Reactive Empirical Bond Order Research Articles

Atomic scale etching of diamond: insights from molecular dynamics simulations

Abstract Diamond is a promising material for multiple applications in quantum information processing and sensing as well as applications in microelectronics. However, diamond devices can be limited by surface defects that compromise charge stability and spin coherence, among others. Improved strategies in plasma etching of diamond could play an important role in minimizing or eliminating these defects. In this work, we explore plasma-assisted atomic scale etching of diamond using argon ions (Ar+), hydrogen ions (H+) and hydrogen atoms (H). We employ classical molecular dynamics (MD) simulations and test several interatomic potentials based on the Reactive Empirical Bond Order (REBO) form with comparisons to a variety of published experimental results. We performed MD simulations of low-energy hydrogen ( ⩽ 50 eV) and argon ( ⩽ 200 eV) ion bombardment of diamond surfaces. Ar+ bombardment can be used to locally smooth initially rough diamond surfaces via the formation of an amorphous C layer, the thickness of which increases with argon ion energy. Subsequent exposure with hydrogen ions (or fast neutrals) will selectively etch this amorphous C layer, leaving the underlying diamond layer mostly intact if the H energy is maintained below about 10 eV. The simulations suggest that combining Ar+ smoothing with selective, near threshold energy H removal of amorphous C can be an effective strategy for diamond surface engineering, leading to more reliable and sensitive diamond color center devices.

Pressure and temperature diagram of C60 from atomistic simulations.

Although widely studied experimentally in the 1990s, the structure and properties of low-dimensional or high-pressure phases of fullerenes have recently been re-examined. Remarkably, recent experiments have shown that transparent, nearly pure amorphous sp3-bonded carbon phases can be obtained by heating a C60 molecular crystal at a high pressure. With the additional aim of testing the ability of three classical carbon potentials reactive empirical bond order, environment-dependent interatomic potential, and reactive force-field to reproduce these results, we investigate the details of the structural transformations undergone by fullerene crystals over a wide range of pressures and temperatures. All the potentials tested show that the initial polymerization of fullerenes is accompanied by negative thermal expansion, albeit in slightly different ranges. However, more significant differences in structural and mechanical properties are observed in the amorphous phases, in particular the sp3 carbon fraction and the existence of layered amorphous carbon. Overall, these results indicate to which extent classical reactive potentials can be used to explore phase transitions over a wide range of pressures and temperatures.

Description of Short-Range Interactions of Carbon-Based Materials with a Combined AIREBO and ZBL Potential.

An accurate description of short-range interactions among atoms is crucial for simulating irradiation effects in applications related to ion modification techniques. A smooth integration of the Ziegler-Biersack-Littmark (ZBL) potential with the adaptive intermolecular reactive empirical bond-order (AIREBO) potential was achieved to accurately describe the short-range interactions for carbon-based materials. The influence of the ZBL connection on potential energy, force, and various AIREBO components, including reactive empirical bond-order (REBO), Lennard-Jones (LJ), and the torsional component, was examined with configurations of the dimer structure, tetrahedron structure, and monolayer graphene. The REBO component is primarily responsible for the repulsive force, while the LJ component is mainly active in long-range interactions. It is shown that under certain conditions, the torsional energy can lead to a strong repulsive force at short range. Molecular dynamics simulations were performed to study the collision process in configurations of the C-C dimer and bulk graphite. Cascade collisions in graphite with kinetic energies of 1 keV and 10 keV for primary knock-on atoms showed that the short-range description can greatly impact the number of generated defects and their morphology.

Molecular Dynamics Simulations of Effects of Geometric Parameters and Temperature on Mechanical Properties of Single-Walled Carbon Nanotubes

Carbon nanotubes (CNTs) are considered an advanced form of carbon. They have superior characteristics in terms of mechanical and thermal properties compared to other available fibers and can be used in various applications, such as supercapacitors, sensors, and artificial muscles. The properties of single-walled carbon nanotubes (SWNTs) are significantly affected by geometric parameters such as chirality and aspect ratio, and testing conditions such as temperature and strain rate. In this study, the effects of geometric parameters and temperature on the mechanical properties of SWNTs were studied by molecular dynamics (MD) simulations using the Large-scaled Atomic/Molecular Massively Parallel Simulator (LAMMPS). Based on the second-generation reactive empirical bond order (REBO) potential, SWNTs of different diameters were tested in tension and compression under different strain rates and temperatures to understand their effects on the mechanical behavior of SWNTs. It was observed that the Young’s modulus and the tensile strength decreases with increasing SWNT tube diameter. As the chiral angle increases, the tensile strength increases, while the Young’s modulus decreases. The simulations were repeated at different temperatures of 300 K, 900 K, 1500 K, 2100 K and different strain rates of 1 × 10−3/ps, 0.75 × 10−3/ps, 0.5 × 10−3/ps, and 0.25 × 10−3/ps to investigate the effects of temperature and strain rate, respectively. The results show that the ultimate tensile strength of SWNTs increases with increasing strain rate. It is also seen that when SWNTs were stretched at higher temperatures, they failed at lower stresses and strains. The compressive behavior results indicate that SWNTs tend to buckle under lower stresses and strains than those under tensile stress. The simulation results were validated by and consistent with previous studies. The presented approach can be applied to investigate the properties of other advanced materials.

Comparison of Interatomic Potentials for Modeling Defects in Graphene Using Molecular Dynamics

In this work, we tested the ability of classical interatomic potentials to describe the energy characteristics of defects of various dimensionality in graphene crystals. Brenner's Reactive Empirical Bond Order potentials (second generation REBO, AIREBO, AIREBO-M), Tersoff potentials, as well as BOP and LCBOP potentials were considered. The data obtained in this work using the molecular dynamics method was compared with literature data obtained using the density functional theory. It is noted that when modeling point and linear defects, the potentials of the REBO family and the LCBOP potential demonstrate the best agreement with the literature data. For modeling pseudo-graphene crystals, the best fit is demonstrated by the Tersoff B-N-C potential, which shows slightly overestimated energy values for linear and point defects, but most accurately describes the geometry of the crystal lattice. The potential of BOP demonstrates its inability to correctly model defect configurations with high densities of eight-member defect rings. When simulating four-member carbon defect rings, most potentials exhibit distortions in the crystal lattice that are not observed in the density functional theory calculations.

Graphullerene: A Fully Atomistic Analysis of the Tensile Characteristics Under Temperature Variation

Based on the molecular dynamics simulations, mechanical properties of the most recently synthesized covalent fullerene network, i.e., graphullerene sheet, under uniaxial tension, are determined. These properties include Young's modulus, ultimate stress, failure strain, tensile toughness, and Poisson's ratio. Due to the lack of existing data on mechanical properties of graphullerene, a comprehensive study is performed to get an insight into the effects of defect density, size, and temperature on all tensile characteristics. In the results, it is indicated that the tensile behavior of graphullerene structure is comparable with graphene allotropes. Moreover, it is observed that the presence of defect reduces stiffness, ultimate stress, and tensile toughness, while it has a negligible effect on the failure strain. It is further observed that unlike stiffness, increasing the temperature reduces other tensile characteristics. Interestingly, it is found that graphullerene exhibits auxetic behavior with negative Poisson's ratio which categorizes it as a unique auxetic material ever synthesized. At the end, three types of potential functions for hydrocarbons including adaptive intermolecular reactive empirical bond order, reactive empirical bond order, and Tersoff are considered to study the tensile behavior of graphullerene. This new auxetic nanosheet is a promising analogous to the existing 2D nanostructures with fascinating potential applications.

Deformation behaviors of hydrogen filled boron nitride and boron nitride - carbon nanotubes: Molecular dynamics simulations of proposed materials for hydrogen storage, gas sensing, and radiation shielding

Boron nitride nanotubes (BNNTs) have been extensively studied for hydrogen storage, sensing, and radiation shielding applications. The deformation behaviors of pristine, defective, and H2-encapsulated closed-end armchair BNNTs and BNNT-carbon nanotube (CNT) heterostructures are investigated using molecular dynamics employing the ReaxFF interatomic potential and a combination of Tersoff, adaptive intermolecular reactive empirical bond order, and Lennard-Jones potentials. The calculated elastic properties for pristine BNNTs and CNTs are in excellent agreement with published reliable data. The nanotube length and diameter, defects, entrapped hydrogen content, and temperature all affect the mechanical properties significantly. The ReaxFF potential predicts H2 molecule adsorption on B atoms of undeformed BNNTs whilst under strain, the H2 molecules show preference for the N atoms. Hydrogen adsorption leads to lowering of the elastic and fracture strength of BNNTs. The ReaxFF potential is shown to be a reliable forcefield for investigating the interaction of H2 molecules with BNNTs and BNNT-CNT heterotubes.

Investigation of fullerene cluster growth mechanisms by carbon atom addition using classical molecular dynamics.

The mechanisms of carbon sticking reactions to C36 and C-C80 fullerenes were investigated with molecular dynamics simulations (MD) using the Second-generation Reactive Empirical Bond Order (SREBO) and Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potentials that were specifically optimized for carbon-carbon interactions. Results showed the existence of three possible sticking configurations where the projectile atom can stick either to one, two or three atoms of the target fullerene. They also showed that although the two potentials give similar magnitudes for the sticking cross-sections, they yield fairly different results as far as sticking mechanisms and configurations at thermal collision-energies, i.e., in the range 0.05-0.5eV, are concerned. While AIREBO, that takes into account the long-range Lennard-Jones interaction, essentially results in a surface-sticking configuration with a single atom of the target fullerene, SREBO potential yields both surface- and two neighbors-sticking (2N-sticking) configurations. The fullerene structure is preserved in the last configuration while it can be recovered by a 2000K annealing in the former configuration. Results obtained with SREBO eventually showed larger sticking probabilities for C36 as compared with C80. In spite of this, the sticking cross-sections obtained for C80 are similar to or even larger than those obtained for C36 due to the larger size of C80 that compensates for its smaller sticking probabilities.

Molecular dynamics simulation of free transverse vibration behavior of single-walled coiled carbon nanotubes

Coiled carbon nanotubes (CCNTs) belong to one of the prominent classes of carbon nanostructures with unique mechanical properties and vibrational behavior due to their helical geometries. In this paper, the free transverse vibration behavior of single-walled CCNTs is investigated by molecular dynamics (MD) simulations and the adaptive intermolecular reactive empirical bond-order (AIREBO) potential under different boundary conditions (B.Cs.). The beating phenomenon is observed in CCNTs due to the presence of longitudinal, transverse, and torsional coupled vibrations and the proximity of their corresponding frequency. Generally, the frequency of the CCNTs decreases by increasing the length () or the number of pitches ( Moreover, the pitch angle plays a more decisive role compared to other geometric parameters. At constant length () of CCNTs, the frequency increases by enhancing the pitch angle Furthermore, the fundamental frequency range of the studied CCNTs is obtained less than 331.9 GHz for lengths greater than 2 nm under different boundary conditions. This indicates that CCNTs have a higher vibration sensitivity than straight CNTs. Therefore, CCNTs can be a proper alternative to straight CNTs in sensors. The results of this study can be used in the design and analysis of nanoelectromechanical systems (NEMs) with CCNTs elements as well as to calibrate continuum mechanics methods.

Effect of CNT’s volume fraction on the mechanical properties of CNT reinforced Al/Cu alloy nanocomposite using molecular dynamics simulation

Aluminum is one of most widely available elements on planet earth. After oxygen and silicon, aluminum is the third available element in the crust of the earth with a contribution of about 8%. It has a unique set of properties for which it has been used heavily in different aspects of life but yet its mechanical properties such as strength and hardness may be enhanced by the addition of other elements to form an alloy or a composite. Carbon nanotubes (CNTs) have attracted a great attention as reinforcements in metal matrix composites due to their extraordinary characteristics such as strength, stiffness, and low density. Aluminum/Copper (Al/Cu) alloy enhanced with CNTs of different volume fractions is the topic of this work which was not addressed previously in the literature. Embedded atom method (EAM) potential was used to model the interaction between aluminum and copper while the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential was used to model the CNT. The interactions between Al/Cu alloy and CNT was modeled using the pairwise Morse potential. Mechanical properties of Al/Cu alloy with alloying percentage of 10% reinforced with single walled carbon nanotube of different volume fractions are studied under tensile loading using molecular dynamics (MD) simulation. The open-source code LAMMPS is used to conduct the MD simulations. An armchair CNT with a chirality of (6,6) is considered for the study. The modulus of elasticity and the ultimate tensile strength of the Al/Cu-CNT nanocomposite are both observed to increase as the volume fraction of the CNT increases. The modulus of elasticity increased by almost 75% as the volume fraction of the CNT increased from 1% to 10%. The failure strains are almost the same in all cases. The results showed a maximum practical threshold of 10% CNT volume fraction to the Al/Cu alloy.

Mechanical properties of pristine and nanocrystalline graphene up to ultra-high temperatures

Despite being the basic building block of most carbon materials used in high and ultra-high temperature applications, the mechanical properties of graphene at such temperatures remain totally unknown. Here we compute these properties using tensile molecular dynamics simulations at temperatures ranging from room temperature to 5000 K (i. e. about the melting temperature) for pristine and nanocrystalline graphene. Mechanical simulations at such temperatures are made possible via the use of the screened environment dependent reactive empirical bond order potential [Perriot et al. Phys. Rev. B 88, 064101 (2013)]. We show that even at the highest temperature, both graphene and nanocrystalline graphene retain high stiffness, strength and fracture strain. Analyses of tensile curves and trajectories (including an analysis of in-plane non-affine atomic displacements) show that both pristine and nanocrystalline graphene show brittle rupture, whatever the temperature, even though some early sign of ductility can be detected for the nanocrystalline graphene at 5000 K. This rules out the possibility of a ductile fracture in graphene at any temperature below the melting point. Finally, the simulations also highlight the importance of bond rotation (Stone-Wales) based defects in the fracture initiation mechanism for pristine graphene. Especially at intermediate temperature (3000 K) crack nucleation in pristine graphene is shown to take place exactly at the moment and place of the first Stone-Wales defect formation.

Relaxation effects in twisted bilayer graphene: A multiscale approach

We present a multiscale density functional theory (DFT) informed molecular dynamics and tight-binding approach to capture the interdependent atomic and electronic structures of twisted bilayer graphene. We calibrate the flat band magic angle to be at ${\ensuremath{\theta}}_{\mathrm{M}}=1.{08}^{\ensuremath{\circ}}$ by rescaling the interlayer tunneling for different atomic structure relaxation models as a way to resolve the indeterminacy of existing atomic and electronic structure models whose predicted magic angles vary widely between $0.{9}^{\ensuremath{\circ}}$ and $1.{3}^{\ensuremath{\circ}}$. The interatomic force fields are built using input from various stacking and interlayer distance-dependent DFT total energies including the exact exchange and random phase approximation ($\mathrm{EXX}+\mathrm{RPA}$). We use a Fermi velocity of ${\ensuremath{\upsilon}}_{\mathrm{F}}\ensuremath{\simeq}{10}^{6}$ m/s for graphene that is enhanced by $\ensuremath{\sim}15%$ over the local density approximation (LDA) values. Based on this atomic and electronic structure model we obtain high-resolution spectral functions comparable with experimental angle-resolved photoemission spectroscopy. Our analysis of the interdependence between the atomic and electronic structures indicates that the intralayer elastic parameters compatible with the DFT-LDA, which are stiffer by $\ensuremath{\sim}30%$ than widely used reactive empirical bond order force fields, can combine with $\mathrm{EXX}+\mathrm{RPA}$ interlayer potentials to yield the magic angle at $\ensuremath{\sim}1.{08}^{\ensuremath{\circ}}$ without further rescaling of the interlayer tunneling.

Thermal conductivities of hydrogen encapsulated boron nitride and hybrid boron nitride – carbon nanotubes using molecular dynamics simulations

In this work, we investigated the thermal conductivities of boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) as potential hydrogen storage materials using atomistic nonequilibrium molecular dynamics (NEMD) simulations. Pristine and defective (containing B/N/C single atom vacancy or B-N double atom vacancies) capped armchair BNNTs and BN-C heteronanotubes have been studied. Furthermore, thermal conduction in BNNT, CNT, and BN(50%)-C(50%) heteronanotubes with varying amounts of encapsulated H2 molecules has been evaluated. We compare the performance of the reactive forcefield ReaxFF with non-reactive Tersoff-adaptive intermolecular reactive empirical bond order (AIREBO)-Lennard-Jones (TALJ) potentials combination. The ReaxFF potential yielded consistently lower thermal conductivity (k) for all the nanotubes. However, it provides a better qualitative prediction of thermal transport in the nanotubes (in terms of k reduction due to defects and entrapped H2, and sensitivity to composition as reflected in the phonon density of states) whereas Tersoff/TALJ potentials provide a better quantitative prediction of k while failing to account for acoustic phonon details. Near-linear mixing rule for k across the CNT-BNNT composition suggested by both potentials indicates smooth interfacial heat transport at the heterojunctions.

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.

Molecular Dynamics Simulation on Collision Frictional Properties of a Molybdenum Disulfide (MoS2) Film in Microgravity Environment

In this paper, a collision friction model for a double-layer MoS2 film is proposed considering the microgravity induced collision in space environment. A modified REBO (Reactive Empirical Bond Order) potential is used to describe interactions among the atoms in the MoS2 film. The collision friction process of the MoS2 film is simulated by vibrations in the y and z directions, and the dependence of average friction force is analyzed. The influence of a single vibration in the y direction on the friction forces can be ignored, while the vibration in the z direction shows great influence. The effects of vibration frequency and amplitude on frictional behaviors of the MoS2 film are investigated. The average friction forces during the collision friction process correlate with the frequency of the vibration in the z direction, and the relationship shows four stages. As the frequency increases, average friction forces show low values in the first stage, and they are increased as the frequency in the second stage. In the third stage, the average friction forces are decreased, and they come to a stable level in the fourth stage. Increasing the vibration amplitude at different frequencies leads to an increase in average friction force, due to that the increased amplitude results in a high indentation depth. The puckering phenomenon occurs at a specific frequency, which is a reason that the average friction force is increased during this collision friction process.

Investigate the effect of temperature, length, width, and thickness of substrate on thermal conductivity of graphene copper nanocomposites

The novel thermal conductivity of graphene-copper (GN/Cu) nanocomposites, which include thermal transport of phonons and electrons, has attracted tremendous attention for the development of next-generation nanoelectronic and optoelectronic device applications. To study the effect of temperature, length, width, and substrate thickness on thermal conductivity, Large-Scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) software has been taken to perform molecular dynamics (MD) simulation. The Green Kubo methods with Embedded Atom Method (EAM), Reactive Empirical Bond Order (REBO), andLenard-Jones (L-J) potentials were used to analyze interactions of atoms. First of all, the thermal conductivity of copper and graphene at 300k temperature has been observed. The thermal conductivity of (GN, GN/Cu (10 0), GN/Cu (110), and GN/Cu (111)) was observed by a wide range of temperature (300K to 1000K). Thermal conductivities of GN, GN/Cu (1 0 0), GN/Cu (1 1 0) and GN/Cu (1 1 1) at 300k were obtained as 1500 Wm-1K -1 ,1204Wm-1K -1 ,1005 Wm -1K -1 ,897 Wm -1K -1 respectively. It was witnessed that the thermal conductivity of GN/Cu (1 0 0), GN/Cu (1 1 0), and GN/Cu (1 1 1) decreased by 80.26%, 67%, 59.8% as compared to graphene by changing temperature. It was also observed that thermal conductivity of GN/Cu (1 0 0), GN/Cu (1 1 0), and GN/Cu (1 1 1) decreased by 231.30%, 221.01%, 231.26% with the change in thickness of the substrate. It was calculated that thermal conductivity of GN/Cu (1 0 0), GN/Cu (1 1 0), GN/Cu (1 1 1) increased by 290.7%, 323.98%, 523.1% within length variation from 7nm to 11.0nm.It was also calculated that thermal conductivity of GN/Cu (1 0 0), GN/Cu (1 1 0), GN/Cu (1 1 1) increased by 290.7%, 323.98%, 523.1% within width variation from 0.8 nm to 2.0 nm. Finally, it was concluded that thermal conductivity increased with the increase of length as well as width. It was investigated that the above-mentioned parameters significantly improve the thermal conductivity of GN/Cu (111), which is beneficial for high-performance nanoelectronic and optoelectronic devices include solar cells.

Effects of Orientation and Temperature on the Tensile Strength of Pristine and Defective Bi-Layer Graphene Sheet – A Molecular Dynamics Study

Molecular dynamics simulations with adaptive intermolecular reactive empirical bond order (AIREBO) potential were carried out to study the effect of temperature and orientation on the tensile strength of pristine and defective bilayer graphene (BLG) sheet. Results obtained reveal that the fall in tensile strength of pristine AA stacked BLG due to the presence of vacancy is significant at room temperature (300 K) but decreases at higher temperature (1073 K). Interestingly, this phenomenon reverses in case of AB stacked BLG, wherein the percentage fall in strength at higher temperature due to defect is more than that at room temperature. In order to understand these discrepancies in the results obtained, three case studies were conducted, and the results obtained suggested that when defects are present in armchair direction, this phenomenon occurs. The study also reveals that in case of AB stacked BLG, zigzag direction is more defect tolerant at room and high temperatures. Interestingly, variation of tensile strength due to the orientation is in good agreement with projections from potential energy concepts and theoretical calculations. We envisage that the study will provide useful information to the device engineers for the optimisation of the mechanical properties and convenient structural adaptation of bilayer graphene while working at wide range of temperatures.

Comparing different multibody reactive potentials for the elastic properties and nonlinear mechanics of the carbon nanostructures

A multiscale model in conjunction with the finite element method is employed to investigate the accuracy of the different reactive empirical bond order potentials involving material and geometric nonlinearities. The four different sets of empirical parameters for the Tersoff-Brenner potential are considered in the present study and among these four sets of empirical parameters, one set is newly proposed herein and three are taken from the available reference sources. The atomistic-continuum coupling is established using kinematics of quadratic type Cauchy-Born rule for establishing the energy density of the unit cell. The finite element method is used to solve the governing equations at the continuum scale. The revised set of empirical parameters is found better for predicting the elastic properties and bond length than the formerly proposed three sets of empirical parameters when compared to those reported using quantum mechanics calculations. The results of the newly proposed potential are compared with DFT based continuum models for a wide range of numerical problems. The proposed potential parameters are tested for a variety of problems such as the energy of the carbon nanotubes with different radii, torsion of the carbon nanotubes, free vibrations of the graphene sheets and carbon nanotubes and numerical simulations of the nanoindentation experiments for circular graphene sheets.

Impact of chosen force fields and applied load on thin film lubrication

The rapid development of molecular dynamics (MD) simulations, as well as classical and reactive atomic potentials, has enabled tribologists to gain new insights into lubrication performance at the fundamental level. However, the impact of adopted potentials on the rheological properties and tribological performance of hydrocarbons has not been researched adequately. This extensive study analyzed the effects of surface structure, applied load, and force field (FF) on the thin film lubrication of hexadecane. The lubricant film became more solid-like as the applied load increased. In particular, with increasing applied load, there was an increase in the velocity slip, shear viscosity, and friction. The degree of ordering structure also changed with the applied load but rather insignificantly. It was also significantly dependent on the surface structure. The chosen FFs significantly influenced the lubrication performance, rheological properties, and molecular structure. The adaptive intermolecular reactive empirical bond order (AIREBO) potential resulted in more significant liquid-like behaviors, and the smallest velocity slip, degree of ordering structure, and shear stress were compared using the optimized potential for liquid simulations of united atoms (OPLS-UAs), condensed-phase optimized molecular potential for atomic simulation studies (COMPASS), and ReaxFF. Generally, classical potentials, such as OPLS-UA and COMPASS, exhibit more solid-like behavior than reactive potentials do. Furthermore, owing to the solid-like behavior, the lubricant temperatures obtained from OPLS-UA and COMPASS were much lower than those obtained from AIREBO and ReaxFF. The increase in shear stress, as well as the decrease in velocity slip with an increase in the surface potential parameter ζ, remained conserved for all chosen FFs, thus indicating that the proposed surface potential parameter ζ for the COMPASS FF can be verified for a wide range of atomic models.

Molecular dynamics simulation of transversely isotropic elastic properties of carbon nanocones

In this paper, transversely isotropic elastic properties of carbon nanocones are studied using molecular dynamics simulation implemented in the large-scale atomic/molecular massively parallel simulator (LAMMPS). All atomic interactions are calculated based on the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential energy functions. To determine the five independent elastic constants, four distinct loading conditions, i. e. uniaxial tensile, longitudinal torsion, in-plane biaxial stretching, and in-plane shear are imposed. The results reveal that Young’s and axial shear moduli are dependent on the apex angle of carbon nanocones, while the effect of the length on them is negligible. Furthermore, the in-plane bulk modulus and in-plane shear constant of these structures increase as their apex angle increases.