By using field-ion microscopy, Ir clusters (2 to 13 atoms) on the close-packed (111) Ir plane were examined. The arrangement of Ir atoms in clusters, in relation to the binding sites for single atoms on Ir(111), was determined in detail. It was found that, in clusters, the atoms sat on nearest-neighbor sites. As the size of the cluster increased, the likelihood of finding cluster atoms at bulk sites (where the addition of atoms caused the normal face-centered cubic structure to advance) also increased. From tetramers to Ir13, compact arrangements which maximized the number of close neighbors were favored. Measurements of the temperature for the onset of dissociation of these clusters revealed an unusual dependence of cluster cohesion upon size. That is, the binding energy increased sharply from dimers to trimers, dropped for tetramers, and then rose again. In clusters which contained 5 or more atoms, the binding energy was relatively insensitive to size. The magnitude of the cohesive energy was also surprising in that the bond energy amounted to only about ¼ of that in a bulk crystal. Diffusion of the clusters over the surface was observed before dissociation, and the trends in the barrier to motion were similar to that for dissociation. However, for small clusters the activation energy rose more slowly than in the case of dissociation and, for larger clusters, it increased more rapidly. Quantitative determinations of the diffusion characteristics of dimers up to pentamers again revealed an anomalously low activation energy for tetramers, while the pre-factor was insensitive to cluster size. This indicated that the diffusion mechanism was the same, regardless of the number of cluster atoms. Detailed observations of cluster motion were made at low temperatures, in order to identify the individual atomic events involved in diffusion. From these, it appeared that diffusion occurred via a series of single atomic jumps. Although surface migration was a reasonably simple process, the cohesive properties of the clusters appeared to be quite sensitive to structure, and were markedly different to predictions which were based upon bulk properties or upon the behavior of clusters in the gas phase.

S.C.Wang, G.Ehrlich: Surface Science, 1990, 239[3], 301-32