Mechanisms of initial alloy formation for Pd on Cu(100) studied by STM.

Abstract

The nucleation and growth of Pd alloys on Cu(100), up to the ordered c(2\ifmmode\times\else\texttimes\fi{}2) structure have been studied using scanning tunneling microscopy. Atomically resolved images at low Pd coverages (0.20 ML) show protrusions, located in the lattice sites of the substrate, that are attributed to individual Pd atoms. A pair- correlation plot shows that there is a strong tendency for them to be located in second-nearest-neighbor positions, resulting in the formation of short chains along the [001] and [010] directions. As the coverage increases, the chains converge to form a c(2\ifmmode\times\else\texttimes\fi{}2) structure, with the squeezed-out Cu atoms nucleating into Cu islands. The Cu islands also form over the alloyed regions, with the result that more than the previously reported 0.5 ML of Pd is required to form the c(2\ifmmode\times\else\texttimes\fi{}2) structure with second-layer growth initiated before a perfect c(2\ifmmode\times\else\texttimes\fi{}2) structure is formed.