Abstract: Recently, nanostructuration has been proposed to improve the performance of phase change memories. This is the case of superlattices composed of amorphous carbon and crystalline germanium telluride, which we have investigated by molecular dynamics. For this, a modified Stillinger–Weber potential is adapted to reproduce their stiffness contrast/impedance ratio. In order to study the effect of the interface interaction, two sets of parameters are used to model the interfaces with different interactions between the two materials using the properties of the softer material or the average properties between the two creating an adaptation of impedance across the layers. The effects of interface roughness and carbon diffusion at grain boundaries are studied. Using equilibrium molecular dynamics as well as the propagation of wave-packets, we show first that without impedance adaptation, the anisotropy is high, and the roughness has a marked impact on the properties. However, the introduction of impedance adaptation destroys those effects on the thermal conductivity. Finally, we show that the periodic texturing of the interface increases the transmission of in-plane transverse phonons. read less

Abstract: The adsorption of organic aromatic molecules, namely aniline, onto graphene oxide is investigated using molecular simulations. The effect of the oxidation level of the graphene oxide sheet as well as the presence of two different halide salts, sodium chloride and sodium iodide, were examined. The aniline molecule in the more-reduced graphene oxide case, in the absence of added salt, showed a slightly greater affinity for the graphene oxide–water interface as compared to the oxidized form. The presence of the iodide ion increased the affinity of the aniline molecule in the reduced case but had the opposite effect for the more-oxidized form. The effect of oxidation and added salt on the interfacial water layer was also examined. read less

Abstract: Vibrational spectroscopy is a nondestructive technique commonly used in chemical and physical analyses to determine atomic structures and associated properties. However, the evaluation and interpretation of spectroscopic profiles based on human-identifiable peaks can be difficult and convoluted. To address this challenge, we present a reliable protocol based on supervised manifold learning techniques meant to connect vibrational spectra to a variety of complex and diverse atomic structure configurations. As an illustration, we examined a large database of virtual vibrational spectroscopy profiles generated from atomistic simulations for silicon structures subjected to different stress, amorphization, and disordering states. We evaluated representative features in those spectra via various linear and nonlinear dimensionality reduction techniques and used the reduced representation of those features with decision trees to correlate them with structural information unavailable through classical human-identifiable peak analysis. We show that our trained model accurately (over 97% accuracy) and robustly (insensitive to noise) disentangles the contribution from the different material states, hence demonstrating a comprehensive decoding of spectroscopic profiles beyond classical (human-identifiable) peak analysis. read less

Abstract: Development of high-performance lithium-ion batteries (LIBs) is boosted by the needs of the modern automotive industry and the wide expansion of all kinds of electronic devices. First of all, improvements should be associated with an increase in the specific capacity and charging rate as well as the cyclic stability of electrode materials. The complexity of experimental anode material selection is now the main limiting factor in improving LIB performance. Computer selection of anode materials based on first-principles and classical molecular dynamics modeling can be considered as the main paths to success. However, even combined anodes cannot always provide high LIB characteristics and it is necessary to resort to their alloying. Transmutation neutron doping (NTD) is the most appropriate way to improve the properties of thin film silicon anodes. In this review, the effectiveness of the NTD procedure for silicene/graphite (nickel) anodes is shown. With moderate P doping (up to 6%), the increase in the capacity of a silicene channel on a Ni substrate can be 15–20%, while maintaining the safety margin of silicene during cycling. This review can serve as a starting point for meaningful selection and optimization of the performance of anode materials. read less

Abstract: Molecular dynamics simulation is among the most significant methods in nanoscale studies. This paper studied the effect of strain rate, temperature, and nanotube chirality on the stress-strain behavior of aluminum/silicon nanotubes (SiNTs) using molecular dynamics simulation. Ultimate tensile stress and Young’s modulus of the nanocomposite were evaluated using molecular dynamics simulation. According to the results, Young’s modulus of the nanocomposite decreased with increasing temperature. Also, Young’s modulus decreased by increasing the strain rate. Next, an experimental approach was used based on the Box–Behnken design. According to the input parameters and the experimental approach, the number of simulations in the software was 39 runs. Overall, it is concluded that the optimal conditions were created at a temperature of 50 K, a strain rate of 0.01/ps, and chirality of (5,5), leading to the elasticity modulus of 137 GPa and the ultimate tensile stress of 11.8 GPa. read less

Abstract: Pure and gold-doped amorphous silicon membranes were irradiated with swift heavy ions (75 MeV Ag or 1.1 GeV Au ions) and studied using small-angle x-ray scattering. The samples that were irradiated with 1.1 GeV Au ions produced a scattering pattern consistent with core-shell-type ion tracks of 2.0 ± 0.1 nm (core) and 7.0 ± 0.3 nm (total) radius irrespective of gold doping, consistent with radii previously observed [9]. The density in the core is nearly 4% different from that of the surrounding material. The entire track (core + shell) is slightly less dense than the surrounding material, yielding an expansion or hammering constant A of 0 . 036 ± 0 . 003 nm 2 per ion track, consistent with the macroscopic “hammering” deformation. No tracks were found in samples irradiated with 75 MeV Ag ions, and no signature specific to the gold impurity doping could be observed. read less

Abstract: Surface interactions between epoxy matrices and graphene nanosheets can directly affect the properties of nanocomposites, and such interactions are crucial to investigate. In this study, molecular dynamics simulation was used to investigate the interaction rate between epoxy thermoset resins (EPON 828, EPON 862, Epoxy Novolac, and Cycloaliphatic Epoxy) with graphene, and Functionalized Graphene with Oxygen Functional Groups. The effect of temperatures and different weight percentage of graphene oxide (1, 3, and 5 wt.%) was studied, and as a result of which it was observed that increasing the weight percentage of graphene oxide improved the interaction energy (adhesion) between epoxy resins and graphene oxide. Also, it decreased the radius of gyration (more contraction of epoxy molecules than graphene oxide nanosheets). In addition, increasing the temperature led to reduction of the interaction energy between epoxy resins and functionalized graphene. Moreover, escalating the temperature did not have any significant effect on the radius of gyration and Epoxy molecules resizing compared to graphene oxide nanosheets. The amount of interaction energy between epoxy resins and pristine graphene is very low in comparison with graphene oxide; therefore, temperature has a small effect on the amount of interaction energy and radius of gyration. Finally, the strongest interaction energy was found to be related to Epoxy Novolac and EPON 862 resins by comparing the interaction energy and radius of gyration between epoxy resins with graphene oxide. Therefore, they were the best candidates for matrixes in epoxy/graphene oxide nanocomposites. read less

Abstract: The irradiation of the target surface by an ultrafast femtosecond (fs) laser pulse produces the extreme non-equilibrium states of matter and subsequent phase transformations. Computational modeling and simulation is a very important tool for gaining insight into the physics processes that govern the laser–matter interactions, and, specifically, for quantitative understanding the laser light absorption, electron–ion energy exchange, spallation, melting, warm dense matter regime, vaporization, and expansion of plasma plume. High-fidelity predictive modeling of a variety of these multi-physics processes that take place at various time and length scales is extremely difficult, requiring the coupled multi-physics and multi-scale models. This topical review covers progress and advances in developing the modeling approaches and performing the state-of-the-art simulations of fs laser-pulse interactions with solids and plasmas. A complete kinetic description of a plasma based on the most accurate Vlasov–Maxwell set of equations is first presented and discussed in detail. After that an exact kinetic model that encompasses the microscopic motions of all the individual particles, their charge and current densities, generated electric and magnetic fields, and the effects of these fields on the motion of charged particles in a plasma is briefly reviewed. The methodology of kinetic particle-in-cell (PIC) approach that is well suitable for computational studies of the non-linear processes in laser–plasma interactions is then presented. The hydrodynamic models used for the description of plasmas under the assumption of a local thermodynamic equilibrium include the two-fluid and two-temperature model and its simplifications. The two-temperature model coupled with molecular dynamics (MD) method is finally discussed. Examples are illustrated from research areas such as applications of the fully kinetic, PIC, hydrodynamic, and MD models to studies of ultrafast laser–matter interactions. Challenges and prospects in the development of computational models and their applications to the modeling of ultrafast intense laser–solid and laser–plasma interactions are overviewed. read less

Abstract: In this paper, the vibrational behavior of the atomic force microscope (AFM) on a graphene sheet sample was analyzed using a multi-scale model. The cantilever and silicone tip base were simulated using continuum mechanics and finite element modeling, while the tip apex was modeled using Tersoff potential and structural mechanics modeling. The modified Morse potential was used to model the single-layer graphene, and the Lennard-jones potential was employed as nonlinear springs to model the interactions between the graphene layers and the tip-sample. In addition, the contact behavior between the tip and graphene was investigated by measuring the friction force during the movement of the tip on the graphene sheet, and the results were compared to those obtained from a molecular dynamics simulation and an experimental test. The friction force between the tip and graphene increased by enhancing the tip radius and the contact surface between the tip and the sample. With the initial distance displacement of the tip from the sample, two curves of the tip oscillation amplitude variations and the tip oscillation and excitation vibration phase shift were plotted. In conclusion, the results of the present multi-scale model are compared with those of the MD simulation and demonstrate the strong correlation between the proposed model and the MD model. read less

Abstract: Silicon carbide (SiC) is a promising structural and cladding material for accident tolerant fuel cladding of nuclear reactor due to its excellent properties. However, when exposed to severe environments (e.g., during neutron irradiation), lattice defects are created in amounts significantly greater than normal concentrations. Then, a series of radiation damage behaviors (e.g., radiation swelling) appear. Accurate understanding of radiation damage of nuclear materials is the key to the design of new fuel cladding materials. Multi-scale computational simulations are often required to understand the physical mechanism of radiation damage. In this work, the effect of neutron irradiation on the volume swelling of cubic-SiC film with 0.3 mm was studied by using the combination of molecular dynamics (MD) and rate theory (RT). It was found that for C-vacancy (CV), C-interstitial (CI), Si-vacancy (SiV), Si-interstitial (SiI), and Si-antisite (SiC), the volume of supercell increases linearly with the increase of concentration of these defects, while the volume of supercell decreases linearly with the increase of defect concentration for C-antisite (CSi). Furthermore, according to the neutron spectrum of a certain reactor, one RT model was constructed to simulate the evolution of point defect under neutron irradiation. Then, the relationship between the volume swelling and the dose of neutrons can be obtained through the results of MD and RT. It was found that swelling typically increases logarithmically with radiation dose and saturates at relatively low doses, and that the critical dose for abrupt transition of volume is consistent with the available experimental data, which indicates that the rate theory model can effectively describe the radiation damage evolution process of SiC. This work not only presents a systematic study on the relationship between various point defect and excess volume, but also gives a good example of multi-scale modelling through coupling the results of binary collision, MD and RT methods, etc., regardless of the multi-scale modelling only focus on the evolution of primary point defects. read less

Abstract: Strain rate is a critical parameter in the mechanical application of nano-devices. A comparative atomistic study on both perfect monocrystalline silicon crystal and silicon nanowire was performed to investigate how the strain rate affects the mechanical response of these silicon structures. Using a rate response model, the strain rate sensitivity and the critical strain rate of two structures were given. The rate-dependent dislocation activities in the fracture process were also discussed, from which the dislocation nucleation and motion were found to play an important role in the low strain rate deformations. Finally, through the comparison of five equivalent stresses, the von Mises stress was verified as a robust yield criterion of the two silicon structures under the strain rate effects. read less

Abstract: This work uses molecular dynamics to simulate the grooving behavior of optical silica glass. The amorphous SiO2 (silica glass) was fabricated using a melting-quenching process, and the critical cut depth, and mechanism of the chip formation, were explored using a molecular dynamics simulation. The analytical results indicated that the tool edge radius affected the critical cut depth and roughness of the machined surface. A larger tool edge radius had a larger equivalent negative rake angle at the same depth of cut, causing a larger critical cut depth, while producing a smoother machined surface. In addition, the uncut chip thickness affected the tangential force and thrust force distribution weighting of the tool. The temperature field analysis revealed that a groove formed by the chip formation mechanism resulted in a higher workpiece temperature, causing the brittle material to exhibit ductile cutting behavior during the nanogrooving process. However, grooves formed by the scratching-indenting mechanism had a lower workpiece temperature. read less

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

Abstract: Most analyses of the mechanical deformation of electrode materials of lithium-ion battery in the framework of continuum mechanics suggest the occurring of structural damage/degradation during the de-lithiation phase and cannot explain the lithiation-induced damage/degradation in electrode materials, as observed experimentally. In this work, we present first-principle analysis of the interaction between two adjacent silicon atoms from the Stillinger–Weber two-body potential and obtain the critical separation between the two silicon atoms for the rupture of Si–Si bonds. Simple calculation of the engineering-tensile strain for the formation of Li–Si intermetallic compounds from the lithiation of silicon reveals that cracking and cavitation in lithiated silicon can occur due to the formation of Li–Si intermetallic compounds. Assuming the proportionality between the net mass flux across the tip surface of a slit crack and the migration rate of the crack tip, we develop analytical formulas for the growth and healing of the slit crack controlled by lithiation and de-lithiation, respectively. It is the combinational effects of the state of charge, the radius of curvature of the crack tip and local electromotive force that determine the cycling-induced growth and healing of surface cracks in lithiated silicon. read less

Abstract: First-principles lattice dynamics is applied to symmetric tilt grain boundaries (GBs) in Al, Si and MgO, with the goal of revealing critical factors in determining excess vibrational entropies at the atomic level. Excess vibrational entropies at GBs are found to vary depending on the substances. Al GBs tend to show larger excess entropies and hence larger temperature dependence of the GB free energies than those in Si and MgO. Most of the Si GBs show small excess entropies. For Al and MgO, atom-projected vibrational entropies are well correlated with bond-length changes at GB cores, and have large positive values as bond lengths increase for GB atoms. This demonstrates that a similar mechanism likely dominates excess vibrational entropies of GBs for both substances, despite their dissimilar bonding nature. For Si GBs, atoms with threefold coordination do not simply follow such a correlation, implying the importance of other factors that are different from bond-length changes. These systematic comparisons will be a foothold for understanding a physical origin of excess entropies at GBs even in more complex substances. read less

Abstract: Molecular dynamics simulation defined as simple state updates over multiple time steps is the main approach to the description of the chemical, mechanical and electrical processes in many real-world application. Currently, most optimization methods focus on simplifying forces or computing parallelization. However, modern general-purpose supercomputers have succeeded on compute-intensive or memory/bandwidth-intensive applications, but the improvement of network bandwidth/latency is fairly limited. The communication overhead is critical, and severely degrades the overall performance and scalability. In this paper, we propose a communication optimization strategy to reduce inter-nodes communication overheads, including a ghost communication mode to reduce the total amount of message, a shift communication algorithm to reduce the total number of messages and a zero-copy RDMA (Remote Direct Memory Access) communication method to reduce inter-nodes memory copy overheads. We implement our ghost communication, shift communication and zero-copy RDMA communication on the third highest performance supercomputer Sunway TaihuLight in the world (before June 2020). We test molecular dynamics simulation of condensed covalent materials and scale the simulation up to 8,519,680 cores to simulate more than 50.4 million silicon atoms, where the parallel efficiency is over 80% on the whole machine. Results show that the communication optimization strategy reduces the communication time by nearly 75% compared with traditional inter-nodes MPI (Message Passing Interface) communication with memory copy and the dimension of the simulated system significantly exceeds the experimentally measurable range. Our simulation enables virtual experiments on real-world applications and makes the technology more accessible to the general scientific users by using general-purpose supercomputers. read less

Abstract: The nonionizing energy loss (NIEL) concept has been used for many years for qualitative estimation of the damage induced by radiation (particles and gamma rays). Different approaches, based on the physics of radiation interaction with the nuclear and electronic subsystems of the target atoms, are used for calculating NIEL values. The simplest model used is the binary collision approximation (BCA), which is applicable for energies larger than the threshold energy for Frenkel pair creation. In this article, we analyze the dependence of the calculated NIEL values (in silicon and germanium) on the basic interaction characteristics: the differential cross section (DCS) of energy transferred to the recoiling atoms and the partition factor between ionization and ion displacement. We estimate the differences between existing approaches and the possible scatter of the NIEL data for Coulomb, elastic, and inelastic nuclear scatterings. We also present new partition factors based on fits to experimental data. The contribution of low-energy cascade zones of displaced atoms (pockets) to the total damage is estimated using results of molecular dynamics (MD) calculation. We find for silicon a total increase of NIEL by 30%–40% relative to the original (BCA) NIEL value. The role of phonon excitations in the subthreshold cascades on the damage value is discussed. read less

Abstract: This paper reveals the existence of a critical separation distance ( d c) beyond which the elastic interactions between a pair of monovacancies in graphene or hexagonal boron nitride become inconsequential for the strength and toughness of the defective lattice. This distance is independent of the chirality of the lattice. For any inter-defect distance higher than d c, the lattice behaves mechanically as if there is a single defect. For a distance less than d c, the defect–defect elastic interactions produce distinctive mechanical behavior depending on the orientation ( θ) of the defect pair relative to the loading direction. Both strength and toughness of the lattice containing a pair of “interacting monovacancies (iMVs)” are either higher or smaller than that of the lattice containing a pair of “non-interacting monovacancies (nMVs),” suggesting the existence of a critical orientation angle θ c. For θ < θ c, the smaller the distance between the iMVs, the higher the toughness and strength compared to the lattice containing nMVs, whereas, for θ ≥ θ c, the smaller the separation distance between the iMVs, the smaller the toughness and strength compared to the lattice containing nMVs. The transitional behavior has a negligible dependence on the chirality of the lattice, which indicates that the crystallographic anisotropy has a much weaker influence on toughness and strength compared to the anisotropy induced by the orientation angle itself. These observations underline an important point that the elastic fields emanating from vacancy defects are highly localized and fully contained within a small region of around 1.5 nm radius. read less

Abstract: We perform a systematic study of thermal resistance/conductance of tilt grain boundaries (GBs) in Si using classical molecular dynamics. The GBs studied are naturally divided into three groups according to the structural units forming the GB core. We find that, within each group, the GB thermal conductivity strongly correlates with the excess GB energy. All three groups predict nearly the same GB conductivity extrapolated to the high-energy limit. This limiting value is close to the thermal conductivity of amorphous Si, suggesting similar heat transport mechanisms. While the lattice thermal conductivity decreases with temperature, the GB conductivity slightly increases. However, at high temperatures it turns over and starts decreasing if the GB structure undergoes a premelting transformation. Analysis of vibrational spectra of GBs resolved along different directions sheds light on the mechanisms of their thermal resistance. The existence of alternating tensile and compressive atomic environments in the GB core gives rise to localized vibrational modes, frequency gaps creating acoustic mismatch with lattice phonons, and anharmonic vibrations of loosely-bound atoms residing in open atomic environments. read less

Abstract: Thermal storage potential and thermal expansion are characteristic properties for extreme applications. ZrB2 is a candidate for advanced applications in aircraft and fusion reactors. This article presents density functional theory calculations of its states, microstructure and quasi-harmonic levels calculations of thermophysical properties. Band structure highlighted dynamical instability with metallic impurities in ZrB2 structure based on frequency modes. The observed projected density of states (PDOS) appropriate 4d orbital of Zr dominated at low frequency both in perfect crystal in the presence or absence of covalent impurities while B 2s and 2p orbitals dominate higher frequency states. Temperature dependency and anisotropy of coefficient of thermal expansion (CTE) were evaluated with various impurities. Various thermodynamic properties like entropy and free energy were explored for degrees of freedom resulting from internal energy changes in the material. Computed results for heat capacity and CTE were compared to available numerical and experimental data. read less

Abstract: Artificial neural-network (ANN) interatomic potentials for simulating atomic structures and energetics of grain boundaries (GBs) in silicon were constructed and integrated into structural optimization and molecular dynamics (MD) algorithms. A training dataset including various atomic environments of symmetric tilt GBs was generated by performing density-functional-theory (DFT) calculations. The ANN potential after training was found to be capable of approximating the potential-energy surface at GBs even with dangling bonds and large atomic displacements at high temperatures, which cannot be well reproduced with empirical interatomic potentials. Additionally, reliability of the ANN potential for molecular simulations was evaluated. GB structures optimized or equilibrated by the ANN molecular simulations were also energetically lower for DFT calculations, without significant errors. The ANN potential is therefore expected to greatly reduce structural-optimization iterations and required time steps to acquire stable or equilibrium GB structures in MD simulations, enabling us to address even large-scale systems of general GBs in silicon, with high accuracy and low computational cost. read less

Abstract: The stability of the system “bi-layer silicene on the graphite substrate” is studied in the molecular dynamics simulation. Silicene sheets are doped with phosphorus, and graphite sheets are doped with nitrogen. Lithium ion moves along a silicene channel with a gap in the range of 0.6–0.8 nm. The time for the ion to pass the channel and leave it decreases with an increase in the channel gap. There is a tendency of the silicene sheets roughness growth with an increase in the gap between silicene sheets (except, 0.75 nm). Doping phosphorus and nitrogen atoms stabilize the silicene and graphite structure. 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: We provide a new reactive force field (ReaxFF) for simulations of silicon oxycarbide (SiCO) ceramics and of their syntheses from inorganic polymer precursors. The validity of the force field is extensively tested against experimental and computational thermochemical data. Its performance in simulation at elevated temperatures is gauged by the results of comprehensive ab initio molecular dynamics simulations. We apply the force field to the formation of amorphous SiCO in a simulated polymer pyrolysis. Modeling results are in good agreement with experimental observations and allow new insights into the formation of graphene segregations embedded in an amorphous oxycarbide matrix. The new ReaxFF for Si–C–O–H compounds enables large-scale and long-time atomistic simulations with unprecedented fidelity. read less

Abstract: The vibrational properties of glasses remain a topic of intense interest due to several unresolved puzzles, including the origin of the Boson peak and the mechanisms of thermal transport. Inelastic scattering measurements have revealed that amorphous solids support collective acoustic excitations with low THz frequencies despite the atomic disorder, but these frequencies are well below most of the thermal vibrational spectrum. Here, we report the observation of acoustic excitations with frequencies up to 10 THz in amorphous silicon. The excitations have atomic-scale wavelengths as short as 6 A and exist well into the thermal vibrational frequencies. Simulations indicate that these high-frequency waves are supported due to the high group velocity and monatomic composition of a-Si, suggesting that other glasses with these characteristics may also exhibit such excitations. Our findings demonstrate that a substantial portion of thermal vibrational modes in amorphous materials can still be described as a phonon gas despite the lack of atomic order. read less

Abstract: In this work, we investigated tensile and compression forces effect on the thermal conductivity of silicon. We used equilibrium molecular dynamics approach for the evaluation of thermal conductivity considering different interatomic potentials. More specifically, we tested Stillinger-Weber, Tersoff, Environment-Dependent Interatomic Potential and Modified Embedded Atom Method potentials for the description of silicon atom motion under different strain and temperature conditions. Additionally, we extracted phonon density of states and dispersion curves from molecular dynamics simulations. These data were used for direct calculations of thermal conductivity considering the kinetic theory approach. Comparison of molecular dynamics and kinetic theory simulations results as a function of strain and temperature allowed us to investigate the different factors affecting the thermal conductivity of strained silicon. read less

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

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

Abstract: This work investigates how the interatomic surface potential influences molecular dynamics (MD)-derived thermal accommodation coefficients (TACs). Iron, copper, and silicon surfaces are considered over a range of temperatures that include their melting points. Several classes of potentials are reviewed, including two-body, three-body, and bond-order force fields. MD-derived densities and visualization of the surfaces are used to explain the differences in the parameterizations of these potentials within the context of gas–surface scattering. Finally, TACs are predicted for a range of gas–surface combinations, and recommended values of the TAC are selected that take into account the robustness and uncertainties of each of the considered parameterizations. Further, it is observed that there is a significant change in the TAC about phase changes that must be taken into account for applications with a large range of surface temperatures. read less

Abstract: Molecular dynamics simulations with Tersoff-Ziegler-Biersack-Littmark (Tersoff-ZBL) potential and adaptive intermolecular reactive empirical bond order (AIREBO) potential are performed to study the substitutional process of silicon ions by bombardment. The silicon ions bombardment of graphene is simulated at energies 100 eV, 100 eV, 68 eV and 67 eV, respectively. All silicon atoms are substitute for the relevant carbon atoms at these energies. And a perfect region of SiC structure in graphene sheet is observed, this approach can viewed as a new preparation of graphene-based SiC electronics in theory. read less

Abstract: Thermal resistance across the interface between touching surfaces is critical for many industrial applications. We developed a network model to predict the macroscopic thermal resistance of mechanically contacting surfaces. Contacting interfaces are fractally rough, with small islands of locally intimate contact separated by regions with a wider gas filled boundary gap. Heat flow across the interface is therefore heterogeneous and thus the contact model is based on a network of thermal resistors representing boundary resistance at local contacts and the access resistance for lateral transport to contacts. Molecular dynamics simulations have been performed to characterize boundary resistance of Silicon Alumina interfaces for testing the sensitivity of thermal resistance to contact opening. Boltzmann transport simulations of access resistance in Si are conducted in the ballistic transport regime. read less

Abstract: The melting curve of silicon is investigated through classical molecular dynamics simulations. We explore pressures from 0 to 20 GPa using the EDIP, Stillinger-Weber, and Tersoff interactomic potentials. Using the Z method, we demonstrate that the predicted melting temperature Tm can be significantly overestimated, depending on the potential chosen. Our results show that none of the potentials explored is able to reproduce the experimental melting curve of silicon by means of the Z-method. However, the EDIP potential does predict the change in the Clapeyron slope, associated with the diamond to β-tin phase transition. read less

Abstract: Understanding the impact dynamics and spreading of molten nanosized droplets on a solid surface is a crucial step towards the design and control of nano-fabrication in many novel applications of nanotechnology. In this context, molecular dynamic (MD) simulations have been conducted to compute temperature and dynamic contact angles of nano-droplets during impact. The evolution of the morphology of a molten metallic nano-droplet impacting on a substrate has been studied using a combination of experimental and simulation techniques. Femtosecond lasers have been used to transfer nanosized gold droplets. Droplet morphology calculated in MD simulations is found to be in good agreement with that seen in scanning electron microscopy (SEM) images. It is found that the spreading of nanoscale molten gold droplets upon impact is enhanced by increasing the droplet impact energy. As observed in experimental data, MD simulation results show that a high droplet-substrate heat transfer rate together with increased wettability of the substrate facilitates spreading and results in a thinner metal deposit after solidification. read less

Abstract: The Spectral Neighbor Analysis Potential (SNAP) is a classical interatomic potential that expresses the energy of each atom as a linear function of selected bispectrum components of the neighbor atoms. An extension of the SNAP form is proposed that includes quadratic terms in the bispectrum components. The extension is shown to provide a large increase in accuracy relative to the linear form, while incurring only a modest increase in computational cost. The mathematical structure of the quadratic SNAP form is similar to the embedded atom method (EAM), with the SNAP bispectrum components serving as counterparts to the two-body density functions in EAM. The effectiveness of the new form is demonstrated using an extensive set of training data for tantalum structures. Similar to artificial neural network potentials, the quadratic SNAP form requires substantially more training data in order to prevent overfitting. The quality of this new potential form is measured through a robust cross-validation analysis. read less

Abstract: Multiphase chemical reactors with characteristic multiscale structures are intrinsically discrete at the elemental scale. However, due to the lack of multiscale models and the limitation of computational capability, such reactors are traditionally treated as continua through straightforward averaging in engineering simulations or as completely discrete systems in theoretical studies. The continuum approach is advantageous in terms of the scale and speed of computation but does not always give good predictions, which is, in many cases, the strength of the discrete approach. On the other hand, however, the discrete approach is too computationally expensive for engineering applications. Developments in computing science and technologies and encouraging progress in multiscale modeling have enabled discrete simulations to be extended to engineering systems and represent a promising approach to virtual process engineering (VPE, or virtual reality in process engineering). In this review, we analyze this emerging trend and emphasize that multiscale discrete simulations (MSDS), that is, considering multiscale structures in discrete simulation through rational coarse-graining and coupling between discrete and continuum methods with the effect of mesoscale structures accounted in both cases, is an effective way forward, which can be complementary to the continuum approach that is being improved by multiscale modeling also. For this purpose, our review is not meant to be a complete summary to the literature on discrete simulation, but rather a demonstration of its feasibility for engineering applications. We therefore discuss the enabling methods and technologies for MSDS, taking granular and particle-fluid flows as typical examples in chemical engineering. We cover the spectrum of modeling, numerical methods, algorithms, software implementation and even hardware-software codesign. The structural consistency among these aspects is shown to be the pivot for the success of MSDS. We conclude that with these developments, MSDS could soon become, among others, a mainstream simulation approach in chemical engineering which enables VPE. read less

Abstract: Ion beam irradiation has recently emerged as a versatile approach to functional materials design. We show in this work that patterned defective regions generated by ion beam irradiation of silicon can create a phonon glass electron crystal (PGEC), a longstanding goal of thermoelectrics. By controlling the effective diameter of and spacing between the defective regions, molecular dynamics simulations suggest a reduction of the thermal conductivity by a factor of $\approx$20 is achievable. Boltzmann theory shows that the thermoelectric power factor remains largely intact in the damaged material. To facilitate the Boltzmann theory, we derive an analytical model for electron scattering with cylindrical defective regions based on partial wave analysis. Together we predict a figure of merit of ZT$\approx$0.5 or more at room temperature for optimally patterned geometries of these silicon metamaterials. These findings indicate that nanostructuring of patterned defective regions in crystalline materials is a viable approach to realize a PGEC, and ion beam irradiation could be a promising fabrication strategy. read less

Abstract: Gas cluster ion beams bring new opportunities for diagnostics and modification of materials surfaces. In this work impact of argon clusters on silicon dioxide has been studied by molecular dynamics simulations and experimentally. We have obtained dependencies of crater size and the SiO2 sputtering yield on cluster size and specific energy. High reactive selectivity of sputtered products has been revealed for a high specific energy of clusters. It can cause modification of the target surface layer composition in case of long time irradiation. Peculiarities of experimental and computational data matching have been discussed.Gas cluster ion beams bring new opportunities for diagnostics and modification of materials surfaces. In this work impact of argon clusters on silicon dioxide has been studied by molecular dynamics simulations and experimentally. We have obtained dependencies of crater size and the SiO2 sputtering yield on cluster size and specific energy. High reactive selectivity of sputtered products has been revealed for a high specific energy of clusters. It can cause modification of the target surface layer composition in case of long time irradiation. Peculiarities of experimental and computational data matching have been discussed. read less

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

Abstract: Using molecular dynamics simulations, we study collisions between amorphous silica nanoparticles. Our silica model contains uncontaminated surfaces, that is, the effect of surface hydroxylation or of adsorbed water layers is excluded. For central collisions, we characterize the boundary between sticking and bouncing collisions as a function of impact velocity and particle size and quantify the coefficient of restitution. We show that the traditional Johnson-Kendall-Roberts (JKR) model provides a valid description of the ingoing trajectory of two grains up to the moment of maximum compression. The distance of closest approach is slightly underestimated by the JKR model, due to the appearance of plasticity in the grains, which shows up in the form of localized shear transformation zones. The JKR model strongly underestimates the contact radius and the collision duration during the outgoing trajectory, evidencing that the breaking of covalent bonds during grain separation is not well described by this model. The adhesive neck formed between the two grains finally collapses while creating narrow filaments joining the grains, which eventually tear. read less

Abstract: The thermal atomic vibrations of amorphous solids can be distinguished by whether they propagate as elastic waves or do not propagate due to lack of atomic periodicity. In a-Si, prior works concluded that nonpropagating waves are the dominant contributors to heat transport, with propagating waves being restricted to frequencies less than a few THz and scattered by anharmonicity. Here, we present a lattice and molecular dynamics analysis of vibrations in a-Si that supports a qualitatively different picture in which propagating elastic waves dominate the thermal conduction and are scattered by local fluctuations of elastic modulus rather than anharmonicity. We explicitly demonstrate the propagating nature of waves up to around 10 THz, and further show that pseudoperiodic structures with homogeneous elastic properties exhibit a marked temperature dependence characteristic of anharmonic interactions. Our work suggests that most heat is carried by propagating elastic waves in a-Si and demonstrates that manipulating local elastic modulus variations is a promising route to realize amorphous materials with extreme thermal properties. read less

Abstract: 1 Molecular dynamics simulation (MDS) to study nanoscale cutting processes Saurav Goel1*, Saeed Zare Chavoshi2 and Adrian Murphy3 1Precision Engineering Institute, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, Bedfordshire, MK430AL, UK 2Mechanical Engineering Department, Imperial College London, London, SW7 2AZ, UK 3School of Mechanical and Aerospace Engineering, Queen’s University, Belfast, BT9 5AH, UK *Corresponding author Tel.: +44 1234754132, Email address: sgoel.diamond@gmail.com read less

Abstract: Today, many fields of rapidly growing research about nanomaterials and nanodevices are dependent on a combined detailed investigation between nanoscience and engineering. Hence, current nanotechnological engineering requires a vital linkage between fundamental research about the nature of the materials, which should be sought in nanoscience, and engineering investigation tools through simulations and modeling in computational nanomechanics. This linkage is necessary to obtain a comprehensive picture of the properties and characteristics of the studied nanomaterials and nanodevices under various conditions. In this chapter, a review of the fundamental concepts of the Newtonian mechanics, including Lagrangian and Hamiltonian functions is provided first. The developed equations of motion of a system with interacting material points are introduced then. After that, based on the physics of nanosystems, which can be applicable in any material phases, basic concepts of molecular dynamic simulations are introduced. Some interatomic potentials from which Morse function is recognized as an accurate definition are discussed in the next step for defining the natural bond length. With the purpose of decreasing computational effort, the cut-off radius is considered to limit atomic interactions to immediate neighbors only. The link between molecular dynamics and quantum mechanics is then explained using a simple classical example of two interacting hydrogen atoms, and the major limitations of the simulation method are discussed. Length and timescale limitation of molecular dynamics simulation technique are the major reasons behind opting multiscale simulations rather than molecular dynamics, which are explained briefly at the final sections of this chapter. read less

Abstract: Nanoscale friction often exhibits hysteresis when load is increased (loading) and then decreased (unloading) and is manifested as larger friction measured during unloading compared to loading for a given load. In this work, the origins of load-dependent friction hysteresis were explored through atomic force microscopy (AFM) experiments of a silicon tip sliding on chemical vapor deposited graphene in air, and molecular dynamics simulations of a model AFM tip on graphene, mimicking both vacuum and humid air environmental conditions. It was found that only simulations with water at the tip-graphene contact reproduced the experimentally observed hysteresis. The mechanisms underlying this friction hysteresis were then investigated in the simulations by varying the graphene-water interaction strength. The size of the water-graphene interface exhibited hysteresis trends consistent with the friction, while measures of other previously proposed mechanisms, such as out-of-plane deformation of the graphene film and irreversible reorganization of the water molecules at the shearing interface, were less correlated to the friction hysteresis. The relationship between the size of the sliding interface and friction observed in the simulations was explained in terms of the varying contact angles in front of and behind the sliding tip, which were larger during loading than unloading. read less

Abstract: Atomically deposited layers of SiO2 and Al2O3 have been recognized as promising coating materials to buffer the volumetric expansion and capacity retention upon the chemo-mechanical cycling of the nanostructured silicon- (Si-) based electrodes. Furthermore, silica (SiO2) is known as a promising candidate for the anode of next-generation lithium ion batteries (LIBs) due to its superior specific charge capacity and low discharge potential similar to Si anodes. In order to describe Li-transport in mixed silica/alumina/silicon systems we developed a ReaxFF potential for Li-Si-O-Al interactions. Using this potential, a series of hybrid grand canonical Monte Carlo (GCMC) and molecular dynamic (MD) simulations were carried out to probe the lithiation behavior of silica structures. The Li transport through both crystalline and amorphous silica was evaluated using the newly optimized force field. The anisotropic diffusivity of Li in crystalline silica cases is demonstrated. The ReaxFF diffusion study also verifies the transferability of the new force field from crystalline to amorphous phases. Our simulation results indicates the capability of the developed force field to examine the energetics and kinetics of lithiation as well as Li transportation within the crystalline/amorphous silica and alumina phases and provide a fundamental understanding on the lithiation reactions involved in the Si electrodes covered by silica/alumina coating layers. read less

Abstract: The universality of the scaling laws that correlate the hydrodynamic slip length and static contact angle was investigated by introducing the concept of the wettability transparency of graphene-coated surfaces. Equilibrium molecular dynamics simulations of droplet wettability for Si(111), Si(100), and graphene-coated silicon surfaces were performed to determine the conditions required to obtain similar contact angles between bare and graphene-coated surfaces (wettability transparency). The hydrodynamic slip length was determined by means of equilibrium calculations for silicon and graphene-coated silicon nanochannels. The results indicate that the slip-wettability scaling laws can be used to describe the slip behavior of the bare silicon nanochannels in general terms; however, clear departures from a general universal description were observed for hydrophobic conditions. In addition, a significant difference in the hydrodynamic slippage was observed under wettability transparency conditions. Alternatively... read less

Abstract: There is a need to choose appropriate interatomic empirical potentials for the molecular dynamics (MD) simulation of nanomachining, so as to represent chip formation and other cutting processes reliably. Popularly applied potentials namely; Lennard-Jones (LJ), Morse, Embedded Atom Method (EAM) and Tersoff were employed in the molecular dynamics simulation of nanometric machining of copper workpiece with diamond tool. The EAM potentials were used for the modelling of the copper-copper atom interactions. The pairs of EAM-Morse and EAM-LJ were used for the workpiece-tool (copper-diamond) atomic interface. The Tersoff potential was used for the carbon-carbon interactions in the diamond tool. Multi-pass simulations were carried out and it was observed that the EAM-LJ and the EAM-Morse pair potentials with the tool modelled as deformable with Tersoff potential were best suitable for the simulation. The former exhibit the lowest cutting forces and the latter has the lowest potential energy. read less

Abstract: Using Molecular-Dynamics simulation we observed the generation of amorphous SiO2 target from a randomly distributed Si and O atoms. We applied a sequence of annealing of the target with various temperature and quenching to room temperature. The relaxation time required by the system to form SiO4 tetrahedral mesh after a relatively long simulation time, is studied. The final amorphous target was analyzed using the radial distribution function method, which can be compared with the available theoretical and experimental data. We found that up to 70% of the target atoms form the tetrahedral SiO4 molecules. The number of formed tetrahedral increases following the growth function and the rate of SiO4 formation follows Arrhenius law, depends on the annealing temperature. The local structure of amorphous SiO2 after this treatment agrees well with those reported in some literatures. read less

Abstract: We demonstrate the applicability of extended cluster expansion technique, GICE, to calculation of a potential energy surface (PES) at discrete position in terms of atomic arrangement with an example of Cu and Cu-Ti binary system on fcc lattice. We find that the proposed CE successfully predicts total energy within error of 0.5meV/atom for Cu and 1.2meV/atom for Cu-Ti with respect to DFT calculation, which indicates that this method can model the PES and possesses potential to formulate physical properties in terms of atomic arrangement. [doi:10.2320/matertrans.M2015024] read less

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

Abstract: The Glenn Research Centre of NASA, USA (www.grc.nasa.gov/WWW/SiC/, silicon carbide electronics) is in pursuit of realizing bulk manufacturing of silicon carbide (SiC), specifically by mechanical means. Single point diamond turning (SPDT) technology which employs diamond (the hardest naturally-occurring material realized to date) as a cutting tool to cut a workpiece is a highly productive manufacturing process. However, machining SiC using SPDT is a complex process and, while several experimental and analytical studies presented to date aid in the understanding of several critical processes of machining SiC, the current knowledge on the ductile behaviour of SiC is still sparse. This is due to a number of simultaneously occurring physical phenomena that may take place on multiple length and time scales. For example, nucleation of dislocation can take place at small inclusions that are of a few atoms in size and once nucleated, the interaction of these nucleations can manifest stresses on the micrometre length scales. The understanding of how these stresses manifest during fracture in the brittle range, or dislocations/phase transformations in the ductile range, is crucial to understanding the brittle–ductile transition in SiC. Furthermore, there is a need to incorporate an appropriate simulation-based approach in the manufacturing research on SiC, owing primarily to the number of uncertainties in the current experimental research that includes wear of the cutting tool, poor controllability of the nano-regime machining scale (effective thickness of cut), and coolant effects (interfacial phenomena between the tool, workpiece/chip and coolant), etc. In this review, these two problems are combined together to posit an improved understanding on the current theoretical knowledge on the SPDT of SiC obtained from molecular dynamics simulation. read less

Abstract: The EON software is designed for simulations of the state-to-state evolution of atomic scale systems over timescales greatly exceeding that of direct classical dynamics. States are defined as collections of atomic configurations from which a minimization of the potential energy gives the same inherent structure. The time evolution is assumed to be governed by rare events, where transitions between states are uncorrelated and infrequent compared with the timescale of atomic vibrations. Several methods for calculating the state-to-state evolution have been implemented in EON, including parallel replica dynamics, hyperdynamics and adaptive kinetic Monte Carlo. Global optimization methods, including simulated annealing, basin hopping and minima hopping are also implemented. The software has a client/server architecture where the computationally intensive evaluations of the interatomic interactions are calculated on the client-side and the state-to-state evolution is managed by the server. The client supports optimization for different computer architectures to maximize computational efficiency. The server is written in Python so that developers have access to the high-level functionality without delving into the computationally intensive components. Communication between the server and clients is abstracted so that calculations can be deployed on a single machine, clusters using a queuing system, large parallel computers using a message passing interface, or within a distributed computing environment. A generic interface to the evaluation of the interatomic interactions is defined so that empirical potentials, such as in LAMMPS, and density functional theory as implemented in VASP and GPAW can be used interchangeably. Examples are given to demonstrate the range of systems that can be modeled, including surface diffusion and island ripening of adsorbed atoms on metal surfaces, molecular diffusion on the surface of ice and global structural optimization of nanoparticles. read less

Abstract: The development of interatomic potentials employing artificial neural networks has seen tremendous progress in recent years. While until recently the applicability of neural network potentials (NNPs) has been restricted to low-dimensional systems, this limitation has now been overcome and high-dimensional NNPs can be used in large-scale molecular dynamics simulations of thousands of atoms. NNPs are constructed by adjusting a set of parameters using data from electronic structure calculations, and in many cases energies and forces can be obtained with very high accuracy. Therefore, NNP-based simulation results are often very close to those gained by a direct application of first-principles methods. In this review, the basic methodology of high-dimensional NNPs will be presented with a special focus on the scope and the remaining limitations of this approach. The development of NNPs requires substantial computational effort as typically thousands of reference calculations are required. Still, if the problem to be studied involves very large systems or long simulation times this overhead is regained quickly. Further, the method is still limited to systems containing about three or four chemical elements due to the rapidly increasing complexity of the configuration space, although many atoms of each species can be present. Due to the ability of NNPs to describe even extremely complex atomic configurations with excellent accuracy irrespective of the nature of the atomic interactions, they represent a general and therefore widely applicable technique, e.g. for addressing problems in materials science, for investigating properties of interfaces, and for studying solvation processes. read less

Abstract: Glassy Eyed In crystalline materials, the collective motion of atoms in one- and two-dimensional defects—like dislocations and stacking faults—controls the response to an applied strain, but how glassy materials change their structure in response to strain is much less clear. Huang et al. (p. 224; see the Perspective by Heyde) used advanced-transmission electron microscopy to investigate the structural rearrangements in a two-dimensional glass, including the basis for shear deformations and the atomic behavior at the glass/liquid interface. Dynamics of individual atoms in a two-dimensional silicate glass have been observed using transmission electron microscopy. [Also see Perspective by Heyde] Structural rearrangements control a wide range of behavior in amorphous materials, and visualizing these atomic-scale rearrangements is critical for developing and refining models for how glasses bend, break, and melt. It is difficult, however, to directly image atomic motion in disordered solids. We demonstrate that using aberration-corrected transmission electron microscopy, we can excite and image atomic rearrangements in a two-dimensional silica glass—revealing a complex dance of elastic and plastic deformations, phase transitions, and their interplay. We identified the strain associated with individual ring rearrangements, observed the role of vacancies in shear deformation, and quantified fluctuations at a glass/liquid interface. These examples illustrate the wide-ranging and fundamental materials physics that can now be studied at atomic-resolution via transmission electron microscopy of two-dimensional glasses. read less

Abstract: The Moores law which predicts that the number of transistors which can be integrated on the computer chip will double every 24 months and which has been the guiding principle for the advancement of the computer industry, is gradually reaching its limit. This is due to the limitations imposed by the laws of physics. Similarly, in the machining sector, Taniguchi predicted an increasing achievable machining precision as a function of time in the 1980s and this prediction is still on course. The question also is, is there a limit to machining and to material removal processes; and how far can this prediction be sustained In this paper, the molecular dynamics (MD) simulation was employed to investigate this limit in the nanomachining of a copper workpiece with a diamond tool. The variation of the depth of cut used was from 0.01nm to 0.5nm. The Embedded Atom Method (EAM) potential was used for the copper-copper interactions in the workpiece; the Lennard-Jones (LJ) potential was used for the copper-carbon (workpiece-tool interface) interactions and the tool (carbon-carbon interactions) was modelled as deformable by using the Tersoff potential. It was observed from the simulation results that no material removal occurred between 0.01nm 0.25nm depth. At the depth of cut of 0.3nm, a layer of atoms appears to be removed or ploughed through by the tool. At a depth of cut less than 0.3nm, the other only phenomenon observed was the squeezing of the atom. The 0.3nm depth of cut is around the diameter of the workpiece-copper atom. So, it may be suggested that the limit of machining may be the removal of the atom of the workpiece. read less

Abstract: This work provides comprehensive modeling for the bond length and angle distributions in random and spontaneously ordered ternary III-V alloys using empirical interaction potentials. The compounds InxGa1−xAs, GaAs1−xSbx, and InxGa1−xP were used as model systems due to their technological importance and the fact that ordered structures were observed experimentally in these materials. For random alloys, we reproduce the bimodal bond length distribution, which allows linear fits with slopes between 0.087 A and 0.1059 A for all bond types. The calculated values for dilute compositions slightly deviate from these functions, causing stronger deformations. In the case of CuPt-ordered structures, the bond length distribution is shown to collapse to four sharp peaks with an area ratio of 1:3:3:1, which originate from a different atom to atom distance within the different (111) planes and perpendicular to these. An essential consequence of this atomic arrangement is the different spacings for the different stacked ... read less

Abstract: Using the non-equilibrium Green’s function techniques with interatomic potentials, we study the temperature dependence and the crossover of thermal conductance from the usual behavior proportional to the cross-sectional area at room temperature to the universal quantized behavior at low temperature for carbon nanotubes, silicon nanowires, and diamond nanowires. We find that this crossover of thermal conductance occurs smoothly for the quasi-one-dimensional materials and its universal behavior is well reproduced by the simplified model characterized by two parameters. read less

Abstract: The minimum depth of cut (MDC) is a major limiting factor on achievable accuracy in nanomachining, because the generated surface roughness is primarily attributed to the ploughing process when the uncut chip thickness is less than the MDC. This paper presents the material removal in a nanomachining process, where a sharp diamond tool with an edge radius of few atoms acts on a crystalline copper workpiece. The molecular dynamics (MD) simulation results show the phenomena of rubbing, ploughing and cutting. The formation of chip occurred from the depth of cut thickness of 1~1.5 nm. Also, the effects of the interatomic potentials on the MDC have been presented. read less

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

Abstract: We present results on parameterization of reactive force field [van Duin, A. C. T.; Dasgupta, S.; Lorant, F.; Goddard, W. A. ReaxFF: A Reactive Force Field for Hydrocarbons. J. Phys. Chem. A 2001, 105, 9396-9409] for investigating the properties of the [Nb6O19Hx]((8-x)-) Lindqvist polyoxoanion, x = 0-8, in water. Force-field parameters were fitted to an extensive data set consisting of structures and energetics obtained at the Perdew-Burke-Ernzerhof density functional level of theory. These parameters can reasonably describe pure water structure as well as water with an excess of H(+) and OH(-) ions. Molecular dynamics simulations were performed on [Nb6O19Hx]((8-x)-), x = 0-8, submerged in bulk water at 298 K. Analysis of the MD trajectories showed facile H atom transfer between the protonated polyoxoanion core and bulk water. The number of oxygen sites labeled with an H atom was found to vary depending on the pH of the solution. Detailed analysis shows that the total number of protons at bridging (terminal), η-O (μ2-O), sites ranges from 3(1) at pH 7, to 2(0) at pH 11, to 1(0) at pH 15. These findings closely reflect available experimental measurements. read less

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

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

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

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

Abstract: This chapter outlines our progress toward developing a first-principles-based hierarchical multiscale, multiparadigm modeling and simulation framework for the characterization and optimization of electronic and chemical properties of nanoscale materials and devices. In our approach, we build from the bottom-up by solving the quantum-mechanical (QM) Schrodinger equation for small systems. The results of these calculations lead to physical parameters that feed into methods capable of spanning longer length and time scale with minimum loss of accuracy. This is achieved by having higher-scale quantities self-consistently derived and optimized from the results at finer scales. In contrast to other methods, we are strictly first-principles-based, and all of our parameters at all scales relate to physically measurable or QM-computable observables. Our approach that is applicable to the forward (materials phenomenology) and inverse (“materials by design”) problems. The inverse problem involves top-down predictions of structures and compositions at a lower scale from desired properties at a higher scale. The advantages of our strategy over experimental- and phenomenological-based modeling and simulation approaches include the following: (1) providing access to details that are difficult or impossible to measure (e.g., excited electronic states in materials undergoing extreme conditions of pressure, temperature, etc.); (2) the ability to make useful predictions outside the range of experiments (i.e., since all calculations are ultimately related to first principles); and (3) providing sound, first-principles-based, steering for experiments. read less

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

Abstract: Paracrystalline Amorphous silicon has traditionally been represented by a continuous random network model in which there is no long-range ordering for the atoms, and some have less than fourfold coordination, which form dangling bonds—a type of defect. Treacy and Borisenko (p. 950; see the Perspective by Gibson) used fluctuation electron microscopy to explain that models including regions of crystalline order are needed to fit the observed local variations in structure. Thus, on the 1- to 2-nanometer-length scale, this material should be thought of as having a paracrystalline structure containing localized crystalline regions. Amorphous silicon is more accurately described by a paracrystalline model, not the idealized continuous random network. It is widely believed that the continuous random network (CRN) model represents the structural topology of amorphous silicon. The key evidence is that the model can reproduce well experimental reduced density functions (RDFs) obtained by diffraction. By using a combination of electron diffraction and fluctuation electron microscopy (FEM) variance data as experimental constraints in a structural relaxation procedure, we show that the CRN is not unique in matching the experimental RDF. We find that inhomogeneous paracrystalline structures containing local cubic ordering at the 10 to 20 angstrom length scale are also fully consistent with the RDF data. Crucially, they also matched the FEM variance data, unlike the CRN model. The paracrystalline model has implications for understanding phase transformation processes in various materials that extend beyond amorphous silicon. read less

Abstract: Using molecular dynamics simulations and a hybrid Tersoff-ZBL potential, the effects of irradiating graphene with a carbon ion at several positions and several energies from 0.1 eV to 100 keV are studied. The simulations show four types of processes: absorption, reflection, transmission, and vacancy formation. At energies below 10 eV, the dominant process is reflection; between 10 and 100 eV, it is absorption; and between 100 eV and 100 keV, the dominant process is transmission. Vacancy formation is a low-probability process that takes place at energies above 30 eV. Three types of defects are found: adatom, single vacancy, and 5–8–5 defect formed from a double-vacancy defect. The simulations provide a fundamental understanding of the graphene carbon bombardment and the parameters to develop graphene devices by controlling defect formation. read less

Abstract: Graphics processing unit (GPU) is becoming a powerful computational tool in scientific and engineering fields. In this paper, for the purpose of the full employment of computing capability, a novel mode for parallel molecular dynamics (MD) simulation is presented and implemented on basis of multiple GPUs and hybrids with central processing units (CPUs). Taking into account the interactions between CPUs, GPUs, and the threads on GPU in a multi-scale and multilevel computational architecture, several cases, such as polycrystalline silicon and heat transfer on the surface of silicon crystals, are provided and taken as model systems to verify the feasibility and validity of the mode. Furthermore, the mode can be extended to MD simulation of other areas such as biology, chemistry and so forth. read less

Abstract: We examine simulated electron microdiffraction patterns from models of thin polycrystalline silicon. The models are made by a Voronoi tessellation of random points in a box. The Voronoi domains are randomly selected to contain either a randomly-oriented cubic crystalline grain or a region of continuous random network material. The microdiffraction simulations from coherent probes of different widths are computed at the ideal kinematical limit, ignoring inelastic and multiple scattering. By examining the normalized intensity variance that is obtained in fluctuation electron microscopy experiments, we confirm that intensity fluctuations increase monotonically with the percentage of crystalline grains in the material. However, anomalously high variance is observed for models that have 100% crystalline grains with no imperfections. We confirm that the reduced normalized variance, V(k,R) − 1, that is associated with four-body correlations at scattering vector k, varies inversely with specimen thickness. Further, for probe sizes R larger than the mean grain size, we confirm that the reduced normalized variance obeys the predicted form given by Gibson et al. [Ultramicroscopy, 83, 169–178 (2000)] for the kinematical coherent scattering limit. read less

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

Abstract: This Letter is devoted to the investigation of the tip, substrate and particle flexibility effects on the elastic deformation, the maximum repulsive force and the topography images in tapping mode (amplitude modulation) atomic force microscopy (TM-AFM). Several quantitative comparisons among the different models are presented and the effects of the elastic deformations on TM-AFM measurement are investigated. read less

Abstract: 1. Introduction Part I. Continuum Mechanics and Thermodynamics: 2. Essential continuum mechanics and thermodynamics Part II. Atomistics: 3. Lattices and crystal structures 4. Quantum mechanics of materials 5. Empirical atomistic models of materials 6. Molecular statics Part III. Atomistic Foundations of Continuum Concepts: 7. Classical equilibrium statistical mechanics 8. Microscopic expressions for continuum fields 9. Molecular dynamics Part IV. Multiscale Methods: 10. What is multiscale modeling? 11. Atomistic constitutive relations for multilattice crystals 12. Atomistic/continuum coupling: static methods 13. Atomistic/continuum coupling: finite temperature and dynamics Appendix References Index. read less

Abstract: Using harmonic and anharmonic force constants extracted from density functional calculations within a supercell, we have developed a relatively simple but general method to compute thermodynamic and thermal properties of any crystal. First, from the harmonic, cubic, and quartic force constants, we construct a force field for molecular dynamics. It is exact in the limit of small atomic displacements and thus does not suffer from inaccuracies inherent in semiempirical potentials such as Stillinger-Weber's. By using the Green-Kubo formula and molecular dynamics simulations, we extract the bulk thermal conductivity. This method is accurate at high temperatures where three-phonon processes need to be included to higher orders, but may suffer from size scaling issues. Next, we use perturbation theory (Fermi golden rule) to extract the phonon lifetimes and compute the thermal conductivity $\ensuremath{\kappa}$ from the relaxation-time approximation. This method is valid at most temperatures, but will overestimate $\ensuremath{\kappa}$ at very high temperatures, where higher-order processes neglected in our calculations also contribute. As a test, these methods are applied to bulk crystalline silicon, and the results are compared and differences are discussed in more detail. The presented methodology paves the way for a systematic approach to model heat transport in solids using multiscale modeling, in which the relaxation time due to anharmonic three-phonon processes is calculated quantitatively, in addition to the usual harmonic properties such as phonon frequencies and group velocities. It also allows the construction of an accurate bulk interatomic potentials database. read less

Abstract: A set of modified embedded-atom method (MEAM) potentials for the interactions between Al, Si, Mg, Cu, and Fe was developed from a combination of each element's MEAM potential in order to study metal alloying. Previously published MEAM parameters of single elements have been improved for better agreement to the generalized stacking fault energy (GSFE) curves when compared with ab initio generated GSFE curves. The MEAM parameters for element pairs were constructed based on the structural and elastic properties of element pairs in the NaCl reference structure garnered from ab initio calculations, with adjustment to reproduce the ab initio heat of formation of the most stable binary compounds. The new MEAM potentials were validated by comparing the formation energies of defects, equilibrium volumes, elastic moduli, and heat of formation for several binary compounds with ab initio simulations and experiments. Single elements in their ground-state crystal structure were subjected to heating to test the potentials at elevated temperatures. An Al potential was modified to avoid formation of an unphysical solid structure at high temperatures. The thermal expansion coefficient of a compound with the composition of AA 6061 alloy was evaluated and compared with experimental values. MEAM potential tests performed in this work, utilizing the universal atomistic simulation environment (ASE), are distributed to facilitate reproducibility of the results. read less

Abstract: Models capable of accurate simulation of the microcantilever dynamics coupled with complex tip-sample interactions are essential for interpretation of the imaging results in non-contact atomic force microscopy (AFM). In the present research, a combination of finite element and molecular dynamics methods are used for modelling the AFM system to overcome the drawbacks of conventional approaches that use a lumped system with van der Waals interaction. To illustrate the ability of the proposed scheme in providing images with atomic resolution, some simulations have been performed. In the conducted simulations, a diamond tip is interacting with nickel samples having different surface plane directions. The results demonstrate the effectiveness of the proposed modelling method for measuring the surface topography with appropriate contrast and atomic details. read less

Abstract: The Voronoi structural evolution of silicon upon melting is investigated using a molecular dynamics simulation. At temperatures below the melting point, the solid state system is identified to have a four-fold coordination structure 〈4,0,0,0〉. As the temperature increases, the five-fold coordination 〈2,3,0,0〉 and six-fold coordination structures 〈2,2,2,0〉 and 〈0,6,0,0〉 are observed. This is explained in terms of increasing atomic displacement due to thermal motion and the trapping of the moving atoms by others. At temperatures above the melting point, nearly all of the four-fold coordination structures grows into multiple-fold coordination ones. read less

Abstract: A two-step unified framework for the evaluation of continuum field expressions from molecular simulations for arbitrary interatomic potentials is presented. First, pointwise continuum fields are obtained using a generalization of the Irving-Kirkwood procedure to arbitrary multibody potentials. Two ambiguities associated with the original Irving-Kirkwood procedure (which was limited to pair potential interactions) are addressed in its generalization. The first ambiguity is due to the nonuniqueness of the decomposition of the force on an atom as a sum of central forces, which is a result of the nonuniqueness of the potential energy representation in terms of distances between the particles. This is in turn related to the shape space of the system. The second ambiguity is due to the nonuniqueness of the energy decomposition between particles. The latter can be completely avoided through an alternate derivation for the energy balance. It is found that the expressions for the specific internal energy and the heat flux obtained through the alternate derivation are quite different from the original Irving-Kirkwood procedure and appear to be more physically reasonable. Next, in the second step of the unified framework, spatial averaging is applied to the pointwise field to obtain the corresponding macroscopic quantities. These lead to expressions suitable for computation in molecular dynamics simulations. It is shown that the important commonly-used microscopic definitions for the stress tensor and heat flux vector are recovered in this process as special cases (generalized to arbitrary multibody potentials). Several numerical experiments are conducted to compare the new expression for the specific internal energy with the original one. read less

Abstract: The thermal conductivity of single-walled silicon carbide nanotubes (SW-SiCNTs) has been investigated by molecular dynamics (MD) simulation using the many-body Tersoff potential. To validate the reliability of the simulations code, the following measures have been taken: The calculated potential energies of SW-SiCNTs and the calculated thermal conductivities of single-walled carbon nanotubes (SWCNTs) are, respectively, compared with available data, and both comparisons are in good agreement. To investigate the size (tube length and diameter), morphology (chirality and the atom arrangement) and temperature dependence of the thermal conductivity of SW-SiCNTs, the thermal conductivities of SW-SiCNTs with different sizes, morphologies and temperatures, are calculated and compared with each other. It is found that (1) as the temperature increases, the thermal conductivity decreases at different rate, which depends on the tube morphology; (2) as long as the length increases, the thermal conductivity increases correspondingly; (3) the thermal conductivity depends on the tube diameter and exhibits a peaking behavior as a function of diameter; (4) atom arrangement strongly affects the thermal conductivity not only in quantity but also in the extent of dependence on chirality; and (5) the thermal conductivity is dependent on the chirality of nanotube with different extent. read less

Abstract: Differential Evolution with Inexact Line Search (DEILS) is proposed to determination of the ground-state geometry of atom clusters. DEILS algorithm adopts probabilistic inexact line search method in acceptance rule of differential evolution to accelerate the convergence as the region of global minimum is approached. More realistic many-body potential energy functions, namely the Tersoff and Tersoff-like semi-empirical potentials for silicon, are considered. Numerical studies indicate that the new algorithm is considerably faster and more reliable than original differential evolution algorithm, especially for large-scale global optimization problem of MBP6/Si(C). Moreover, some ground-state solutions, which are superior to the known best solution given in literature, are reported. read less

Abstract: The development of ultra-precision processes which can achieve excellent surface finish and tolerance at the nanometre level is now a critical requirement for many industrial applications. At present, it is very difficult to observe the diverse microscopic physical phenomena occurring in nanometric machining through experiments. The use of molecular dynamics (MD) simulation has proved to be an effective tool for the prediction and the analysis of these processes at the nanometre scale. The crucial task in a MD simulation is the selection of the potential function. The lack of clear understanding about the scope and the limitations of a given potential function may lead to nonsensical results. This article presents the backgrounds of popular potentials used in the modelling of materials processes and the algorithms for the solution of the equations encountered in the simulation. Current applications of MD in abrasive machining are reviewed. read less

Abstract: In plasma processes such as reactive ion etching and thin film deposition for microelectronics device fabrication, atomic-level control of surface morphologies and compositions of processed materials has become increasingly important as the device sizes diminish to the nano-meter range. While various species such as ions, neutral radicals, electrons and photons simultaneously hit the material surface in a plasma, the plasma-surface interactions can be best understood if individual elementary processes such as interaction of specific species with the surface at specific incident energy are studied separately under well controlled conditions. In this article, a molecular dynamics (MD) simulation technique is reviewed as a means to analyse plasma-surface interactions in such a manner and some sample simulation results for polymer etching and diamond-like carbon (DLC) deposition are presented. read less

Abstract: We present a model of frequency-changeable carbon-nanotube (CNT) oscillator and its dynamic properties are investigated via classical molecular dynamics simulations. The operating frequency of the CNT oscillator can be changed by manipulating the intertube gap. The oscillations of the coretubes were found in two modes; the coretube oscillated between two outertubes or in only one outertube. When the gap is small and the kinetic energy of the coretube is high, the coretube oscillates between two outertubes. When the gap is large and the kinetic energy of the coretube is low, the coretube oscillates in only one outertube. The changes of the gap dominantly influenced the frequency rather than the changes of the initial velocities of the coretube. The bandwidths by intertube gap engineering can be enhanced more than the bandwidths achieved by initial velocity engineering by several tens of gigahertzs read less

Abstract: A mechanical single molecular switch using specific metallic broken carbon nanotubes (BCNTs) as electrodes is designed. It can operate by simply pressing one of the electrodes mechanically with robust performance. The device has been modeled by combining molecular dynamics simulations and first principles calculations. With the help of molecular dynamic simulations, a realistic description of the broken ends of the BCNT is obtained, while high ON/OFF conductance ratio has been obtained from nonequilibrium Green’s function calculations. A microscopic mechanism is suggested for the switch behavior. read less

Abstract: The problem of the determination of the minimum energy configuration of an arrangement of $N$ point particles under the interaction of their interatomic forces is discussed. The interatomic force is described by a classical many body potential, namely the Tersoff potential for silicon. We propose a global optimization algorithm for minimization of energy of clusters of particles using Tersoff potential. The algorithm combines the topographical differential evolution (TDE) with the modified recursive procedure of the recursive differential evolution (RDE) algorithm. It also introduces an initialization procedure for the population set. Two important features of the new algorithm are that it makes use of the \lq graph minima' for local search, and that the initial population set is generated with low function values. The global minima of clusters consisting of up to 20 particles are investigated. The new algorithm is compared with a recent genetic algorithm. read less

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

Abstract: This paper reports a molecular dynamics simulation of chemo-mechanical grinding (CMG) of silicon wafer by controlling the interatomic potential parameters to imitate the chemo-mechanical or mechano-chemical reactions between an abrasive grain and a Si wafer. Some comparisons between diamond grinding and CMG were made by using the proposed simulation model. From the simulation results, reductions of surface damages, wears of abrasive grain and scratching forces in CMG were confirmed to be same as observed in actual experiments by a CeO2 abrasive wheel, and the availability of proposed simulation model was verified. read less

Abstract: A general method for the development of potential-energy hypersurfaces is presented. The method combines a many-body expansion to represent the potential-energy surface with two-layer neural networks (NN) for each M-body term in the summations. The total number of NNs required is significantly reduced by employing a moiety energy approximation. An algorithm is presented that efficiently adjusts all the coupled NN parameters to the database for the surface. Application of the method to four different systems of increasing complexity shows that the fitting accuracy of the method is good to excellent. For some cases, it exceeds that available by other methods currently in literature. The method is illustrated by fitting large databases of ab initio energies for Si(n) (n=3,4,...,7) clusters obtained from density functional theory calculations and for vinyl bromide (C(2)H(3)Br) and all products for dissociation into six open reaction channels (12 if the reverse reactions are counted as separate open channels) that include C-H and C-Br bond scissions, three-center HBr dissociation, and three-center H(2) dissociation. The vinyl bromide database comprises the ab initio energies of 71 969 configurations computed at MP4(SDQ) level with a 6-31G(d,p) basis set for the carbon and hydrogen atoms and Huzinaga's (4333/433/4) basis set augmented with split outer s and p orbitals (43321/4321/4) and a polarization f orbital with an exponent of 0.5 for the bromine atom. It is found that an expansion truncated after the three-body terms is sufficient to fit the Si(5) system with a mean absolute testing set error of 5.693x10(-4) eV. Expansions truncated after the four-body terms for Si(n) (n=3,4,5) and Si(n) (n=3,4,...,7) provide fits whose mean absolute testing set errors are 0.0056 and 0.0212 eV, respectively. For vinyl bromide, a many-body expansion truncated after the four-body terms provides fitting accuracy with mean absolute testing set errors that range between 0.0782 and 0.0808 eV. These errors correspond to mean percent errors that fall in the range 0.98%-1.01%. Our best result using the present method truncated after the four-body summation with 16 NNs yields a testing set error that is 20.3% higher than that obtained using a 15-dimensional (15-140-1) NN to fit the vinyl bromide database. This appears to be the price of the added simplicity of the many-body expansion procedure. read less

Abstract: We have carried out a classical molecular dynamics study to quantify the conditions under which damage is generated by ion implantation in silicon at energies below the displacement threshold. The obtained results have been used to construct a general framework for damage generation which captures the transition from ballistic (high above the displacement threshold) to thermal (around and below the displacement threshold) regime. The model, implemented in a binary collision code, has been successfully used to simulate monatomic and especially molecular implantations, where nonlinear effects occur. It reproduces the amount and morphology of generated damage at atomic level in good agreement with classical molecular dynamics simulations but with a computational gain factor of ∼103 to ∼104. The incorporation of this damage model to process simulators will improve the prediction of amorphization conditions and provide a convenient tool for simulating molecular implants not available to date. Although this wor... read less

Abstract: We investigated the characteristics of a capacitive nano-accelerometer based on carbon nanotube by means of classical molecular dynamics simulations. The position of the telescoping nanotube was controlled by the externally applied force and the feedback sensing was achieved from the capacitance change. Considering energy dissipation, the oscillation features of the nano-accelerometers were similar, regardless of their initial displacements. The capacitance variations, which were almost linearly proportional to the applied acceleration, were monitored within an error tolerance. read less

Abstract: An interatomic potential model for Si–Br systems has been developed for performing classical molecular dynamics (MD) simulations. This model enables us to simulate atomic-scale reaction dynamics during Si etching processes by Br+-containing plasmas such as HBr and Br2 plasmas, which are frequently utilized in state-of-the-art techniques for the fabrication of semiconductor devices. Our potential form is based on the well-known Stillinger–Weber potential function, and the model parameters were systematically determined from a database of potential energies obtained from ab initio quantum-chemical calculations using GAUSSIAN03. For parameter fitting, we propose an improved linear scheme that does not require any complicated nonlinear fitting as that in previous studies [H. Ohta and S. Hamaguchi, J. Chem. Phys. 115, 6679 (2001)]. In this paper, we present the potential derivation and simulation results of bombardment of a Si(100) surface using a monoenergetic Br+ beam. read less

Abstract: We have developed a new ReaxFF reactive force field to describe accurately reactions of hydrocarbons with vanadium oxide catalysts. The ReaxFF force field parameters have been fit to a large quantum mechanics (QM) training set containing over 700 structures and energetics related to bond dissociations, angle and dihedral distortions, and reactions between hydrocarbons and vanadium oxide clusters. In addition, the training set contains charge distributions for small vanadium oxide clusters and the stabilities of condensed-phase systems. We find that ReaxFF reproduces accurately the QM training set for structures and energetics of small clusters. Most important is that ReaxFF describes accurately the energetics for various oxidation states of the condensed phases, including V2O5, VO2, and V2O3 in addition to metallic V (V0). To demonstrate the capability of the ReaxFF force field for describing catalytic processes involving vanadium oxides, we performed molecular dynamics (MD) simulation for reactions of a ... read less

Abstract: The migration of point defects in silicon and the corresponding atomic mobility are investigated by classical molecular dynamics simulations using the Stillinger-Weber potential and the Tersoff potential. In contrast to most of the previous studies both the point defect diffusivity and the self-diffusion coefficient per defect are calculated separately so that the diffusion-correlation factor can be determined. Simulations with both the Stillinger-Weber and the Tersoff potential show that vacancy migration is characterized by the transformation of the tetrahedral vacancy to the split vacancy and vice versa and the diffusion-correlation factor is about 0.5. This value was also derived by the statistical diffusion theory under the assumption of the same migration mechanism. The mechanisms of self-interstitial migration are more complex. The detailed study, including a visual analysis and investigations with the nudged elastic band method, reveals a variety of transformations between different self-interstitial configurations. Molecular dynamics simulations using the Stillinger-Weber potential show, that the self-interstitial migration is dominated by a dumbbell mechanism, whereas the interstitialcy mechanism prevails with the Tersoff potental. The corresponding values of the correlation factor are different, namely 0.59 and 0.69 for the dumbbell and the interstitialcy mechanism, respectively. The latter value is nearly equal to that obtainedmore » by the statistical theory which assumes the interstitialcy mechanism. Recent analysis of experimental results demonstrated, that in the framework of state-of-the-art diffusion and reaction models the best interpretation of point defect data can be given by assuming . The comparison with the present atomistic study leads to the conclusion that a dumbbell mechanism governs the self-interstitial migration in Si. Simulations using the Stillinger-Weber potential reveal two dominating migration paths which are characterized by transformation between the extended dumbbell and the dumbbell and vice versa. This process occurs either in a single {110} plane or includes a change into an equivalent {110} plane.« less read less

Abstract: Using a combination of quantum and classical computational approaches, we model the electronic structure in amorphous silicon in order to gain an understanding of the microscopic atomic configurations responsible for light-induced degradation of solar cells. We demonstrate that regions of strained silicon bonds could be as important as dangling bonds for creating traps for charge carriers. Further, our results show that defects are preferentially formed when a region in the amorphous silicon contains both a hole and a light-induced excitation. These results are consistent with the puzzling dependencies on temperature, time, and pressure observed experimentally. read less

Abstract: In this paper, thermodynamical properties of crystalline silicon under strain are calculated using classical molecular dynamics (MD) simulations based on the Tersoff interatomic potential. The Helmholtz free energy of the silicon crystal under strain is calculated by using the ensemble method developed by Frenkel and Ladd (1984). To account for quantum corrections under strain in the classical MD simulations, we propose an approach where the quantum corrections to the internal energy and the Helmholtz free energy are obtained by using the corresponding energy deviation between the classical and quantum harmonic oscillators. We calculate the variation of thermodynamic properties with temperature and strain and compare them with results obtained by using the quasi-harmonic model in the reciprocal space. read less

Abstract: The numerical calculation for Ge/Si(001) heteroepitaxial growth is performed. We adopt the most widely used Stillinger–Weber potential, and the island energies of the three types, two-dimensional island, pyramid and dome, are explored as a function of the lateral size. These island energies are compared with each other to find the island morphology which has the lowest energy. Then, a growth history of the most stable growth mode is searched. Although the result reproduces qualitatively the Stranski–Krastanov growth as observed in the experiments, quantitative differences between our result and experiments in the critical wet layer thickness and the island morphology are found. read less

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

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

Abstract: Surface viscous flow, surface diffusion, and ballistic effects have recently been discussed as possible atomic-scale mechanisms to explain the dramatic smoothing reactions observed during keV ion bombardment of amorphous surfaces. By employing multiscale modeling, viz. a combination of molecular dynamics and continuum rate equations, we compare the relevance of the individual processes at room temperature. This is achieved by calculating diffusion constants, viscosities, and lateral transport due to momentum transfer. Depending on the surface structure size, we find the dominance of surface viscous flow or ballistic effects. The findings are found to be valid for both strong and fragile glasses, as represented by amorphous Si and CuTi, respectively. read less

Abstract: A very high probability $(\ensuremath{\sim}60%)$ for H abstraction induced by $\mathrm{Si}{\mathrm{H}}_{3}$ thermal impacts on the $\mathrm{Si}(001)(2\ifmmode\times\else\texttimes\fi{}1)$ hydrogenated surface is reported, as a consequence of the Eley-Rideal mechanism by which a silane molecule is formed. The reaction probability is computed within a fully dynamical approach. After running a limited set of ab initio Car-Parrinello simulations to validate a suitable empirical potential, an actual probability for the mechanism was estimated by averaging over thousands of classical molecular dynamics simulations. The probability of H abstraction is shown to be quite constant in the typical experimental range for plasma-enhanced chemical vapor deposition. Very low evidence for insertion of the radical into surface Si-Si bonds is found. 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: In this study, the nucleation mechanism of carbon nanocones is investigated using molecular dynamics (MD) simulations and structural analyses and is compared with that of carbon nanotubes. It is shown that the structural stability of carbon nanocones is sensitive to the cone apex angle. Specifically, an increase in the conical angle results in a moderate improvement in the structural stability of the nanocone as a result of a lower strain energy in the capped mantle. The simulation results also show that the melting temperature of the nanocone increases with increasing conical angle. Furthermore, it is observed that a metastable tube-like structure is formed in carbon nanocones with a lower conical angle at temperatures ranging from 2400 to 3600 K. Finally, the numerical simulations reveal that the mechanical properties of carbon nanocones under nanoindentation are strongly dependent on the conical angle. For carbon nanocones with a large conical angle, the high deformation-promoted reactivity and reversible mechanical response have been performed due to highly symmetrical networks. read less

Abstract: A dynamic-charge, many-body potential for the Si/SiO{sub 2} system, based on an extended Tersoff potential for semiconductors, is proposed and implemented. The validity of the potential function is tested for both pure silicon and for five polymorphs of silica, for which good agreement is found between the calculated and experimental structural parameters and energies. The dynamic charge transfer intrinsic to the potential function allows the interface properties to be captured automatically, as demonstrated for the silicon/{beta}-cristobalite interface. read less

Abstract: To investigate the effect of tool geometry on single-crystal silicon nano-cutting, parallel molecular dynamics (MD) simulations are carried out with different tool rake angles. In this study, a parallel arithmetic based on mechanism of spatial decomposition together with MD is applied to simulate nano-cutting processes of single-crystal silicon (100) plane by using a single-crystal diamond tool. The simulation results show that tool rake angle has great effects on cutting forces and subsurface stress, and the effect of tool rake angle variation on work-piece potential energy is not evident while cutting single-crystal Silicon (100) plane. Moreover, the analysis of cutting forces and potential energy show that there is not evident dislocation in the nano-cutting. read less

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

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

Abstract: Coordination-dependent interatomic potentials are proposed for silicon oxides and oxynitrides\char22{}also hydrogenated ones\char22{}with a functional form based on the widely used Tersoff silicon potential. They are intended for an accurate sampling of the configurational space of realistic silicon oxynitride systems and their interfaces with silicon, including defects and changes of oxidation states. The parameters, which are given in the text, are obtained by simultaneously mapping forces and energies onto the results of density-functional-theory calculations performed for a set of diverse systems and configurations and a wide composition range. Application to a larger set of systems and configurations shows the transferability of these augmented Tersoff potentials and their validity in predicting bulk lattice parameters, energetics of defect relaxation, and vibrational spectra. read less

Abstract: The molecular dynamics simulation of nanoscale cantilevers made of pure crystalline silicon with different lattice conditions is presented. Young's moduli for various sized specimen is obtained by simulating clamped-free cantilever beam vibrations and static tensile responses. Young's modulus decreases monotonically as the thickness of the specimen decreases. Although significant discrepancies exist between the simulated and experimentally determined Young's modulus, incorporating a minute amount of voids in the specimen during simulation offers a partial account of this discrepancy. The dependence of the Young's modulus on dimensional scaling is then applied to estimate thermal fluctuations of the cantilever under various temperatures, sizes, and lattice conditions and shows excellent agreement with the theoretical estimate based on the equipartition theorem. Finally, the applicability of the nanocantilevers as molecular mass sensors is demonstrated by simulating the change in the first flexural mode frequency as the number of silicon molecules placed at the tip of the cantilever is varied. The results show good agreement with the theoretical predictions of the Euler-Bernoulli beam vibration model. read less

Abstract: We investigated the substrate effect of carbon nanotube (CNT) oscillators using classical molecular dynamics simulations. Double-walled CNT oscillators on {100} gold surface were considered. The nanotube–gold interactions induced the compressive deformations of the outer nanotube and affected the transitional velocity and the energy dissipation of the nanotube oscillator. When the inner nanotube was extruded from the outer nanotube, the central regions of the outer nanotube were compressed by the nanotube–gold interactions and then, these compressive forces pushed out the inner nanotube and finally, the transitional velocity of the inner nanotube was slightly increased at the edges regions. Since the energy dissipation of the nanotube oscillator on gold surface was higher than that in vapor, the decrease of the transitional velocity for the nanotube oscillator on gold surface was greater than that for the nanotube oscillator in vapor. read less

Abstract: Recent experiments and calculations have highlighted the important role of surface-energy (gamma) anisotropy in governing island formation in the Ge/Si(001) system. To further elucidate the factors determining this anisotropy, we perform atomistic and continuum calculations of the orientation dependence of gamma for strained-Ge surfaces near (001), accounting for the presence of dimer-vacancy lines (DVLs). The net effect of DVLs is found to be a substantial reduction in the magnitude of the slope of gamma vs orientation angle, relative to the highly negative value derived for non-DVL, dimer-reconstructed, strained-Ge(001) surfaces. The present results thus point to an important role of DVLs in stabilizing the (001) surface orientation of a strained-Ge wetting layer. read less

Abstract: Thermal stability and molecular electronic properties of a single walled, bamboo shaped carbon nanotube has been investigated. Molecular dynamics method is applied to investigate thermal stability, and electronic properties are calculated at the Extended Huckel level. Although bamboo shaped carbon nanotubes observed in experimental literature are multi-walled, it is shown that the suggested structural model in this work, which is single-walled, is also both thermodynamically and energetically stable. Bamboo shape of the model investigated is due to periodical coronene-like spacers. The resultant structure is compartmented, having geometrical aberrations in the vicinity of spacers. There is no degradation in the average coordination number. The geometrical aberrations in the vicinity of spacers is due to curvature induced by the pentagons of the resultant geometry. read less

Abstract: We have studied the coalescence mechanism of impurity atoms implanted into a crystalline target at low temperatures, paying attention to the dimer formation mechanism. A classical molecular-dynamics (MD) calculations was used in the case of 1 keV B ion implantation at 100 K into a crystalline silicon (c-Si) target that was supersaturated with pre-embedded B atoms at a concentration of 3 at. %. The initial phase of dimer formation was investigated in terms of the temperature distribution and the degradation of the long-range order (LRO) parameters defined in the pixel mapping (PM) method. One result was the locally high temperature and big temperature gradient that revealed the acceleration of collisional diffusion of light impurity atoms in the host medium. The other finding was the decrease of the LRO parameters. This fact associated with the number of self-interstitial atoms (SIAs) implies a deformed potential-field in a crystal under ion irradiation. These results indicate that dynamically enhanced diffusion or collisional-diffusion is the origin of dimer formation. read less

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

Abstract: Sin clusters in the size range n = 4–30 have been investigated using a combination of global structure optimization methods with DFT and ab initio calculations. One of the central aims is to provide explanations for the structural transition from prolate to spherical outer shapes at about n = 25, as observed in ion mobility measurements. Firstly, several existing empirical potentials for silicon and a newly generated variant of one of them were better adapted to small silicon clusters, by global optimization of their parameters. The best resulting empirical potentials were then employed in global cluster structure optimizations. The most promising structures from this stage were relaxed further at the DFT level with the hybrid B3LYP functional. For the resulting structures, single point energies have been calculated at the LMP2 level with a reasonable medium-sized basis set, cc-pVTZ. These DFT and LMP2 calculations were also carried out for the best structures proposed in the literature, including the most recent ones, to obtain the currently best and most complete overall picture of the structural preferences of silicon clusters. In agreement with recent findings, results obtained at the DFT level do support the shape transition from prolate to spherical structures, beginning with Si26 (albeit not completely without problems). In stark contrast, at the LMP2 level, the dominance of spherical structures after the transition region could not be confirmed. Instead, just as below the transition region, prolate isomers are obtained as the lowest-energy structures for n ≤ 29. We conclude that higher (probably multireference) levels of theoretical treatments are needed before the puzzle of the silicon cluster shape transition at n = 25 can safely be considered as explained. read less

Abstract: We show that when the graphene planes of graphite are uniformly expanded, thereby increasing the CC bond length to $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, the ${\ensuremath{\sigma}}^{*}$ edge onset of the energy-loss near-edge structure (ELNES) spectrum shifts to lower energies by almost $5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, meanwhile the ${\ensuremath{\pi}}^{*}$ edge shifts by less than $0.2\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The shift of the ${\ensuremath{\sigma}}^{*}$ edge demonstrates that for bond lengths which are typical of some carbon systems such as amorphous carbon, it is possible to find ${\ensuremath{\sigma}}^{*}$ features in the ELNES spectra at energies as low as $286\char21{}288\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Calculations on 64-atom amorphous carbon $(a\text{\ensuremath{-}}\mathrm{C})$ and amorphous carbon nitride model structures characterized by a wide range of bond lengths confirm this. Most of the $s{p}^{2}∕s{p}^{3}$ quantification techniques that are available overlook this issue of ${\ensuremath{\sigma}}^{*}$ contamination of the ${\ensuremath{\pi}}^{*}$ region and assume that all features within this energy range are entirely of ${\ensuremath{\pi}}^{*}$ origin. We show that the effect of bond length variation on the ${\ensuremath{\pi}}^{*}$ spectrum of graphite and $a\text{\ensuremath{-}}\mathrm{C}$ is minor, thereby supporting the reliability of the former spectrum for $s{p}^{2}∕s{p}^{3}$ quantification purposes, as was recently demonstrated [see J. T. Titantah and D. Lamoen, Phys. Rev. B 70, 075115 (2004)]. read less

Abstract: A comprehensive study on the migration of di- and tri-interstitials in silicon is performed using classical molecular dynamics simulations with a Stillinger-Weber potential. At first the structures and energetics of the di- and the tri-interstitial are investigated, and the accuracy of the interatomic potential is tested by comparing the results with literature data obtained by tight-binding and density-functional-theory calculations. Then the migration is investigated for temperatures between 800 and 1600 K. Very long simulation times, large computational cells and different initial conditions are considered. The defect diffusivity, the self-diffusion coefficient per defect and the corresponding effective migration barriers are calculated. Compared to a mono-interstitial, the di-interstitial migrates faster, whereas the tri-interstitial diffuses slower. The mobility of the di- and the mono-interstitial is higher than the mobility of the lattice atoms during the diffusion of these defects. On the other hand, the tri-interstitial mobility is lower than the corresponding atomic mobility. The migration mechanism of the di-interstitial shows a pronounced dependence on the temperature. At low temperature a high mobility on zigzag-like lines along a axis within a {l_brace}110{r_brace} plane is found, whereas the change between equivalent directions or equivalent {l_brace}110{r_brace} planes occurs seldomly and requires a long simulationmore » time, but the rate of directional change increases with increasing temperature. During the diffusion within {l_brace}110{r_brace} planes the di-interstitial moves like a wave packet so that the atomic mobility is lower than that of the defect. On the other hand, the change between equivalent {l_brace}110{r_brace} migration planes is characterized by frequent atomic rearrangements. The visual analysis of the tri-interstitial diffusion reveals complex migration mechanisms and a high atomic mobility. The diffusivities and effective migration barriers obtained are compared with the few data from the literature. The implications of the present results for the explanation of experimental data on defect evolution and migration are discussed.« less read less

Abstract: Mechanical stresses and morphology during growth and ion bombardment of amorphous Ge thin films are investigated by a combination of in situ stress measurements and molecular dynamics computer simulations. Strong compressive stresses are generated during irradiation that subsequently lead to severe swelling. The simulations indicate that interstitial-mediated viscous flow in combination with well-localized vacancy defects are the main ingredients responsible for the observed phenomena. read less

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

Abstract: A nanoelectromechanical model based on atomistic simulations including charge transfer was investigated. Classical molecular dynamics method combined with continuum electric models could be applied to a carbon-nanotube nanoelectromechanical memory device that could be characterized by carbon-nanotube bending performance by atomistic capacitive and interatomic forces. The capacitance of the carbon atom was changed with the height of the carbon atom. We performed MD simulations for a suspended (5,5) carbon-nanotube-bridge with the length of 11.567 nm (LCNT) and the depth of the trench of 0.9 ~ 1.5 nm (H). After the carbon-nanotube collided on the gold surface, the carbon-nanotube-bridge oscillated on the gold surface with amplitude of ~1 Å, and the amplitude gradually decreased. When H ≤ 1.3 nm, the carbon-nanotube-bridge continually contacted with the gold surface after the first collision. When H ≥ 1.4 nm, the carbon-nanotube-bridge stably contacted with the gold surface after several rebounds. As H increased, the threshold voltage linearly increased. As the applied bias increased, the transition time exponentially decreased at each trench depth. When H / LCNT was below 0.13, the carbon-nanotube nanoelectromechanical memories were permanent nonvolatile memory devices, whereas the carbon-nanotube nanoelectromechanical memories were volatile memory or switching devices when H / LCNT was above 0.14. The turn-on voltages and tunneling resistances obtained from our simulations are compatible to those obtained from previous experimental and theoretical results. read less

Abstract: We investigated a nonvolatile nanomemory element based on boron nitride nanopeapods using molecular dynamics simulations. The studied system was composed of two boron-nitride nanotubes filled Cu electrodes and fully ionized endo-fullerenes. The two boron-nitride nanotubes were placed face to face and the endo-fullerenes came and went between the two boron-nitride nanotubes under alternatively applied force fields. Since the endo-fullerenes encapsulated in the boron-nitride nanotubes hardly escape from the boron-nitride nanotubes, the studied system can be considered to be a nonvolatile memory device. The minimum potential energies of the memory element were found near the fullerenes attached copper electrodes and the activation energy barrier was . Several switching processes were investigated for external force fields using molecular dynamics simulations. The bit flips were achieved from the external force field of above . read less

Abstract: Instabilities in semiconductor heterostructure growth can be exploited for the self-organized formation of nanostructures, allowing for carrier confinement in all three spatial dimensions. Beside the description of various growth modes, the experimental characterization of structural properties, such as size and shape, chemical composition, and strain distribution is presented. The authors discuss the calculation of strain fields, which play an important role in the formation of such nanostructures and also influence their structural and optoelectronic properties. Several specific materials systems are surveyed together with important applications. read less

Abstract: This paper presents a short overview of the methods used for the study of mechanics at small scales. The key issue to be tackled is the presence of multiple scales starting from the atomic scale. The methods outlined include continuum, atomistic and mixed methods. read less

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

Abstract: A comparative study has been performed for silicon microclusters, Si3 and Si4, considering fifteen different empirical potential energy functions. It has been found that only two of the empirical potential energy functions give linear structure more stable for Si3, the remaining potential functions give triangular structure as more stable. In the case of Si4 microclusters eight potential functions give open tetrahedral structure as more stable, two functions give perfect tetrahedral as more stable, three functions give square structure as more stable, and two functions give linear structure as more stable. read less

Abstract: The use of molecular–dynamics simulations to understand the ejection processes of particles from surfaces after energetic ion bombardment is discussed. Substrates considered include metals, covalent and ionic materials, polymers and molecular solids. It is shown how the simulations can be used to aid interpretation of experimental results by providing the underlying mechanisms behind the ejection processes. read less

Abstract: We have investigated the structural properties and the thermal behavior of single-wall GaN nanotubes using atomistic simulations based on the Tersoff-type potential. The Tersoff potential for GaN has effectively described the properties of GaN nanotubes. The caloric curves of single-wall GaN nanotubes were divided into three regions corresponding to nanotube, disintegrating range and vapor. Since the stability or the stiffness of the tube decreased with increasing curving strain energy of sheet-to-tube, the disintegration temperatures of GaN nanotubes were closely related to the curving strain energy of sheet-to-tube. read less

Abstract: A multi-scale modeling, from ab-initio calculations, through Monte Carlo diffusion simulators, to the continuum models, is necessary to describe the complexity of dopant and defect interactions for process simulators. The advantages that make continuum simulators the standard in industrial applications are maintained only on the basis of the simplicity of the physical models. The inclusion of complex interactions in Kinetic Monte Carlo methods is not a problem from the computational point of view, but they demand a large number of parameters. The fabrication of small devices brings up complex physical mechanisms, for which atomistic simulations seem more appropriate than continuum methods. read less

Abstract: A multiscale method combining atomistic and continuum regions is presented to predict the static response of Nanoelectromechanical (NEM) switches. The atomistic and continuum regions are combined by using a Schwartz technique with either overlapped subdomains and Dirichlet-Dirichlet type boundary conditions, or non-overlapped subdomains and Dirichlet-Neumann type boundary conditions. The continuum regions are treated by nonlinear elastic theories and the atomistic regions are treated by the molecular dynamics (MD) method. The accuracy of the multiscale approach is checked. by comparing results obtained with the MD technique for the entire device or geometry. Finally, the convergence behavior of the multiscale approach is investigated in detail. read less

Abstract: Using plane wave pseudopotential density functional theory calculations we have identified for the first time precursor states for hydrogen atom chemisorption on the Si(100)-(2×1) surface. These states exist above clean, partially, and fully monohydride-adsorbed surface dimers. In all three cases the dimer bond is broken in the trapped state. A study of the energetics for atomic desorption, abstraction, chemisorption, and migration was carried out. We find that “hot” hydrogen atoms of energies up to approximately 1.3–1.9 eV can be trapped on the surface. These atoms are highly mobile, and we obtained energetics consistent with experimental data from which precursor-mediated adsorption mechanisms have been inferred. The existence of these states provides an understanding of the non-Langmuirian atomic hydrogen adsorption probability, and also underscores the importance of lattice distortions in the interactions of hydrogen with the silicon surface. read less

Abstract: Evolutionary computation techniques (in particular, genetic algorithms) have been applied to optimize the structure of microclusters. Various empirical potential energy functions have been used to describe the interactions among the atoms in the clusters. A comparative study of silicon microclusters has been performed. read less

Abstract: The structural properties of carbon nanorods obtained from diamond crystal have been investigated by performing molecular-dynamics computer simulations. Calculations have been realized by using an empirical many-body potential energy function for carbon. Diamond nanorods have been generated from three low-index planes of diamond crystal. It has been found that the average coordination number, cross-section geometry, and surface orientation from which the nanorod is generated play a role in the stability of diamond nanorods under heat treatment. The most stable diamond nanorod has been obtained from the (111) surface. read less

Abstract: A recently developed potential function for covalent materials (Phys. Stat. Sol.B212, 9 (1999)) is used to simulate the surface adsorption, and diffusion of Si adtom and ad-dimer on the Si(001) surface. We calculate the formation energies and diffusion activation energies of several possible binding sites. The predicted stable and metastable configurations and diffusion paths of Si ad-atom and Si ad-dimer on Si(001)-(2×1) surface are in agreement with that from the first principle calculations or experiments. read less

Abstract: We have performed a comparative study of Si(001) surface reconstruction employing molecular dynamics simulation using the interatomic potentials of Stillinger–Weber, Tersoff and Bazant–Kaxiras. Simulations were carried out for temperatures at 300 K and 1000 K using each of these three potentials. At 300 K, the three potentials were found to generate surface features comprising mainly the simple (2 × 1) reconstruction. At 1000 K, more complex reconstruction similar to the p(2 × 2) and c(2 × 2) patterns was observed on the surfaces of Stillinger–Weber and Tersoff crystals while the surface generated on Bazant–Kaxiras crystal is characterized by disorderliness with no identifiable pattern of reconstruction. read less

Abstract: The authors envision computational nanotechnology's role in developing the next generation of multifunctional materials and molecular-scale electronic and computing devices, sensors, actuators, and machines. They briefly review computational techniques and provide a few recent examples derived from computer simulations of carbon nanotube-based molecular nanotechnology. The four core areas are: molecular-scale, ultralightweight, extremely strong, functional or smart materials; molecular-scale or nanoscale electronics with possibilities for quantum computing; molecular-scale sensors or actuators; and molecular machines or motors with synthetic materials. The underlying molecular-scale building blocks in all four areas are fullerenes and carbon nanotube-based molecular materials. Only the different aspects of their physical, chemical, mechanical, and electronic properties create the many applications possible with these materials in vastly different areas. read less

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

Abstract: The Tersoff potential and a recently developed potential function for covalent materials (phys. stat. sol. (b) 212, 9 (1999)) are used to simulate the reconstruction of Si(001) surface. We obtain a dimered (2 x 1) reconstruction with an asymmetric rearrangement of atoms in deeper layers in Z-direction using Tersoffs potential and an asymmetric buckled dimered (2 × 1) reconstruction using the recently developed potential. The latter is in agreement with results from the first principles calculations or experiments. read less

Abstract: A new method combining the finite element method (FEM) and the molecular dynamics (MD) for the complicated diamond-like structure of silicon is proposed. For simultaneous simulation, the patch model was used to exchange displacement information in both directions. A one-to-one correspondence of atoms and nodes is impossible for a silicon lattice, therefore, the atom was embedded in an isoparametric element. The influence of internal displacement which is a additional displacement to the continuum one, was taken into consideration. Martin’s method was applied to calculate the internal displacement and elastic constants. The conjugate gradient method was used for MD, the Newton-Raphson method was used for FEM to efficiently find the stable state, and the acceleration condition was set to raise convergence. The verification model showed that the smooth transition of displacement and stress was realized in the boundary region of FEM and MD. These values showed good agreement with the elastic solution. read less

Abstract: We fit an empirical potential for silicon using the modified embedded atom (MEAM) functional form, which contains a nonlinear function of a sum of pairwise and three-body terms. The three-body term is similar to the Stillinger-Weber form. We parametrized our model using five cubic splines, each with 10 fitting parameters, and fitted the parameters to a large database using the force-matching method. Our model provides a reasonable description of energetics for all atomic coordinations, Z, from the dimer (Z = 1) to fcc and hcp (Z = 12). It accurately reproduces phonons and elastic constants, as well as point defect energetics. It also provides a good description of reconstruction energetics for both the 30° and 90° partial dislocations. Unlike previous models, our model accurately predicts formation energies and geometries of interstitial complexes - small clusters, interstitial-chain and planar {311} defects. read less

Abstract: We review the challenges for atomic scale modeling of alternative gate dielectric stacks. We begin by highlighting recent achievements of state-of-the-art atomistic simulations of the Si-SiO/sub 2/ system, showing how such calculations have elucidated the microscopic origins of several important experimental phenomena. For the benefit of readers who may be unfamiliar with the simulation tools, we overview and compare the relevant methods. We then describe the difficulties encountered in extending these approaches to investigate high-k dielectric stacks, pointing out exciting research directions aimed at overcoming these challenges. We conclude by presenting a roadmap of computational goals for atomic scale modeling of alternative gate dielectrics. read less

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

Abstract: The empirical potentials used for defect simulation in silicon are fitted on elastic or phonon properties. Usually they are not able to take into account all the distortions present in a defected material. On the basis of the potential proposed by Vanderbilt, Taole and Narasimhan a method is presented to separate the contribution of distortions linked to elastic properties from the contribution of distortions linked to phonon properties. The method is applied to grain boundaries in silicon simulated by potentials in the harmonic approximation. The phonon contribution is found slightly predominant with respect to the elastic one. read less

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

Abstract: Atomic resolution Z-contrast scanning transmission electron microscopy reveals preferential nucleation of electron-beam-induced damage in select atomic columns of a Si tilt grain boundary. Atomic scale simulations find that the region of initial damage nucleation corresponds to columns where the formation energies of vacancies and vacancy complexes are very low. The calculations further predict that vacancy accumulation in certain pairs of columns can induce a structural transformation to low-density dislocation “pipes” with all atoms fourfold coordinated. read less

Abstract: A new method combining the finite element method (FEM) and the molecular dynamics (MD) for silicon is proposed. For simultaneous simulation, the patch model was used to exchange displacement information in both directions. A one-to-one correspondence of atoms and nodes is impossible for silicon lattice, therefore the atom was embedded in isoparametric element. The influence of internal displacement which is the additional displacement to the continuum one was taken into consideration. Martin's method was applied to calculate internal displacement and elastic constants. The verification model showed that the smooth transition of displacement and stress was realized in the boundary region of FEM and MD. These value showed good agreement with elastic solution. read less

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

Abstract: The Brenner hydrocarbon potential was extended recently to include interactions with silicon. This extended Brenner potential has now been improved by the fitting of bond order correction terms, and the introduction of an adjustable parameter into the angular function. The new potential gives an excellent description of small Si m H n molecules and radicals. Its treatment of the low index surfaces of silicon and β-SiC is also significantly improved, although the recently proposed non-dimerized structure for the silicon terminated (001) surface of β-SiC is not described properly. Calculations of the chemisorption of C2H2 and CH3 onto the (001) surfaces of silicon and β-SiC using this improved potential are reported. Also presented are some initial results of molecular dynamics simulations of the Si(111) 7 × 7:CH3 and hydrogenated Si(001) 2 × 1:C2H2 chemisorption systems. read less

Abstract: The effects of stress on the Raman frequencies of crystalline silicon are studied using molecular dynamics simulation both for uniaxial stress along the[100]direction and for biaxial stress which is isotropic in the(001)plane. The Tersoff potential is used to represent the interaction among the silicon atoms. Simulation results showed that the tensile stress causes the Raman frequencies to decrease. The conversion coefficients that are needed for converting the shifts of the Raman frequencies into the stress were obtained by comparing the simulation results with the dynamical equations for optical modes. The values obtained for the coefficients agreed well with the experimental values obtained by other works. The obtained relationship between the uniaxial stress and the Raman frequency for vibration in the[001]direction also agreed well with the experimental result we obtained using microscopic Raman spectroscopy. read less

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

Abstract: When considering the mechanical behaviour of materials an important property is the tensor of elastic moduli. Recently, local elastic moduli of interfaces have been defined and studied for metallic materials [1 to 3]. In these works grain boundaries are regarded as heterogeneous continua composed of ‘phases’ associated with individual atoms which possess elastic moduli identified with the atomic-level moduli evaluated at corresponding atomic positions. From this representation it is possible to define the ‘effective’ moduli of the grain boundary region. In this paper this concept is developed for materials with covalent character of bonding, specifically silicon. Using the Tersoff's potential [4, 5], the atomic-level and effective elastic moduli of the interfacial region have been evaluated for three alternate structures of the Σ = 3 (112-)/[11-0] tilt boundary. These calculations are then compared with the continuum bounds on the effective moduli evaluated using the classical minimum-energy principles of elasticity. The effective moduli calculated in the atomistic framework are generally within the continuum bounds and thus the present study demonstrates that the heterogeneous continuum model of the interfaces is appropriate for the description of the elastic properties of grain boundaries in silicon. An important aspect addressed in this study is the uniqueness of interfacial elastic moduli since their evaluation involves the energy associated with an atom which cannot be defined uniquely. The calculations have been made for two different partitions of the total energy into energies associated with individual atoms. These two partitions lead to almost identical results for the effective moduli and continuum bounds when the tensor of the atomic-level moduli is positive definite. When some atomic-level moduli are not positive definite the results may depend on the chosen energy partition. read less

Abstract: The tight-binding method of modelling materials lies between the very accurate, very expensive, ab initio methods and the fast but limited empirical methods. When compared with ab initio methods, tight-binding is typically two to three orders of magnitude faster, but suffers from a reduction in transferability due to the approximations made; when compared with empirical methods, tight-binding is two to three orders of magnitude slower, but the quantum mechanical nature of bonding is retained, ensuring that the angular nature of bonding is correctly described far from equilibrium structures. Tight-binding is therefore useful for the large number of situations in which quantum mechanical effects are significant, but the system size makes ab initio calculations impractical. In this paper we review the theoretical basis of the tight-binding method, and the range of approaches used to exactly or approximately solve the tight-binding equations. We then consider a representative selection of the huge number of systems which have been studied using tight-binding, identifying the physical characteristics that favour a particular tight-binding method, with examples drawn from metallic, semiconducting and ionic systems. Looking beyond standard tight-binding methods we then review the work which has been done to improve the accuracy and transferability of tight-binding, and moving in the opposite direction we consider the relationship between tight-binding and empirical models. read less

Abstract: AbstractAn improved molecular dynamics technique that allows reduction of the computation time required in ion bombardment simulations is presented. This technique has been used to study the influence of the target temperature and structure on the argon sputtering of silicon. Molecular dynamics simulations of l keV Ar+ ion bombardment of silicon were carried out for several types of sample: (100) crystalline at 0 K, (100) crystalline at 300 K, and amorphous at 300 K. The yield of the sputtering process and the energy distribution of the sputtered atoms have been obtained. These results show that the sputtering process depends on the target surface binding energy which, in turn, is very sensitive to the structure of the sample surface. read less

Abstract: The interaction of hdyrogenated Si(001) surfaces is studied by means of molecular dynamics using an empirical potential. Above a certain critical external force covalent bonds may be formed between the surfaces even at room temperature, leaving a hydrogenated interface. The critical force is related to the assumptions of the molecular dynamics, thus scaling with the potential, heat transfer, boundary conditions, and the weak long-range interaction omitted. Below this critical force, the hydrogen–hydrogen interactions prevent covalent bonding. read less

Abstract: ▪ Abstract Growth of thin films from atoms deposited from the gas phase is intrinsically a non-equilibrium phenomenon dictated by a competition between kinetics and thermodynamics. Precise control of the growth becomes possible only after achieving an understanding of this competition. In this review, we present an atomistic view of the various kinetic aspects in a model system, the epitaxy of Si on Si(001), as revealed by scanning tunneling microscopy and total-energy calculations. Fundamentally important issues investigated include adsorption dynamics and energetics, adatom diffusion, nucleation, sticking, and detachment. We also briefly discuss the inverse process of growth, removal by sputtering or etching. We aim our discussions to an understanding at a quantitative level whenever possible. read less

Abstract: A simple quantum mechanical model (tight binding) offers an attractive starting point for developing expressions for the total energy and atomic forces to be used in molecular dynamics simulations. A number of methods are presented for implementing tight binding in an efficient manner, and their relative merits discussed. read less

Abstract: The interatomic potentials of Stillinger-Weber and Tersoff were incorporated into the randomization-and-relaxation model, which was originally developed for modelling amorphous silicon by using the Keating interatomic potential. The inclusion of more recent and more complicated interatomic potentials resulted in a more sophisticated set of bond switching rules which form the basis for the randomization-and-relaxation algorithm. This improved model was then used to model small isolated amorphous zones which are produced by individual heavy ions during ion implantation in silicon. The temperature evolution during zone creation was calculated by using idealized thermal spike model. The structure and stability of these amorphous zones was examined with respect to the energy of incoming ion and with respect to the interatomic potential employed. It was established that significantly lower spike energy is required to create a stable amorphous region than in the simulation where the Keating potential wa... read less

Abstract: Ion assistance has been very successful in modern processes of physical or chemical vapour deposition, as it promotes the production of high-quality films and, in particular, the formation of new thin film materials with extreme properties [1–5]. In contrast to a broad range of experimental experience, the basic understanding of the effects of ion bombardment during thin film deposition is still in the state of a beginning. Therefore, the field is still mostly relying on broad empirical investigations rather than focused studies of optimization, which would be based on the basic understanding of the underlying mechanisms. Such studies require both well-directed experiments and a reliable modelling. The slow progress in this area is related to the large degree of complexity associated with the interplay of surface mechanisms, and both energetic and thermal processes in the near-surface bulk, which may be of physical and chemical nature. Consequently, the ability of analytical models is rather limi... read less

Abstract: Bombardment of solids with keV atoms leads to violent collisions with subsequent ejection of target particles. This review discusses how classical molecular dynamics simulations designed to describe the bombardment events can give insight into microscopic processes where not only classical but also quantum effects such as electronic excitation and organic reactions play an important role. By incorporating a simple excitation/de-excitation model into the simulation, we have shown that collisional events are important to describe the distribution of excited state atoms measured experimentally. Molecular dynamics simulations employing a reactive many-body potential of small hydrocarbon molecules adsorbed on a metal surface predict the occurrence of various collision induced organic reactions prior to ejection. Lateral motion of particles in the region right above the surface plays an important role in signal enhancement. The calculations predict several processes such as direct molecular ejection, d... read less

Abstract: The following is an edited version of the Von Hippel Award address, given by recipient Sir Alan Cottrell at the 1996 MRS Fall Meeting. Cottrell received the Materials Research Society’s highest honor for “converting crystal dislocations from a hand-waving hypothesis to a rigorous discipline, transforming the understanding of brittle fracture, making varied and crucial advances in the theory of radiation damage, and for transforming the teaching of materials science throughout the academic world through his pioneering textbooks.” Cottrell is an honorable distinguished research Fellow in the Department of Materials Science and Metallurgy, University of Cambridge. read less

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

Abstract: The problem of nonequilibrium phase transformations is reviewed for compression of diamond, zincblende, and amorphous tetrahedral semiconductors and for decompression of their high-pressure phases. The importance of lattice or network dynamical properties for the nature of nonequilibrium transformations is shown. Using low-temperature quenching of high-pressure phases, we obtained new Crystalline phases for Ge and Ge-Gash solid solutions, which have X-ray patterns very similar to corresponding data for rhombohedral R8 silicon. We established that a number of anomalous features, like elastic softness, pressure-induced geometrical distortion, and strong bulk modulus softening (δB/δP > 0), distinguish the pressure behavior of the amorphous tetrahedral network in a-GaSb from its crystalline counterpart. read less

Abstract: The Si(001)/SiO2 (tridymite) interface has been simulated using a Monte-Carlo method. It has been shown that in this way reasonable values of angles and interatomic distances are obtained. The oxygen defect formation energy dependence with different vacancy sites has been studied. Because of coulombic interactions, the formation of vacancies is much easier in the vicinity of the interface. read less

Abstract: Nucleation of misfit dislocations in Ge/(001)Si heterostructures is investigated theoretically by an atomistic model based on the Stillinger-Weber potential (Stillinger and Weber 1985, Phys. Rev. B, 31, 5262). Both 60° and 90° dislocations are considered, and the energy is calculated as a function of distance of dislocations from the free surface in a thin-film heterostructure. The critical thicknesses of the dislocation nucleation obtained from the atomistic simulation are larger than the previously reported results of the continuum analysis, and we attribute this difference mainly to core energy of dislocations. The activation bamer for dislocation nucleation from the surface is estimated from the variation of energy with distance of a dislocation from the surface. The calculated activation energy is much larger than the thermal energy at normal growth temperatures. We also discuss the interaction between two 60° dislocations and the formation of a 90° dislocation at the interface by a dislocat... read less

Abstract: Recent experimental and theoretical studies of exotic forms of tetrahedrally coordinated semiconductors are reviewed. These unusual phases are synthesized as long-lived metastable forms of the elemental semiconductors silicon and germanium by the application and subsequent removal of high pressure. Rather than being simply crystallographic oddities, the bonding arrangements in these phases show many similarities to those found in amorphous semiconductors. As a result, these dense structures have been used as so-called 'complex crystal' models for the amorphous state. Advances in experimental and computational techniques have recently allowed for detailed study of the structural, electronic and vibrational properties of these phases to be made under variable temperature and pressure conditions. In view of the considerable difficulties associated with performing theoretical studies of non-crystalline solids, the BC8 and ST12 structures are useful in that an understanding of their properties provides insight into the essential physics of amorphous tetrahedral semiconductors. read less

Abstract: We investigate atomic relocation processes in silicon at OK, initiated by an internal 100eV silicon recoil. The molecular dynamics code MODYSEM is used, based on a Tersoff potential for silicon. A fitting procedure was used for the generation of 8 potential valid over the whole energy range of interest. The contribution of the collisional, spontaneous relaxation and thermalization stages to the atomic relocation process are discussed. A threshold distance for the definition of relocated atoms is determined, which separates atomic displacements into stable and unstable (or transient) groups. The atomic mixing process is quantified in terms of the first and second spatial moments over the relocation cross-section. These moments depend on the criterion used to define a relocated Si atom, with short-distance thermal-like atomic displacements, which appear during the thermalization stage, dominating the values of the spatial moments. However, the moments of the relocation cross-section calculated by c... read less

Abstract: An empirical TersofF-type interatomic potential has been developed for describing Si-H interactions. The potential gives a reasonable fit to bond lengths, angles and energetics of silicon hydride molecules and hydrogen-terminated silicon surfaces. The frequencies of most vibrational modes are within 15% of the experimental and ab initio theory values. The potential is computationally efficient and suitable for molecular dynamics investigations of various processing treatments of hydrogen-terminated silicon surfaces. read less

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

Abstract: Analytic expressions for the shear constants of sp-valent zincblende and f.c.c. structure types are obtained using a first-nearest-neighbour bond order potential. Novel expressions for the tetragonal and trigonal zincblende shear constant C' and C 44 are derived. ImportantlyC' is found to vary as the cube of the bond order. The angular character of the bond order potential is shown to remove the anisotropy constraint of C 44/C' = 2 for f.c.c. lattices within a nearest-neighbour model. read less

Abstract: The binding energies of equivalent, symmetric lattices such as the three-dimensional f.c.c., b.c.c., simple-cubic and diamond structures, the two-dimensional hexagonal, square and graphite layers and the one-dimensional linear chain are compared using a first-nearest-neighbour embedding potential for the bond order which retains the first term in a recently derived many-atom expansion. Unexpectedly we find that the bond energy of angularly dependent sp-valent systems with identical dimensionality shows the same square-root dependence on coordination number as that predicted by the conventional embedded atom potential or the angularly independent bond order potentials for s-valent systems. Thus the inclusion of the predicted angular character in the first term of the many-atom expansion for the bond order does not provide any additional differentiation between the binding energies of different isodimensional structure types. read less

Abstract: The results obtained by a recently proposed empirical potential for silicon which includes four-body terms are compared with the results of quantum-mechanical tight-binding calculations. In particular, the ground-state energy and structure of the Si33 cluster were computed by both methods. By performing an equivalent calculation using only up to three-body interactions the authors demonstrate that the four-body term is absolutely necessary in order to achieve good agreement with the quantum method. read less

Abstract: The relative structural stability of s- and sp-valent systems is examined within the fourth-moment approximation to the recently derived tight binding bond order potentials. At this low level of approximation the authors find that the application of a sum rule constraint to the choice of terminator is necessary to get good results. In particular, the competition between graphite, diamond and simple cubic sp-valent lattices is modelled well by the new angularly dependent bond order potentials. read less

Abstract: Classical trajectory calculations were employed to study the reaction of acetylene with dimer sites on the Si(100) surface at 105 K. Two types of potential energy functions were combined to describe interactions for different regions of the model surface. A quantum mechanical potential based on the semiempirical AM1 Hamiltonian was used to describe interactions between C2H2 and a portion of the silicon surface, while an empirically parametrized potential was developed to extend the size of the surface and simulate the dynamics of the surrounding silicon atoms. Reactions of acetylene approaching different sites were investigated, directly above a surface dimer, and between atoms from separate dimers. In all cases, the outcome of C2H2 surface collisions was controlled by the amount of translational energy possessed by the incoming molecule. Acetylene molecules with high translational energy reacted with silicon dimers to form surface species with either one or two Si–C bonds. Those molecules with low transl... read less

Abstract: We clarify the local structure in a model silicon glass by use of Voronoi-polyhedron analysis. The glass is produced by molecular dynamics with a Stillinger-Weber potential. The atoms in the glass are nearly distinguishable: there are about 200 types in the system with 216 atoms. The analysis clarifies that the polyhedra are formed by a small number of large-area polygons or by a large number of small-area polygons. This feature is different from those in Lennard-Jones glasses or metallic glasses and is attributed to the loose-packed structure even in the glass state, in which the atoms still have directional bonding read less

Abstract: Low energy (50 eV) implantation of boron and silicon into crystalline Si through the {110} and {100} faces is studied by a molecular dynamics simulation and the results compared with a binary-collision crystalline computer model and SIMS data. It is found that long-range channelling of the B particles takes place and that their ranges far exceed those predicted by transport theory in random media or Monte-Carlo computer models. It is found that channelling of B occurs only in the 〈110〉 direction. The Si atoms which are displaced by more than the nearest-neighbour spacing always originate from near to the surface as a result of direct knock-ons from ions which do not channel. These displacements show a distinct angular dependence because of the crystalline nature of the solid. For the Si implantation, the attractive forces between the incoming ion and the crystal atoms play an important role in limiting the ion range. read less

Abstract: A semiclassical interatomic potential for carbon is discussed which is based on the proximity cell (the Wigner-Seitz cell) around each atom. It introduces three internal degrees of freedom per atom, representing the magnitude and direction of the p orbital that is not involved in sp hybridization. Its direct interpolation between sp2 and sp3 configurations combined with good elastic properties allows its use on problematic defects, such as the interplanar interstitial in graphite, which is given as an example. read less

Abstract: Molecular-dynamics simulations have been performed for the keV particlebombardment of Si/l brace/110/r brace/ and Si/l brace/100/r brace/ using a many-body potential developed byTersoff. Energy and angle distributions are presented along with an analysis ofthe important ejection mechanisms. We have developed a computer logic that onlyintegrates the equations of motion of the atoms that are struck, thusdecreasing the computer time by a factor of 3 from a completemolecular-dynamics simulation. read less

Abstract: Two-dimensional (2D) materials have attracted extensive research interest in various applications in recent years due to their superior thermal, electrical, and optical properties, making them preferable for potential electronic and optoelectronic applications. These 2D materials form Van der Waals interfaces with common substrate materials due to fabrication and/or device requirements. Since the generated heat during the operation of the devices cause degradation and reliability concerns, interface thermal boundary conductance (TBCs) and in-plane thermal conductivities of the interfaces should be well understood for proper thermal management. Herein, we investigate the TBC and in-plane thermal conductivities of the Van der Waals interfaces of 2D materials by approach to-equilibrium molecular dynamics (AEMD) and non-equilibrium molecular dynamics (NEMD) simulations. Our results show that the TBC is higher for the interfaces with stronger phonon DOS and lattice match. Also, the increasing number of 2D material layers increases the TBC of the interface. The results also showed that the thermal conductivity of the materials forming the interface could affect each other's in-plane thermal conductivity. Changes in thermal conductivities of individual in-plane thermal conductivities can be as high as 70%. Change in thermal conductivity depends on the difference in thermal conductivities of materials in contact and only visible in the vicinity of the interface. Thermal management strategies should pay attention to the trade-off between the changes in individual thermal conductivities and TBC of the interfaces. read less

Abstract: Epitaxially grown ultra-thin Si layers are often used to passivate Ge surfaces in the high-k gate module of (strained) Ge FinFET and Gate All Around devices. We use Si 4 H 10 as Si precursor as it enables epitaxial Si growth at temperatures down to 330 ◦ C. C-V characteristics of blanket capacitors made on Ge virtual substrates point to the presence of an optimal Si thickness. In case of compressively strained Ge fin structures, the Si growth results in non-uniform and high strain levels in the strained Ge fin. These strain levels have been calculated for different shapes of the Ge fin and in function of the grown Si thickness. The high strain is the driving force for potential (unwanted) Ge surface reflow during Si deposition. The Ge surface reflow is strongly affected by the strength of the H-passivation during Si-capping and can be avoided by carefully selected process conditions. read less

Abstract: The intrinsic stress at hetero interface is one of key factors to induce the structural instability of micropatterning in semiconductor devices, and it is essential to quantitatively predict the intrinsic stress in terms of atomic structure. In our previous study, we established the method to predict the lateral undulation buckling of micropattern using continuum buckling theory and finite element method. However, there is a possibility that several-nanometer surface oxidized layer, which is formed during oxygen plasma etching process, involves the intrinsic stress of the order of 1 GPa and affects the buckling criteria. Since the experimental measurement of the stress of such a nano-scale layer is beyond the present technology, an atomistic simulation could be a tool to discuss the possibility. In this study, we make a molecular dynamics model for the plasma etching and predict the intrinsic stress of the oxidized layer on the surface of amorphous silicon (a-Si). Our approach reveals that the layer thickness depends on the incident energy of oxygen and that the intrinsic stress exceeds 1 GPa. Therefore, we conclude that it would be possible for the oxidized a-Si film to trigger the buckling of micropatterning even if the initial a-Si structure involves no stress. read less

Abstract: Abstract: One of the most recent developments in ceramics has been the distribution of multiple phases in a ceramic composite at the nanoscopic length scale. An advanced nanocomposite microstructure such as that of polycrystalline silicon carbide (SiC)–silicon nitride (Si3N4) nanocomposites contains multiple length scales with grain boundary thickness of the order of 50 nm, SiC particle sizes of the order of 200–300 nm and Si3N4 grain sizes of the order of 0.8–1.5 μm. Designing the microstructure of such a composite for a targeted set of material properties is, therefore, a daunting task. Since the microstructure involves multiple length scales, multiscale analyses based material design is an appropriate approach for such a task. With this view, this chapter presents an overview of the current state of the art and work performed in this area. read less

Abstract: Carbon nanotubes (CNTs) are of great interest for load-­‐bearing applications because of their excellent mechanical properties. While much effort has been made in the last decade in order to address problems that obscure the applications of CNTs with their remarkable properties fully exploited from both experimental and theoretical perspectives, some fundamental issues regarding the nanomechanical behavior of individual CNTs at noncritical stress, the interaction between CNTs in their assembled forms and, along with the development of a method for effectively dispersing CNTs in aqueous and polymer media with their intrinsic properties retained are far from being settled. In this study, we first focus on probing the fracture mechanisms of CNTs creep using classical molecular dynamics (MD) and nudged elastic band (NEB) methods. The long-­‐timescale microstructural evolution of CNTs at relatively low external stress is modeled by dividing the continuous process into a series of successive discrete transitions between metastable states. Our results indicate that there exist bifurcation states of the failure mechanism in armchair CNT: brittle-­‐type fracture dominates the fracture if external stress exceeds 42.2 GPa for a (8, 8) CNT; alternatively, plastic deformation caused by the nucleation and diffusion of a specific read less

Abstract: In this work, we demonstrated engineered modification of propagation of thermal phonons, i.e. at THz frequencies, using phononic crystals. This work combined theoretical work at Sandia National Laboratories, the University of New Mexico, the University of Colorado Boulder, and Carnegie Mellon University; the MESA fabrication facilities at Sandia; and the microfabrication facilities at UNM to produce world-leading control of phonon propagation in silicon at frequencies up to 3 THz. These efforts culminated in a dramatic reduction in the thermal conductivity of silicon using phononic crystals by a factor of almost 30 as compared with the bulk value, and about 6 as compared with an unpatterned slab of the same thickness. This work represents a revolutionary advance in the engineering of thermoelectric materials for optimal, high-ZT performance. We have demonstrated the significant reduction of the thermal conductivity of silicon using phononic crystal structuring using MEMS-compatible fabrication techniques and in a planar platform that is amenable to integration with typical microelectronic systems. The measured reduction in thermal conductivity as compared to bulk silicon was about a factor of 20 in the cross-plane direction [26], and a factor of 6 in the in-plane direction. Since the electrical conductivity was only reduced by a corresponding factor of about 3 due to the removal of conductive material (i.e., porosity), and the Seebeck coefficient should remain constant as an intrinsic material property, this corresponds to an effective enhancement in ZT by a factor of 2. Given the number of papers in literature devoted to only a small, incremental change in ZT, the ability to boost the ZT of a material by a factor of 2 simply by reducing thermal conductivity is groundbreaking. The results in this work were obtained using silicon, a material that has benefitted from enormous interest in the microelectronics industry and that has a fairly large thermoelectric power factor. In addition, the techniques and scientific understanding developed in the research can be applied to a wide range of materials, with the caveat that the thermal conductivity of such a material be dominated by phonon, rather than electron, transport. In particular, this includes several thermoelectric materials with attractive properties at elevated temperatures (i.e., greater than room temperature), such as silicon germanium and silicon carbide. It is reasonable that phononic crystal patterning could be used for high-temperature thermoelectric devices using such materials, with applications in energy scavenging via waste-heat recovery and thermoelectric cooling for high-performance microelectronic circuits. The only part of the ZT picture missing in this work was the experimental measurement of the Seebeck coefficient of our phononic crystal devices. While a first-order approximation indicates that the Seebeck coefficient should not change significantly from that of bulk silicon, we were not able to actually verify this assumption within the timeframe of the project. Additionally, with regards to future high-temperature applications of this technology, we plan to measure the thermal conductivity reduction factor of our phononic crystals as elevated temperatures to confirm that it does not diminish, given that the nominal thermal conductivity of most semiconductors, including silicon, decreases with temperature above room temperature. We hope to have the opportunity to address these concerns and further advance the state-of-the-art of thermoelectric materials in future projects. read less

Abstract: Generally, it is difficult to develop practical interatomic potentials which can be used in classical molecular dynamics calculations, since a method for development had not been established. In order to solve such problems, we had proposed a framework to develop interatomic potentials. However, the framework had not been widely used due to the difficulty of coding a process to optimize potential-parameters, which was involved in the framework. In this work, a practical software to optimize potential-parameters for solid systems, was developed based on real-coded genetic algorithm (GA). Fitness (=optimization-function) in GA for potential-parameter optimization could be calculated in practical time, and a concept of the corresponded module in GA-code was proposed. The developed software has an extensible structure: a user can define a new crystal-structure, or a new potential function to use in potential-parameter optimization by defining a corresponded class for that in separated header files. Convenient interfacial class to calculate equilibrium material properties of crystals and that to calculate material properties of user-inputted atomic-structures were prepared to define optimization-function. Several sample input files and header files were available for easy starting. As a result of distribution, several effective interatomic potentials were developed by using the software. As a sample of the development of interatomic potential, Tersoff-type potential for SiO2 systems were shown. read less

Abstract: Molecular dynamics simulations are performed to estimate acoustical and optical phonon relaxation times, dispersion relations, group velocities, and specific heat of silicon needed to solve the Boltzmann transport equation (BTE) at 300 K and 1000 K. The relaxation times are calculated from the temporal decay of the autocorrelation function of the fluctuation of total energy of each normal mode in the ⟨100⟩ family of directions, where the total energy of each mode is obtained from the normal mode decomposition of the motion of the silicon atoms over a period of time. Additionally, silicon dispersion relations are directly determined from the equipartition theorem obtained from the normal mode decomposition. The impact of the anharmonic nature of the potential energy function on the thermal expansion of the crystal is determined by computing the lattice parameter at the cited temperatures using a NPT (i.e., constant number of atoms, pressure, and temperature) ensemble, and are compared with experimental values reported in the literature and with those computed analytically using the quasiharmonic approximation. The dependence of the relaxation times with respect to the frequency is identified with two functions that follow the functional form of the relaxation time expressions reported in the literature. From these functions a simplified version of relaxation times for each normal mode is extracted. Properties, such as group and phase velocities, thermal conductivity, and mean free path, needed to further develop a methodology for the thermal analysis of electronic devices (i.e., from nano- to macroscales) are determined once the relaxation times and dispersion relations are obtained. The thermal properties are validated by comparing the BTE-based thermal conductivity against the predictions obtained from the Green–Kubo method. It is found that the relaxation times closely resemble the ones obtained from perturbation theory at high temperatures; the contribution to the thermal conductivity of the transverse acoustic, longitudinal acoustic, and longitudinal optical modes being approximately 30%, 60%, and 10%, respectively, and the contribution of the transverse optical mode negligible. read less

Abstract: As in our previous work [1] nanoporous silicon periodic supercells with 1000 atoms but now with 80 % porosity were constructed using the Tersoff potential and our novel approach [2]. The approach consists first in constructing a crystalline diamond-like supercell with a density (volume) close to the real value, and then lowering the density by increasing the volume, subjecting the resulting periodic supercell to Tersoff-based molecular dynamics processes at a temperature of 300 K, followed by geometry relaxation [1]. As in the ab initio approach [2] the resulting samples are also essentially amorphous and display pores along some of the crystallographic directions. We report the radial (pair) distribution function (RDF), g(r), the bond angle distribution, the pore structure where prominent and a computational prediction for the vibrational density of states for this structure. We then compare it to the 50 % porous sample presented in Ref [1]. The soft acoustic phonons are displaced towards lower energy in the 80 % porosity sample whereas the optical modes are displaced towards higher energies. The pseudo gap, existing in the 50 % porous sample, is depleted even more in the 80 % sample indicating a tendency towards the creation of a phonon gap for higher porosity materials. Some conjectures that point to the possible engineering of porous materials to produce predetermined phonon properties are discussed. read less

Abstract: The Tersoff potential is one of the most widely used interatomic potentials for silicon. However, its poor descrip-tion of the elastic constants and melting point of diamond silicon is well known. In this research, a new bond-order type interatomic potential has been developed that can reproduce the elastic constants and melting point of diamond silicon as well as the cohesive energies and equilibrium bond lengths of polytypes of silicon. We improved the original Tersoff potential function through the introduction of a flexible angular dependent term. In order to increase the robustness of the potential, systems that include a wide range of local atomic environments are employed for fitting. Optimized potential parameters were found using a genetic algorithm. The elastic constants and melting point of diamond silicon calculated using the developed potential turned out to be C 11 = 166.4GPa, C 12 = 65.3GPa, C 44 = 77.1GPa and T m = 1681K. It was also found that only elastic constants can be reproduced using the original Tersoff potential function, and that our proposed angular dependent term is a key to reproducing the melting point. read less

Abstract: In this study, we use a combiend method of a classical molecular dynamics with the Tersoff potential and an ab-initio calculation based on density functional theory. This combination method can provide quantitative evaluation of the surface energy and surface stress of well-relaxed amorphous silicon in addition to its structure. Using this method, a surface energy of 1.05 ± 0.14 J/m2 and a surface stress of 1.5 ± 1.2N/m was obtained. This calculation also led to a new discovery of the microscopic characteristic of a-Si surfaces, which was not revealed through the use of an empirical potential. It was shown that there are two types of threefold coordinated atoms at the surface region; one with p3-like bonding and the other with sp2-like bonding. In addition, the investigation indicated that the microstructures of these coordination defects were different from those of threefold coordinated atoms observed in the bulk region. read less

Abstract: Martin's method, which is used to determine the internal displacement of atomic systems and elastic constants, is applied to the Tersoff potential. The potential is modified to provide an accurate description of the high-temperature elastic properties of silicon. The elastic constants of crystalline silicon were investigated at both low and high temperatures. Results were verified using the statistical thermodynamic method, i. e., ‘Fluctuation formula’. It was found that values of elastic constants and the influence of the internal displacement are valid. However, at high temperatures the gap becomes larger owing to the thermal fluctuation. Since the convergence of the Martin's method is faster by about two orders, it is the more effective method. It was also found that the fluctuation term includes the effects of the internal displacement and thermal fluctuation. read less

Abstract: We first provide an overview of some predominant theoretical methods currently used for predicting thermal conductivity of thin dielectric films: the equation of radiative transfer, the temperature‐dependent thermal conductivity theories based on the Callaway model, and the molecular dynamics simulation. This overview also highlights temporal and spatial scale issues by looking at a unified theory that bridges physical issues presented in the Fourier and Cattaneo models. This newly developed unified theory is the so‐called C‐ and F‐processes constitutive model. This model introduces the notion of a new dimensionless heat conduction model number, which is the ratio of the thermal conductivity of the fast heat carrier F‐processes to the total thermal conductivity comprised of both the fast heat carriers F‐processes and the slow heat carriers C‐processes. Illustrative numerical examples for prediction of thermal conductivity in thin films are primarily presented. read less

Abstract: A hemispherical asperity moving over a flat plane is simulated based on classical molecular dynamics. The asperity and the plane consist of silicon atoms whose interactions are governed by the Tersoff three-body potential. The gap between the asperity and the plane is maintained to produce attractive normal force in order to investigate the adhesive friction and wear. The simulation focuses on the influence of crystallographic orientation of the contacting surfaces and the moving direction. It is demonstrated that the adhesive friction and wear are lower when crystallographic orientations of the contacting surfaces are different, and also depend on the moving direction relative to the crystal1ographic orientation. read less

Abstract: We show by molecular dynamics simulations, that the radial distribution function of an amorphous material does not change significantly by introducing a considerable volume fraction of nanocrystals. The nanocrystals, embedded in a continuous random network, ensure a certain degree of medium range order in the amorphous material. Our simulations, which are on germanium, show that microcrystals smaller than 2 nm can comprise at least 20 % of the volume without significantly changing the radial distribution function from that of pure continuous random network. By increasing the size of the crystals, without altering the crystal to amorphous volume ratio, the radial distribution changes. The molecular dynamics simulations show that the nanocrystals are unchanged at low temperature. At higher temperature the mobility and critical size of the grains increase, transforming the sub-critical crystalline grains into the surrounding continuous random network matrix. read less

Abstract: The authors have calculated the displacement-threshold energy E(d) for point-defect production in Si and SiC using empirical potentials, tight-binding, and first-principles methods. They show that -- depending on the knock-on direction -- 64-atom simulation cells can be sufficient to allow a nearly finite-size-effect-free calculation, thus making the use of first-principles methods possible. They use molecular dynamics (MD) techniques and propose the use of a sudden approximation which agrees reasonably well with the MD results for selected directions and which allows estimates of Ed without employing an MD simulation and the use of computationally demanding first-principles methods. Comparing the results with experiment, the authors find the full self-consistent first-principles method in conjunction with the sudden approximation to be a reliable and easy method to predict E{sub d}. Furthermore, they have examined the temperature dependence of E{sub d} for C in SiC and found it to be negligible. read less

Abstract: A new theoretical model of potential-energy functions for atomic solids has been developed. An angular factor has been included in this model and its effect has been discussed. Using this new model a new preliminary potential for silicon crystal has been derived. The calculated phonon dispersion curve along the [q00] direction, using this new potential, has been given. A good agreement has been found with experiment. read less

Abstract: We have implemented a modified diffusion limited aggregation model to simulate the porous silicon structure obtained by electro-chemical dissolution. The fractal structures thus obtained have been used as starting configurations from which a fully equilibrated structure was determined by the method of molecular dynamics. read less

Abstract: The phase behavior of silicon is studied using the Modified Embedded Atom Method (MEAM) proposed by Baskes, Nelson and Wright. We find this model to quantitatively reproduce aspects of both the solid and liquid phases with an accuracy comparable to the widely-used Stillinger-Weber (SW) potential, thus providing an opportunity to examine the consistency of results obtained previously using the SW model. Although the models are very different, they both produce solid-liquid interfaces on both silicon (100) and (111) which have very similar morphologies. We find that the MEAM predicts the melting point of silicon to be 1445K, or 14% lower than the experimental value. The model also predicts the heat of melting to be 34.9 kJ/mol, 45% lower than the experimental value of 50.6 kJ/mol, and a liquid density which is 5.4% larger than that of the solid at the melting point, which is the density ratio found by experiment. The liquid density is found to be too low with respect to experiment. We also suggest a correction which might be applied to the MEAM model to improve its description of the liquid phase. read less

Abstract: The molecular dynamics method was used to simulate the fracture process of monocrystalline silicon with different sizes of point defect under a constant strain rate. The mechanism of the defect size on the mechanical properties of monocrystalline silicon was also investigated. The results suggested that the point defect significantly reduces the yield strength of monocrystalline silicon. The relationships between the yield strength variation and the size of point defect fitted an exponential function. By statistically analyzing the internal stress in monocrystalline silicon, it was found that the stress concentration induced by the point defect led to the decrease in the yield strength. A comparison between the theoretical strength given by the four theories of strength and actual strength proved that the Mises theory was the best theory of strength to describe the yield strength of monocrystalline silicon. The dynamic evolution process of Mises stress and dislocation showed that the fracture was caused by the concentration effect of Mises stress and dislocation slip. Finally, the fractured microstructures were similar to a kind of two-dimensional grid which distributed along the cleavage planes while visualizing the specimens. The results of this article provide a reference for evaluating the size effects of point defects on the mechanical properties of monocrystalline silicon. read less

Abstract: Machine learning models are poised to make a transformative impact on chemical sciences by dramatically accelerating computational algorithms and amplifying insights available from computational chemistry methods. However, achieving this requires a confluence and coaction of expertise in computer science and physical sciences. This Review is written for new and experienced researchers working at the intersection of both fields. We first provide concise tutorials of computational chemistry and machine learning methods, showing how insights involving both can be achieved. We follow with a critical review of noteworthy applications that demonstrate how computational chemistry and machine learning can be used together to provide insightful (and useful) predictions in molecular and materials modeling, retrosyntheses, catalysis, and drug design. read less

Abstract: In the present article, using the methods of mathematical modeling, the thermal conductivity of silicon was obtained in a wide temperature range (0.3 ≼ T ≼ 3 kK), including the region of semiconductor-metal phase transformations. As it is known, there are two mechanisms of heat transfer in a solid: elastic lattice vibrations and free electrons, therefore, in the study of the thermal conductivity of silicon, the lattice and electronic components were taken into account. The lattice (phonon) thermal conductivity in this work was determined within the framework of the atomistic approach. The Stillinger–Weber and Kumagai–Izumi–Hara–Sakai interaction potentials were used for modeling. The results of the comparison of the phonon thermal conductivity obtained from the simulation results with the used interaction potentials are presented. The modeling of the thermal conductivity of the electronic subsystem of silicon with intrinsic conductivity in this work is based on the use of the quantum statistics of the electron gas using the Fermi–Dirac integrals. The total thermal conductivity of silicon, obtained as the sum of the electronic and phonon components, is compared with the experimental data. read less

Abstract: Applying classical molecular dynamics simulations, we report the effects of length ( λ) and orientation ( θ) of a line-defect on strength and toughness in defective 2D hexagonal boron nitride. Results reveal the existence of a “transition angle,” θ t = 2.47 °, at which both toughness and strength are insensitive to the finite length of the defect in an infinite domain. For θ θ t, they show the opposite behavior. Examination of the stress-fields shows that θ-dependent variation in stress-localization at the edges of the line-defect and symmetry-breaking of the stress-fields with respect to the defect-axis govern the disparate θ-dependent behavior. For θ θ t, the stress-intensity at the edges is strongly localized at the opposite sides of the line-defect. The stress-intensity increases asymptotically with the increasing defect-length and reduces the strength and toughness of the defective lattice. The stress-localization, however, saturates at a “saturation angle” of around 60 ° for strength and 30 ° for toughness. Additionally, there exists a critical defect-length λ c = 60 A, below which there is a strong θ-dependent variation in elastic interactions between the edges, affecting strength and toughness substantially. For λ > λ c, the elastic interactions saturate and make both strength and toughness insensitive to the change in the length of the defect. read less

Abstract: Despite the considerable efforts made to use silicon anodes and composites based on them in lithium-ion batteries, it is still not possible to overcome the difficulties associated with low conductivity, a decrease in the bulk energy density, and side reactions. In the present work, a new design of an electrochemical cell, whose anode is made in the form of silicene on a graphite substrate, is presented. The whole system was subjected to transmutation neutron doping. The molecular dynamics method was used to study the intercalation and deintercalation of lithium in a phosphorus-doped silicene channel. The maximum uniform filling of the channel with lithium is achieved at 3% and 6% P-doping of silicene. The high mobility of Li atoms in the channel creates the prerequisites for the fast charging of the battery. The method of statistical geometry revealed the irregular nature of the packing of lithium atoms in the channel. Stresses in the channel walls arising during its maximum filling with lithium are significantly inferior to the tensile strength even in the presence of polyvacancies in doped silicene. The proposed design of the electrochemical cell is safe to operate. read less

Abstract: This work is aimed at analysis of methods for the developing of new thermoelectric systems including those based on the so-called locally resonant "nanophonic" metamaterials. These materials, with their experimental realization, are intended for use in thermoelectric systems, since they have a large value of the thermoelectric Q-factor ZT. The increase in Q is achieved by reducing the thermal conductivity of the material, which leads to an increase in the value of ZT, if the electrical conductivity remains the same. The reduction in thermal conductivity, without corresponding reduction in electrical conductivity, is achieved by choosing a configuration of thermoelectric materials consisting of a thin film with a periodic set of columns mounted on a free surface. Such a configuration qualitatively changes the base phonon spectrum of a thin film due to the hybridization mechanism between local column resonances and underlying lattice scattering. Numerical methods are used to study changes in the phonon spectrum and, as a consequence, a change (decrease) in the value of thermal conductivity depending on variations in the system architecture. read less

Abstract: Molecular dynamics (MD) has experienced a significant growth in the recent decades. Simulating systems consisting of hundreds of thousands of atoms is a routine task of computational chemistry researchers nowadays. Thanks to the straightforwardly parallelizable structure of the algorithms, the most promising method to speed‐up MD calculations is exploiting the large‐scale processing power offered by the parallel hardware architecture of graphics processing units or GPUs. Programming GPUs is becoming easier with general‐purpose GPU computing frameworks and higher levels of abstraction. In the recent years, implementing MD simulations on graphics processors has gained a large interest, with multiple popular software packages including some form of GPU‐acceleration support. Different approaches have been developed regarding various aspects of the algorithms, with important differences in the specific solutions. Focusing on published works in the field of classical MD, we describe the chosen implementation methods and algorithmic techniques used for porting to GPU, as well as how recent advances of GPU architectures will provide even more optimization possibilities in the future. read less

Abstract: ABSTRACT During single-point diamond turning of hard and brittle materials, tool wear is a dominant factor that influences machinability. In the wear process, micro grooves on the flank face is an important character of tool wear, which leads to the formation of subcutting edges and the ductile to brittle transition. In this paper, classical molecular dynamic simulations of nanometric cutting of silicon by a diamond tool with V-shape grooves were carried out to explore the effect of groove geometry on the workpiece and tool integrity. The evolution of tool wear and machined surface integrity was discussed. Simulation result shows that grooves have a significant influence on the stress and temperature distributions of the tool, which has a great influence on tool deterioration. Grooves with sharp edges will lead to severe tool wear and bring deep subsurface damage of the machined surface. However, the subsurface damage of the machined surface can be restrained with blunt grooves since the pressure is reduced. With a comprehensive understanding and controlling of groove, tool wear can be suppressed and high-quality surface can be achieved. read less

Abstract: This paper presents a quantitative understanding of toughness and strength anisotropy in 3 C-SiC under uniaxial deformation. We consider four high-symmetry crystallographic directions including [100], [110], [111], and [ 11 2 ¯ ] for loading, and find that both toughness and strength are the maximum along the [100] direction and the minimum along the [111] direction. The maximum anisotropy in crack nucleation-toughness is 145% and in fracture toughness 126%, relative to the [111] direction. The corresponding anisotropies in fracture strain and fracture strength are found to be 62% and 36%, respectively. An atomistic analysis shows that bonds deform uniformly for loading along the [100] direction, whereas for loading along the [110], [111], or [ 11 2 ¯] directions, bonds deform nonuniformly and it breaks the symmetry of the local atomic structure. The nonuniform bond deformation creates different sets of bond lengths and forms the atomistic basis for the direction-dependent mechanical behavior. The simulations are conducted with four different interatomic potentials including the Stillinger-Weber, Tersoff, Vashishta, and Environment Dependent Interatomic Potentials. It is found that only the Stillinger-Weber potential exhibits first-principles accurate strength and toughness as well as brittlelike fracture. Also, there is a sizeable difference among the potentials in terms of the crack nucleation toughness and strength. We find the difference to originate from the dissimilarity in the forcing function and its derivative in the nonlinear regime of mechanical deformation. A mathematical analysis suggests that it is essential for the forcing function to accurately represent the first-principles accurate forcing function, at least up to the maximum bond force, to produce accurate fracture properties and patterns. read less

Abstract: Locally, the atomic structure in well-annealed amorphous silicon appears similar to that of crystalline silicon. We address here the question whether a point defect, specifically a vacancy, in amorphous silicon also resembles that in the crystal. From density-functional theory calculations of a large number of nearly defect-free configurations, relaxed after an atom has been removed, we conclude that there is little similarity. The analysis is based on formation energy, relaxation energy, bond lengths, bond angles, Vorono\"{\i} volume, coordination, atomic charge, and electronic gap states. All these quantities span a large and continuous range in amorphous silicon and while the removal of an atom leads to the formation of one to two bond defects and to a lowering of the local atomic density, the relaxation of the bonding network is highly effective, and the signature of the vacancy generally unlike that of a vacancy in the crystal. read less

Abstract: Molecular dynamics is an extensively utilized computational tool for solids, liquids and molecules simulation. Currently, much research on molecular dynamics simulation focuses on simplifying forces or parallelizing tasks to reduce the overheads of forces computation. However, the molecular dynamics simulation still remains challenging since the communication and neighbor list construction are time-consuming in the existing algorithm. In this paper, we propose a swMD optimization strategy including a new communication mode called ghost communication to reduce superfluous communication overheads and an innovative neighbor list algorithm to improve the construction efficiency of it. Moreover, we accelerate computation by utilizing many-core resources on Sunway Taihulight and present an auto-tuning Producer-Consumer pairing algorithm to make neighbor list construction happen in fast register communication. Compared to traditional methods, swMD optimization strategy obtains a maximal 82.2% and an average of 79.4% performance improvement. We also evaluate the scalability up to 266,240 cores and the results demonstrate the high efficiency of swMD optimization strategy on communication, computation and neighbor list construction respectively. read less

Abstract: Based on the molecular dynamics software package CovalentMD 2.0, the fastest molecular dynamics simulation for covalent crystalline silicon with bond‐order potentials has been implemented on the third highest performance supercomputer “Sunway TaihuLight” in the world (before June 2019), and already obtained 16.0 Pflops (1015 floating point operation per second) in double precision for the simulation of crystalline silicon, which is recordly high for rigorous atomistic simulation of covalent materials. The simulations used up to 160,768 64‐core processors, totally nearly 10.3 million cores, to simulate more than 137 billion silicon atoms, where the parallel efficiency is over 80% on the whole machine. The running performance on a single processor reached 15.1% of its theoretical peak at highest. The longitudinal dimension of the simulated system is far beyond the range with scale‐dependent properties, while the lateral dimension significantly exceeds the experimentally measurable range. Our simulation enables virtual experiments on real‐world nanostructured materials and devices for predicting macroscale properties and behaviors from microscale structures directly, bringing about many exciting new possibilities in nanotechnology, information technology, electronics and renewable energies, etc. © 2019 Wiley Periodicals, Inc. read less

Abstract: Silicon is an important material and many empirical interatomic potentials have been developed for atomistic simulations of it. Among them, the Tersoff potential and its variants are the most popular ones. However, all the existing Tersoff-like potentials fail to reproduce the experimentally measured thermal conductivity of diamond silicon. Here we propose a modified Tersoff potential and develop an efficient open source code called GPUGA (graphics processing units genetic algorithm) based on the genetic algorithm and use it to fit the potential parameters against energy, virial and force data from quantum density functional theory calculations. This potential, which is implemented in the efficient open source GPUMD (graphics processing units molecular dynamics) code, gives significantly improved descriptions of the thermal conductivity and phonon dispersion of diamond silicon as compared to previous Tersoff potentials and at the same time well reproduces the elastic constants. Furthermore, we find that quantum effects on the thermal conductivity of diamond silicon at room temperature are non-negligible but small: using classical statistics underestimates the thermal conductivity by about 10% as compared to using quantum statistics. read less

Abstract: We present a deep-learning approach for modeling the atomic structure of amorphous silicon ( a -Si). While accurate models of disordered systems require an ab initio description of the energy landscape which severely limits the attainable system size, large-scale models rely on empirical potentials, at the price of reduced reliability and a computational load that is still restricting for many purposes. In this paper, we explore an approach based on deep learning, particularly generative modeling that could reconcile both requirements of accuracy and efficiency by learning structural features from data. When trained on a set of observations, such models can generate new structures very efficiently with the desired level of accuracy, as determined by the data set. We first validate our approach by training a convolutional neural network to approximate the potential-energy surface of a -Si, as given by the Stillinger-Weber potential, which results in a root-mean-square error of 5.05 meV per atom—about 0 . 16% of the atomic energy. We then train a deep generative model, the Wasserstein autoencoder, for the generation of a -Si configurations. Our approach leads to models which exhibit some of the essential features of a -Si while possessing too much structural disorder, thus suggesting that the method is viable; we indicate avenues for improving it towards the generation of state-of-the-art structures. read less

Abstract: Epitaxial graphene grown by thermal Si decomposition of Silicon Carbide appears in different morphological variants, depending on the production conditions: the strongly rugged buffer layer, retaining a considerable amount of sp3 hybridized buffer layer, the softly corrugated graphene monolayer and the rather flat quasi free standing monolayer with sparse small pits pinned to localized electronic states. Therefore, graphene on SiC is not a single material, but a set of materials with different morphologies depending on the environmental conditions during the synthesis. In all cases the distortion from the ideal flat structure seem to follow to some extent specific symmetries, which appear to preserve some memory of the interaction with the SiC bulk, even in the cases in which the sheet is substantially decoupled from it. Defects bear interesting properties, e.g. localized hot spots of reactivity and localized electronic states with specific energy depending on their nature and morphology, while their possible symmetric location is an added value for applications. Therefore, being capable of controlling the morphology, concentration, symmetry and location of the defects would allow tailoring this material for specific applications. Based on ab initio calculations and simulations, we first describe in detail the morphology of the different systems, and, subsequently, we formulate hypotheses on the relationship between morphology and the formation process. We finally suggest future simulation studies capable of revealing the still unclear steps. These should give indication on how to tune the environmental conditions to control the final morphology of the sample and specifically design this material. read less

Abstract: The present study introduces a facile single-source precursor preparative access to bamboo-like multiwalled carbon nanotubes (MWCNTs) highly dispersed within a mesoporous silica-rich matrix. The metal-free single-source precursor was synthesized via a one-pot sol-gel process using tetramethyl orthosilicate (TMOS) and 4,4'-dihydroxybiphenyl (DHBP) and converted subsequently via pyrolysis under an argon atmosphere into MWCNT/silica nanocomposites. The in situ segregation of the highly defective bamboo-like MWCNTs was carefully investigated and has been shown to occur within the mesopores of the silica-rich matrix at relatively low temperatures and without the use of a metal catalyst. The experimental results have been supported by extensive computational simulations, which correlate the molecular architecture of the single-source precursor with the structural features of the carbon phase segregating from the silica matrix. Furthermore, the role of hydrogen in the stability of the prepared nanocomposites as well as in the high-temperature evolution and morphology of the segregated MWCNTs has been discussed based on vibrational spectroscopy, calorimetric studies and empirical potential calculations. The results obtained within the present study may allow for designing highly-defined nanocarbon-containing composites with tailored structural features and property profiles. read less

Abstract: In this study, we have investigated delivery of cisplatin as the anticancer drug molecules in different carbon nanotubes (CNTs) in the gas phase using molecular dynamics simulation. We examined the shape and composition of the releasing agent by using the different nanowires and nanoclusters. We also investigated the doping effect on the drug delivery process using N‐, Si, B‐, and Fe‐doped CNTs. Different thermodynamics, structural, and dynamical properties have been studied by using the pure and different doped CNTs in this study. Our results show that the doping of the CNT has significant effect on the rate of the drug releasing process regardless of the composition of the releasing agent. © 2019 Wiley Periodicals, Inc. read less

Abstract: We report that single interatomic potential, developed using Gaussian regression of density functional theory calculation data, has high accuracy and flexibility to describe phonon transport with ab initio accuracy in two different atomistic configurations: perfect crystalline Si and crystalline Si with vacancies. The high accuracy of second- and third-order force constants from the Gaussian approximation potential (GAP) are demonstrated with phonon dispersion, Gruneisen parameter, three-phonon scattering rate, phonon-vacancy scattering rate, and thermal conductivity, all of which are very close to the results from density functional theory calculation. We also show that the widely used empirical potentials (Stillinger-Weber and Tersoff) produce much larger errors compared to the GAP. The computational cost of GAP is higher than the two empirical potentials, but five orders of magnitude lower than the density functional theory calculation. Our work shows that GAP can provide a new opportunity for studying phonon transport in partially disordered crystalline phases with the high predictive power of ab initio calculation but at a feasible computational cost. 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: The nucleation and growth of SiC precipitates in liquid silicon is important in the crystallization of silicon used for the photovoltaic industry. These processes depend strongly on the carbon concentration as well as the equilibrium solubility relative to the precipitate phase. Here, using a suite of statistical thermodynamic techniques, we calculate the solubility of carbon atoms in liquid silicon relative to the β-SiC phase. We employ several available empirical potentials to assess whether these potentials may reasonably be used to computationally analyze SiC precipitation. We find that some of the Tersoff-type potentials provide an excellent picture for carbon solubility in liquid silicon but, because of their severe silicon melting point overestimation, are limited to high temperatures where the carbon solubility is several percent, a value that is irrelevant for typical solidification conditions. Based on chemical potential calculations for pure silicon, we suggest that this well-known issue is confined to the description of the liquid phase and demonstrate that some recent potential models for silicon might address this weakness while preserving the excellent description of the carbon-silicon interaction found in the existing models. read less

Abstract: The central approximation made in classical molecular dynamics simulation of materials is the interatomic potential used to calculate the forces on the atoms. Great effort and ingenuity is required to construct viable functional forms and find accurate parametrizations for potentials using traditional approaches. Machine learning has emerged as an effective alternative approach to develop accurate and robust interatomic potentials. Starting with a very general model form, the potential is learned directly from a database of electronic structure calculations and therefore can be viewed as a multiscale link between quantum and classical atomistic simulations. Risk of inaccurate extrapolation exists outside the narrow range of time and length scales where the two methods can be directly compared. In this work, we use the spectral neighbor analysis potential (SNAP) and show how a fit can be produced with minimal interpolation errors which is also robust in extrapolating beyond training. To demonstrate the method, we have developed a tungsten-beryllium potential suitable for the full range of binary compositions. Subsequently, large-scale molecular dynamics simulations were performed of high energy Be atom implantation onto the (001) surface of solid tungsten. The machine learned W-Be potential generates a population of implantation structures consistent with quantum calculations of defect formation energies. A very shallow ($l2\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) average Be implantation depth is predicted which may explain ITER diverter degradation in the presence of beryllium. read less

Abstract: We investigated the mechanical properties of <100>-oriented square cross-sectional silicon nanowires under tension and compression, with a focus on the effect of side surface orientation. Two types of silicon nanowires (i.e., nanowires with four {100} side surfaces and those with four {110} side surfaces) were simulated by molecular dynamics simulations at a temperature of 300 K. The deformation mechanism exhibited no dependence on the side surface orientation, while the tensile strength and compressive strength did. Brittle cleavage was observed under tension, whereas dislocation nucleation was witnessed under compression. Silicon nanowires with {100} side surfaces had a lower tensile strength but higher compressive strength. The effect of side surface orientation became stronger as the nanowire width decreased. The obtained results may provide some insight into the design of silicon-based nano-devices. read less

Abstract: Interatomic potentials are widely used in computational materials science, in particular for simulations that are too computationally expensive for density functional theory (DFT). Most interatomic potentials have a limited application range and often there is very limited information available regarding their performance for specific simulations. We carried out high-throughput calculations for molybdenum and silicon with DFT and a number of interatomic potentials. We compare the DFT reference calculations and experimental data to the predictions of the interatomic potentials. We focus on a large number of basic materials properties, including the cohesive energy, atomic volume, elastic coefficients, vibrational properties, thermodynamic properties, surface energies and vacancy formation energies, which enables a detailed discussion of the performance of the different potentials. We further analyze correlations between properties as obtained from DFT calculations and how interatomic potentials reproduce these correlations, and suggest a general measure for quantifying the accuracy and transferability of an interatomic potential. From our analysis we do not establish a clearcut ranking of the potentials as each potential has its strengths and weaknesses. It is therefore essential to assess the properties of a potential carefully before application of the potential in a specific simulation. The data presented here will be useful for selecting a potential for simulations of Mo or Si. read less

Abstract: Engineering wear models are generally empirical and lack connections to the physical processes of debris generation at the nanoscale to microscale. Here, we thus analyze wear particle formation for sliding interfaces in dry contact with full and reduced adhesion. Depending on the material and interface properties and the local slopes of the surfaces, we find that colliding surface asperities can either deform plastically, form wear particles, or slip along the contact junction surface without significant damage. We propose a mechanism map as a function of material properties and local geometry, and confirm it using quasi-two-dimensional and three-dimensional molecular dynamics and finite-element simulations on an amorphous, siliconlike model material. The framework developed in the present paper conceptually ties the regimes of weak and strong interfacial adhesion together and can explain that even unlubricated sliding contacts do not necessarily lead to catastrophic wear rates. A salient result of the present paper is an analytical expression of a critical length scale, which incorporates interface properties and roughness parameters. Therefore, our findings provide a theoretical framework and a quantitative map to predict deformation mechanisms at individual contacts. In particular, contact junctions of sizes above the critical length scale contribute to the debris formation. read less

Abstract: A multi-scale computational methodology based on the density functional theory and molecular dynamics has been used to investigate the rheological properties of super critical CO2 with CuO nano-particle (NP). Density functional theory which treats the electron density as the central variable has been used to explore the adsorption of CO2 molecules on the two most stable CuO surfaces [i.e., (111) and (011)] at absolute zero. The results of this theory would provide valuable information to make CuO NPs with the surface where the CO2 adsorption is maximum in order to have a stronger mono-layer of adsorbed CO2 molecules on the surface of the NP which is the most crucial factor in formation of a stable nanofluid. The results show that the CO2 molecule is adsorbed more strongly on the (011) surface with an adsorption energy of -99.06 kJ/mol compared to the (111) surface. A computational methodology based on molecular dynamics has been used to evaluate the enhancement of the rheological properties of the super-critical CO2 liquid based nanofluid at different temperatures and pressures. In this scale, first, the CO2 liquid has been modeled by employing the condensed-phase optimized molecular potentials for atomistic simulation studies (COMPASS) force field potential and the fluid properties computed are in excellent agreement with the literature and experiment values. Second, the nanofluid has been modeled in order to study the enhancement of the fluid properties with the CuO NPs. The charged optimized many-body force field potential has been employed to consider the effect of the charge transferring between the NPs and liquid molecules and breaking of existing bonds and the formation of new bonds. The COMPASS force field potential is also employed for the interactions between CO2 molecules. The combination of these potentials is quite a new approach for the study of the super-critical (SC)-CO2 based nanofluid. The results show that the viscosity of the SC-CO2 is enhanced between 1.3 and 2.5 times under the temperature and pressure conditions studied. read less

Abstract: To achieve quasi-atomic precision etching of thin film materials in advanced transistors, a method based on light ion implantation and consisting of two sequential steps—(1) surface modification in hydrogen ICP or CCP plasmas, and (2) selective removal of modified layers in wet solutions or remote plasmas—was recently proposed. In this paper, to better understand this process (called Smart Etch thereafter), molecular dynamics simulations are performed to study the modification of crystalline Si (1 0 0) substrates under low-energy (x  =  1–3) ion implantation and mixed ion/H radical bombardment. In agreement with the experiments, simulations of (x  =  1–3) ion bombardment of Si show a self-limited implantation with a surface evolution composed of two stages: a rapid volume modification (with no etching) followed by a slow saturation and the formation of a stable [a:Si–H] layer at a steady state. The mechanisms of ion-induced damage (Si–Si bond breaking, formation of H2 molecules, creation of SiHx groups) are investigated and allow us to bring new insights to both the well-known Smart Cut™ and more recent Smart Etch technologies. Si exposure to both ions and H radicals (H2 plasma) also shows a self-limited transformation but the modified layers are simultaneously etched during the implantation. Both the modified layer thickness and the ion dose required to reach the steady state increase with the ion energy. The ion composition and the radical-to-ion flux ratio (Γ) must be considered as well. In particular, the etch rate strongly increases with Γ, compromising even the possibility to achieve a Smart Etch of silicon. read less

Abstract: Strength and toughness are two crucial mechanical properties of a solid that determine its ability to function reliably without undergoing failure in extreme conditions. While hexagonal boron nitride (hBN) is known to be elastically isotropic in the linear regime of mechanical deformation, its directional response to extreme mechanical loading remains less understood. Here, using a combination of density functional theory calculations and molecular dynamics simulations, we show that strength and crack nucleation toughness of pristine hBN are strongly anisotropic and chirality dependent. They vary nonlinearly with the chirality of the lattice under symmetry breaking deformation, and the anisotropic behavior is retained over a large temperature range with a decreasing trend at higher temperatures. An atomistic analysis reveals that bond deformation and associated distortion of electron density are nonuniform in the nonlinear regime of mechanical deformation, irrespective of the loading direction. This nonuniformity forms the physical basis for the observed anisotropy under static conditions, whereas reduction in nonuniformity and thermal softening reduce anisotropy at higher temperatures. The chirality-dependent anisotropic effects are well predicted by inverse cubic polynomials.Strength and toughness are two crucial mechanical properties of a solid that determine its ability to function reliably without undergoing failure in extreme conditions. While hexagonal boron nitride (hBN) is known to be elastically isotropic in the linear regime of mechanical deformation, its directional response to extreme mechanical loading remains less understood. Here, using a combination of density functional theory calculations and molecular dynamics simulations, we show that strength and crack nucleation toughness of pristine hBN are strongly anisotropic and chirality dependent. They vary nonlinearly with the chirality of the lattice under symmetry breaking deformation, and the anisotropic behavior is retained over a large temperature range with a decreasing trend at higher temperatures. An atomistic analysis reveals that bond deformation and associated distortion of electron density are nonuniform in the nonlinear regime of mechanical deformation, irrespective of the loading direction. This nonun... read less

Abstract: Molecular dynamics simulations have been used for decades to investigate continuum mechanics failure to give the correct distribution of stress near discontinuities, such as holes and crack tips. In this paper, stress distribution around elliptical holes in a sheet material has been examined in an atomistic and a continuum model. Atomistic interactions are described by the Tersoff potential tuned for carbon. Calculations were conducted for the problem of stress distribution around the elliptic hole in a 2D graphene sheet subjected to the gradually increasing uniaxial tension load. The atomistic stress is calculated as spatial average utilizing Hardy’s formulation. The results have been compared with the Kirsch solution for stress concentration at the edge of the circular hole. A quantitative measure for switching from atomistic to continuum model and vice versa has been proposed. Routes toward the effective data-driven coupling of macroand micromechanical models where continuum mechanics approach fails are pointed out. read less

Abstract: Two models are proposed to predict the evolution of shear band width as a function of applied strain for simulated glasses mechanically deformed in simple shear. The first model arises from dimensional analysis and an assumption that band broadening is controlled by the strain rate inside the shear band. The second model describes the shear band as a pulled front propagating into an unsteady state, the dynamics of which are described using the effective temperature shear transformation zone (ET-STZ) theory. Both models are compared to three simulated systems: a two-dimensional binary Lennard-Jones glass, a Cu64Zr36 glass modeled using an embedded atom method (EAM) potential, and a Si glass modeled using the Stillinger-Weber potential. Shear bands form in all systems across a variety of quench rates. Depending on the case these bands either appear to broaden indefinitely or to saturate to a finite width. The shear band strain rate model appears to apply only when band growth is unconstrained, indicating the dominance of a single time scale in the early stages of band development. The front propagation model, which reduces to the other model in the early stages of band broadening, also applies to cases in which the band width saturates, suggesting that competition between the rate of shear-induced configurational disordering and thermal relaxation sets a maximum width for shear bands in a variety of materials systems. read less

Abstract: Radiation can drive the electrons in a material out of thermal equilibrium with the nuclei, producing hot, transient electronic states that modify the interatomic potential energy surface. We present a rigorous formulation of two-temperature molecular dynamics that can accommodate these electronic effects in the form of electronic-temperature-dependent force fields. Such a force field is presented for silicon, which has been constructed to reproduce the ab initio-derived thermodynamics of the diamond phase for electronic temperatures up to 2.5eV, as well as the structural dynamics observed experimentally under nonequilibrium conditions in the femtosecond regime. This includes nonthermal melting on a subpicosecond timescale to a liquidlike state for electronic temperatures above ∼1eV. The methods presented in this paper lay a rigorous foundation for the large-scale atomistic modeling of electronically driven structural dynamics with potential applications spanning the entire domain of radiation damage. read less

Abstract: : We carried out molecular dynamics simulations at various temperatures to predict the thermal conductivity and the thermal conductance of graphene and hexagonal boron-nitride (h-BN) thin films. Therefore, several models with six different grain boundary configurations ranging from 33–140 nm in length were generated. We compared our predicted thermal conductivity of pristine graphene and h-BN with previously conducted experimental data and obtained good agreement. Finally, we computed the thermal conductance of graphene and h-BN sheets for six different grain boundary configurations, five sheet lengths ranging from 33 to 140 nm and three temperatures (i.e., 300 K, 500 K and 700 K). The results show that the thermal conductance remains nearly constant with varying length and temperature for each grain boundary. read less

Abstract: Simulations are performed using the reverse non-equilibrium molecular dynamics (rNEMD) method and the Stillinger-Weber (SW) potential to determine the input parameters for achieving ±1% convergence of the calculated thermal conductivity of silicon. These parameters are then used to investigate the effects of the interatomic potentials of SW, Tersoff II, Environment Dependent Interatomic Potential (EDIP), Second Nearest Neighbor, Modified Embedded-Atom Method (MEAM), and Highly Optimized Empirical Potential MEAM on determining the bulk thermal conductivity as a function of temperature (400–1000 K). At temperatures > 400 K, data collection and swap periods of 15 ns and 150 fs, system size ≥6 × 6 UC2 and system lengths ≥192 UC are adequate for ±1% convergence with all potentials, regardless of the time step size (0.1–0.5 fs). This is also true at 400 K, except for the SW potential, which requires a data collection period ≥30 ns. The calculated bulk thermal conductivities using the rNEMD method and the EDIP potential are close to, but lower than experimental values. The 10% difference at 400 K increases gradually to 20% at 1000 K.Simulations are performed using the reverse non-equilibrium molecular dynamics (rNEMD) method and the Stillinger-Weber (SW) potential to determine the input parameters for achieving ±1% convergence of the calculated thermal conductivity of silicon. These parameters are then used to investigate the effects of the interatomic potentials of SW, Tersoff II, Environment Dependent Interatomic Potential (EDIP), Second Nearest Neighbor, Modified Embedded-Atom Method (MEAM), and Highly Optimized Empirical Potential MEAM on determining the bulk thermal conductivity as a function of temperature (400–1000 K). At temperatures > 400 K, data collection and swap periods of 15 ns and 150 fs, system size ≥6 × 6 UC2 and system lengths ≥192 UC are adequate for ±1% convergence with all potentials, regardless of the time step size (0.1–0.5 fs). This is also true at 400 K, except for the SW potential, which requires a data collection period ≥30 ns. The calculated bulk thermal conductivities using the rNEMD method and the EDIP... read less

Abstract: Using an atomistic technique consistent with continuum balance laws and drawing on classical fracture mechanics theory, we estimate the resistance to fracture propagation of amorphous silica. We discuss correspondence and deviations from classical linear elastic fracture mechanics theory including size dependence, rigid/floppy modes of deformation, and the effects of surface energy and stress. read less

Abstract: The success of first principles electronic structure calculation for predictive modeling in chemistry, solid state physics, and materials science is constrained by the limitations on simulated length and time scales due to computational cost and its scaling. Techniques based on machine learning ideas for interpolating the Born-Oppenheimer potential energy surface without explicitly describing electrons have recently shown great promise, but accurately and efficiently fitting the physically relevant space of configurations has remained a challenging goal. Here we present a Gaussian Approximation Potential for silicon that achieves this milestone, accurately reproducing density functional theory reference results for a wide range of observable properties, including crystal, liquid, and amorphous bulk phases, as well as point, line, and plane defects. We demonstrate that this new potential enables calculations that would be extremely expensive with a first principles electronic structure method, such as finite temperature phase boundary lines, self-diffusivity in the liquid, formation of the amorphous by slow quench, and dynamic brittle fracture. We show that the uncertainty quantification inherent to the Gaussian process regression framework gives a qualitative estimate of the potential's accuracy for a given atomic configuration. The success of this model shows that it is indeed possible to create a useful machine-learning-based interatomic potential that comprehensively describes a material, and serves as a template for the development of such models in the future. read less

Abstract: Vibrational energy transport in disordered media is of fundamental importance to several fields spanning from sustainable energy to biomedicine to thermal management. This work investigates hybrid ordered/disordered nanocomposites that consist of crystalline membranes decorated by regularly patterned disordered regions formed by ion beam irradiation. The presence of the disordered regions results in reduced thermal conductivity, rendering these systems of interest for use as nanostructured thermoelectrics and thermal device components, yet their vibrational properties are not well understood. Here, the mechanism of vibrational transport and the reason underlying the observed reduction is established in detail. The hybrid systems are found to exhibit glass‐crystal duality in vibrational transport. Lattice dynamics reveals substantial hybridization between the localized and delocalized modes, which induces avoided crossings and harmonic broadening in the dispersion. Allen/Feldman theory shows that the hybridization and avoided crossings are the dominant drivers of the reduction. Anharmonic scattering is also enhanced in the patterned nanocomposites, further contributing to the reduction. The systems exhibit features reminiscent of both nanophononic materials and locally resonant nanophononic metamaterials, but operate in a manner distinct to both. These findings indicate that such “patterned disorder” can be a promising strategy to tailor vibrational transport through hybrid nanostructures. read less

Abstract: The surface reactivity and hydrophilicity of silicate materials are key properties for various industrial applications. However, the structural origin of their affinity for water remains unclear. Here, based on reactive molecular dynamics simulations of a series of artificial glassy silica surfaces annealed at various temperatures and subsequently exposed to water, we show that silica exhibits a hydrophilic-to-hydrophobic transition driven by its silanol surface density. By applying topological constraint theory, we show that the surface reactivity and hydrophilic/hydrophobic character of silica are controlled by the atomic topology of its surface. This suggests that novel silicate materials with tailored reactivity and hydrophilicity could be developed through the topological nanoengineering of their surface. read less

Abstract: Two-dimensional molybdenum disulfide (MoS2) is a promising material for the next generation of switchable transistors and photodetectors. In order to perform large-scale molecular simulations of the mechanical and thermal behavior of MoS2-based devices, an accurate interatomic potential is required. To this end, we have developed a Stillinger-Weber potential for monolayer MoS2. The potential parameters are optimized to reproduce the geometry (bond lengths and bond angles) of MoS2 in its equilibrium state and to match as closely as possible the forces acting on the atoms along a dynamical trajectory obtained from ab initio molecular dynamics. Verification calculations indicate that the new potential accurately predicts important material properties including the strain dependence of the cohesive energy, the elastic constants, and the linear thermal expansion coefficient. The uncertainty in the potential parameters is determined using a Fisher information theory analysis. It is found that the parameters are... read less

Abstract: The diffusivity of carbon atoms in liquid silicon and their equilibrium distribution between the silicon melt and crystal phases are key, but unfortunately not precisely known parameters for the global models of silicon solidification processes. In this study, we apply a suite of molecular simulation tools, driven by multiple empirical potential models, to compute diffusion and segregation coefficients of carbon at the silicon melting temperature. We generally find good consistency across the potential model predictions, although some exceptions are identified and discussed. We also find good agreement with the range of available experimental measurements of segregation coefficients. However, the carbon diffusion coefficients we compute are significantly lower than the values typically assumed in continuum models of impurity distribution. Overall, we show that currently available empirical potential models may be useful, at least semi-quantitatively, for studying carbon (and possibly other impurity) trans... read less

Abstract: The crystalline-Si/amorphous-SiO2 (c-Si/a-SiO2) interface is an important system used in many applications, ranging from transistors to solar cells. The transition region of the c-Si/a-SiO2 interface plays a critical role in determining the band alignment between the two regions. However, the question of how this interface band offset is affected by the transition region thickness and its local atomic arrangement is yet to be fully investigated. Here, by controlling the parameters of the classical Monte Carlo bond switching algorithm, we have generated the atomic structures of the interfaces with various thicknesses, as well as containing Si at different oxidation states. A hybrid functional method, as shown by our calculations to reproduce the GW and experimental results for bulk Si and SiO2, was used to calculate the electronic structure of the heterojunction. This allowed us to study the correlation between the interface band characterization and its atomic structures. We found that although the systems with different thicknesses showed quite different atomic structures near the transition region, the calculated band offset tended to be the same, unaffected by the details of the interfacial structure. Our band offset calculation agrees well with the experimental measurements. This robustness of the interfacial electronic structure to its interfacial atomic details could be another reason for the success of the c-Si/a-SiO2 interface in Si-based electronic applications. Nevertheless, when a reactive force field is used to generate the a-SiO2 and c-Si/a-SiO2 interfaces, the band offset significantly deviates from the experimental values by about 1 eV. read less

Abstract: Ultrafast laser annealing of ion implanted Si has led to thermodynamically unexpected large {001} self-interstitial loops, and the failure of Ostwald ripening models for describing self-interstitial cluster growth. We have carried out molecular dynamics simulations in combination with focused experiments in order to demonstrate that at temperatures close to the melting point, self-interstitial rich Si is driven into dense liquidlike droplets that are highly mobile within the solid crystalline Si matrix. These liquid droplets grow by a coalescence mechanism and eventually transform into {001} loops through a liquid-to-solid phase transition in the nanosecond time scale. read less

Abstract: Shock wave interactions with defects, such as pores, are known to play a key role in the chemical initiation of energetic materials. The shock response of hexanitrostilbene is studied through a combination of large scale reactive molecular dynamics and mesoscale hydrodynamic simulations. In order to extend our simulation capability at the mesoscale to include weak shock conditions (< 6 GPa), atomistic simulations of pore collapse are used to define a strain rate dependent strength model. Comparing these simulation methods allows us to impose physically-reasonable constraints on the mesoscale model parameters. In doing so, we have been able to study shock waves interacting with pores as a function of this viscoplastic material response. We find that the pore collapse behavior of weak shocks is characteristically different to that of strong shocks. read less

Abstract: We use non-equilibrium molecular dynamics oscillatory shear simulations to study frequency-dependent viscoelastic damping spanning nearly six decades in frequency range (MHz to THz), in a wide range of model glasses including binary glasses such as Cu-Zr metallic glass (MG), Wahnstrom glass and amorphous silica, and unary glasses, namely, Dzugutov glass and amorphous silicon. First, for the Cu-Zr MG, we elucidate the role of quench rate, number of shear cycles, shear amplitude, and shear temperature on the damping characteristics. We observe striking commonalities in damping characteristics for all glasses studied—(i) a peak in the loss modulus in the high-frequency regime (∼THz) and (ii) persistent damping in the low-frequency regime (extending down to 10 s of MHz). The high-frequency peak is seen to overlap with the range of natural vibrational frequencies for each glass, and arises from coupling between the excited harmonic vibrational modes. On the other hand, persistent damping at intermediate and lo... read less

Abstract: We present two different methods which both enable large-scale first-principles device simulations including electron-phonon coupling (EPC). The methods are based on Density Functional Theory and Nonequilibrium Greens Functions (DFT- NEGF) calculations of electron transport. The inelastic current is in both methods calculated in a post-processing step to a self consistent DFT calculation. The first method is based on first order perturbation theory in the EPC self-energy within the Lowest Order Expansion (LOE) approximation. The method requires calculation of the first-principles EPC in the device region and it includes the effect of each phonon mode on the current perturbatively. This approach is made practical by calculating the EPC of the device region using a smaller periodic reference system. In addition, the phonon modes are assembled into a small number of energy intervals in which phonon modes are described collectively. The second method involves calculating the electron transmission for a single configuration where the atoms are displaced according to the phonon temperature of the system. Thus, this method has a computational cost equivalent to conventional elastic transport calculations. Both methods have been implemented in the Atomistix ToolKit (ATK) and we apply the methods for calculating the inelastic current in a silicon n-i-n junction and for calculation of phonon limited mobilities of silicon nanowires. read less

Abstract: Reactive potentials are becoming increasingly popular as they are expected to bridge the gap between ab initio and classical molecular dynamics. However, their applicability and potential benefits to model multicomponent glass networks are yet to be assessed. Here, an archetypal modified silicate glass, sodium silicate glass, is simulated using the ReaxFF potential. The predicted structure is critically evaluated and compared to that obtained by a classical potential and experimental data. Our results indicate that ReaxFF offers an improved description of the atomic structure, both at the short- and medium- range. Particularly, owing to its bond order form that dynamically adjusts potential energies according to the local atomic environment, ReaxFF reproduces the effect of modifiers on the Si–O network, a demonstration of its good transferability to various compositions. Overall, ReaxFF provides a promising alternative to classical and ab initio methods for simulating complex structures and processes for multi-component silicate glasses. read less

Abstract: Despite a high technical relevance and 35 years of observation, self-organized morphogenesis of nanoporous sponge-like amorphous structures during exposure of selected covalent materials to energetic ions is still insufficiently understood. Due to the presence and absence of these effects in amorphous Ge and Si, respectively, the Ge-Si alloy system constitutes an ideal testbed to track down the underlying physics at the atomic scale. This is realized within the present study by a combination of tailored experiments and extensive molecular dynamics computer modeling. The swelling capabilities of a variety of interaction potentials for the Ge-Si system and its elemental constituents are scrutinized with respect to the experimental observations and related to relevant physical properties of the model systems. This allows to identify defect kinetics in combination with a moderate radiation induced fluidity as key ingredients for nanopore morphogenesis. Cast in a simple quantitative model, it enables to account for both experimental as well as computational results, thus paving the way for a design by understanding approach in synthesis. read less

Abstract: Machine learning interatomic potentials (MLIPs) based on a large dataset obtained by density functional theory (DFT) calculation have been developed recently. This study gives both conceptual and practical bases for the high accuracy of MLIPs, although MLIPs have been considered to be simply an accurate black-box description of atomic energy. We also construct the most accurate MLIP of the elemental Ti ever reported using a linearized MLIP framework and many angular-dependent descriptors, which also corresponds to a generalization of the modified embedded atom method (MEAM) potential. read less

Abstract: The properties of nanometric-sized helium bubbles in silicon have been investigated using both spatially resolved electron-energy-loss spectroscopy combined with a recently developed method, and molecular-dynamics simulations. The experiments allowed for an accurate determination of size, aspect ratio, and helium density for a large number of single bubbles, whose diameters ranged from 6 to 20 nm. Very high helium densities, from 60 to 180 He nm(-3), have been measured depending on the conditions, in stark contrast with previous investigations of helium bubbles in metal with similar sizes. To supplement experiments on a smaller scale, and to obtain insights into the silicon matrix state, atomistic calculations have been performed for helium bubbles in the diameter range 1-13 nm. Molecular-dynamics simulations revealed that the maximum attainable helium density is critically related to the strength of the silicon matrix, which tends to yield by amorphization at the highest density levels. Calculations give helium density values for isolated single bubbles that are typically lower than measurements. However, excellent agreement is recovered when the interactions between bubbles and the presence of helium interstitials in the matrix are taken into account. Both experiments and numerical simulations suggest that the Laplace-Young law cannot be used to predict helium density in nanometric-sized bubbles in a covalent material such as silicon. read less

Abstract: In this paper, we mainly focused on analyzing the thermoelectric property i.e. figure of merit of different nanostructured materials in room temperature (300–310 K). Here we studied the transition-metal dichalcogenides, particularly Molybdenum Disulfide (MoS2); Metal Oxides, specifically Zinc Oxide (ZnO); and conventional semiconductor materials, i.e. n-type and p-type Silicon (Si) and Silicon Germanium (SiGe). At first, we calculated the electrical conductance (Ge), by using electronic density functional theory (DFT). Similarly, we calculated the thermal conductance (κ) using Tersoff empirical potential (TEP) model. With these calculated values of Ge and κ and the Seebeck coefficient (S), we calculated the figure of merit (ZT) at different room temperatures. The main findings of our research were the increased ZT of MoS2, which is slightly larger than p-type Si while, 2∼3 times larger than ZnO and 100∼103 times larger than conventionally used SiGe and n-type Si at room temperatures. We have further investigated a thermoelectric generator (TEG) device with these materials to validate our result. read less

Abstract: Statistical learning based on a local representation of atomic structures provides a universal model of chemical stability. Determining the stability of molecules and condensed phases is the cornerstone of atomistic modeling, underpinning our understanding of chemical and materials properties and transformations. We show that a machine-learning model, based on a local description of chemical environments and Bayesian statistical learning, provides a unified framework to predict atomic-scale properties. It captures the quantum mechanical effects governing the complex surface reconstructions of silicon, predicts the stability of different classes of molecules with chemical accuracy, and distinguishes active and inactive protein ligands with more than 99% reliability. The universality and the systematic nature of our framework provide new insight into the potential energy surface of materials and molecules. read less

Abstract: An optimized interatomic potential has been constructed for silicon using a modified Tersoff model. The potential reproduces a wide range of properties of Si and improves over existing potentials with respect to point defect structures and energies, surface energies and reconstructions, thermal expansion, melting temperature, and other properties. The proposed potential is compared with three other potentials from the literature. The potentials demonstrate reasonable agreement with first-principles binding energies of small Si clusters as well as single-layer and bilayer silicenes. The four potentials are used to evaluate the thermal stability of free-standing silicenes in the form of nanoribbons, nanoflakes, and nanotubes. While single-layer silicene is found to be mechanically stable at zero Kelvin, it is predicted to become unstable and collapse at room temperature. By contrast, the bilayer silicene demonstrates a larger bending rigidity and remains stable at and even above room temperature. The results suggest that bilayer silicene might exist in a free-standing form at ambient conditions. read less

Abstract: Using molecular dynamics phonon wave packet simulations, we study phonon transmission across hexagonal (h)-BN and amorphous silica (a-SiO2) nanoscopic thin films sandwiched by two crystalline leads. Due to the phonon interference effect, the frequency-dependent phonon transmission coefficient in the case of the crystalline film (Si|h-BN|Al heterostructure) exhibits a strongly oscillatory behavior. In the case of the amorphous film (Si|a-SiO2|Al and Si|a-SiO2|Si heterostructures), in spite of structural disorder, the phonon transmission coefficient also exhibits oscillatory behavior at low frequencies (up to ∼1.2 THz), with a period of oscillation consistent with the prediction from the two-beam interference equation. Above 1.2 THz, however, the phonon interference effect is greatly weakened by the diffuse scattering of higher-frequency phonons within an a-SiO2 thin film and at the two interfaces confining the a-SiO2 thin film. read less

Abstract: We performed a characterization of the energetic and structural features of amorphous Si using classical molecular dynamics simulations. We generated amorphous Si samples from different procedures: quenching liquid silicon, accumulating the damage generated by subsequent energetic recoils, and accumulating point defects. The obtained energetic and structural features of these types of samples are analyzed to elucidate which procedure provides a more realistic a-Si structure. read less

Abstract: The modeling of self-interstitial defects evolution is key for process and device optimization. For a self-interstitial cluster of a given size, several configurations or topologies exist, but conventional models assume that the minimum energy one is instantaneously reached. The existence of significant energy barriers for configurational transitions may change the picture of defect evolution in non-equilibrium processes (such as ion implantation), and contribute to explain anomalous defect observations. In this work, we present a method to determine the energy barriers for topological transitions among small self-interstitial defects, which is applied to characterize the Si self-interstitial and the di-interstitial cluster. read less

Abstract: The majority of the silicon devices contain amorphous phase and amorphous/crystalline interfaces which both considerably affect the transport of energy carriers as phonons and electrons. In this article, we investigate the impact of amorphous phases (both amorphous silicon and amorphous SiO2) of silicon nanoporous membranes on their thermal properties via molecular dynamics simulations. We show that a small fraction of amorphous phase reduces dramatically the thermal transport. One can even create nanostructured materials with subamorphous thermal conductivity, while keeping an important crystalline fraction. In general, the a-SiO2 shell around the pores reduces the thermal conductivity by a factor of five to ten compared to a-Si shell. The phonon density of states for several systems is also given to give the impact of the amorphisation on the phonon modes. read less

Abstract: Owing to its intrinsically lubricious property, graphene has a high potential to be an atomically thin solid lubricant for sliding interfaces. Despite its ultrahigh breaking strength at the nanoscale, graphene often fails to maintain its integrity when subjected to macroscale tribological tests. To reveal the true wear characteristics of graphene, a nanoscale diamond tip was used to scratch monolayer graphene mechanically exfoliated to SiO2 substrates. Our experimental results show that while graphene exhibited extraordinary wear resistance in the interior region, it could be easily damaged at the step edge under a much lower normal load (∼2 orders of magnitude smaller). Similar behavior with substantially reduced wear resistance at the edge was also observed for monatomic graphene layer on graphite surface. Using molecular dynamics simulations, we attributed this markedly weak wear resistance at the step edge to two primary mechanisms, i.e., atom-by-atom adhesive wear and peel induced rupture. Our findings shed light on the paradox that graphene is nanoscopically strong yet macroscopically weak. As step edge is ubiquitous for two-dimensional materials at the macroscale, our study also provides a guiding direction for maximizing the mechanical and tribological performance of these atomically thin materials. read less

Abstract: Molecular dynamic (MD) simulation method was applied to investigate the surface quality and brittle–ductile transition of monocrystalline silicon with a diamond tool during the elliptical vibration-assisted nanocutting (EVANC) and traditional nanocutting process. In the simulations, the interaction between silicon atoms in the specimen was modeled by the Tersoff potential, whereas the Morse potential was for the description of the interactions between silicon atoms in the specimen and carbon atoms in the diamond tool. In this study, we discovered that EVANC not only changed the brittle-mode cutting into the ductile-mode cutting, but also made the phase transformation layer thinner than that in the traditional nanocutting, which leads to a better surface finish and a large rate of removal of materials. Herein, stress analysis showed that the stress-affected region of the workpiece processed by EVANC was smaller than that of the workpiece processed by the traditional nanocutting. The temperature also increased during the EVANC process. This may soften the silicon material and make the cutting easier. In EVANC, the tangential force and normal force decreased because of the change in the brittle–ductile transition. From the simulation results, EVANC removed the material in the ductile mode, which could increase the removal rate, improve the surface finish, and decrease the cutting force to reduce the tool wear. In conclusion, EVANC has positive effects on the machinability and surface finish of the silicon material. read less

Abstract: We calculate the phonon-limited carrier mobility in (001) Si films with a fully atomistic framework based on a tight-binding (TB) model for the electronic structure, a valence-force-field model for the phonons, and the Boltzmann transport equation. This framework reproduces the electron and phonon bands over the whole first Brillouin zone and accounts for all possible carrier-phonon scattering processes. It can also handle one-dimensional (wires) and three-dimensional (bulk) structures and therefore provides a consistent description of the effects of dimensionality on the phonon-limited mobilities. We first discuss the dependence of the electron and hole mobilities on the film thickness and carrier density. The mobility tends to decrease with decreasing film thickness and increasing carrier density, as the structural and electric confinement enhances the electron-phonon interactions. We then compare hydrogen-passivated and oxidized films in order to understand the impact of surface passivation on the mobi... read less

Abstract: A charge-transfer interatomic potential, based on the hybrid-Tersoff potential that incorporates a covalent-ionic mixed-bond nature, was developed to reproduce the growth process of the thermal oxidation of silicon. A fitting process was employed with various reference structures sampled by MD. Actively exploring and learning the wide-range of phase space enabled us to develop a robust interatomic potential. Our interatomic potential reproduced the bulk properties of Si and SiO2 polymorphs well, in addition to the radial distribution function and bond angle distribution of amorphous SiO2. The covalent-ionic mixed-bond nature of the interatomic potential well reproduced the dissociation process of an oxygen molecule on the Si/SiO2 interface. The initial oxidation simulation was performed on the silicon surface. We grew the amorphous SiO2 layer by incorporating the oxygen molecules into the silicon network at the interface. The density of the SiO2 layer and the charge distribution at the interface showed go... read less

Abstract: In this work we propose a methodology to analyze the elastic energy interaction at the atomic level between Si self-interstitials and extended defects in crystalline Si. The representation of this energy in maps in 2D planes shows the anisotropic nature of the elastic interaction. This elastic energy maps can be used to understand diffusion trajectories of Si self-interstitials around extended defects obtained from classical molecular dynamics simulations. The combined analysis of these trajectories and the elastic energy maps shows preferential capture directions around extended defects. 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: Nonequilibrium molecular dynamics simulation has been a powerful tool for studying the thermophysical properties of bulk silicon and silicon nanowires. Nevertheless, usually limited by the capacity and capability of computational resources, the traditional longitudinal and transverse simulation sizes are evidently restricted in a narrow range much less than the experimental scales, which seriously hinders the exploration of the thermal properties. In this research, based on a powerful and efficient molecular dynamics (MD) simulation method, the computation of thermal conductivity beyond the known Casimir size limits is realized. The longitudinal dimensions of the simulations significantly exceed the micrometer scale. More importantly, the lateral characteristic sizes are much larger than 10 nanometers, explicitly comparable with the silicon nanowires fabricated and measured experimentally, whereas the traditional simulation size is several nanometers. The powerful virtual experimental measurement provided in our simulations achieves the direct prediction of the thermal conductivity of bulk silicon and real-scale silicon nanowires, and delineates the complete longitudinal size dependence of their thermal conductivities, especially at the elusive mesoscopic scale. Furthermore, the presented measurement paves an exciting and promising way to explore in depth the thermophysical properties of other bulk covalent solids and their low-dimensional structures, such as nanowires and nanosheets. read less

Abstract: The stability of kesterite Cu2ZnSnS4 (CZTS) under a range of compositions leads to the formation of a number of stable defects that appear to be necessary for high efficiency photovoltaic applications. In this work, the impact of the presence of these defects on the thermal conductivity of CZTS thin films has been explored. Thermal conductivities of CZTS thin films, prepared by pulsed laser deposition with differing compositions, were measured from 80 K to room temperature using the 3ω-method. The temperature dependence of the thermal conductivity indicates that the phonon mean free path is limited by strain field induced point defect scattering from sulfur vacancies in sulfur deficient thin films. The sulfurization of these films in a 10% N2 + H2S ambient at 500 °C increased the sulfur content of the films, reducing the concentration of sulfur vacancies, and produced a negligible change in grain size with an unexpected factor of 5 increase in phonon boundary scattering. This, along with anisotropies in the x-ray diffraction peak profiles of the sulfurized films, suggests that the phonon mean free path in sulfurized films is limited by the presence of cation exchange induced stacking faults. The resulting room temperature thermal conductivities for sulfurized and sulfur deficient thin films were found to be 4.0 W/m K and 0.9 W/m K, respectively. read less

Abstract: This report summarizes the progress on the interatomic potential development of triuranium-disilicide (U3Si2) for molecular dynamics (MD) simulations. The development is based on the Tersoff type potentials for single element U and Si. The Si potential is taken from the literature and a Tersoff type U potential is developed in this project. With the primary focus on the U3Si2 phase, some other U-Si systems such as U3Si are also included as a test of the transferability of the potentials for binary U-Si phases. Based on the potentials for unary U and Si, two sets of parameters for the binary U-Si system are developed using the Tersoff mixing rules and the cross-term fitting, respectively. The cross-term potential is found to give better results on the enthalpy of formation, lattice constants and elastic constants than those produced by the Tersoff mixing potential, with the reference data taken from either experiments or density functional theory (DFT) calculations. In particular, the results on the formation enthalpy and lattice constants for the U3Si2 phase and lattice constants for the high temperature U3Si (h-U3Si) phase generated by the cross-term potential agree well with experimental data. Reasonable agreements are also reached on the elastic constants of U3Si2,more » on the formation enthalpy for the low temperature U3Si (m-U3Si) and h-U3Si phases, and on the lattice constants of m-U3Si phase. All these phases are predicted to be mechanically stable. The unary U potential is tested for three metallic U phases (α, β, γ). The potential is found capable to predict the cohesive energies well against experimental data for all three phases. It matches reasonably with previous experiments on the lattice constants and elastic constants of αU.« less read less

Abstract: Semiconductor heterostructures are well characterized experimentally and provide a solid basis for electronic and optoelectronic devices ranging from single interface to complex superlattice structures. Yet, structural and electronic models commonly describe the material properties in a continuum approach, which neglects the crystalline structure, as well as potential local variations of the composition and resulting strain. Empirical interaction potentials provide an efficient way to model chemical bonds and therefore allow a structural description of multi‐layer structures. This work provides a detailed introduction on methods to minimize the total energy of semiconductor heterostructures at an atomistic level. We present an algorithm to minimize the total energy and generate optimized interface configurations. The relaxed structures are then evaluated with respect to interfacial strain, where different strain calculation methods are evaluated and compared with experimental data. read less

Abstract: The mechanical response of symmetric tilt grain boundaries (GBs) in silicon bicrystals under shear loading are characterized using molecular dynamics simulations. It is seen that under shear, high-angle GBs namely Σ5 and Σ13 having a rotation axis [0 0 1] demonstrate coupled GB motion, such that the displacement of grains parallel to the GB interface is accompanied by normal GB motion. An atomic-scale characterization revealed that concerted rotations of silicon tetrahedra within the GB are the primary mechanisms leading to the coupled GB motion. Interestingly, so far, this phenomenon has only been examined in detail for metallic systems. A distinguishing feature of the coupled GB motion observed for the silicon symmetric tilt bicrystals as compared to metallic bicrystals is the fact that in the absence of shear, spontaneous coupled motion is not observed at high temperatures. read less

Abstract: Bond-order potentials (BOPs) have been used successfully in simulations of a wide range of processes. A brief overview of bond-order potentials is provided which focuses on the reactive empirical bond-order (REBO) potential for hydrocarbons (Brenner et al 2002 J. Phys.: Condens. Matter 14 783) and the large number of useful potentials it has spawned. Two specific extensions of the REBO potential that make use of its formalism are discussed. First, the 2B-SiCH potential (Schall and Harrison 2013 J. Phys. Chem. C 117 1323) makes the appropriate changes to the hydrocarbon REBO potential so that three atom types, Si, C, and H, can be modeled. Second, we recently added the electronegative element O, along with the associated charge terms, to the adaptive intermolecular REBO (AIREBO) potential (Stuart et al 2000 J. Chem. Phys. 112 6472). The resulting qAIREBO potential (Knippenberg et al 2012 J. Chem. Phys. 136 164701) makes use of the bond-order potential/split-charge (BOP/SQE) equilibration method (Mikulski et al 2009 J. Chem. Phys. 131 241105) and the Lagrangian approach to charge dynamics (Rick et al 1994 J. Chem. Phys. 101 6141). The integration of these two techniques allows for atomic charges to evolve with time during MD simulations: as a result, chemical reactions can be modeled in C-, O-, and H-containing systems. The usefulness of the 2B-SiCH potential for tribological investigations is demonstrated in molecular dynamics (MD) simulations of axisymmetric tips composed of Si and SiC placed in sliding contact with diamond(1 1 1) surfaces with varying amounts of hydrogen termination. The qAIREBO potential is used to investigate confinement of sub-monolayer coverages of water between nanostructured surfaces. read less

Abstract: The nature of disorder in amorphous silicon (a-Si) is explored by investigating the spatial arrangement and energies of coordination defects in a numerical model. Spatial correlations between structural defects are examined on the basis of a parameter that quantifies the probability for two sites to share a bond. Pentacoordinated atoms are found to be the dominant coordination defects. They show a tendency to cluster, and about 17% of them are linked through three-membered rings. As for tricoordinated sites, they are less numerous, and tend to be distant by at least two bond lengths. Typical local geometries associated to under and overcoordinated atoms are extracted from the model and described using partial bond angle distributions. An estimate of the formation energies of structural defects is provided. Using molecular-dynamics calculations, we simulate the implantation of high-energy atoms in the initial structure in order to study the effect of relaxation on the coordination defects and their environments. read less

Abstract: By using molecular dynamics technique, we have computed the effective cross-plane thermal conductivity of a single cascade of a quantum cascade laser diode. Additionally, the local phonon density of states was also computed. The Tersoff potential was used with coefficients found from the literature for inter-atomic forces, and the Green-Kubo relation was used to compute the conductivity from the integral of the system heat flux autocorrelation. The computed conductivity lies in the same range as measurements found in the literature, but the local density of states deviated from expected values based on local diode composition, suggesting that previous assumptions made about electron-phonon coupling are incorrect. read less

Abstract: Progress has recently been made in developing reactive force fields to describe chemical reactions in systems too large for quantum mechanical (QM) methods. In particular, ReaxFF, a force field with parameters that are obtained solely from fitting QM reaction data, has been used to predict structures and properties of many materials. Important applications require, however, determination of the final structures produced by such complex processes as chemical vapor deposition, atomic layer deposition, and formation of ceramic films by pyrolysis of polymers. This requires the force field to properly describe the formation of other products of the process, in addition to yielding the final structure of the material. We describe a strategy for accomplishing this and present an example of its use for forming amorphous SiC films that have a wide variety of applications. Extensive reactive molecular dynamics (MD) simulations have been carried out to simulate the pyrolysis of hydridopolycarbosilane. The reaction products all agree with the experimental data. After removing the reaction products, the system is cooled down to room temperature at which it produces amorphous SiC film, for which the computed radial distribution function, x-ray diffraction pattern, and the equation of state describing the three main SiC polytypes agree with the data and with the QM calculations. Extensive MD simulations have also been carried out to compute other structural properties, as well the effective diffusivities of light gases in the amorphous SiC film. read less

Abstract: We study computationally the formation of thermodynamics and morphology of silicon self-interstitial clusters using a suite of methods driven by a recent parameterization of the Tersoff empirical potential. Formation free energies and cluster capture zones are computed across a wide range of cluster sizes (2 < Ni < 150) and temperatures (0.65 < T/Tm < 1). Self-interstitial clusters above a critical size (Ni ∼ 25) are found to exhibit complex morphological behavior in which clusters can assume either a variety of disordered, three-dimensional configurations, or one of two macroscopically distinct planar configurations. The latter correspond to the well-known Frank and perfect dislocation loops observed experimentally in ion-implanted silicon. The relative importance of the different cluster morphologies is a function of cluster size and temperature and is dictated by a balance between energetic and entropic forces. The competition between these thermodynamic forces produces a sharp transition between the t... read less

Abstract: Tetrahedral substances, such as silicon, water, germanium, and silica, share various unusual phase behaviors. Among them, the so-called polyamorphism, i.e., the existence of more than one amorphous form, has been intensively investigated in the last three decades. In this work, we study the metastable relations between amorphous states of silicon in a wide range of pressures, using Monte Carlo simulations. Our results indicate that the two amorphous forms of silicon at high pressures, the high density amorphous (HDA) and the very high density amorphous (VHDA), can be decompressed from high pressure (∼20 GPa) down to the tensile regime, where both convert into the same low density amorphous. Such behavior is also observed in ice. While at high pressure (∼20 GPa), HDA is less stable than VHDA, at the pressure of 10 GPa both forms exhibit similar stability. On the other hand, at much lower pressure (∼5 GPa), HDA and VHDA are no longer the most stable forms, and, upon isobaric annealing, an even less dense form of amorphous silicon emerges, the expanded high density amorphous, again in close similarity to what occurs in ice. read less

Abstract: This review article presents a discussion of theoretical progress made over the past several decades towards our understanding of thermoelectric properties of materials. Particular emphasis is placed upon describing recent progress in ‘tuning’ phonon properties of nanocomposite materials for gaining enhancement of the thermoelectric figure of merit. read less

Abstract: It is known that M23C6(M = Cr/Fe) behavior in heat-resistant ferritic steels affects the strength of the material at high temperature. The ability to garner direct information regarding the atomic motion using classical molecular dynamics simulations is useful for investigating the M23C6 behavior in heat-resistant ferritic steels. For such classical molecular dynamics calculations, a suitable interatomic potential is needed. To satisfy this requirement, an empirical bond-order-type interatomic potential for Fe-Cr-C systems was developed because the three main elements to simulate the M23C6 behavior in heat-resistant ferritic steels are Fe, Cr, and C. The angular-dependent term, which applies only in non-metallic systems, was determined based on the similarity between a Finnis-Sinclair-type embedded-atom-method interatomic potential and a Tersoff-type bond-order potential. The potential parameters were determined such that the material properties of Fe-Cr-C systems were reproduced. These properties include... read less

Abstract: We present an atomistic-continuum model to simulate ultrashort laser-induced melting processes in semiconductor solids on the example of silicon. The kinetics of transient non-equilibrium phase transition mechanisms is addressed with a Molecular Dynamics method at atomic level, whereas the laser light absorption, strong generated electron-phonon non-equilibrium, fast diffusion and heat conduction due to photo-excited free carriers are accounted for in the continuum. We give a detailed description of the model, which is then applied to study the mechanism of short laser pulse melting of free standing Si films. The effect of laser-induced pressure and temperature of the lattice on the melting kinetics is investigated. Two competing melting mechanisms, heterogeneous and homogeneous, were identified. Apart of classical heterogeneous melting mechanism, the nucleation of the liquid phase homogeneously inside the material significantly contributes to the melting process. The simulations showed, that due to the open diamond structure of the crystal, the laser-generated internal compressive stresses reduce the crystal stability against the homogeneous melting. Consequently, the latter can take a massive character within several picoseconds upon the laser heating. Due to negative volume of melting of modeled Si material, -7.5%, the material contracts upon the phase transition, relaxes the compressive stresses and the subsequent melting proceeds heterogeneously until the excess of thermal energy is consumed. The threshold fluence value, at which homogeneous nucleation of liquid starts contributing to the classical heterogeneous propagation of the solid-liquid interface, is found from the series of simulations at different laser input fluences. On the example of Si, the laser melting kinetics of semiconductors was found to be noticeably different from that of metals with fcc crystal structure. read less

Abstract: Molecular dynamics simulations are performed to clarify the extreme strain rate and temperature dependence of the mechanical behaviors of nano silicon nitride thin layers in a basal plane under tension. It is found that fracture stresses show almost no change with increasing strain rate. However, fracture strains decrease gradually due to the appearance of additional N(2c)-Si bond breaking defects in the deformation process. With increasing loading temperature, there is a noticeable drop in fracture stress and fracture strain. In the low temperature range, roughness phases can be observed owing to a combination of factors such as configuration evolution and energy change. read less

Abstract: A systematic method to calculate anharmonic force constants of crystals is presented. The method employs the direct-method approach, where anharmonic force constants are extracted from the trajectory of first-principles molecular dynamics simulations at high temperature. The method is applied to Si where accurate cubic and quartic force constants are obtained. We observe that higher-order correction is crucial to obtain accurate force constants from the trajectory with large atomic displacements. The calculated harmonic and anharmonic force constants are, then, combined with the Boltzmann transport equation (BTE) and non-equilibrium molecular dynamics (NEMD) methods in calculating the thermal conductivity. The BTE approach successfully predicts the lattice thermal conductivity of bulk Si, whereas NEMD shows considerable underestimates. To evaluate the linear extrapolation method employed in NEMD to estimate bulk values, we analyze the size dependence in NEMD based on BTE calculations. We observe strong nonlinearity in the size dependence of NEMD in Si, which can be ascribed to acoustic phonons having long mean-free-paths and carrying considerable heat. Subsequently, we also apply the whole method to a thermoelectric material Mg2Si and demonstrate the reliability of the NEMD method for systems with low thermal conductivities. read less

Abstract: We introduce a new ab initio derived reactive potential for the simulation of CdTe within density functional theory (DFT) and apply it to calculate both static and dynamical properties of a number of systems (bulk solid, defective structures, liquid, surfaces) at finite temperature. In particular, we also consider cases with low sulfur concentration (CdTe:S). The analysis of DFT and classical molecular dynamics (MD) simulations performed with the same protocol leads to stringent performance tests and to a detailed comparison of the two schemes. Metadynamics techniques are used to empower both Car-Parrinello and classical molecular dynamics for the simulation of activated processes. For the latter, we consider surface reconstruction and sulfur diffusion in the bulk. The same procedures are applied using previously proposed force fields for CdTe and CdTeS materials, thus allowing for a detailed comparison of the various schemes. read less

Abstract: A nanoindentation simulation using molecular dynamic (MD) method was carried out to investigate the hardness behavior of monocrystalline silicon with a spherical diamond indenter. In this study, Tersoff potential was used to model the interaction of silicon atoms in the specimen, and Morse potential was used to model the interaction between silicon atoms in the specimen and carbon atoms in the indenter. Simulation results indicate that the silicon in the indentation zone undergoes phase transformation from diamond cubic structure to body-centred tetragonal and amorphous structure upon loading of the diamond indenter. After the unloading of the indenter, the crystal lattice reconstructs, and the indented surface with a residual dimple forms due to unrecoverable plastic deformation. Comparison of the hardness of three different crystal surfaces of monocrystalline silicon shows that the (0 0 1) surface behaves the hardest, and the (1 1 1) surface behaves the softest. As for the influence of the indentation temperature, simulation results show that the silicon material softens and adhesiveness of silicon increases at higher indentation temperatures. read less

Abstract: Molecular dynamics simulations of low-energy (5–100 eV) Cl+ and Cl2+ bombardment on (100) Si surfaces are performed to investigate the impact of plasma dissociation and very low-energy ions (5–10 eV) in chlorine pulsed plasmas used for silicon etch applications. Ion bombardment leads to an initial rapid chlorination of the Si surface followed by the formation of a stable SiClx mixed layer and a constant etch yield at steady state. The SiClx layer thickness increases with ion energy (from 0.7 ± 0.2 nm at 5 eV to 4 ± 0.5 nm at 100 eV) but decreases for Cl2+ bombardment (compared to Cl+), due to the fragmentation of Cl2+ molecular ions into atomic Cl species with reduced energies [one X eV Cl +  two 2X eV Cl2+]. The Si etch yield is larger for Cl2+ than Cl+ bombardment at high-energy (Ei > 25 eV) but larger for Cl+ than Cl2+ bombardment at low-energy (Ei  two 2X eV Cl2+]. The Si etch yield is larger for Cl2+ than Cl+ bombardment at high-energy (Ei > 25 eV) but larger for Cl+ than Cl2+ bombardment at low-energy (Ei < 25 eV) due to threshold effects. And the higher the ion energy, the less saturated the etch products. Results suggest that weakly dissociated chlorine ... read less

Abstract: We determined the Stillinger-Weber interatomic potential parameters for Si/N/H system based on first principles density functional calculations. This new potential can be used to perform classical molecular dynamics simulation for silicon nitride deposition on Si substrate. During the first principles calculations, cluster models have been carefully and systematically chosen to make sampling of the interatomic potential supersurface more thoroughly. Global optimization method was used to fit the ab initio data into Stillinger-Weber form. We used a recursive method to perform the classical molecular dynamics simulations for silicon nitride (SiN) film growth on Si substrate with SiH4/NH3 gas mixtures. During the simulation, we could clearly observe the silicon nitride film growth progress. In this paper, we present the details of potential derivation and simulation results with different SiH4:NH3 ratios. It is demonstrated that this new potential is suitable to describe the surface reactions of the Si/N/H s... read less

Abstract: In this paper we present a systematic and well controlled procedure for building atomistic amorphous/crystalline interfaces in silicon, dedicated to the molecular dynamics simulations of superlattices and core/shell nanowires. The obtained structures depend on the technique used to generate the amorphous phase and their overall quality is estimated through comparisons with structural information and interfacial energies available from experimental and theoretical results. While most of the related studies focus on a single planar interface, we consider here both the generation of multiple superlattice planar interfaces and core/shell nanowire structures. The proposed method provides periodic homogeneous and reproducible, atomically sharp and defect free interface configurations at low temperature and pressure. We also illustrate how the method may be used to predict the thermal transport properties of composite crystalline/amorphous superlattices. read less

Abstract: Building on previous charge-optimized many-body (COMB) potentials for metallic α-U and gaseous O2, we have developed a new potential for UO2, which also allows the simulation of U–UO2–O2 systems. The UO2 lattice parameter, elastic constants and formation energies of stoichiometric and non-stoichiometric intrinsic defects are well reproduced. Moreover, this is the first rigid-ion potential that produces the correct deviation of the Cauchy relation, as well as the first classical interatomic potential that is able to determine the defect energies of non-stoichiometric intrinsic point defects in UO2 with an appropriate reference state. The oxygen molecule interstitial in the α-U structure is shown to decompose, with some U–O bonds approaching the natural bond length of perfect UO2. Finally, we demonstrate the capability of this COMB potential to simulate a complex system by performing a simulation of the α-U + O2 → UO2 phase transformation. We also identify a possible mechanism for uranium oxidation and the orientation of the resulting fluorite UO2 structure relative to the coordinate system of orthorhombic α-U. 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: We grew graphene epitaxially on 6H-SiC(0001) substrate by the simulated annealing method. The mechanisms that govern the growth process were investigated by testing two empirical potentials, namely, the widely used Tersoff potential [J. Tersoff, Phys. Rev. B 39, 5566 (1989)] and its more refined version published years later by Erhart and Albe [Phys. Rev. B 71, 035211 (2005)]. Upon contrasting the results obtained by these two potentials, we found that the potential proposed by Erhart and Albe is generally more physical and realistic, since the annealing temperature at which the graphene structure just coming into view at approximately 1200 K is unambiguously predicted and close to the experimentally observed pit formation at 1298 K within which the graphene nucleates. We evaluated the reasonableness of our layers of graphene by calculating carbon-carbon (i) average bond-length, (ii) binding energy, and (iii) pair correlation function. Also, we compared with related experiments the various distance of separation parameters between the overlaid layers of graphene and substrate surface. read less

Abstract: Stainless steels found in real-world applications usually have some C content in the base Fe–Cr alloy, resulting in hard and dislocation-pinning carbides—Fe3C (cementite) and Cr23C6—being present in the finished steel product. The higher complexity of the steel microstructure has implications, for example, for the elastic properties and the evolution of defects such as Frenkel pairs and dislocations. This makes it necessary to re-evaluate the effects of basic radiation phenomena and not simply to rely on results obtained from purely metallic Fe–Cr alloys. In this report, an analytical interatomic potential parameterization in the Abell–Brenner–Tersoff form for the entire Fe–Cr–C system is presented to enable such calculations. The potential reproduces, for example, the lattice parameter(s), formation energies and elastic properties of the principal Fe and Cr carbides (Fe3C, Fe5C2, Fe7C3, Cr3C2, Cr7C3, Cr23C6), the Fe–Cr mixing energy curve, formation energies of simple C point defects in Fe and Cr, and the martensite lattice anisotropy, with fair to excellent agreement with empirical results. Tests of the predictive power of the potential show, for example, that Fe–Cr nanowires and bulk samples become elastically stiffer with increasing Cr and C concentrations. High-concentration nanowires also fracture at shorter relative elongations than wires made of pure Fe. Also, tests with Fe3C inclusions show that these act as obstacles for edge dislocations moving through otherwise pure Fe. read less

Abstract: Although multicrystalline silicon (mc-Si) is currently the most widely used material for fabricating photovoltaic cells, its electrical properties remain limited by several types of defects, which interact in complex ways that are not yet fully understood. A particularly important phenomenon is the interaction between grain boundaries and intrinsic point defects or impurity atoms, such as carbon, oxygen, nitrogen, and various types of metals. Here, we use empirical molecular dynamics to study the interactions of Σ3{111}, Σ9{221}, and Σ27{552} twin boundaries, which account for over 50% of all grain boundaries in mc-Si, with self-interstitials, vacancies, and substitutional carbon atoms. It is shown that twin boundary-point defect interaction energies increase with twinning order and that they are predominantly attractive. We also find that twin boundary interactions with substitutional carbon are highly spatially heterogeneous, exhibiting alternating repulsive-attractive regions that correlate strongly wi... 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: A review of radiation-induced displacement damage effects in semiconductor devices is presented, with emphasis placed on silicon technology. The history of displacement damage studies is summarized, and damage production mechanisms are discussed. Properties of defect clusters and isolated defects are addressed. Displacement damage effects in materials and devices are considered, including effects produced in silicon particle detectors, visible imaging arrays, and solar cells. Additional topics examined include NIEL scaling, carrier concentration changes, random telegraph signals, radiation hardness assurance, and simulation methods for displacement damage. Areas needing further study are noted. read less

Abstract: We have developed a reactive force field within the ReaxFF framework to accurately describe reactions involving aluminum–molybdenum alloy, which are part parameters of Al–O–Mo ternary system metastable intermolecular composites. The parameters are optimized from a training set, whose data come from density functional theory (DFT) calculations and experimental value, such as heat of formation, geometry data, and equation of states, which are reproduced well by ReaxFF. Body-centered cubic molybdenum’s surface energy, vacancy formation, and two transformational paths, Bain and trigonal paths are calculated to validate the ReaxFF ability describing the defects and deformations. Some structures’ elastic constant and phonon are calculated by DFT and ReaxFF to predict the structures’ mechanics and kinetic stability. All those results indicate that the fitted parameters can describe the energy difference of various structures under various circumstances and generally represent the diffusion property but cannot reproduce the elasticity and phonon spectra so well. read less