MECHANISMS OF INITIAL ALLOY FORMATION FOR PD ON CU(100) STUDIED BY STM

The nucleation and growth of Pd alloys on Cu(100), up to the ordered c (2 X 2) structure have been studied using scanning tunneling microscop y. Atomically resolved images at low Pd coverages (0.20 ML) show protr usions, located in the lattice sites of the substrate, that are attrib uted 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 X 2) structure, with the squeezed-out Cu atoms nucleatin g 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 X 2) structure with second-layer growth init iated before a perfect c(2 x 2) structure is formed.