Surface structure of Pd(111) with less than half a monolayer of Zn

We have characterized the structural properties of submonolayer amounts of Zn on Pd(111) using scanning tunneling microscopy (STM) and spot-profile analysis low energy electron diffraction (SPA-LEED). Following room temperature deposition of E0.06 monolayers (ML) Zn onto Pd(111), we observe the substitution of Zn for Pd in the surface layer. At E0.20 ML of deposited Zn, STM reveals a locally ordered phase with a 2/sqrt3x2/sqrt3R30deg unit cell located near Zn substitutions; SPA-LEED patterns reveal the same periodicity. We attribute this phase to the metastable bonding of atoms or clusters predominantly in hollow sites surrounding Zn substitutions in the surface layer. At Therta=0.4 ML, STM images reveal local 2/sqrt3x2/sqrt3R30deg and (2x1) ordering on surfaces annealed to 350 K. At coverages near 0.5 ML, both STM and SPA-LEED show the onset of the formation of the (2x1) ordering associated with the Zn: Pd 1 : 1 alloy phase. At all coverages, the surface is dominated by island growth; the islands’ size and density is shown to depend critically on annealing at temperatures as low as 350 K. These results provide insight into the structural features of a Zn/Pd(111) coverage regime that has been much debated in recent years.