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 (√3 × √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 × 2) unit cell including adatoms
and vacancies at room temperature need to be revisited.