Ripening of monolayer vacancy pits on metal surfaces: Pathways, energetics, and size-scaling for Ag(111) versus Ag(100)

Scanning tunneling microscopy studies have revealed that monolayer-deep vacancy pits typically coarsen at 300 K via Smoluchowski ripening (SR) on Ag(111) surfaces and via Ostwald ripening (OR) on Ag(100) surfaces. We elucidate the underlying atomistic processes, the relevant energetics with some input from density functional theory analysis, and also the scaling of the ripening rate with mean pit size. Size scaling for SR reflects the size dependence of the pit diffusion coefficient, so we also discuss observed deviations from classical theories. SR dominates OR for pits on Ag(111) primarily due to its significantly lower effective energy barrier. However, the effective barrier for OR is not lower than that for SR for pits on Ag(100), and one must also account for the distinct size scaling of these pathways to explain the dominance of OR. We also briefly discuss the dependence on temperature of the dominant ripening pathway and the ripening behavior for adatom islands.