Kinetic Monte Carlo simulation of vacancy and adatom diffusion in Ag(111) with a monolayer C60 adsorbate.

摘要

Recently, clean Ag(111) surfaces with monolayer C60 adsorbates have been studied with scanning tunneling microscopy and low energy electron diffraction. These studies revealed that the C60 molecules form a commensurate (2√3 x 2√3)R30 °phase on the Ag(111) substrate and when observed with STM, the C60 molecules appear either "bright" or "dim." LEED studies showed that these two species of C60 are a result of the C60 taking two different orientations on the Ag substrate, one of which only occurs when the C60 is located over an Ag lattice vacancy. STM also shows the bright and dim C60 molecules change location over time. This dynamic "flipping" behavior implies that vacancy diffusion in the Ag lattice is taking place. Here, using the kinetic Monte Carlo algorithm, I model the diffusion of vacancies in the Ag lattice. Additionally, more complex simulations involving creation/destruction of vacancies/adatoms, spatial C 60 flip correlation, adatom diffusion, adatom interactions with vacancies, and "superbright" C60 molecules are also presented and studied. Datasets collected from these simulations are compared to experimental data on the flipping rate of the C60 molecules vs. temperature, the bright/dim C60 ratio vs. temperature, and the "superbright" flipping rate of the C60 molecules vs. temperature. Additionally, the system of C60 on Au(111) is studied using the models developed in this dissertation. For vacancy diffusion, it is concluded that the number of vacancies in the lattice is not fixed as the temperature of the lattice changes. The source/sink of vacancies is shown to most likely be the domain boundaries of the system, as models allowing this type of diffusion approximate the experimental results and show spatially correlated flipping. A strong equilibrium number of vacancies forms in the lattice when vacancy diffusion through domain boundaries is allowed. Bulk vacancy diffusion is shown to be highly unlikely. Spatial flipping correlation is studied closely and it is concluded that it is directly related to vacancy number. Adatoms are also included in the simulation and allowed to diffuse in the hollow sites of the surface lattice. These serve to increase the flipping correlation and decrease the flipping rate. It is shown that adatoms forming a triangular pedestal beneath C60 is likely the cause of the superbright C60. Finally, this model is shown to also accurately simulate the system of C 60 on Au(111).