Buried Pd slows self-diffusion on Cu(001)

Using low-energy electron microscopy, we determine that self-diffusion of the Cu(001) surface is slowed by the presence of a c(2×2)-Pd buried surface alloy. We probe surface diffusion using Cu-adatom island-ripening measurements. On alloyed surfaces, the island decay rate decreases monotonically as the Pd concentration is increased up to ∼0.5 monolayer (ML), where the 2×2 buried alloy is Pd saturated. We propose that the Pd slows island ripening by inhibiting the diffusion of surface vacancies across terraces. For dilute alloys (0.2-ML Pd), this conclusion is supported by density-functional theory calculations, which show that surface vacancies migrate more slowly owing to an attraction to isolated buried Pd atoms. The results illustrate a fundamental mechanism by which even a dilute alloy thin-film coating may act to inhibit surface-diffusion-mediated processes, such as electromigration.