A Molecular Dynamics Study of the Role of Adatoms in SAMs of Methylthiolate on Au(111): A New Force Field Parameterized from Ab Initio Calculations

Starting from ab initio calculations and using a force matching procedure, we have developed a new force field for molecular dynamics simulations of self-assembled monolayers of methylthiolate (MT) on Au(111) surfaces. This new force field is able to reproduce several observed features of SAMs of MT on Au(111) surface, such as the formation of gold vacancy islands and the (root 3 x root 3)R30 lattice. We have studied the dynamics of Au adatoms and monatomic vacancies on the Au(111) surface for the SAM of MT at room temperature. It is observed that monatomic vacancies coarsen to form large vacancy islands while the adatoms group to form clusters. Both results are in agreement with experiments. At elevated temperatures, Au adatoms that are lifted from the surface leave an atomic vacancy on it. The liquid-like diffusion of gold adatoms on the SAM surface occurs by hopping between pairs of methylthiolate to which the adatom is temporarily bound. Our findings indicate that structural models of the c(4 x 2) unit cell including adatoms and vacancies at room temperature need to be revisited.