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

Abstract: Thermoelectric materials convert heat into electricity through thermally driven charge transport in solids or vice versa for cooling. To compete with conventional energy‐conversion technologies, a thermoelectric material must possess the properties of both an electrical conductor and a thermal insulator. However, these properties are normally mutually exclusive because of the interconnection between scattering mechanisms for charge carriers and phonons. Recent theoretical investigations on sub‐device scales have revealed that nanopillars attached to a membrane exhibit a multitude of local phonon resonances, spanning the full spectrum, that couple with the heat‐carrying phonons in the membrane and cause a reduction in the in‐plane thermal conductivity, with no expected change in the electrical properties because the nanopillars are outside the pathway of voltage generation and charge transport. Here this effect is demonstrated experimentally for the first time by investigating device‐scale suspended silicon membranes with GaN nanopillars grown on the surface. The nanopillars cause up to 21% reduction in the thermal conductivity while the power factor remains unaffected, thus demonstrating an unprecedented decoupling in the semiconductor's thermoelectric properties. The measured thermal conductivity behavior for coalesced nanopillars and corresponding lattice‐dynamics calculations provide evidence that the reductions are mechanistically tied to the phonon resonances. This finding paves the way for high‐efficiency solid‐state energy recovery and cooling. read less

Abstract: Hybrid structures comprised of graphene domains embedded in larger hexagonal boron nitride (h-BN) nanosheets were first synthesized in 2013. However, the existing theoretical investigations on them have only considered relaxed structures. In this work, we use Density Functional Theory (DFT) and Molecular Dynamics (MD) simulations to investigate the mechanical and electronic properties of this type of nanosheet under strain. Our results reveal that the Young's modulus of the hybrid sheets depends only on the relative concentration of graphene and h-BN in the structure, showing little dependence on the shape of the domain or the size of the structure for a given concentration. Regarding the tensile strength, we obtained higher values using triangular graphene domains. We find that the studied systems can withstand large strain values (between 15% and 22%) before fracture, which always begins at the weaker C–B bonds located at the interface between the two materials. Concerning the electronic properties, we find that by combining composition and strain, we can produce hybrid sheets with band gaps spanning an extensive range of values (between 1.0 eV and 3.5 eV). Our results also show that the band gap depends more on the composition than on the external strain, particularly for structures with low carbon concentration. The combination of atomic-scale thickness, high ultimate strain, and adjustable band gap suggests applications of h-BN nanosheets with graphene domains in wearable electronics. read less

Abstract: Self-synchronization is a ubiquitous phenomenon in nature, in which oscillators are collectively locked in frequency and phase through mutual interactions. While self-synchronization requires the forced excitation of at least one of the oscillators, we demonstrate that this mechanism spontaneously appears due to activation from thermal fluctuations. By performing molecular dynamics simulations, we demonstrate self-synchronization in a platform supporting doped silicon resonator nanopillars having different eigenfrequencies. We find that pillar’s vibrations are spontaneously converging to the same frequency and phase. In addition, the dependencies on the intrinsic frequency difference and the coupling strength agree well with the Kuramoto model predictions. More interestingly, we find that a balance between energy dissipation resulting from phonon–phonon scattering and potential energy between oscillators is reached to maintain synchronization. The balance could be suppressed by increasing the membrane size. While microscopic stochastic motions are known to follow random probability distributions, we finally prove that they can also yield coherent collective motions via self-synchronization. read less

Abstract: Since it is now possible to record vibrational spectra at nanometer scales in the electron microscope, it is of interest to explore whether extended defects in crystals such as dislocations or grain boundaries will result in measurable changes of the phonon densities of states (dos) that are reflected in the spectra. Phonon densities of states were calculated for a set of high angle grain boundaries in silicon. The boundaries are modeled by supercells with up to 160 atoms, and the vibrational densities of states were calculated by taking the Fourier transform of the velocity–velocity autocorrelation function from molecular dynamics simulations with larger supercells doubled in all three directions. In selected cases, the results were checked on the original supercells by comparison with the densities of states obtained by diagonalizing the dynamical matrix calculated using density functional theory. Near the core of the grain boundary, the height of the optic phonon peak in the dos at 60 meV was suppressed relative to features due to acoustic phonons that are largely unchanged relative to their bulk values. This can be attributed to the variation in the strength of bonds in grain boundary core regions where there is a range of bond lengths. read less

Abstract: Thermal transport of nanocrystalline Si is of great importance for the application of thermoelectrics. A better understanding of the modal thermal conductivity of nanocrystalline Si will be expected to benefit the efficiency of thermoelectrics. In this work, the variance reduced Monte Carlo simulation with full band of phonon dispersion is applied to study the modal thermal conductivity of nanocrystalline Si. Importantly, the phonon modal transmissions across the grain boundaries which are modeled by the amorphous Si interface are calculated by the mode-resolved atomistic Greens function method. The predicted ratios of thermal conductivity of nanocrystalline Si to that of bulk Si agree well with that of the experimental measurements in a wide range of grain size. The thermal conductivity of nanocrystalline Si is decreased from 54 percent to 3 percent and the contribution of phonons with mean free path larger than the grain size increases from 30 percent to 96 percnet as the grain size decreases from 550 nm to 10 nm. This work demonstrates that the full band Monte Carlo simulation using phonon modal transmission by the mode-resolved atomistic Greens function method can capture the phonon transport picture in complex nanostructures, and therefore can provide guidance for designing high performance Si based thermoelectrics. read less

Abstract: Al is one of the most widely applicable metallic materials due to its advanced properties. However, its main drawback is its strength, which is relatively low compared to ferrous alloys. This issue may be resolved using different approaches. In the present work, a heterostructure of Al substrate with a modified surface with carbon nanotubes (CNTs) was studied. This heterostructure was obtained using the laser-oriented deposition technique. The obtained results showed a slight reduction in the reflectivity of the obtained Al substrate with embedded CNTs compared to pure Al. Additionally, the obtained surface heterostructure showed enhancement in microhardness and higher hydrophobicity. Simulation of the CNT embedding process revealed that CNT penetration strongly depends on the diameter. Hence, the penetration increases when the diameter decreases. read less

Abstract: The interfacial properties of ZnO nanowire (NW)/carbon fiber-reinforced epoxy composites are investigated using molecular dynamics (MD) simulations. An atomistic representative volume element (RVE) is developed in which a single ZnO NW is aligned on carbon fiber and embedded in the cross-linked epoxy. Effects of ZnO NWs on the fiber-matrix adhesion are studied by evaluating the fiber and the enhanced matrix interaction. The traction-separation behavior in both sliding mode (shear separation) and opening mode (normal separation) is evaluated. The cohesive parameters, including the peak traction and adhesion energy, are calculated in each mode. Different numbers of cross-linked epoxy units in the system are studied and validated. The interfacial properties of the hybrid system are compared with the simulated bare RVE containing fiber and epoxy. MD results showed that the interfacial strength is increased from 485 MPa to 1066 MPa with the ZnO NWs. The adhesion energy in both opening and sliding modes is significantly improved by growing ZnO NWs on the carbon fibers. In addition, the hybrid system shows more rate-independent behavior compared with the bare system in the opening mode. read less

Abstract: Методом нерiвноважної молекулярної динамiки дослiджено процеси теплового транспорту в Si нанонитках, покритих оболонкою аморфного SiO2. Розглянуто вплив товщини аморфного шару, радiуса кристалiчного кремнiєвого ядра I температури на величину коефiцiєнта теплопровiдностi нанониток. Встановлено, що збiльшення товщини аморфної оболонки зумовлює зменшення теплопровiдностi Si/SiO2 нанониток типу ядро-оболонка. Результати також показують, що теплопровiднiсть Si/SiO2 нанониток при 300 К зростає зi збiльшенням площi поперечного перерiзу кристалiчного Si ядра. Виявлено, що температурна залежнiсть коефiцiєнта теплопровiдностi Si/SiO2 нанониток типу ядро-оболонка є суттєво слабшою, нiж в кристалiчних кремнiєвих нанонитках. Показано, що така вiдмiннiсть є результатом рiзних домiнуючих механiзмiв фононного розсiювання в нанонитках. Отриманi результати демонструють, що нанонитки Si/SiO2 є перспективним матерiалом для термоелектричних застосувань. read less

Abstract: Recently synthesized hexagonal group IV materials are a promising platform to realize efficient light emission that is closely integrated with electronics. A high crystal quality is essential to assess the intrinsic electronic and optical properties of these materials unaffected by structural defects. Here, we identify a previously unknown partial planar defect in materials with a type I3 basal stacking fault and investigate its structural and electronic properties. Electron microscopy and atomistic modeling are used to reconstruct and visualize this stacking fault and its terminating dislocations in the crystal. From band structure calculations coupled to photoluminescence measurements, we conclude that the I3 defect does not create states within the hex-Ge and hex-Si band gap. Therefore, the defect is not detrimental to the optoelectronic properties of the hex-SiGe materials family. Finally, highlighting the properties of this defect can be of great interest to the community of hex-III-Ns, where this defect is also present. read less

Abstract: A non-equilibrium molecular dynamics simulation method is conducted to study the thermal conductivity (TC) of silicon nanowires (SiNWs) with different types of defects. The impacts of defect position, porosity, temperature, and length on the TC of SiNWs are analyzed. The numerical results indicate that SiNWs with surface defects have higher TC than SiNWs with inner defects, the TC of SiNWs gradually decreases with the increase of porosity and temperature, and the impact of temperature on the TC of SiNWs with defects is weaker than the impact on the TC of SiNWs with no defects. The TC of SiNWs increases as their length increases. SiNWs with no defects have the highest corresponding frequency of low-frequency peaks of phonon density of states; however, when SiNWs have inner defects, the lowest frequency is observed. Under the same porosity, the average phonon participation of SiNWs with surface defects is higher than that of SiNWs with inner defects. read less

Abstract: Anomalous heat transport in one-dimensional nanostructures, such as nanotubes and nanowires, is a widely debated problem in condensed matter and statistical physics, with contradicting pieces of evidence from experiments and simulations. Using a comprehensive modeling approach, comprised of lattice dynamics and molecular dynamics simulations, we proved that the infinite length limit of the thermal conductivity of a (10,0) single-wall carbon nanotube is finite but this limit is reached only for macroscopic lengths due to a thermal phonon mean free path of several millimeters. Our calculations showed that the extremely high thermal conductivity of this system at room temperature is dictated by quantum effects. Modal analysis showed that the divergent nature of thermal conductivity, observed in one-dimensional model systems, is suppressed in carbon nanotubes by anharmonic scattering channels provided by the flexural and optical modes with polarization in the plane orthogonal to the transport direction. read less

Abstract: In this paper we expand our previous study on phonon thermal rectification (TR) exhibited in a hybrid graphene-carbon nitride system (G−C3N) to investigate the system’s behavior under a wider range of temperature differences, between the two employed baths, and the effects of media-interface geometry on the rectification factor. Our simulation results reveal a sigmoid relation between TR and temperature difference, with a sample-size depending upper asymptote occurring at generally large temperature differences. The achieved TR values are significant and go up to around 120% for ΔT = 150 K. Furthermore, the consideration of varying media-interface geometries yields a non-negligible effect on TR and highlights areas for further investigation. Finally, calculations of Kapitza resistance at the G-C3N interface are performed for assisting us in the understanding of interface-geometry effects on TR. read less

Abstract: We study the surface elastic response of pure Ni, the random alloy FeNiCr and an average FeNiCr alloy in terms of the surface lattice Green’s function. We propose a scheme for computing per-site Green’s function and study their per-site variations. The average FeNiCr alloys accurately reproduces the mean Green’s function of the full random alloy. Variation around this mean is largest near the edge of the surface Brillouin-zone and decays as q −2 with wavevector q towards the Γ-point. We also present expressions for the continuum surface Green’s function of anisotropic solids of finite and infinite thickness and show that the atomistic Green’s function approaches continuum near the Γ-point. Our results are a first step towards efficient contact calculations and Peierls–Nabarro type models for dislocations in high-entropy alloys. read less

Abstract: Tuning the thermal conductivity of silicon nanowires (Si-NWs) is essential for realization of future thermoelectric devices. The corresponding management of thermal transport is strongly related to the scattering of phonons, which are the primary heat carriers in Si-NWs. Using the molecular dynamics method, we find that the scattering of phonons from internal body defects is stronger than that from surface structures in the low-porosity range. Based on our simulations, we propose the concept of an exponential decay in thermal conductivity with porosity, specifically in the low-porosity range. In contrast, the thermal conductivity of Si-NWs with a higher porosity approaches the amorphous limit, and is insensitive to specific phonon scattering processes. Our findings contribute to a better understanding of the tuning of thermal conductivity in Si-NWs by means of patterned nanostructures, and may provide valuable insights into the optimal design of one-dimensional thermoelectric materials. read less

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

Abstract: While machine learning (ML) has shown increasing effectiveness in optimizing materials properties under known physics, its application in challenging conventional wisdom and discovering new physics still remains challenging due to its interpolative nature. In this work, we demonstrate the potential of using ML for such applications by implementing an adaptive ML-accelerated search process that can discover unexpected lattice thermal conductivity ($\kappa_l$) enhancement instead of reduction in aperiodic superlattices (SLs) as compared to periodic superlattices. We use non-equilibrium molecular dynamics (NEMD) simulations for high-fidelity calculations of $\kappa_l$ for a small fraction of SLs in the search space, along with a convolutional neural network (CNN) which can rapidly predict $\kappa_l$ for a large number of structures. To ensure accurate prediction by the CNN for the target unknown structures, we iteratively identify aperiodic SLs containing structural features which lead to locally enhanced thermal transport, and include them as additional training data for the CNN in each iteration. As a result, our CNN can accurately predict the high $\kappa_l$ of aperiodic SLs that are absent from the initial training dataset, which allows us to identify the previously unseen exceptional structures. The identified RML structures exhibit increased coherent phonon contribution to thermal conductivity owing to the presence of closely spaced interfaces. Our work describes a general purpose machine learning approach for identifying low-probability-of-occurrence exceptional solutions within an extremely large subspace and discovering the underlying physics. read less

Abstract: We propose a way to properly interpret the apparent thermal conductivity obtained for finite systems using equilibrium molecular dynamics simulations (EMD) with fixed or open boundary conditions in the transport direction. In such systems the heat current autocorrelation function develops negative values after a correlation time which is proportional to the length of the simulation cell in the transport direction. Accordingly, the running thermal conductivity develops a maximum value at the same correlation time and eventually decays to zero. By comparing EMD with nonequilibrium molecular dynamics (NEMD) simulations, we conclude that the maximum thermal conductivity from EMD in a system with domain length $2L$ is equal to the thermal conductivity from NEMD in a system with domain length $L$. This facilitates the use of nonperiodic-boundary EMD for thermal transport in finite samples in close correspondence to NEMD. read less

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

Abstract: Glasses usually represent the lower limit for the thermal conductivity of solids, but a fundamental understanding of lattice heat transport in amorphous materials can provide design rules to beat such a limit. Here we investigate the role of mass disorder in glasses by studying amorphous silicon-germanium alloy (a-${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$) over the full range of atomic concentration from $x=0$ to $x=1$, using molecular dynamics and the quasiharmonic Green-Kubo lattice dynamics formalism. We find that the thermal conductivity of a-${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ as a function of $x$ exhibits a smoother U shape than in crystalline mass-disordered alloys. The main contribution to the initial drop of thermal conductivity at low Ge concentration stems from the localization of otherwise extended modes that make up the lowest 8% of the population by frequency. Contributions from intermediate frequency modes are decreased more gradually with increasing Ge to reach a broad minimum thermal conductivity between concentrations of Ge from $x=0.25$ to 0.75. Modal analysis unravels the correlations among localization, line broadening, and the contribution to thermal transport of modes within different frequency ranges. read less

Abstract: Advances in materials informatics (MI), which combines material property calculations/measurements and informatics algorithms, have realized properties in the nanostructures of thermal functional materials beyond what is accessible using empirical approaches based on physical instincts and models. In this Tutorial, we introduce technological procedures and underlying knowledge of MI combining thermal transport calculations and machine learning using an optimization problem of superlattice structures as an example (sample script available in the supplement). To provide fundamental guidance on how to use MI, we describe practical details about descriptors, objective functions, property calculators, machine learning (Bayesian optimization) algorithms, and optimization efficiencies. We then briefly review the recent successful applications of MI to design thermoelectric and thermal radiation materials. Finally, we summarize and provide future perspectives about the topic. read less

Abstract: Understanding heat transport in semiconductors and insulators is of fundamental importance because of its technological impact in electronics and renewable energy harvesting and conversion. Anharmonic Lattice Dynamics provides a powerful framework for the description of heat transport at the nanoscale. One of the advantages of this method is that it naturally includes quantum effects due to atoms vibrations, which are needed to compute the thermal properties of semiconductors widely used in nanotechnology, like silicon and carbon, even at room temperature. While the heat transport picture substantially differs between amorphous and crystalline semiconductors from a microscopic standpoint, a unified approach to simulate both crystals and glasses has been devised. Here we introduce a unified workflow, which implements both the Boltzmann Transport equation (BTE) and the Quasi Harmonic Green-Kubo (QHGK) methods. We discuss how the theory can be optimized to exploit modern parallel architectures, and how it is implemented in $\kappa ALDo$: a versatile and scalable open-source software to compute phonon transport in solids. This approach is applied to crystalline and partially disordered silicon-based systems, including bulk silicon and clathrates, and on silicon-germanium alloy clathrates with largely reduced thermal conductivity. read less

Abstract: In this study, we performed nanoindentation test using the molecular dynamic (MD) approach on a selected thin film of palladium, vanadium, copper and niobium coated on the vanadium substrate at a loading rate of 0.5 Å/ps. The thermosetting control is applied with temperature variance from 300 to 700 K to study the mechanical characteristics of the selected thin films. The effects of temperature on the structure of the material, piling-up phenomena and sinking-in occurrence were considered. The simulation results of the analysis and the experimental results published in this literature were well correlated. The analysis of temperature demonstrated an understanding of the impact of the behaviour. As the temperature decreases, the indentation load increases for loading and unloading processes. Hence, this increases the strength of the material. In addition, the results demonstrate that the modulus of elasticity and thin-film hardness decreases in the order of niobium, vanadium, copper and palladium as the temperature increases. read less

Abstract: Silicon doping is an effective way to modulate the bandgap of graphene that might open the door for graphene to the semiconductor industries. However, the mechanical properties of silicon doped graphene (SiG) also plays an important role to realize its full potential application in the electronics industry. Electronic and optical properties of silicon doped graphene are well studied, but, our understanding of mechanical and fracture properties of the doped structure is still in its infancy. In this study, molecular dynamics (MD) simulations are conducted to investigate the tensile properties of SiG by varying the concentration of silicon. It is found that as the concentration of silicon increases, both fracture stress and strain of graphene reduces substantially. Our MD results also suggest that only 5% of silicon doping can reduce the Young's modulus of graphene by ∼15.5% along the armchair direction and ∼13.5% along the zigzag direction. Tensile properties of silicon doped graphene have been compared with boron and nitrogen doped graphene. The effect of temperature, defects and crack length on the stress–strain behavior of SiG has also been investigated. Temperature studies reveal that SiG is less sensitive to temperature compared to free stranding graphene, additionally, increasing temperature causes deterioration of both fracture stress and strain of SiG. Both defects and cracks reduce the fracture stress and fracture strain of SiG remarkably, but the sensitivity to defects and cracks for SiG is larger compared to graphene. Fracture toughness of pre-cracked SiG has been investigated and results from MD simulations are compared with Griffith's theory. It has been found that for nano-cracks, SiG with larger crack length deviates more from Griffith's criterion and the degree of deviation is larger compared to graphene. Fracture phenomenon of pre-cracked SiG and the effect of strain rate on the tensile properties of SiG have been reported as well. These results will aid the design of SiG based semiconducting nanodevices. read less

Abstract: Si/Ge nanostructures have attracted much attention since they are compatible with current microelectronics technology. The geometry and composition variations can be used to tune their electronic properties. Here, we introduce a 3D Si/Ge superlattice, metalattice, made of more volumetric meta-atoms and thinner metabonds between them. Its size varies from a few tens to hundreds of nanometers and can be taken as a mesoscale physics platform. We intend to bring a metallic character to such an alloy metalattice. This requires that the quantum confinement and chemical composition act in a complementary way. The tight-binding method is employed and it is confirmed that a 3D uniform density of states across the whole metalattice is possible. Search for the preferred electronic structure now transforms to the problem of finding the appropriate geometry. read less

Abstract: A molecular dynamics model of the diamond abrasive polishing the single crystal silicon is established. Crystal surfaces of the single crystal silicon in the Y-direction are (010), (011), and (111) surfaces, respectively. The effects of crystallographic orientations on polishing the non-continuous single crystal silicon surfaces are discussed from the aspects of surface morphology, displacement, polishing force, and phase transformation. The simulation results show that the Si(010) surface accumulates chips more easily than Si(011) and Si(111) surfaces. Si(010) and Si(011) workpieces are deformed in the entire pore walls on the entry areas of pores, while the Si(111) workpiece is a local large deformation on entry areas of the pores. Comparing the recovery value of the displacement in different workpieces, it can be seen that the elastic deformation of the A side in the Si(011) workpiece is larger than that of the A side in other workpieces. Pores cause the tangential force and normal force to fluctuate. The fluctuation range of the tangential force is small, and the fluctuation range of the normal force is large. Crystallographic orientations mainly affect the position where the tangential force reaches the maximum and minimum values and the magnitude of the decrease in the tangential force near the pores. The position of the normal force reaching the maximum and minimum values near the pores is basically the same, and different crystallographic orientations have no obvious effect on the drop of the normal force, except for a slight fluctuation in the value. The high-pressure phase transformation is the main way to change the crystal structure. The Si(111) surface is the cleavage surface of single crystal silicon, and the total number of main phase transformation atoms on the Si(111) surface is the largest among the three types of workpieces. In addition, the phase transformation in Si(010) and Si(011) workpieces extends to the bottom of pores, and the Si(111) workpiece does not extend to the bottom of pores. read less

Abstract: Molecular dynamics simulations (MDs) are carried out for predicting platinum Proton Exchange Membrane (PEM) fuel cell nanocatalyst growth on a model carbon electrode. The aim is to provide a one-shot simulation of the entire multistep process of deposition in the context of plasma sputtering, from sputtering of the target catalyst/transport to the electrode substrate/deposition on the porous electrode. The plasma processing reactor is reduced to nanoscale dimensions for tractable MDs using scale reduction of the plasma phase and requesting identical collision numbers in experiments and the simulation box. The present simulations reproduce the role of plasma pressure for the plasma phase growth of nanocatalysts (here, platinum). read less

Abstract: Wave-like heat propagation is shown to emerge as a consequence of a rapidly varying temperature field in Ge. Second sound is known as the thermal transport regime where heat is carried by temperature waves. Its experimental observation was previously restricted to a small number of materials, usually in rather narrow temperature windows. We show that it is possible to overcome these limitations by driving the system with a rapidly varying temperature field. High-frequency second sound is demonstrated in bulk natural Ge between 7 K and room temperature by studying the phase lag of the thermal response under a harmonic high-frequency external thermal excitation and addressing the relaxation time and the propagation velocity of the heat waves. These results provide a route to investigate the potential of wave-like heat transport in almost any material, opening opportunities to control heat through its oscillatory nature. read less

Abstract: Electrical and thermal properties of atomically thin two-dimensional (2D) materials are affected by their environment, e.g. through remote phonon scattering or dielectric screening. However, while it is known that mobility and thermal conductivity (TC) of graphene are reduced on a substrate, these effects are much less explored in 2D semiconductors such as MoS2. Here, we use molecular dynamics to understand TC changes in monolayer (1L) and bilayer (2L) MoS2 by comparing suspended, supported, and encased structures. The TC of monolayer MoS2 is reduced from ∼117 W m−1 K−1 when suspended, to ∼31 W m−1 K−1 when supported by SiO2, at 300 K. Encasing 1L MoS2 in SiO2 further reduces its TC down to ∼22 W m−1 K−1. In contrast, the TC of 2L MoS2 is not as drastically reduced, being >50% higher than 1L both when supported and encased. These effects are due to phonon scattering with remote vibrational modes of the substrate, which are partly screened in 2L MoS2. We also examine the TC of 1L MoS2 across a wide range of temperatures (300 K to 700 K) and defect densities (up to 5 × 1013 cm−2), finding that the substrate reduces the dependence of TC on these factors. Taken together, these are important findings for all applications which will use 2D semiconductors supported or encased by insulators, instead of freely suspended. read less

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

Abstract: The development of new fabrication technologies for Ge–Si thermoelectric materials requires a corresponding theoretical description of physical processes lying behind the synthesis. In the present paper, we investigated the interdiffusion of Si and Ge atoms at the Ge/Si interface, which takes place during spark plasma sintering of Ge and Si powders for fabrication of thermoelectric bulk. The calculation was performed using numerical simulation based on the classical molecular dynamics method. The diffusion coefficients of Si in Ge and vice versa were found at sintering temperatures of 900 K–1300 K and an external pressure of 7 MPa. The calculation results were used to analyze the experimental data derived from the measurements of Ge and Si profiles at the interface of thin Ge/Si plates subjected to spark plasma sintering at the temperature of 1160 K (887 °C). The comparison of measured and calculated diffusion profiles has shown good agreement with one another. read less

Abstract: The temperature-dependent transversely isotropic elastic properties of multi-walled boron nitride nanotubes (MWBNNTs) were determined using molecular dynamics simulations with a three-body Tersoff potential force field. These elastic properties were calculated by applying the four different loading conditions on MWBNNTs: uniaxial tension, torsional moment, in-plane biaxial tension and in-plane shear. The effect of chirality, number of layers and aspect ratio (AR) were taken into consideration. The results reveal that the elastic constants of MWBNNTs decrease as their number of layers increase. The elastic moduli of MWBNNTs do not depend on the AR, but are a function of chirality. Furthermore, the effect of temperature on the transversely isotropic elastic constants of MWBNNTs was studied. Higher temperature considerably affects the mechanical properties of MWBNNTs. For instance, the reduction in the values of axial Young’s, longitudinal shear, plane-strain bulk and in-plane shear moduli of MWBNNTs was found to be by approximately 10% due to the increase in temperature. The results reveal that the mechanical properties and failure behavior of MWBNNTs significantly depend on the number of layers, chirality and temperature. The finding of this work can be utilized for engineering MWBNNT-based advanced nanocomposite structures for specific application under thermal environment. read less

Abstract: A recent study has highlighted an existing controversy among experimental measurements and theoretical models on the size-dependent elastic behavior of silicon nanowires. Some measurements have depicted a significant size-dependent elastic response, while several studies report a negligible change on the elastic modulus of silicon nanowires through size reduction. To address such contrast, this work studies the surface stress contribution on the size-dependent elastic behavior of silicon nanowires. Molecular dynamics simulations are employed to investigate the influence of size, crystal orientation, boundary condition, and the residual surface stress on the incorporation of the surface stress in the mechanical properties of silicon nanowires. This is accomplished by a primary atomic stress analysis. The implication of the surface stress on the bending behavior is then calculated for silicon nanowires along ⟨ 100 ⟩ and ⟨ 110 ⟩ crystal orientations having { 100 } and { 100 } / { 110 } transverse surfaces, respectively. This study demonstrates, for the first time, the role played by the surface stress to reduce the elastic modulus of ⟨ 110 ⟩ silicon nanowires, which is comparable with experimental measurements on wires with the same size and crystal orientation. The present work enlightens the incorporation of the surface stress on the mechanical behavior of silicon nanowires for the explanation of existing studies and implementation for future investigations. read less

Abstract: Nano-size confinement induces many intriguing non-Fourier heat conduction phenomena, such as nonlinear temperature gradients, temperature jumps near the contacts, and size-dependent thermal conductivity. Over the past decades, these effects have been studied and interpreted by nonequilibrium molecular dynamics (NEMD) and phonon Boltzmann transport equation (BTE) simulations separately, but no theory that unifies these two methods has ever been established. In this work, we unify these methods using a quantitative mode-level comparison and demonstrate that they are equivalent for various thermostats. We show that different thermostats result in different non-Fourier thermal transport characteristics due to the different mode-level phonon excitations inside the thermostats, which explains the different size-dependent thermal conductivities calculated using different reservoirs, even though they give the same bulk thermal conductivity. Specifically, the Langevin thermostat behaves like a thermalizing boundary in phonon BTE and provides mode-level thermal-equilibrium phonon outlets, while the Nose-Hoover chain thermostat and velocity rescaling method behave like biased reservoirs, which provide a spatially uniform heat generation and mode-level nonequilibrium phonon outlets. These findings explain why different experimental measurement methods can yield different size-dependent thermal conductivity. They also indicate that the thermal conductivity of materials can be tuned for various applications by specifically designing thermostats. The unification of NEMD and phonon BTE will largely facilitate the study of thermal transport in complex systems in the future by, e.g., replacing computationally unaffordable first-principles NEMD simulations with computationally less expensive spectral BTE simulations. read less

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

Abstract: Thermal boundary conductance (TBC) is important for heat dissipation in light-emitting diodes (LEDs). In this study, we predicted the TBC between the high thermal conductivity boron arsenide (BAs) and silicon (Si) by nonequilibrium molecular dynamics (MD) simulations. From the thermal conductivity accumulation function with respect to phonon frequency, the dominant phonon frequencies for heat conduction in BAs are extremely different from those in Si. However, our nonequilibrium MD simulations indicated that the TBC of the BAs/Si interface was still high compared to most other interfaces, even though there was a major frequency mismatch in the thermal conductivity accumulation function between BAs and Si. The primary reason for the high TBC is the overlap of phonon density of states between BAs and Si in the frequency range of 5–8 THz. The range of predicted TBC of the BAs/Si interface was between 200 and 300 MW/m2 K in the temperature range of 300–700 K, and the values of the TBC were not sensitive to the temperature. We also found that the TBCs in Si/BAs and Si/Ge interfaces were close to each other considering the simulation uncertainty. This work indicates BAs as an excellent material for heat dissipation across the interfaces. read less

Abstract: The single-walled carbon nanotube (CNT) is a quasi-1D material with ultra high thermal conductivity. It is known that the heat conduction is closely related with the energy diffusion and the information of heat conduction can be obtained by examining the energy diffusion behavior. The correlated energy distribution CE(i,t) calculated in diffusion process gives the whole spatial distribution of energy spreading at every correlation time which is much better than the traditional heat conduction method where only a number of thermal conductivity is provided. In previous study the anomalous super-diffusion of energy has been reported for CNT via a non-equilibrium diffusion method. Here we apply the more sophisticate equilibrium diffusion method to investigate the energy diffusion behavior in CNT with much longer length up to m with much higher accuracy. At room temperature, the super-diffusion of energy described by the mean square displacement of energy is found to be not far away from ballistic diffusion which is consistent with the experimentally measured ultra high thermal conductivity for CNT. With the equilibrium diffusion method, the diffusion of momentum has also been performed for CNT and ballistic diffusion is found which can be viewed as a support for anomalous super energy diffusion. The momentum diffusion can also give accurate information of sound velocity which is determined as m s−1 at room temperature. In particular, the very weak temperature coefficient of sound velocity can be obtained as m (. The normalized temperature coefficient of sound velocity can be related with the normalized temperature coefficient of Raman frequency shifts experimentally measured as . The obtained value via the momentum diffusion method is consistent with the experimental measurements. read less

Abstract: Graphene exhibits excellent mechanical properties under atomically thin thickness, which made it very suitable for nanoelectromechanical systems that had high requirements for the thickness of coatings. The epitaxial bilayer graphene on the 4H-SiC (0001) surface presents high stiffness and hardness comparable to diamond. However, due to structural transition occurring at the nanoscale, it is difficult to elucidate reinforcement mechanisms using experimental methods. Here, we applied molecular dynamics simulations to study nanoindentation of epitaxial carbon-film-covered 4H-SiC (0001) surfaces. Because a weak interaction potential existed between graphene layers at indentation depth (h < 0.8 Å) that far smaller than interlayer distance, the epitaxial bilayer graphene does not allow the 4H-SiC to exceed its intrinsic stiffness. When the indentation depth h ≥6.45 Å, the sp3 hybridized bonds formed on the interlayer of graphene, which leads to fewer amorphous atoms in the sample of 4H-SiC and exhibits stronger stiffness, in comparison with bare 4H-SiC. This strongly suggests the existence of sp3 bonds contributing to the surface strengthening. Meanwhile, we found that the comprehensive mechanical properties of nanocomposites with hydrogenated diamond-like films were superior to those of nanocomposites with other carbon films at high temperatures. read less

Abstract: Three dimensional (3D) graphene-CNT hybrid structures (GCNTs) are promising materials for applications including capacitors and gas storage and separation devices, however until now their thermal conductance mechanism has scarcely been studied. These hybrid nanomaterials are particularly suitable as next-generation thermal interface materials due to the excellent thermal properties of carbon nanotubes and single atomic layer graphene. In this paper, the out-of-plane thermal conductivities of GCNTs, graphene nanomesh (GNM), and graphene sheets are investigated using molecular dynamics (MD) simulations which apply the Green-Kubo method. Distinct from GNMs and graphene sheets, the GCNTs exhibit a relatively high out-of-plane thermal conductivity, stemming from the CNTs' ability to accelerate the energy flow. However, the GCNT out-of-plane thermal conductivity is still far lower than that of pristine graphene due to extreme phonon localizations, which are concentrated on the graphene-CNT junction regions as evidenced by the participation ratio, phonon vibrational density of states, and overlap energy. This study provides microscopic insight into the GCNT heat transfer mechanism and offers design guidelines for application of GCNTs in thermal management devices. read less

Abstract: A nanoindenter was used to compress individual particles of hydrogenated amorphous silicon (a-Si:H) ranging in diameter from 290 nm to 780 nm. The colloidal synthesis used to produce the particles enables the hydrogen content to be manipulated over a wide range, from about 5 at. % to 50 at. %, making these a-Si:H particles promising for applications in lithium ion batteries, hydrogen storage, and optical metamaterials. Force-displacement curves generated using a tungsten probe flattened with focused ion beam exhibited elastic and then plastic deformations, followed by fracture and crushing of the particles. For particles with 5% and 50% H, Young's moduli, yield strengths, and compressive strengths were 73.5(±19.5) GPa, 5.8 GPa, and 3.2(±0.1)–9.3(±0.6) GPa and 31.2(±9.0) GPa, 2.5 GPa, and 1.8 (±0.3)–5.3(±0.8) GPa, respectively. Particles with more hydrogen were significantly more compliant and weaker. This is consistent with atomistically detailed molecular dynamics simulations, which revealed compression forms of an interphase of H atom clusters that weakens the material.A nanoindenter was used to compress individual particles of hydrogenated amorphous silicon (a-Si:H) ranging in diameter from 290 nm to 780 nm. The colloidal synthesis used to produce the particles enables the hydrogen content to be manipulated over a wide range, from about 5 at. % to 50 at. %, making these a-Si:H particles promising for applications in lithium ion batteries, hydrogen storage, and optical metamaterials. Force-displacement curves generated using a tungsten probe flattened with focused ion beam exhibited elastic and then plastic deformations, followed by fracture and crushing of the particles. For particles with 5% and 50% H, Young's moduli, yield strengths, and compressive strengths were 73.5(±19.5) GPa, 5.8 GPa, and 3.2(±0.1)–9.3(±0.6) GPa and 31.2(±9.0) GPa, 2.5 GPa, and 1.8 (±0.3)–5.3(±0.8) GPa, respectively. Particles with more hydrogen were significantly more compliant and weaker. This is consistent with atomistically detailed molecular dynamics simulations, which revealed compression ... read less

Abstract: Anisotropic phonon transport along different lattice directions of two-dimensional (2D) materials has been observed, however, the effect decreases with increasing the thickness beyond a few atomic layers. Here we establish a novel mechanism to induce anisotropic phonon transport in quasi-2D materials with isotropic symmetry. The phonon propagation is guided by resonance hybridization with surface nanostructures. We demonstrate that the thermal conductivity of 3 nm-thick silicon membrane with surface nanofins is greater by $\sim50\%$ parallel to the fins than that perpendicular to the fins. read less

Abstract: Nowadays 2D materials like Graphene, Silicene, Stanene and single layer transition metal dichalcogenides (e.g., MoS2, WSe2,MoTe2,), are drawing significant attention in the research arena due to their superior electrical, thermal and opto-electronic properties to their bulk counterparts. In this study, we have investigated the thermal transport properties of single layer zigzag gallium nitride (GaN) nanoribbon using equilibrium molecular dynamics simulations. The calculated room temperature thermal conductivity of $20\ \mathbf{nm} \times 2\ \mathbf{nm}$ single layer GaN nanoribbon using tersoff inter-atomic potential is 2.04 W/m-K. The temperature and sample size dependence of thermal conductivity have also been studied. For a particular sample size, the thermal conductivity of GaN nanoribbon decreases with increasing temperatures. On the other hand, an opposite pattern is observed for length variation i.e. thermal conductivity increases with the increase in ribbon length keeping the temperature constant. Our study further includes the investigation of the thermal transport of defected GaN nanoribbon. The thermal conductivity of defected GaN sample has been estimated by incorporating defects of different concentration [1% to 5%] for different operating temperatures [100K to 500K]. Our study shows that the thermal conductivity reduces drastically with the increase of defect concentration. We have also calculated phonon density of states (PDOS) for pristine and defected GaN nanoribbon to provide better understanding of these phenomena. Our study would be helpful for further investigation of thermal transport in single layer GaN based devices. read less

Abstract: Screw dislocations are known to impede the thermal transport of homogeneous nanowires by reducing the phonon relaxation time without affecting the phonon group velocity. By using molecular dynamics simulations in this study, we show that the impact of screw dislocation on the thermal conductivity of the SiGe superlattice nanowires depends on the period length. The analysis of phonon transmission spectra and phonon mean free paths indicate that strong phonon-screw dislocation scatterings occur for phonons in the frequency range of 3-8 THz. The screw dislocations change the phonon scattering mechanisms, which is the main cause of the thermal conductivity reduction. Contrary to the case of homogeneous nanowires, a sizable decrease in the phonon group velocity is found in superlattices with screw dislocations. This phenomenon is attributed to the larger number of Si-Ge bonds in the vicinity of the interface due to the slipping of the atomic planes. In contrast to the decreased thermal conductivity, the phonon propagation in the interface region of the nanowires is enhanced by screw dislocations. Our findings provide critical insights into the understanding of dislocation-heat transfer relationship in materials, especially in heterostructures where interfaces are vital for thermal transport. read less

Abstract: Recently, two-dimensional silicon carbide (2D-SiC) has attracted considerable interest due to its exotic electronic and optical properties. Here, we explore the thermal properties of 2D-SiC using reverse non-equilibrium molecular dynamics simulation. At room temperature, a thermal conductivity of ∼313 W mK−1 is obtained for 2D-SiC which is one order higher than that of silicene. Above room temperature, the thermal conductivity deviates the normal 1/T law and shows an anomalous slowly decreasing behavior. To elucidate the variation of thermal conductivity, the phonon modes at different length and temperature are quantified using Fourier transform of the velocity auto-correlation of atoms. The calculated phonon density of states at high temperature shows a shrinking and softening of the peaks, which induces the anomaly in the thermal conductivity. On the other hand, quantum corrections are applied to avoid the freezing effects of phonon modes on the thermal conductivity at low temperature. In addition, the effect of potential on the thermal conductivity calculation is also studied by employing original and optimized Tersoff potentials. These findings provide a means for better understating as well as designing the efficient thermal management of 2D-SiC based electronics and optoelectronics in near future. read less

Abstract: In the molecular dynamics study of short-pulsed laser processing of semiconductors, potential models capable of describing the atomistic behavior during high electronic excitations is the most critical issue at the current stage. This study of the molecular dynamics adopts the Tersoff-potential model to analyze the ultrafast laser processing of silicon. The model was modified to include electronic excitation effects by reducing the attraction of the antibonding state by half. It offers an excellent description of the experimental behavior during nonthermal melting. Subpicosecond melting is achieved above certain threshold levels of superheating and carrier density as required in experiments. Energy conservation is demonstrated with a bandgap energy of the order obtained in experiments. The modification of the potential mimics an absorption of bandgap energy and a subsequent lattice heating on a time scale within 0.3 ps. The melting kinetics establishes a correlation between nonthermal melting and thermal bulk melting. For superheating of less than two, the electronic melting of bond softening proceeds via homogeneous nucleation. The associated thermal theory, corrected with a limit on the nucleus radius to bond length, is still valid for the higher superheating regime. The original Tersoff model shows that this superheating by a factor of two is isothermal for spallation—the lowest-energy ablative mechanism. Its proximity to the evaporating point suggests the role of thermal boiling during spallation.In the molecular dynamics study of short-pulsed laser processing of semiconductors, potential models capable of describing the atomistic behavior during high electronic excitations is the most critical issue at the current stage. This study of the molecular dynamics adopts the Tersoff-potential model to analyze the ultrafast laser processing of silicon. The model was modified to include electronic excitation effects by reducing the attraction of the antibonding state by half. It offers an excellent description of the experimental behavior during nonthermal melting. Subpicosecond melting is achieved above certain threshold levels of superheating and carrier density as required in experiments. Energy conservation is demonstrated with a bandgap energy of the order obtained in experiments. The modification of the potential mimics an absorption of bandgap energy and a subsequent lattice heating on a time scale within 0.3 ps. The melting kinetics establishes a correlation between nonthermal melting and thermal ... read less

Abstract: Boron nitride honeycomb structure is a new three-dimensional material similar to carbon honeycomb, which has attracted a great deal of attention due to its special structure and properties. In this paper, the tensile mechanical properties of boron nitride honeycomb structures in the zigzag, armchair and axial directions are studied at room temperature by using molecular dynamics simulations. Effects of temperature and strain rate on mechanical properties are also discussed. According to the observed tensile mechanical properties, the piezoelectric effect in the zigzag direction was analyzed for boron nitride honeycomb structures. The obtained results showed that the failure strains of boron nitride honeycomb structures under tensile loading were up to 0.83, 0.78 and 0.55 in the armchair, zigzag and axial directions, respectively, at room temperature. These findings indicated that boron nitride honeycomb structures have excellent ductility at room temperature. Moreover, temperature had a significant effect on the mechanical and tensile mechanical properties of boron nitride honeycomb structures, which can be improved by lowering the temperature within a certain range. In addition, strain rate affected the maximum tensile strength and failure strain of boron nitride honeycomb structures. Furthermore, due to the unique polarization of boron nitride honeycomb structures, they possessed an excellent piezoelectric effect. The piezoelectric coefficient e obtained from molecular dynamics was 0.702 C/m2, which was lower than that of the monolayer boron nitride honeycomb structures, e=0.79 C/m2. Such excellent piezoelectric properties and failure strain detected in boron nitride honeycomb structures suggest a broad prospect for the application of these new materials in novel nanodevices with ultrahigh tensile mechanical properties and ultralight-weight materials. read less

Abstract: Hydrogen incorporation in the fabrication of amorphous Si (a-Si) plays an important role in improving its electronic and optical properties. An important question is how H interacts with the a-Si atomic network, and consequently affects the electronic properties of a-Si. The common assumption is that the role of H is to passivate the dangling bonds (DBs) of the a-Si structure, which subsequently leads to a reduction in the density of midgap sates and localized states within the mobility gap. In the present work, we first employ a combined molecular dynamic (MD) and density functional theory (DFT) method to create stable configurations of a-Si:H, and then analyze the atomic and electronic structure to investigate which structural defects interact with H, and how the electronic structure changes with H addition. We show that in contrast with the simple dangling bond picture, atoms bonded by highly strained bonds (SBs) are significantly affected by the addition of H, in terms of the lowest energy configuration, with similar if not greater importance to that of dangling bonds in passivating a-Si. We find that H atoms decrease the density of mid-gap states of a-Si by bonding to the Si atoms with SBs. Our results also indicate that Si atoms with SBs creates highly localized orbitals in the mobility gap of a-Si and a-Si:H, and the bonding of H atoms to them can significantly decrease the degree of orbital localization. The results demonstrate the beneficial effects of hydrogenation of a-Si in terms of reducing the overall strain energy of the a-Si network, with commensurate reduction of mid-gap states and orbital localization. read less

Abstract: Nonequilibrium molecular dynamics (NEMD) has been extensively used to study thermal transport at various length scales in many materials. In this method, two local thermostats at different temperatures are used to generate a nonequilibrium steady state with a constant heat flux. Conventionally, the thermal conductivity of a finite system is calculated as the ratio between the heat flux and the temperature gradient extracted from the linear part of the temperature profile away from the local thermostats. Here, we show that, with a proper choice of the thermostat, the nonlinear part of the temperature profile should actually not be excluded in thermal transport calculations. We compare NEMD results against those from the atomistic Green's function method in the ballistic regime and those from the homogeneous nonequilibrium molecular dynamics method in the ballistic-to-diffusive regime. These comparisons suggest that in all the transport regimes, one should directly calculate the thermal conductance from the temperature difference between the heat source and sink and, if needed, convert it into the thermal conductivity by multiplying it with the system length. Furthermore, we find that the Langevin thermostat outperforms the Nosé-Hoover (chain) thermostat in NEMD simulations because of its stochastic and local nature. We show that this is particularly important for studying asymmetric carbon-based nanostructures, for which the Nosé-Hoover thermostat can produce artifacts leading to unphysical thermal rectification. read less

Abstract: Molecular dynamics (MD) simulation was employed to examine the deformation and phase transformation of mono-crystalline Si nanowire (SiNW) subjected to tensile stress. The techniques of coordination number (CN) and centro-symmetry parameter (CSP) were used to monitor and elucidate the detailed mechanisms of the phase transformation throughout the loading process in which the evolution of structural phase change and the dislocation pattern were identified. Therefore, the relationship between phase transformation and dislocation pattern was established and illustrated. In addition, the electrical resistance and conductivity of SiNW were evaluated by using the concept of virtual electric source during loading and unloading similar to in situ electrical measurements. The effects of temperature on phase transformation of mono-crystalline SiNWs for three different crystallographically oriented surfaces were investigated and discussed. Simulation results show that, with the increase of applied stress, the dislocations are initiated first and then the phase transformation such that the total energy of the system tends to approach a minimum level. Moreover, the electrical resistance of (001)- rather than (011)- and (111)-oriented SiNWs was changed before failure. As the stress level of the (001) SiNW reaches 24 GPa, a significant amount of metallic Si-II and amorphous phases is produced from the semiconducting Si-I phase and leads to a pronounced decrease of electrical resistance. It was also found that as the temperature of the system is higher than 500 K, the electrical resistance of (001) SiNW is significantly reduced through the process of axial elongation. read less

Abstract: We studied the modal contributions to heat conduction across an interface between crystalline Si and amorphous SiO2, using the interface conductance modal analysis (ICMA) method. Our results show that >70% of the thermal interface conductance (TIC) arises from the extended modes. Using ICMA, we could also determine the contribution of interfacial modes to the TIC. Interestingly, we observed that although the number of these modes is 15%). Such an observation shows the non-negligible role of localized modes in facilitating heat conduction across systems with interfaces between dissimilar materials, specifically in a system that is straightforward to fabricate and study experimentally. Our observations suggest that neglecting the contribution of localized modes would be an oversimplification of the actual mechanisms at play. Determining the individual mode contributions is therefore of vital importance, since these values are directly utilized in predicting the temperature dependent TIC, which is important to silicon on insulator technologies with a myriad of applications within microelectronics and optoelectronics.We studied the modal contributions to heat conduction across an interface between crystalline Si and amorphous SiO2, using the interface conductance modal analysis (ICMA) method. Our results show that >70% of the thermal interface conductance (TIC) arises from the extended modes. Using ICMA, we could also determine the contribution of interfacial modes to the TIC. Interestingly, we observed that although the number of these modes is 15%). Such an observation shows the non-negligible role of localized modes in facilitating heat conduction across systems with interfaces between dissimilar materials, specifically in a system that is straightforward to fabricate and study experimentally. Our observations suggest that neglecting the contribution of localized modes would be an oversimplification of the actual mechanisms at play. Determining the individual mode contributions is therefore of vital importance, since these values are directly... read less

Abstract: A primary challenge to use silicon nanowires as a truly potential building block in nanoscale devices is the implementation of scale effects into operational performance. Therefore, surface stress effects—as a direct result of size reduction—on transport properties became a major field of study. Previous computational simulations have focused so far on geometrical parameters with symmetrical cross sections, while silicon nanowires with nonsymmetrical cross sections are the major result of top-down fabrication techniques. A recent study has drawn a new aspect on the role played by the surface stress with a torsional profile on silicon nanowires to address the existing controversy from experimental and computational studies. Motivated by its success, the implications of this surface stress profile on the tensile properties of silicon nanowires are studied through molecular dynamics simulations. Deformation associated with the surface stress is computed for different length-to-thickness and width-to-thickness ratios. Then, tensile properties are investigated for a constant strain rate. Atomic calculations are carried out on silicon nanowires along the ⟨ 100 ⟩ crystal orientation for fixed-fixed and fixed-free boundary conditions. A combination of compressive uniaxial surface stress and torsional surface stress contributes to the mechanical behavior of silicon nanowires. A transition on elastic properties is obtained through changing the cross section from square to rectangular configuration. Further to addressing the controversy regarding the contribution of the surface stress on the mechanical properties, limits associated with available analytical approaches are highlighted for silicon nanowires.A primary challenge to use silicon nanowires as a truly potential building block in nanoscale devices is the implementation of scale effects into operational performance. Therefore, surface stress effects—as a direct result of size reduction—on transport properties became a major field of study. Previous computational simulations have focused so far on geometrical parameters with symmetrical cross sections, while silicon nanowires with nonsymmetrical cross sections are the major result of top-down fabrication techniques. A recent study has drawn a new aspect on the role played by the surface stress with a torsional profile on silicon nanowires to address the existing controversy from experimental and computational studies. Motivated by its success, the implications of this surface stress profile on the tensile properties of silicon nanowires are studied through molecular dynamics simulations. Deformation associated with the surface stress is computed for different length-to-thickness and width-to-thicknes... read less

Abstract: In situ X-ray diffraction with advanced X-ray sources offers unique opportunities for investigating materials properties under extreme conditions such as shock-wave loading. Here, Singh's theory for deducing high-pressure density and strength from two-dimensional (2D) diffraction patterns is rigorously examined with large-scale molecular dynamics simulations of isothermal compression and shock-wave compression. Two representative solids are explored: nanocrystalline Ta and diamond. Analysis of simulated 2D X-ray diffraction patterns is compared against direct molecular dynamics simulation results. Singh's method is highly accurate for density measurement (within 1%) and reasonable for strength measurement (within 10%), and can be used for such measurements on nanocrystalline and polycrystalline solids under extreme conditions (e.g. in the megabar regime). read less

Abstract: Curvature induced by mechanical deformation in nanostructures has been found to significantly affect their stability and reliability during applications. In this work, we investigated the effects of curvature induced by mechanical bending on the thermal properties of silicon nanowire (SiNW) by using molecular dynamics simulations. By examining the relationship between the curved geometry and local temperature/heat flux distribution, we found that there is no temperature gradient/heat flux along the radial direction of the bent SiNW, and the local heat current density along the circumferential direction varies with the radius of curvature. Interestingly, a ∼10% reduction in the thermal conductivity is found in the bent SiNW due to the depression of long-wavelength phonons caused by its inhomogeneous deformation. The present work demonstrates that the curvature induced by mechanical bending can be used to modulate the thermal conductivity of SiNWs.Curvature induced by mechanical deformation in nanostructures has been found to significantly affect their stability and reliability during applications. In this work, we investigated the effects of curvature induced by mechanical bending on the thermal properties of silicon nanowire (SiNW) by using molecular dynamics simulations. By examining the relationship between the curved geometry and local temperature/heat flux distribution, we found that there is no temperature gradient/heat flux along the radial direction of the bent SiNW, and the local heat current density along the circumferential direction varies with the radius of curvature. Interestingly, a ∼10% reduction in the thermal conductivity is found in the bent SiNW due to the depression of long-wavelength phonons caused by its inhomogeneous deformation. The present work demonstrates that the curvature induced by mechanical bending can be used to modulate the thermal conductivity of SiNWs. read less

Abstract: Enhancing thermal energy transport is critical for the applications of 2-dimensional materials. Here, we explored the methods of enhancing the interfacial thermal energy transport across the graphene (GR)/hexagonal boron nitride (h-BN) heterostructure interface, and revealed the enhancement mechanisms of interfacial thermal energy transport by applying non-equilibrium molecular dynamics (NEMD) simulations. The computational results indicated that both doping and interface topography optimization could effectively improve the interfacial thermal conductance (ITC) of the GR/h-BN heterostructure. In particular, the enhancement of the zigzag interface topography led to a much better result than the other methods. Doping and interface topography optimization increased the overlap of the phonon density of states (PDOS). Temperature had a negligible effect on the ITC of the GR/h-BN heterostructure when the temperature exceeded 600 K. read less

Abstract: Deposition of atoms or molecules on a solid surface is a flexible way to prepare various novel two-dimensional materials if the growth conditions, such as suitable surface and optimum temperature, could be predicted theoretically. However, prediction challenges modern theory of material design because the free energy criteria can hardly be applied to this issue due to the long-standing problem in statistical physics of the calculations of the free energy. Herein, we present an approach to the problem by the demonstrations of graphene and γ-graphyne on the surface of copper crystal, as well as silicene on a silver substrate. Compared with previous state-of-the-art algorithms for calculations of the free energy, our approach is capable of achieving computational precisions at least 10-times higher, which was confirmed by molecular dynamics simulations, and working at least four orders of magnitude faster, which enables us to obtain free energy based on ab initio calculations of the interaction potential instead of the empirical one. The approach was applied to predict the optimum conditions for silicene growth on different surfaces of solid silver based on density functional theory, and the results are in good agreement with previous experimental observations. read less

Abstract: The vibration behaviors of van der Waals (vdW) heterostructures are studied based on molecular dynamics (MD) simulations and continuum mechanics modelling in this paper. Graphene/hexagonal boron nitride and graphene/silicene systems are considered as two typical examples of heterostructures studied here. Our MD results show that the resonance frequency of vdW heterostructures grows as their layer number increases and tends to be saturated when the layer number is relatively large. These findings deviate from results of the conventional composite beam (CB) model of vdW heterostructures. By abandoning the assumptions in the CB model, we propose a novel multiple beam (MB) model giving a result that agrees well with MD results. We find from the MB model that compared to other factors the interlayer shearing effect plays the key role in determining the resonance behaviors of vdW heterostructures. Considering this fact, we further simplify the MB model to a much simpler form which gives a simple but precise description of the vibration behaviors of vdW heterostructures. This simplified MB model suggests that the resonance frequency of vdW heterostructures can be optimized by changing their total mass, the sum of bending stiffness of their component layers, and the sum of interlayer shear modulus of their vdW layers.The vibration behaviors of van der Waals (vdW) heterostructures are studied based on molecular dynamics (MD) simulations and continuum mechanics modelling in this paper. Graphene/hexagonal boron nitride and graphene/silicene systems are considered as two typical examples of heterostructures studied here. Our MD results show that the resonance frequency of vdW heterostructures grows as their layer number increases and tends to be saturated when the layer number is relatively large. These findings deviate from results of the conventional composite beam (CB) model of vdW heterostructures. By abandoning the assumptions in the CB model, we propose a novel multiple beam (MB) model giving a result that agrees well with MD results. We find from the MB model that compared to other factors the interlayer shearing effect plays the key role in determining the resonance behaviors of vdW heterostructures. Considering this fact, we further simplify the MB model to a much simpler form which gives a simple but precise des... read less

Abstract: As a crucial part in thermal management, interfacial thermal transport is still not well understood. In this paper, we employ the newly developed modal nonequilibrium molecular dynamics to study the Si/Ge interfacial thermal transport and clarify several long-standing issues. We find that the few atomic layers at the interface are dominated by interfacial modes, which act as a bridge that connects the bulk Si and Ge modes. Such bridging effect boosts the inelastic transport to contribute more than 50% to the total thermal conductance even at room temperature. The apparent inelastic transport can even allow effective four-phonon processes across the interface when the mass difference between the two materials is large. Surprisingly, optical phonon modes can contribute equal or more thermal conductance than the acoustic modes due to the bridging effect. From the modal temperature analysis, we find that the phonon modes are in strong thermal nonequilibrium near the interface, which impedes the thermal transport. The widely used Landauer approach that does not consider the phonon nonequilibrium can lead to inaccurate results. We have modified the Landauer approach to include the inelastic transmission and modal thermal nonequilibrium. The approach is used to analyze our modal NEMD results, and the mode-dependent phonon transmission function that includes inelastic scattering has been derived. Our results unveil the fundamental thermal transport physics across interfaces and will shed light on the future engineering in thermal management. It provides a method of calculating modal phonon transmission functions that includes inelastic scattering from molecular dynamics. read less

Abstract: Silicon carbide (SiC), especially 4H-SiC, is an ideal semiconductor in power electronics due to its outstanding electrical and thermal properties. It has high hardness and brittleness, which makes it difficult to machine. To understand the nanomachining characteristics of off-axis 4H-SiC and provide suggestions on 4H-SiC substrate thinning, the nanocutting process of 4 ∘ off-axis 4H-SiC was simulated by molecular dynamics. The results showed that the stacking fault induced by cutting propagates in the basal plane, and propagates deep into the SiC workpiece when the angle between the cutting direction and the c-axis is smaller than 90 ∘ . Bond reconstruction is found near the slip plane. The cutting depth is also a key parameter in nanocutting. With smaller cutting depth, machining is more like scratching than cutting. With larger cutting depth, more atoms are involved in the cutting, cutting force and workpiece temperature are higher, and more defects exist. read less

Abstract: Molecular dynamics simulations were carried out for different structural models of the Si/3C-SiC interface using the Tersoff SiC potential that can model both Si and SiC. We find that the bonding at the Si/3C-SiC interface has a strong effect on the crystallization of the Si phase and that a degree of intermixing is present between the two materials with some C atoms migrating from the 3C-SiC (hereinafter referred to as SiC) into the Si region. The degree of intermixing is likely to exhibit a strong dependence on the temperature and most likely also increases with time, which would lead to changes in the Si/SiC interface during the life of the Si/SiC composite. The inter-mixing also creates disorder and defects of threefold and fivefold bonded atoms in the vicinity of the interfaces. In particular, {111} 1 2 ⟨ 110 ⟩ misfit dislocations were formed at all three types of interfaces [(100), (110), and (111)] in order to relieve the local stress due to lattice mismatch. Additionally, the Si(110)/SiC(110) and Si(111)/SiC(111) interfaces prepared at higher temperatures show the formation of the {111} 1 6 ⟨ 112 ⟩ partial dislocation which arises due to intrinsic stacking faults. We find that the bonding at the crystalline(c) c-Si/SiC interface is weaker than that in bulk crystalline Si, whereas bonding at the amorphous(a)-Si/SiC interface is stronger than that in amorphous Si. Therefore, the rupture in the yield stress occurs at the vicinity of the Si/SiC interface and in the Si region for the a-Si/SiC systems, respectively. Finally, for both bulk and Si/SiC interface systems, a strong variation of the yield strength with temperature was observed.Molecular dynamics simulations were carried out for different structural models of the Si/3C-SiC interface using the Tersoff SiC potential that can model both Si and SiC. We find that the bonding at the Si/3C-SiC interface has a strong effect on the crystallization of the Si phase and that a degree of intermixing is present between the two materials with some C atoms migrating from the 3C-SiC (hereinafter referred to as SiC) into the Si region. The degree of intermixing is likely to exhibit a strong dependence on the temperature and most likely also increases with time, which would lead to changes in the Si/SiC interface during the life of the Si/SiC composite. The inter-mixing also creates disorder and defects of threefold and fivefold bonded atoms in the vicinity of the interfaces. In particular, {111} 1 2 ⟨ 110 ⟩ misfit dislocations were formed at all three types of interfaces [(100), (110), and (111)] in order to relieve the local stress due to lattice mismatch. Additionally, the Si(110)/SiC(110) a... read less

Abstract: Silicon oxycarbide (SiCO)-derived porous carbon is a novel class of nano-porous material with unique properties including highly sensitive gas detection. In this letter, SiCO-derived porous carbon (porous SiCO) structures are successfully reproduced by simulating the etching process in experiments. Then, the gas sensing performance of SiCO-derived carbon with different porous morphologies is investigated. The calculated adsorption energy, Mulliken charge transfer, bandgap, and adsorption distance indicate that SiCO-derived porous carbon exhibits a higher sensitivity toward NO2 gas than CO, 2, and acetone in accordance with experimental conclusions. Moreover, the porous SiCO with the largest SSA and PV shows the most excellent NO2 sensing performance. The adsorption of NO2 leads to the appearance of a new strong peak at the edge of the conduction band, resulting in obvious changes of the conductivity of the systems, which is necessary for NO2 detection. read less

Abstract: Based on the Tersoff potential, molecular dynamics simulations for the growth of Si crystals along the 〈112〉 orientation were carried out to investigate the atomic configurations of dislocations and twins. Two typical configurations were observed. One is a sandwich structure which has two twin boundaries and several rows of atoms normally arranged and is ended by a Shockley dislocation with a Burgers vector of 〈112〉/6. The other is composed of two intersecting stacking faults and a Lomer–Cottrell dislocation with a Burgers vector of 〈110〉/6. The two configurations can combine together and form complex atomic configurations in Si crystal. And the two configurations have similar formation processes. Firstly, two twin boundaries or a stacking fault forms in a {111} facet of solid–liquid interface. Then, a dislocation nucleates after the following crystal atom planes dock with each other or the second stacking fault forms. The formation of Shockley dislocation takes a longer time than that of Lomer–Cottrell dislocation due to a larger lattice mismatch ( 1 ¯ 1 ¯ 1 ) of planes. read less

Abstract: In this work we present a molecular dynamics investigation of thermal transport in a silica-gallium nitride nanocomposite. A surprising enhancement of the thermal conductivity for crystalline volume fractions larger than 5% is found, which cannot be predicted by an effective medium approach, not even including percolation effects, the model systematically leading to an underestimation of the effective thermal conductivity. The behavior can instead be reproduced if an effective volume fraction twice larger than the real one is assumed, which translates into a percolation effect surprisingly stronger than the usual one. Such a scenario can be understood in terms of a phonon tunneling between inclusions, enhanced by the iso-orientation of all particles. Indeed, if a misorientation is introduced, the thermal conductivity strongly decreases. We also show that a percolating nanocomposite clearly stands in a different position than other nanocomposites, where thermal transport is dominated by the interface scattering and where parameters such as the interface density play a major role, differently from our case. read less

Abstract: We introduce an analytical method to predict the slip length (Ls) in cylindrical nanopores using equilibrium molecular dynamics (EMD) simulations, following the approach proposed by Sokhan and Quirke for planar channels []. Using this approach, we determined the slip length of water in carbon nanotubes (CNTs) of various diameters. The slip length predicted from our method shows excellent agreement with the results obtained from nonequilibrium molecular dynamics (NEMD) simulations. The data show a monotonically decreasing slip length with an increasing nanotube diameter. The proposed EMD method can be used to precisely estimate slip length in high slip cylindrical systems, whereas, Ls calculated from NEMD is highly sensitive to the velocity profile and may cause large statistical errors due to large velocity slip at the channel surface. We also demonstrated the validity of the EMD method in a BNNT-water system, where the slip length is very small compared to that in a CNT pore of similar diameter. The developed method enables us to calculate the interfacial friction coefficient directly from EMD simulations, while friction can be estimated using NEMD by performing simulations at various external driving forces, thereby increasing the overall computational time. The EMD analysis revealed a curvature dependence in the friction coefficient, which induces the slip length dependency on the tube diameter. Conversely, in flat graphene nanopores, both Ls and friction coefficient show no strong dependency on the channel width. read less

Abstract: Si nanowires (NWs) have been experimentally proven to be better candidates for thermoelectric applications than other Si-based nanomaterials due to their extremely low thermal conductivity (TC). Designing and manufacturing high-performance Si NW-based thermoelectrics require a systematic and robust understanding of their thermal transport properties. Therefore, various experimentally observed Si NWs, i.e., pure Si NWs, five-fold twinned Si NWs (5T-Si NWs) and alloyed Si NWs are systematically studied here. Our results show that TC of circular-section Si NWs is clearly lower than that of rectangular-section Si NWs, which is caused by the coarser external boundary (EB). Introducing a five-fold twin boundary (TB) in Si NWs can lead to 3–4 times reduction of the TC. Furthermore, Ge-doping can result in further enormous TC reduction, and the TC can be lowered to an extreme value (1.4 W mK−1); this value approaches the lowest limit of the TC of Si-based nanomaterials (1 W mK−1) and is lower than that of pure amorphous Si NWs (2.4 W mK−1). After combining with lattice dynamics analysis, we notice that compared to EB, five-fold TB not only leads to the usual boundary-phonon scattering, but also results in resonance to reduce the phonon group velocity of low-frequency phonons due to its symmetrical structure, i.e., the symmetrical TB can weaken TCs in two ways: vibration and scattering, which enable it to show greater potential in the field of thermoelectrics. There are three major reasons for the extremely low TC of 5T alloyed Si NWs: (1) the hybridization effect caused by the resonance of the five symmetrical independent twin domains. (2) The greater reduction of TC contributed by TB when compared to that of EB. (3) The appearance of non-propagating phonons caused by Ge-doping. These results reveal that the combined implementation of five-fold TB and Ge-doping, which can enormously block the phonon transport in the entire frequency range, is an effective method to reduce TC and consequently to enhance the figure-of-merit (ZT). In general, our investigations can be expected to offer some useful guidance for the enhancement of ZT of conventional silicon materials. read less

Abstract: Recent breakthroughs in the synthesis of boron nitride nanotubes (BNNTs) attracted researchers' attention again for developing their nanocomposites. This is due to the fact that BNNT possesses wide band gap (~5.5 eV, independent of geometry), strong hardness, chemically and thermally stable, and excellent piezoelectric properties than its carbon-based counterpart. Furthermore, BNNTs have comparable mechanical and thermal properties compared to carbon nanotubes. As the first of its kind, this study reports the transversely isotropic elastic properties of pristine and vacancy defected BNNTs within the framework of MD simulations using a Tersoff force field. This is achieved by imposing axial extension, twist, in-plane biaxial tension, and in-plane shear to the pristine and defective BNNTs. Our results reveal that vacancy concentration of 2% affects profoundly the axial Young's, shear, plane strain bulk and in-plane shear moduli of BNNTs, and decrease their respective values by 14%, 25%, 14% and 18%. The current fundamental study highlights the important role played by vacancy defected BNNTs in determining their mechanical behaviours as fillers in multifunctional nanocomposites. read less

Abstract: Two-dimensional (2D) materials are envisaged as ultra-thin solid lubricants for nanomechanical systems. So far, their frictional properties at the nanoscale have been studied by standard friction force microscopy. However, lateral manipulation of nanoparticles is a more suitable method to study the dependence of friction on the crystallography of two contacting surfaces. Still, such experiments are lacking. In this study, we combine atomic force microscopy (AFM) based lateral manipulation and molecular dynamics simulations in order to investigate the movements of organic needle-like nanocrystallites grown by van der Waals epitaxy on graphene and hexagonal boron nitride. We observe that nanoneedle fragments - when pushed by an AFM tip - do not move along the original pushing directions. Instead, they slide on the 2D materials preferentially along the needles' growth directions, which act as invisible rails along commensurate directions. Further, when the nanocrystallites were rotated by applying a torque with the AFM tip across the preferential sliding directions, we find an increase of the torsional signal of the AFM cantilever. We demonstrate in conjunction with simulations that both, the significant friction anisotropy and preferential sliding directions are determined by the complex epitaxial relation and arise from the commensurate and incommensurate states between the organic nanocrystallites and the 2D materials. read less

Abstract: At the nanoscale, thermal transport across the interface between two lattice insulators can be described by the transmission of bulk phonons and depends on the crystallographic structure of the interface and the bulk crystal lattice. In this tutorial, we give an account of how an extension of the Atomistic Green's Function (AGF) method based on the concept of the Bloch matrix can be used to model the transmission of individual phonon modes and allow us to determine the wavelength and polarization dependence of the phonon transmission. Within this framework, we can explicitly establish the relationship between the phonon transmission coefficient and dispersion. Details of the numerical methods used in the extended AGF method are provided. To illustrate how the extended AGF method can be applied to yield insights into individual phonon transmission, we study the (16,0)/(8,0) carbon nanotube intramolecular junction. The method presented here sheds light on the modal contribution to interfacial thermal transport between solids. read less

Abstract: It is well known that the optical materials are unique and perspective. Optical materials and the devices based on them are operated in the broad spectral range: In the UV spectral range (where the wavelength l is approximately placed in the range of ~ 0.1 - 0.4 microns), in the VIS spectral range (l ~ 0.5 - 0.75 microns), and in the IR spectral range (l is larger than the 0.75-1 microns). These materials can be considered to resolve the different complicated tasks. To study optical materials different techniques and methods should be scrupulously used. Among different applied methods namely the laser oriented technique and nanostructuration approach have some unique features. It can be considered as the effective dominant approach in order to reveal the change of all basic physical-chemical characteristics of the materials. Our own steps in this direction have partially been recently shown too. In the current paper, advantages of the modification of optical material surfaces via a nanotechnology approach will be shown. The surface relief change provokes the spectral, mechanical and wetting phenomena changes. A CO2-laser is applied to modify the optical materials surfaces under the condition when the carbon nanotubes are deposited in vertical position at the materials surfaces. This process permits to organize covalent bonding between the carbon atoms and the model matrix ones. An emphasis will be given on the surface modifications of the materials, such as: LiF, CaF2, KBr, BaF2, Sc, some polymer surface, etc. Mechanisms responsible for the spectral characteristics change, mechanical hardness as well as the increase of the wetting angle will be discussed. The area of the application of the materials studied can be increased. read less

Abstract: Nanoconfined water plays a significant role in nature. In recent years, many efforts have been dedicated to determine how the properties of nano-confined water change in nanopores and how these changes influence different systems, such as biological systems, and water flow in different media, such as cements and zeolites. In this work, we have simulated water molecules with different densities between two parallel graphene sheets with different distances at 300 K. Various thermodynamic, structural, and dynamical properties of the confined water molecules have been investigated using the SPC/E and TIP4P models. Our results showed that the confined systems are more stable when the pore size is between 6 and 7 A. The energy of the confined water molecules also decreases with increasing density up to 0.6 g cc−1. Our observations showed that the confined water molecules tend to form square and rhombus structures at higher densities and tend to form pentagonal and hexagonal structures at lower densities. Pentagonal and hexagonal structures can be also more frequently observed for confined water molecules at smaller pore sizes. The most stable structure of the confined water molecules based on the SPC/E model is the rhombic structure. Our results also showed that the most stable state of the TIP4P model has fewer rhombus structures than that of the SPC/E model. Also, the TIP4P model presents greater self-diffusion values (especially at large pore sizes) than the SPC/E model. We also recognized a nanostructure phase transition from a region of high hydrogen bonding (HB) to a region of low HB by decreasing the density below around 0.6 g cc−1. The region of high HB is more ordered and contains more rhombus or square structures. read less

Abstract: We critically readdress the definition of thermal boundary resistance at an interface between two semiconductors. By means of atomistic simulations we provide evidence that the widely used Kapitza formalism predicts thermal boundary resistance values in good agreement with the more rigorous Onsager non-equilibrium thermodynamics picture. The latter is, however, better suited to provide physical insight on interface thermal rectification phenomena. We identify the factors that determine the temperature profile across the interface and the source of interface thermal rectification. To this end we perform non-equilibrium molecular dynamics computational experiments on a Si-Ge system with a graded compositional interface of varying thickness, considering thermal bias of different sign. read less

Abstract: Molecular dynamics simulations of the seeded solidification of silicon along <100>, <110>, <111> and <112> directions have been carried out. The Tersoff potential is adopted for computing atomic interaction. The control of uniaxial strains in the seed crystals is enabled in the simulations. The results show that the dislocation forms stochastically at the crystal/melt interface, with the highest probability of the formation in <111> growth, which agrees with the prediction from a previously proposed twinning-associated dislocation formation mechanism. Applications of the strains within a certain range are found to inhibit the {111}-twinning-associated dislocation formation, while beyond this range they are found to induce dislocation formation by different mechanisms. read less

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

Abstract: In polymer nanocomposites, the interface region between the matrix and the fillers has been identified as a key interaction region that strongly determines the properties of the final material. Determining its structure is crucial from several points of view, from modeling (i.e., properties prediction) to materials science (i.e., understanding properties/structure relationships). In the presented paper, a method for characterizing the interface region of polymer nanocomposites is described using molecular dynamics (MD) simulations. In particular, the structure of the polymer within the interface region together with its dimension in terms of thickness were analyzed through density profiles. Epoxy resin nanocomposites based on diglycidyl ether of bisphenol A (DGEBA) were studied using this approach, and the interface region with triple walled carbon nanotubes (TWCNT) and carbon fibers (CF) was characterized. The effect of carbon nanotube diameter, type of hardener, and effect of epoxy resin cross-linking degree on interface thickness were analyzed using MD models. From this analysis no general rule on the effect of these parameters on the interface thickness could be established, since in some cases overlapping effects between the analyzed parameters were observed, and each specific case needs to be analyzed independently in detail. Results show that the diameter has an impact on interface thickness, but this effect is affected by the cross-linking degree of the epoxy resin. The type of hardener also has a certain influence on the interface thickness. read less

Abstract: Resonance properties such as the resonance frequency, the sensitivity, and the intrinsic dissipation of boron nitride nanotube (BNNT) based resonators are investigated in this work based on molecular dynamics simulations together with density functional theory calculations. A remarkable resonance property comparable to their carbon nanotube (CNT) counterparts is found in the present BNNT based resonators. Moreover, due to the unique piezoelectric characteristic of BNNTs, the resonance properties of BNNT based resonators can be efficiently tailored by external electric fields. It is found that when a negative electric field is applied, the resonance frequency and the sensitivity of BNNT based resonators can be significantly enhanced. This effect is attributed to the fact that due to the inverse piezoelectric response the applied negative electric field will induce a residual tensile stress in BNNTs and thus enhance their equivalent stiffness. Meanwhile, it is also found that the intrinsic dissipation of BNNT based resonators can be mitigated by a positive external electric field, since under this condition the thermoelastic dissipation and the phonon-phonon scattering of BNNTs are both reduced by the piezoelectric effect. Such unique piezoelectrically tunable resonance properties in BNNT based resonators render them have a broader spectrum of applications than their conventional CNT counterparts.Resonance properties such as the resonance frequency, the sensitivity, and the intrinsic dissipation of boron nitride nanotube (BNNT) based resonators are investigated in this work based on molecular dynamics simulations together with density functional theory calculations. A remarkable resonance property comparable to their carbon nanotube (CNT) counterparts is found in the present BNNT based resonators. Moreover, due to the unique piezoelectric characteristic of BNNTs, the resonance properties of BNNT based resonators can be efficiently tailored by external electric fields. It is found that when a negative electric field is applied, the resonance frequency and the sensitivity of BNNT based resonators can be significantly enhanced. This effect is attributed to the fact that due to the inverse piezoelectric response the applied negative electric field will induce a residual tensile stress in BNNTs and thus enhance their equivalent stiffness. Meanwhile, it is also found that the intrinsic dissipation of BN... read less

Abstract: Amorphization, phase transformation, and dilation cracking are 3 major deformation/failure mechanisms of monocrystalline 6H-SiC. This paper studies their critical formation conditions and mechanisms under hydrostatic pressure and uniaxial compression and tension with the aid of large-scale molecular dynamics simulations. It was found that under hydrostatic pressure the major deformation mechanism is amorphization, that under uniaxial compression the major mechanism turns to phase transformation at low temperature and amorphization at high temperature, and that under uniaxial tension the dominating mechanism becomes dilation cracking. Increasing the temperature reduces the thresholds significantly and brings about a heterogeneous deformation mode. The study further concluded that these deformation mechanisms and their thresholds can be predicted theoretically. read less

Abstract: Background: Graphitization behavior of diamond has received an increasing interest in nanoscale machining of some hard and brittle materials. Diamond has always been an important and excellent tool material in cutting area. However, the graphitization of the diamond tool is inevitable when it was used in special conditions. It is indicated that the graphitization of diamond crystal has great influence on the wear resistance of diamond cutting tool. The graphitization behavior needs to be investigated extensively in nanoscale with an atomic view. Molecular dynamics simulation provides a useful tool for understanding of the graphitization mechanism of diamond. The investigation on graphi-tization behavior of single crystal diamond can also provide a useful reference for the application of diamond cutting tool. Materials and Methods: In this paper, a molecular dynamics (MD) diamond crystal model is built to examine the graphitization behavior of diamond under various conditions. The sixfold ring method was employed to identify the structural characteristics of graphite and diamond. The effects of temperature and crystal orientation on the graphitization of diamond have been revealed. Considering the effect of temperature, the anisotropy of diamond graphitization against various crystal planes is presented and discussed carefully. The nano-metric cutting model of diamond tool evaluated by the sixfold ring meth-od also proves the graphitization mechanisms in atomic view. Results: Results indicate that the sixfold ring method is a reliable method to evaluate the graphitization behavior of diamond crystal. There exists a critical temperature of the graphitization of diamond. The results also show that {111} plane is more easy to get graphitization as compared with other crystal planes. However, {100} plane of diamond model presents the highest anti-graphitization property. Conclusion: The obtained results have provided the in-depth understanding on the wear of diamond tool in nano-metric machining and underpin the development of diamond cutting tool read less

Abstract: Manipulating thermal transport by designing materials with control of their properties over the entire spectral range of vibrational frequencies would provide a unique path to create solids with designer thermal conductivities. Traditional routes of nanostructuring to reduce the vibrational thermal conductivity of solids typically target narrow bands of the vibrational energy spectrum, which is often based on the characteristic dimensions of the nanostructure. In this work, we demonstrate the ability to simultaneously impact the phonon transport of both highand low-frequency modes by creating defects that act as both high-frequency phonon scattering sites while coherently manipulating low-frequency waves via resonance with the long wavelength phonons. We use atomistic simulations to identify fullerenes functionalized on the sidewalls of carbon nanotubes (CNT) as efficient phonon blocks realized through localized resonances that appear due to hybridization between the modes of the fullerene and the underlying CNT. We show that with a large surface coverage and high periodicity in the inclusion of the covalently bonded fullerenes, the thermal conductivity of individual CNTs can be lowered by more than an order of magnitude, thus providing a large tunability in the thermal transport across these materials. We prescribe the large reduction in thermal conductivity to a combination of resonant phonon localization effects leading to phonon band anticrossings and vibrational scattering effects due to the inclusion of the strongly bonded fullerene molecules. read less

Abstract: Si/Ge nanowires are considered to be promising candidates as efficient thermoelectric materials due to their remarkable thermal insulating performance over bulk counterparts. In this study, thermal insulating performance of Si/Ge nanowires of randomly organized layer thickness, called random layer nanowires (RLNWs), is systematically investigated and compared against superlattice nanowires (SLNWs).The thermal conductivity (TC) of these structures is evaluated via non-equilibrium molecular dynamic simulations, and more informative insight is gained by normal mode decomposition and lattice dynamics calculations. It is demonstrated that the modes in random layer structures, in general, exhibit similar characteristics except the degree of localization to the corresponding superlattice counterparts by comparing the mode spectral energy densities, relaxation times, density of states, and participation ratios. For all physical and geometrical conditions investigated here, RLNWs show improved thermal insulating performance over corresponding SLNWs. More importantly, a RLNW of low mean layer thickness attains even lower TC than the corresponding Si/Ge alloy nanowire indicating the effectiveness of the random layer arrangements. An anomalous trend in TC of RLNWs (larger than the bulk counterpart) is observed at higher cross-sectional widths, and it is explained as a competing effect of phonon localization and wall scattering. Moreover, it is illustrated that the effectiveness of thermal insulating performance of RLNW depends on the fraction of coherent phonons that exist and how effectively those phonons are subject to localization under different cases.Si/Ge nanowires are considered to be promising candidates as efficient thermoelectric materials due to their remarkable thermal insulating performance over bulk counterparts. In this study, thermal insulating performance of Si/Ge nanowires of randomly organized layer thickness, called random layer nanowires (RLNWs), is systematically investigated and compared against superlattice nanowires (SLNWs).The thermal conductivity (TC) of these structures is evaluated via non-equilibrium molecular dynamic simulations, and more informative insight is gained by normal mode decomposition and lattice dynamics calculations. It is demonstrated that the modes in random layer structures, in general, exhibit similar characteristics except the degree of localization to the corresponding superlattice counterparts by comparing the mode spectral energy densities, relaxation times, density of states, and participation ratios. For all physical and geometrical conditions investigated here, RLNWs show improved thermal insulating p... read less

Abstract: Inspired by the interesting phenomenon that biological systems have the inherent skill to generate significant bioelectricity when the salt content in fluids flows over highly selective ion channels on cell membranes, in this study, the flow-induced voltage is investigated by driving the pure bulk room-temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]) flowing over a graphene nano-channel consisting of two parallel single-layered graphene sheets using molecular dynamics simulation for the first time. Considering the combined effect of cations and anions in the adsorbed layer on the free charge carriers of the graphene surfaces (the interactions are 12.0 and 7.0 kJ mol−1 per cation/anion and graphene, respectively) and the characteristic of Coulomb's law, we have developed an advanced equation that can effectively and accurately calculate the flow-induced voltage of RTIL and graphene nano-channel system on the nano-scale. A maximum flow-induced voltage of 2.3 μV is obtained from this nano-scaled system because the free charge carrier on the graphene channel surfaces is dragged along the pure bulk RTIL's direction of movement. A saturation of the flow-induced voltage with increased flow velocity is observed, and this saturation can be attributed to the balance between the external driving force and viscous resistance arising from the internal RTIL and graphene nano-channel. Further analysis shows that the flow-induced voltages gradually increase towards saturation from 1.9 to 2.1 μV or decrease from 2.3 to 2.1 μV when the distance between the two parallel single-layered graphene or the area of single-layered graphene of the nano-channel increases from 1 to 5 nm or from 1 to 25 nm2, respectively. Additionally, the influence of the system temperature (viscosity) and average flow velocity on the flow-induced voltage is investigated. read less

Abstract: We use a phase field crystal model to generate large-scale bicrystalline and polycrystalline single-layer hexagonal boron nitride (h-BN) samples and employ molecular dynamics (MD) simulations with the Tersoff many-body potential to study their heat transport properties. The Kapitza thermal resistance across individual h-BN grain boundaries is calculated using the inhomogeneous nonequilibrium MD method. The resistance displays strong dependence on the tilt angle, the line tension and the defect density of the grain boundaries. We also calculate the thermal conductivity of pristine h-BN and polycrystalline h-BN with different grain sizes using an efficient homogeneous nonequilibrium MD method. The in-plane and the out-of-plane (flexural) phonons exhibit different grain size scalings of the thermal conductivity in polycrystalline h-BN and the extracted Kapitza conductance is close to that of large-tilt-angle grain boundaries in bicrystals. read less

Abstract: Thermoelectric transport properties of Ti doped or adsorbed monolayer blue phosphorene are investigated by density functional theory combined with the nonequilibrium Green’s function formalism. The thermal giant magnetoresistance and a nearly 100% spin polarization which solely relies on the temperature gradient of electrodes without bias or gate voltage are observed. Moreover, the spin Seebeck effect is also found. Furthermore, taking into account the electronic and phonon dispersion, the thermoelectric merit for Ti doping in the monolayer blue phosphorene at room temperature is also studied, the maximum value of thermoelectric merit can reach 1.01 near the Fermi level. The results indicate that Ti doped or adsorbed monolayer blue phosphorene has potential application in both spintronics and spin caloritronics. read less

Abstract: While the microscopic structure of defected solid crystalline materials has significant impact on their physical properties, efficient and accurate determination of a given polycrystalline microstructure remains a challenge. In this paper, we present a highly generalizable and reliable variational method to achieve this goal for two-dimensional crystalline and quasicrystalline materials. The method is benchmarked and optimized successfully using a variety of large-scale systems of defected solids, including periodic structures and quasicrystalline symmetries to quantify their microstructural characteristics, e.g., grain size and lattice misorientation distributions. We find that many microstructural properties show universal features independent of the underlying symmetries. read less

Abstract: Interstitial defects are inevitably present in doped semiconductors that enable modern-day electronic, optoelectronic or thermoelectric technologies. Understanding of stability of interstitials and their bonding mechanisms in the silicon lattice was accomplished only recently with the advent of first-principles modeling techniques, supported by powerful experimental methods. However, much less attention has been paid to the effect of different naturally occurring interstitials on the thermal and electrical properties of silicon. In this work, we present a systematic study of the variability of heat and charge transport properties of bulk silicon, in the presence of randomly distributed interstitial defects (Si, Ge, C and Li). We find through atomistic lattice dynamics and molecular dynamics modeling studies that, interstitial defects scatter heat-carrying phonons to suppress thermal transport-1.56% of randomly distributed Ge and Li interstitials reduce the thermal conductivity of silicon by $\sim$ 30 and 34 times, respectively. Using first principles density functional theory and semi-classical Boltzmann transport theory, we compute electronic transport coefficients of bulk Si with 1.56% Ge, C, Si and Li interstitials, in hexagonal, tetrahedral, split-interstitial and bond-centered sites. We demonstrate that hexagonal-Si and hexagonal-Ge interstitials minimally impact charge transport. To complete the study, we predict the thermoelectric property of an experimentally realizable bulk Si sample that contains Ge interstitials in different symmetry sites. Our research establishes a direct relationship between the variability of structures dictated by fabrication processes and heat and charge transport properties of silicon. The relationship provides guidance to accurately estimate performance of Si-based materials for various technological applications. read less

Abstract: Most recently, boron–graphdiyne, a π-conjugated two-dimensional (2D) structure made from a merely sp carbon skeleton connected with boron atoms was successfully experimentally realized through a bottom-up synthetic strategy. Motivated by this exciting experimental advance, we conducted density functional theory (DFT) and classical molecular dynamics simulations to study the mechanical, thermal conductivity and stability, electronic and optical properties of single-layer B-graphdiyne. We particularly analyzed the application of this novel 2D material as an anode for Li, Na, Mg and Ca ion storage. Uniaxial tensile simulation results reveal that B-graphdiyne owing to its porous structure and flexibility can yield superstretchability. The single-layer B-graphdiyne was found to exhibit a semiconducting electronic character, with a narrow band-gap of 1.15 eV based on the HSE06 prediction. It was confirmed that mechanical straining can be employed to further tune the optical absorbance and electronic band-gap of B-graphdiyne. Ab initio molecular dynamics results reveal that B-graphdiyne can withstand high temperatures, like 2500 K. The thermal conductivity of suspended single-layer B-graphdiyne was predicted to be very low, ∼2.5 W mK−1 at room temperature. Our first-principles results reveal the outstanding prospect of B-graphdiyne as an anode material with ultrahigh charge capacities of 808 mA h g−1, 5174 mA hg−1 and 3557 mA h g−1 for Na, Ca and Li ion storage, respectively. The comprehensive insight provided by this investigation highlights the outstanding physics of B-graphdiyne nanomembranes, and suggests them as highly promising candidates for the design of novel stretchable nanoelectronics and energy storage devices. read less

Abstract: Si and Si/Ge based nanostructures of reduced lattice thermal conductivity are widely attractive for developing efficient thermoelectric materials. In this study, we demonstrate the reduced thermal conductivity of Si nanotwinned random layer (NTRL) structures over corresponding superlattice and twin-free counterparts. The participation ratio analysis of vibrational modes shows that a possible cause of thermal conductivity reduction is phonon localization due to the random arrangement of twin boundaries. Via non-equilibrium molecular dynamic simulations, it is shown that ~23 and ~27% reductions over superlattice counterparts and ~55 and 53% over twin-free counterparts can be attained for the structures of total lengths of 90 and 170 nm, respectively. Furthermore, a random twin boundary distribution is applied for Si/Ge random layer structures seeking further reduction of thermal conductivity. A significant reduction in thermal conductivity of Si/Ge structures exceeding the thermal insulating performance of the corresponding amorphous Si structure by ~31% for a total length of 90 nm can be achieved. This reduction is as high as ~98% compared to the twin-free Si counterpart. It is demonstrated that application of randomly organised nanoscale twin boundaries is a promising nanostructuring strategy towards developing efficient Si and Si/Ge based thermoelectric materials in the future. read less

Abstract: The interface between nanoscale films plays a very important role in semiconductor industry. In this paper, the interfacial thermal resistance (Kapitza resistance) of a crystalline and amorphous silicon carbide (SiC) heterojunction has been investigated by using molecular dynamics simulations. It is found that Kapitza resistance at crystalline and amorphous SiC interface depends on the interfacial coupling strength remarkably. Kapitza resistance in the strong interfacial coupling is significantly lower than that in weak coupling. The thickness of the heterojunction and temperature dependence of Kapitza resistance have also been examined. The results have shown that the Kapitza resistance decreases monotonically with the increase of temperature (from 300K to 800K). Moreover, Kapitza resistance can be effectively tuned by cross-plane strain. A 5% compressive strain is able to reduce the Kapitza resistance by 380% in weak coupling case. In contrast, a 5% tensile strain can increase Kapitza resistance by 13%. Our study provides useful guidance to the thermal management and heat dissipation across nanoscale crystalline and amorphous silicon carbide interface, in particular, for the design of silicon carbide nanowire based nano electronics devices. read less

Abstract: Silicon carbide (SiC) is gradually entering into commercialization as one of the next-gen semiconductor materials. In order to improve its yield and quality, it is important to understand its nanometric grinding mechanism. Silicon (Si) has been used in semiconductor industry for several decades, and its nanometric machining mechanism has been widely studied. However, the machining mechanism of Si can't be directly applied to SiC due to the differences in their physical properties. In this paper, nanometric grinding simulations of monocrystalline Si, 4H-SiC, and 6H-SiC have been performed by molecular dynamics. Rigid and dynamic cutting tool models have been applied and assessed. The results show that rigid cutting tool model can be applied to the simulations of Si for simplicity and dynamic cutting tool model is more appropriate for the simulations of SiC. Grinding force of SiC is much larger than that of Si, and the difference in grinding force between 4H-SiC and 6H-SiC is small. read less

Abstract: Single- and multilayer graphene sheets (MLGSs) are projectile-resisting materials that can be bombarded by nanoparticles to produce graphene sheets of various sizes and distributions of nanopores. These sheets are used in a variety of applications, including DNA sequencing, water desalination, and phase separation. Here, the impact-withstanding efficiency of graphene nanosheets and the primary factors affecting creation of nanopores in these sheets were studied using a molecular dynamics method. The velocity of impacting nanoparticles and resulting displacement in graphene nanosheets are not sufficient criteria for evaluating the impact resistance of sheets with more than six layers. Instead, visual inspection of the bottom side of a graphene sheet should be used. Self-healing is the most important aspect of MLGSs because it closes the paths of penetrating nanoparticles in the upper layers of the sheets. For nanosheets with few layers, self-healing is observed only at very small nanoparticle velocities; however, when the number of layers is more than six, self-healing occurs even at high nanoparticle velocities. In nanoribbon simulations, it was found that layer boundaries improve resistance against projectile impacts that create well-defined oval shapes. By increasing the distance between layers, the carbon atoms of each layer experience more collisions with the carbon atoms of other layers. Thus, increasing the interlayer distance causes the number of unwanted collisions between carbon atoms to increase and the graphene nanosheets to disintegrate. Additionally, as the circularity of nanopores increases, they become more circular and homogeneous, in turn increasing interlayer spacing, the impact-withstanding efficiency of the sheets, and the circular shape of created nanopores. read less

Abstract: We investigate thermally driven water droplet transport on graphene and hexagonal boron nitride (h-BN) surfaces using molecular dynamics simulations. The two surfaces considered here have different wettabilities with a significant difference in the mode of droplet transport. The water droplet travels along a straighter path on the h-BN sheet than on graphene. The h-BN surface produced a higher driving force on the droplet than the graphene surface. The water droplet is found to move faster on h-BN surface compared to graphene surface. The instantaneous contact angle was monitored as a measure of droplet deformation during thermal transport. The characteristics of the droplet motion on both surfaces is determined through the moment scaling spectrum. The water droplet on h-BN surface showed the attributes of the super-diffusive process, whereas it was sub-diffusive on the graphene surface. read less

Abstract: The precision and crack-free surface of brittle silicon carbide (SiC) ceramic was achieved in the nanoscale ductile grinding. However, the nanoscale scratching mechanism and the root causes of SiC ductile response, especially in the atomistic aspects, have not been fully understood yet. In this study, the SiC atomistic scale scratching mechanism was investigated by single diamond grain scratching simulation based on molecular dynamics. The results indicated that the ductile scratching process of SiC could be achieved in the nanoscale depth of cut through the phase transition to an amorphous structure with few hexagonal diamond structure. Furthermore, the silicon atoms in SiC could penetrate into diamond grain which may cause wear of diamond grain. It was further found out that the chip material in the front of grain flowed along the grain side surface to form the groove protrusion as the scratching speed increases. The higher scratching speed promoted more atoms to transfer into the amorphous structure an... read less

Abstract: Silicon carbide (SiC) is nowadays a major material for applications in high power electronics, quantum optics, or nitride semiconductors growth. Mastering the surface of SiC substrate is crucial to obtain reproducible results. Previous studies on the 6H-SiC(0001) surface have determined several reconstructions, including the (√ 3× √ 3)-R30 • and the (3×3). Here, we introduce a process of progressive Si enrichment that leads to the formation of two reconstructions, the giant (12×12) and the (4×8). From electron diffraction and tunneling microscopy completed by molecular dynamics simulations, we build models introducing a type of Si adatom bridging two Si surface atoms. Using these Si bridges, we also propose a structure for two other reconstructions, the (2 √ 3×2 √ 3)-R30 • and the (2 √ 3×2 √ 13). We show that five reconstructions follow each other with Si coverage ranging from 1 and 1.444 monolayer. This result opens the way to greatly improve the control of 6H-SiC(0001) at the atomic scale. read less

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

Abstract: Mono-elemental two-dimensional (2D) crystals (graphene, silicene, germanene, stanene, and so on), termed 2D-Xenes, have been brought to the forefront of scientific research. The stability and electronic properties of 2D-Xenes are main challenges in developing practical devices. Therefore, in this review, we focus on 2D free-standing group-IV graphene analogs (graphene quantum dots, silicane, and germanane) and the functionalization of these sheets with organic moieties, which could be handled under ambient conditions. We highlight the present results and future opportunities, functions and applications, and novel device concepts. read less

Abstract: The effect of abrasive shape on the three-body abrasion behaviors of monocrystalline silicon was investigated via molecular dynamics. The axial ratio of abrasive particle varied from 1.00 to 0.40 to mimic abrasive shape. It has been observed that the particle’s movement became sliding instead of rolling when the axial ratio was smaller than a critical value 0.46. In the abrasion process, the friction force and normal force showed an approximately sinusoid-like fluctuation for the rolling ellipsoidal particles, while the front cutting of particle caused that friction force increased and became larger than normal force for sliding particles. The phase transformation process was tracked under different particle’ movement patterns. The Si-II and Bct5 phase producing in loading process can partially transform to Si-III/Si-XII phase, and backtrack to original crystal silicon under pressure release, which also occurred in the abrasion process. The secondary phase transformation showed difference for particles’ rolling and sliding movements after three-body abrasion. The rolling of particle induced the periodical and inhomogeneous deformation of substrates, while the sliding benefited producing high-quality surface in chemical mechanical polishing (CMP) process. This study aimed to construct a more precise model to understand the wear mechanism benefits evaluating the micro-electro-mechanical systems (MEMS) wear and CMP process of crystal materials. read less

Abstract: Extensive high resolution transmission and scanning transmission electron microscopy observations were performed in In(Ga)N/GaN multi-quantum well short period superlattices comprising two-dimensional quantum wells (QWs) of nominal thicknesses 1, 2, and 4 monolayers (MLs) in order to obtain a correlation between their average composition, geometry, and strain. The high angle annular dark field Z-contrast observations were quantified for such layers, regarding the indium content of the QWs, and were correlated to their strain state using peak finding and geometrical phase analysis. Image simulations taking into thorough account the experimental imaging conditions were employed in order to associate the observed Z-contrast to the indium content. Energetically relaxed supercells calculated with a Tersoff empirical interatomic potential were used as the input for such simulations. We found a deviation from the tetragonal distortion prescribed by continuum elasticity for thin films, i.e., the strain in the rel... read less

Abstract: We have studied the formation of a “host-guest” type сarcerand with a double-layer silicene (DL-S) as a host and an unstable antiaromatic cyclobutadiene as a guest. By employing the methods of MM+, РМ3 and Monte-Carlo, there has been studied the positioning of molecules of cyclobutadiene in a DL-S depending on intercalate concentration and intercalation temperature. At that the deformation vibrations of the DL-S crystal grate do not exceed 0.017 nm, and the vibrations of the intercalate molecules do not exceed 0.025 nm which provides for configuration and conformation stability of the system. The silicene planes do not move relatively each other and the order of the silicon atoms between the planes remains the same – AB… (similar to the silicene single crystal). When initially heated from 0 to ~273 K, the systems energy grows gradually, then rises sharply between 273–300 K and 350–400 K, then, with the temperature growth, it reaches a plateau which proves its high stability up to ~420 K. In the temperature range 0–273 K there appears physical sorption while chemisorption is observed at higher temperature (~300 K) which is peculiar of π-π interactions of classical aromatic and quasiaromatic cyclic and heterocyclic systems. There have been calculated the UV-spectra of the DL-S depending on the intercalate concentration in terms of the modified Benes-Hilderbrand method. There has been shown that the association constant of the system studied is 380 l·mol –1 , with computation accuracy ≥ 0.977. read less

Abstract: Mechanical properties such as hardness and modulus of sodium borosilicate (NBS) glasses in irradiation conditions were studied extensively in recent years. With irradiation of heavy ions, a trend that the hardness of NBS glasses decreased and then stabilized with increase of dose has been reported. Variations in network structures were suggested for the decrease of hardness after irradiation. However, details of these variations in a network of glass are not clear yet. In this paper, molecular dynamics was applied to simulate the network variations in a type of NBS glass and the changes in hardness after xenon irradiation. The simulation results indicated that hardness variation decreased with fluence in an exponential law, which was consistent with experimental results. The origin of hardness decrease after irradiation might be attributed to the break of Biv-O links that could be derived from the (1) decrease of average coordinate number of boron, (2) decrease of Si-O-Biv bonds, and (3) increase of non-bridging oxygen. read less

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

Abstract: Strain variation within a nanoparticle plays a crucial role in tuning its properties. High Resolution Transmission Electron Microscopy (HRTEM) images of a nanoparticle supported on amorphous carbon film are used to determine the strain variation. Experimental measurements in this present study on a single crystalline silver nanoparticle exhibited an unexpected high strain variation. Generally, the influence of carbon film is not accounted for during interpretation of measured strain variation. However, experimental observations raise the question whether the supporting carbon film alters the measured strain variation. In order to address this, strain variation within a simulated Ag nanoparticle supported on an amorphous carbon is measured with varying film thicknesses. The results show that supporting carbon film thickness introduces an artefact leading to more strain variation than what is present within an unsupported nanoparticle. Moreover, the variation increases with increasing supporting carbon film thickness. This effect is more pronounced in a thinner nanoparticle. Without considering this influence, the interpretation of strain within a nanoparticle may introduce severe errors which in turn will affect the tunability of desirable properties for different applications. Since strain measurement depends on the accuracy of the atomic position, the interpretation of any result using the atomic position from HRTEM images of a nanoparticle should consider the influence of supporting film. read less

Abstract: Molecular-dynamics simulations for silicon, hydrogen- and hydroxyl-terminated silicon in contact with liquid water, at 220 and 300 K, display water-density ‘ordering’ along the laboratory z-axis, emphasising the hydrophobicity of the different systems and the position of this first adsorbed layer. Density of states (DOS) of the oxygen and proton velocity correlation functions (VACFs) and infrared (IR) spectra of the first monolayer of adsorbed water, calculated via Fourier transformation, indicate similarities to more confined, ice-like dynamical behaviour (redolent of ice). It was observed that good qualitative agreement is obtained between the DOS for this first layer in all systems. The DOS for the lower-frequency zone indicates that for the interface studied (i.e., the first layer near the surface), the water molecules try to organise in a similar form, and that this form is intermediate between liquid water and ice. For IR spectra, scrutiny of the position of the highest-intensity peaks for the stret... read less

Abstract: The shock-induced pore collapsing and hot spot formation processes of plastic bonded explosives are simulated by molecular dynamics. After shock loading, the temperature field, pressure field, particle velocity field, energy field, plastic work field, and plastic temperature field are calculated by using the virtual grid method. A set of microscopic parameters about the hot spot are evaluated, including the pore collapsing time, pore collapsing speed, plastic work, and hot spot radius. The physical models to describe the energy dissipation and temperature relaxation behaviors of the hot spot are developed. We find that the hot spot formation consists of three steps: pore collapsing, work-heat transition, and temperature relaxation. The pore collapsing speed is proportional to the piston speed, and the temperature relaxation time is proportional to the square of the hot spot radius. read less

Abstract: Due to similar atomic bonding and electronic structure to graphene, hexagonal boron nitride (h-BN) has broad application prospects such as the design of next generation energy efficient nano-electronic devices. Practical design and efficient performance of these devices based on h-BN nanostructures would require proper thermal characterization of h-BN nanostructures. Hence, in this study we have performed equilibrium molecular dynamics (EMD) simulation using an optimized Tersoff-type interatomic potential to model the thermal transport of nanometer sized zigzag hexagonal boron nitride nanoribbons (h-BNNRs). We have investigated the thermal conductivity of h-BNNRs as a function of temperature, length and width. Thermal conductivity of h-BNNRs shows strong temperature dependence. With increasing width, thermal conductivity increases while an opposite pattern is observed with the increase in length. Our study on h-BNNRs shows considerably lower thermal conductivity compared to GNRs. To elucidate these aspect... read less

Abstract: Composition graded nanowires (NWs) have attracted increasing research interest in the application of optoelectronic devices, due to their graded bandgaps caused by the changing composition. However, the thermal transport property of composition graded NWs is not clear, which is critical for their potential applications in electronics and thermoelectrics. In this Letter taking SiGe NW as an example, we explore the thermal transport property of composition graded NWs. Molecular dynamics simulations reveal that the thermal conductivities (κ) of the composition graded SiGe NWs can be reduced up to 57% compared with that of the corresponding SiGe NW with abrupt interfaces. The κ reduction stems from the shortening of phonon mean free paths due to the inhomogeneous composition distributions. The phonon wave packet propagation analysis reveals that the composition gradient can reflect more than 70% of the wave packet energy, and phonon localization is observed in the composition graded region. Our findings suggest a promising prospect of composition graded NWs in the use of thermoelectrics and high temperature coatings, where low thermal conductivity is expected.Composition graded nanowires (NWs) have attracted increasing research interest in the application of optoelectronic devices, due to their graded bandgaps caused by the changing composition. However, the thermal transport property of composition graded NWs is not clear, which is critical for their potential applications in electronics and thermoelectrics. In this Letter taking SiGe NW as an example, we explore the thermal transport property of composition graded NWs. Molecular dynamics simulations reveal that the thermal conductivities (κ) of the composition graded SiGe NWs can be reduced up to 57% compared with that of the corresponding SiGe NW with abrupt interfaces. The κ reduction stems from the shortening of phonon mean free paths due to the inhomogeneous composition distributions. The phonon wave packet propagation analysis reveals that the composition gradient can reflect more than 70% of the wave packet energy, and phonon localization is observed in the composition graded region. Our findings sugge... read less

Abstract: In this paper, molecular dynamics (MD) simulations and analytical calculations are performed to study the influence of the elastocaloric effect (ECE) on the piezoelectric potential of hexagonal boron nitride (BN) nanotubes. To take into account the ECE in the simulations and calculations, the adiabatic condition is required. To reach this goal, the heat transfer between the BN nanotubes and their environment is excluded in the present study. In MD simulations, we find a large ECE in BN nanotubes, which will make the temperature of the BN nanotubes greatly change after external loads are applied on them. Moreover, the piezoelectric and dielectric properties of BN nanotubes calculated from MD simulations are found to be strongly dependent on the temperature. The temperature-dependent piezoelectric and dielectric properties together with the ECE are thus considered in the analytical calculations of the piezoelectric potential of BN nanotubes. The obtained analytical results reveal that the large ECE in BN nanotubes will make the piezoelectric potential of BN nanotubes strongly depend on the loading path of external loads. Specifically, stretching a BN nanotube is found to be more efficient than compressing the nanotube to generate the piezoelectric potential. These results are expected to significantly expand the knowledge of the electromechanical behaviours of piezoelectric nanomaterials and provide important guidelines for the optimum design of piezotronics nanodevices. read less

Abstract: Grain boundaries in graphene are inherent in wafer-scale samples prepared by chemical vapor deposition. They can strongly influence the mechanical properties and electronic and heat transport in graphene. In this work, we employ extensive molecular dynamics simulations to study thermal transport in large suspended polycrystalline graphene samples. Samples of different controlled grain sizes are prepared by a recently developed efficient multiscale approach based on the phase field crystal model. In contrast to previous works, our results show that the scaling of the thermal conductivity with the grain size implies bimodal behavior with two effective Kapitza lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a Kapitza length that is an order of magnitude larger than that of the in-plane phonons. We also show that, to get quantitative agreement with the most recent experiments, quantum corrections need to be applied to both the Kapitza conductance of grain boundaries and the thermal conductivity of pristine graphene, and the corresponding Kapitza lengths must be renormalized accordingly. read less

Abstract: We have studied collisions between tetraphenylporphyrin cations and He or Ne at center-of-mass energies in the range 50-110 eV. The experimental results were interpreted in view of density functional theory calculations of dissociation energies and classical molecular dynamics simulations of how the molecules respond to the He/Ne impact. We demonstrate that prompt atom knockout strongly contributes to the total destruction cross sections. Such impulse driven processes typically yield highly reactive fragments and are expected to be important for collisions with any molecular system in this collision energy range, but have earlier been very difficult to isolate for biomolecules. read less

Abstract: We use equilibrium molecular dynamics simulations to determine the relative role of compositional and structural disorder in a phononic thermal conductivity reduction by studying three 50-50 SiGe alloy structures: ordered alloys, disordered alloys, and amorphous alloys, as well as pure amorphous Si and Ge structures for reference. While both types of disorders significantly reduce thermal conductivity, structural disorder is much more effective to this aim. The examination of phonon lifetimes in disordered alloys shows high values in a low frequency regime governed by Umklapp scattering that are reduced rapidly with increasing frequency following Rayleigh scattering behavior. The local properties analysis reveals that the structural disorder leads to elastic heterogeneities that are significantly larger than density heterogeneities, which is likely the key reason for amorphous semiconductor alloys having lower thermal conductivity than disordered alloys. Temperature dependence of thermal conductivity indi... read less

Abstract: Complex materials design is often represented as a black-box combinatorial optimization problem. In this paper, we present a novel python library called MDTS (Materials Design using Tree Search). Our algorithm employs a Monte Carlo tree search approach, which has shown exceptional performance in computer Go game. Unlike evolutionary algorithms that require user intervention to set parameters appropriately, MDTS has no tuning parameters and works autonomously in various problems. In comparison to a Bayesian optimization package, our algorithm showed competitive search efficiency and superior scalability. We succeeded in designing large Silicon-Germanium (Si-Ge) alloy structures that Bayesian optimization could not deal with due to excessive computational cost. MDTS is available at https://github.com/tsudalab/MDTS. Graphical Abstract read less

Abstract: Semiconductor nanowires are potential candidates for applications in quantum information processing, Josephson junctions, and field-effect transistors. Therefore, this study focused on the effects of covering a germanium nanowire (GeNW) with a single wall carbon nanotube (CNT) on the stress-strain diagram, failure points, and Young's modulus using molecular dynamics simulations. To describe the interactions between atoms in the system, we used Tersoff potential. Also, a Nose-Hoover thermostat was employed to control temperature of the system. The stress-strain curves of GeNW and GeNW inside CNT (CNT-GeNW) were obtained at various temperatures, radii, and strain velocities. It was found that coverage of GeNW with CNT led to 2–6 fold improved Young's modulus. It was also determined that a significant part of the Young's modulus in CNT-GeNW is due to the presence of CNT. Moreover, we defined the behavior of Young's modulus of GeNW as well as CNT-GeNW in the [100], [110], and [111] crystallography direction a... read less

Abstract: Amorphous silicon oxycarbide is considered as a promising anode material for new generation of lithium-ion batteries, and figuring out the lithiation mechanism is crucial for its application. In this work, first principle calculations are performed to study the atomic structures, formation energy and lithiation voltage of homogeneous SiC2/5O6/5. The interpretation of radial distribution, angular distribution and coordinate number suggests that the Si–O bond tends to break and the Li2O will form at the beginning of lithiation, then the LixO and the LiySi form with increasing Li concentration, which makes a major contribution to the capacity of SiC2/5O6/5. By the Li content dependence of the formation energies curve, the theoretical specific capacity of SiC2/5O6/5 is predicted as 1415 mA h g−1, which is comparable to the reversible capacity of 900 mA h g−1 in experiments. Both the formation energies and the voltage curves suggest lithium is preferable in incorporation with SiC2/5O6/5, and this is attributed to the formation of LixO and LiySi. read less

Abstract: In the current chapter, the results of the optimized organic and inorganic materials features have been presented and discussed under the conditions when the material volumetric body and their surfaces have been structured. The dramatic change of the main characteristics of the inorganic matrix which surface has been modified with oriented carbon nanotubes and additionally treated by surface electromagnetic waves has been established. The transmittance and reflection spectral change, of the micro-hardness and of the wetting angle increase have been discussed due to the covalent bonding of the carbon nanotubes with the near-surface materials layers. Analytic and molecular dynamics simulations have supported the data. The essential change of the basic macro-parameters of the organic matrix, including the liquid crystal one, via their structuration with the nanoand/or bio-objects such as the fullerenes, carbon nanotubes, shungites, quantum dots, graphene oxides, DNA has been found. The spectral, photorefractive, and photoconductive characteristics modification has been discussed due to the drastic increase of the dipole moment. The laser-induced change of the refractive index has been considered as the indicator of the basic materials macro-parameters changing. It has been predicted that a scientific knowledge and the technology advances can be useful for the solar energy, display technique, for the system to absorb the gas and impurities, for the schemes with the compacted information recording as well as for the biomedicine. read less

Abstract: The formation of nanoscale voids in amorphous-germanium (a-Ge), and their size and shape evolution under ultra-fast thermal spikes within an ion track of swift heavy ion, is meticulously expatiated using experimental and theoretical approaches. Two step energetic ion irradiation processes were used to fabricate novel and distinct embedded nanovoids within bulk Ge. The ‘bow-tie’ shape of voids formed in a single ion track tends to attain a spherical shape as the ion tracks overlap at a fluence of about 1  ×  1012 ions cm−2. The void assumes a prolate spheroid shape with major axis along the ion trajectory at sufficiently high ion fluences. Small angle x-ray scattering can provide complementary information about the primary stage of void formation hence this technique is applied for monitoring simultaneously their formation and growth dynamics. The results are supported by the investigation of cross-sectional transmission and scanning electron micrographs. The multi-time-scale theoretical approach corroborates the experimental findings and relates the bow-tie shape void formation to density variations as a result of melting and resolidification of Ge within the region of thermal spike generated along an ion track, plus non-isotropic stresses generated towards the end of the thermal spike. read less

Abstract: One way to reduce the lattice thermal conductivity of solids is to induce additional phonon–surface scattering through nanostructures. However, the way in which phonons interact with surfaces, especially at the atomic level, is not well understood at present. In this work, we perform two-dimensional atomistic wave-packet simulations to investigate angular-resolved phonon reflection at a surface. Different surface morphologies, including smooth surfaces, periodically rough surfaces, and surfaces with amorphous coatings, are considered. For a smooth surface, mode conversion can occur after reflection, with the resulting wave-packet energy distribution depending on the surface condition and the polarization of the incident phonon. At a periodically rough surface, the reflected wave-packet distribution does not follow the well-known Ziman model but shows a nonmonotonic dependence on the depth of the surface roughness. When an amorphous layer is attached to a smooth surface, the incident wave packet is absorbe... read less

Abstract: Monocrystalline silicon is the foundation of the computer industry, so it has a great significance to study the ultra-high precision machining of silicon. Molecular dynamics has been proved as a very effective method for the study of ultra-precision machining in nanoscale. During the grinding of brittle materials in nano-level, there are some unique phenomena such as brittle-ductile transition. To study the machining mechanism in nanometric grinding of monocrystalline silicon, the subsurface damage of oriented Monocrystalline silicon under different grinding speeds were investigated by means of molecular dynamics simulations. The interactions between different atoms are described by the Morse and Tersoff potential. Based on analyzing the mechanism of diamond tool extrusion induced silicon lattice slip and distortion, the grinding process is explained. The movement of atoms and phase transformation are studied. The results show that there is not enough time for atoms beneath the tool to rearrange... read less

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

Abstract: Microporous carbide-derived carbons are an important structural class for various technological applications. We present two possible strategies based on molecular dynamics simulations for modeling microporous amorphous carbon. In addition, we have investigated the influence of the precursor structure and simulation parameters on the porosity of the final model structure. We observed a minor influence of the precursor structure on the porosity and found that the structural properties such as pore size and hybridization in the modeled carbon structures agree well with experimental findings. Moreover, CO2 adsorption isotherms have been simulated using Monte Carlo simulations for comparsion with experimental data. In this context, we have also considered partially oxidized carbon structures for which an increased uptake of CO2 was observed. read less

Abstract: Various theoretical and experimental methods are utilized to investigate the thermal conductivity of nanostructured materials; this is a critical parameter to increase performance of thermoelectric devices. Among these methods, equilibrium molecular dynamics (EMD) is an accurate technique to predict lattice thermal conductivity. In this study, by means of systematic EMD simulations, thermal conductivity of bulk Si-Ge structures (pristine, alloy and superlattice) and their nanostructured one dimensional forms with square and circular cross-section geometries (asymmetric and symmetric) are calculated for different crystallographic directions. A comprehensive temperature analysis is evaluated for selected structures as well. The results show that one-dimensional structures are superior candidates in terms of their low lattice thermal conductivity and thermal conductivity tunability by nanostructuring, such as by diameter modulation, interface roughness, periodicity and number of interfaces. We find that thermal conductivity decreases with smaller diameters or cross section areas. Furthermore, interface roughness decreases thermal conductivity with a profound impact. Moreover, we predicted that there is a specific periodicity that gives minimum thermal conductivity in symmetric superlattice structures. The decreasing thermal conductivity is due to the reducing phonon movement in the system due to the effect of the number of interfaces that determine regimes of ballistic and wave transport phenomena. In some nanostructures, such as nanowire superlattices, thermal conductivity of the Si/Ge system can be reduced to nearly twice that of an amorphous silicon thermal conductivity. Additionally, it is found that one crystal orientation, 100, is better than the 111 crystal orientation in one-dimensional and bulk SiGe systems. Our results clearly point out the importance of lattice thermal conductivity engineering in bulk and nanostructures to produce high-performance thermoelectric materials. Graphical Abstract read less

Abstract: In this study, a series of large-scale molecular dynamics simulations have been performed to study the nanometric cutting of single crystal silicon with a laser-fabricated nanostructured diamond tool. The material removal behavior of the workpiece using a structured diamond tool cutting is studied. The effects of groove direction, depth, width, factor, and shape on material deformation are carefully investigated by analyzing normal stresses, shear stress, von Mises stress, hydrostatic stress, phase transformation, cutting temperature, cutting force and friction coefficients. Simulation results show that a cutting tool groove orientation of 60° produces a smaller cutting force, less cutting heat, more beta-silicon phase, and less von Mises stress and hydrostatic stress. Moreover, tools with a smaller groove orientation, groove depth and groove width, and larger groove factor lead to more ductile cutting and an increased material removal rate. However, a cutting tool with a smaller groove width results in more heat during the nanoscale cutting process. In addition, the average temperature of the subsurface increases as groove factor increases, showing that a tool groove accelerates heat dissipation to the subsurface atoms. Furthermore, this V-shape groove cutting is shown to improve material removal ability in nanoscale cutting. read less

Abstract: It is possible to confine vibrational modes to a crystal by encapsulating it within thin disordered layers with the same average properties as the crystal. This is not due to an impedance mismatch between materials but, rather, to higher order moments in the distribution of density and stiffness in the disordered phase—i.e. it is a result of material substructure. The concept is elucidated in an idealized one-dimensional setting and then demonstrated for a realistic nanocrystalline geometry. This offers the prospect of specifically engineering higher order property distributions as an alternate means of managing phonons. read less

Abstract: Understanding the structural evolution of covalent systems under rapid cooling is very important to establish a comprehensive solidification theory. Herein, we conducted molecular dynamics simulations to investigate the crystallization of silicon-germanium (SiGe) alloys. It was found that during crystallization, the saturation and orientation of covalent bonds are satisfied in order, resulting in three phase transitions. The saturation is satisfied during a continuous phase transition that occurs in the super-cooled liquid state. When the orientation was satisfied at the local scale, a novel state, the critical-nuclei crystalline (CNC) phase was obtained, where the local diamond structures increase in number with time and ultimately stabilize at an average size at the critical value. Finally with a coordinated rearrangement of atoms, the orientation is satisfied globally and a stable diamond crystal is produced. For SiGe alloys this CNC phase is universal and rather stable, and the stable temperature range has a certain relationship with the cooling rate and number fraction of atoms. This novel pathway is believed to be universal for such materials including carbon. The CNC state can explain the observation that diamond can be obtained without high pressure. These findings will significantly advance the understanding of the mechanism of phase transition, particularly for covalently bonded materials. read less

Abstract: Silicon carbide (SiC) is a semiconductor with excellent mechanical and physical properties. We study the thermal transport in SiC by using non-equilibrium molecular dynamics simulations. The work is focused on the effects of twin boundaries and temperature on the thermal conductivity of 3C-SiC. We find that compared to perfect SiC, twinned SiC has a markedly reduced thermal conductivity when the twin boundary spacing is less than 100 nm. The Si–Si twin boundary is more effective to phonon scattering than the C–C twin boundary. We also find that the phonon scattering effect of twin boundary decreases with increasing temperature. Our findings provide insights into the thermal management of SiC-based electronic devices and thermoelectric applications. read less

Abstract: ATK-ForceField is a software package for atomistic simulations using classical interatomic potentials. It is implemented as a part of the Atomistix ToolKit (ATK), which is a Python programming environment that makes it easy to create and analyze both standard and highly customized simulations. This paper will focus on the atomic interaction potentials, molecular dynamics, and geometry optimization features of the software, however, many more advanced modeling features are available. The implementation details of these algorithms and their computational performance will be shown. We present three illustrative examples of the types of calculations that are possible with ATK-ForceField: modeling thermal transport properties in a silicon germanium crystal, vapor deposition of selenium molecules on a selenium surface, and a simulation of creep in a copper polycrystal. read less

Abstract: In this study, we investigated the plasma-induced damage in silicon trench etching. The damage was measured by detecting the dark current, which was a very small leakage current thermally generated from silicon crystal defects. The results indicated that both the amount and depth of the sidewall damage in the trenches were almost the same as those of the bottom damage. From the results of analysis and simulation, we considered that the crystal defects inducing isotropic damage were mainly formed by hydrogen ions diffusing in the silicon trenches. read less

Abstract: We investigate the electronic structure of a Palladium nanoparticle that is partially embedded in a matrix of silicon carbonitride. From classical molecular dynamics simulations we first obtain a representative atomic structure. This geometry then serves as input to density-functional theory calculations that allow us to access the electronic structure of the combined system of particle and matrix. In order to make the computations feasible, we devise a subsystem strategy for calculating the relevant electronic properties. We analyze the Kohn-Sham density of states and pay particular attention to d-states which are prone to be affected by electronic self-interaction. We find that the density of states close to the Fermi level is dominated by states that originate from the Palladium nanoparticle. The matrix has little direct effect on the electronic structure of the metal. Our results contribute to explaining why silicon carbonitride does not have detrimental effects on the catalytic properties of palladium particles and can serve positively as a stabilizing mechanical support. read less

Abstract: We compute the interfacial free energy of a silicon system in contact with flat and structured walls by molecular dynamics simulation. The thermodynamics integration method, previously applied to Lennard-Jones potentials [R. Benjamin and J. Horbach, J. Chem. Phys. 137, 044707 (2012)], has been extended and implemented in Tersoff potentials with two-body and three-body interactions taken into consideration. The thermodynamic integration scheme includes two steps. In the first step, the bulk Tersoff system is reversibly transformed to a state where it interacts with a structureless flat wall, and in a second step, the flat structureless wall is reversibly transformed into an atomistic SiO2 wall. Interfacial energies for liquid silicon-wall interfaces and crystal silicon-wall interfaces have been calculated. The calculated interfacial energies have been employed to predict the nucleation mechanisms in a slab of liquid silicon confined by two walls and compared with MD simulation results. read less

Abstract: The structures of the Si/Cu heterogenous interface impacted by a nanoindenter with different incident angles and depths are investigated in detail using molecular dynamics simulation. The simulation results suggest that for certain incident angles, the nanoindenter with increasing depth can firstly increase the stress of each atom at the interface and it then introduces more serious structural deformation of the Si/Cu heterogenous interface. A nanoindenter with increasing incident angle (absolute value) can increase the length of the Si or Cu extended atom layer. It is worth mentioning that when the incident angle of the nanoindenter is between −45° and 45°, these Si or Cu atoms near the nanoindenter reach a stable state, which has a lower stress and a shorter length of the Si or Cu extended atom layer than those of the other incident angles. This may give a direction to the planarizing process of very large scale integration circuits manufacture. read less

Abstract: The majority of intuition on phonon transport has been derived from studies of homogenous crystalline solids, where the atomic composition and structure are periodic. For this specific class of materials, the solutions to the equations of motions for the atoms (in the harmonic limit) result in plane wave modulated velocity fields for the normal modes of vibration. However, it has been known for several decades that whenever a system lacks periodicity, either compositional or structural, the normal modes of vibration can still be determined (in the harmonic limit), but the solutions take on different characteristics and many modes may not be plane wave modulated. Previous work has classified the types of vibrations into three primary categories, namely, propagons, diffusions, and locons. One can use the participation ratio to distinguish locons, from propagons and diffusons, which measures the extent to which a mode is localized. However, distinguishing between propagons and diffusons has remained a challe... read less

Abstract: We report on the thermal boundary resistances across crystalline and amorphous confined thin films and the thermal conductivities of amorphous/crystalline superlattices for Si/Ge systems as determined via non-equilibrium molecular dynamics simulations. Thermal resistances across disordered Si or Ge thin films increase with increasing length of the interfacial thin films and in general demonstrate higher thermal boundary resistances in comparison to ordered films. However, for films ≲3 nm, the resistances are highly dependent on the spectral overlap of the density of states between the film and leads. Furthermore, the resistances at a single amorphous/crystalline interface in these structures are much lower than those at interfaces between the corresponding crystalline materials, suggesting that diffusive scattering at an interface could result in higher energy transmissions in these systems. We use these findings, together with the fact that high mass ratios between amorphous and crystalline materials can... read less

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

Abstract: Tuning the surface boundary structures of crystal nanowires (NWs) is one of the most popular ways to control the thermal conductivity of NWs. In this paper, non-equilibrium molecular dynamic simulations (NEMD) and time domain normal mode analysis (TDNMA) are performed to investigate the thermal transport properties of Ar (length of 52.9 nm, width of 4.23 nm) and Ge (56.5 nm long and 4.52 nm wide) NWs with different surface boundary structures. Results show that both the total and modal thermal conductivity have a dramatic difference among the NWs with the same diameter (the maximal and minimal thermal conductivity of Ar (Ge) NWs are 1.25 (15.3) and 0.60 (5.64) W/mK, respectively), which suggests the efficiency of nanoscale thermoelectric devices and heat dissipation devices can be largely tuned by only changing the boundary orientation. The fundamental mechanism responsible for such a discrepancy is found to mainly originate from different phonon group velocity (combined with phonon relaxation time for Ar NWs). The results are quite helpful for understanding the boundary scattering effect on thermal transport and also facilitating the design of nanoscale devices by tailoring the boundary structures. read less

Abstract: We present a new force-field potential that describes the interlayer interactions in heterojunctions based on graphene and hexagonal boron nitride (h-BN). The potential consists of a long-range attractive term and a short-range anisotropic repulsive term. Its parameters are calibrated against reference binding and sliding energy profiles for a set of finite dimer systems and the periodic graphene/h-BN bilayer, obtained from density functional theory using a screened-exchange hybrid functional augmented by a many-body dispersion treatment of long-range correlation. Transferability of the parametrization is demonstrated by considering the binding energy of bulk graphene/h-BN alternating stacks. Benchmark calculations for the superlattice formed when relaxing the supported periodic heterogeneous bilayer provide good agreement with both experimental results and previous computational studies. For a free-standing bilayer we predict a highly corrugated relaxed structure. This, in turn, is expected to strongly alter the physical properties of the underlying monolayers. Our results demonstrate the potential of the developed force-field to model the structural, mechanical, tribological, and dynamic properties of layered heterostructures based on graphene and h-BN. read less

Abstract: Based on the Tersoff potential, molecular dynamics simulations have been performed to investigate the kinetic coefficients and growth velocities of Si (100), (110), (111), and (112) planes. The sequences of the kinetic coefficients and growth velocities are μ(100) > μ(110) > μ(112) > μ(111) and v(100) > v(110) > v(112) > v(111), respectively, which are not consistent with the sequences of the interface energies, interplanar spacings, and melting points of the four planes. However, they agree well with the sequences of the distributions and diffusion coefficients of the melting atoms near the solid–liquid interfaces. It indicates that the atomic distributions and diffusion coefficients affected by the crystal orientations determine the anisotropic growth of silicon. The formation of stacking fault structure will further decrease the growth velocity of the Si (111) plane. read less

Abstract: We perform computational experiments using nonequilibrium molecular dynamics simulations, showing that the interface between two solid materials can be described as an autonomous thermodynamic system. We verify the local equilibrium and give support to the Gibbs description of the interface also away from the global equilibrium. In doing so, we reconcile the common formulation of the thermal boundary resistance as the ratio between the temperature discontinuity at the interface and the heat flux with a more rigorous derivation from nonequilibrium thermodynamics. We also show that thermal boundary resistance of a junction between two pure solid materials can be regarded as an interface property, depending solely on the interface temperature, as implicitly assumed in some widely used continuum models, such as the acoustic mismatch model. Thermal rectification can be understood on the basis of different interface temperatures for the two flow directions. read less

Abstract: In this paper, normal and tangential coefficients of restitution for gaseous molecules colliding with layerwise (single and few layers) graphene nanosheets were calculated using non-equilibrium molecular dynamics method. The normal and tangential coefficients of restitution depend on impact angle, velocity and landing position of projectiles. These parameters were evaluated computationally by implementation of several operations. Some differences were observed between proposed problem and macro-scale nature. The most notable difference was proportionality between the restitution coefficients and impact velocity, while in the case of macro-scale systems, they have inverse relation. Number of layers of graphene substrate does not contribute significantly to the normal and tangential coefficients of restitution. On the other hand, except for hydrogen, where its normal coefficient of restitution is approximately unity, the normal coefficient of restitution for other gasses increases as impact angle increases. Furthermore, tangential coefficients of restitution reduce with impact angle. These results can offer additional insights for further understanding the impact mechanisms and bombardment-related phenomena in low-dimensional materials. Presented results could be utilised to optimise some applied operations such as porosity generation via bombardment of layered nanostructures, gas detection process, thermal management and pressure sensor applications. read less

Abstract: In this work, the phenomenon of the voltage generation is explored by using the molecular dynamics simulations, which is performed by driving a nano-sized droplet of room temperature ionic liquids moving along the monolayer graphene sheet for the first time. The studies show that the cations and anions of the droplet will move with velocity nonlinearly increasing to saturation arising by the force balance. The traditional equation for calculating the induced voltage is developed by taking the charge density into consideration, and larger induced voltages in μV-scale are obtained from the nano-size simulation systems based on the ionic liquids (ILs) for its enhanced ionic drifting velocities. It is also derived that the viscosity acts as a reduction for the induced voltage by comparing systems composed of two types of ILs with different viscosity and temperature. read less

Abstract: We perform a large-scale molecular dynamics simulation of nanoindentation on the (100), (110), and (111) oriented silicon surface to investigate the orientation-dependent mechanical behavior and phase transformation of monocrystalline silicon. The results show both the remarkable anisotropic mechanical behavior and structure phase transformation of monocrystalline silicon. The mechanical behavior of the (110) and (111) oriented surfaces are similar (has a high indentation modulus, low critical indentation depth for the onset of plastic deformation) but quite different from the (100) oriented surface. The mechanical behavior is carefully linked to the phase transformation. The formation of crystalline bct5 phase and β-Si phase is the fundamental phase transformation mechanism for (100) oriented surface. But, a large number of amorphous silicon can be found beneath the indenter for (110) and (111) oriented surface beside the bct5 phase and β-Si phase. The β-Si phase region is relatively small for (110) and ... read less

Abstract: The positioning of tricarbonyl(cyclopentadienyl)manganese molecules in double‐walled (5.5)@(10.10) carbon nanotubes depending on their concentration and temperature was studied using the methods of molecular dynamics, semi‐empirical quantum‐chemical parameterized model number 3 and Monte‐Carlo. The molecules were found to form stable bonds with the carbon nanotubes walls, with a tendency between intercalate stability and the carbon nanotubes structure. A temperature increase (above ˜460 K) causes gradual bond ruining followed by extrusion of interwall intercalate. Further temperature increase up to 600–750 K is characterised with intercalate external surface desorption, stabilising the whole system and keeping the interwall intercalate only. Double‐walled carbon nanotubes UV‐spectra depending on the intercalate concentration and association constant of the “double‐walled carbon nanotubes‐intercalate” system were calculated. A combination of unique optical, electrical and magnetic behaviour of cyclopentadienyl complexes with their ability to form high‐stable intercalate with carbon nanotubes opens a prospect of their application in nanotechnology. read less

Abstract: Calculations based on the density functional theory and empirical molecular dynamics are performed to investigate interlayer interaction, electronic structure and thermal transport of a bilayer heterostructure consisting of silicene and hexagonal boron nitride (h-BN). In this heterostructure, the two layers are found to interact weakly via a non-covalent binding. As a result, the Dirac cone of silicene is preserved with the Dirac cone point being located exactly at the Fermi level, and only a small amount of electrons are transferred from h-BN to silicene, suggesting that silicene dominates the electronic transport. Molecular dynamics calculation results demonstrate that the heat current along h-BN is six times of that along silicene, suggesting that h-BN dominates the thermal transport. This decoupled role of h-BN and silicene in thermal and electronic transport suggests that the BN-silicene bilayer heterostructure is promising for thermoelectric applications. read less

Abstract: For many decades, phonon transport at interfaces has been interpreted in terms of phonons impinging on an interface and subsequently transmitting a certain fraction of their energy into the other material. It has also been largely assumed that when one joins two bulk materials, interfacial phonon transport can be described in terms of the modes that exist in each material separately. However, a new formalism for calculating the modal contributions to thermal interface conductance with full inclusion of anharmonicity has been recently developed, which now offers a means for checking the validity of this assumption. Here, we examine the assumption of using the bulk materials' modes to describe the interfacial transport. The results indicate that when two materials are joined, a new set of vibrational modes are required to correctly describe the transport. As the modes are analyzed, certain classifications emerge and some of the most important modes are localized at the interface and can exhibit large conduc... read less

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

Abstract: In this effort, we examined the piezoelectric properties of the multi-walled boron nitride nanotubes (BNNTs), and the influence of the tube layer number on these properties using molecular dynamics (MD) simulations. The results reveal that the piezoelectric coefficient is positive for BNNTs with odd numbers of layers but negative for those with even numbers of layers. For both, odd and even cases, the magnitude of the piezoelectric coefficient is found to decrease with increasing layer number of BNNTs. A simple continuum mechanics model is developed and reveals that the observed layer number-dependent piezoelectric coefficient of the multi-walled BNNTs mainly originates from their unique inversely stacked structure. In addition, integrating the continuum mechanics model with the MD data leads to an explicit expression for the piezoelectric coefficient of multi-walled BNNTs, which can be easily used to predict the piezoelectric coefficient of BNNTs with various layer numbers. read less

Abstract: Carbon nanotubes/polyamide (PA) nanocomposite thin films have become very attractive as reverse osmosis (RO) membranes. In this work, we used molecular dynamics to simulate the influence of single walled carbon nanotubes (SWCNTs) in the polyamide molecular structure as a model case of a carbon nanotubes/polyamide nanocomposite RO membrane. It was found that the addition of SWCNTs decreases the pore size of the composite membrane and increases the Na and Cl ion rejection. Analysis of the radial distribution function of water confined in the pores of the membranes shows that SWCNT+PA nanocomposite membranes also exhibit smaller clusters of water molecules within the membrane, thus suggesting a dense membrane structure (SWCNT+PA composite membranes were 3.9% denser than bare PA). The results provide new insights into the fabrication of novel membranes reinforced with tubular structures for enhanced desalination performance. read less

Abstract: Understanding and controlling heat transport in molecular junctions would provide new routes to design nanoscale coupled electronic and phononic devices. Using first-principles full quantum calculations, we tune thermal conductance of a molecular junction by mechanically compressing and extending a short alkane chain connected to graphene leads. We find that the thermal conductance of the compressed junction drops by half in comparison to the extended junction, making it possible to turn on and off the heat current. The low conductance of the off state does not vary by further approaching the leads and stems from the suppression of the transmission of the in-plane transverse and longitudinal channels. Furthermore, we show that misalignment of the leads does not reduce the conductance ratio. These results also contribute to the general understanding of thermal transport in molecular junctions. read less

Abstract: GaAs/AlGaAs core–shell nanowires (NWs) were grown on Si(111) by Ga-assisted molecular beam epitaxy via the vapor–liquid–solid mechanism. High-resolution and scanning transmission electron microscopy observations showed that NWs were predominantly zinc-blende single crystals of hexagonal shape, grown along the [111] direction. GaAs core NWs emerged from the Si surface and subsequently, the NW growth front advanced by a continuous sequence of (111) rotational twins, while the AlGaAs shell lattice was perfectly aligned with the core lattice. Occasionally, single or multiple stacking faults induced wurtzite structure NW pockets. The AlGaAs shell occupied at least half of the NW’s projected diameter, while the average Al content of the shell, estimated by energy dispersive x-ray analysis, was x = 0.35. Furthermore, molecular dynamics simulations of hexagonal cross-section NW slices, under a new parametrization of the Tersoff interatomic potential for AlAs, showed increased atom relaxation at the hexagon vertices of the shell. This, in conjunction with the compressively strained Al0.35Ga0.65As shell close to the GaAs core, can trigger a kinetic surface mechanism that could drive Al adatoms to accumulate at the relaxed sites of the shell, namely along the diagonals of the shell’s hexagon. Moreover, the absence of long-range stresses in the GaAs/Al0.35Ga0.65As core–shell system may account for a highly stable heterostructure. The latter was consolidated by temperature-dependent photoluminescence spectroscopy. read less

Abstract: We investigate the structural, mechanical, and electronic properties of graphite-like amorphous carbon coating on bulky silicon to examine whether it can improve the durability of the silicon anodes of lithium-ion batteries using molecular dynamics simulations and ab-initio electronic structure calculations. Structural models of carbon coating are constructed using molecular dynamics simulations of atomic carbon deposition with low incident energies (1–16 eV). As the incident energy decreases, the ratio of sp2 carbons increases, that of sp3 decreases, and the carbon films become more porous. The films prepared with very low incident energy contain lithium-ion conducting channels. Also, those films are electrically conductive to supplement the poor conductivity of silicon and can restore their structure after large deformation to accommodate the volume change during the operations. As a result of this study, we suggest that graphite-like porous carbon coating on silicon will extend the lifetime of the silicon anodes of lithium-ion batteries. read less

Abstract: The present study employs the method of atomistic simulation to estimate the thermal stress experienced by Si/Ge and Ge/Si, ultrathin, core/shell nanowires with fixed ends. The underlying technique involves the computation of Young’s modulus and the linear coefficient of thermal expansion through separate simulations. These two material parameters are combined to obtain the thermal stress on the nanowires. In addition, the thermally induced stress is perceived in the context of buckling instability. The analysis provides a trade-off between the geometrical and operational parameters of the nanostructures. The proposed methodology can be extended to other materials and structures and helps with the prediction of the conditions under which a nanowire-based device might possibly fail due to elastic instability. read less

Abstract: Coherent phonon heat conduction has recently been confirmed experimentally in superlattice structures. Such traveling coherent phonon waves in superlattices lead to a linear increase in thermal conductivity as the number of periods increases. For applications such as thermal insulation or thermoelectrics, minimization of the phonon coherent effect is desirable. In this work, we use molecular dynamics simulations to study how to control coherent heat conduction in superlattices (SLs). It is found that either aperiodic SLs or SLs with rough interfaces can significantly disrupt coherent heat conduction when the interface densities are high. For sample thickness less than 125 nm, aperiodic SLs with perfect interfaces are found to have the lowest thermal conductivity. We use the atomic Green’s function method to examine the phonon dynamics. The impact of either aperiodicity or interface roughness is attributed to reduced transmittance. Such impact diminishes as the interface density reduces. read less

Abstract: Owing to the superior thermal conductivity of graphene, nanocomposites with graphene fillers dispersed in a polymer matrix become promising in thermal management applications, e.g. serving as thermal interface materials (TIMs) in high power microelectronic devices. However, the thermal conductivity of graphene-based nanocomposites is constrained by the high interfacial thermal resistance between the graphene fillers and polymer matrix. This research focuses on changing graphene–paraffin interfacial thermal transport by employing various treatment methods. Using molecular dynamics (MD) simulations, the effectiveness of hydrogenation, defecting and doping on reducing the graphene–paraffin interfacial thermal resistance is closely investigated. We found that the interfacial thermal resistance can be considerably reduced by the hydrogenation of graphene, while it is insensitive to defecting and doping. From the simulation results of the graphene–paraffin nanocomposites under tensile loading, a lower Young’s modulus and lower tensile strength are observed for the paraffin filled with hydrogenated graphene. The results clearly show that the hydrogenation of graphene exerts opposite effects on the thermal and mechanical properties of graphene–paraffin nanocomposites. Thus hydrogenation is suggested to be used wisely in the graphene–paraffin nanocomposite so as to improve its interfacial thermal conductance at the minimum cost of its mechanical strength. read less

Abstract: We performed MD simulations of nanoindentation on a single layer graphene nanoribbon in order to obtain the tip-position-dependent mechanical properties of the graphene nanoribbon. A correlation between the load and the indentation depth was constructed. Also, the bending rigidity, the Young’s modulus and the strength of the graphene nanoribbon were obtained. Our results yielded the tip position dependence of the Young’s modulus for the graphene nanoribbon. The Young’s modulus of the graphene nanoribbon was calculated by using the lowest value when the tip’s position was in the center region of the graphene nanoribbon, and as the tip’s position moved away from the center, the Young’s modulus increased. These results gave interesting insight as to why the Young’s moduli obtained from experiments were different from one another. The change in the mechanical properties of the graphene nanoribbon due to the change in the tip’s position can also lead to a novel graphene-based nanoelectromechanical device. read less

Abstract: This study reports the molecular dynamics simulation of multilayer graphene to investigate the influence of deflection, layers, temperature and stack patterns on the determination of Young’s modulus using nanoindentation models. Results reveal that the Young’s modulus of graphene is not consistent under different strain scopes, with the value being much larger in small deflection scopes. However, the values of Young’s modulus of multilayer graphene were rather consistent irrespective of the different stack patterns. Conversely, the relationship between Young’s modulus and temperature is almost linear in the region of 300–1000 K, with the value of Young’s modulus increasing proportionally with temperature. The derived value of Young’s modulus is about 1.00 TPa for multilayer graphene, in good agreement with monolayer graphene and close to the measured values of some experiments. read less

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

Abstract: Molecular dynamics (MD) simulations are performed to study the correlation between electron irradiation defects and applied stress in graphene. The electron irradiation effect is introduced by the binary collision model in the MD simulation. By applying a tensile stress to graphene, the number of adatom-vacancy (AV) and Stone–Wales (SW) defects increase under electron irradiation, while the number of single-vacancy defects is not noticeably affected by the applied stress. Both the activation and formation energies of an AV defect and the activation energy of an SW defect decrease when a tensile stress is applied to graphene. Applying tensile stress also relaxes the compression stress associated with SW defect formation. These effects induced by the applied stress cause the increase in AV and SW defect formation under electron irradiation. read less

Abstract: In heat assisted magnetic recording (HAMR) system, heating of the hard disk magnetic layer is carried out by applying laser rays during the movement of the read/write head over the carbon overcoat for the purpose of reading and writing on its magnetic layer. Depletion of PFPE Zdol occurs because of thermal decomposition and desorption on a DLC substrate due to laser heating and this model is developed using the coarse-grained bead spring based on quartic and van der Waals interaction potential. The effects of temperature on the bond breaking phenomenon of PFPE Zdol due to thermal decomposition and thermal desorption were studied. To support the reliability of the present simulation results by a quartic potential, the end bead density and total bead density on a DLC substrate obtained by the finitely extensible non-linear elastic (FENE) and quartic potential are shown in a comparative manner. read less

Abstract: Heat transport in one kind of double-bond linear chains of fullerenes (C60's) is investigated by the classical nonequilibrium molecular dynamics method. It is found that the negative differential thermal resistance (NDTR) is more likely to occur at larger temperature difference and shorter length. In addition, with the increase of the length, the thermal conductivity of the chains increases, and NDTR region shrinks and vanishes in the end. The temperature profiles reveal that a large temperature jump exists at a high-temperature boundary of the chains when NDTR occurs. These results may be helpful for designing thermal devices where low-dimensional C60 polymers can be used. read less

Abstract: The wettability of graphitic carbon and silicon surfaces was numerically and theoretically investigated. A multi-response method has been developed for the analysis of conventional molecular dynamics (MD) simulations of droplets wettability. The contact angle and indicators of the quality of the computations are tracked as a function of the data sets analyzed over time. This method of analysis allows accurate calculations of the contact angle obtained from the MD simulations. Analytical models were also developed for the calculation of the work of adhesion using the mean-field theory, accounting for the interfacial entropy changes. A calibration method is proposed to provide better predictions of the respective contact angles under different solid-liquid interaction potentials. Estimations of the binding energy between a water monomer and graphite match those previously reported. In addition, a breakdown in the relationship between the binding energy and the contact angle was observed. The macroscopic contact angles obtained from the MD simulations were found to match those predicted by the mean-field model for graphite under different wettability conditions, as well as the contact angles of Si(100) and Si(111) surfaces. Finally, an assessment of the effect of the Lennard-Jones cutoff radius was conducted to provide guidelines for future comparisons between numerical simulations and analytical models of wettability. read less

Abstract: In the present study, the effects of various types of strain and indium concentration on the total energy and optoelectronic properties of GaN nanowires (NWs) with embedded InxGa1−xN nanodisks (NDs) are examined. In particular, the bi-axial, hydrostatic, and uniaxial strain states of the embedded InxGa1−xN NDs are investigated for multiple In concentrations. Density functional theory is employed to calculate the band structure of the NWs. The theoretical analysis finds that the supercell-size-dependent characteristics calculated for our 972-atom NW models are very close to the infinite supercell-size limit. It is established that the embedded InxGa1−xN NDs do not induce deep states in the band gap of the NWs. A bowing parameter of 1.82 eV is derived from our analysis in the quadratic Vegard's formula for the band gaps at the various In concentrations of the investigated InxGa1−xN NDs in GaN NW structures. It is concluded that up to ∼10% of In, the hydrostatic strain state is competitive with the bi-axial ... read less

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

Abstract: The results of computer simulation studies of transition metal and silicon clusters published in the last decade are summarized. Comparative analysis of the stability and thermal evolution of nanoclusters is performed depending on the preparation method, type of bonds, atom packing, coherence of the constituent nanofragments, surface morphology and change in the relationship between the short- and long-range ordering with increasing size. Taking account of the substrate nature and dimensionality of the cluster disperse systems being simulated, most important structure-dependent kinetic and mechanical characteristics are discussed, including specific temperature ranges of disordering corresponding to isomerization and quasi-melting. The bibliography includes 263 references. read less

Abstract: This work investigates the nanoscale friction between diamond-structure silicon (Si) and diamond via molecular dynamics simulation. The interaction between the interfaces is considered as strong covalent bonds. The effects of load, sliding velocity, temperature and lattice orientation are investigated. Results show that the friction can be divided into two stages: the static friction and the kinetic friction. During the static friction stage, the load, lattice orientation and temperature dramatically affects the friction by changing the elastic limit of Si. Large elastic deformation is induced in the Si block, which eventually leads to the formation of a thin layer of amorphous Si near the Si-diamond interface and thus the beginning of the kinetic friction stage. During the kinetic friction stage, only temperature and velocity have an effect on the friction. The investigation of the microstructural evolution of Si demonstrated that the kinetic friction can be categorized into two modes (stick-slip and smooth sliding) depending on the temperature of the fracture region. read less

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

Abstract: A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively. read less

Abstract: We study Si-SiC core-shell nanowires by means of electronic structure first-principles calculations. We show that the strain induced by the growth of a lattice-mismatched SiC shell can drive a semiconductor-metal transition, which in the case of ultrathin Si cores is already observed for shells of more than one monolayer. Core-shell nanowires with thicker cores, however, remain semiconducting even when four SiC monolayers are grown, paving the way to versatile, biocompatible nanowire-based sensors. read less

Abstract: Directing the morphological evolution of one-dimensional materials in order to tune their properties for a variety of practical applications in optical sensing and solar cells is an ongoing effort. Here, we establish a systematic method for exerting control over the morphology of nanowires (NWs) grown via a vapour–solid–solid (VSS) process from different metal catalysts. We use germanium, a technologically important material, to demonstrate how catalysts influence the axial growth rate of a crystalline core against the lateral vapour deposition of an amorphous shell which in turn deforms the NWs into straight, tapered or spiral geometries due to interfacial stress. Finite element method (FEM) and molecular dynamic (MD) simulations are further utilized to confirm the proposed mechanism of deformation in crystalline–amorphous core–shell NWs. read less

Abstract: The electronic and transport properties of the (10, 0) single-walled carbon nanotube are studied by performing first-principles calculations and semi-classical Boltzmann theory. It is found that the (10, 0) tube exhibits a considerably large Seebeck coefficient and electrical conductivity which are highly desirable for good thermoelectric materials. Together with the lattice thermal conductivity predicted by non-equilibrium molecular dynamics simulations, the room temperature ZT value of the (10, 0) tube is estimated to be 0.15 for p-type carriers. Moreover, the ZT value exhibits strong temperature dependence and can reach to 0.77 at 1000 K. Such a ZT value can be further enhanced to as high as 1.9 by isotopic substitution and chemisorptions of hydrogen on the tube surface. read less

Abstract: Defect evolution in a single crystal silicon which is implanted with hydrogen atoms and then annealed is investigated in the present paper by means of molecular dynamics simulation. By introducing defect density based on statistical average, this work aims to quantitatively examine defect nucleation and growth at nanoscale during annealing in Smart-Cut® technology. Research focus is put on the effects of the implantation energy, hydrogen implantation dose and annealing temperature on defect density in the statistical region. It is found that most defects nucleate and grow at the annealing stage, and that defect density increases with the increase of the annealing temperature and the decrease of the hydrogen implantation dose. In addition, the enhancement and the impediment effects of stress field on defect density in the annealing process are discussed. read less

Abstract: In this study, the fracture of single-layered graphene sheets (SLGSs) with edge crack under simple tension is investigated using molecular dynamics simulations, and the variations in fracture strength of SLGSs with crack length, strain rate and temperature are analysed. It is found that the existing edge crack weakens mechanical properties of SLGSs. Fracture strength and strain decrease with the increase in crack length and temperature, but increase with the increase in strain rate. It is also shown that shorter initial cracks propagate faster than longer initial cracks, but shorter initial cracks begin propagating at higher axial strain at a certain temperature and strain rate. And cracks are found to propagate faster in higher strain rates. read less

Abstract: This report deals with studies concerning vacancy related defects created in silicon due to implantation of 200 keV per atom aluminium and its molecular ions up to a plurality of 4. The depth profiles of vacancy defects in samples in their as implanted condition are carried out by Doppler broadening spectroscopy using low energy positron beams. In contrast to studies in the literature reporting a progressive increase in damage with plurality, implantation of aluminium atomic and molecular ions up to Al3, resulted in production of similar concentration of vacancy defects. However, a drastic increase in vacancy defects is observed due to Al4 implantation. The observed behavioural trend with respect to plurality has even translated to the number of vacancies locked in vacancy clusters, as determined through gold labelling experiments. The impact of aluminium atomic and molecular ions simulated using MD showed a monotonic increase in production of vacancy defects for cluster sizes up to 4. The trend in damage... read less

Abstract: The stable positions, binding energies, and dynamic properties of Li impurity in the presence of a 90° partial dislocation in Si have been studied by using the multi-scale simulation method. The corresponding results are compared with the defect-free Si crystal in order to reflect how the dislocation defect affects the performances of Li-ion batteries (LIBs) at the atomic level. It is found that the inserted Li atom in the dislocation core and nearest regions is more stable, since the binding energies are 0.13 eV to 0.52 eV larger than the bulk Si. Moreover, it is easier for Li atom to diffuse into those defect areas and harder to diffuse out. Thus, Li dopant may tend to congregate in the dislocation core and nearest regions. On the other side, the 90° partial dislocation can glide in the {111} plane accompanied by the diffusion of Li impurity along the pentagon ring of core. In addition, the spacious heptagon ring of dislocation core can lower the migration barrier of Li atom from 0.63 eV to 0.34 eV, whi... read less

Abstract: Graphene on silicon with silicon dioxide quantum dots is a promising opto-electronic material. The optical band gap and the corresponding optical conductivity are estimated using the density functional approach with the combination of molecular dynamics. The regular repeating unit cell of graphene silicon nano-texture is identified using the classical molecular dynamics simulations. Electronic calculations predict the optical band gap is around 0.2 eV and the optical conductivity is identified to be 0.3 times the quantum conductance. read less

Abstract: Phononic computing is emerging as an alternative computing paradigm to the conventional electronic and optical computing. In this study, we propose and analyze various phononic interconnects, such as nano-scaled phononic resonators, waveguides and switches, on the 〈111〉 surface of 3C-SiC and 3C-GeSi with substitutional and vacancy defects. This is achieved by simultaneously introducing defects of various types, and by varying their specific locations on the surface. To calculate the intrinsic and the defect-induced vibrational properties, such as the phononic bandgap and the variation in the phonon spectra, the total phonon density of states (TPDOS) and the partial phonon density of states (PPDOS) were calculated using molecular dynamics simulations with semi-empirical potentials. The proposed phononic interconnects, in conjunction with electronic and/or photonic interconnects, can be used in the current and future devices. read less

Abstract: Molecular dynamics simulations were performed to study defect formation and transformation in graphene under electron irradiation. The single-vacancy was the most frequently formed defect and the number of defects did not depend on the defect formation energy for normal incidence. The single-vacancy transformed to other types of defects and migrated in graphene by heating. The recovery energies of adatom-vacancy and pentagon–heptagon defects were relatively small. The Stone–Wales defect was the most stable, and did not easily recover. In the single atomic chain formation process from graphene by electron irradiation, competition between defect formation by electron collision and the recovery by heating was observed. read less

Abstract: We report experimental total, absolute, fragmentation cross sections for anthracene C14H10, acridine C13H9N, and phenazine C12H8N2 ions colliding with He at center-of-mass energies close to 100 eV. In addition, we report results for the same ions colliding with Ne, Ar, and Xe at higher energies. The total fragmentation cross sections for these three ions are the same within error bars for a given target. The measured fragment mass distributions reveal significant contributions from both delayed (≫10(-12) s) statistical fragmentation processes as well as non-statistical, prompt (∼10(-15) s), single atom knockout processes. The latter dominate and are often followed by secondary statistical fragmentation. Classical Molecular Dynamics (MD) simulations yield separate cross sections for prompt and delayed fragmentation which are consistent with the experimental results. The intensity of the single C/N-loss peak, the signature of non-statistical fragmentation, decreases with the number of N atoms in the parent ion. The fragment intensity distributions for losses of more than one C or N atom are rather similar for C14H10 and C13H9N but differ strongly for C12H8N2 where weak C-N bonds often remain in the fragments after the first fragmentation step. This greatly increases their probability to fragment further. Distributions of internal energy remaining in the fragments after knockout are obtained from the MD simulations. read less

Abstract: Thermoelectric properties of three-dimensional covalently connected carbon networks are investigated by using first-principles calculation, Boltzmann transport theory, and nonequilibrium molecular dynamics simulations. It is found that the electronic transport of such networks exhibit "ballistic transport" behavior, similar to single carbon nanotubes. The thermoelectric performance of network structures is significantly enhanced relative to one-dimensional carbon nanotubes, owing to the high power factor and largely reduced thermal conductivity. The ZT value of carbon network (9,0) at intermediate temperature can be increased to 0.78 by n-type doping with a carrier concentration of 3.9 x 10(19) cm(-3). Therefore carbon networks are expected to be potential candidates for eco-friendly thermoelectric materials. read less

Abstract: In this article, molecular dynamics simulation was performed to study the heat transport in secondary particles chain of silica aerogel. The two adjacent particles as the basic heat transport unit were modelled to characterize the heat transfer through the calculation of thermal resistance and vibrational density of states (VDOS). The total thermal resistance of two contact particles was predicted by non-equilibrium molecular dynamics simulations (NEMD). The defects were formed by deleting atoms in the system randomly first and performing heating and quenching process afterwards to achieve the DLCA (diffusive limited cluster-cluster aggregation) process. This kind of treatment showed a very reasonable prediction of thermal conductivity for the silica aerogels compared with the experimental values. The heat transport was great suppressed as the contact length increased or defect concentration increased. The constrain effect of heat transport was much significant when contact length fraction was in the small range ( 0.5). Also, as the contact length increased, the role of joint thermal resistance played in the constraint of heat transport was increasing. However, the defect concentration did not affect the share of joint thermal resistance as the contact length did. VDOS of the system was calculated by numerical method to characterize the heat transport from atomic vibration view. The smaller contact length and greater defect concentration primarily affected the longitudinal acoustic modes, which ultimately influenced the heat transport between the adjacent particles. read less

Abstract: The thermoelectric properties of two typical SiGe nanotubes are investigated using a combination of density functional theory, Boltzmann transport theory, and molecular dynamics simulations. Unlike carbon nanotubes, these SiGe nanotubes tend to have gear-like geometry, and both the (6, 6) and (10, 0) tubes are semiconducting with direct band gaps. The calculated Seebeck coefficients as well as the relaxation time of these SiGe nanotubes are significantly larger than those of bulk thermoelectric materials. Together with smaller lattice thermal conductivity caused by phonon boundary and alloy scattering, these SiGe nanotubes can exhibit very good thermoelectric performance. Moreover, there are strong chirality, temperature and diameter dependences of the ZT values, which can be optimized to 4.9 at room temperature and further enhanced to 5.4 at 400 K for the armchair (6, 6) tube. read less

Abstract: It is shown by informatics that the high frequency short ranged modes exert a significant influence in impeding thermal transport through isotope substituted graphene nanoribbons. Using eigenvalue decomposition methods, we have extracted features in the phonon density of states spectra that reveal correlations between isotope substitution and phonon modes. This study also provides a data driven computational framework for the linking of materials chemistry and transport properties in 2D systems. read less

Abstract: Nanoelectromechanical systems (NEMSs) based on carbon nanotubes (CNTs) enjoy the considerable interest of researchers for various applications. In particular, CNT oscillators have attracted much attention for applications such as ultrafast optical filters and nanosensors. Here, we investigated the resonance frequencies of CNT oscillators with a telescoping outertube via molecular dynamics simulations. Their resonance frequencies could be engineered by changing the telescoping length of the outertube, and the distributions vs. the telescoping length could be fitted by using a normal distribution. While the maximum resonance frequencies were maintained with the same initial velocity, they could be changed by adjusting the telescoping length of the outertube. Engineering the resonance frequency of CNT oscillators via telescoping outertube as a controllable parameter can provide an advanced feature for CNT oscillators to be utilized as NEMS components. read less

Abstract: Apolipoprotein E4 (Apo E4) is the major genetic risk factor in the causation of Alzheimer's disease (AD). In this study we utilize virtual screening of the world's largest traditional Chinese medicine (TCM) database and investigate potential compounds for the inhibition of ApoE4. We present the top three TCM candidates: Solapalmitine, Isodesacetyluvaricin, and Budmunchiamine L5 for further investigation. Dynamics analysis and molecular dynamics (MD) simulation were used to simulate protein-ligand complexes for observing the interactions and protein variations. Budmunchiamine L5 did not have the highest score from virtual screening; however, the dynamics pose is similar to the initial docking pose after MD simulation. Trajectory analysis reveals that Budmunchiamine L5 was stable over all simulation times. The migration distance of Budmunchiamine L5 illustrates that docked ligands are not variable from the initial docked site. Interestingly, Arg158 was observed to form H-bonds with Budmunchiamine L5 in the docking pose and MD snapshot, which indicates that the TCM compounds could stably bind to ApoE4. Our results show that Budmunchiamine L5 has good absorption, blood brain barrier (BBB) penetration, and less toxicity according to absorption, distribution, metabolism, excretion, and toxicity (ADMET) prediction and could, therefore, be safely used for developing novel ApoE4 inhibitors. read less

Abstract: Carbon is a surprising material in all respects. In this Review, we present results of metadynamics calculations for crystal structure prediction of novel carbon polymorphs with even and odd rings. So-called superhard graphite results from cold-compressing graphite. We review the results of molecular dynamics simulations on the graphite-to-diamond phase transition, that yields Oganov's M-carbon. The latter, like some of the modifications predicted by metadynamics, features five- and seven-membered odd rings. We identify the role of odd rings at times of phase nucleation, and we speculate on the preference for odd-membered rings over even motifs in a context of nucleation and phase growth. read less

Abstract: Using molecular dynamics (MD) simulation, we consider the stable structure of a partially unzipped carbon nanotube, in which a graphene nanoribbon is formed at the tip. We characterize the shape of the junction between a single carbon nanotube and a graphene nanoribbon using three parameters: the radius of curvature, bend, and twist-rotation. The increase in the radius of curvature is proportional to the square of the distance from the boundary between the carbon nanotube and the graphene nanoribbon, and this can be explained by using continuous mechanics for a thin plate. The oscillations of the graphene nanoribbon at room temperature are also taken into consideration. read less

Abstract: Synergizing graphene on silicon based nanostructures is pivotal in advancing nano-electronic device technology. A combination of molecular dynamics and density functional theory has been used to predict the electronic energy band structure and photo-emission spectrum for graphene-Si system with silicon as a substrate for graphene. The equilibrium geometry of the system after energy minimization is obtained from molecular dynamics simulations. For the stable geometry obtained, density functional theory calculations are employed to determine the energy band structure and dielectric constant of the system. Further the work function of the system which is a direct consequence of photoemission spectrum is calculated from the energy band structure using random phase approximations. read less

Abstract: In this work, we examine the vibrational and mechanical properties of clamped‐clamped rectangular SixGe1−x and Si/SixGe1−x nanowires (NWs) using molecular dynamics simulations. A virtual atomic force microscope nanotip is used to drive the vibration. The frequency response, the beat vibration phenomenon and the calculation of mechanical properties such as quality factor Q and Young's modulus E are thoroughly analyzed. The influence of added mass and hydrogen passivation on the NW resonator performance is also demonstrated. The decreasing frequency trend with increasing Ge concentration was different for binary alloys and alloy superlattices, while it remained unaffected by the superlattice period. The beat vibration phenomenon driven by a single excitation was observed at elevated temperatures for all studied configurations. The resonance frequency decreases linearly with increasing temperature whereas the Q‐factor follows a power law decrease. The Young's modulus E obtained through stress–strain computational experiments is found to be overestimated compared to the classical beam theory. Frequency decreases linearly as more atoms are added to the resonator. For low temperatures, the quality factor of composite Si/Ge resonators increased from 104 to 105 at T = 23 K, after performing hydrogen passivation on the NW free surfaces. read less

Abstract: Silicene, a graphene-like two-dimensional silicon, has attracted great attention due to its fascinating electronic properties similar to graphene and its compatibility with existing semiconducting technology. So far, the effects of temperature and strain rate on its mechanical properties remain unexplored. We investigate the mechanical properties of silicene under uniaxial tensile deformation by using molecular dynamics simulations. We find that the fracture strength and fracture strain of silicene are much higher than those of bulk silicon, though the Young's modulus of silicene is lower than that of bulk silicon. An increase in temperature decreases the fracture strength and fracture strain of silicene significantly, while an increase in strain rate enhances them slightly. The fracture process of silicene is also studied and brittle fracture behavior is observed in the simulations. read less

Abstract: Effect of defects on negative differential thermal resistance (NDTR) is investigated using non-equilibrium molecular dynamics method for the symmetric graphene nanoribbons (GNRs) with defects. Remarkably, it is found that NDTR will disappear as the number of defects increases. In addition, heat current of GNRs with defects is larger than that of the pristine GNRs when the temperature difference is larger. The results are explained qualitatively utilizing the theory of phonon spectra mismatch. The work may be useful in designing thermal-nano devices on the basis of GNRs. read less

Abstract: To analyze the thermal transport properties in core-shell nanowires, we calculate systematically the distributions of heat flux in InAs/GaAs and GaAs/InAs core-shell nanowires by using nonequilibrium molecular dynamics simulations. The results show that for InAs/GaAs core-shell nanowires, the heat current tends to transport in the shell, while for GaAs/InAs core-shell nanowires the heat current tends to transport through the core. Moreover, a simple equation is presented to describe the relationship of the thermal conductance among the core, the tubular shell, and core-shell nanowire. It is suggested that the core-shell nanowires can be served as heat cable. read less

Abstract: A detailed theoretical model for thermoelectric transport perpendicular to the multilayers of a Si–Ge heterostructure is presented. The electronic structure of a three-dimensional superlattice, consisting of a regular array of Ge quantum dots in each layer, capped by Si layers, is calculated using an atomistic tight-binding approach. The Seebeck coefficient, the electric conductivity and the contribution of the electrons to the thermal conductivity for n-doped samples are worked out within Boltzmann transport theory. Using experimental literature data for the lattice thermal conductivity, we determine the temperature dependence of the figure of merit ZT. A nonlinear increase of ZT with temperature is found, with ZT > 2 at T = 1000 K in highly doped samples. Moreover, we find an enhanced thermoelectric power factor already at room temperature and below, which is due to highly mobile electrons in strain-induced conductive channels. read less

Abstract: We developed a classical potential to model phase-change materials based on the binary chalcogenide alloy of GeTe that are currently exploited for memory applications. Our potential is based on the recently proposed extension of the Tersoff potential plus additional terms to better reproduce the structure of the amorphous and the crystalline phases of GeTe. The parameters defining the potential reported in this work were fitted to reproduce the energies and forces of a database of reference structures obtained via density-functional theory molecular-dynamics simulations. This paper reports on the method used to construct the potential and on its validation against first-principles calculations either available in literature or part of this work. We found that the structural properties of amorphous GeTe were well reproduced. The advantage of the current implementation toward more flexible neural network-based methods is that most of the parameters can be reconnected to physical properties. Moreover, the relatively small number of parameters results in a simple implementation and facilitates the introductions of further interactions among additional species. read less

Abstract: Controlling the bandgap of carbon nanostructures is a key factor in the development of mainstream applications of carbon-based nanoelectronic devices. This is particularly important in the cases where it is desired that the carbon nanos- tructures are the active elements, as opposed to being conductive leads between other elements of the device. Here, we report density functional theory calculations of the effect of silicon impurities on the electronic properties of carbon nanotubes (CNTs). We have found that Si adatoms can open up a bandgap in intrinsically metallic CNTs, even when the linear density of Si atoms is low enough that they do not create an adatom chain along the tube. The bandgap opened in metallic CNTs can range up to approximately 0.47 eV, depending on adsorption site, on the linear density of Si adatoms, and on the chirality of the nanotube. We have found that a lower spatial symmetry of the charge transfer between adatom and CNT leads to a higher value of the bandgap opened, which indicates that the physical origin of the bandgap lies in the reduced spatial symmetry of the charge transferred. read less

Abstract: The rapid solidification of liquid silicon carbide (SiC) is studied by molecular dynamic simulation using the Tersoff potential. The structural properties of liquid and amorphous SiC are analyzed by the radial distribution function, angular distribution function, coordination number, and visualization technology. Results show that both heteronuclear and homonuclear bonds exist and no atomic segregation occurs during solidification. The bond angles of silicon and carbon atoms are distributed at around 109° and 120°, respectively, and the average coordination number is <4. Threefold carbon atoms and fourfold silicon atoms are linked together by six typical structures and ultimately form a random network of amorphous structure. The simulated results help understand the structural properties of liquid and amorphous SiC, as well as other similar semiconductor alloys. read less

Abstract: The formation dynamics of nano-sized hydrogenated Si (Si : H) clusters and their interaction with the Si(1 0 0) substrate have been investigated with molecular dynamics simulation using the Tersoff potential. Several nm-sized clusters comprising H atoms are obtained during rapid cooling of atomic Si and H vapour mixtures. Upon surface impingement, the Si : H clusters display higher degrees of deformation and atomic self-ordering than the Si clusters formed with no H atom incorporation. This is due to the movement of the high potential H atoms toward the cluster surface during deformation, not to the local heat generation by the atomic H recombination within the cluster. read less

Abstract: Two-dimensional Au islands of different thicknesses grown on graphene/Ru(0001) were used to study the corrugation of the moire structure of graphene/Ru(0001) and discriminate between its mainly structural or electronic character. A comparison of the apparent corrugation measured by scanning tunneling microscopy (STM) for different Au thicknesses with results of elasticity theory equations applied to a gold film over a corrugated substrate shows that the corrugation observed for the graphene/Ru(0001) moire is of structural nature rather than electronic. STM showed a large value for the corrugation of the first Au monolayer on graphene/Ru(0001), 1.7 A; using density functional theory calculations, we explain this large corrugation of the Au monolayer as the result of a strong (weak) binding of the Au layer at the valley (hill) regions of the graphene/Ru(0001) moire structure and infer an actual corrugation of the graphene/Ru(0001) moire structure of ∼1.2 A from the measured corrugation of the Au monolayer. read less

Abstract: Molecular dynamics simulations have been performed to study the correlation between electron irradiation defects and applied stress in single-walled carbon nanotubes. The electron irradiation effect is modeled based on the Monte Carlo method using binary collision theory. The simulations show that the number of knock-on defects is not significantly affected by applied stress. Electron collision causes early stage defects such as bond breaking and punched-out atoms. The applied stresses become nonequilibrium and concentrates around the defect site. The biased stresses become driving forces and cause bond shifts and rotations, which results in the formation of pentagon–heptagon pairs and Stone–Wales defects. read less

Abstract: In this paper, we report a non-equilibrium molecular dynamics study on the length-dependent lattice thermal conductivity of graphene with lengths up to 16 μm at room temperature. In the molecular dynamics simulations, whether the non-equilibrium systems reach the steady states is rigorously investigated, and the times to reach the steady states are found to drastically increase with the lengths of graphene. From the ballistic to the diffusive regime, the lattice thermal conductivities are explicitly calculated and found to keep increasing in a wide range of lengths with finally showing a converging behavior at 16 μm. That obtained macroscopic value of the lattice thermal conductivity of graphene is 3200 W/mK. read less

Abstract: Silicene is a monolayer of silicon atoms arranged in honeycomb lattice similar to graphene. We study the thermal transport in silicene by using non-equilibrium molecular dynamics simulations. We focus on the effects of tensile strain and isotopic doping on the thermal conductivity, in order to tune the thermal conductivity of silicene. We find that the thermal conductivity of silicene, which is shown to be only about 20% of that of bulk silicon, increases at small tensile strains but decreases at large strains. We also find that isotopic doping of silicene results in a U-shaped change of the thermal conductivity for the isotope concentration varying from 0% to 100%. We further show that ordered doping (isotope superlattice) leads to a much larger reduction in thermal conductivity than random doping. Our findings are important for the thermal management in silicene-based electronic devices and for thermoelectric applications of silicene. read less

Abstract: Ion tracks formed in amorphous Ge by swift heavy-ion irradiation have been identified with experiment and modeling to yield unambiguous evidence of tracks in an amorphous semiconductor. Their underdense core and overdense shell result from quenched-in radially outward material flow. Following a solid-to-liquid phase transformation, the volume contraction necessary to accommodate the high-density molten phase produces voids, potentially the precursors to porosity, along the ion direction. Their bow-tie shape, reproduced by simulation, results from radially inward resolidification. read less

Abstract: Metallic silicon nanowire and quantum dots are promising low dimensional materials for a great range of applications. A critical issue is their quality-controlled, cost-effective fabrication. This paper presents a simple method for making seamlessly integrated tunable metallic silicon nanowires and quantum dots in the subsurface of mono-crystalline silicon by mechanical scratching. The study predicted, with the aid of the molecular dynamics analysis, that arrays of stable metallic bct-5 silicon nanowires and conductive quantum dots could be produced in the subsurface of silicon by scratching the {001} surface along a ⟨110⟩ direction. The dimension and spacing of the nanowires and quantum dots can easily be controlled by adjusting the distance between scratching tips, the size of the tips, and their depth-of-cut. It was also shown that the metallic bct-5 silicon is stable under a residual octahedral shear stress of 5 to 8 GPa. read less

Abstract: We report molecular dynamics simulations of the nanomechanical properties and fracture mechanisms of β-Si3N4 thin layers in a prismatic plane under uniaxial tension. It is found that the thin layers in the y loading direction display a linear stress-strain relationship at ε < 0.021, and afterward, the stress increases nonlinearly with the strain until fracture occurs. However, for the z direction, the linear response is located at ε < 0.051. The calculated fracture stresses and strains of the thin layers increase with strain rates both in both directions. The thin layers exhibit the higher Young's modulus of 0.345 TPa in the z direction, higher than that in the y direction. The origins of crack derive from N(2c-1)-Si and N(6h-1)-Si bonds for the y and z loading directions, respectively. read less

Abstract: Results on stress analysis for single-crystal diamonds are presented. Isolated crystals were studied by Raman mapping and depth profiling techniques, using confocal microscopy. Diamonds were deposited on molybdenum and tantalum by hot filament and microwave CVD methods at growth rates between 10 and 30 μm·h-1. Crystals from 10 to 40 μm size were examined. Local stress was evaluated by analyzing the position, broadening and splitting of the 1332 cm-1 Raman peak in a 3D mapping. For the (001) orientation, the most stressed zone was found at the center of the crystal base, close to the interface with the substrate: a Raman peak around 1340 cm-1 was measured, corresponding to a pressure c.a. 3 GPa, according to our dynamical calculations. This peak disappears few microns out of the center, suggesting that this highly concentrated stress sector was the nucleation zone of the crystal. A shifting and slight broadening of the 1332 cm-1 band was observed in the rest of the crystal. The causes of these effects are discussed: they proved not to be due to anisotropic stress but to refractive effects. Same results were found for different crystal sizes and growth rates. read less

Abstract: Quantum dot triple junction solar cells (QD TJSCs) have potential for higher efficiency for space and terrestrial applications. Extended absorption in the QD layers can increase efficiency by increasing the short circuit current density of the device, as long as carrier extraction remains efficient and quality of the bulk material remains high. Experimental studies have been conducted to quantify the carrier extraction probability from quantum confined levels and bulk material. One studies present insight to the carrier extraction mechanisms from the quantum confined states through the use of temperature dependent measurements. A second study analyses the loss in carrier collection probability in the bulk material by investigating the change in minority carrier lifetimes and surface recombination velocity throughout the device. Recent studies for space applications have shown response from quantum structures to have increased radiation tolerance. The role strain and bonding strength within the quantum structures play in improving the radiation tolerance is investigated. The combination of sufficiently good bulk material and device enhancement from the quantum confinement leads to temperature dependent measurements that show TJSCs outperform baseline TJSCs near and above 60°C. Insight into the physical mechanisms behind this phenomenon is presented. read less

Abstract: Irradiation is known to bring new features in one-dimensional nano materials. In this study, we used molecular dynamics simulations to investigate the irradiation effects on twined SiC nanowires. Defects tend to accumulate from outside toward inside of the twined SiC nanowires with increasing irradiation dose, leading to a transition from brittle to ductile failure under tensile load. Atomic chains are formed in the ductile failure process. The first-principles calculations show that most of the atomic chains are metallic. read less

Abstract: The thermal conductivity of diameter and polytype modulated SiC nanowires is predicted using nonequilibrium molecular dynamics. For the polytype modulated nanowires, the two main SiC polytypes, zinc blende (3C) and wurtzite (2H) were considered. We show that the thermal conductivity of the diameter modulated nanowires may be even smaller than that of the constant diameter nanowire with the small section. This remarkable reduction in thermal conduction is attributed to a significant thermal boundary resistance displayed by the constriction, as measured by independent molecular-dynamics simulations. The constriction resistance is related to the confinement of low-frequency modes, as shown by vibrational density-of-states calculations. We used Monte Carlo simulations to conclude that the value of the constriction resistance may be explained by the specular reflections of this class of modes on the surface surrounding the constriction. read less

Abstract: Various methods have been used to study the thermal conductivity of nanocomposites which are playing increasing roles in energy conversion and thermal management. However, when the size of particle inclusions is on the order of several nanometers, the existing macro- and meso-scale analytical methods cannot be used to predict the thermal conductivity of nanocomposites due to the existence of both phonon wave interference and particle scattering effects. In this study, equilibrium molecular dynamics (EMD) is explored to study the thermal conductivity of Si/Ge nanocomposites. We found that EMD can be used to study the thermal conductivity of nanocomposites when multiple nanoparticles are included to avoid the artificial effect of simulation domain sizes. We then calculated the thermal conductivity of Si/Ge nanocomposites with different volumetric ratio and particle size at 300 K. The result shows that the thermal conductivity of Si/Ge nanocomposites first decreases and then increases with decreasing particl... read less

Abstract: We studied the nanoindentation of monolayer graphene by molecular dynamics simulations. It is found that the response of graphene to indentation is deflection dependent. In small deflection range, the response obeys point load model, while large-deflection indentation follows the sphere load model. Hence, we proposed to make sectional fittings and use different response models in different deflection ranges. In this way, a consistent Young's modulus is obtained that is almost independent of the size ratio of intender to graphene and the pretensions of graphene. The calculated Young's modulus is about 1.00 TPa, in good agreement with the experiments. read less

Abstract: Various models were previously used to predict interfacial thermal conductance of vertical carbon nanotube (CNT)-silicon interfaces, but the predicted values were several orders of magnitude off the experimental data. In this work, we show that the CNT filling fraction (the ratio of contact area to the surface area of the substrate) is the key to remedy this discrepancy. Using molecular dynamics, we have identified an upper limit of thermal interface conductance for C-Si interface which is around 1.25 GW/m2K, corresponding to a 100% filling fraction of carbon nanotube or graphene nanoribbon on substrate. By extrapolating to low filling fraction (∼1%) that was measured in experiments, our predicted interfacial thermal conductance agrees with experimental data for vertical CNT arrays grown on silicon substrate (∼3 MW/m2 K). Meanwhile, thermal rectification of more than 20% has been found at these C-Si interfaces. We observed that this is strongly dependent on the interfacial temperature drop than the fillin... read less

Abstract: The mechanical properties and failure mechanisms of the β-Si3N4 thin layers in basal plane under uniaxial tension are investigated by using molecular dynamics simulations. It is found that the thin layers display a nonlinear stress-strain relationship first at e < 0.06, and then a linear response at 0.06 < e < 0.09, and finally the stresses increase nonlinearly with the strains until fracture occurs. The fracture stresses and strains increase with increasing the side lengths of the thin layers, and the trend is same for Young's moduli accompanying little anisotropy. The deterioration in mechanical properties derives from the N6h-Si bonds where the fracture is initiated. read less

Abstract: In this work, we report a theoretical study on thermal conductivity of graphene nanoribbons by using molecular dynamics simulation. It is found that the thermal conductivity (κ) increases with the length (L) as, κ∝Lβ, even when the length is up to 600 nm. Moreover, thermal conductivities of curling and twisted graphene nanoribbons are investigated. In contrast to the obvious dependence on sample length, thermal conductivity is not sensitive to these types of geometry deformation due to the superior flexibility of graphenes. Our results predict that curling graphene nanoribbons may have advantages in suspended single-layer heat dissipation devices. read less

Abstract: Molecular dynamics simulations have been used to study the structural modification of graphene by electron irradiation. The authors used the Monte Carlo method to introduce the interaction between incident electrons and carbon atoms in graphene. Then, the effects of electron energy and incident angle on irradiation defects in single-layer graphene were studied, and the cutting of single-layer graphene using different methods of electron irradiation was compared. Following this, the authors simulated the process of single atom chain formation from single-layer graphene using electron irradiation. They also demonstrated the formation of three-dimensional structures, such as tubular structures and nanotube junctions, in bilayer graphene by electron irradiation. The simulations show the capability of structural modification of graphene to a variety of nanostructures by electron irradiation.Molecular dynamics simulations have been used to study the structural modification of graphene by electron irradiation. The authors used the Monte Carlo method to introduce the interaction between incident electrons and carbon atoms in graphene. Then, the effects of electron energy and incident angle on irradiation defects in single-layer graphene were studied, and the cutting of single-layer graphene using different methods of electron irradiation was compared. Following this, the authors simulated the process of single atom chain formation from single-layer graphene using electron irradiation. They also demonstrated the formation of three-dimensional structures, such as tubular structures and nanotube junctions, in bilayer graphene by electron irradiation. The simulations show the capability of structural modification of graphene to a variety of nanostructures by electron irradiation. read less

Abstract: Using ab initio and molecular dynamics simulations with semi-empirical potentials, the phonon density of states (PnDOS) of graphene with different types of defects such as substitution atoms (Si), carbon isotopes (12C and 14C), and vacancies was calculated. The main interest was to investigate the possibility to generate phononic band gaps (PBGs) in the PnDOS of graphene, since the derived structures may have sufficiently low thermal conductivity and find applications in improved thermoelectric materials. From all the studied defect types, the silicon substitution is the only one that creates PBGs. read less

Abstract: The engineering of nanostructured materials with very low thermal conductivity is a necessary step toward the realization of efficient thermoelectric devices. We report here the main results of an investigation with nonequilibrium molecular dynamics simulations on thermal transport in Si/Ge superlattice nanowires aiming at taking advantage of the inherent one dimensionality and the combined presence of surface and interfacial phonon scattering to yield ultralow values for their thermal conductivity. Our calculations revealed that the thermal conductivity of a Si/Ge superlattice nanowire varies nonmonotonically with both the Si/Ge lattice periodic length and the nanowire cross-sectional width. The optimal periodic length corresponds to an order of magnitude (92%) decrease in thermal conductivity at room temperature, compared to pristine single-crystalline Si nanowires. We also identified two competing mechanisms governing the thermal transport in superlattice nanowires, responsible for this nonmonotonic behavior: interface modulation in the longitudinal direction significantly depressing the phonon group velocities and hindering heat conduction, and coherent phonons occurring at extremely short periodic lengths counteracting the interface effect and facilitating thermal transport. Our results show trends for superlattice nanowire design for efficient thermoelectrics. read less

Abstract: In this paper we present the results of empirical potential and density functional theory (DFT) studies of models of interfaces between amorphous silicon (a-Si) or hydrogenated amorphous Si (a-Si:H) and crystalline Si (c-Si) on three unreconstructed silicon surfaces, namely (100), (110) and (111). In preparing models of a-Si on c-Si, melting simulations are run with classical molecular dynamics (MD) at 3000 K for 10 ps to melt part of the crystalline surface and the structure is quenched to 300 K using a quench rate of 6 × 10(12) K s(-1) and finally relaxed with DFT. Incorporating the optimum hydrogen content in a-Si to passivate undercoordinated Si, followed by DFT relaxation, produces hydrogenated amorphous silicon on crystalline surfaces, a-Si:H/c-Si. The (100) surface is the least stable crystalline surface and forms the thickest amorphous Si region, while the most stable (110) surface forms the smallest amorphous region. Calculated radial distribution functions (RDF) in the amorphous and crystalline layers are consistent with a-Si and c-Si and indicate a structural interface region one layer thick. The electronic density of states shows an evolution from c-Si to a-Si (or a-Si:H), with a larger electronic interface layer, suggesting that the electronic properties are more strongly perturbed by interface formation compared to the atomic structure. The computed optical absorption spectra show strong effects arising from the formation of different a-Si and a-Si:H regions in different Si surfaces. read less

Abstract: This study uses molecular dynamics simulation to examine the geometric criteria and stability of forming a perfect carbon nanotorus without pentagon-heptagon defects or surface buckles. Various nanotube diameters and nanoring diameters of both armchair and zigzag nanotori were relaxed at room temperature, and the equilibrated atomic configurations were inspected. This study uses the coordinate parameter, which illustrates the atomic arrangement around each atom, as an indicator of buckles to avoid misjudgment caused by transient or thermal disturbance. For each nanotube diameter, there exists a critical nanoring diameter beyond which the perfect carbon nanotori can form. This study examines the binding potential energy and deformation energy of the relaxed nanotorus model, showing that the critical nanoring diameter cannot be easily predicted through critical energy consideration because buckling is a form of structural instability. Results show that the structural stability of a perfect nanoring primarily depends on the nanotube diameter and nanoring diameter, whereas its chirality has little effect, and one empirical relation is fitted to determine the critical nanoring diameters. read less

Abstract: Using the nonequilibrium Green's function method and nonequilibrium molecular dynamics simulations, we discuss the possibility of using silicene nanoribbons (SiNRs) as high performance thermoelectric materials. It is found that SiNRs are structurally stable if the edge atoms are passivated by hydrogen, and those with armchair edges usually exhibit much better thermoelectric performance than their zigzag counterparts. The room temperature ZT value of armchair SiNRs shows a width-dependent oscillating decay, while it decreases slowly with increasing ribbon width for the zigzag SiNRs. In addition, there is a strong temperature dependence of the thermoelectric performance of these SiNRs. Our theoretical calculations indicate that by optimizing the doping level and applied temperature, the ZT value of SiNRs could be enhanced to as high as 4.9 which suggests their very appealing thermoelectric applications. read less

Abstract: Morphologies of AgI1–xBrx (0 ≤ x ≤ 1) nanocrystalline structures formed in carbon single-wall nanotubes (SWNT), of diameter d = 11.5–17.6 A, have been investigated by molecular dynamics simulation. For AgI1–xBrx in a (10, 10) carbon SWNT (d = 13.54 A), ionic motion characteristics at different temperatures have been studied. Calculations confirm the experimentally based suggestion that structural differences between AgI and AgBr in carbon SWNTs are less pronounced than in the bulk crystals. According to the simulation results, in tubes taken out from the melt, AgBr and AgI1–xBrx tend to form hexagonal nanotubes after annealing, similar to those formed by AgI. A superionic state, with significant silver ion mobility against a stable anion sublattice, can be observed in the simulated AgI1–xBrx@SWNT; the superionic conduction temperature range shifts downward with increasing bromine content. At temperatures below and just above the nanocrystal melting point, ion migration is faster in more bromine-rich AgI1–... read less

Abstract: Ge-Si core-shell nanowires with surface disorder are shown to be very promising candidates for thermoelectric applications. In atomistic calculations we find that surface roughness decreases the phonon thermal conductance significantly. On the contrary, the hole states are confined to the Ge core and are thereby shielded from the surface disorder, resulting in large electronic conductance values even in the presence of surface disorder. This decoupling of the electronic and phonon transport is very favorable for thermoelectric purposes, giving rise to promising room temperature figure of merits ZT > 2. It is also found that the Ge-Si core-shell wires perform better than pure Si nanowires. read less

Abstract: When a graphene sheet is transferred onto a Si-terminated SiC(0001) substrate, covalent Si–C bonds form at the interface. These interfacial bonds may form domains with a regular lattice pattern. The domain size is dictated by the lattice mismatch between graphene and its substrate. In the present work, a strategy of strain engineering is employed to eliminate the lattice mismatch to achieve a perfect epitaxy between the graphene and the substrate. Both molecular mechanics simulations and density function calculations confirm the perfect epitaxial pattern formed between a prestrained graphene sheet and its underlying silicon-carbide substrate. We further examine the edge effect on the disruption of the epitaxial pattern of prestrained graphene nanoribbons. It is found that that the perfect epitaxial pattern is disrupted only along the narrow regions near the nanoribbon edges, and the size of the affected region of the arm-chair ribbon is about 3 times larger than that of the zigzag ribbon. read less

Abstract: Molecular dynamics (MD) simulations based on the Tersoff potential have been developed to study the laser-induced crystallization of amorphous silicon (a-Si) film with ultrathin thickness to form size-controllable Si nano-dots. The influences of laser fluence and a-Si film thickness on the crystallization process were discussed. Classic nucleation theory was used to explain the results of the MD simulations. The constrain effect of a-Si films thickness on the formation of Si nano-dots was evaluated accordingly. read less

Abstract: Silicon carbide nanowires (NWs) are promising candidates for structural applications owing to their excellent mechanical, thermal, and electronic properties. The effect of amorphous carbon coatings on the mechanical behavior of the nanowires was studied via molecular dynamics methods at room temperature. The results show that the amorphous carbon coatings can shield opening cracks on silicon carbide nanowires, making them damage-tolerant. With increasing the defect size, the tensile strength and fracture energy of uncoated silicon carbide nanowires rapidly decrease; however, the properties of coated nanowires maintain nearly constant. Increasing the coating thickness leads to a brittle-to-ductile transition for the nanowires. Careful tailoring of the coatings permits engineering of these nanostructures for higher strength and damage tolerance at submicron scales. read less

Abstract: With the help of computer simulations we have studied the crystallization kinetics of amorphous silicon in solid phase epitaxial (SPE) and random nucleation growth processes. Our simulations employing classical molecular dynamics and first principles methods suggest qualitatively similar behavior in both processes. Pressure is found to reduce the difference in molar volumes and coordination numbers between the amorphous and crystalline phases, which in turn lowers the energy barrier of crystallization. The activation energy for the SPE growth of four coordinated diamond phase is found to reach a minimum (a maximum in growth rates) close to 10 GPa when its density becomes equal to that of the amorphous phase. The crystallization temperatures of successive high pressure phases of silicon are found to decrease, offering a possible explanation for the pressure induced crystallization reported in this material. read less

Abstract: Interaction between grain boundaries and radiation is studied in 3C-SiC by conducting molecular dynamics cascade simulations on bicrystal samples with different misorientation angles. The damage in the in-grain regions was found to be unaffected by the grain boundary type and is comparable to damage in single crystal SiC. Radiation-induced chemical disorder in the grain boundary regions is quantified using the homonuclear to heteronuclear bond ratio (χ). We found that χ increases nearly monotonically with the misorientation angle, which behavior has been attributed to the decreasing distance between the grain boundary dislocation cores with an increasing misorientation angle. The change in the chemical disorder due to irradiation was found to be independent of the type of the grain boundary. read less

Abstract: To clarify the growth mechanism of the lateral growth of Ge in the rapid-melting-growth process, two types of molecular-dynamics simulation were investigated in this study. One was the nucleation of Si1-xGex (0 ≤x ≤1) from supercooled melts, and the other is the growth rate of supercooled Si1-xGex melts using a crystalline Si1-xGex seed. The incubation time is found to be minimum at approximately 0.70 Tm (Tm: melting temperature for Si1-xGex). No nucleation was found when the temperature was higher than 0.75 Tm. The crystal growth rates of Si1-xGex peaked between 0.90 Tm and 0.94 Tm for both the [100] and [111] orientations. These results suggest that 0.90 Tm to 0.94 Tm of Si1-xGex (x = 1) is an optimum temperature range to grow crystalline Ge in the rapid-melting-growth process. read less

Abstract: This paper presents a novel physical phenomenon—heat wave propagation—at the atomic scale by investigating the collision of C60 molecules with a graphene sample through molecular dynamics (MD) simulation. A correlation between mechanical wave and temperature variation has been captured at the early stage of collision to demonstrate that temperature variations behave in a wave motion, which contradicts the concept in classical continuum mechanics, whereas later temperature variations exhibit the properties of a diffusion equation. This intriguing result, called wave-diffusion duality, offers an insight into the thermomechanical coupling phenomenon of nanodevices. DOI: 10.1061/(ASCE)NM.2153-5477.0000044. © 2012 American Society of Civil Engineers. CE Database subject headings: Nanotechnology; Wave propagation; Simulation; Diffusion; Temperature effects. Author keywords: Molecular dynamics; Graphene; Buckyball; Collision; Heat wave; Wave-diffusion duality. read less

Abstract: The intrusion process of a C60 ball into a sliding contact space with an included angle made up by two silicon substrates (100) was simulated using the molecular dynamics approach. The simulation was carried out using Tersoff potential of C and Si atoms at room temperature of 300 K. The included angle was defined as initial entry angle changing from 20° to 90° in the simulation for studying the effect of the initial entry angle during the intrusion process. The dependence of the initial entry angle on the number of sticking Si atoms of upper substrate was calculated. The results showed that the number of sticking atoms increased with the increasing of initial entry angle, and the number of sticking atoms was divided into three regions with different slopes, which could be used to evaluate the intrusion performance of a C60 ball into the sliding contact space. Copyright © 2011 John Wiley & Sons, Ltd. read less

Abstract: Understanding the structural stability of carbon nanostructure under heat treatment is critical for tailoring the thermal properties of carbon-based material at small length scales. We investigate the heat resistance of the single carbon nanoball (C60) and carbon nanoonions (C20@C80, C20@C80@C180, C20@C80@C180C320) by performing molecular dynamics simulations. An empirical many-body potential function, Tersoff potential, for carbon is employed to calculate the interaction force among carbon atoms. Simulation results shows that carbon nanoonions are less resistive against heat treatment than single carbon nanoballs. Single carbon nanoballs such C60 can resist heat treatment up to 5600 K, however, carbon nanoonions break down after 5100 K. This intriguing result offers insights into understanding the thermal-mechanical coupling phenomena of nanodevices and the complex process of fullerenes' formation. read less

Abstract: Ag defects in Σ3 grain boundary of SiC were analyzed to test the hypothesis that Ag release from TRISO fuel particles can occur through grain boundary diffusion. Although Σ3 grain boundaries cannot provide a connected path through the crystal, they are studied here to provide guidance for overall trends in grain boundary vs. bulk Ag transport. Formation energies of Ag defects are found to be 2 − 4 eV lower in the grain boundaries than in the bulk, indicating a strong tendency for Ag to segregate to the grain boundaries. Diffusion of Ag along Σ3 was found to be dramatically faster than through the bulk. At 1600 ◦ C, which is a temperature relevant for TRISO accident conditions, Ag diffusion coefficients are predicted to be 3 . 7 × 10 − 18 m 2 /s and 3 . 9 × 10 − 29 m 2 /s in the Σ3 grain boundary and bulk, respectively. While at this temperature Σ3 diffusion is still two orders of magnitude slower than diffusion estimated from integral release measurements, they values are close enough to suggest that grain boundary diffusion is a plausible mechanism for release of Ag from intact SiC coatings. The remaining discrepancies in the diffusion coefficients could be possibly bridged by considering high-energy grain boundaries, which are expected to have diffusivity faster than Σ3 and which provide a connected percolating path through polycrystalline SiC. } because of the special bulk-like structure of this GB provides an approximate lower bound on diffusion coefficient along GBs in SiC. The results were compared to bulk studies previously reported by Shrader et al . 10 . We found that for a typical grain diameter (0.8 µ m) of SiC used in TRISO application there is a strong segregation of Ag to the Σ3 GB. Specifically, we found that, based on Σ3 energies, the fraction of time that Ag spends in the GB is 99.999% at 1200 ◦ C and 99.988% at 1600 ◦ C. This strong segregation, combined with the slow diffusion in bulk SiC 10 , means that GB pathways will dominate diffusion of this fission product through a polycrystalline SiC (assuming no cracks or pores that enable faster are present). used formation neutral both in GB and in the bulk. read less

Abstract: Molecular dynamics simulation is performed to study electron-irradiation effects in carbon nanomaterials on substrates. The interaction between an incident electron and a carbon atom in target nanomaterials is introduced by the Monte Carlo method. Collisions of the backscattered electrons from the substrate are also introduced. The distributions of energy and the exit angle of backscattered electrons are calculated using Monte Carlo simulation of electron scattering in the substrate. Structural changes become more remarkable when the carbon nanomaterials are on the substrates. The threshold energy and the characteristics of structural changes by backscattered electrons are also discussed. read less

Abstract: Outstanding thermal transport properties of carbon nanotubes (CNTs) qualify them as possible candidates to be used as thermal management units in electronic devices. However, significant variations in the thermal conductivity (κ) measurements of individual CNTs restrict their utilizations for this purpose. In order to address the possible sources of this large deviation and to propose a route to solve this discrepancy, we systematically investigate the effects of varying concentrations of randomly distributed multiple defects (single and double vacancies, Stone-Wales defects) on the phonon transport properties of armchair and zigzag CNTs with lengths ranging between a few hundred nanometers to several micrometers, using both nonequilibrium molecular dynamics and atomistic Green's function methods. Our results show that, for both armchair and zigzag CNTs, κ converges nearly to the same values with different types of defects, at all lengths considered in this study. On the basis of the detailed mean free path analysis, this behavior is explained with the fact that intermediate and high frequency phonons are filtered out by defect scattering, while low frequency phonons are transmitted quasi-ballistically even for several micrometer long CNTs. Furthermore, an analysis of variances in κ for different defect concentrations indicates that defect scattering at low defect concentrations could be the source of large experimental variances, and by taking advantage of the possibility to create a controlled concentration of defects by electron or ion irradiation, it is possible to standardize κ with minimizing the variance. Our results imply the possibility of phonon engineering in nanostructured graphene based materials by controlling the defect concentration. read less

Abstract: The electronic transport of three kinds of ultrasmall single-wall carbon nanotubes are studied by using nonequilibrium Green’s function method. It is found that the transmission function displays a clear stepwise structure that gives the number of electron channels. The calculated power factor of these nanotubes can be optimized to much higher values in a wide temperature range. Using nonequilibrium molecule dynamics simulations, the lattice thermal conductivity of these nanotubes are predicated with quantum correction. Our calculations indicate that the (4,2) tube has relatively higher room temperature figure of merit (ZT value) compared with those of the (5,0) and (3,3) tubes. Moreover, the thermoelectric performance of these nanotubes can be greatly enhanced by surface design, formation of bundles, increasing the tube length, and so on, which significantly reduce the phonon and electron-derived thermal conductance. read less

Abstract: Mono-crystalline silicon experiences various phase transformations under different loading conditions. This paper reveals, with the aid of molecular dynamics simulations, that scratching the silicon {001} surface along the [110] direction under a load of 0.8 µN or more would produce stable 5 coordinated body centered tetragonal (bct-5) silicon in the subsurface. By examining the effect of this bct-5 silicon on indentation, it was found that the resistant to deformation of bct-5 silicon is higher than a-Si but lower than diamond Si. read less

Abstract: This study establishes that the effective thermal conductivity keff of crystalline nanoporous silicon is strongly affected not only by the porosity f ν and the system’s length Lz but also by the pore interfacial area concentration Ai. The thermal conductivity of crystalline nanoporous silicon was predicted using non-equilibrium molecular dynamics simulations. The Stillinger-Weber potential for silicon was used to simulate the interatomic interactions. Spherical pores organized in a simple cubic lattice were introduced in a crystalline silicon matrix by removing atoms within selected regions of the simulation cell. Effects of the (i) system length ranging from 13 to 130 nm, (ii) pore diameter varying between 1.74 and 5.86 nm, and (iii) porosity ranging from 8% to 38%, on thermal conductivity were investigated. A physics-based model was also developed by combining kinetic theory and the coherent potential approximation. The effective thermal conductivity was proportional to (1 − 1.5f ν) and inversely propor... read less

Abstract: This paper describes a practical framework for using multilayer feedforward neural networks to simultaneously fit both a function and its first derivatives. This framework involves two steps. The first step is to train the network to optimize a performance index, which includes both the error in fitting the function and the error in fitting the derivatives. The second step is to prune the network by removing neurons that cause overfitting and then to retrain it. This paper describes two novel types of overfitting that are only observed when simultaneously fitting both a function and its first derivatives. A new pruning algorithm is proposed to eliminate these types of overfitting. Experimental results show that the pruning algorithm successfully eliminates the overfitting and produces the smoothest responses and the best generalization among all the training algorithms that we have tested. read less

Abstract: In order to fulfill the stringent requirements for ultra-shallow junction formation and proper defect removal needed for future Si devices, molecular and cold implants have arisen as new technological strategies for dopant incorporation. In this work we have used different atomistic simulation techniques within a multiscale scheme to study the phenomena governing the damage generation in these types of implants, and to develop models that can help to optimize the fabrication of future Si devices. read less

Abstract: Classical molecular dynamics simulations have been performed for crystalline germanium with the aim to estimate the thermal effects within the first three coordination shells and their influence on the single-scattering and multiple-scattering contributions to the Ge K-edge extended x-ray absorption fine structure (EXAFS). read less

Abstract: Using molecular dynamics, we study the role of the surface facets of III–V nanowires on their failure during tensile deformation. We find that wurtzite (WZ) nanowires can withstand higher levels of stress and strain at failure compared to zinc-blende (ZB) nanowires. We observe that it is easier to nucleate a crack on a ZB nanowire due to the stress singularities that occur at the intersection of two opposing {111} facets. In WZ nanowires, we observe that cracks always nucleate at the intersection between two adjacent {112} surface facets. We explain these phenomena using fracture mechanics techniques based on energetics of crack formation. read less

Abstract: We present molecular and lattice dynamics calculations of the thermal conductivity of nanoporous silicon, and we show that it may attain values 10-20 times smaller than in bulk Si for porosities and surface-to-volume ratios similar to those obtained in recently fabricated nanomeshes. Further reduction of almost an order of magnitude is obtained in thin films with thickness of 20 nm, in agreement with experiment. We show that the presence of pores has two main effects on heat carriers: appearance of non-propagating, diffusive modes and reduction of the group velocity of propagating modes. The former effect is enhanced by the presence of disorder at the pore surfaces, while the latter is enhanced by decreasing film thickness. read less

Abstract: We investigated impact of type of crystal defects in multicrystalline Si (mc-Si) on electrical properties and their change after gettering process of impurities. A bundle of dislocations gives negative impact on the gettering process, while Σ3 grain boundaries does not affect at all. In addition, we categorized random grain boundaries in mc-Si by the contact angle between adjacent dendrite crystals to form the grain boundary. Change in the contrast of photoluminescence intensity around the grain boundary was found to systematically vary by the contact angle, which showed good correlation with calculated interface energy of the grain boundary. Grain boundaries with low interface energy are concluded to be preferable to weaken recombination activity by the gettering process and improvement of solar cell performance based on mc-Si. read less

Abstract: Thermal dissipation in molecular electronic devices is a critical issue for the proper functioning of such devices. In this work, molecular dynamics (MD) simulations were carried out to study the thermal energy transport in GaAs-SAM (self-assembly monolayer)-GaAs junctions, with alkanedithiols being the SAM molecules. In order to characterize the molecule-GaAs interface, ab initio density functional theory (DFT) was used to study the structural and binding properties of alkanethiolates on GaAs(001) surfaces. Parameters of classical potentials, which were used to model the molecule-GaAs interactions, were obtained by fitting to the results from the DFT calculations. Then, nonequilibrium MD (NEMD) simulations were performed to reveal the GaAs-SAM interfacial thermal conductance at different temperatures. The results from this work showed that the GaAs-SAM interfaces are the major sources of thermal resistance in the GaAs-SAM-GaAs junctions. The delocalized phonon modes carry thermal energy efficiently insid... read less

Abstract: Dispersing nanoparticles in a polymer can enhance both mechanical and transport properties. Nanocomposites with high thermal conductivity could be obtained by using thermally conductive nanoparticles. Carbon-based nanoparticles are extremely promising, although high resistances to heat transfer from the nanoparticles to the polymer matrix could cause significant limitations. This work focuses on graphene sheets (GS) dispersed within n-octane. Although pristine GS agglomerate, equilibrium molecular dynamic simulations suggest that when the GS are functionalized with short branched hydrocarbons along the GS edges, they remain well dispersed. Results are reported from equilibrium and non-equilibrium molecular dynamics simulations to assess the effective interactions between dispersed GS, the self-assembly of GS, and the heat transfer through the GS–octane nanocomposite. Tools are designed to understand the effect of GS size, solvent molecular weight and molecular architecture on GS dispersability and GS–octane thermal conductivity. Evidence is provided for the formation of nematic phases when the GS volume fraction increases within octane. The atomic-level results are input for a coarse-grained Monte Carlo simulation that predicts anisotropic thermal conductivity for GS-based composites when the GS show nematic phases. read less

Abstract: Analysis of ultrahigh frequency nanomechanical resonators, which are based on double-walled carbon nanotubes (DWCNTs) with various wall lengths, was carried out via classical molecular dynamics simulations. In the case of the inner wall entirely encapsulated inside the outer wall, the outer wall vibration has a significant effect on the vibration of the DWCNT; while in the case of the inner wall longer than the outer wall, the vibration of the extruded inner wall has a substantially stronger effect on the DWCNT vibration. It is shown that variations of the DWCNT resonance frequency with different wall lengths can be well fitted by Pearson VII and Gauss distribution functions. This result is potentially useful for developing design guidelines for making very fine tuners using DWCNT resonators of various wall lengths. read less

Abstract: Free energy calculation is a challenging problem in molecular dynamics (MD). Its key issue is to get entropy. However, entropy seems difficult to be obtained by MD directly, but requires support from other methods, such as phonon theory and cluster variation method, etc. In this work, we find a way to obtain free energy by a single MD task. It is derived from phonon theory and fluctuation theory first, and then we prove that it does not rely on phonon theory but can be independent. Finally, we use this method to study Si, find the results in agreement with experiment. read less

Abstract: The structure of aqueous sodium dodecyl sulfate (SDS) surfactant aggregates formed on small graphene sheets and graphene nanoribbons has been studied using all-atom molecular dynamics simulations. Because the edges of the carbonaceous supports confine laterally the surfactant aggregates, by changing the size of the support (diameter of graphene sheets and width of graphene nanoribbons) it is possible to investigate lateral confinement effects on the aggregate morphology. The results are compared to those available on graphite, with no lateral confinement. Aqueous SDS aggregates were studied on 2.0 nm, 5.0 nm, and 10.0 nm circular graphene sheets and on 2.0 and 5.0 nm wide graphene nanoribbons. For the first time our results show that, because of lateral confinement provided by the graphene edges, SDS yields multiple layers, hemispheres, hemicylinders or multiple hemispheres depending on the graphene size and shape. Results are quantified in terms of morphology of the surfactant aggregates, order parameter of the adsorbed surfactant aggregates, and number of water molecules at contact with the carbonaceous support. read less

Abstract: We discuss strain simulations of quantum dot structures covered with a GaSbAs strain reducing capping layer in the presence of Sb segregation. Cross Sectional Scanning Tunneling Microscopy shows strong Sb and In segregation in the material surrounding the quantum dot. Using the three layer model originally proposed for the SiGe system by D. J. Godbey, M. G. Ancona, J. Vac. Sci. Technol. A 15, 976 (1997) we accurately calculate the segregation profile and include a non uniform composition to our models. Using atomistic modeling, we present strain maps of the quantum dot structures that show the propagation of the strain into the GaAs region is strongly affected by the shape and composition of the strain reduction layer. read less

Abstract: The purpose of this article is to present a multiscale finite element method that captures nanoscale surface stress effects on the dynamic mechanical behavior of nanomaterials. The method is based upon arguments from crystal elasticity, i.e. the Cauchy–Born rule, but significantly extends the capability of the standard Cauchy–Born rule by accounting for critical nanoscale surface stress effects, which are well known to have a significant effect on the mechanics of crystalline nanostructures. We present the governing equations of motion including surface stress effects, and demonstrate that the methodology is general and thus enables simulations of both metallic and semiconducting nanostructures. The numerical examples on elastic wave propagation and dynamic tensile and compressive loading show the ability of the proposed approach to capture surface stress effects on the dynamic behavior of both metallic and semiconducting nanowires, and demonstrate the advantages of the proposed approach in studying the deformation of nanostructures at strain rates and time scales that are inaccessible to classical molecular dynamics simulations. Copyright © 2010 John Wiley & Sons, Ltd. read less

Abstract: In this work we extend classical molecular dynamics by coupling it with an electron transport model known as the two temperature model. This energy balance between free electrons and phonons was first proposed in 1956 by Kaganov et al. but has recently been utilized as a framework for coupling molecular dynamics to a continuum description of electron transport. Using finite element domain decomposition techniques from our previous work as a basis, we develop a coupling scheme that preserves energy and has local control of temperature and energy flux via a Gaussian isokinetic thermostat. Unlike the previous work on this subject, we employ an efficient, implicit time integrator for the fast electron transport which enables larger stable time steps than the explicit schemes commonly used. A number of example simulations are given that validate the method, including Joule heating of a copper nanowire and laser excitation of a suspended carbon nanotube with its ends embedded in a conducting substrate. Published in 2010 by John Wiley & Sons, Ltd. read less

Abstract: We present an extension to a constant-pressure molecular dynamics method for ellipsoidal finite systems that allows one to deal with nano-objects of cylindrical and cuboidal shapes. The method is based on the inclusion of a pressure x volume term in the system Lagrangian, where the volume is defined as a function of the eigenvalues of the inertia tensor. We illustrate how such a method works for selected systems, including CdSe nanocrystals and nanorods, carbon nanotubes and NaCl nanocrystals over a range of pressures. We also assess its performance in comparison with the use of an auxiliary pressure transmitting medium. read less

Abstract: We have investigated the thermal conductivity of various ice nanotubes (Ice-NTs) using the nonequilibrium molecular dynamics method. The results indicate that Ice-NTs have an unusually high thermal conductivity compared to that of the bulk ices. The thermal conductivity is sensitive to temperature, tube length and diameter, while being insensitive to polarization. We have also studied the confinement effect from single-walled carbon nanotubes (SWCNs). A very remarkable increase in the thermal conductivity is further observed after the Ice-NTs are confined in SWCNs. read less

Abstract: Indentation at very low load rate showed region of constant volume with releasing load in crystalline (c-)Si, indicating a direct observation of liquidlike amorphous phase which is incompressible under pressure. Signature of amorphization is also confirmed from load dependent indentation study where increased amount of amorphized phase is made responsible for the increasing elastic recovery of the sample with increasing load. Ex situ Raman study confirmed the presence of amorphous phase at the center of indentation. The molecular dynamic simulation has been employed to demonstrate that the effect of indentation velocities has a direct influence on c-Si during nanoindentation processes. read less

Abstract: The vibrational properties of various double-walled carbon nanotube resonators were investigated via a classical molecular dynamics approach. The fundamental frequencies of double-walled carbon nanotube resonators with short outer tubes were closely related to the intertube spacing and the chirality of the outer tubes. Even though the length of the outer tube was 1 nm, the vibration of the outer tube affected that of the inner tube. For nanotubes with similar intertube spacing, both maximum frequencies were similar. For their corresponding maximum frequencies, the lengths of the outer tubes of the zigzag nanotubes were slightly less than those of the armchair ones. read less

Abstract: The schematics of a gigahertz-range tuner is addressed as an application of a telescoping multi-walled carbon nanotube (CNT) that can be used repeatedly, and its dynamic operation is investigated via classical molecular dynamics simulations based on a (5,5)(10,10) double-walled CNT. Fine control of the telescoped length of the double-walled CNT enables its resonance frequency to be matched to one of the signal frequencies, and the telescoped nanotube can be tuned to its resonance frequency for use as a component of a bandpass filter. read less

Abstract: In this paper, the damping characteristics of epoxy resin containing aligned or randomly oriented carbon nanotube (CNT) ropes are investigated via a multiscale analysis approach. The shear strengths at the inter-tube and tube-resi n interfaces are calculated using molecular dynamics simulations of nanotube pullouts before being applied to a micromechanical damping model. In the micromechanical model, the composite is described as a three-phase system composed of a resin, a resi n sheath acting as a shear transfer zone, and a carbon nanotube rope. The concept of stick-slip motion is used to describe the load transfer behavior between carbon nanotubes in a rope as well as between nanotubes and the surrounding sheath. Both the energy dissipations f rom the viscoelastic polymer matrix and from the stick-slip motion are included in the over all structural damping characteristics. The effect of nanorope alignment on damping characteristics is also presented. Nomenclature 1 E = Young’s modulus in three-element standard solid model for viscoelastic resin 2 E = Young’s modulus in three-element standard solid model for viscoelastic resin eq E = equivalent Yong’s modulus for carbon nanotube as a solid cylinder rp f = volume fraction of carbon nanotube rope G = complex shear modulus of viscoelastic resin eq G = equivalent shear modulus for carbon nanotube as a solid cylinder K = bulk modulus of viscoelastic resin a L = length of composite unit cell nt L = length of carbon nanotube eff t s l - = effective length of SWNT/sheath debonding nt R = radius of carbon nanotube rp R = radius of carbon nanotube rope sh R = outer radius of the sheath a read less

Abstract: Equilibrium molecular dynamics combined with the Green-Kubo formula can be used to calculate the thermal conductivity of materials such as germanium and carbon. The foundation of this calculation is extracting the heat current from the results and implementing it into the Green-Kubo formula. This work considers all formulations from the literature that calculate the heat current for the Tersoff potential, the interatomic potential most applicable to semiconductor materials. The formulations for the heat current are described, and results for germanium and carbon are presented. The formulations are compared with respect to how well they capture the physics of the Tersoff potential and how well the calculated value of the thermal conductivity reflects the experimentally measured value. read less

Abstract: Recently, the best 65 nm Ge p-channel metal-oxide-semiconductor field-effect transistor (pMOSFET) performance has been reported with a standard Si complementary metal-oxide-semiconductor HfO 2 gate stack module. The Ge passivation is based on a thin, fully strained epitaxial Si layer grown on the Ge surface. We investigate in more detail how the device performance (hole mobility, I on , D it, V t , etc.) depends on the characteristics of this Si layer. We found that surface segregation of Ge through the Si layer takes place during the growth, which turns out to be determining for the interfacial trap density and distribution in the finalized gate stack. Based on a better understanding of the fundamentals of the Si deposition process, we optimize the process by switching to another Si precursor and lowering the deposition temperature. This results in a 4 times lower D it and improved device performance. read less

Abstract: Switching processes of carbon-based resistive memory cells are simulated on a fully atomistic level by the molecular dynamics (MD) method and the Extended-Hückel-Theory-based Non Equilibrium Green's Function (EHT-NEGF) method. Graphitic filament breakage and re-growth are found to be responsible for the switching of resistance of carbon-based memory. Key parameters that affect the switching speed of a memory cell are studied and trade-off between speed and power is discussed. read less

Abstract: Molecular dynamics simulations of Ar ion collision on a Si surface using an optimized potential function were carried out in the case of the acceleration voltages of 50keV for Ar ions. A hillock structure was formed by the Ar ion impact on the Si surface. The height of the structure calculated by the simulations corresponded to those of the experiments. The height of the structure was found to be proportional to the fluence of Ar ions. The amorphous structural region was expanded by the progress of the interface region between the amorphous structure and the crystalline structure with increasing the fluence in the depth direction. read less

Abstract: We report an unexpected characteristic of dislocation cores in silicon. Using first-principles calculations, we show that all of the stable core configurations for a nondissociated 60 degrees dislocation are sessile. The only glissile configuration, previously obtained by nucleation from surfaces, surprisingly corresponds to an unstable core. As a result, the 60 degrees dislocation motion is solely driven by stress, with no thermal activation. We predict that this original feature could be relevant in situations for which large stresses occur, such as mechanical deformation at room temperature. Our work also suggests that postmortem observations of stable dislocations could be misleading and that mobile unstable dislocation cores should be taken into account in theoretical investigations. read less

Abstract: Propagation of a heat pulse in (10,0) zig-zag carbon nanotubes, modeled by the Brenner-II and Tersoff bond-order potentials, respectively, is investigated using a molecular dynamics simulation. The longitudinal acoustic mode, twisting phonon mode, and second sound waves are observed in the simulation. The time variations of speed and intensity of the above three phonon modes are in good agreement with the previous works based on the Brenner-I potential. Higher speed and weaker peak intensity are observed in the simulation of the Tersoff potential. The inherent over-binding of radicals and the non-local effects in Tersoff's covalent-bonding formula may play an important role in the heat pulse propagating simulation. read less

Abstract: This paper explores the evolution mechanisms of metastable phases during the nanoindentation on monocrystalline silicon. Both the molecular dynamics (MD) and the in situ scanning spreading resistance microscopy (SSRM) analyses were carried out on Si(100) orientation, and for the first time, experimental verification was achieved quantitatively at the same nanoscopic scale. It was found that under equivalent indentation loads, the MD prediction agrees extremely well with the result experimentally measured using SSRM, in terms of the depth of the residual indentation marks and the onset, evolution and dimension variation of the metastable phases, such as β-Sn. A new six-coordinated silicon phase, Si-XIII, transformed directly from Si-I was discovered. The investigation showed that there is a critical size of contact between the indenter and silicon, beyond which a crystal particle of distorted diamond structure will emerge in between the indenter and the amorphous phase upon unloading. read less

Abstract: Owing to their outstanding mechanical, tribological, electronic transport, chemical and biocompatibility properties, the ultrananocrystalline diamond (UNCD) films grown by the microwave plasma chemical vapor deposition method under hydrogen-poor conditions have become the subject of intense research interests over the past decade. Based on the available computational capabilities and experimental data, a combined kinetic Monte Carlo (KMC) and molecular dynamics (MD) procedure has been developed for large-scale atomistic simulation of the responses of polycrystalline UNCD films under various loading conditions. The mechanical responses of resulting UNCD film have been investigated by applying displacement-controlled loading in the MD simulation box. Recently, a systematic study is being performed to understand the combined effects of grain size, loading rate, temperature, imperfection, loading path and history on the material strengths and failure patterns of both pure and nitrogen-doped UNCD films. Furthermore, recent MD simulation results of the notch size effect on the failure mechanism of nano-scale hierarchical structures consisting of one-dimensional members arranged in parallel will also be discussed to better design MEMS devices. read less

Abstract: Cluster expansions have proven to be a valuable tool in alloy theory and other problems in materials science but the generation of cluster expansions can be a computationally expensive and time-consuming process. We present a Bayesian framework for developing cluster expansions that explicitly incorporates physical insight into the fitting procedure. We demonstrate how existing methods fit within this framework and use the framework to develop methods that significantly improve the predictive power of cluster expansions for a given training set size. The key to the methods is to apply physical insight and cross validation to develop physically meaningful prior probability distributions for the cluster expansion coefficients. We use the Bayesian approach to develop an efficient method for generating cluster expansions for low-symmetry systems such as surfaces and nanoparticles. read less

Abstract: Molecular dynamics simulations are presented that provide evidence for the existence of diluted interfaces in nanoglasses, which is a class of material that can be synthesized by consolidating glassy nanoparticles. By comparing simulations of a covalently bonded Ge nanoglass and a metallic CuZr nanoglass, we show that the delocalization of the excess free volume initially located within the interfaces depends on the flow strain of the material. Our results suggest that the density distribution within a nanoglass can be controlled by the initial particle size and the annealing conditions. Therefore, nanoglasses represent an alternative route for controlling the properties of glassy materials. read less

Abstract: The low energy deposition of silver cluster cations with 561 (±5) atoms on a cold fullerene covered gold surface has been studied bot h y scanning tunneling microscopy and molecular dynamics simulation. The s p cial properties of the C60/Au(111) surface result in a noticeable fixation of the clusters without a significant change of the cluster shape. Upon heati ng to room temperature we observe a flattening or shrinking of the cluster sa mples due to thermal activation. Similar changes were observed also for mass selecte d Ag clusters with other sizes. For comparison we also studied Ag islands of simila r size, grown by low temperature deposition of Ag atoms and subsequent a n ealing. A completely different behavior is observed with much broader si ze distributions and a qualitatively different response to annealing. [2] Stefanie Duffe, Lukas Patryarcha, Benedikt Sieb en, Chunrong Yin, Bernd von Issendorff, Michael Moseler, and Heinz Hövel, Finite size effect for the penetration of nanoscopi barriers in metal-carbon systems , submitted for publication Abstract: Room temperature (RT) nanostructure stability is of paramount importance for applications in catalysis [3,4], sensors [5] or mag netic recording devises [6]. Metal particles on carbon nanosubstrates are of par ticular interest [7,8]. Here, a well defined representative is studied. Size select d Ag309±3 clusters were softlanded on C60 films supported on Au(111) and graphite is studied . For one monolayer (ML) C60 on Au(111), the Ag clusters shrunk within minutes at RT. Two ML C60 on Au(111) or the use of HOPG below one ML C 60 stabilizes the clusters over days at RT. Supported by atomistic ca lculations these results reveal a finite-size effect for the penetration of a nanosco pi barrier in which ametal dimmer, still in contact with the metal cluster, ge ts into the attractive potential of the metal substrate. This lowers the energy barrier for an atom-by-atom cluster decay at RT. Similar effects will be important for ther nanosystems in technology or biology. Room temperature (RT) nanostructure stability is of paramount importance for applications in catalysis [3,4], sensors [5] or mag netic recording devises [6]. Metal particles on carbon nanosubstrates are of par ticular interest [7,8]. Here, a well defined representative is studied. Size select d Ag309±3 clusters were softlanded on C60 films supported on Au(111) and graphite is studied . For one monolayer (ML) C60 on Au(111), the Ag clusters shrunk within minutes at RT. Two ML C60 on Au(111) or the use of HOPG below one ML C 60 stabilizes the clusters over days at RT. Supported by atomistic ca lculations these results reveal a finite-size effect for the penetration of a nanosco pi barrier in which ametal dimmer, still in contact with the metal cluster, ge ts into the attractive potential of the metal substrate. This lowers the energy barrier for an atom-by-atom cluster decay at RT. Similar effects will be important for ther nanosystems in technology or biology. read less

Abstract: The use of multiscale modeling concepts and simulation techniques to study the destabilization of an ultrathin layer of oxide interface between a metal substrate and the surrounding environment is considered. Of particular interest are chemomechanical behavior of this interface in the context of a molecular-level description of stress corrosion cracking. Motivated by our previous molecular dynamics simulations of unit processes in materials strength and toughness, we examine the challenges of dealing with chemical reactivity on an equal footing with mechanical deformation, (a) understanding electron transfer processes using first-principles methods, (b) modeling cation transport and associated charged defect migration kinetics, and (c) simulation of pit nucleation and intergranular deformation to initiate the breakdown of the oxide interlayer. These problems illustrate a level of multi-scale complexity that would be practically impossible to attack by other means; they also point to a perspective framework that could guide future research in the broad computational science community. read less

Abstract: A detailed investigation of plastic relaxation onset in heteroepitaxial SiGe islands on Si(001) is presented. The strain field induced by a straight misfit-dislocation segment is modeled by finite-element-method (FEM) calculations in three dimensions, fully taking into account the interaction with the multifaceted free surfaces of realistic islands. The total elastic energies before and after the placement of a $60\ifmmode^\circ\else\textdegree\fi{}$ dislocation segment in the most favorable position are therefore evaluated by a full FEM approach, for different island sizes and compositions. The critical volumes with composition for inserting the dislocation are finally obtained and successfully compared with the data in a report by Marzegalli et al. [Phys. Rev. Lett. 99, 235505 (2007)], where experimental values are compared to a simpler approach. read less

Abstract: This paper investigates the scale effect of indenter tip radius on the deformation of silicon under nanoindentation using molecular dynamics simulation. It was found that with larger diamond tips a six-coordinated silicon phase different from β-silicon on loading and a diamond like crystal beneath the indenter on unloading would appear as a result of the indentation stressing. This is a new phenomenon that has not been observed previously. read less

Abstract: By means of Tersoff and Morse potentials, a three-dimensional molecular dynamics simulation is performed to study atomic force microscopy cutting on silicon monocrystal surface. The interatomic forces between the workpiece and the pin tool and the atoms of workpiece themselves are calculated. A screw dislocation is introduced into workpiece Si. It is found that motion of dislocations does not occur during the atomic force microscopy cutting processing. Simulation results show that the shear stress acting on dislocation is far below the yield strength of Si. read less

Abstract: We explore the frequency characteristics of triple-walled carbon nanotube (TWCNT) oscillators using molecular dynamics simulations. The fast Fourier transform results of the TWCNT oscillators show both primary (f1) and minor (f2) peaks. The frequency characteristics of TWCNT oscillators are closely related to the amplitude (A1) of the primary peak. Both f1 and f2 linearly increase as a function of A1 for A1−0.5<0.45, whereas f1 and f2 slightly decrease as a function of A1 for A1−0.5>0.45. f1 for all TWCNT oscillators is always less than the frequency of the double-walled CNT oscillators, while f2 is less than the operating frequencies of double-walled CNT oscillators for A1−0.5<0.3. As a function of A1−0.5, f2 was almost two times higher than f1. read less

Abstract: The deformation pathway of silicon induced by nanoindentation is investigated in detail at the atomic level using molecular dynamics. Due to the complex stresses associated with the directional loading along a specific crystallographic orientation, the initial Si I lattice is transformed into two different high-pressure phases, namely, Si II and BCT5-Si. The Si II phase, where atoms have the six nearest neighbors, is generated through the tetragonal deformation caused by the compressive loading along the [001] direction. In contrast, the BCT5-Si phase, where each silicon atom has the five nearest neighbors, is formed by flattening the initially stepped sixfold rings of the diamond lattice onto the (110) plane of the BCT lattice. These reconstructive transformations are accomplished only by adding additional bonds and do not involve any bond breaking. read less

Abstract: This letter reports the size dependence of twin formation energy in cubic SiC at the nanoscale, while the bulk value is nearly zero. Atomic edges surround the twin boundary of SiC nanowires, and they are responsible for the finite twin formation energy and its size dependence. Based on classical molecular statics calculations, the average formation energy of convex and concave edges is 73meV∕nm. Effectively, the twin formation energy is inversely proportional to the length of edges surrounding the twin boundary. Results of this letter make it possible to understand large separations of twin boundaries in SiC nanowires. read less

Abstract: Using molecular dynamics simulations with Tersoff reactive many-body potential for Si-Si, Si-C, and C-C interactions, we have calculated the thermal conductance at the interfaces between carbon nanotube (CNT) and silicon at different applied pressures. The interfaces are formed by axially compressing and indenting capped or uncapped CNTs against 2 x 1 reconstructed Si surfaces. The results show an increase in the interfacial thermal conductance with applied pressure for interfaces with both capped and uncapped CNTs. At low applied pressure, the thermal conductance at interface with uncapped CNTs is found to be much higher than that at interface with capped CNTs. Our results demonstrate that the contact area or the number of bonds formed between the CNT and Si substrate is key to the interfacial thermal conductance, which can be increased by either applying pressure or by opening the CNT caps that usually form in the synthesis process. The temperature and size dependences of interfacial thermal conductance are also simulated. These findings have important technological implications for the application of vertically aligned CNTs as thermal interface materials. read less

Abstract: Based upon the Slater–Koster tight‐binding model and the gradient approximation, we investigate the linear optical properties of radially deformed single‐walled carbon nanotubes (SWNTs). It is found that, for the armchair tubes, the radial pressure may make the absorption peaks exhibited in the perfect tube shift to lower energy and eventually vanish, which is consistent with the experimental results observed in the optical spectra of squashed SWNT films. In addition, a lot of new and pressure‐ and orientation‐dependent absorption peaks emerge in the optical spectra of the radially deformed tubes. Finally, for comparison, the dependence of optical properties of deformed tubes on chirality is discussed simply. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) read less

Abstract: Molecular dynamics simulations and quantum transport theory are employed to study the temperature-dependent electrical properties of individual (12,0) zigzag and (5,5) armchair carbon nanotubes deposited on silicon substrates. The results demonstrate that the magnitude of the leakage current depends on the length of the nanotube. Furthermore, the leakage current is generated periodically along the length of the nanotube. Finally, the results indicate that given an appropriate value of the applied bias voltage, the induced current varies linearly with the temperature over specific temperature ranges. As a result, the temperature can be inversely derived from the measured current signal. Overall, the results show that the (12,0) zigzag and (5,5) armchair carbon nanotubes are suitable for temperature sensing applications over temperature ranges of 200–420 K and 300–440 K, respectively. read less

Abstract: We investigate the bending of nanometer thick Si cantilevers induced by chemisorption of H atoms and acetylene molecules, using atomistic simulations and continuum theories. We show that the bending curvature of Si nanocantilevers does not follow the classical Stoney formula, but agrees well with a modified Stoney formula that we have derived. Our studies reveal the dominant role of atomic structure and surface stress in governing the bending behavior of nanofilms, and demonstrate that the modified Stoney formula has to be used for the calibration of nanomechanochemical sensors in detecting trace amounts of molecules. read less

Abstract: The Molecular dynamics (MD) method was used to predict the thermal-stability and tensile properties of two single-walled Si nanotubes that are hydrogenated outside and both inside and outside respectively, i.e. the Sio–H and Siio–H nanotubes. Further, the axial-tensile properties of the two Si–H nanotubes were discussed by comparison with one (14,14) carbon nanotube. The obtained results show that: (1) the two Si–H nanotubes both have the Si skeletons with the structure similar to the {110} planes of single-crystal silicon, and they can stably exist only at the temperature lower than 200 and 125 K respectively and (2) the Sio–H and Siio–H nanotubes, respectively, have the tensile strength of 4.0 and 1.2 GPa as well as the fracture strain of 0.35 and 0.32; both their tensile strength and fracture strain are much lower than the corresponding ones of the (14,14) carbon-tube. read less

Abstract: An effective interatomic interaction potential for SiC is proposed. The potential consists of two-body and three-body covalent interactions. The two-body potential includes steric repulsions due to atomic sizes, Coulomb interactions resulting from charge transfer between atoms, charge-induced dipole-interactions due to the electronic polarizability of ions, and induced dipole-dipole (van der Waals) interactions. The covalent characters of the Si–C–Si and C–Si–C bonds are described by the three-body potential. The proposed three-body interaction potential is a modification of the Stillinger-Weber form proposed to describe Si. Using the molecular dynamics method, the interaction potential is used to study structural, elastic, and dynamical properties of crystalline (3C), amorphous, and liquid states of SiC for several densities and temperatures. The structural energy for cubic (3C) structure has the lowest energy, followed by the wurtzite (2H) and rock-salt (RS) structures. The pressure for the structural t... read less

Abstract: Fracture of silicon and germanium nanowires in tension at room temperature is studied by molecular dynamics simulations using several interatomic potential models. While some potentials predict brittle fracture initiated by crack nucleation from the surface, most potentials predict ductile fracture initiated by dislocation nucleation and slip. A simple parameter based on the ratio between the ideal tensile strength and the ideal shear strength is found to correlate very well with the observed brittle versus ductile behaviours for all the potentials used in this study. This parameter is then computed by ab initio methods, which predict brittle fracture at room temperature. A brittle-to-ductile transition (BDT) is observed in MD simulations at higher temperature. The BDT mechanism in semiconductor nanowires is different from that in the bulk, due to the lack of a pre-existing macrocrack that is always assumed in bulk BDT models. read less

Abstract: We demonstrate, by theoretical analysis and molecular dynamics simulation, a mechanism for fabricating nanotubes by self-bending of nanofilms under intrinsic surface-stress imbalance due to surface reconstruction. A freestanding Si nanofilm may spontaneously bend itself into a nanotube without external stress load, and a bilayer SiGe nanofilm may bend into a nanotube with Ge as the inner layer, opposite of the normal bending configuration defined by misfit strain. Such rolled-up nanotubes can accommodate a high level of strain, even beyond the magnitude of lattice mismatch, greatly modifying the tube electronic and optoelectronic properties. read less

Abstract: We present a methodology for the investigation of dislocation energetics in segregated alloys based on Monte Carlo simulations which equilibrate the topology and composition of the dislocation core and its surroundings. An environment-dependent partitioning of the system total energy into atomic contributions allows us to link the atomistic picture to continuum elasticity theory. The method is applied to extract core energies and radii of 60 degrees glide dislocations in segregated SiGe alloys which are inaccessible by other methods. read less

Abstract: The mechanisms of coalescence and T-junction formation of carbon nanotubes are analyzed using actionderived molecular dynamics. The control of kinetic energy in addition to the total energy leads to the determination of the minimum-energy atomistic pathway for each of these processes. Particularly, we find that the unit merging process of two carbon nanotubes consists of four sequential generalized Stone-Wales transformations occurring in four hexagon-heptagon pairs around the jointed part. In addition, we show that a single carbon atom may play the role of an autocatalyst, which significantly reduces the global activation energy barrier of the merging process. For T junction formation, two different models are chosen for simulation. One contains defects near the point of junction formation, while the other consists of two perfect nanotubes plus two additional carbon atoms. Our results indicate that the coalescence and junction formation of nanotubes may occur more easily than theoretically predicted in the presence of additional carbon atoms at moderate temperatures. read less

Abstract: A double-walled carbon nanotube (CNT) oscillator encapsulating a copper nanowire has been investigated using molecular dynamics simulations. Our simulation results show that the excess energy due to the interactions between the copper nanowire and the outer CNT were around 1% of the excess of van der Waals energy between the inner and the outer CNTs. The classical oscillation theory and the theory given by Zheng et al (2002 Phys. Rev. Lett. 88 045503) provide a fairly good estimate of the mass-dependent frequency of a CNT oscillator encapsulating a metal nanowire. The nanotube oscillator encapsulating a metal nanowire is found to be more dependent on the encapsulated metal mass than the metal–carbon interaction. read less

Abstract: This study uses molecular dynamics simulations to investigate the intrinsic thermal vibrations of a single-walled carbon nanotube (SWNT) modelled as a clamped cantilever. Using an elastic model defined in terms of the tube length, the tube radius and the tube temperature, the standard deviation of the vibrational amplitude of the tube’s free end is calculated and the Young’s modulus of the SWNT evaluated. The numerical results reveal that the value of the Young’s modulus is independent of the tube length, but decreases with increasing tube radius. At large tube radii, the Young’s modulus value approaches the in-plane modulus of graphene, which can be regarded as an SWNT of infinitely large radius. The results also indicate that the Young’s modulus is insensitive to changes in the tube temperature at temperatures of less than approximately 1100 K, but decreases significantly at higher temperatures. read less

Abstract: This paper deals with ab initio calculations relating to the atomic and electronic structure of the β-SiC(100)(3 × 2)–H surface. The results lead to the interpretation that electronic states associated with H atoms are responsible for a metallic behaviour, when saturation in the H deposition on the clean β-SiC(100)(3 × 2) is effected, a feature observed experimentally by Derycke et al. Although confirming the experimentally observed electronic behaviour, the present results differ as regards the H atom positions. Atomic structural properties were calculated and compared with previous ones available in the literature. read less

Abstract: Using a coarse molecular-dynamics (CMD) approach with an appropriate choice of coarse variable (order parameter), we map the underlying effective free-energy landscape for the melting of a crystalline solid. Implementation of this approach provides a means for constructing effective free-energy landscapes of structural transitions in condensed matter. The predictions of the approach for the thermodynamic melting point of a model silicon system are in excellent agreement with those of ``traditional'' techniques for melting-point calculations, as well as with literature values. read less

Abstract: The radiative recombination processes in dislocated float zone silicon samples deformed under gigapascal stresses were studied by photoluminescence (PL) spectroscopy. The observed shuffle dislocations present a reconstructed core and their generation is accompanied by the introduction of point defects and point defect clusters, whose signature is evident in the PL spectra. A broad band around 1eV is the only PL feature which could be directly related to shuffle dislocations and it is explained conjecturing strain field induced gap changes, as confirmed by molecular dynamics simulations. read less

Abstract: Structural phase transformations of silicon during nanoindentation were investigated in detail at the atomic level. The molecular dynamics simulations of nanoindentation on the (100) and (111) surface of single crystalline silicon were simulated, and this supported the theoretical prediction of the anisotropic behaviour of structural phase transformations. Simulations showed that microscopic aspects of phase transformation varied according to the crystallographic orientation of the contact surface and were directly linked to the slip system of silicon. In the transformed region along the centreline, the crystalline structure of Si-II and the amorphous structure were observed when silicon was loaded in the [100] and [111] directions, respectively. Simultaneously, metastable phases with fourfold coordination, such as Si-III and Si-XII, were formed by the inhomogeneous distortion in the slip direction of silicon and observed along the direction. Additionally, our results indicated that the deviatoric stress added to the hydrostatic pressure induced by loading was an indispensable factor for the structural phase transformation to Si-II during nanoindentation on the (100) surface. read less

Abstract: By means of thorough atomistic simulations an energy-based theory, named quantized fracture mechanics, is commented and validated. This approach modifies continuum linear elastic fracture mechanics by introducing the hypothesis of discrete crack propagation, taking into account the discreteness of the crystal lattice. We investigate at an atomistic level the crack energy resistance for a matrix of silicon carbide with an isolated crack, and the effect on the stress at the crack tip due to a second phase particle. In both cases our results show that, while atomistic simulations provide the most basic level of understanding of mechanical behavior in nanostructured brittle materials, quantized fracture mechanics is able to effectively incorporate the main lattice-related feature, thus enlarging the realm of continuum modeling. read less

Abstract: Unlike in conventional cutting processes, where the undeformed chip thickness is significant compared to the cutting tool edge radius, in nanoscale cutting processes, the undeformed chip thickness is very small, on the nanoscales. Therefore, the tool edge radius can not been ignored. It has been found that there is a brittle-ductile transition in cutting of brittle materials when the cutting tool edge radius is reduced to nanoscale and the undeformed chip thickness is smaller than the tool edge radius. In order to better understand the mechanism of the transition, a molecular dynamics (MD) method, which is different from continuous linear mechanics, is employed to model and simulate the nanoscale ductile mode cutting process of monocrystalline silicon wafer. The simulated variation of the cutting forces with the tool cutting edge radius is compared with the results of the cutting force from experimental cutting tests and they show a good agreement. In the simulated results, it can be seen that the thrust force is much larger than the cutting force in cutting. The simulated results also denote that the resultant force in the cutting process is not uniformly distributed along the cutting tool edge. In the simulation, the elastic springback of small thickness is observed on the machined workpiece surface. read less

Abstract: We investigated a carbon nanoribbon (CNR) using atomistic simulations based on Tersoff–Brenner potential function. The CNR was obtained from a compressed (5,5) carbon nanotube (CNT). The obtained CNR had a cross-sectional view as a binocular telescope structure composed of both sp2 and sp3 bonds. One carbon atom per ten carbon atoms had sp3 bond. For the optimized structures, the residual forces on the CNR were 3-order higher than that on the CNR and the lattice constant of the CNR was higher 0.0624 Å than that of the CNT along the tube axis. The Young's modulus of the CNR was the same as that of the CNT whereas the critical strain of the CNR was significantly lower than that of the CNT because the residual stresses on the CNR was very higher than those on the CNT. The tensile force curve vs. the strain of the CNT was slightly higher than that of the CNR. read less

Abstract: This paper utilizes molecular-dynamics simulations to investigate the mechanical characteristics of a suspended (10, 10) single-walled carbon nanotube (SWCNT) during atomic force microscopy (AFM) nanoindentation at different temperatures. Spontaneous topological transition of the Stone-Wales (SW) defects is clearly observed in the indentation process. The present results indicate that under AFM-bending deformation, the mechanical properties of the SWCNT, e.g., the bending strength, are dependent on the wrapping angle. In addition, it is also found that the radial dependence of the reduced formation energy of the SW defects is reasonably insensitive only for the small tubes. However, for tube diameters greater than 2.4 nm [corresponding to the (18, 18) CNT], the SW defects tend to be more radius sensitive. The results indicate that the bending strength decreases significantly with increasing temperature. This study also investigates the variation in the mechanical properties of the nanotube with the density of C60 encapsulated within the nanotube at various temperatures. It is found that, at lower temperatures, the bending strength of the C60-filled nanotube increases with C60 density. However, the reverse tendency is observed at higher temperatures. Finally, the "sharpest tip" phenomena between the probe and the tube wall and the elastic recovery of the nanotube during the retraction process are also investigated. read less

Abstract: Using molecular dynamics simulations, based on Tersoff potentials, we show that at typical experimental temperatures high compressive strain regimes suppress the formation of partial glide dislocations, while enhancing the gliding of the shuffle segments. Despite being qualitative in nature, these results suggest that strain relaxation in thin virtual substrates at high misfit may occur with a different modality than in thick graded layers, as indicated by preliminary experimental results by low-energy plasma enhanced chemical vapor deposition. read less

Abstract: We investigated the electroemission of endo-fullerenes from a carbon nanotube using classical molecular dynamics simulations based on the Tersoff–Brenner potential and Lennard-Jones potentials. The height of the potential energy barrier was closely related to the threshold electrostatic field intensity. The length of the empty nanospace inside the carbon peapod was important for the endo-fullerene to obtain the kinetic energy above the potential energy barrier. As the number of the endo-fullerenes increased, since the correlated collisions between the endo-fullerenes increased, the kinetic energy transfers between the endo-fullerenes were very important for the endo-fullerene electroemission from the carbon peapod. As the empty nanospace in the peapod increased, the electroemission of the endo-fullerenes from the peapod could be easily achieved in the low electrostatic field intensity. Both the initial configurations of the endo-fullerenes in the carbon peapod and the external electrostatic field intensity very closely affected the endo-fullerene electroemission from the carbon peapod. read less

Abstract: This study adopts a classical molecular dynamics (MD) simulation with the realistic Tersoff many-body potential model to investigate the mechanical properties of gallium nitride (GaN) nanotubes. The investigation focuses primarily on the mechanical properties of (n,0) and (n,n) GaN nanotubes since these particular nanotubes represent two extreme cases. The present results indicate that under small strain conditions, mechanical properties such as Young’s modulus are insensitive to the wrapping angle. Conversely, the wrapping angle has a significant influence upon these mechanical properties under large strain conditions. It is demonstrated that (9,0) GaN nanotubes are far less resistant to bond rotation. Under large tensile strain conditions, due to the unfavourable bond orientations induced by Stone–Wales (SW) transformation, the bonds in (n,0) GaN tubes quickly degenerate. Moreover, the present results suggest that the tensile strength of a nanotube is strongly sensitive to the temperature and strain rate. Regarding the fatigue test, this study uses a standard theoretical model to derive curves of amplitude stress versus number of cycles for the current nanotubes. The results demonstrate that the fatigue limit of GaN nanotubes increases with increasing temperature. read less

Abstract: The molecular dynamics (MD) simulation employing a Tersoff potential was performed to examine the nanomechanical behavior of the � SiC nanowire in tension. The elongation was much larger than that of the bulk � -SiC. We observed non-homogeneous deformation, and the fracture behavior was found to depend on size, orientation and temperature of the specimen. The Young’s modulus calculated in this study generally decreased with temperatures and increased with the radius, namely, the diameter of the � -SiC nanowire as long as the length scale remained the same. The initial orientation was found to have a more serious effect on the Young’s modulus than size and temperature. The [111] Young’s modulus is much higher than that of the [001] orientation. The fracture of the � -SiC nanowire in the [001] orientation showed two different modes, which is brittle at 100 K and ductile at 300 and 500 K. The ductile fracture was accompanied by formation of an atomic chain. In the [111] orientation, it was always fractured in the ductile mode and thus an atomic chain was formed before rupture. read less

Abstract: Molecular dynamics (MD) simulations have been carried out in order to investigate morphological changes in three different crystallographically oriented silicon surfaces during nanoindentation. Transformations to fivefold and sixfold coordinated structures are observed during the contact loading. However, the atoms are arranged in a complex mix of different local structures rather than forming a new crystalline phase. This region forms a thin layer of around 15 A under the contact area. The thermal vibration of the atoms is found to have an important role in the recovery of the damage region during tip retraction. read less

Abstract: The current understanding of the relaxation and reconstruction of low-index cubic SiC surfaces, as it derives from first-principles calculations, is briefly reviewed in comparison with surface-sensitive experimental data. The calculated structural properties are obtained from ab initio total energy and grand canonical potential minimization in the framework of the local density and generalized gradient approximations of density functional theory. Characteristic surface structural properties are related to the surface electronic structure and to the ionicity of the underlying bulk crystal. For a number of cubic surfaces, there is good agreement between first-principles results and the data. In other cases, most noticeably for Si-terminated SiC(001) surfaces, there is still considerable controversy with respect to the atomic and electronic structure in both experiment and theory. read less

Abstract: We have investigated the transitions between disordered phases in supercooled liquid silicon using computer simulations. The thermodynamic properties were directly obtained from the free energy, which was computed using the recently proposed reversible scaling method. The calculated free energies of the crystalline and liquid phases of silicon at zero pressure, obtained using the environment dependent interatomic potential, are in excellent agreement with the available experimental data. The results show that, at zero pressure, a weak first-order liquid-liquid transition occurs at 1135 K and a continuous liquid-amorphous transition takes place at 843 K. These results are consistent with the existence of a second critical point for the liquid-liquid transition at a negative pressure. read less

Abstract: The molecular-dynamics simulation approach [Morris and Song, J. Chem. Phys. 116, 9352 (2002)] is employed to calculate the melting lines for two model systems of silicon: the Stillinger–Weber (SW) model and the Tersoff-89 model. To address the anisotropic stress problem indicated in the previous paper, a slightly improved simulation procedure is used to prepare the coexisting solid and liquid phases at the thermodynamic equilibrium. For the SW silicon, the calculated melting temperature Tm at zero pressure agrees with that based on the free-energy calculation [Broughton and Li, Phys. Rev. B 35, 9120 (1987)]. The dependence of Tm at zero pressure on the selected solid surface orientation is also examined. The relative difference between Tm calculated based on the sharp Si (111)/liquid interface and the faceted Si (100)/liquid interface is less than 1%. Both models predict that the melting line exhibits a negative slope, which is consistent with the fact that the molar volume of the solid is larger than tha... read less

Abstract: In this work the Monte Carlo method with an empirical potential model for atomic interactions is applied to study reconstruction and intermixing at a Ge-covered Si(001) surface. We investigate the structure and energetics of the $2\ifmmode\times\else\texttimes\fi{}n$ reconstruction which serves as a strain-relief mechanism. The optimal value of n is found to be strongly dependent on the thickness of the Ge overlayer. Si-Ge intermixing is studied using a direct simulation method which includes entropic effects. Ge occupation probabilities in subsurface layers are evaluated as a function of Ge coverage at different temperatures. The results show that strain-relief driven intermixing has a pronounced effect on the surface reconstruction once the Ge coverage reaches a full layer. We also evaluate the effect of temperature on the distribution of Ge in subsurface layers and discuss effects due to kinetic limitations. In agreement with experiments, the study provides a description of the interplay between reconstruction and intermixing at Ge-covered Si(001). read less

Abstract: Quasi-single-domain 3C-SiC films have been successfully grown on nominally on-axis Si(001) substrate. The starting surface is either of 2×1 quasi-single-domain or of 2×1+1×2 double-domain. The point here is to use dc-resistive heating of the substrate and to form a low-temperature (650 °C) interfacial buffer layer using monomethylsilane (H3 C-SiH3). The dc resistive heating serves to form a single-domain Si(001)-2×1 or 1×2 starting surface or to develop a single-domain 3C-SiC(001)-2×3 or 3×2 surface on a 2×1+1×2 double-domain Si(001) substrate. When a single-domain Si(001) starting surface is utilized, it is not the dc polarity during growth but the surface reconstruction of the starting surface that determines the dominant domain in the 3C-SiC film. The thickness of the single-domain 3C-SiC film is as thin as ∼45–200 nm, which is about three orders of magnitude smaller than that required in a previous study (>5 μm). read less

Abstract: Fracture toughness of silicon crystals has been investigated by indentation methods, and their surface energy has been calculated using molecular dynamics (MD). When a conical indenter was forced into a (001) silicon wafer at room temperature, {110} cracks were mainly introduced from the indent, indicating that fracture occurs most easily along the {110} plane among the crystallographic planes of the h001i zone. To confirm this orientation dependence, surface energies for those planes were computed using molecular dynamics. The surface energy read less

Abstract: Despite the large lattice mismatch, SiC is often grown on top of Si substrates. An effective mechanism for accommodating this mismatch is the formation of an array of pure edge dislocations at the interface. We have performed a comparative study of possible core structures of the misfit dislocation via molecular dynamics simulations. We present and discuss the results for the energetics and the structural properties of the most stable configuration. read less

Abstract: We examine the role of atomistic simula- tions in multiscale modeling of mechanical behavior of stressed solids. Theoretical strength is defined through modes of structural instability which, in the long wave- length limit, are specified by criteria involving elastic stiffness coefficients and the applied stress; more gen- erally, strength can be characterized by the onset of soft vibrational modes in the deformed lattice. Alternatively, MD simulation of stress-strain response provides a direct measure of the effects of small-scale microstructure on strength, as illustrated by results on SiC in single crystal, amorphous, and nanocrystalline phases. A Hall-Petch type scaling is introduced to estimate strength of labora- tory specimens containing microstructural flaws of cer- tain critical size. A preliminary simulation of Cu thin film nano-indentation is described as a means of probing the ideal shear strength. The challenge of formulating a local measure of the driving force for defect motion is briefly discussed. read less

Abstract: In this letter, the strain field below uncapped Ge islands of a different shape on a Si(001) substrate is estimated by molecular dynamics simulations at a realistic scale. Comparison to the Fourier transform maps of transmission electron micrographs, recently reported in literature, shows a very good agreement. We point out that the complex deformation in silicon, just below the edges of the Ge islands, is far from being uniaxial. The stress distribution generated by such a strain determines the range of interdot repulsion. read less

Abstract: Structural characteristics of amorphous silicon (a-Si) have been examined by molecular-dynamics calculations using the Tersoff interatomic potential. It was confirmed that the computer-generated atomic configurations reproduce well the structural and dynamical properties of a-Si obtained experimentally. The a-Si networks contained two types of structural defect: threefold coordinated Si atoms (dangling bonds) and fivefold coordinated ones (floating bonds). The average bond length increased with the coordination number. Bond angles were distributed around 120° for the threefold coordinations, suggesting the existence of the atomic clusters constructed by sp2 bonding. On the other hand, they had peaks at ~60° and 90° for the fivefold coordinated atoms. Partial radial distribution functions revealed that the floating bonds have a tendency to cluster in the a-Si network. read less

Abstract: Diffuse x-ray scattering is a powerful means to study the structure of defects in crystalline solids. The traditional analysis of diffuse x-ray scattering experiments relies on analytical and numerical methods which are not well suited for studying complicated defect configurations. We present here an atomistic simulation method by which the diffuse x-ray scattering can be calculated for an arbitrary finite-sized defect in any material where reliable interatomic force models exist. We present results of the method for point defects, defect clusters and dislocations in semiconductors and metals, and show that surface effects on diffuse scattering, which might be important for the investigation of shallow implantation damage, will be negligible in most practical cases. We also compare the results with x-ray experiments on defects in semiconductors to demonstrate how the method can be used to understand complex damage configurations. read less

Abstract: In this thesis, the impact process of cluster ions on solid surfaces was studied using molecular dynamics (MD) simulation. Cluster is an aggregated material which consists of a few to thousand atoms. The impact process of cluster ion on solid surface is of great interest because the e ect of impact by cluster ion cannot be explained by the summation of individual monomer ions, and it is termed as `nonlinear e ect.' In order to understand the nonlinear e ect by cluster, the dynamics of collisional process between cluster and solid surface should be examined. MD simulation is one method of computer simulation to solve numerically the Newton's equation of motion for each atom in the system using di erence equation technique, so MD can make it possible to trace the time evolution of coordinates and velocity for every atom with high resolution. The basic theory of molecular dynamics and the acceleration method are described in chapter 2. For this study, the original MD program was developed, which can accelerate the calculation speed of collisional process of high-energy atoms with a solid surface by applying di erent timestep to each atom depending on its velocity. Due to this acceleration technique and recent progress in computers, it can be possible to simulate the system with a large number of atoms, or more than hundred simulations in order to obtain statistics. In the following chapters, MD simulation is used to examine the impacts of various types of clusters on a number of well de ned substrates. Chapter 3 describes the typical impact process of cluster on solid surface examined using large argon cluster and silicon substrate. The di erences between cluster and monomer impact are shown in penetration range, damage formation and sputtering. The energy dependence of penetration depth of cluster was examined and it was fond that the penetration depth is pro- read less

Abstract: The character of relaxation of atoms around a vacancy in cubic silicon carbide is determined with the help of the empirical potential of Tersoff. The recursion method of Haydock and Nex is applied to calculate the density of states derived from atoms situated around the defect. The outward relaxation of the lattice surrounding a empty site is established. The lattice relaxation results in the shift of gap states toward the conduction band. Vacancy levels of carbon at 0.5 eV and silicon at 0.45 and 1.98 eV are revealed in the band gap. The obtained results are compared with the experimental ones and with the data of other calculations. The work shows the importance of taking into account the lattice relaxation in examining vacancy states in semiconducting compounds. read less

Abstract: A recently introduced semiempirical methodology is used to model and simulate silicon via molecular dynamics. This approach is capable of grasping essential qualitative and quantitative features of the coupling between the electronic coordinates and the geometric structure. Properties of the bulk diamond crystal, the melt and amorphous solid states are obtained using optimization techniques and molecular dynamics simulations. The pair distribution function of the amorphous state is in excellent agreement with experimental and other molecular dynamics simulation results. read less

Abstract: The Si adatom adsorption and diffusion on the fully relaxed Si{100}(2×1) surface is studied by a combination of molecular dynamics simulations with Tersoff’s potential for the Si interactions, a simplified transition state theory of Voter and lattice gas simulations. Six local minima for adsorption are found on the surface and the activation energies between each are determined. The Arrhenius behavior for the macroscopic diffusion is found to be D=5.67×10−3 exp(−0.75 eV/kT) cm2/s. In addition, it is found that the adatom diffusion is strongly anisotropic in nature and the direction of easy diffusion is perpendicular to the dimers (i.e., parallel to the dimer rows) of the original surface. The minimum energy path for the diffusion is found to be activated by the local unreconstruction (dimer opening) of the otherwise fully reconstructed surface. read less

Abstract: The water-based cutting fluid plays an important role in cooling and lubricating during cutting process. In order to analyze the role of water in the cutting process from the microscopic view, this paper used molecular dynamics simulation to establish the cutting model with water lubrication by covering a water layer on the surface of iron workpiece. By comparing the cutting heat and friction coefficient under dry cutting and wet cutting, it is found that: water molecules will enter the gap between the tool and the workpiec, preventing the direct contact between the carbon atoms and the iron atoms, thereby reducing the friction coefficient. At the same time, wet cutting can reduce the surface temperature of the workpiece and play a role in cooling and lubricating. read less

Abstract: Defect evolution in a single crystal silicon which is implanted with hydrogen atoms and then annealed is investigated in the present paper by means of molecular dynamics simulation. By introducing defect density based on statistical average, this work aims to quantitatively examine defect nucleation and growth at nanoscale during annealing in Smart-Cut® technology. Research focus is put on the effects of the implantation energy, hydrogen implantation dose and annealing temperature on defect density in the statistical region. It is found that most defects nucleate and grow at the annealing stage, and that defect density increases with the increase of the annealing temperature and the decrease of the hydrogen implantation dose. In addition, the enhancement and the impediment effects of stress field on defect density in the annealing process are discussed. read less

Abstract: The class of Complex Metallic Alloys (CMAs) is interesting for its wide range of physical properties. There are materials that exhibit high hardness at low density or good corrosion resistance, which is important for technological applications. Other compounds are superconductors, have strong anisotropic transport coefficients or exhibit a novel magnetic memory effect. The theoretical investigation of CMAs is often very challenging because of their inherent complexity and large unit cells with up to several thousand atoms. Molecular dynamics simulations with classical interaction potentials are well suited for this task – they can handle hundreds of thousands of atoms in reasonable time. Such simulations can provide insight into static and dynamic processes at finite temperatures on an atomistic level. The accuracy of these simulations depends strongly on the quality of the employed interaction potentials. To generate physically relevant potentials the force-matching method can be applied. A computer code called potfit has been developed at the Institute for Theoretical and Applied Physics (ITAP) especially for this task. It uses a large database of quantum-mechanically calculated reference data, forces on individual atoms and cohesive energies, to generate effective potentials. The parameters of the potential are optimized in such a way that the reference data are reproduced as accurately as possible. The potfit program has been greatly enhanced as part of this thesis. The optimization of analytic potentials, new interaction models as well as a new optimization algorithm were implemented. Potentials for two different complex metallic alloy systems have been generated and used to study their properties with molecular dynamics simulations. The first system is an approximant to the decagonal Al-Pd-Mn quasicrystal. A potential which can reproduce the cohesive energy with high accuracy was generated. With the help of this potential a refinement of the experimentally poorly determined structure model could be performed. The second class of potentials was fitted for intermetallic clathrate systems. They have interesting thermoelectric properties which originate from their special structure. Silicon- and germanium-based clathrate potentials were derived and the influence of the complex structure on the thermal conductivity has been studied. Komplexe Intermetallische Verbindungen (CMAs) sind aufgrund ihrer vielfaltigen physikalischen Eigenschaften sehr interessant fur technologische Anwendungen. Dabei ist z.B. hohe Harte bei geringer Dichte und Korrosionsbestandigkeit wichtig. Neben Supraleitern gibt es Materialien mit anisotropen Transporteigenschaften oder einem neuartigen magnetischen Memory Effekt. Theoretische Untersuchungen von CMAs stellen durch ihre inharente Komplexitat und die riesigen Einheitszellen mit mehreren tausend Atomen oft eine grose Herausforderung dar. Molekulardynamiksimulationen mit effektiven Potenzialen konnen dazu eingesetzt werden; sie ermoglichen die Berechnung von hunderttausenden von Atome in annehmbarer Zeit. Damit kann ein Einblick in sowohl statische als auch dynamische Prozesse auf atomarer Ebene gewonnen werden. Die Ergebenisse solcher Simulationen hangen jedoch sehr stark von der Qualitat der eingesetzten Wechselwirkung ab. Um physikalisch gerechtfertigte Potenziale zu erzeugen, kann die Force-Matching-Methode angewandt werden. Dazu wurde am Institut fur Theoretische und Angewandte Physik (ITAP) ein Programm mit dem Namen potfit entwickelt. Es verwendet eine grose Datenbank von quantenmechanisch berechneten Referenzgrosen wie z.B. Krafte auf die einzelnen Atome und die Kohasionsenergie, um effektive Potenziale zu generieren. Die freien Parameter des Potenzials werden optimiert, um die Referenzdaten so gut wie moglich zu reproduzieren. Fur diese Arbeit wurde potfit deutlich erweitert. Es konnen nun analytisch definierte Potenziale optimiert werden, neue Wechselwirkungen wurden implementiert und ein neuer Optimierungsalgorithmus wurde hinzugefugt. Damit wurden effektive Potenziale fur zwei verschiedene CMA Systeme gefittet und deren Eigenschaften mit Molekulardynamik untersucht. Fur die Approximanten eines decagonalen Al-Pd-Mn Quasikristalls, den Xi-Phasen, wurde ein Potenzial fur die Strukturbestimmung erzeugt. Es kann die Kohasionsenergien verschiedener Strukturen mit groser Genauigkeit wiedergeben. Ein aus experimentellen Daten ungenau bestimmtes Strukturmodell konnte damit erheblich verbessert werden. Auserdem wurden Potenziale fur Intermetallische Klathrate erzeugt. Diese Systeme besitzen interessante thermoelektrische Eigenschaften aufgrund ihrer speziellen Kafigstruktur. Effektive Wechselwirkungen fur silizium- und germaniumbasierte Klathrate wurden erzeugt. Damit wurde der Einfluss der komplexen Struktur auf die thermische Leitfahigkeit des Gitters untersucht. read less

Abstract: We have performed molecular dynamics (MD) simulations to investigate the effect of SiC bond formation on fluence‐dependent results in 20 keV C60 bombardment of Si. Sputter depth profiling experiments of C60 on Si have produced atypical results, which are thought to be caused by the strong covalent bonds that are formed between the C atoms in the projectile and Si atoms in the substrate. A recently developed scheme developed by Russo, et al. 8 has been adapted to perform MD simulations of 150 successive impacts of 20 keV C60 on Si, which corresponds to a total fluence of 2.64 × 1013 impacts/cm2. In order to isolate the effects of SiC bond formation, the same set of trajectories is calculated with and without the attractive SiC potential energy terms. When SiC bonds are able to form, nearly all the C atoms from the projectile are incorporated into the substrate. When the possibility of SiC bond formation is removed, most of the C atoms are backscattered into the vacuum. The cumulative result is that the substrate with SiC bonds contains a factor of twenty times more C atoms, which are located below the surface. Copyright © 2010 John Wiley & Sons, Ltd. read less

Abstract: By introducing internal degree, the deformation of hexagonal noncentrosymmetric crystal sheet can be described by the revised Cauchy–Born rule based on atomic potential. The instability criterion is deduced to investigate the inhomogeneous dislocation nucleation behavior of the crystal sheet under simple loading. The anisotropic characters of dislocation nucleation under uniaxial tension are studied by using the continuum method associated with the instability criterion. The results show a strong relationship between yield stress and crystal sheet chirality. The results also indicate that the instability criterion has sufficient ability to capture the dislocation nucleation site and expansion. To observe the internal dislocation phenomenon, the prediction of the dislocation nucleation site and expansion domain is illustrated by MD simulations. The developed method is another way to explain the dislocation nucleation phenomenon. read less

Abstract: For better understanding of the essential removal mechanisms of brittle material and stress distribution at high speed and ultrasonic elliptical vibration assisted cutting conditions at the atomic level, the single crystal silicon, expected as a next generation semiconductor material for wide band cap, high-voltage and low-loss power devices, MEMS components and so on, is analyzed by molecular dynamics computer simulation for its nanometer behavior through cutting force and subsurface stress distribution in the condition of similar ultrasonic elliptical vibration cutting. In this simulation, the cutting tool is assumed to be one of rigid single crystal diamond. The atomic behavior in a plane corresponding to Silicon (100) plane is simulated for dealing with a plane strain problem where the three-dimensional effect of inter-atomic force is considered. The results show that the cutting forces varies following a similar sinusoid different from the conventional cutting and the atomic layers below the machined surface are deformed and have residual stress read less

Abstract: This paper presents both experimental and theoretical studies on the atomic structure changes of monocrystalline silicon brought about by surface nano-modification. The experiment revealed amorphous transformations with boundaries featuring faceting along {111} planes near the sample surface, which were altered to a random nature at the bottom of the transformation zone. The deformation outside the zone was minor near the surface, but advanced to heavy bending, extensive dislocations and plane shifting in the depth of the samples. Theoretical analysis closely reproduced this deformation, highlighting some scaling effects. read less

Abstract: Molecular dynamics computer simulations have been employed with the Tersoff interatomic potential to examine the atomic structure of (a/2) 〈110〉 screw dislocations forming regular two-dimensional arrays in silicon. The main attention is focused on the atomic configurations of dislocation intersections. The dislocations are assumed to be undissociated, following HREM observations on the low-angle (001) twist boundaries produced by silicon wafer bonding in ultrahigh vacuum. It is shown that cores of the dislocation intersections are formed by closed characteristic groups of atoms (extended point defects). The symmetry of these defects strongly depends on the fact whether the screw dislocation arrays generate a twist or a shear boundary. read less

Abstract: SiC devices have been typically subjected to extreme environments and complex stresses during operation, such as intense radiation and large diurnal amplitude differences on the lunar surface. Radiation displacement damage may lead to degradation or failure of the performance of semiconductor devices. In this paper, the effects of temperature and incidence angle on the irradiation cascade effect of 6H-SiC were investigated separately using the principles of molecular dynamics. Temperatures were set to 100 K, 150 K, 200 K, 250 K, 300 K, 350 K, 400 K and 450 K. The incidence direction was parallel to the specified crystal plane, with angles of 8°, 15°, 30°, 45°, 60° and 75° to the negative direction of the Z-axis. In this paper, the six types of defects were counted, and the microscopic distribution images and trajectories of each type of defect were extracted. The results show a linear relationship between the peak of the Frenkel pair and temperature. The recombination rate of Frenkel pairs depends on the local temperature and degree of aggregation at the center of the cascade collision. Increasing the angle of incidence first inhibits and then promotes the production of total defects and Frenkel pairs. The lowest number of total defects, Frenkel pairs and antisite defects are produced at a 45° incident angle. At an incidence angle of 75°, larger size hollow clusters and anti-clusters are more likely to appear in the 6H-SiC. read less

Abstract: The structure of the two-dimensional BN containing 9941 atoms has been studied by classical molecular dynamics simulation with Tersoff potential. The periodic boundary condition is applied to the two x and y directions, while the z direction is free. The analysis results of the function of total energy per atom and heat capacity, mean squared displacement, diffusion coefficient, radial distribution function, distribution of coordination number, angle distribution, and ring statistics show that the melting point of the material is about 4600 K. This value is higher than the experimental value as well as the previous simulation results. The observations also show that the melting process begins at the corners and edges and then spreads across the face of the model. The breakage of the B-N bond leads to the formation of clusters of N2 molecules and B with different sizes. read less

Abstract: Silicon carbide (SiC) is a promising material for thermoelectric power generation. The characterization of thermal transport properties is essential to understanding their applications in thermoelectric devices. The existence of stacking faults, which originate from the “wrong” stacking sequences of Si–C bilayers, is a general feature of SiC. However, the effects of stacking faults on the thermal properties of SiC are not well understood. In this study, we evaluated the accuracy of Tersoff, MEAM, and GW potentials in describing the thermal transport of SiC. Additionally, the thermal conductivity of 3C/4H-SiC nanowires was investigated using non-equilibrium molecular dynamics simulations (NEMD). Our results show that thermal conductivity exhibits an increase and then saturation as the total lengths of the 3C/4H-SiC nanowires vary from 23.9 nm to 95.6 nm, showing the size effect of molecular dynamics simulations of the thermal conductivity. There is a minimum thermal conductivity, as a function of uniform period length, of the 3C/4H-SiC nanowires. However, the thermal conductivities of nanowires weakly depend on the gradient period lengths and the ratio of 3C/4H. Additionally, the thermal conductivity of 3C/4H-SiC nanowires decreases continuously from compressive strain to tensile strain. The reduction in thermal conductivity suggests that 3C/4H-SiC nanowires have potential applications in advanced thermoelectric devices. Our study provides insights into the thermal transport properties of SiC nanowires and can guide the development of SiC-based thermoelectric materials. read less

Abstract: Sputtered Cu/Si thin films were experimentally prepared at different sputtering pressures and characterized using X-ray diffraction (XRD) and an atomic force microscope (AFM). Simultaneously, an application-oriented simulation approach for magnetron sputtering deposition was proposed in this work. In this integrated multiscale simulation, the sputtered atom transport was modeled using the Monte Carlo (MC) and molecular dynamics (MD) coupling method, and the deposition of sputtered atoms was simulated using the MD method. This application-oriented simulation approach was used to simulate the growth of Cu/Si(100) thin films at different sputtering pressures. The experimental results unveiled that, as the sputtering pressure decreased from 2 to 0.15 Pa, the surface roughness of Cu thin films gradually decreased; (111)-oriented grains were dominant in Cu thin films and the crystal quality of the Cu thin film was gradually improved. The simulation results were consistent with the experimental characterization results. The simulation results revealed that the transformation of the film growth mode from the Volmer–Weber growth mode to the two-dimensional layered growth mode resulted in a decrease in the surface roughness of Cu thin films; the increase in the amorphous compound CuSix and the hcp copper silicide with the decrease in the sputtering pressure was responsible for the improvement of the crystal quality of the Cu thin film. This work proposed a more realistic, integrated simulation scheme for magnetron sputtering deposition, providing theoretical guidance for the efficient preparation of high-quality sputtered films. read less

Abstract: Due to the lack of appropriate experimental methods for imaging the evolution of the microstructure of materials at the growth conditions, our understanding of the physical behavior of crystal growth and defect formation during the vapor deposition growth of SiC crystals is still rather limited. In the present work, the vapor deposition growth of SiC crystal on a 4H-SiC substrate has been investigated by the molecular dynamics (MD) computer simulation method. Three different lattice planes of 4H-SiC ((0001), (112-0) and (1-100)) were selected as the surface of the substrate, and three different temperatures for substrate (2200 K, 2300 K and 2400 K) were used in growth simulations. The characteristics of the formation of different polytypes of SiC and dislocations in the grown crystals were examined. The results show that the SiC crystals were grown by a subsurface nucleation and growth mode in the vapor deposition process. For substrates with (0001) plane as the surface, the 3C-SiC single crystal was obtained in the deposited thin film. For substrates with (112-0) or (1-100) plane as the surface, the 4H-SiC single crystal was obtained instead. The temperature of the substrate was found to have a significant effect on the dislocation density generated in the grown crystals. The mechanism for the formation of Frank partial dislocations during the growth of SiC crystals has been analyzed, for which the importance of the diffusivity of atoms on the surface layer in growth has been highlighted, and it gives a good explanation of the temperature effect on dislocation formation in the grown crystals. These results can be helpful for experimental vapor deposition growth of SiC single crystals and epitaxial layers of high quality. read less

Abstract: Silicon carbide is successfully implemented in semiconductor technology; it is also used in systems operating under aggressive environmental conditions, including high temperatures and radiation exposure. In the present work, molecular dynamics modeling of the electrolytic deposition of silicon carbide films on copper, nickel, and graphite substrates in a fluoride melt is carried out. Various mechanisms of SiC film growth on graphite and metal substrates were observed. Two types of potentials (Tersoff and Morse) are used to describe the interaction between the film and the graphite substrate. In the case of the Morse potential, a 1.5 times higher adhesion energy of the SiC film to graphite and a higher crystallinity of the film was observed than is the case of the Tersoff potential. The growth rate of clusters on metal substrates has been determined. The detailed structure of the films was studied by the method of statistical geometry based on the construction of Voronoi polyhedra. The film growth based on the use of the Morse potential is compared with a heteroepitaxial electrodeposition model. The results of this work are important for the development of a technology for obtaining thin films of silicon carbide with stable chemical properties, high thermal conductivity, low thermal expansion coefficient, and good wear resistance. read less

Abstract: Based on the non-equilibrium molecular dynamics simulation, a Cu/amorphous diamond/crystalline diamond sandwich model was established to investigate the effects of the amorphous degree diamond surface and the thickness of the amorphous layer on the thermal boundary conductance of Cu/crystalline diamond. The simulation results show that the thermal boundary conductance can be enhanced by diamond surface amorphization, and increases with the increase of the amorphous degree. For the fully amorphous layer, the thermal boundary conductance increases gradually with the increase of the thickness of the amorphous layer and can be enhanced up to 4 times. The analysis of the vibrational density of states, overlap energy and phonon participation ratio shows that the diamond surface amorphization promotes the vibrational coupling between diamond and Cu atoms at low frequencies, as well as the occurrence of phonon inelastic scattering, and thus improves the thermal transport capacity of interface. read less

Abstract: This paper investigated the micromechanical behavior of different 6H-SiC/Al systems during the uniaxial tensile loading by using molecular dynamics simulations. The results showed that the interface models responded diversely to the tensile stress when the four low-index surfaces of the Al were used as the variables of the joint surfaces. In terms of their stress–strain properties, the SiC(0001)/Al(001) models exhibited the highest tensile strength and the smallest elongation, while the other models produced certain deformations to relieve the excessive strain, thus increasing the elongation. The SiC(0001)/Al(110) models exhibited the largest elongations among all the models. From the aspect of their deformation characteristics, the SiC(0001)/Al(001) model performed almost no plastic deformation and dislocations during the tensile process. The deformation of the SiC(0001)/Al(110) model was dominated by the slip of the 1/6 <112> Shockley partial dislocations, which contributed to the intersecting stacking faults in the model. The SiC(0001)/Al(111) model produced a large number of dislocations under the tensile loading. Dislocation entanglement was also found in the model. Meanwhile, a unique defect structure consisting of three 1/6 <110> stair-rod dislocations and three stacking faults were found in the model. The plastic deformation in the SiC(0001)/Al(112) interface model was restricted by the L-C lock and was carried out along the 1/6 <110> stair-rod dislocations’ direction. These results reveal the interfacial micromechanical behaviors of the 6H-SiC/Al composites and demonstrate the complexity of the deformation systems of the interfaces under stress. read less

Abstract: Molecular dynamics simulation has become a major theoretical analysis method for ultra precision machining of various materials. Large Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) is used to construct a molecular dynamics model for cutting polycrystalline Fe-Cr-W alloy with CBN tool. The geometric parameters of the tool model are: the rake angle is −10°, the clearance angle is 7°, and the tool tip radius is 1.5 nm, including 8570 atoms. The size of the workpiece model is 24 nm × 10 nm × 10 nm, including 203,893 atoms. Ovito is used to visually analyze the influence of cutting parameters on the machinability of workpieces. The results show that the extrusion of the tool on the workpiece makes the atoms of the workpiece move and become chips and machined surfaces. Excessive cutting speed and depth will produce large hydrostatic stress, large cutting force, high cutting temperature and deteriorate the machined surface. read less

Abstract: In order to study the material removal mechanism of Fe–C alloy surfaces in the particle microcutting process, the molecular dynamics method was used to study the material deformation and removal rules during the particle microcutting process. By analyzing and discussing the particle cutting force, atomic energy, atomic displacement, lattice structure, and dislocation in the particle microcutting process under different cutting velocities, the material removal mechanism is revealed. The results show that the atomic binding energy of Fe–C alloy increases with an increase in particle cutting velocity. The cutting force of particles and atomic potential energy of the workpiece increase obviously. The accumulated strain energy and dislocation energy in the lattice increase, the lattice deformation becomes more severe, and the material is prone to plastic deformation. The atoms form atomic groups at the front of the particle and are then remove from the surface of Fe–C alloy in the form of chips. read less

Abstract: Silicon carbide (SiC) is a promising semiconductor material for making high-performance power electronics with higher withstand voltage and lower loss. The development of cost-effective machining technology for fabricating SiC wafers requires a complete understanding of the deformation and removal mechanism. In this study, molecular dynamics (MD) simulations were carried out to investigate the origins of the differences in elastic–plastic deformation characteristics of the SiC polytypes, including 3C-SiC, 4H-SiC and 6H-SiC, during nanoindentation. The atomic structures, pair correlation function and dislocation distribution during nanoindentation were extracted and analyzed. The main factors that cause elastic–plastic deformation have been revealed. The simulation results show that the deformation mechanisms of SiC polytypes are all dominated by amorphous phase transformation and dislocation behaviors. Most of the amorphous atoms recovered after completed unload. Dislocation analysis shows that the dislocations of 3C-SiC are mainly perfect dislocations during loading, while the perfect dislocations in 4H-SiC and 6H-SiC are relatively few. In addition, 4H-SiC also formed two types of stacking faults. read less

Abstract: To explore the effect of nanoindentation temperature on the plastic deformation of 3C-SiC, it is possible to analyze the 3C-SiC load-displacement changes at different temperatures and the dislocation propagation in the plastic deformation stage. The 3C-SiC nanoindentation model is established on the basis of molecular dynamics interatomic interaction potential. The model combines the 3C-SiC crystal structure to optimize the Vashishta potential function and modifies the relaxation system, system boundary, and other simulated environmental factors. The plastic deformation process of 3C-SiC at different temperatures is analyzed from multiple angles such as the load-displacement curve, the stress distribution during the plastic deformation stage of the matrix, and the formation and growth of specimen dislocations. During the pressing process, intermolecular dislocations and stress are concentrated in the elastic-plastic deformation zone. The load value of the elastic-plastic deformation zone under high temperature environment is generally higher, and the energy of the dislocation loop will be released. In the plastic deformation zone, the dislocation loop will break under the action of high temperature environmental load. The premature release of energy will cause the load value to drop. During the pressing process, the bearing capacity of 3C-SiC polycrystalline will decrease as the temperature rises. Plastic deformation occurs inside the material, and dislocations nucleate and expand from the grain boundary to the crystal and finally form a U-shaped dislocation ring. read less

Abstract: To investigate the subsurface damage of 6H-SiC nanofriction, this paper uses molecular dynamics analysis to analyze the loading process of friction 6H-SiC surfaces, thus providing an in-depth analysis of the formation mechanism of subsurface damage from microscopic crystal structure deformation characteristics. This paper constructs a diamond friction 6H-SiC nanomodel, combining the radial distribution function, dislocation extraction method, and diamond identification method with experimental analysis to verify the dislocation evolution process, stress distribution, and crack extension to investigate the subsurface damage mechanism. During the friction process, the kinetic and potential energies as well as the temperature of the 6H-SiC workpiece basically tend to rise, accompanied by the generation of dislocated lumps and cracks on the sides of the 6H-SiC workpiece. The stresses generated by friction during the plastic deformation phase lead to dislocations in the vicinity of the diamond tip friction, and the process of dislocation nucleation expansion is accompanied by energy exchange. Dislocation formation is found to be the basis for crack generation, and cracks and peeled blocks constitute the subsurface damage of 6H-SiC workpieces by diamond identification methods. Friction experiments validate microscopic crystal changes against macroscopic crack generation, which complements the analysis of the damage mechanism of the simulated 6H-sic nanofriction subsurface. read less

Abstract: ABSTRACT In this article, the mechanical properties of the boron-nitride nano-sheet (BNNS)-reinforced aluminium (Al) nanocomposite are estimated by using molecular dynamics (MD) models. Different nanoscale representative volume elements (NRVEs) have been considered to predict the stiffness and strength properties of BNNS-Al NRVE under tensile and shear loading conditions. A comparison is also made between the stress–strain behaviour of graphene sheet (GS)-Al and BNNS-Al nanocomposites. BNNS-Al nanocomposites are found to be more ductile and less stiff than GS-Al nanocomposite. Effect of layering of BNNS on the stress–strain response is observed and found that, in contrast to the layering of graphene sheets, the stiffness properties of BNNS-reinforced Al nanocomposite are not sensitive to the layering of BNNS. It is established that the layering of BNNS marginally enhance the mechanical properties of the nanocomposite. It is also found that the chirality of BNNS marginally affects the stress–strain response of the nanocomposite irrespective of the loading condition i.e. tensile/shear loading. read less

Abstract: ABSTRACT In this computational study, the effect of the shape and size of graphene oxide (GO) and SiO2 nanoparticles on the shear thickening Behaviour of the fluids was reported with the molecular dynamics (MD) approach. For this purpose, the viscosity of fluids with C, Si, O, and H atomic arrangements was determined by Tersoff and Lenard-Jones (LJ) interatomic force fields. Atomic stability of the simulated structures was detected after 1.000.000 time-steps, demonstrating the validity of the PEG-400-based STF. Additionally, MD simulation results indicated that addition of zigzag GO and cubic SiO2 nanoparticles to the pristine fluid would maximise the viscosity of this atomic structure. Numerically, by adding these nanostructures, the viscosity of the simulated fluid was converged to 88 Pa. s and 94 Pa. s, respectively. The jamming viscosity (discontinuous shear-thickening) changes occurred in 70 and 80 s−1 shear rates by adding GO and SiO2 nanoparticles to the pristine fluid. read less

Abstract: The molecular dynamics (MD) model of nano-indentation process was established to study the crack evolution in single crystal during nano-indentation. Two workpieces with different cracks and one workpiece with no crack were selected for indentation simulation in this study. The parameters of atom displacement, coordination number (CN), temperature, potential energy and loading force in the indentation process are analyzed in detail. Cracks were found to close during nano-indentation. Two modes of crack closure are observed: cooperative displacement and indentation failure. The existence of cracks will affect the size of transformation zone and the coordination number of atoms after indentation. Besides, the existence of cracks will reduce the increase of temperature and potential energy, and the closing mode of cracks is found to affect the value of indentation load. In addition, the change of stress with indentation depth at crack tip is calculated by theoretical model. The calculated stress curves reveal the evolution trend of cracks during indentation. These results provide guidance for the production of silicon wafer with higher surface quality. read less

Abstract: Single-crystal copper (Cu), whose atom arrangement is in the same direction and has no grain boundary, is widely used in defense technology, civil electronics and network communication. As a diamond turnable material, fan-shaped patterns appear on the machined surface, which affects the machined surface quality and the optical function it carries. Previous studies on the surface generation mechanism in single-point diamond turning (SPDT) of Cu were limited to experimental analysis, while there is a lack of fundamental understanding of the fan-shaped pattern generation mechanism and suppression method. In the present study, the different fan-shaped patterns, surface quality, cutting force and chip morphology of the typical crystal planes (100), (110) and (111) planes of Cu were studied by both theoretical and experimental analyses. A molecular dynamics (MD) simulation was conducted to present the fundamental generation mechanism of the fan-shaped patterns from atom arrangement directions and its angle change with the main cutting direction, while a cutting dynamics model was established to simulate the generation of fan-shaped patterns on the machined surface. Based on theoretical and experimental analysis, it was found that the atom density arrangement directions of Cu and its angle change with the main cutting direction of SPDT caused fluctuations in the friction coefficient, which further caused the vibration of the cutting system and generated the fan-shaped patterns. The SPDT of crystal planes (100) can achieve the best surface quality. The present research provides a fundamental understanding of fan-shaped pattern formation on the machined surface, and provides an instruction for machining Cu to obtain better surface quality. read less

Abstract: Single crystal silicon carbide (SiC) is widely used for optoelectronics applications. Due to the anisotropic characteristics of single crystal materials, the C face and Si face of single crystal SiC have different physical properties, which may fit for particular application purposes. This paper presents an investigation of the material removal and associated subsurface defects in a set of scratching tests on the C face and Si face of 4H-SiC and 6H-SiC materials using molecular dynamics simulations. The investigation reveals that the sample material deformation consists of plastic, amorphous transformations and dislocation slips that may be prone to brittle split. The results showed that the material removal at the C face is more effective with less amorphous deformation than that at the Si face. Such a phenomenon in scratching relates to the dislocations on the basal plane (0001) of the SiC crystal. Subsurface defects were reduced by applying scratching cut depths equal to integer multiples of a half molecular lattice thickness, which formed a foundation for selecting machining control parameters for the best surface quality. read less

Abstract: In this study, molecular dynamics simulation was conducted to investigate the frictional behaviors between diamond tool and zirconium (Zr) substrates at the nanoscale. The effects of grain size on friction and wear were discussed under different sliding velocities. The simulation results showed that the friction forces had similar variation tendencies under different sliding velocities. Besides, the friction responses were stronger at high sliding velocities because of the atomic adhesion while the ploughing effect was more obvious at slower sliding velocity. Moreover, both the friction forces and the wear amounts increased with the decrease in the average grain sizes of the substrates. To explain this phenomenon, the internal mechanism was investigated by using the dislocation extract algorithm and the atomic displacement analyses. The results showed that the [0001]-oriented single crystalline substrate was prone to form continuous dislocation structures moving tangentially along the sliding direction due to the characteristic of Zr's slip systems, whereas grain boundaries conducted the deformation further into the polycrystalline substrates, increasing the contact areas and causing atomic accumulation in front, both resulted in stronger friction responses and wear. Accordingly, with the decrease in average grain sizes, the substrates experienced more severe subsurface damage and the deformation mechanism of nanocrystalline Zr had evolved from dislocation emission to grain boundary rotation and sliding. read less

Abstract: In the nanoscale dimensions, semiconductor nanowires such as germanium nanowires (GeNWs) are appropriate candidates for using field-effect transistors, Josephson junctions, sensors and so on. However, such uses require detailed knowledge of the physical properties of GeNW. Thus we investigated the thermal conductivity and stress-strain diagram of GeNW with lattice vacancy and linear imperfection. Non-equilibrium molecular dynamics simulation as numerical method was employed in this research. The three body Tersoff potential was employed to describe interaction between germanium atoms in GeNW. Two types of defects, point and linear, were applied to the nanowire. The Nose-Hoover thermostat was employed to control temperature of the system. We then studied thermal conductivity and Young’s modulus in three crystallography directions [100], [110] and [111]. Our MD results showed that in the case of 8% point vacancy, the thermal conductivity decreased greater than 70% and Yong’s modulus decreased about 25% for three crystallography directions. read less

Abstract: The flank faces of diamond cutting tools are characterized by the groove wear when the tools are used to machine single crystal silicon workpieces precisely, which significantly affects the quality of the machined surface. Many researchers confirmed the existence of SiC hard particles and inferred that hard particles scratch led to the groove wear on tools flank face. However, little literature can be found to reveal the formation process of tools groove wear. Therefore, in this paper, a scratch model of SiC hard particles and diamond tool is proposed by molecular dynamics simulation method to investigate the formation mechanism of the groove wear on the tools flank face. The two basic scratch conditions, namely mechanical scratch and rolling scratch, are utilized to simulate hard particles motion on diamond tools surface respectively, and the tools wear are suggested by the variation in coordination numbers. It can be concluded that owing to the high temperature, the locally softened performance of diamond tools combined with continuous effects of SiC hard particles are the main factors that lead to the formation of the groove wear on the tools flank face. read less

Abstract: Carbon nanotubes (CNTs) possess unique structural properties which can be modified by several methods such as partial and full atom substitution. In this method also known as doping, novel hybrid tubular structures with desired and new characteristics can be synthesized. To this end, Boron (B) and Nitrogen (N) are selected as dopants and then by using molecular dynamics (MD) simulations the structural behavior of the new heteronanotubes are investigated. Moreover, the buckling behavior of these novel nanotube alongside the pure CNT and BN nanotube (BNNT) are studied. Apparently, the critical forces of the newly formed structures are computed between those of pure CNT and BNNT. Further, a combination of the doped structures and pure ones are used to simulate hybrid double-wall nanotubes and then their buckling response under axial compressive load is studied. Attained results demonstrated that double-walled hybrid structures possess mechanical stabilities lower and higher than pure double-walled CNT and BNNT, respectively. read less

Abstract: The reinforcement of various materials by nanofillers as nanocomposites has recently received the attention of many researchers. In the present research, molecular dynamics simulations are used to investigate the influence of nanowire (NW)/carbon nanotube (CNT) reinforcement on the buckling behavior of metallic glass matrix nanocomposites (MGMNCs). The buckling characteristics of nanocomposites made by adding Cu NWs, CNTs and Cu NW-encapsulated CNTs to metallic glass matrices are studied. The results demonstrate that MG alloys comprising just two elements (Cu and Zr) with higher Cu percentage have higher mechanical stability. Also, it is observed that adding NW leads to a negative effect on the buckling behavior, while adding CNT and NW-encapsulated CNT considerably increases the buckling force and strain of the metallic glass models. Moreover, it is found that the filled CNT is the most effective nanofiller for amending the buckling behavior of metallic glasses. Furthermore, as the size of nanofillers gets larger, the critical force increases and the critical strain decreases. read less

Abstract: Molecular dynamics (MD) simulation is used to study the subsurface quality and material removal of single crystal silicon with a diamond tool during ultrasonic elliptical vibration assisted cutting (UEVAC), 1D ultrasonic vibration assisted cutting (1D UVAC) and traditional cutting (TC) process. In the simulations, a long-range analytical bond order potential is used to describe the interaction inside the silicon specimen, providing a more accurate depiction of the atomic scale mechanisms of ductile plasticity, brittle fracture, and structural changes in silicon. The results show that UEVAC and 1D UVAC in cutting brittle material silicon causes a much smaller cutting force, much lower von Mises stress at the subsurface, larger material remove rate, lower compressive normal stress σ x x and σ y y , and smaller shear stress τ x y . In addition, the hydrostatic stress of subsurface for TC and 1D UVAC is much higher than that for UEVAC, which results in fewer Si-II and bct5-Si formed from the original Si-I in UEVAC. Moreover, the number of other atoms for UEVAC is overall smaller than those of using TC and 1D UVAC, which confirms that UEVAC produces a better subsurface. And atomic flow field analysis shows that the UEVAC tends to cut silicon in a more ductile mode. Besides, the temperature in front of tool edge and below the tool flank face of TC is much higher. This means that 1D UVAC and UEVAC have a positive effect on the tool life. However, the temperature in subsurface zone is overall larger, which reveals that 1D UVAC and UEVAC have a negative effect on the subsurface temperature. read less

Abstract: The mechanical properties of a material can be positively or negatively affected by its applied or residual stress. In this article, a series of molecular dynamic simulations were adopted to investigate the nanoindentation response of monocrystalline copper under tensile pre-deformation. Nanoindentation simulation under stress-free condition was compared with those under pre-tension strain values of 1.2%, 2.4% and 3.6%. Load–displacement curves with hardness value and recovery rates of total work for nanoindentation based on various tensile pre-deformations were obtained and discussed. It indicated that tensile pre-deformations resulted in a higher potential energy in substrate and a lower external energy will be introduced to realize the same elastic or plastic deformation during indentation. Moreover, the evolution of interior defects during indentation was also observed and analysed. The results showed that tensile pre-strain can influence dislocation nucleation behaviour of material during indentation. This article proposed a special molecular dynamic simulation method to characterize the mechanical properties of the material under tensile pre-deformations via nanoindentation, which gives an effective approach to characterize residual stresses in micro- and nanoscale and will have promising application in mechanical characterization of Microelectro Mechanical Systems devices and structures. Further analysis based on experiments will be done in our further research work. read less

Abstract: Mechanical polishing is one of the essential attributes of nanofinishing. To maintain precision during nanofinishing process, the mechanical polishing needs to be studied and analyzed at nanometric scale. In view of this, a set of molecular dynamics simulation has been carried out to analyze the process behavior and its effects on abrasive particles. After simulation, it is observed that the finishing force and velocity damage the abrasive particle by changing its phase from diamond cubic to graphite. Thus, the abrasive particles need replacement in a scheduled time-bound manner. In addition, a strategy has been proposed for efficient and economic polishing. read less

Abstract: The subsurface damage has a siginificant effect on the strength of the wafer. To investigate the effect of the grinding speed and depth of cut on the surface and subsurface damage, a series of large scale MD simulation of nano grinding of silicon is performed. The results show that the monocrystalline silicon lattice undergoes extrusion and shear deformation, lattice reconstruction and amorphous phase transformation are also observed during the grinding process. It also turned that the grinding speed has an optimal value approximately 160m/s in the range of 50–400 m/s. By considering the variation of subsurface damage thickness for various depths of grinding at the grinding speed of 160m/s. The results show that the subsurface damage are gradually increasing from 9.5 Å to 20.5 Å with the augment of the depth of cut from 5 Å to 30 Å. Based on the above conclusions, it can be predicted that smaller depth of cut could reduce subsurface damage. read less

Abstract: The paper focuses on the surface damage of quartz glass in multiple grinding, so as to find out the machining parameters that can improve the surface quality of quartz glass. Molecular dynamics (MD) method is adopted to machine the quartz glass. Firstly, the initial grinding is done on quartz glass with the depth of 12 Å. Based on the initial grinding, no feed grinding processes are done for three times separately and the feed grinding processes are carried out on the damage layer left by the previous process. By the coordination number (CN), machined surface topographies of quartz glass are gained and regions of densification are marked. Moreover, the damage layer thickness of different machined surface is also calculated. By analyzing the density of different surface damage layers, the regulation of the density distribution is obtained. Finally, the nanoindentation hardness is gained by different load-displacement curves in nanoindentation simulation. The results show that the first no feed grinding and the second feed grinding can improve the accuracy and quality of grinding. Too many no feed grinding processes and other feed grinding processes will induce serious damage of the machined surface, which is clearly showed in the obvious increase in the density, hardness and thickness of the damage layer. At last, the results of the density analysis and nanoindentation also proved that the densification and hardness of quartz glass cannot increase unlimited. The results can be applied in the ultra-precision grinding of quartz glass to control the thickness of damage layer and improve the quality of processing. read less

Abstract: Layer-by-layer assembled graphene oxide (GO) has been considered as a high-efficiency novel membrane material. However, its performance of water permeation and ion rejection remains largely unresolved. Herein we constructed a model of a GO membrane using laminate nanochannels with aligned flexible multilayered GO sheets, on which functional groups were randomly distributed based on the Lerf–Klinowski model. The water permeation and ion rejection in the flexible GO membranes with various pore widths and surface oxidization degrees were simulated. Our results indicate water flow rate in the GO nanochannels is significantly slowed, which is quantitatively equivalent with the prediction using the no-slip Poiseuille equation. The simulated results suggest the capillary channels within GO stacked laminated membranes might not always work as the major flow route for water to permeate. It is observed that confined water structure becomes more disordered and loose within the corrugated GO nanochannels. The interfa... read less

Abstract: Single point diamond tools are commonly used for ultraprecision machining. At high cutting speeds, frictional contact and local heat may cause material damage to the diamond tool. The diamond crystal is softened and its mechanical strength decreases with the increase in temperature. Plastic deformation of diamonds was recently reported in some experimental studies. In this work, a molecular dynamics (MD) simulation was implemented to predict the deformation of single crystal diamond at various temperatures. Diamond is brittle at room temperature, however, it starts to exhibit plastic dislocation at a temperature above 1200 K under a confining pressure. The condition in ultraprecision machining is indeed a temperature gradient distribution at the tool tip, between the maximum temperature at the tool-workpiece interface and the average temperature at the core. The simulation results predicted that diamond deformed plastically under the gradient between 1500K and 860K. It is surprising that secondary cracks were resulted from the gradient, as comparing to a single slip obtained in an evenly distributed temperature. Bond dissociation nucleated the fractures along the (111) shuffle planes, perfect dislocation merely occurred in the hot zone and sp3-to-sp2 disorder at the cool zone. The temperature gradient created a lattice mismatch and nucleated the secondary cracks. The results give an insight that a catastrophic fracture and local material damage can occur at a diamond tool tip at the cutting temperature above 1200 K, due to softening and graphitization. read less

Abstract: . This study successfully simulated the single crystal copper nanocutting by a rigid body /elastic tools with nose radius at the nitrogen gas environment using molecular dynamics, and analyzed the workpiece temperature distribution and dislocation during nanocutting. After simulations, it can be found that when cutting with the elastic body tool, the tool itself was still distorted slightly, however, the cutting results of the elastic tool and the rigid body tool of the tool are not the same. The chip temperature was highest near the central rake and nose.The workpiece temperature when the elastic body tool cutting was lower; the temperature in the nose and rake plane is the highest, the more away from the nose, the lower the temperature. read less

Abstract: Carrying out molecular dynamics (MD) simulations is a relevant way to understand growth phenomena at the atomic scale. Initial conditions are defined for reproducing deposition conditions of plasma sputtering experiments. Two case studies are developed to highlight the implementation of MD simulations in the context of plasma sputtering deposition: ZrxCu1−x metallic glass and AlCoCrCuFeNi high entropy alloy thin films deposited onto silicon. Effects of depositing atom kinetic energies and atomic composition are studied in order to predict the evolution of morphologies and atomic structure of MD grown thin films. Experimental and simulated x-ray diffraction patterns are compared. read less

Abstract: A three-dimensional molecular dynamics (MD) simulation is conducted to investigate the effect of the abrasive rotating velocity on monocrystalline silicon specimen by mechanical polishing at atomistic scale. By monitoring relative positions of atoms in the monocrystalline silicon specimen, the microstructure of monocrystalline silicon is clearly identified and analyzed. The simulation results show that better machined surface quality is obtained and more phase transformation atoms occur with small abrasive rotating velocity. When the abrasive rotating is high, the surface quality deteriorates and amorphous layer thickens.These results provide us an effective approach to analyze the mechanism of material deformation and the formation of the machined surface after ultra-precision polishing. read less

Abstract: This study successfully simulated the single crystal copper nanocutting with a rigid body tool at the nitrogen gas environment using molecular dynamics, and analyzed the workpiece stress distribution and dislocation during nanocutting. After simulations, a diamond rigid tool with a completely sharp produce a shear plane during cutting. The distribution of equivalent stress was greatest at the shear zone and that residual stress occurred on the machined surface. And the stress gets smaller as the distance from the chip surface is farther. read less

Abstract: This study examines the vibrational behaviors of both armchair and zigzag carbon nanotubes (CNTs). The natural longitudinal/flexural/torsional/radial frequencies of CNTs were extracted and analyzed simultaneously from an equilibrium molecular dynamics (MD) simulation without imposing any initial modal displacement or force. Initial random atomic velocities, which were assigned to fit the simulated temperature, could be considered as an excitation on CNTs composing of wide range of spatial frequencies. The position and velocity of each atom at every time step was calculated using finite difference algorithm, and fast Fourier transform (FFT) was used to perform time-to-frequency domain transform. The effects of CNT length, radius, chirality, and boundary condition on the vibrational behaviors of CNTs were systematically examined. Moreover, the simulated natural frequencies and mode shapes were compared with the predictions based on continuum theories, i.e., rod, Euler–Bernoulli beam and nonlocal Timoshenko beam, to examine their applicability in nanostructures. read less

Abstract: An efficient and highly scalable bond-order potential code has been developed for the molecular dynamics simulation of bulk silicon, reaching 1.87 Pflops (floating point operations per second) in single precision on 7168 graphic processing units (GPUs) of the Tianhe-1A system. Furthermore, by coupling GPUs and central processing units, we also simulated surface reconstruction of crystalline silicon at the sub-millimeter scale with more than 110 billion atoms, reaching 1.17 Pflops in single precision plus 92.1 Tflops in double precision on the entire Tianhe-1A system. Such simulations can provide unprecedented insight into a variety of microscopic behaviors or structures, such as doping, defects, grain boundaries, and surface reactions. read less

Abstract: We present a parameterization of the Stillinger-Weber potential to describe the interatomic interactions within single-layer MoS2 (SLMoS2). The potential parameters are fitted to an experimentally obtained phonon spectrum, and the resulting empirical potential provides a good description for the energy gap and the crossover in the phonon spectrum. Using this potential, we perform classical molecular dynamics simulations to study chirality, size, and strain effects on the Young's modulus and the thermal conductivity of SLMoS2. We demonstrate the importance of the free edges on the mechanical and thermal properties of SLMoS2 nanoribbons. Specifically, while edge effects are found to reduce the Young's modulus of SLMoS2 nanoribbons, the free edges also reduce the thermal stability of SLMoS2 nanoribbons, which may induce melting well below the bulk melt temperature. Finally, uniaxial strain is found to efficiently manipulate the thermal conductivity of infinite, periodic SLMoS2. read less

Abstract: A series of three-dimensional molecular dynamics (MD) simulations of nanoindentation are conducted to investigate the deformation behavior and phase transformation of monocrystalline silicon with different size hemispherical diamond indenters on (010) crystal plane. The technique of coordination number (CN) is employed to elucidate the detailed mechanism of phase transformation in the monocrystalline silicon. The simulation results show that the phase transformation varies according to the different radii indenters. In the phase transformation region beneath the indenter, the crystalline structures of Si-II, Si-XIII, and amorphous phase structures are observed. In addition, the results indicate that phase transformation with large indenters is not same with the small indenter. The six-coordinated silicon phase, Si-XIII, transformed from Si-I is identified. The phases of Si-II and Si-XIII, which have the same coordinate number, are successfully extracted from the transformation region during nanoindentation and amorphous phase will emerge upon unloading. read less

Abstract: Contact surface of nanoscale sliding friction represent some new features that are different from the macro scale sliding friction, which need to seek new analysis methods. Molecular dynamics simulation is an effective method to describe microscopic phenomena. Therefore, Molecular dynamics method was used to study mechanical behavior of contact surface of nanoscale sliding friction. A molecular dynamics model of hemisphere sphere sliding on rectangular solid plane was built. State change of the micro contact area and friction force variation in the process of sliding friction were observed and analyzed after solution and simulation. The results show that, at the beginning position of the sliding, with different contact depth, contact action region of hemisphere and plane generated the atoms displacement, re-arranged and close-packed accumulation is also different. The deeper the contact depth is, the greater the atoms close-packed accumulation is, and the greater the contact deformation is. In the process of sliding friction, the contact surface of the basal body has produced lattice destruction, surface upheaval and silicon atoms close-packed accumulation, and then formed furrow scratches. At the same time the silicon atoms of the hemisphere generated atomic migration obviously and adhered on the basal body surface. The top of the hemisphere was torn and peeled, which resulted in wear. The deeper contact depth is, the more loss of the material of the hemisphere is, and wear become heavier. The curve of friction force and sliding displacement in different contact depths shows that the deeper contact depth is, the greater friction force is. The friction force increases quickly at the beginning of the sliding. Then the friction force remains steady relatively at stable sliding phase. In subsequent sliding process, due to hemisphere was worn and the original contact surface changed in size, shape and configuration state, friction force decreases obviously. Besides, in process of sliding friction, due to stick-slip effect, friction force appears obviously fluctuations. Moreover, if the sliding speed is large the changes of sliding speed have less effect on friction force when the nanoscale sphere sliding on the plane at the different speeds. read less

Abstract: The present reactive molecular dynamics (RMD) simulations discuss the formation of interphase regions in cured polymer adhesives. The latter are obtained from the curing of reactive liquid mixtures composed of pentafunctional linkers and bifunctional monomers in contact with idealized surfaces. The present reactive scheme mimics the one of epoxies with amine linkers, i.e., processes investigated experimentally by Possart and co-workers. Generic RMD simulations are performed in a coarse-grained (CG) resolution to evaluate basic principles in curing characterized by preferential interactions. The creation of linker-rich domains is promoted by preferential surface-linker as well as linker-linker interactions in the reactive mixtures. The dimension of the interphase both in the starting mixture and the cured network depends on these preferential interactions which lead to a retardation of the curing velocity. This retardation behavior is mapped by conversion curves as a function of the number of reactive steps and by the spatially resolved profiles of the connected linkers. Although derived by generic potentials, the simulated reduction of the curing velocity is in agreement with experimental results in epoxies. The chosen interactions also imply a smaller number of linker bonds in the interphase than in the bulk region. The present RMD approach offers insight into key parameters of curing processes under the influence of preferential surface interactions coupled to selective attractions in the liquid starting mixture. read less

Abstract: Wear of diamond tool is also very serious, which affects the surface quality of the machined work material, even if ductile mode where an undeformed chip thickness is at a nanoscale is used. During the cutting process, the crystal structure in the cutting zone is destroyed under the high pressures applied by the diamond tool. The silicon atoms adhering to the tool surface reconstruct to be in a crystal state under the effect of adhesion and pressures. read less

Abstract: The tensile and compressive behaviour of silicon carbide nanocones (SiCNCs) under axial strain has been investigated using classic molecular dynamics simulations. For the tensile behaviour, the influences of the cone height on the failure strain, and failure force as well as failure strain energy have been systematically explored. Both the failure strain and the strain energy of SiCNCs are found to decrease with the increasing cone height. However, the failure force is marginally affected by the cone height. For the compressive behaviour of the SiCNCs, the deformation patterns in the buckling and postbuckling stages are extensively examined. The influences of the cone height on compressive behaviour have been explored. It is noted that the increased cone height has significant effect on the critical strain, and critical force as well as critical strain energy per atom of SiCNCs. read less

Abstract: Molecular dynamics simulations are performed on the atomic origin of the growth process of graphite‐like carbon film on silicon substrate. The microstructure, mass density, and internal stress of as‐deposited films are investigated systematically. A strong energy dependence of microstructure and stress is revealed by varying the impact energy of the incident atoms (in the range 1–120 eV). As the impact energy is increased, the film internal stress converts from tensile stress to compressive stress, which is in agreement with the experimental results, and the bonding of C‐Si in the film is also increased for more substrate atoms are sputtered into the grown film. At the incident energy 40 eV, a densification of the deposited material is observed and the properties such as density, sp3 fraction, and compressive stress all reach their maximums. In addition, the effect of impact energy on the surface roughness is also studied. The surface morphology of the film exhibits different characteristics with different incident energy. When the energy is low (<40 eV), the surface roughness is reduced with the increasing of incident energy, and it reaches the minimum at 50 eV. Copyright © 2012 John Wiley & Sons, Ltd. read less

Abstract: The shear instability of the nanoscrystalline 3C-SiC during nanometric cutting at a cutting speed of 100 m/s has been investigated using molecular dynamics simulation. The deviatoric stress in the cutting zone was found to cause sp3-sp2 disorder resulting in the local formation of SiC-graphene and Herzfeld-Mott transitions of 3C-SiC at much lower transition pressures than that required under pure compression. Besides explaining the ductility of SiC at 1500 K, this is a promising phenomenon in general nanoscale engineering of SiC. It shows that modifying the tetrahedral bonding of 3C-SiC, which would otherwise require sophisticated pressure cells, can be achieved more easily by introducing non-hydrostatic stress conditions. read less

Abstract: Thermal transport in Si/Ge nano-composites, consisting of crystalline silicon as matrix and aligned germanium nanowires as inclusions, is investigated here through non-equilibrium and equilibrium molecular dynamics (MD) simulations. Our results show increasing of temperature gradient at the interface between silicon and germanium, which is limited in an interfacial phase of few lattices. A thermal boundary phase is included explicitly in our three-phase model, in companion with the modified effective medium theory, to be compared with MD simulation results with various nanowire concentrations. The results suggest that the presence of nanowires leads to a dramatic decrease of κ for heat transfer across nanowires arising from interfacial phase, while along the interfaces, the reduction of phonon mean free path due to interfacial scattering lowers κ of silicon matrix and germanium nanowires. read less

Abstract: Three dimensional molecular dynamics simulation on the nanocutting of monocrystalline silicon is carried out to investigate the material deformation behaviors and atomic motion characteristics of the machined workpiece. A deformation criterion is developed to determine the material deformation and phase transformation behavior in the subsurface layer based on the single-atom potential energy variations. The results show that the machined chips suffer a complex phase transformation and eventually present an amorphous structure caused by the plastic deformation behavior. A polycrystalline structure is obtained on the machined surface. Both plastic and elastic deformation simultaneously takes place on the machined surface, and elastic deformation takes place under the machined surface. In order to further unveil the mechanism of nanocutting process, the displacements of all atoms are also simulated. The simulation results shows that different atomic motions occur in different regions in the workpiece, and the chips formations occur via extrusion. read less

Abstract: Ideal shear strength under superimposed normal stress of cubic covalent crystals (C, Si, Ge, and SiC) is evaluated by ab initio density functional theory calculation. Shear directions in [ ] and [ ] on the (111) plane are examined. The critical shear stress along the former direction is lower than that along the latter in all the crystals unless the hydrostatic tension is extremely high. In both the [ ]-shear and [ ]-shear, critical shear stress is increased by compression in C but is decreased in the other crystals. The different response of the critical shear stress to normal stress is due to the strength of the bond-order term, i.e. dependence of the short-range interatomic attraction on the bond-angle. read less

Abstract: A growing scientific effort is being devoted to the study of nanoscale interface aspects such as thin-film adhesion, abrasive wear and nanofriction at surfaces by using the nanoscratching technique but there remain immense challenges. In this paper, a three-dimensional (3D) model is suggested for the molecular dynamics (MD) simulation and experimental verification of nanoscratching initiated from nano-indentation, carried out using atomic force microscope (AFM) indenters on Al-film/Si-substrate systems. Hybrid potentials such as Morse and Tersoff, and embedded atom methods (EAM) are taken into account together for the first time in this MD simulation (for three scratching conditions: e.g. orientation, depth and speed, and the relationship between forces and related parameters) in order to determine the mechanisms of nanoscratching phenomena. Salient features such as nanoscratching velocity, direction and depth - as well as indenter shape- and size-dependent functions such as scratch hardness, wear and coefficient of friction - are also examined. A remarkable conclusion is that the coefficient of friction clearly depends upon the tool rake-angle and therefore increases sharply for a large negative angle. read less

Abstract: We developed the computer simulation method to study growth of SiC at the SiC(0001)/Si1-xCx interface based on the Monte Carlo method. Energy is calculated by using the Tersoff potential and the lattice spacing is sub-divided to enable the structural relaxation in a dicrete manner. Before making an attempt for the atomic difusion via the species exchange process in the Metropolis alogrithm, local relaxation is carried out to locate atoms at the local minima of the potential surface. Then, parallel computation is carried out to thermally equilibrate a system. read less

Abstract: The growth of amorphous carbon films via deposition is investigated using molecular dynamics simulation with a modified Tersoff potential. The impact energy of carbon atoms ranges from 1 to 50 eV and the temperature of the diamond substrate is 300 K. The effects of the incident energy on the growth dynamics and film structure are studied in a detail. Simulation results show that the mobility of surface atoms in the cascade region is enhanced by impacting energetic carbon ions, especially at moderate energy, which favors the growth of denser and smoother films with better adhesion to the substrate. Our results agree qualitatively with the experimental observation. read less

Abstract: Tool wear not only changes its geometry accuracy and integrity, but also decrease machining precision and surface integrity of workpiece that affect using performance and service life of workpiece in ultra-precision machining. Scholars made a lot of experimental researches and stimulant analyses, but there is a great difference on the wear mechanism, especially on the nano-scale wear mechanism. In this paper, the three-dimensional simulation model is built to simulate nano-metric cutting of a single crystal silicon with a non-rigid right-angle diamond tool with 0 rake angle and 0 clearance angle by the molecular dynamics (MD) simulation approach, which is used to investigate the diamond tool wear during the nano-metric cutting process. A Tersoff potential is employed for the interaction between carbon-carbon atoms, silicon-silicon atoms and carbon-silicon atoms. The tool gets the high alternating shear stress, the tool wear firstly presents at the cutting edge where intension is low. At the corner the tool is splitted along the {1 1 1} crystal plane, which forms the tipping. The wear at the flank face is the structure transformation of diamond that the diamond structure transforms into the sheet graphite structure. Owing to the tool wear the cutting force increases. read less

Abstract: In this paper, we show that combining Multiwavelength Anomalous Diffraction (MAD) and Diffraction Anomalous Fine Structure (DAFS) spectroscopy, in grazing incidence geometry, allows to obtain structural properties (strain and composition) of semiconductor nanostructures. We report results obtained on dome-shaped Ge nano-islands grown on Si(001) surfaces and AlGaN nanowires grown on Si(100). It is shown that, in the case of sharp interfaces, MAD alone can not determine the mean Ge content in the region of the substrate-island interface and needs to be combined with Extented-DAFS measurements. read less

Abstract: Nanoindentation on ion-induced damage of silicon surface was performed using molecular dynamics. The simulation revealed interaction between a ion-induced damage substrate and a indentation. Two types of silicon substrates are used. One is silicon crystal, and another is amorphous silicon which damaged by argon ion bombardment. The initial velocity of the ion bombardment was 3.807×105 m/sec at 30 keV. The computation volume (8.00 nm × 8.00 nm × 17.24 nm) consisted of 57600 Si atoms with Si (100) surface. Each silicon substrate was pressed by the indentation and the force acting on the indentation head were investigated. The force of the ion-induced substrate is smaller than that of the crystalline substrate. Consequently, it turned out that the internal structure of silicon with ion-induced damage can be examined by the force distribution of the indentation. read less

Abstract: A numerical study is performed to investigate the effect of loading history on the size-dependent material properties of pure and nitrogen-doped ultrananocrystalline diamond (UNCD) specimens under different shear loading paths. A simple procedure with combined kinetic Monte Carlo and molecular dynamics methods is adopted to form a polycrystalline UNCD with an artificial grain boundary (GB). By randomly adding different numbers of nitrogen atoms into the GBs, the effects of nitrogen doping and GB width on the UNCD responses under pre-compressioned/tensioned shear loading conditions are studied. It appears that the loading history has certain influence on the size-dependent material properties of UNCD, which provides useful information for formulating a multiscale constitutive model with applications to general cases. read less

Abstract: Pure carbon nanotube (CNT) oscillators are compared to the corresponding CNT oscillators encapsulating copper nanowires (Cu@CNTs) by molecular dynamics simulations. The classical oscillation theory provides a fairly good estimate of the mass dependence of the operating frequency when the CNT surface is not deformed by the Cu nanowire. The structural deformations of the CNT induced by the encapsulated copper nanowire have a greater effect on the oscillation frequency than the mass of the copper nanowire. The excess forces of the Cu@CNT oscillator are slightly higher than those of the CNT oscillator and the excess van der Waals forces induced by the inter-wall interactions are 17 times higher than the excess forces induced by the Cu nanowire–CNT interactions. read less

Abstract: It is difficult for brittle materials to acquire high machined surface quality as they have low fracture strength. But in the case of nanometric cutting technology, nanometer level machined surface is often acquired by means of ductile material remove mode. In order to investigate physical essence of ductile machining process, this paper carries out molecular dynamics (MD) analysis of nanometric cutting single crystal silicon. The result shows that huge hydrostatic pressure induced in the local area which lead to the silicon atom transform from classical diamond structure ( α silicon) to metal structure ( β silicon). At the same time, the stress concentration is avoided by uniformly distributed pressure in the cutting area. These two important factors together result in the ductile machining of silicon and then acquire super-smooth surface. read less

Abstract: Dynamic damage response characteristics of an amorphous silicon carbide target due to hypersonic velocity impacts of diamond projectiles are investigated using molecular dynamics simulations. In a certain range of radii of the projectile, four distinct regimes of damage are uncovered and summarized in a penetration depth diagram. The regimes correspond to shallow crater formation, deep penetration into the target, deep penetration with local melting of the target, and complete disintegration of the projectile. In the third regime, a logarithmic dependence of the penetration depth as a function of the projectile velocity has been found and explained by an analytical model. read less

Abstract: Using first principles molecular dynamics and Nudged Elastic Band calculations, we have investigated the effect of irradiation on cubic silicon carbide at the atomic scale, and in particular the formation of Frenkel pairs, and the crystal recovery after thermal treatment. Threshold displacement energies have been determined for C and Si sublattice, and the stability and structure of the formed Frenkel pairs are described. The activation energies for annealing these defects have then been computed and compared with experiments. read less

Abstract: Focused Ga+ ion beams are routinely used at high incident angles for specimen preparation. Molecular dynamics simulations of 2 and 30keV Ga bombardment of Si(011) at a grazing angle of 88° were conducted to assess sputtering characteristics and damage depth. The bombardment of atomically flat surfaces and surfaces with vacancies shows little energy transfer yielding ion reflection. The bombardment of surfaces with adatoms allows for the coupling of the energy of motion parallel to the surface into the substrate resulting in sputtering. The adatom and one other Si atom eject, and motion in the substrate occurs down to a depth of 13A. Experimental evidence shows that sputtering is a reality, suggesting that an atomically flat surface is never achieved. read less

Abstract: An advanced nanocomposite microstructure such as that of polycrystalline Silicon Carbide (SiC)-Silicon Nitride (Si3N4) nanocomposites contains multiple lengthscales with grain boundary (GB) thickness of the order of 50 nm, SiC particle sizes of the order of 200300 nm and Si3N4 grain sizes of the order of 0.8 to 1.5 μm. Recent developments in failure analyses of such materials focus on continuum calculations with an account of the corresponding atomistic deformation mechanisms. In the presented research one such analysis approach is applied to analyze continuum level deformation in polycrystalline SiCSi3N4 nanocomposites. The continuum bilinear cohesive law is motivated from the atomistic SiC-Si3N4 interfacial separation analyses. The cohesive finite element method (CFEM) based analyses of dynamic fracture at a loading rate of 2 m/sec in bi-modal SiC-Si3N4 nanocomposites with an explicit account of the multiple length scales associated with GBs, second phase (SiC particles), and the primary phase (Si3N4 matrix) are performed. For CFEM analyses bimodal polycrystalline SiC-Si3N4 nanocomposite structures are generated with grain sizes of Si3N4 in the range of 0.8 to 1.5 μm and SiC particle size varying between 200 nm and 300 nm. The volume fraction of the SiC phase is fixed at 30%. In order to analyze the effect of GBs each sample of SiC-Si3N4 nanocomposite has two corresponding meshes: one with finite element (FE) mesh resolving GBs and the other with FE mesh neglecting GBs. Since, a given unique set of phase morphology defining parameters (such as location of SiC particles, SiC or GB distribution etc.) corresponds to a multiple sets of morphologies, three different random sets of morphologies are used to characterize the material behavior corresponding to one unique set of phase morphology parameters. Analyses clearly show that the GBs have a strong effect on dynamic fracture in the nanocomposites. read less

Abstract: We have investigated the nucleation and crystallization processes of molten silicon (Si) on SiO2 substrates by performing molecular dynamics (MD) simulations based on the modified Tersoff potential parameters. A heat flow that leads to a steady fall of the local temperature in the molten Si is achieved by determining the atomic movements with the combination of Langevin and Newton equations. Good agreement is reached between the predictions of temperatures based on the kinetic energies and the velocity distributions of atoms at local regions. The results of simulations revealed that the (111) plane of the Si nuclei formed at the surface was predominantly parallel to the substrate of MD cell. The surface energies of the (100), (110), and (111) planes of Si at 77 K were calculated to be 2.27, 1.52, and 1.20 J∕m2, respectively, and they were in good agreement with the experimental results. The lowest value of surface energy, 1.20 J∕m2, for the (111) plane at 1700 K was obtained under the condition of elastic... read less

Abstract: Molecular-dynamics simulations of crystalline (c), nanocrystalline (nc), and amorphous (a) silicon carbides and silicon were carried out to investigate their vibrational and mechanical properties. The atomic configurations, vibrational spectra, and stress-strain curves were calculated at room temperature. In the case of the nanocrystalline structures, these characteristics were analyzed as functions of grain size. Young's and bulk modul and yield and flow stresses were determined from uniaxial deformation of samples under periodic boundary constraints and from experiments on rod extension. For silicon carbides, Young's modulus and flow stress decrease in the sequence ``c-nc-a,'' and with decreasing grain size, which is attributed to a weakening of the Si--C bonds in the amorphous matrix. The enhancement of the strength properties of the homopolar nc--Si structures is attributed to their deformation anisotropy. The calculated vibrational spectra and Young's moduli are in rather good agreement with the corresponding experimental characteristics. read less

Abstract: Heteroepitaxial growth in the $\mathrm{Ge}∕\mathrm{Si}$ (001) system is known to lead to the formation of pyramid-like ``hut'' islands with {105}-oriented facets. Recent calculations of island formation energies in this system have suggested that edge energies lead to an important contribution to the barrier to island formation at small sizes. Here we provide an independent calculation of the magnitude of the average edge energy for $\mathrm{Ge}∕\mathrm{Si}(001)$ by matching the results of atomistic simulations to continuum theory for the energy of faceted surfaces. We consider an infinitely long Ge island, or wire, bounded by {105} facets with the recently proposed rebonded-step-model reconstruction, on a (001) wetting-layer terrace with the $2\ifmmode\times\else\texttimes\fi{}8$ dimer-vacancy-line reconstruction. To perform these calculations we derive models for edge structures between {105} facets and between {105} and (001) facets, leading in both cases to atomic coordinations with no more than one dangling bond per atom. For these model edge structures we obtain an average value for the edge energy on the order of $10\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\mathrm{\AA{}}$. read less

Abstract: A simulation system for nanoscale ductile mode cutting of monocrystalline silicon has been developed in thi study using the Molecular Dynamics (MD) method for better understanding of the ductile mode cutting mechanism. In the model of this simulation system, the initial atom positions of silicon workpiece material are arranged according to the crystal lattice structure, the atomic interactive actions of silicon are based on the Tersoff potential, the diamond cutting tool is assumed to be undeformable, the tool cutting edge is realistically modelled to have a finite radius, and the motions of the atoms in the chip formation zone are determined by Newton's equations of motion. The simulated variation of the cutting forces with the tool cutting edge radius is compared with the results of experimental cutting tests to substantiate the developed simulation system and the results show a good agreement with analytical findings. read less

Abstract: Single wall carbon nanotubes (SWCN's) have attracted scientific interest as a result of their remarkable mechanical and electrical properties. Metallic SWCN's can carry electrical current due to $\ensuremath{\pi}$ electrons propagating along their graphitelike surface. This work theoretically considers the effects resulting from the possibility of connecting nanotubes to a partially hydrogenated Si-rich $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Si}\mathrm{C}(100)$ $(3\ifmmode\times\else\texttimes\fi{}2)$ surface. This is done with the perspective that they can improve the metallic character of the surface. Calculations were performed using a first-principles pseudopotential methodology within the density-functional theory and local-density approximation. Results indicates the formation of a covalent bond between nanotube carbon atoms and surface silicon atoms. The structural and electronic properties of this theoretical system are presented and comparison with previous calculations for related systems is also done. read less

Abstract: Since no physical approach can be employed to study the mechanism in micro cutting, the molecular dynamics simulation is becoming more and more important. In this study, the results of molecular dynamics modeling and analysis on the nanometric machining on silicon surface are presented. According to the simulation, some phenomena in the nanometric cutting process are found. First, surface elastic rebound happens on the cut surface after cutter moving away. The value of the surface elastic rebound is calculated in the simulation. Second, the atoms near the corner of work piece swirl up following the cutter moving direction at the initial stage of removing atoms from the work piece. Third, the simulation results show that no matter how small material removal is, the burr is always formed at the edge of work piece. read less

Abstract: The thermal stability of the first-neighbor antisite pair configurations in 3C- and 4H-SiC is investigated by a comprehensive atomistic study. At first the structure and energetics of these defects is determined in order to check the accuracy of the Gao-Weber interatomic potential used. The results are comparable with literature data obtained by the density-functional theory. Then, the lifetime of the antisite pair configurations is calculated for temperatures between 800 and 2500 K. Both in 3C- and 4H-SiC the thermal stability of the antisite pairs is rather low. In contrast to previous theoretical interpretations, the antisite pair can be therefore not correlated with the DI photoluminescence center that is stable to above 2000 K. The atomic mechanisms during the recombination of the antisite pair in 3C-SiC and of three antisite pair configurations in 4H-SiC is a modified concerted exchange. Due to the different sizes of the silicon and the carbon atoms, this process is not identical with the concerted exchange in Si. Two intermediate metastable configurations found during the recombination are similar to the bond defect in Si. Since the SiC lattice contains two types of atoms, there are also two different types of bond defects. The two bond defects canmore » be considered as the result of the incomplete recombination of a carbon vacancy and a neighboring mixed dumbbell interstitial. For selected temperatures the thermal stability of the antisite pair in 3C-SiC is investigated by molecular dynamics simulations that are based on the density-functional theory. Their results are very similar to those of the atomistic study, i.e. the Gao-Weber potential describes the antisite pair and its recombination reasonably well. The antisite pair in 4H-SiC with the two atoms on hexagonal sites has a slightly different formation energy than the other three antisite pair configurations in 4H-SiC. Its lifetime shows another dependence on the temperature, and its recombination is characterized by a separate motion of atoms.« less read less

Abstract: This paper presents the molecular dynamics (MD) simulation of nano-indentation of diamond-like carbon (DLC) coating on silicon substrates. It is found that the mechanisms of nanoindentation of coated systems on the nanometre scale defers considerably from the same process on the micrometre scale. The coating thickness affects the mechanisms of plastic deformation both in the coating and the substrate. read less

Abstract: The present paper reports on molecular dynamics simulations of the formation process of Si quantum dots (Si QDs). Icosahedral Si QDs are formed spontaneously by freezing 274-, 280-, and 323-atom Si droplets. We find that the initialization of pentagonal channels leads into the overall icosahedral structure. We also study the melting behavior of the 280-atom icosahedral Si QD. We find that the melting point is reduced more than 15% compared with that of bulk Si. A possible approach to synthesize icosahedral Si QDs is discussed. The formation of the icosahedral structure could be expected in other systems characterized by tetrahedral bonding network. read less

Abstract: Brittle fracture dynamics for three low-index crack surfaces, i.e., (110), (111), and (100), in crystalline cubic silicon carbide (3C-SiC) is studied using molecular dynamics simulation. The results exhibit significant orientation dependence: (110) fracture propagates in a cleavage manner; (111) fracture involves slip in the {111¯} planes; and crack branching is observed in (001) fracture. Calculated critical energy release rates, which characterize fracture toughness, are compared with available experimental and ab initio calculation data. read less

Abstract: Molecular dynamics (MD) simulation can play a significant role in addressing a number of machining problems at the atomic scale. This simulation, unlike other simulation techniques, can provide new data and insights on nanometric machining; which cannot be obtained readily in any other theory or experiment. In this paper, some fundamental problems of mechanism are investigated in the nanometric cutting with the aid of molecular dynamics simulation, and the single-crystal silicon is chosen as the material. The study showed that the purely elastic deformation took place in a very narrow range in the initial stage of process of nanometric cutting. Shortly after that, dislocation appeared. And then, amorphous silicon came into being under high hydrostatic pressure. Significant change of volume of silicon specimen is observed, and it is considered that the change occur attribute to phase transition from a diamond silicon to a body-centered tetragonal silicon. The study also indicated that the temperature distributing of silicon in nanometric machining exhibited similarity to conventional machining. read less

Abstract: For a better understanding of essential mechanisms of material removal at extremely small depth of cut and of the brittle-ductile transition in the material removal process of monocrystalline silicon, nonometric deformation behaviour in three-point bending of defect-free monocrystalline silicon is analysed by molecular dynamics (MD) computer simulation. MD simulations show that plastic deformation takes place through a phase transformation from diamond to amorphous structures. The critical octahedral shearing stress for phase transformation is estimated to be 12–14 GPa. In the deformed region, a crack nucleus on atomic scale can be generated owing to thermally activated vibration of atoms. After the crack nucleus, the crack extension takes place under a certain stress field. The crack initition takes place when a tensile stress reaches a certain critical value of about 30 GPa at the crack nucleus. The critical values for plastic deformation and crack initiation depend on crystal orientation and hydrostatic pressure. It is shown that there can also be critical criteria of the stress field to determine whether plastic deformation or crack initiation would predomiantly take place. When the plastic deformation proceeds to a crack initiation, ductile mode machining can be realized. read less

Abstract: Mechanical characteristics of a suspended (10,10) single-walled carbon nanotube (SWCNT) during atomic force microscopy (AFM) nanoindentation are investigated at different temperatures by molecular dynamics simulations. The results indicate that the Young modulus of the (10,10) SWCNT under temperatures of 300?600?K is 1.2?1.3?TPa. As the temperature increases, the Young modulus of the SWCNT increases, but the axial strain of the SWCNT decreases. The strain-induced spontaneous formation of the Stone?Wales defects and the rippled behaviour under inhomogeneous stress are studied. The rippled behaviour of the SWCNT is enhanced with the increasing axial strain. The adhesive phenomenon between the probe and the nanotube and the elastic recovery of the nanotube during the retraction are also investigated. read less

Abstract: For better understanding of the essential mechanisms of material removal at extremely small depth of cut and ductile-brittle transition in material removal process of monocrystalline silicon carbide, which is expected as a next generation semiconductor material for wide band gap, high-voltage and low-loss power devices, nanometric deformation behavior in three-point bending of defect-free monocrystalline silicon carbide is analyzed by molecular dynamics (MD) computer simulation. The MD simulation results show that plastic deformation takes place through a phase transformation from cubic zinc sulfide to amorphous structures. The critical octahedral shearing stress for phase transformation is estimated to be 24 to 38 GPa. In the deformed region, a crack nucleus in atomic scale can be generated due to thermally activated vibration of atoms. After the crack nucleus, the crack extension takes place under certain stress field. The crack initiation takes place when a tensile stress reaches a certain critical value of approximately 67 GPa at the crack nucleus. The critical values for plastic deformation and crack initiation depend on crystal orientation and hydrostatic pressure. The results also show that there can be a number of critical criteria of the stress field to determine which processes plastic deformation or crack initiation predominantly takes place. When the plastic deformation proceeds to a crack initiation, ductile mode machining can be realized. Introduction There are increasing demands for ductile mode machining of brittle materials, especially of industrial key materials such as silicon and silicon carbide, with higher form accuracy and better surface quality together with lower costs. On the other hand, it is empirically well known that chip is removed in ductile mode under the depth of cut smaller than a critical value intrinsic to the work material. To realize ductile mode machining on any brittle material, it is essential to understand mechanisms of material removal at extremely small depth of cut. In general, brittle fracture takes place by extension of pre-existing micro-cracks. It depends on balance of two different energy states associated with crack extension and local plastic deformation whether brittle fracture or plastic deformation takes place. The former is the excess surface energy for extension of pre-existing crack, the latter the energy for local plastic deformation of corresponding site. However, mechanisms of ductile-brittle transition of defect-free monocrystalline silicon carbide have not been fully understood. Diamond indenter (Rigid) Loading (V=50m/s) Thermostat atoms (293 K) Periodic boundary SiC (100) read less

Abstract: We have performed classical molecular dynamics simulations of amorphous Si–C–N materials. The dependence of the local order and of the microstructure on the chemical composition was investigated. Our simulations show that for a stoichiometric nitrogen/silicon ratio equal to or higher than 4/3, the amorphous ceramic separates into different amorphous domains, namely C-rich, SiN-rich, and SiC-rich phases. Below this ratio, the material is composed of mixed structures, homogeneously spread within the material. For a very particular composition range, we found that carbon atoms crystallize into monoatomic graphitic layers surrounding the SiN-rich domains. read less

Abstract: Molecular-dynamics simulations of crystal growth from melted Si have been performed using the Tersoff potential to analyze the defect formation and migration processes. We observed that a five-membered ring generated at the solid/liquid interface initiated self-interstitial defect formation. The formation and migration energies of the obtained defect were calculated to be 3.85 and 0.94 eV, respectively. The sum of the calculated formation and migration energies, 4.79 eV, is in good agreement with the experimental activation energy of self-diffusion in crystal Si. read less

Abstract: Structures of symmetrical tilt grain boundaries and antiphase boundaries in β-silicon carbide are investigated by means of classical molecular dynamics simulations using the Tersoff potential. A structural unit model is used to classify the possible interface structures. Structures and energies are given for 〈001〉 tilt grain boundaries in the angular range 0° ≤ θ ≤ 53.15° and for 〈110〉 tilt grain boundaries in the range 0° ≤ θ ≤ 70.53° of tilt angles. It is found that among 〈110〉 boundaries, those containing antiphase boundaries are the most stable ones in an intermediate angular range. read less

Abstract: Monte Carlo simulation (MCS) on the thermal properties in Gd–Mg alloy becomes essential as there are only limited experiments available. A realistic Johnson potential is used to workout the specific heats for various temperatures and hence the Debye temperature. The results from the present simulation technique are very well compared with our shell model calculation. The need of better X-ray measurements for Debye–Waller factor and Debye temperature other than the measurements of Subadhra and Sirdeshmukh, is discussed in detail. read less

Abstract: The process of energetic C atom deposition on Si (001)-(2×1) is studied by the molecular dynamics method using the semi-empirical many-bond Tersoff potential. It is found that the incident energy of the carbon atom has an important effect on the collision process and its diffusion process on the substrate. Most of the incident energy of the carbon atom is transferred to the substrate atoms within the initial two vibration periods of substrate atoms and its value increases with the incident energy. The spreading distance and penetration depth of the incident atom increasing with the incident energy are also identified. The simulated results imply that an important effect of energy of incident carbon on the film growth at low substrate temperature provides activation energy for silicon carbide formation through the vibration enhancement of local substrate atoms. In addition, suppressing carbon atom inhomogeneous collection and dispensing with the silicon diffusion process may be effectively promoted by the spreading and penetration of the energetic carbon atom in the silicon substrate. read less

Abstract: Molecular dynamics (MD) simulations of CF3+ ion bombardment of Si predict the formation of a steady-state fluorocarbosilyl mixing layer that actively participates in the etching of the underlying Si. The active nature of this mixing layer has been characterized by computing atomic residence time distributions (RTDs) for adsorbed fluorine and carbon. The average residence time of carbon in the layers is seen to increase dramatically as ion energy increases, while that of fluorine is not sensitive to ion energy. The overall RTDs compare well with those of an ideal stirred tank. A simple “well-mixed” transient mass balance model is presented. The phenomenology of this model is based on interpretations of the MD results. The model correctly predicts the evolution of atomic concentrations in the mixing layer. Both the MD and model results shed new light on how CF3+ ions etch Si. read less

Abstract: Monte Carlo simulation of an FCC lattice-gas model is carried out to study order-disorder phase transitions. To study an actual Cu-Au alloys as quantitatively as possible, a Finnis-Sinclair-type potential, which has been used widely for molecular dynamics (MD) simulations, is mapped onto the FCC model by using the potential renormalization technique proposed by one of us. Using this renormalized potential, we find that the linear expansion coefficient of Cu and Au crystals and the transition temperatures are greatly improved when compared with the case of using the MD potential directly on the lattice. read less

Abstract: Brittle fracture of β‐SiC (polytype 3C) under hydrostatic tension has been modeled by molecular dynamics simulation using an interatomic potential function that treats the solid as fully covalent. The critical stress at which the lattice becomes structurally unstable is shown to agree quantitatively with that predicted by stability analysis based on elastic stiffness coefficients. The instability mode is the spinodal (vanishing of bulk modulus), and decohesion occurs as spontaneous nucleation of cracking on {111} shuffle planes. Atomic relaxation on the newly generated cracked surfaces appears to take place immediately following crack opening. read less

Abstract: Mechanism of SiC heteroepitaxial growth by the carbonization of the Si(001) surface was studied at the atomic scale using molecular dynamics (MD) simulations and molecular beam epitaxy (MBE) experiments. Heteroepitaxial growth of single crystal 3c‐SiC on the Si(001) surface (3c‐SiC[001]∥Si[001] and 3c‐SiC[110]∥Si[110]) was observed in both the MD simulations and MBE experiments. Breaking of the Si—Si bonds and shrinkage of the [110] Si rows with C atoms are possible mechanisms for the heteroepitaxial growth of SiC on Si(001). Microscopic structures and mechanisms of the twin formations and pit formations are discussed. Ultraviolet light irradiation is proposed and confirmed to enhance the epitaxial growth of SiC in the MBE experiments. read less

Abstract: The structures of the (100) surfaces of silicon and germanium generally have been interpreted in a static manner in the past. We present molecular dynamics (MD) simulations that show these surfaces to consist of a mixture of rapidly interconverting buckled and unbuckled dimers. Over a time average, the surface is found to have long p(2×1) rows of symmetric, unbuckled dimers, as seen in recent scanning tunneling microscopy images of silicon. However, higher order unit cells are observed in He scattering and low energy electron diffraction experiments at low temperatures. We present a dynamical interpretation of the structure to explain both sets of observations. The simulations have been performed on different size slabs at both constant energy and constant temperature utilizing a new method for effective removal of heat from an exothermic system while retaining the correct dynamics. Several different interaction potentials were analyzed in an attempt to find the most realistic one for simulations of these surfaces. The effect of surface defects and annealing were also investigated. The surface phonon densities of states were calculated and for Si(100) are in good agreement with experiments and other theoretical treatments. Such simulations and structural analyses are reported for the first time for Ge(100). read less

Abstract: This study establishes that the effective thermal conductivity keff of crystalline nanoporous silicon is strongly affected not only by the porosity fv and the system’s length Lz but also by the pore interfacial area concentration Ai . The thermal conductivity of crystalline nanoporous silicon was predicted using non-equilibrium molecular dynamics (NEMD) simulations. The Stillinger-Weber potential for silicon was used to simulate the interatomic interactions. Spherical pores organized in a simple cubic lattice were introduced in a crystalline silicon matrix by removing atoms within selected regions of the simulation cell. Effects of the (i) system length ranging from 13 to 130 nm, (ii) pore diameter varying between 1.74 and 5.86 nm, and (iii) porosity ranging from 8% to 38%, on thermal conductivity were investigated. A physics-based model was also developed by combining kinetic theory and the coherent potential approximation. The effective thermal conductivity was proportional to (1 –1.5 fv ) and inversely proportional to the sum (Ai /4 +1 /Lz ). This model was in excellent agreement with the thermal conductivity of nanoporous silicon predicted by MD simulations for spherical pores (present study) as well as for cylindrical pores and vacancy defects reported in the literature. These results will be useful in designing nanostructured materials with desired thermal conductivity by tuning their morphology.Copyright © 2011 by ASME read less

Abstract: In order to study an unknown mechanism of defect initiation in Hertz indentation of monocrystalline silicon with no preexisting defect, analytical solution controlled-MD simulation proposed by the authors has been carried out using an MD model embedded on the surface of silicon. It has been shown that a defect that will become a ring crack can be generated just outside the outer periphery of the contact area between the silicon and indenter. The mechanism of the defect initiation is that the dynamic force associating with acoustic waves transforms monocrystal structure to polycrystal one at the outer periphery of the contact area and this phase change under a macroscopic/static tensile stress acting there triggers cross slips which result in micro-void defects. read less

Abstract: Although extensive studies have been carried out on structural analyses of amorphous silicon-germanium (a-Si1-xGex) alloys, there are still controversies concerning short-range order in a-Si1-xGex alloys. In this study, we examined amorphous structures of Si1-xGex alloys by molecular-dynamics (MD) simulations using the Tersoff interatomic potential. Amorphous networks were prepared by rapid quenching from liquid Si1-xGex alloys. Computer-generated atomic configurations consisted of tetrahedral networks with complete chemical disorder in the first coordination shell. The bond lengths of Si-Si, Si-Ge and Ge-Ge pairs slightly increased with the Ge composition, and a weaker composition dependence was observed in the bond lengths of Si-Si and Si-Ge pairs than that for Ge-Ge pair. These results were in good agreement with those obtained experimentally, suggesting that the MD calculations based on the Tersoff potential is useful for the structural analysis of a-Si1-xGex alloys. It was confirmed that the Ge-Ge bond length and bond angle surrounding Ge atoms are more distorted. This is attributed to the difference between Si and Ge in the bond-stretching and in the bond-bending forces. read less

Abstract: We have performed molecular dynamics simulations of adatom diffusion on the SiC(001) surface and found that the barriers for carbon adatoms is less than that for silicon adatoms. The diffusion paths were also found to be temperature dependent and at high temperatures the adatom diffusion constant was found to of the order of 10−5 cm2 s−1 read less