Atom Movement and Binding on Surface Clusters: Pt on Pt(111) Clusters

Binding and diffusion of single platinum atoms adsorbed on platinum clusters on Pt(111) have been explored using helium-cooled field ion microscopy. Pt atoms deposited on Pt(111) clusters at $T\sim 20$ K are found at both fcc and hcp sites. At hcp sites, atoms are more weakly bound; after annealing at temperatures as low as 45 K, they occupy fcc sites exclusively. The cluster surface itself is not uniform, however. An empty zone separates the central region from the cluster edges: Atoms in the center have to overcome an energy barrier just to cross this empty zone and reach the cluster edges.