Boris Ni

A second-generation reactive empirical bond order (REBO) potential energy expression for hydrocarbons

A second-generation potential energy function for solid carbon and hydrocarbon molecules that is based on an empirical bond order formalism is presented. This potential allows for covalent bond breaking and forming with associated changes in atomic hybridization within a classical potential, producing a powerful method for modelling complex chemistry in large many-atom systems. This revised potential contains improved analytic functions and an extended database relative to an earlier version (Brenner D W 1990 Phys. Rev. B 42 9458). These lead to a significantly better description of bond energies, lengths, and force constants for hydrocarbon molecules, as well as elastic properties, interstitial defect energies, and surface energies for diamond.

Tribological properties of carbon nanotube bundles predicted from atomistic simulations

Abstract Classical molecular dynamics simulations are used to investigate the responses of bundles of single-walled carbon nanotubes to compressive and shear forces between two sliding diamond surfaces. The forces on the atoms in the simulations are determined using a many-body reactive empirical potential for hydrocarbons coupled to Lennard–Jones potentials. The simulations predict that the nanotubes can be subjected to high shear forces prior to wear because of their flexibility. The response to the applied shear forces is sliding of the bundle or a combination of sliding and rolling, where the exact responses depend on the orientation and bonding of the nanotube bundle to the sliding surfaces. No rolling of carbon nanotubes against other nanotubes in the bundle is predicted to occur in any of the orientations considered.

Effect of polyatomic ion structure on thin-film growth: Experiments and molecular dynamics simulations

The experiments described here examine 25–100 eV CF3+ and C3F5+ ion modification of a polystyrene (PS) surface, as analyzed by x-ray photoelectron spectroscopy. The molecular dynamics computer simulations probe the structurally and chemically similar reactions of 20–100 eV CH3+ and C3H5+ with PS. CF3+ and C3F5+ each form a distribution of different fluorocarbon (FC) functional groups on PS in amounts dependent upon the incident ion energy, structure, and fluence. Both ions deposit mostly intact upon the surface at 25 eV, although they also undergo some crosslinking upon deposition. Fragmentation of the two ions increases as the ion energies are increased to 50 eV. Both ions show increases in total fluorine and fluorinated carbon content when changing the ion energy from 25 to 50 eV. The simulations predict that CH3+ and C3H5+ behave in a similar fashion to their FC analogs, remaining mostly intact and either embedding or scattering from the surface without reacting at 20 eV. At 50 and 100 eV, the simulati...

Thermal conductance across grain boundaries in diamond from molecular dynamics simulation

We determine the dependence of the interfacial conductance on twist angle for (001) symmetric twist grain boundaries (GBs) in diamond. We find that the conductances are extremely large, ranging from 7.7to17.6GW∕m2K. Nevertheless, when normalized to the single-crystal conductivity, the resulting Kapitza lengths are actually longer in diamond than in Si, indicating that the diamond GBs are relatively worse conductors of heat. This result is consistent with the poorer bonding across the diamond grain boundaries. We find that the interfacial conductance and Kapitza length can be well fitted by an extended Read-Shockley model.

Study of angular influence of C3H5+ ion deposition on polystyrene surfaces using molecular dynamics simulations

The influence of incident angle on the interaction of polyatomic hydrocarbon ions (C3H5+) with polystyrene surfaces is examined using classical molecular dynamics simulations. The forces are determined using the reactive empirical bond order method developed by Tersoff and parametrized by Brenner. The total incident energy is 50 eV and the angles considered are 0° (normal to the surface), 15°, 45°, and 75°. At each angle, the outcomes of 80 trajectories are compiled and averaged. The results show that intact ions scatter from the surface in only 2% of the trajectories and that the ions dissociate in 61% of the trajectories at normal incidence. At 75°, intact ions scatter away in 56% and they dissociate in only 30% of the trajectories. The largest total amount of carbon is deposited at normal incident angles. However, more ions or ion fragments are predicted to remain near the surface (penetrate 3.5–5.5 A) at 45°. This is because ion fragments tend to penetrate more deeply (6–7 A) into the surface at small...

Comparison of growth of hydrocarbon thin films by molecular-beam and cluster-beam deposition: atomistic simulations

Molecular dynamics simulations have been performed to study the difference in the growth of polyethylene thin films via neutral ethylene molecular-beam and cluster-beam depositions. The collisions occurred at incident energies of 25 and 50 eV/molecule on a hydrogen-terminated diamond (111) surface. A many-body empirical bond order potential for hydrocarbons was used to model the interatomic interactions in the system that allowed for bonds to be broken and formed over the course of the simulations. In contrast to the expectations that the cluster beam would provide significantly enhanced thin-film nucleation, the results indicate few differences in thin-film adhesion that occurs as a result of the deposition. The role of substrate temperature on thin-film nucleation is also investigated and found to be negligible over a 1200° range. Finally, structural relaxation of thin-film fragments is investigated through energy minimization with AM1. The structures of the fragments are found to change slightly from the relaxed structures predicted in the molecular dynamics simulations.

Chemical functionalization and modification of carbon nanotubes through ion bombardment

Classical molecular dynamics simulations have been performed to investigate the chemical functionalization of single-walled (SWNT) and double-walled carbon nanotubes (DWNT) through CH3+ ion bombardment at 10, 45, and 80 eV. The simulations show that the process is highly efficient and that chemical functionalization occurs at every incident ion energy considered. However, significant differences in the response of the SWNTs and DWNTs are predicted from the simulations. At 45 and 80 eV defect formation and cross-linking between nearby nanotubes occurs. These new defect structures could substantially alter the mechanical and electrical properties of nanotubes