopic material properties of composites have been investigated in detail. read less

Abstract: The development of an approximation method that rigorously averages small‐scale atomistic physics and embeds them in large‐scale mechanics is the principal aim of this work. This paper presents a general computational procedure based on homogenization to average frozen nanoscale atomistics and couple them to the equations of continuum hyperelasticity. The proposed application is to nanopatterned systems in which complex atomic configurations are organized in a repeating periodic array. The finite element method is used to solve the equations at the large scale, but the small‐scale equation is representative of lattice‐statics. The method is predicated on a quasistatic zero‐temperature assumption and, through homogenization, leads to a coupled set of variational equations. The numerical procedure is presented in detail, and 2‐D examples of ultra thin film layers of carbon one atom thick are shown to illustrate its applicability. Homogenization naturally gives rise to an inner displacement term with which point defects are explicitly modelled and their non‐linear interactions with global states of multiaxial strain are studied. Published in 2004 by John Wiley & Sons, Ltd. read less

Abstract: The processes involved in soot precursor formation exhibit a wide range of timescales, spanning pico- or nanoseconds for intramolecular processes that can occur on a particle surface to milliseconds for the formation of the first soot precursors. To accurately describe the soot formation process, it is important to model the reactions happening at different timescales. The use of atomistic models allows this. The code, named KMC/MD, combines the strengths of Kinetic Monte Carlo for long-time sampling, and Molecular Dynamics for relaxation processes. It enables the investigation of physical as well as chemical properties of the carbonaceous nanoparticles formed, such as particle morphology and concentration of free radicals. read less

Abstract: Macroscopic fracture parameters are investigated using molecular mechanics simulations for a graphene sheet containing atomic-scale cracks. In the discrete atomistic modeling the interatomic forces are described based on Tersoff-Brenner potential. Elastic energy release rates of the graphene sheet under symmetric (Mode I) and antisymmetric (Mode II) small deformation are directly calculated from global energy approach and local force approach using the principle of virtual work respectively. The energy release rates are also calculated through homogenized material properties based on linear elastic fracture mechanics. The results show good agreement between discrete atomistic and continuum mechanics modeling for fracture parameters and deformed crack surface profile. This establishes connections of fracture parameters between microscopic and macroscopic description of fracture in covalently bonded solids. read less

Abstract: The potential non-equivalent defects in both 3C- and 4H-SiC are classified by a new method that is based on symmetry considerations. In 4H-SiC their number is considerably higher than in 3C-SiC, since the hexagonal symmetry leads to diversification. The different theoretical methods hitherto used to investigate defects in 3C-SiC are critically reviewed. Classical MD simulations with a recently developed interatomic potential are employed to investigate the stability, structure and energetics of the large number of non-equivalent defects that may exist in 4H-SiC. Most of the potential defect configurations in 4H-SiC are found to be stable. The interstitials between hexagonal and trigonal rings, which do not exist in 3C-SiC, are characteristic for 4H-SiC and other hexagonal polytypes. The structure and energetics of some complex and anisotropic dumbbells depend strongly on the polytype. On the other hand, polytypism does not have a significant influence on the properties of the more compact and isotropic defects, such as vacancies, antisites, hexagonal interstitials, and many dumbbells. The results allow conclusions to be drawn about the energy hierarchy of the defects. read less

Abstract: A method is developed based on an additive modification to the first Lagrangian elasticity tensor to make the finite element method for hyperelasticity viable at the atomic length scale in the context of lattice statics. Through the definition of an overlap region, the close-ranged atomic interaction energies are consistently summed over the boundary of each finite element. These energies are subsequently used to additively modify the conventional material property tensor that comes from the second derivative of the stored energy function. The summation over element boundaries, as opposed to atom clusters, allows the mesh and nodes to be defined independently from the atoms. The method is developed with a specific form of the Tersoff-Brenner potential for carbon. The method correctly predicts the in-plane deformation behavior of a single graphite sheet subjected to displacement boundary conditions. Estimated plane elasticity properties agree with experimental data from the literature. Quenched molecular dynamics results are used to validate the method for homogeneous and inhomogeneous loading constraints. keyword: Nanomechanics, multiscale modeling, lattice mechanics, continuum mechanics, finite element method. read less

Abstract: A model for H2 inside single-walled carbon nanotubes is outlined. ARPACK (the Arnoldi package), a robust iterative matrix-vector eigenvalue software library, is used to determine the allowed quantum states of H2 inside various carbon nanotubes. This information is used to construct the equilibrium constants for H2 adsorption as a function of temperature for a variety of CNTs. read less

Abstract: We explore quantum interference effects in the electronic transport properties of single-walled carbon nanotube junctions. Remarkable changes in the electrical conductance are found by varying the length of one of the junction's arms. Owing to the relatively high electron mobility in carbon nanotubes, we show that a double-slit-like electron interferometer may be fabricated by joining two Y-shaped carbon nanotube junctions. These nanostructures have very interesting electrical characteristics that may be manipulated by altering geometrical aspects of the junctions. read less

Abstract: We present a computational approach to obtain classical atomic trajectories for given initial and final atomic configurations. By introducing an additional penalty function to the action of Passerone and Parrinello [Phys,Rev. Lett. 87, 108302 (2001)] the quality of atomic trajectories is greatly improved in terms of the Onsager-Machlup action. We demonstrate that this variant of the action is useful for improving path quality and consequently for atomic trajectory annealing. We utilize the one-way multigrid method as an efficient relaxation method for the construction of trajectories. The implementation of the proposed approach to a general system is quite straightforward as in the case of ordinary molecular dynamics simulations, i.e., the only requirement is to evaluate the potential energy and the atomic forces. read less

Abstract: Using an empirical bond-order potential, molecular dynamics (MD) simulations, and lattice dynamics calculations, we study the thermal expansion of diamond, graphite and single-walled carbon nanotubes. MD simulations demonstrate that, while the C-C bond length increases at a similar rate with increasing temperature in all structures, the thermal expansion coefficient varies greatly in a manner consistent with experiment. An analysis of the mode-dependent Gr\"uneisen parameters provides a detailed picture of how structure influences the competition between various vibrational modes associated with negative and positive Gr\"uneisen parameters in determining the overall thermal expansion coefficient. read less

Abstract: Monte Carlo simulations, supplemented by ab initio calculations, shed light onto the energetics and thermodynamic stability of nanostructured amorphous carbon. The interaction of the embedded nanocrystals with the host amorphous matrix is shown to determine in a large degree the stability and the relative energy differences among carbon phases. Diamonds are stable structures in matrices with ${\mathrm{sp}}^{3}$ fraction over 60%. Schwarzites are stable in low-coordinated networks. Other ${\mathrm{sp}}^{2}$-bonded structures are metastable. read less

Abstract: The experimental data relating to the second- and third-order elasticity and the zone-center optic modes of hexagonal graphite are reviewed and some amendments proposed. A modified Keating model involving three sets of interactions, one planar and two interlayer, has been developed. The harmonic parameters, four planar and seven interlayer, have been fitted by least-squares procedures to five second-order elastic constants, five zone-center optic-mode frequencies and two assumptions relating to internal strain. The anharmonic parameters comprise three planar and three interlayer ones. They have been fitted to the pressure derivatives of the five second-order constants and of three of the optic-mode frequencies. The full spectrum of inner elastic constants and internal strain tensors is given, the composition of the second- and third-order elastic constants is exposed, and the corresponding elastic compliances calculated. A pressure-induced phase transition is correctly predicted at around 16 GPa. read less

Abstract: Recent high-temperature studies of Raman-active modes in single-walled carbon nanotube (SWNT) bundles report a softening of the radial and tangential band frequencies with increasing sample temperature. A few speculations have been proposed in the past to explain the origin of these frequency downshifts. In the present study, based on experimental data and the results of molecular dynamics simulations, we estimate the contributions from three factors that may be responsible for the observed temperature dependence of the radial breathing mode frequency $[{\ensuremath{\omega}}_{\mathrm{RBM}}(T)].$ These factors include thermal expansion of individual SWNTs in the radial direction, softening of the C-C (intratubular) bonds, and softening of the van der Waals intertubular interactions in SWNT bundles. Based on our analysis, we find that the first factor plays a minor role due to the very small value of the radial thermal expansion coefficient of SWNTs. On the contrary, the temperature-induced softening of the intra- and intertubular bonds contributes significantly to the temperature-dependent shift of ${\ensuremath{\omega}}_{\mathrm{RBM}}(T).$ For nanotubes with diameters $(d)g~1.34\mathrm{nm},$ the contribution due to the radial thermal expansion is \ensuremath{\leqslant}4% over the temperature range used in this study. Interestingly, this contribution increases to \ensuremath{\geqslant}10% in the case of nanotubes having $dl~0.89\mathrm{nm}$ due to the relatively larger curvature of these nanotubes. The contributions from the softening of the intra- and intertubular bonds are approximately equal. These two factors together contribute a total of about \ensuremath{\sim}95% and 90%, respectively, for SWNTs having $dg~1.34\mathrm{nm}$ and \ensuremath{\leqslant}0.89 nm. read less

Abstract: Soon after the discovery of carbon nanotubes, it was realized that the theoretically predicted mechanical properties of these interesting structures--including high strength, high stiffness, low density and structural perfection--could make them ideal for a wealth of technological applications. The experimental verification, and in some cases refutation, of these predictions, along with a number of computer simulation methods applied to their modeling, has led over the past decade to an improved but by no means complete understanding of the mechanics of carbon nanotubes. We review the theoretical predictions and discuss the experimental techniques that are most often used for the challenging tasks of visualizing and manipulating these tiny structures. We also outline the computational approaches that have been taken, including ab initio quantum mechanical simulations, classical molecular dynamics, and continuum models. The development of multiscale and multiphysics models and simulation tools naturally arises as a result of the link between basic scientific research and engineering application; while this issue is still under intensive study, we present here some of the approaches to this topic. Our concentration throughout is on the exploration of mechanical properties such as Young's modulus, bending stiffness, buckling criteria, and tensile and compressive strengths. Finally, we discuss several examples of exciting applications that take advantage of these properties, including nanoropes, filled nanotubes, nanoelectromechanical systems, nanosensors, and nanotube-reinforced polymers. This review article cites 349 references. read less

Abstract: Nonlinear elastic properties of single-wall carbon nanotubes subjected to large-scale tensile deformation were investigated using the second-generation Brenner potential. The energy change of the nanotubes was found to be a cubic function of tensile strains. In contrast to values of in-plane stiffness C based on small strains reported previously in the literature, the present study showed that C is linearly dependent on axial tensile deformation. C is also affected by the chirality of the tubes to some extent when axial strains are large. Consequently, these tubes should be considered in the context of anisotropic elastic shell models in the event of employing continuum mechanics in the analysis of nanotubes. read less

Abstract: Chemical adsorption and desorption of hydrogen atoms on single-walled carbon nanotubes (SWNTs) are investigated by using molecular dynamics simulations. It is found that the adsorption and desorption energy of hydrogen atoms depend on the hydrogen coverage and the diameter of the SWNTs. Hydrogen-adsorption geometry at the coverage of 1.0 is more energetically stable. The adsorption energy decreases with the increasing diameter of the armchair tubes. The adsorption and desorption energy of hydrogen atoms can be modified reversibly by externally radial deformation. The averaged C-H bond energy on the high curvature sites of the deformed tube increases with increasing radial deformation, while that on the low curvature sites decreases. read less

Abstract: An improved interatomic potential for tetrahedral carbon is presented. This potential is of the Stillinger-Weber (SW) type and has been determined from calculations performed on a select group of small hydrocarbon molecules, chosen for their similarities to the tetrahedral lattice of bulk diamond. Counterpoise corrected Hartree-Fock (HF) and second-order Møller-Plesset perturbation theory (MP2) calculations were performed on ethane, 2,2-dimethylpropane (neo-pentane, (C5H12), 2-dimethyl-3-dimethylbutane (neobutane, C8H18) and cyclohexane (C6H12) in order to determine the two-body (stretching) and three-body (bond bending) energies. The suitability of these molecules to model the properties of diamond was determined by comparison of CC bond length, well depth, CCC bond angle, simultaneous stretch and bend energy and force constants to those of bulk diamond. It was found that neopentane provided the best overall description of tetrahedral bonded carbon. The ab initio derived stretch and bend energies were fitted to the SW potential energy terms and the SW parameters calculated. The newly parametrized SW potential was then evaluated by calculating the stretch force constants, elastic constants and the X-point phonon modes of bulk diamond. read less

Abstract: In this paper high-resolution electron microscopy investigations and molecular dynamics simulations of GeSi nanocrystals buried in 4H-SiC are performed, showing that the experimentally observed shapes of the GeSi nanocrystals are strongly correlated with their orientational relationships. Measurements of the lattice spacing suggest that the nanocrystals are strained. Quantum confinement in selected nanocrystals has been detected using spatially-resolved electron energy loss spectroscopy performed in conjunction with atomic resolution annular dark-field scanning TEM. read less

Abstract: The environment-dependent interaction potential is a transferable empirical potential for carbon which is well suited for studying disordered systems. Ab initio data are used to motivate and parametrize the functional form, which includes environment-dependence in the pair and triple terms, and a generalized aspherical coordination describing dihedral rotation and non-bonded π-repulsion. Simulations of liquid carbon compare very favourably with Car-Parrinello calculations, while amorphous networks generated by liquid quench have properties superior to Tersoff, Brenner and orthogonal tight-binding calculations. The efficiency of the method enables the first simulations of tetrahedral amorphous carbon by deposition, and a new model for the formation of diamond-like bonding is presented. read less

Abstract: The effect of chirality on the structural stability of single-wall carbon nanotubes have been investigated by performing molecular-dynamics computer simulations. Calculations have been realized by using an empirical many-body potential energy function for carbon. It has been found that carbon nanotube in chiral structure is more stable under heat treatment relative to zigzag and armchair models. The diameter of the tubes is slightly enlarged under heat treatment. read less

Abstract: The structural properties of single and multi-wall carbon nanotubes and the formation of carbon nanorods from multi-wall carbon nanotubes have been investigated by performing molecular-dynamics computer simulations. Calculations have been realized by using an empirical many-body potential energy function for carbon. It has been found that carbon nanorod formation takes place with smallest possible multi-wall nanotubes under heat treatment. On the other hand, it has been also found that single-wall carbon nanotubes are stronger than the multi-wall nanotubes against heat treatment. read less

Abstract: Simulation methods have been used to study the miscibility ofCuxAg1 − xI based on a Tersoff potential. Monte Carlo calculations show that CuxAg1 − xI is a complete solid solution. This result agrees well with experiments using NMR and X-ray diffractions methods. Structural, elastic and thermodynamic properties are also predicted at 0.25, 0.5 and 0.75 using molecular dynamics simulations. read less

Abstract: The coalescence of single-walled nanotubes is studied in situ under electron irradiation at high temperature in a transmission electron microscope. The merging process is investigated at the atomic level, using tight-binding molecular dynamics and Monte Carlo simulations. Vacancies induce coalescence via a zipper-like mechanism, imposing a continuous reorganization of atoms on individual tube lattices along adjacent tubes. Other topological defects induce the polymerization of tubes. Coalescence seems to be restricted to tubes with the same chirality, explaining the low frequency of occurrence of this event. read less

Abstract: The effect of the periodic boundary condition (PBC) on the structure and energetics of nanotubes has been investigated by performing molecular-dynamics computer simulation. Calculations have been realized by using an empirical many-body potential energy function for carbon. A single-wall carbon nanotube has been considered in the simulations. It has been found that the periodic boundary condition has no effect at low temperature (1 K), however, it plays an important role even at intermediate temperature (300 K). read less

Abstract: A method that gives the parameters of advanced Tersoff interatomic potentials for describing nonequilibrium atomic structures has been developed. This method uses a genetic algorithm to optimize the Tersoff potential parameters fitted to first-principles-calculated cohesive energies of various carbon systems, including bulk systems with atomic defects and amorphous, surface, or cluster systems under stress. These optimized parameters converge towards a set of Tersoff potential parameters that well describes not only crystals but also amorphous systems. read less

Abstract: This paper reviews the fundamental ideas related to the concept of local atomic stresses and their application to the study of strain fields in ta-C. The calculations are based on Monte Carlo simulations within the empirical potential approach. We find that the stress distributions in ta-C are high inhomogeneous, both in the bulk as well as in the surface and interface regions. There is a close relationship between local stress and hybridization. The most probable stress state for fourfold sites is compression, while threefold sites prefer to be under tension. Local-atomic behavior dominates longer-ranged stress conditions. Interdiffusion following thermal annealing is an important factor for the reduction of compressive stress in ta-C films grown on silicon substrates. read less

Abstract: ▪ Abstract The synthesis of nanocrystalline diamond films from carbon-containing noble gas plasmas is described. The nanocrystallinity is the result of new growth and nucleation mechanisms, which involve the insertion of C2, carbon dimer, into carbon-carbon and carbon-hydrogen bonds, resulting in hetereogeneous nucleation rates on the order 1010 cm−2 s−1. Extensive characterization studies led to the conclusion that phase-pure diamond is produced with a microstructure consisting of randomly oriented 3–15-nm crystallites. By adjusting the noble gas/hydrogen ratio in the gas mixture, a continuous transition from micro- to nanocrystallinity is achieved. Up to 10% of the total carbon in the nanocrystalline films is located at 2 to 4 atom-wide grain boundaries. Because the grain boundary carbon is π-bonded, the mechanical, electrical, and optical properties of nanocrystalline diamond are profoundly altered. Nanocrystalline diamond films are unique new materials with applications in fields as diverse as tribolo... read less

Abstract: We present computational aspects of Molecular Dynamics calculations of thermal properties of diamond using the Brenner potential. Parallelization was essential in order to carry out these calculations on samples of suitable sizes. Our implementation uses MPI on a multi-processor machine such as the IBM SP2. Three aspects of parallelization of the Brenner potential are discussed in depth. These are its long-range nature, the need for different parallelization algorithms for forces and neighbors, and the relative expense of force calculations compared to that of data communication. The efficiency of parallelization is presented as a function of different approaches to these issues as well as of cell size and number of processors employed in the calculation. In the calculations presented here, information from almost half of the atoms were needed by each processor even when 16 processors were used. This made it worthwhile to avoid unnecessary complications by making data from all atoms available to all processors. Superlinear speedup was achieved for four processors (by avoiding paging) with 512 atom samples, and 5ps long trajectories were calculated (for 5120 atom samples) in 53 hours using 16 processors; 514 hours would have been needed to complete this calculation using a serial program. Finally, we discuss and make available a set of routines that enable MPI-based codes such as ours to be debugged on scalar machines. read less

Abstract: Based on a series of ab initio studies we have pointed out the remarkable structural stability of nanotubular and quasiplanar boron clusters, and postulated the existence of novel layered, tubular, and quasicrystalline boron solids built from elemental subunits. The present study illustrates and predicts qualitative structural and electronic properties for various models of nanotubular and layered boron solids, and compares them to well-known tubular and layered forms of pure carbon and mixed boron compounds. read less

Abstract: The atomic structures of a few representative large-unit-cell grain boundaries thought to largely determine the behavior of nanocrystalline diamond are determined via Monte-Carlo simulation. In these highly disordered grain boundaries up to 80% of the C atoms exhibit local sp2 bonding. However, because the three-coordinated C atoms are poorly connected to each-other, graphite-like electrical conduction through the grain boundaries is unlikely without 'bridging' impurities. Surprisingly, based on their fracture energies, the high-energy, large-unit-cell boundaries are more stable against brittle decohesion into free surfaces than low-energy ones and perhaps even the perfect crystal. read less

Abstract: The formation of point defects in diamond induced by an energetic displacement of a carbon atom out of its lattice site and the relaxation of the thereby disrupted crystal are studied by molecular dynamics simulations with the Tersoff potential. The displacement energy for Frenkel pair creation is calculated to be 52 eV, in agreement with experiments. It is found that the split interstitial, with a bonding configuration which resembles graphite, is the most stable defect, and the disrupted region around it is rich in sp2-like (graphitic) bonds. This region extends several nanometers and is likely to be the elementary electrically conductive cell experimentally found in radiation-damaged diamond. read less

Abstract: The reaction channels and the reaction products in the collisional reactions between two C60 molecules are found to be sensitively dependent on the relative orientation between the colliding partners. The detailed collision processes and the stable structures of the products are studied by using a molecular dynamics simulation method combined with a Langevin molecular dynamics scheme. The defects of the reaction products are also analyzed. read less

Abstract: We have generated a new model for the structure of tetrahedral amorphous carbon using a modified reverse Monte Carlo modelling method. The novel feature of this approach is the definition of three different types of carbon atom, corresponding to tetrahedral, planar and linear bonding conformations. The particular strengths of the method are the large model size (3000 atoms), that all the possible arrangements of and bonds are allowed, and that no interatomic potential is required. For the first time we have determined the distribution of bonded sites within the predominantly disordered tetrahedral structure, and we find that they form polymer-like chains and small clusters which connect the bonded regions. read less

Abstract: Scanning probe microscopy experiments show that ion irradiation of (0001) graphite results in the formation of isolated defects comprising of a few tens of atoms. We use molecular dynamics simulations and density-functional theory calculations to study the formation probabilities of these defects. We identify different defect structures which correspond to experimentally observed hillocks on graphite surfaces. We find that the predominant source of defects are vacancies and interlayer interstitials, and identify a three-atom carbon ring defect on the graphite surface. read less

Abstract: A semiempirical potential is developed for modelling both the chemistry and the bulk properties of C[sbnd]Si[sbnd]H systems based on the Tersoff formulation. The potentials are compared with the known energetics of small Si[sbnd]H[sbnd]C clusters with good results. The potential is used to investigate the interaction of Ca with hydrogenated crystal surfaces in the energy range 100-250 eV. The simulations show that a wide variety of interactions is possible. The molecule can stick on the surface either directly or by bouncing across the surface. Reflection from the surface is also possible. read less

Abstract: Electron configurations close to the tetrahedral hybridization are found in pure amorphous carbon at a concentration which depends on preparation conditions. Tetrahedral bonding at levels of approximately 80% is found in amorphous carbons formed from beams of carbon ions with energies in a `window' between 20 eV and approximately 500 eV. Suitable techniques for its formation include cathodic arc deposition, ion beam deposition and laser ablation. Similar material appears to be formed by pressure treatment of fullerene precursors and by displacement damage in diamond. Highly tetrahedral forms of amorphous carbon (ta-C) show electronic, optical and mechanical properties which approach those of diamond and are quite different from amorphous carbons with low content. Useful techniques for determining the content include electron energy loss spectroscopy, electron and neutron diffraction and Raman spectroscopy. Considerable progress has been made in the understanding of this material by simulating its structure in the computer with a range of techniques from empirical potentials to ab initio quantum mechanics. The structure shows departures from an idealized glassy state of diamond which would have a random tetrahedral network structure as used to describe amorphous silicon and germanium. A surprising feature of the structure simulated using ab initio methods is the presence of small rings containing three or four carbon atoms. The electronic and optical properties are strongly influenced by the residual of carbon. Applications to electronic devices are at an early stage with the demonstration of photoconductivity and some simple junction devices. Applications as a wear resistant coating are promising, since the theoretically predicted high values of elastic constants, comparable to but less than those of diamond, are achieved experimentally, together with low friction coefficients. read less

Abstract: It is demonstrated how new microscopes with atomic resolution in combination with modern fast computers and computational techniques can be used in a complementary way in the analysis and explanation of crystal growth on surfaces. Examples are given of spiral formation, fractal growth, fullerene formation and the growth of C60 films. Zusammenfassung. Es wird gezeigt, wie neue Mikroskopiertechniken mit atomarer Auflosung in Kombination mit modernen schnellen Computern und neuen Computertechniken genutzt werden konnen, um auf Oberflachen Kristallwachstum zu analysieren und erklaren. Beispiele fur Spiralformationen, Fraktalwachstum, Fullerenebildung und Wachstum von C60 Filmen werden diskutiert. read less

Abstract: Formations of endohedral complexes in collisions of H2 and D2 with C60 molecules have been studied using a molecular dynamics simulation method. H@C60 complex is produced when a H2 molecule collides with a C60 molecule along the direction from the center of a hexagonal ring into the center of the C60 at collision energy E0 = 40 eV. At the same collision condition, a D2 molecule colliding with a C60 molecule forms a D2@C60 complex. read less

Abstract: The importance of defects in the formation of graphitic structures such as fullerenes, nanotubes, onion-like structures, helically coiled nanotubes and hypothetical periodic structures is analyzed. The structures mentioned above can also present defects which preserve their connectivity and topology. We have calculated the stability of Stone-Wales type defects in different shapes of graphitic structures finding that the energy required for these defects is not very high. read less

Abstract: Feynman path-integral Monte Carlo simulations have been performed to study finite-temperature properties of the C60 molecule in a temperature range between 50 and 800 K. The interaction between the C atoms was modelled by the empirical Tersoff potential that reliably reproduces several properties of fullerenes like the binding energy and the alternation between the long and short C-C bonds. We show that the delocalization of the C nuclei is comparable to the difference between the lengths of the long and short bonds. The importance of quantum effects for the C nuclei is illustrated by comparing classical and quantum results for properties like the total energy and the radial and angular distribution functions. read less

Abstract: We study the electronic structure of C 78 and that of C 78 -graphite cointercalation compound. First we show the electronic structure of five isomers of C 78 . Their geometric structures have been optimized by an empirical model potential and their electronic structure has been calculated using the tight-binding model. The C 2 v -symmetry C 78 , a major isomer experimentally extracted, is found to have a considerably deep lowest unoccupied state. Using this C 2 v C 78 , we design a stage-1 C 78 -graphite cointercalation compound. Its electronic structure calculated with the tight-binding model shows that the deep lowest unoccupied state of the C 2 v C 78 causes charge transfer from a graphite sheet to C 78 . Although the material is formed with only carbon atoms, C 78 -graphite cointercalation compound is the hole doped graphite intercalation compound. read less

Abstract: We point out the important role of coordination-dependent terms in tight-binding potentials for carbon structures, as required by the strong changes in orbital hybridization, especially for low-coordination phases. We discuss the performances of two forms of potential on the basis of the cohesion energy and molecular dynamics predictions, as compared with existing first-principles results. read less

Abstract: Surface relaxation and lattice dynamics of (100) Si have been studied using Tersoff-Dodson type Si potential. The average temperature of the lattice is studied as well. The temperature fluctuates with a frequency of 9.5 × 1012 Hz, that is about the average frequency of the optical phonons in Si. The (100) Si surface relaxes inward by 0.86 Å, and a reduction of 19% in the first interlayer spacing is found. read less

Abstract: The molecular mechanics computer program is designed to study the structure of fullerenes with icosahedral symmetry and carbon nanotubes. The program takes full advantage of symmetry. The program predicts meaningful conformations, energies of formation for symmetrical fullerenes as well as for the carbon nanotubes for which also the elastic properties are calculated. The systematic IUPAC nomenclature of fullerenes and fullerenes derivatives is discussed. Comments are made on terminology questions and on the chemical non-aromaticity of fullerenes. read less

Abstract: As most technologically important metals will form oxides readily, any complete study of adhesion at real metal surfaces must include the metal-oxide interface. The role of this ubiquitous oxide layer cannot be overlooked, as the adhesive properties of the oxide or oxide-metal system can be expected to differ profoundly from the adhesive properties of a bare metal surface. We report on the development of a computational method for molecular-dynamics simulations, which explicitly includes variable charge transfer between anions and cations. This method is found to be capable of describing the elastic properties, surface energies, and surface relaxation of crystalline metal oxides accurately. We discuss in detail results using this method for \ensuremath{\alpha}-alumina and several of its low-index faces. read less

Abstract: Experiments and molecular dynamics (MD) simulations of low-energy (10-30 eV) H interactions with the C60 molecule have been carried out in order to investigate the possible chemical combinations at these energies. It is found that the preferred minimum-energy state is for the H atom to be attached to the outside of the molecule but that some trapping of the H atom within the cage should be possible if the C60 molecule has only a small initial energy. As the excitation of the molecule increases, trapping becomes less likely. The trajectories calculated by MD show a number of different interactions: (i) reflection of the H atom (ii) transmission of the H atom through the structure, (iii) implantation within the molecule, (iv) attachment of the H atom to the outside of the structure and (v) initial implantation and attachment of the H atom to the inside of the structure followed by bond-breaking of the C atoms and final attachment of the H atom externally. The experimental results of the interaction process with 'energetic' C60 molecules so far have shown no evidence of the existence of a trapped H atom at the centre of the C60 cage, but indicate strong evidence for externally bonded H atoms. read less

Abstract: The interaction of fullerene molecules with silicon crystal surfaces is modelled using molecular dynamics. The results are compared to similar interactions with graphite surfaces. In contrast to the results for graphite, it is found that the molecule rarely reflects intact from the surface. When reflection does occur it is always at near grazing incidence with impact energies less than 300 eV. At normal incidence and similar energies the molecule remains intact, but becomes embedded in the surface layers of the Si lattice. Grazing incidence ( approximately=75-80 degrees to the surface normal) at energies of a few hundred eV results in the C60 molecule becoming trapped in the surface binding potential. The molecule can roll across the surface for up to one revolution before coming to rest. At energies of greater than approximately=500 eV, at grazing incidence, the molecule breaks up on impact with the majority of the constituent atoms reflected. Normal incidence with impact energies in excess of 1 keV leads to disintegration of the C60 molecule and sputtering from the crystal, with the ejection of atoms and larger SixCy molecules. This is especially evident at energies greater than 4 keV where high-energy deposition near the impact point creates a crater surrounded by a hot disordered region from which Si atoms can be thermally ejected for times up to the order of 1 ps. read less

Abstract: To obtain qualitatively different amorphous carbon networks we carried out molecular dynamics simulations at different mean atomic densities using the Tersoff potential. Differences among the radial distribution functions obtained as well as among the ratio of threefold- and fourfold-coordinated atoms according to the mean densities are discussed. The electronic stability of the models is investigated by means of their total densities of states which are characterized by significant defect bands. These bands are ascribed to the uncorrelated free-atomic p orbitals occurring in the models. The effect may be quantified by a simple pi bonding analysis which estimates the relative number of non-bonding and of bonding and antibonding pi states by calculating the number of unbonded hybrid orbitals and topological pi defects. For a justification, a number of local densities of states are calculated. read less

Abstract: The formation process of C120-complex in C60-C60 collision has been clearly demonstrated by a molecular dynamics simulation. The complex, with a peanut-shell-like structure, is in a quite stable dynamical state. The results are consistent with recent observations. read less

Abstract: The interaction of C60 molecules with a graphite surface is modelled using molecular dynamics. At normal incidence it is found that the C60 molecule is reflected intact at energies up to about 250 eV and a depression wave spreads radially from the point of impact across the graphite surface preceded by hypersonic fronts which transmit small amounts of energy. At energies of 1 keV and 6 keV the molecule implodes as it enters the crystal creating a particle of dense amorphous carbon beneath the surface. Very little sputtering occurs at normal incidence at energies of up to 6 keV. At 6 keV and at an incidence angle of 60° to the normal, the molecule breaks up on impact and some sputtering is observed. At 15 keV the sputtering yield increases and the surface ruptures in the central region where carbon clusters and chains are ejected. Outside this region the bonds remain intact but surface begins to separate from the second layer with a raised travelling wave propagating from the impact point. read less

Abstract: The concept of the characteristic curve in physical adsorption and the rule of its temperature invariance is traced from the potential theory of adsorption by Polanyi to the Frenkel–Halsey–Hill equation, the theory of volume filling of micropores by Dubinin et al. and the theory of adsorption on heterogeneous surfaces. A computer simulation of an irregular atomic configuration at the surface of amorphous carbon is presented. In the submonolayer region, the isotherms of argon adsorption simulated on that surface are shown to correspond to the Freundlich equation and are close to the experimental isotherms on a diamond dust sample. Simulated isosteric heats of adsorption are also reasonably close to the experimental data for argon on a real carbon black. The BET C constant for the simulated isotherm is lower than for the real isotherm on untreated carbon black. However, the isotherms at two temperatures can be described by one absolute isotherm of adsorption. read less

Abstract: An interatomic potential for carbon is developed that is based on an empirical tight-binding approach. The model reproduces accurately the energy-versus-volume diagram of carbon polytypes and gives a good description of the phonons and elastic constants for carbon in the diamond and graphite structures. To test the transferability of the model to different environments further, the authors performed molecular-dynamics simulations to study the liquid phase and the properties of small carbon microclusters. The results obtained are in good agreement with those obtained from ab initio calculations. read less

Abstract: Measurements of the first-order Raman spectrum in homoepitaxially grown synthetic diamond for the temperature range of 300 to 1200 K are presented. Similar measurements for natural type IIa diamond for the temperature range of 300 to 2000 K are also given. Both the Stokes and anti-Stokes components are analyzed for their intensity, Raman shift, and width variation with temperature. The depolarization of the Raman signal at elevated temperatures was found to be the same as that at room-temperature. The synthetic diamond Raman shift indicated the presence of internal stress. The experimental first-order Raman shifts for natural diamond, using units of cm-1 and absolute temperature, are conveniently expressed as (Delta)(nu) = a1T2 + a2T + a3 with the coefficients found to be -1.124 X 10-5 cm-1 K-2, -6.71 X 10-3 cm-1 K-1, and 1334.5 cm-1, respectively. read less

Abstract: Ab initio Hartree–Fock self-consistent-field-theory calculations have been performed on neopentane, a C5 cluster, subjected to bond stretching and bending distortions. The potential energy surface was fitted by a Stillinger-Weber potential reparametrized for carbon in the diamond form. This potential provides a good description of the C–C interaction and should be useful in molecular dynamics, simulations of diamond. read less

Abstract: : Measurements of the first order Raman spectrum in natural type IIa diamond for the temperature range of 293 K to 1850 K are presented. Both the stokes and anti-Stokes components are analyzed for their intensity, Raman shift, and width variation with temperature. Optical pyrometry was used to make the temperature measurements, the results of which were independently confirmed by the Stokes to anti-Stokes intensity ratios. The shift and width variations with temperature are in general agreement with the molecular dynamics simulation of C.Z. Wang, C.T. Chan, and K.M.Ho. Heating of the samples to temperatures as high as 1850 K in vacuo could be performed without any evidence of polymorphic conversion to graphite, also in agreement with previous investigations. read less

Abstract: Melting of diamond at high pressure and the properties of liquid carbon at pressures greater than 1 megabar were investigated with a first-principles molecular dynamics technique. The results indicate an increase of the diamond melting temperature with pressure, which is opposite to the behavior of silicon and germanium. This is contrary to long-held assumptions, but agrees with recent experiments, and has important implications for geology and astrophysics. As is the case for the solid phase of carbon at low temperature, which changes greatly with pressure from graphite to diamond, the structural and bonding properties of liquid carbon vary strongly with pressure. read less

Abstract: To better understand and improve reactive processes on nickel surfaces such as the catalytic steam reforming of hydrocarbons, the decomposition of hydrocarbons at fuel cell anodes, and the growth of carbon nanotubes, we have performed atomistic studies of hydrocarbon adsorption and decomposition on low index nickel surfaces and nickel catalyst nanoparticles. Quantum mechanics (QM) calculations utilizing the PBE flavor of density functional theory (DFT) were performed on all CH_x and C_2H_y species to determine their structures and energies on Ni(111). In good agreement with experiments, we find that CH is the most stable form of CH_x on Ni(111). It is a stable intermediate in both methane dehydrogenation and CO methanation, while CH(2,ad) is only stable during methanation. We also find that nickel surface atoms play an important catalytic role in C-H bond formation and cleavage. For the C_2H_y species we find a low surface coverage decomposition pathway proceeding through CHCH_(ad), the most stable intermediate, and a high surface coverage pathway which proceeds through CCH_(3,ad), the next most stable intermediate. Our enthalpies along these pathways are consistent with experimental observations. To extend our study to larger systems and longer time scales, we have developed the ReaxFF reactive force field to describe hydrocarbon decomposition and reformation on nickel catalyst surfaces. The ReaxFF parameters were fit to geometries and energy surfaces from DFT calculations involving a large number of reaction pathways and equations of state for nickel, nickel carbides, and various hydrocarbon species chemisorbed on Ni(111), Ni(110) and Ni(100). The resulting ReaxFF description was validated against additional DFT data to demonstrate its accuracy, and used to perform reaction dynamics (RD) simulations on methyl decomposition for comparison with experiment. Finally ReaxFF RD simulations were applied to the chemisorption and decomposition of six different hydrocarbons (methane, acetylene, ethylene, benzene, cyclohexane and propylene) on a 468 atom nickel nanoparticle. These simulations realistically model hydrocarbon feedstock decomposition and provide reaction pathways relevant to this part of the carbon nanotube growth process. They show that C-C π bonds provide a low barrier pathway for chemisorption, and that the low energy of subsurface C is an important driving force in breaking C-C bonds. read less

Abstract: Nanowires and ultra-thin films have wide applications in the quickly developed nanotechnology and nanoscience. However, their Young’s modulus varies with the size, which is seemingly contradictory to the conventional continuum elasticity. Investigating and understanding the underlying mechanism of the size-dependent elastic properties in nanomaterials is of both academic and practical significance. In this work, both theoretical modeling and virtual experiments have been made on this issue. A nanoelement, from the traction free bulk lattice, undergoes an initial relaxation, during which its morphology changes and energy reduces, which is an emphasis in this developed methodology and is a distinction from almost other existing models. With different definitions of surfaces and edges, two models for a nanomaterial – a nanowire or an ultra-thin film – are derived based on the same thermodynamics framework. Comparing with most of others’ treatment, Model I has an advantage to mathematically treat a surface phase to be two-dimensional and an edge phase to be one-dimensional. Under external loadings, the initial relaxed state is taken as the reference. Experimentally, relaxation and tension/compression tests in different loading directions have been conducted on SiC, Si and Cu crystalline nanowires with different cross-sectional sizes and ultra-thin films with different thicknesses by Molecular Dynamics (MD) simulations. This systematic study successfully illustrates the intrinsic mechanism of the size-dependent Young’s modulus in nanomaterials and the proposed methodology facilitate characterizing mechanical properties of nanomaterials to some extent when continuum concepts, such as, surface energy and surface elastic constants, are used. read less

Abstract: We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for about 3,000 atoms near the crack tip while the other 100,000 atoms of the model system are described with a simple nonreactive force field. The ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. This model has been successfully used to study crack dynamics in silicon, capable of reproducing key experimental results such as orientation dependence of crack dynamics (Buehler et al., Phys. Rev. Lett. 2006). In this article, we focus on crack speeds as a function of loading and crack propagation mechanisms. We find that the steady state crack speed does not increase continuously with applied load, but instead jumps to a finite value immediately after the critical load, followed by a regime of slow increase. Our results quantitatively reproduce experimental observations of crack speeds during fracture in silicon along the (111) planes, confirming the existence of lattice trapping effects. We observe similar effects in the (110) crack direction. read less

Abstract: We report a study of dynamic cracking in a silicon single crystal in which the ReaxFF reactive force field is used for ∼3,000 atoms near the crack tip while the other 100,000 atoms of the model system are described with a simple nonreactive force field. The ReaxFF is completely derived from quantum mechanical calculations of simple silicon systems without any empirical parameters. Our results reproduce experimental observations of fracture in silicon including details of crack dynamics for loading in the [110] orientations, such as dynamical instabilities with increasing crack velocity. We also observe formation of secondary microcracks ahead of the moving mother crack. We conclude with a study of Si(bulk)-O2 systems, showing that Si becomes more brittle in oxygen environments, as known from experiment. read less

Abstract: Recent developments of meshfree and particle methods and their applications in applied mechanics are surveyed. Three major methodologies have been reviewed. First, smoothed particle hydrodynamics ~SPH! is discussed as a representative of a non-local kernel, strong form collocation approach. Second, mesh-free Galerkin methods, which have been an active research area in recent years, are reviewed. Third, some applications of molecular dynamics ~MD! in applied mechanics are discussed. The emphases of this survey are placed on simulations of finite deformations, fracture, strain localization of solids; incompressible as well as compressible flows; and applications of multiscale methods and nano-scale mechanics. This review article includes 397 references. @DOI: 10.1115/1.1431547# read less

Abstract: Using the Lennard-Jones interaction potential, we have studied the in-tube carbon-chain structure doped into single-wall carbon nanotubes (SWCNTs). Through minimizing the potential energy of the doped system, it is found that the optimal structure of the doping carbon chain is spiral, but not a straight line, when the radius of the SWCNT is larger than about 4.30A. read less

Abstract: Optimisation of geometries of all 40 fullerene isomers of C40, using methods from molecular mechanics and tight-binding to full abinitio SCF and DFT approaches, confirms minimisation of pentagon adjacency as a major factor in relative stability. The consensus predictions of 11 out of 12 methods are that the isomer of lowest total energy is the D2 cage with the smallest possible adjacency count, and that energies rise linearly with the number of adjacencies. Quantum mechanical methods predict a slope of 80–100 kJ mol-1 per adjacency. Molecular mechanics methods are outliers, with the Tersoff potential giving a different minimum and its Brenner modification a poor correlation and much smaller penalty. read less

Abstract: We describe the development and applications of a new electronic structure method that uses a real-space grid as a basis. Multigrid techniques provide preconditioning and convergence acceleration at all length scales and therefore lead to particularly efficient algorithms. The salient points of our implementation include: (i) new compact discretization schemes in real space for systems with cubic, orthorhombic, and hexagonal symmetry and (ii) new multilevel algorithms for the iterative solution of Kohn–Sham and Poisson equations. The accuracy of the discretizations was tested by direct comparison with plane-wave calculations, when possible, and the results were in excellent agreement in all cases. These techniques are very suitable for use on massively parallel computers and in O(N) methods. Tests on the Cray-T3D have shown nearly linear scaling of the execution time up to the maximum number of processors (512). The above methodology was tested on a large number of systems, such as the C60 molecule, diamond, Si and GaN supercells, and quantum molecular dynamics simulations for Si. Large-scale applications include a simulation of surface melting of Si and investigations of electronic and structural properties of surfaces, interfaces, and biomolecules. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 531–543, 1997 read less

Abstract: We report on the development of a novel computational method for molecular dynamics simulations which explicitly includes variable charge transfer between anions and cations. This method is found to be capable of describing the elastic properties, surface energies, and surface relaxation of crystalline metal-oxides accurately. We present results for a simulation of adhesive failure at a model metal/oxide heterophase interface between an aluminum (111) face and an α-alumina (0001) face. Our results indicate that this approach can provide physically realistic empirical potentials for future simulations on mixed metal/metal-oxide systems. read less

Abstract: An empirical potential-energy function comprising two- and three-body terms, whose parameters have been determined from the properties of diamond and graphite, is used to study the structures and energies of carbon microclusters.The binding energy per atom of smaller linear clusters increases monotonically with the number of atoms, whereas cyclic clusters display an optimal energy per atom for six-membered rings. The energies of fullerenes are sensitive to nuclearity and shape, with icosahedral C60 and D5h C70 being the most stable clusters. The potential predicts the binding energy of C60 to be 7.25 eV per atom, in good agreement with experimental measurements.For larger clusters, spherical fragments of cubic bulk structures have been investigated; diamond fragments become relatively more stable than other cubic fragments for more than approximately 100 atoms. Open nanotubes are found to be most stable for circumferences containing five hexagons. Vibrational frequencies were calculated and correlated with experimental results for some clusters. read less

Abstract: Molecular dynamics computer simulations of the growth processes and microstructural properties of amorphous carbon (a:C) and amorphous hydrogenated carbon (a:CH) ultra-thin films have been performed. Films 1 to 10 nm thick were grown on a diamond (100) surface using Brenner`s bond-order potential for hydrocarbons. The stoichiometry, radial distribution function, chemical bonding (amount of sp{sub 2} and sp{sub 3} hybridization) and residual stress are presented. read less

Abstract: We discuss the development of interaction potentials which explicitly allow for charge transfer in metallic oxides. The charge transfer is calculated self-consistently using a charge equilibration approach, which allows the amount of charge transferred to respond to the electrostatic environment. We model the metal-metal, metal-oxygen, and oxygen-oxygen interactions with Rydberg function pair potentials. By fitting the Rydberg potential parameters to the elastic and structural constants of the material, we arrive at an efficient model for the simulation of metallic oxides. We demonstrate the applicability of the model by describing some preliminary results on the rutile phase of titanium dioxide. read less

Abstract: Computer simulation is a major tool for studying the interactions of swift ions with solids which underlie processes such as particle backscattering, ion implantation, radiation damage, and sputtering. Numerical models are classed as molecular dynamics or binary collision models, along with some intermediate types. Binary collision models are divided into those for crystalline targets and those for structureless ones. The foundations of such models are reviewed, including interatomic potentials, electron excitations, and relationships among the various types of codes. Some topics of current interest are summarized. read less

Abstract: Carbon nanotubes can serve as one-dimensional nanoreactors for the in-tube synthesis of various nanostructures. Experimental observations have shown that chains, inner tubes, or nanoribbons can grow by the thermal decomposition of organic/organometallic molecules encapsulated in carbon nanotubes. The result of the process depends on the temperature, the diameter of the nanotube, and the type and amount of material introduced inside the tube. Nanoribbons are particularly promising materials for nanoelectronics. Motivated by recent experimental results observing the formation of carbon nanoribbons inside carbon nanotubes, molecular dynamics calculations were performed with the open source LAMMPS code to investigate the reactions between carbon atoms confined within a single-walled carbon nanotube. Our results show that the interatomic potentials behave differently in quasi-one-dimensional simulations of nanotube-confined space than in three-dimensional simulations. In particular, the Tersoff potential performs better than the widely used Reactive Force Field potential in describing the formation of carbon nanoribbons inside nanotubes. We also found a temperature window where the nanoribbons were formed with the fewest defects, i.e., with the largest flatness and the most hexagons, which is in agreement with the experimental temperature range. read less

Abstract: The atomic‐scale cracking mechanism in clay is vital in discovering the cracking mechanism of clay at the continuum scale in that clay is a nanomaterial. In this article, we investigate mechanisms of modes I and II crack propagations in pyrophyllite and Ca‐montmorillonite with water adsorption through reactive molecular dynamics (MD) with a bond‐order force field. Clay water adsorption is considered by adding water molecules to the clay surface. During the equilibration stage, water adsorption could cause bending deformation of the predefined edge crack region. The relatively small orientating angle of water molecules indicates the formation of hydrogen bonds in the crack propagation process. The peak number density of adsorbed water decreases with the increasing strains. The atomistic structure evolution of the crack tip under loading is analyzed to interpret the nanoscale crack propagation mechanism. The numerical results show that the crack tip first gets blunted with a significant increase in the radius of the curvature of the crack tip and a slight change in crack length. The crack tip blunting is studied by tracking the crack tip opening distance and O–Si–O angle in the tetrahedral Si–O cell in modes I and II cracks. We compare bond‐breaking behaviors between Al–O and Si–O. It is found that Si–O bond breaking is primarily responsible for crack propagation. The critical stress intensity factor and critical energy release rate are determined from MD simulation results. read less

Abstract: We study the performance of eleven reactive force fields (ReaxFF), which can be used to study sp2 carbon systems. Among them a new hybrid ReaxFF is proposed combining two others and introducing two different types of C atoms. The advantages of that potential are discussed. We analyze the behavior of ReaxFFs with respect to 1) the structural and mechanical properties of graphene, its response to strain and phonon dispersion relation; 2) the energetics of (n, 0) and (n, n) carbon nanotubes (CNTs), their mechanical properties and response to strain up to fracture; 3) the energetics of the icosahedral C60 fullerene and the 40 C40 fullerene isomers. Seven of them provide not very realistic predictions for graphene, which made us focusing on the remaining, which provide reasonable results for 1) the structure, energy and phonon band structure of graphene, 2) the energetics of CNTs versus their diameter and 3) the energy of C60 and the trend of the energy of the C40 fullerene isomers versus their pentagon adjacencies, in accordance with density functional theory (DFT) calculations and/or experimental data. Moreover, the predicted fracture strain, ultimate tensile strength and strain values of CNTs are inside the range of experimental values, although overestimated with respect to DFT. However, they underestimate the Young’s modulus, overestimate the Poisson’s ratio of both graphene and CNTs and they display anomalous behavior of the stress - strain and Poisson’s ratio - strain curves, whose origin needs further investigation. read less

Abstract: Two-dimensionally extended amorphous carbon (“amorphous graphene”) is a prototype system for disorder in 2D, showing a rich and complex configurational space that is yet to be fully understood. Here we explore the nature of amorphous graphene with an atomistic machine-learning (ML) model. We create structural models by introducing defects into ordered graphene through Monte-Carlo bond switching, defining acceptance criteria using the machine-learned local, atomic energies associated with a defect, as well as the nearest-neighbor (NN) environments. We find that physically meaningful structural models arise from ML atomic energies in this way, ranging from continuous random networks to paracrystalline structures. Our results show that ML atomic energies can be used to guide Monte-Carlo structural searches in principle, and that their predictions of local stability can be linked to short- and medium-range order in amorphous graphene. We expect that the former point will be relevant more generally to the study of amorphous materials, and that the latter has wider implications for the interpretation of ML potential models. 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: In this study, single groove nanoscratch experiments using a friction force microscope (FFM) with a monocrystalline diamond tip were conducted on a c-plane sapphire wafer to analyze the ductile-regime removal and deformation mechanism including the anisotropy. Various characteristics, such as scratch force, depth, and specific energy for each representative scratch direction on the c-plane of sapphire, were manifested by the FFM, and the results of the specific scratch energy showed a trend of six-fold symmetry on taking lower values than those of the other scratch directions when the scratch directions correspond to the basal slip directions as 0001〈112¯0〉. Since this can be due to the effect of most probably basal slip or less probably basal twinning on the c-plane, a molecular dynamics (MD) simulation of zinc, which is one of the hexagonal close-packed (hcp) crystals with similar slip/twining systems, was attempted to clarify the phenomena. The comparison results between the nanoscratch experiment and the MD simulation revealed that both the specific scratch energy and the burr height were minimized when scratched in the direction of the basal slip. Therefore, it was found that both the machining efficiency and the accuracy could be improved by scratching in the direction of the basal slip in the single groove nanoscratch of c-plane sapphire. read less

Abstract: Carbon materials and their unique properties have been extensively studied by molecular dynamics, thanks to the wide range of available carbon bond order potentials (CBOPs). Recently, with the increase in popularity of machine learning (ML), potentials such as Gaussian approximation potential (GAP), trained using ML, can accurately predict results for carbon. However, selecting the right potential is crucial as each performs differently for different carbon allotropes, and these differences can lead to inaccurate results. This work compares the widely used CBOPs and the GAP-20 ML potential with density functional theory results, including lattice constants, cohesive energies, defect formation energies, van der Waals interactions, thermal stabilities, and mechanical properties for different carbon allotropes. We find that GAP-20 can more accurately predict the structure, defect properties, and formation energies for a variety of crystalline phase carbon compared to CBOPs. Importantly, GAP-20 can simulate the thermal stability of C60 and the fracture of carbon nanotubes and graphene accurately, where CBOPs struggle. However, similar to CBOPs, GAP-20 is unable to accurately account for van der Waals interactions. Despite this, we find that GAP-20 outperforms all CBOPs assessed here and is at present the most suitable potential for studying thermal and mechanical properties for pristine and defective carbon. read less

Abstract: This paper presents an investigation aimed at understanding the flaw size sensitivity in amorphous silica nanostructures. The investigation is carried out in LAMMPS via reactive molecular dynamics analyses by employing ReaxFF–SiO, a bond order-based force field. First, a validated procedure is developed to build the amorphous silica nanostructures via a melt, quench, and equilibration process. This procedure is seen to correctly reproduce the molecular structure as well as mechanical properties of silica. The best agreement to experimental data is obtained by using non-periodic boundary conditions with the isothermal–isobaric ensemble. The validated model is then used to analyze crack propagation in amorphous silica samples with varying flaw sizes subjected to mode I tensile fracture. The analyses reveal a marked transition from flaw sensitive to insensitive behavior with decreasing flaw size. The transition flaw size is found to be 20–25 A. Fracture propagation is found to be accompanied by the formation of several single atom thick strands near the crack tip, previously reported as “stress fibers.” This is proposed as a viable mechanism causing blunting of an initially sharp crack, analogous to blunting of a macroscale crack by an inelastic damage zone. The nanoscale fracture process zone estimated by probing near crack tip stresses is found to nearly equal the transition flaw size, providing an explanation for the transitional behavior. A semi-empirical, transitional flaw size effect law rooted in quasibrittle fracture mechanics is derived based on asymptotic matching and is found to capture well the nanoscale transitional behavior. read less

Abstract: Since the success of monolayer graphene exfoliation, two-dimensional (2D) materials have been extensively studied due to their unique structures and unprecedented properties. Among these fascinating studies, the most predominant focus has been on their atomic structures, defects, and mechanical behaviors and properties, which serve as the basis for the practical applications of 2D materials. In this review, we first highlight the atomic structures of various 2D materials and the structural and energy features of some common defects. We then summarize the recent advances made in experimental, computational, and theoretical studies on the mechanical properties and behaviors of 2D materials. We mainly emphasized the underlying deformation and fracture mechanisms and the influences of various defects on mechanical behaviors and properties, which boost the emergence and development of topological design and defect engineering. We also further introduce the piezoelectric and flexoelectric behaviors of specific 2D materials to address the coupling between mechanical and electronic properties in 2D materials and the interactions between 2D crystals and substrates or between different 2D monolayers in heterostructures. Finally, we provide a perspective and outlook for future studies on the mechanical behaviors and properties of 2D materials. read less

Abstract: Density functional tight binding (DFTB) is an attractive method for accelerated quantum simulations of condensed matter due to its enhanced computational efficiency over standard density functional theory (DFT) approaches. However, DFTB models can be challenging to determine for individual systems of interest, especially for metallic and interfacial systems where different bonding arrangements can lead to significant changes in electronic states. In this regard, we have created a rapid-screening approach for determining systematically improvable DFTB interaction potentials that can yield transferable models for a variety of conditions. Our method leverages a recent reactive molecular dynamics force field where many-body interactions are represented by linear combinations of Chebyshev polynomials. This allows for the efficient creation of multi-center representations with relative ease, requiring only a small investment in initial DFT calculations. We have focused our workflow on TiH2 as a model system and show that a relatively small training set based on unit-cell-sized calculations yields a model accurate for both bulk and surface properties. Our approach is easy to implement and can yield reliable DFTB models over a broad range of thermodynamic conditions, where physical and chemical properties can be difficult to interrogate directly and there is historically a significant reliance on theoretical approaches for interpretation and validation of experimental results. read less

Abstract: Sorting and assigning phonon branches (e.g., longitudinal acoustic) of phonon modes is important for characterizing the phonon bands of a crystal and the determination of phonon properties such as the Grüneisan parameter and group velocity. To do this, the phonon band indices (including the longitudinal and transverse acoustic) have to be assigned correctly to all phonon modes across a path or paths in the Brillouin zone. As our solution to this challenging problem, we propose a computationally efficient and robust two-stage hybrid method that combines two approaches with their own merits. The first is the perturbative approach in which we connect the modes using degenerate perturbation theory. In the second approach, we use numerical fitting based on least-squares fits to circumvent local connectivity errors at or near exact degenerate modes. The method can be easily generalized to other condensed matter problems involving Hermitian matrix operators such as electronic bands in tight-binding Hamiltonians or in a standard density-functional calculation, and photonic bands in photonic crystals. read less

Abstract: Context. Collisions of nanoparticles (NPs) occur in dust clouds and protoplanetary disks. Aims. Sticking collisions lead to the growth of NPs, in contrast to bouncing or even fragmentation events and we aim to explore these processes in amorphous carbon NPs. Methods. Using molecular-dynamics simulations, we studied central collisions between amorphous carbon NPs that had radii in the range of 6.5–20 nm and velocities of 100–3000 m s−1, and with varying sp3 content (20–55%). Results. We find that the collisions are always sticking. The contact radius formed surpasses the estimate provided by the traditional Johnson-Kendall-Roberts model, pointing at the dominant influence of attractive forces between the NPs. Plasticity occurs via shear-transformation zones. In addition, we find bond rearrangements in the collision zone. Low-sp3 material (sp3 ≤ 40%) is compressed to sp3 > 50%. On the other hand, for the highest sp3 fraction, 55%, graphitization starts in the collision zone leading to low-density and even porous material. Conclusions. Collisions of amorphous carbon NPs lead to an increased porosity, atomic surface roughness, and changed hybridization that affect the mechanical and optical properties of the collided NPs. read less

Abstract: We examine the evolution of events occurring when a Li metal surface is in contact with a 2 M solution of a Li salt in a solvent or mixture of solvents, via classical molecular dynamics simulations with a reactive force field allowing bond breaking and bond forming. The main events include Li oxidation and electrolyte reduction along with expansion of the Li surface layers forming a porous phase that is the basis for the formation of the solid-electrolyte interphase (SEI) components. Nucleation of the main SEI components (LiF, Li oxides, and some organics) is characterized. The analysis clearly reveals the details of these physical–chemical events as a function of time, during 20 nanoseconds. The effects of the chemistry of the electrolyte on Li oxidation and dissolution in the liquid electrolyte, and SEI nucleation and structure are identified by testing two salts: LiPF6 and LiCF3SO3, and various solvents including ethers and carbonates and mixtures of them. The kinetics and thermodynamics of Li6F, the core nuclei in the LiF crystal, are studied by analysis of the MD trajectories, and via density functional theory calculations respectively. The SEI formed in this computational experiment is the “native” film that would form upon contact of the Li foil with the liquid electrolyte. As such, this work is the first in a series of computational experiments that will help elucidate the intricate interphase layer formed during battery cycling using metal anodes. read less

Abstract: Hybrid organic–inorganic perovskite (HOIP) materials have attracted significant attention in photovoltaics, light emission, photodetection, etc. Based on the prototype metal halide perovskite crystal, there is a huge space for tuning the composition and crystal structure of this material, which would provide great potential to render multiple physical properties beyond the ongoing emphasis on the optoelectronic property. Recently, the two-dimensional (2D) HOIPs have emerged as a potential candidate for a new class of ferroelectrics with high Curie temperature and spontaneous polarization. Room-temperature solution-processability further makes HOIP a promising alternative to traditional oxide ferroelectrics such as BaTiO3 and PbTiO3. In this perspective, we focus on the molecular aspects of 2D HOIPs, their correlation with macroscopic properties, as well as the material design rules assisted by advanced simulation tools (e.g., machine learning and atomistic modeling techniques). The perspective provides a comprehensive discussion on the structural origin of ferroelectricity, current progress in the design of new materials, and potential opportunities and challenges with emerging materials. We expect that this perspective will provide inspiration for innovation in 2D HOIP ferroelectrics. read less

Abstract: We introduce an interatomic potential for hexagonal boron nitride (hBN) based on the Gaussian approximation potential (GAP) machine learning methodology. The potential is based on a training set of... read less

Abstract: We present an accurate machine learning (ML) model for atomistic simulations of carbon, constructed using the Gaussian approximation potential (GAP) methodology. The potential, named GAP-20, describes the properties of the bulk crystalline and amorphous phases, crystal surfaces, and defect structures with an accuracy approaching that of direct ab initio simulation, but at a significantly reduced cost. We combine structural databases for amorphous carbon and graphene, which we extend substantially by adding suitable configurations, for example, for defects in graphene and other nanostructures. The final potential is fitted to reference data computed using the optB88-vdW density functional theory (DFT) functional. Dispersion interactions, which are crucial to describe multilayer carbonaceous materials, are therefore implicitly included. We additionally account for long-range dispersion interactions using a semianalytical two-body term and show that an improved model can be obtained through an optimization of the many-body smooth overlap of atomic positions descriptor. We rigorously test the potential on lattice parameters, bond lengths, formation energies, and phonon dispersions of numerous carbon allotropes. We compare the formation energies of an extensive set of defect structures, surfaces, and surface reconstructions to DFT reference calculations. The present work demonstrates the ability to combine, in the same ML model, the previously attained flexibility required for amorphous carbon [V. L. Deringer and G. Csányi, Phys. Rev. B 95, 094203 (2017)] with the high numerical accuracy necessary for crystalline graphene [Rowe et al., Phys. Rev. B 97, 054303 (2018)], thereby providing an interatomic potential that will be applicable to a wide range of applications concerning diverse forms of bulk and nanostructured carbon. read less