Palladium diffusion and cluster nucleation on Ta(110).

Abstract

The surface diffusion and initial stages of cluster nucleation of Pd adatoms on Ta(110) are investigated by field ion microscopy (FIM). The results indicate that Pd atoms become mobile on Ta(110) at 180 K and diffuse with an activation energy of 0.49\ifmmode\pm\else\textpm\fi{}0.02 eV. The structure of small Pd clusters on the Ta(110) plane depends upon the number of Pd adatoms present on the plane. If fewer than nine Pd atoms are deposited on the plane, the stable configuration is a linear chain oriented along a close-packed direction of the substrate. Two-dimensional island clusters containing fewer than nine atoms are metastable. For nine or more adatoms, however, the stable configuration is a two-dimensional island cluster and the linear chain cluster is metastable. Field ion images of the two-dimensional island clusters indicate that the Pd overlayer is pseudomorphic up to, and including, approximate monolayer coverages. Two-dimensional Pd clusters migrate as a unit on Ta(110) at temperatures from \ensuremath{\sim}250 to 325 K. Above \ensuremath{\sim}350 K the clusters rapidly disappear from the plane. The results of this investigation are compared to other FIM data related to the nucleation of metal-adatom clusters.