Tersoff Potential Research Articles

Thermal rectification in nozzle-like graphene/boron nitride nanoribbons: A molecular dynamics simulation

Innovations in manufacture of graphene-based nano-devices are principally the outcome of engineering of the nanostructure, while advanced nano-transistors, thermal logic nano-circuits, and thermal nano-diodes are multi-component tailor-made nanomaterials with precise molecular layout. To achieve such delicate graphene-based nanostructures, it is essential to optimize both materials and geometrical parameters. Herein we introduce nozzle-like graphene (G)/boron nitride (BN) nanostructures with very high unidirectional thermal rectification efficiency applying classical molecular dynamics (MD) simulations using Tersoff potential. A series of nozzle-like G/BN and BN/G nanoribbons with variable throat width, L (5–50 Å) and convergence angle, ϴ (20-90°) under average T = 300 K andΔT = 40 K (temperature differences between the thermal baths) situation were simulated to provide a complete image of thermal rectification for such nozzle-like nanostructures to be intended as thermal nano-diodes. Thermal conductivity and rectification analyses unveiled nozzle-like G/BN devices with a comparatively excellent unidirectional thermal rectification of ∼ 25%, at L = 40 Å and ϴ = 60°, which was conceptualized in view of phonon scattering perspective. We believe that the outcome of this survey would open new avenues in manufacturing tailor-made graphene-based nano-diodes.

Evolution of Microstructure during Rapid Solidification of SiC under High Pressure

The microstructure evolution of liquid silicon carbide (SiC) during rapid solidification under different pressure values is simulated with the Tersoff potential using molecular dynamics. The structure evolution characteristics of SiC are analyzed by considering the pair distribution function, bond angle distribution, coordination number, and the diagrams of the microstructure during rapid solidification. The results show that the average energy of atoms gradually increases with pressure. When the pressure reaches 100 GPa, the average energy of the atom is greater than the average energy of the atom in the initial liquid state. Under different pressures, the diffusion of atoms tends to remain stable at a temperature of about 3700 K. The application of pressure has a major impact on the arrangement of atoms, except on the third-nearest neighbor, while the impact on the nearest neighbor and the second-nearest neighbor is relatively small. The pressure increases the medium-range order of the system. The coordination numbers of Si and C atoms gradually decrease with the decrease in temperature and increase in pressure. Pressure changes the microstructure of the SiC amorphous system after solidification, and the density can be increased by adjusting the coordination number of atoms. As the pressure increases, the SiC amorphous system exhibits a dense structure with coordination numbers of 4, 5, 6, and 7.

Defect structure classification of neutron-irradiated graphite using supervised machine learning

Molecular dynamics simulations were performed to predict the behavior of graphite atoms under neutron irradiation using large-scale atomic/molecular massively parallel simulator (LAMMPS) package with adaptive intermolecular reactive empirical bond order (AIREBOM) potential. Defect structures of graphite were compared with results from previous studies by means of density functional theory (DFT) calculations. The quantitative relation between primary knock-on atom (PKA) energy and irradiation damage on graphite was calculated.and the effect of PKA direction on the amount of defects is estimated by counting displaced atoms. Defects are classified into four groups: structural defects, energy defects, vacancies, and near-defect structures, where a structural defect is further subdivided into six types by decision tree method which is one of the supervised machine learning techniques.

Multi-reward Reinforcement Learning Based Bond-Order Potential to Study Strain-Assisted Phase Transitions in Phosphorene.

We introduce a multi-reward reinforcement learning (RL) approach to train a flexible bond-order potential (BOP) for 2D phosphorene based on ab initio training data sets. Our approach is based on a continuous action space Monte Carlo tree search algorithm that is general and scalable and presents an efficient multiobjective optimization scheme for high-dimensional materials design problems. As a proof-of-concept, we deploy this scheme to parametrize multiple structural and dynamical properties of 2D phosphorene polymorphs. Our RL-trained BOP model adequately captures the structure, energetics, transformation barriers, equation of state, elastic constants, and phonon dispersions of various 2D P polymorphs. We use this model to probe the impact of temperature and strain rate on the phase transition from black (α-P) to blue phosphorene (β-P) through molecular dynamics simulations. A decrease in critical strain for this phase transition with increase in temperature is observed, and the underlying atomistic mechanisms are discussed.

OpenACC + Athread collaborative optimization of Silicon-Crystal application on Sunway TaihuLight

The Silicon-Crystal application based on molecular dynamics (MD) is used to simulate the thermal conductivity of the crystal, which adopts the Tersoff potential to simulate the trajectory of the silicon crystal. Based on the OpenACC version, to better solve the problem of discrete memory access and write dependency, task pipeline optimization and the interval graph coloring scheduling method are proposed. Also, the part of codes on CPEs is vectorized by the SIMD command to further improve the computational performance. After the collaborative development of OpenACC+Athread, the performance has been improved by 16.68 times and achieves 2.34X speedup compared with the OpenACC version. Moreover, the application is expanded to 66,560 cores and can simulate reactions of 268,435,456 silicon atoms.

Development of a physically-informed neural network interatomic potential for tantalum

Large-scale atomistic simulations of materials heavily rely on interatomic potentials, which predict the system energy and atomic forces. One of the recent developments in the field is constructing interatomic potentials by machine-learning (ML) methods. ML potentials predict the energy and forces by numerical interpolation using a large reference database generated by quantum-mechanical calculations. While high accuracy of interpolation can be achieved, extrapolation to unknown atomic environments is unpredictable. The recently proposed physically-informed neural network (PINN) model improves the transferability by combining a neural network regression with a physics-based bond-order interatomic potential. Here, we demonstrate that general-purpose PINN potentials can be developed for body-centered cubic (BCC) metals. The proposed PINN potential for tantalum reproduces the reference energies within 2.8 meV/atom. It accurately predicts a broad spectrum of physical properties of Ta, including (but not limited to) lattice dynamics, thermal expansion, energies of point and extended defects, the dislocation core structure and the Peierls barrier, the melting temperature, the structure of liquid Ta, and the liquid surface tension. The potential enables large-scale simulations of physical and mechanical behavior of Ta with nearly first-principles accuracy while being orders of magnitude faster. This approach can be readily extended to other BCC metals.

Basal dislocations in MAX phases: Core structure and mobility

The MAX phases, layered ternary carbides and nitrides, plastically deform via exclusively basal slip systems. This constraint leads to their well-documented plastic anisotropy and deformation mechanisms such as kinking. Basal dislocations govern the incipient plastic deformation, and therefore understanding them is a key step to utilizing the unique properties of MAX phases. As such, we performed the first fully atomistic calculations of the core structures and mobilities of basal dislocations using a bond-order interatomic potential. Calculations show that the weakly bonded A-layer can have a large impact on core structure, with several configurations exhibiting non-planar cores and being an order of magnitude less mobile than their planar counterparts. Additionally, due to the unique stacking sequence and stacking faults in MAX phases, basal dislocations preferentially nucleate and glide as same-signed pairs on adjacent slip planes. These pairs dissociate as zonal dislocations over much larger distances, enabling greater core spreading and therefore increased mobility. Edge pairs have Peierls stresses more closely resembling those in metals than in ceramics, suggesting they control the incipient plasticity of MAX phases at lower temperatures. Overall, these results provide a foundation for better understanding MAX phase incipient plasticity through basal dislocations and thus enabling them to be better engineered for specific applications.

Fracture toughness of single layer boronitrene sheet using MD simulations

In linear elastic fracture mechanics, the critical stress intensity factor (CSIF) is related to the surface energy calculated from the bond energy in the reference configuration. For nanomaterials, the difference between the thus computed CSIF and that found from other methods is attributed to lattice trapping. We show here that the energy release rate (and hence the CSIF) determined from the energy of bonds on the crack surface in the current configuration agrees with that estimated by the traditional methods employing the fracture stress and the initial crack length. We demonstrate this by using molecular dynamics simulations with the Tersoff potential by studying crack propagation in a pre-cracked monolayer boronitrene.

Interatomic Fe–Cr potential for modeling kinetics on Fe surfaces

To enable accurate molecular dynamics simulations of iron–chromium alloys with surfaces, we develop, based on density-functional-theory (DFT) calculations, a new interatomic Fe–Cr potential in the Tersoff formalism. Contrary to previous potential models, which have been designed for bulk Fe–Cr, we extend our potential fitting database to include not only conventional bulk properties but also surface-segregation energies of Cr in bcc Fe. In terms of reproducing our DFT results for the bulk properties, the new potential is found to be superior to the previously developed Tersoff potential and competitive with the concentration-dependent and two-band embedded-atom-method potentials. For Cr segregation toward the (100) surface of an Fe–Cr alloy, only the new potential agrees with our DFT calculations in predicting preferential segregation of Cr to the topmost surface layer, instead of the second layer preferred by the other potentials. We expect this rectification to foster future research, e.g., on the mechanisms of corrosion resistance of stainless steels at the atomic level.

Configurational Forces in Bond Order Potentials

AbstractIn this contribution, a configurational mechanics framework is elaborated to assess the applicability of atomistic configurational forces in fracture of crystalline lattices. To this end, an analytical interatomic potential is reformulated in terms of the material positions occupied by the atoms participating in two‐ and three‐body interactions. It is demonstrated that such a potential satisfies the requirements of invariances i.e., translational, rotational and parity. The focus of this work is developing the configurational setting for the bond order Tersoff potential. Two‐dimensional pre‐cracked mono‐layer graphene modelled with the Tersoff potential is chosen to study the configurational force approach in determining energy release during crack propagation into the lattice.

Probing the displacement damage mechanism in Si, Ge, GaAs by defects evolution analysis

With the rapid development of space technology, the requirements for the radiation resistance of solar cells are higher and higher.A clear understanding of the radiation damage mechanism of materials is very important for radiation hardening of space solar cells. Therefore, molecular dynamics (MD) method was utilized to simulate the irradiated defects evolution in mainly used materials (Si, Ge, and GaAs) for multi-junction solar cells. By simulating the cascade collision process took placed in different materials under different interatomic potential, it is determined that Tersoff potential was more suitable for radiation damage simulation of semiconductor materials.By comparing the number of Frenkel pair defects (FPs) produced in the three materials during irradiation, the radiation resistance of each material was obtained. The MD results were verified by comparing with empirical NRT model.The displacement evolution of off-site atoms and the final clusters formed by point defects, i.e. interstitials and vacancies were further quantitatively analyzed. The atomic image of irradiated defects provides new physical insights for displacement damage mechanism and gives better understanding for explaining the degradation of irradiated multi-junction solar cells.

Thermal expansion of continuous random networks of carbon

Homogeneous and isotropic continuous random networks (CRNs) of carbon were created spanning a large portion of hybridization space, from sp3-rich configurations up to 70% sp2 carbon. For each of these CRNs, the volumetric coefficient of thermal expansion (CTE) was calculated according to the quasi-harmonic approximation using the Brenner potential. The CTE of H-free, sp3-rich CRNs is similar to that of diamond and almost constant up to about 50% sp2-C. The CTE at 300 K varies from about 3.5 ×10−6K−1 to 5.5×10−6K−1, linearly increasing with the sp2-C content for CRNs above ca. 40% sp2-C, in good agreement with previous experimental studies. A small fraction of 10% to 15% of the CRNs, irrespective of composition, exhibit volumetric negative thermal expansion (NTE) over a limited temperature range around 100 K to 200 K.

Structural evolution of SiC sheet in a graphene-based in-plane hybrid system upon heating using molecular dynamics simulation

In-plane hybrid system containing graphene and silicon carbide (SiC) sheets (graphene-SiC-graphene) is studied via molecular dynamics simulation. The size of graphene sheets is 10,000 atoms/layer while the one of SiC sheet is 5700 atoms. The structural evolution of the SiC sheet in the graphene-SiC-graphene system is studied upon heating from 50 K to 3400 K via the Tersoff potentials. Some main results of the SiC sheet in the hybrid system are found, such as, the phase transition exhibits Devil's staircase type related to the SiC sheet from the fully in-plane graphene-SiC-graphene hybrid system, the phase transition initiates at temperature of 1750 K, the melting criterion is calculated (γ3=0.033) to classify the solid-like and the liquid-like atoms, the liquid-like atoms have tendency to form clusters. The formation of the clusters of the free-standing SiC and the SiC nanoribbon is also calculated to have an entire view about the influence of interactions at the boundaries on the formation of the clusters of SiC sheet in the hybrid system. The atomic mechanism of the phase transition from crystalline to liquid states is studied based on the radial distribution functions, the coordination number, and the angular distributions.

Atomistic modelling of the immiscible Fe–Bi system from a constructed bond order potential

An analytical bond-order potential (BOP) of Fe–Bi has been constructed and has been validated to have a better performance than the Fe–Bi potentials already published in the literature. Molecular dynamics simulations based on this BOP has been then conducted to investigate the ground-state properties of Bi, structural stability of the Fe–Bi binary system, and the effect of Bi on mechanical properties of BCC Fe. It is found that the present BOP could accurately predict the ground-state A7 structure of Bi and its structural parameters, and that a uniform amorphous structure of Fe100−x Bi x could be formed when Bi is located in the composition range of 26 ⩽ x < 70. In addition, simulations also reveal that the addition of a very small percentage of Bi would cause a considerable decrease of tensile strength and critical strain of BCC Fe upon uniaxial tensile loading. The obtained results are in nice agreement with similar experimental observations in the literature.

Phonon thermal transport in diamond and lonsdaleite: A comparative study of empirical potentials

The stacking sequence significantly affects the thermal conductivity of diamond. However, the inaccuracy of some widely used empirical potentials to calculate the anharmonic phonon transport in diamond, hinders the use of molecular dynamics simulations from revealing the detailed mechanism of phonon transport in diamond and other related materials with different stacking sequences. In this paper, a comparative study of four common carbon empirical potentials - Tersoff, Airebo, 2nd-rebo, and COMPASS - is performed to simulate the phonon thermal transport of diamond and lonsdaleite. The results show that the Tersoff potential, together with Airebo and 2nd-rebo potentials, is not suitable for describing the phonon transport mechanism in different diamond polytypes, both for harmonic phonon dispersion and anharmonic phonon scattering caused by stacking disorders. COMPASS potential gives a good fit for the acoustic phonon branches in all directions of diamond and accurately describes the band gaps between acoustic and optical branches, and is capable of representing the lattice dynamics and phonon thermal transport in diamond and lonsdaleite. The COMPASS potential is suitable for simulating the phonon thermal transport properties in diamond and lonsdaleite, and thus it is suitable for simulating the stacking disorder effect on the phonon thermal transport in diamond.

Atomistic configurational forces in crystalline fracture

Configurational atomistic forces contribute to the configurational mechanics (i.e. non-equilibrium) problem that determines the release of total potential energy of an atomistic system upon variation of the atomistic positions relative to the initial atomic configuration. These forces drive energetically favorable irreversible re-organizations of the material body, and thus characterize the tendency of crystalline defects to propagate. In this work, we provide new expressions for the atomistic configurational forces for two realistic interatomic potentials, i.e. the embedded atom potential (EAM) for metals, and second generation reactive bond order (REBO-II) potential for hydrocarbons. We present a range of numerical examples involving quasistatic fracture for both FCC metals and mono and bi-layer graphene at zero Kelvin that demonstrate the ability to predict defect nucleation and evolution using the proposed atomistic configurational mechanics approach. Furthermore, we provide the contributions for each potential including two-body stretching, three-body mixed-mode stretching-bending, and four-body mixed-mode stretching-bending-twisting terms that make towards defect nucleation and propagation.

Mechanical and thermal stability of armchair and zig-zag carbon sheets using classical MD simulation with Tersoff potential

In this work, the molecular dynamics method is implemented to study the temperature and edge effects on the atomic crack's growth in graphene nanosheet. Graphene nanosheets with armchair and zig-zag edges are simulated to this mechanical procedure. In our calculations, the atomic interactions of simulated carbon atoms are based on Tersoff potential. Simulation results show that nanosheets' edge is an important parameter in crack growth. Physically, graphene with armchair edge shows more resistance against atomic cracking in atomic simulations. In this structure, the final length of crack is 23.60 Å which is smaller than the zig-zag one at 300 K. Furthermore, the effects of temperature on the atomic crack of graphene nanosheets are studied. Our results show that by increasing the temperature from 300 K to 350 K, the atomic movements of carbon atoms increase; so, we concluded that the atomic stability of graphene nanosheets decreases by temperature increasing. Numerically, by 50 K temperature increasing in armchair/zig-zag graphene nanosheet, the crack length in this structure reaches to 28.32 Å/30.34 Å from 23.60 Å/28.91 Å value.

Elucidation of thermo-mechanical properties of silicon nanowires from a molecular dynamics perspective

The thermo-mechanical properties of silicon nanowires are of great importance in the state of the art silicon technologies. Herein, thermal and mechanical properties of silicon nanowires along [110] direction are illustrated by a molecular dynamics approach using Tersoff potential. Different lengths ranging from 10 to 45 nm and cross section widths from 2.2 nm to 6.5 nm as well as different temperatures from 200 to 500 K were studied. In all the cases, the corresponding thermal conductivities of silicon nanowires were found to be much smaller than the bulk silicon counterpart, in the range of ~1–5 W/(m.K), and strongly dependent on size while less sensitive to temperature. The elastic modulus, fractural strain and stress were also investigated upon various size and temperatures indicating dramatically lower properties at higher temperature and as a result of nano-scaling. The presented findings on <110> silicon nanowires sheds further lights on the current understandings on thermo-mechanical behavior of silicon nanowires for applications such as in Li-ion batteries and provide complementary information for the relevant data bank.

Effect of Diameter, Length, and Chirality on the Properties of Silicon Nanotubes

The mechanical properties of nanostructures are a researcher’s favorite topics. In the meantime, the mechanical and physical properties of the two dimensional structures and the nanotubes have attracted greater attention due to their wide application. Si (Si) nanotubes are structures consisting of Si atoms that are arranged as honeycombs. This structure has created some special properties in Si nanotubes. In this paper, Young’s modulus values and stress strain diagrams of Si nanotubes are investigated using molecular dynamics method and the Tersoff potential. Then, the changes effect of size and dimension was investigated for a closer look. For this purpose, the effect of nanotube diameter, length, and chirality shift from zigzag to armchair were studied. The results showed that the fracture stress of nanotube decreased with increasing the length of Si nanotube. It was also shown that the armchair structure was stronger than the zigzag. The effect of diameter change on the mechanical properties was also investigated and it was observed that no specific order could be found between the diameter changes with the Si nanotube strength. The results were in good agreement with other studies.

A molecular dynamics simulation of inhomogeneous silicon–germanium nucleation from supersaturated vapor mixtures

The inhomogeneous nucleation of silicon–germanium (Si–Ge) systems from supersaturated vapor mixtures was investigated using molecular dynamics simulations. Isothermal simulation runs were performed using the Tersoff potential at various supersaturations and temperatures. We focused on the inhomogeneous dynamics, nucleation rate, and critical cluster size, as well as the effect of inhomogeneity on the quantitative results. The study showed that Si atoms nucleate much faster than Ge atoms. This may lead to the inhomogeneity and final production of Si-rich critical clusters. Such inhomogeneity may also stem from the different chemical properties of Si and Ge atoms. Under the tested conditions, the nucleation rates were within 1033–1036 J/m−3 s−1. They were influenced significantly by the supersaturation and slightly by the temperature. The critical size of 2.5–4.5 atoms was heavily dependent on both the supersaturation and temperature. Our results are generally consistent with those from other nucleating systems using the same method. The inhomogeneity of the Si–Ge system has no significant effect on the nucleation rate but may contribute to smaller critical cluster sizes at low temperatures.