Structure, stability, and surface diffusion of clusters: Ir x on Ir(111)

Abstract

The properties of clusters adsorbed on a crystal made of the same atoms are of interest in understanding crystal growth. Using the field ion microscope, we have therefore examined iridium clusters containing from 2 to 13 atoms held on the close-packed iridium (111) plane. The arrangement of iridium atoms in the clusters in relation to the binding sites for single atoms on Ir(111) has been determined in detailed mapping experiments. In all the clusters examined, atoms sit in nearest-neighbor sites. As the size of the cluster is increased, the likelihood of finding cluster atoms at bulk sites, at which addition of atoms propagates the normal fcc structure, also increases. From tetramers to Ir 13, compact arrangements maximizing the number of close neighbors are favored. Measurements of the temperature for the onset of dissociation of these clusters reveal an unusual dependence of cluster cohesion upon size: the binding energy increases sharply from dimers to trimers, drops for tetramers, and then rises again. For clusters containing 5 or more atoms, the binding energy is relatively insensitive to size. The magnitude of the cohesive energy is also surprising; the bond energy amounts to only ∼ 1 4 that in a bulk crystal. Diffusion of the clusters over the surface is observed prior to dissociation, and the trends in the barrier to motion are similar to those in dissociation. However, for small clusters the activation energy rises more slowly than in dissociation, and for larger clusters it increases more rapidly. Quantitative determinations of the diffusion characteristics for dimers through pentamers again reveal an anomalously low activation energy for tetramers, but the prefactor is insensitive to cluster size, indicating that the mechanism of diffusion is the same independent of the number of cluster atoms. Detailed observations of cluster motion have been made at low temperatures, to identify the individual atomic events in diffusion. From these it appears that diffusion occurs by a series of single atom jumps. Although surface migration is found to be a reasonably simple process, the cohesive properties of these clusters appear quite sensitive to structure, and are markedly different from expectations based either on bulk properties, or on the behavior of clusters in the gas phase.