Ab initio study of step formation and self-diffusion on Ag(100)

TL;DR

This study uses density functional theory to analyze step formation energies and self-diffusion mechanisms on Ag(100), explaining smooth growth behavior and providing insights applicable to other fcc metal surfaces.

Contribution

It offers the first ab initio calculations of step stability and diffusion barriers on Ag(100), clarifying the atomic processes involved in surface growth.

Findings

{111}-faceted steps are more stable than {110}-faceted steps.

Adatoms diffuse mainly by hopping on flat surfaces.

No significant step-edge barrier to descent was found.

Abstract

Using the plane wave pseudopotential method we performed density functional theory calculations on the stability of steps and self-diffusion processes on Ag(100). Our calculated step formation energies show that the {111}-faceted step is more stable than the {110}-faceted step. In accordance with experimental observations we find that the equilibrium island shape should be octagonal very close to a square with predominately {111}-faceted steps. For the (100) surface of fcc metals atomic migration proceeds by a hopping or an exchange process. For Ag(100) we find that adatoms diffuse across flat surfaces preferentially by hopping. Adatoms approaching the close-packed {111}-faceted step edges descend from the upper terrace to the lower level by an atomic exchange with an energy barrier almost identical to the diffusion barrier on flat surface regions. Thus, within our numerical accuracy (approx +- 0.05 eV) there is no additional step-edge barrier to descent. This provides a natural explanation for the experimental observations of the smooth two-dimensional growth in homoepitaxy of Ag(100). Inspection of experimental results of other fcc crystal surfaces indicates that our result holds quite generally.