Kinetic Monte-Carlo Simulation of Substrate Vacancy Diffusion in C$_{60}$ on Ag(111)

Recently, clean Ag(111) surfaces with monolayer C$_{60}$ adsorbates have been studied with scanning tunneling microscopy and low energy electron diffraction. These studies revealed that the C$_{60}$ forms a commensurate $(2\sqrt{3}\times2\sqrt{3})R30^\circ$ phase on the Ag(111) substrate and when observed with STM, the C$_{60}$ molecules appear either ``bright'' or ``dim.'' LEED studies showed that these two species of C$_{60}$ are a result of the C$_{60}$ taking two different orientations on the Ag substrate, one of which only occurs when the C$_{60}$ is located over an Ag lattice vacancy. STM also shows the bright and dim C$_{60}$ molecules change location over time. This ``flipping'' behavior implies that vacancy diffusion in the Ag lattice is taking place. Here, using the kinetic Monte-Carlo algorithm, we model the diffusion of vacancies in the Ag lattice. Data collected from simulations is compared to experimental data on the flipping rate of the C$_{60}$ vs. temperature and the bright/dim C$_{60}$ ratio vs. temperature. Our model tells us that intralayer vacancy diffusion is taking place and that adsorption of C$_{60}$ on Ag(111) results in vacancy creation in the Ag(111) surface. Additional density functional theory calculations support the conclusions of the model.