Scanning tunnelling microscopy revealed that large 2-dimensional Ag clusters could diffuse on Ag(100). The room-temperature diffusivity was of the order of 10-17cm2/s, and varied little for clusters ranging from 100 to 720 atoms in size. This weak variation ruled out peripheral diffusion as the main mechanism of cluster diffusion. It instead suggested the operation of a 2-dimensional evaporation-condensation mechanism. This conclusion was compatible with the energetics of atomic-scale processes within the cluster and with the dissolution of small clusters which was observed at low coverages.

J.M.Wen, S.L.Chang, J.W.Burnett, J.W.Evans, P.A.Thiel: Physical Review Letters, 1994, 73[19], 2591-4