Mixed resolution model for C60 cluster bombardment of solid benzene

Molecular dynamics simulations of C₆₀ cluster bombardment have been instrumental in elucidating physical phenomena related to the sputtering process; however, chemical phenomena can also play an important role in C₆₀ cluster bombardment of molecular solids. Therefore, a mixed resolution model of C₆₀ cluster bombardment is being developed, where the reactive zone is represented by an all atom region, and the remaining part of the target is described by a coarse-grained representation. A reactive many body potential describes the interactions among atoms; whereas, pair potentials describe the interactions between coarse-grained beads and between coarse-grained beads and atoms. Solid benzene is used to develop the methodology of blending the potentials. The blending of potentials is evaluated by the differences in the velocities of the pressure waves (generated by the C₆₀ impact) between the all atom benzene, coarse-grained benzene and the mixed resolution benzene systems. Initial testing with 1 keV C₆₀ cluster bombardment simulations show a smooth transition between regions.     

A statistical analysis of the lateral displacement of Si atoms in molecular dynamics simulations of successive bombardment with 20-keV C60 projectiles

An important factor that determines the possible lateral resolution in sputter depth profiling experiments is ion induced lateral displacement of substrate atoms. Molecular dynamics (MD) simulations are performed to model the successive bombardment of Si with 20 keV C₆₀ at normal incidence. A statistical analysis of the lateral displacement of atoms that originate from the topmost layer is presented and discussed. From these results, it is determined that the motion is isotropic and can be described mathematically by a simple diffusion equation. A “diffusion coefficient” for lateral displacement is determined to be 3.5 Å²/impact. This value can be used to calculate the average lateral distance moved as a function of the number of impacts. The maximum distance an atom may move is limited by the time that it remains on the surface before it is sputtered. After 800 impacts, 99% of atoms from the topmost layer have been removed, and the average distance moved by these atoms is predicted to be 100 Å. Although the behavior can be described mathematically by the diffusion equation, the behavior of the atoms is different than what is thought of as normal diffusion. Atoms are displaced a large distance due to infrequent large hops.