DIFFUSION OF LARGE 2-DIMENSIONAL AG CLUSTERS ON AG(1OO)

Scanning tunneling microscopy shows that large two-dimensional Ag clus ters on Ag(100) can diffuse. The value of the diffusion coefficient at room temperature is of order 10(-17) cm(2) s(-1) and varies little, i f at all, with cluster size in the range studied, 100 to 720 atoms per cluster. This weak variation rules out periphery diffusion as the mai n mechanism of cluster diffusion, suggesting instead two-dimensional e vaporation-condensation. This conclusion is compatible with the energe tics of atomic-scale events within the cluster and with the dissolutio n of small clusters observed at low coverages.