Self-diffusion on low-index metallic surfaces: Ag and Au(100) and (111)

Using molecular-dynamics simulations and the embedded-atom method, we study the homodiffusion of single adatoms on flat Ag and Au (100) and (111) surfaces. Our results for the (111) surfaces indicate that when the thermal energies of the atoms become larger than the energy barriers, diffusion can no longer be represented by a simple random walk since correlations between successive jumps become important. We present a simple model that takes into account these correlated jumps and reproduces the molecular dynamics data very well. We also demonstrate that knowledge of the energy barriers is not sufficient to determine the preferred mechanism for diffusion on the (100) surface, since the prefactors for the various mechanisms can vary significantly from the value that is usually assumed. The ability of a simple transition-state theory to describe diffusion is also rested. We find, in the cases considered here, that the static barrier is equivalent to the dynamical activation energy and that the prefactor is also well described as long as the relaxation of the substrate remains small.