Size-evolution towards metallic behavior in nano-sized gold and platinum clusters as revealed by Au-197 Mossbauer spectroscopy

Four types of molecular Au-55 clusters, with different types of ligands, have been studied using Au-197 Mossbauer spectroscopy. Emission Mossbauer measurements were also performed on the larger Pt-309 cluster, in which Au-197 was produced by neutron activation. There is a large influence of the ligands on the charge densities at the surface atoms of the metal atom cluster cores. For the Au-55 clusters, the inner core atoms have a different char ge density at the nucleus than the bulk metal, and it depends also slightly on the type of ligands on it. For the Pt-309 compound, the inner core atoms have the same charge density at the nucleus as in the corresponding bulk metal. We, therefore, conclude that the inner core of the Pt-309 cluster is metallic with respect to the electronic charge densities, while this is not yet the case for the Au-55 cluster.