The binding and diffusion of single Pt atoms that were adsorbed at Pt clusters on (111) surfaces were studied by means of He-cooled field ion microscopy. The Pt atoms which were deposited onto (111) clusters at about 20K were found at both face-centered cubic and hexagonal close-packed sites. The atoms were more weakly bound at hexagonal close-packed sites. After annealing at temperatures as low as 45K, they occupied only face-centered cubic sites. However, the cluster surface itself was not uniform. An empty zone separated the central region from the cluster edges. Atoms in the center had to overcome an energy barrier in order to cross this empty zone and reach the cluster edges.

A.Gölzhäuser, G.Ehrlich: Physical Review Letters, 1996, 77[7], 1334-7