Platinum clusters on Pt(111) have been examined using the field ion microscope to provide atomistic information about their stability and mobility, which is required for understanding the macroscopic kinetics of crystal growth. From the temperature dependence of the diffusivity D, the diffusion barrier ED and the prefactor D0 have been ascertained for clusters made up of three to seven Pt atoms. The diffusion barrier rises monotonically with cluster size, unlike the behavior of iridium clusters on Ir(111). The prefactor D0 of the largest cluster, Pt7, is unusual, exceeding that of Pt6 by two orders of magnitude. Dissociation of Pt clusters has been examined separately. Activation energies for dissociation, deduced from the temperature at which clusters disappear, also rise monotonically with size. Even at low temperatures like 350 K, diffusion as well as dissociation are found to be significant for all clusters up to and including pentamers, and can affect the kinetics of growth.
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