Adaptive Intermolecular Reactive Empirical Bond Order Research Articles

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.

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.

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.

Stress-strain behaviour of graphene reinforced aluminum nanocomposite under compressive loading using molecular dynamics

In the present study, the graphene sheet (GS) - embedded aluminium (Al) composite under compressive strain rate is analyzed using molecular dynamics (MD) simulation. The effects of pin-hole and layering of GSs on compressive strengthening behaviour of the composite material are predicted. MD simulation has been performed with the aid of open-source code LAMMPS by modelling a periodic system of GS-Al nanocomposite unit cells. Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) and Embedded atom method (EAM) potentials are employed to model the interactions between carbon atoms and the atoms of Al respectively. The simulation results show that the compressive strength of nanocomposite material is substantially larger than that of pure Al material. It is also found that the compressive elastic modulus of GS-Al nanocomposite material deteriorates because of pin-holed GS on the contrary to layered GS reinforced nanocomposites.

Influence of vacancy defects on vibration analysis of graphene sheets applying isogeometric method: Molecular and continuum approaches

The main objective of this research paper is to consider vibration analysis of vacancy defected graphene sheet as a nonisotropic structure via molecular dynamic and continuum approaches. The influence of structural defects on the vibration of graphene sheets is considered by applying the mechanical properties of defected graphene sheets. Molecular dynamic simulations have been performed to estimate the mechanical properties of graphene as a nonisotropic structure with single- and double- vacancy defects using open source well-known software i.e., large-scale atomic/molecular massively parallel simulator (LAMMPS). The interactions between the carbon atoms are modelled using Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential. An isogeometric analysis (IGA) based upon non-uniform rational B-spline (NURBS) is employed for approximation of single-layered graphene sheets deflection field and the governing equations are derived using nonlocal elasticity theory. The dependence of small-scale effects, chirality and different defect types on vibrational characteristic of graphene sheets is investigated in this comprehensive research work. In addition, numerical results are validated and compared with those achieved using other analysis, where an excellent agreement is found. The interesting results indicate that increasing the number of missing atoms can lead to decrease the natural frequencies of graphene sheets. It is seen that the degree of the detrimental effects differ with defect type. The Young\'s and shear modulus of the graphene with SV defects are much smaller than graphene with DV defects. It is also observed that Single Vacancy (SV) clusters cause more reduction in the natural frequencies of SLGS than Double Vacancy (DV) clusters. The effectiveness and the accuracy of the present IGA approach have been demonstrated and it is shown that the IGA is efficient, robust and accurate in terms of nanoplate problems.

Molecular dynamics study of the material property changes induced by accumulated point defects in graphite

Molecular dynamics simulations were performed on pristine graphite using an adaptive intermolecular reactive empirical bond-order (AIREBO) potential, which introduced various concentrations of point defects and different sizes in defect clusters. Induced by the accumulation of mono-vacancy and self-interstitial atoms (SIA), the changes to the geometry, density, Young’s modulus, fracture strength, and coefficient of thermal expansion (CTE) of graphite were simulated over a range of defect concentrations and cluster sizes. We found that accumulated Frenkel pairs lead to clearly-observed changes to the geometric and elastic properties in graphite crystal, but a relatively negligible effect on CTE. As expected, changing size of defect clusters with a given concentration relates to the variation of material properties. The property changes simulated in the present work are qualitatively consistent with the experimental observations. The results are expected to provide some insights into the link between cascade-induced point defects and experimentally irradiation-assisted material property changes.

Vibrational Analysis of Fullerene Hydrides Using AIREBO Potential

In this paper, the vibrational properties of fullerene hydrides with the chemical formula C60H2n are investigated using a method based on the potential energy of the molecule. The potential used in this methodology is AIREBO (Adaptive Intermolecular Reactive Empirical Bond Order). Using this interatomic potential, some of the most important frequencies of the fullerene hydrides, such as the breathing mode frequency, were calculated and then analyzed. It was observed that in addition to the number of hydrogen atoms in the structure, their position on the C60 cage has a significant effect on the natural frequency corresponding to a particular mode shape. The results obtained by this method have been compared and validated with quantum mechanics and experimental observations. The simulations results demonstrate that the proposed method is capable of calculating the vibrational properties of fullerene hydrides with high precision and low computational cost.

Fracture Properties of Graphene‐Coated Silicon for Photovoltaics

AbstractThe possibility of replacing the conductive gridline deposited on solar cells by highly electrically conductive graphene is opening new perspectives for the future generation of photovoltaics. Besides enhanced electric performance, graphene can also have a role in the resistance of silicon against cracking. Here, the influence of depositing graphene on the silicon surface, on the fracture properties of silicon, is investigated. To pin‐point the influence of graphene, fracture properties estimated from molecular dynamics simulations of three different cases in uniaxial tension are compared. In the first case, the fracture properties of silicon alone are estimated in relation to different initial defect sizes. Second, the same simulations are repeated by depositing graphene on the silicon surface. Atomic interactions in the composite structure are modeled using the combined adaptive inter‐molecular reactive empirical bond order (AIREBO) and Tersoff potential functions. Improvement of about 780% in the Young's modulus of silicon is achieved after coating with graphene. Furthermore, to study the influence of realistic initial defects in graphene, a third set of simulations is considered by repeating the previous tests but with initial cracks through graphene and silicon. Predictions show that graphene can be highly beneficial in strengthening and repairing micro‐cracked silicon to decrease electrical power losses caused by cracks.

On the atomistic energetics of carbon nanotube collapse from AIREBO potential

Molecular dynamics simulations based on the adaptive intermolecular reactive empirical bond order (AIREBO) were performed to probe hydrostatic pressure induced collapse of single-walled and double-walled carbon nanotubes. It was unveiled that the torsion term, which is a specific potential component involved in the AIREBO scheme, plays a vital role in stabilizing fully collapsed cross-sections of the carbon nanotubes. Evolution of the cross-sectional deformation along the loading-unloading curve was also elucidated, showing strong dependence on the presence of a structural defect on the outer carbon wall.

Molecular Dynamics Simulations of Si ion Substituted Graphene by Bombardment

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.

Thermal rectification in partially hydrogenated graphene with grain boundary, a non-equilibrium molecular dynamics study

In the present study, we have tried to investigate thermal conductivity and thermal rectification in the partially hydrogenated graphene sheet with grain boundary through non-equilibrium molecular dynamics simulation method. To this end, the 3-body Tersoff and the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential were employed. The results revealed that the grain boundary plays an important role in phonon scattering, which flows to the other side of grain boundary. It was observed that temperature drops significantly across a grain boundary that leads to the thermal resistance. It has been also proved that thermal rectification depends on the temperature difference between two ends of the system. Also in such a system, a tuning factor for the tuning of the thermal rectification has been introduced by randomly removing hydrogen atoms of the partially hydrogenated graphene sheet with grain boundary.

Mechanical Behaviour of Graphene Reinforced Aluminum Nano composites

In this paper, molecular dynamics (MD) simulations are performed on graphene-aluminum (GS-Al) nanocomposite. The mechanical properties of the nanocomposite are investigated by the application of uni-axial load on one end of the representative volume element (RVE) and fixing the other end. The interactions between the atoms of Al are modelled using Embedded Atom Method (EAM) potentials, whereas Adaptive Intermolecular Reactive Empirical Bond-Order (AIREBO) potential is used for the interactions among carbon atoms and these pair potentials are coupled with the Lennard-Jones (LJ) potential. The result shows that the incorporation of Gn into the Al matrix can increase the Young's modulus of the nanocomposite substantially. The nanocomposite containing 6.7 vol.%of GS exhibits Young’s modulus of 143.8 GPa and 116.8 GPa along longitudinal and transverse directions, respectively that are 82.8% and 46.5% higher than pure Al. Results from the molecular dynamics simulations are also compared with analytical results obtained from semi-empirical Halphin-Tsai (H-T) model and the Rule of Mixtures (ROM).

Based on the molecular dynamics characteristic research of heat conduction of graphyne nanoribbons with vacancy defects

As a kind of nano-material, graphyne nanoribbon has some physical properties and its properties should be studied for its better usage. In the process of preparing graphyne nanoribbons, it is possible that vacancy defects exist in the lattice structure, which will affect the physical properties of the graphyne nanoribbons. The flotation of graphyne is closer to the actual situation in engineering than the complete graphyne nanoribbons, and the diversity of vacancy defects can lead to various thermal conductivities, so it is very important to simulate the effects of various vacancy defects on thermal conductivity. In order to better predicte and control heat transfer characteristics of graphyne nanoribbons, this paper focuses on the effects of vacancy defects on the heat transfer characteristics of graphyne nanoribbons. According to the different cutting directions of graphyne nanoribbons, two different types of graphyne nanoribbons are obtained, i.e., armchair type and zigzag type. We compare the effects of vacancy defects on the thermal conductivity of two different chiral graphynes nanoribbons to improve the persuasiveness of the conclusion. In this paper, non-equilibrium molecular dynamics method is adopted, by applying periodic boundary conditions in the length direction of the nanoribbons, the interaction between the carbon-carbon atoms is described based on a potential function of adaptive intermolecular reactive empirical bond order (AIREBO). At 300 K, the effects of single vacancy defect in the acetylene chain, single vacancy defect in the benzene ring or double vacancy defects in the acetylene chain on the thermal conductivities of single-layer graphyne nanoribbons are simulated. Fourier's law is used to calculate the thermal conductivities of graphyne nanoribbons. The simulation results show that for the thermal conductivity of graphyne nanoribbons in a-few-dozen nanometer range:1) as a result of the phonon scattering and enhanced phonon Umklapp process, the graphyne nanoribbons with vacancy defects will cause the thermal conductivity to decrease and becomes lower than that of the complete graphyne nanoribbons; 2) due to the difference in phonon density-of-states matching degree, the vacancy defect in the benzene ring of graphyne nanoribbons has a greater effect on the thermal conductivity than that of vacancy defect in the acetylene chain of graphyne nanoribbons, the vacancy defects have a strong influence on the thermal conductivity of in the acetylene chain of graphyne nanoribbons; 3) because of the influence of size effect, the thermal conductivity of graphyne nanoribbon increases with length increasing. In this paper, the research of the thermal conductivity of graphyne nanoribbon provides the reference for controlling their thermal conductivity on a certain scale.

Classical molecular dynamics simulation on the dynamical properties of H2on silicene layer

This study investigates the diffusion of hydrogen molecule physisorbed on the surface of silicene nanoribbon (SiNR)using the classical molecular dynamic (MD) simulation in LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). The interactions between silicon atoms are modeled using the modified Tersoff potential, the Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential for hydrogen – hydrogen interaction and the Lennard – Jones potential for the physisorbed H2 on SiNR. By varying the temperatures (60 K Δ 130 K), we observed that the Δxdisplacement of H 2 on the surface SiNR shows a Brownian motion on a Lennard-Jones potential and a Gaussian probability distribution can be plotted describing the diffusion of H 2 . The calculated mean square displacement (MSD) was approximately increasing in time and the activation energy barrier for diffusion has been found to be 43.23meV.

Molecular dynamics simulations on deformation and fracture of bi-layer graphene with different stacking pattern under tension

Based on AIREBO (Adaptive Intermolecular Reactive Empirical Bond Order) potential, molecular dynamics simulations (MDs) are performed to study the mechanical behavior of AB- and AA-stacked bi-layer graphene films (BGFs) under tension. Stress–strain relationship is established and deformation mechanism is investigated via morphology analysis. It is found that AA-stacked BGFs show wavy folds, i.e. the structural instability, and the local structure of AB-stacked BGFs transforms into AA-stacked ones during free relaxation. The values of the Young's modulus obtained for AA-stacked zigzag and armchair BGFs are 797.2 GPa and 727.4 GPa, and those of their AB-stacked counterparts are 646.7 GPa and 603.5 GPa, respectively. In comparison with single-layer graphene, low anisotropy is observed for BGFs, especially AB-stacked ones. During the tensile deformation, hexagonal cells at the edge of BGFs are found to transform into pentagonal rings and the number of such defects increases with the rise of tensile strain.

Atomistic modeling of the stiffness, strength and charge-induced actuation of graphene nanofoams

To exploit the excellent electrical and mechanical properties of graphene, three-dimensional nanoporous graphene architectures have appeared recently. These graphene nanofoams are conductive and have very large internal surface areas, making them attractive for application as low-voltage electrochemical actuators. The key feature that connects the excess-charge-modified interatomic bond length to the overall strains is the three-dimensional spatial distribution of the graphene ribbons. This is governed by the competition between interlayer van der Waals forces and intralayer charge-affected covalent interactions. Here we explore this competition through atomistic modeling of ordered graphene hexagonal honeycombs. We modify the adaptive intermolecular reactive empirical bond order (AIREBO) potential to account for the excess-charge-induced change in equilibrium bond length of the carbon atoms. We use the charge-strain results of single graphene ribbons obtained by density functional theory (DFT) calculations to calibrate the atomistic AIREBO potentials. We investigate the effect of edges stresses and excess charge on the dimensional and morphological changes of free-standing graphene ribbons. We study the effect of internal surface area and relative density on the actuation stroke and work density and compare the graphene nanofoams with several nanoporous gold systems. Our results reveal that the graphene nanofoams have a smaller actuation stroke per unit charge density than random nanoporous gold, but that more mechanical work can be generated.

Recent developments and simulations utilizing bond-order potentials

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.