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 ligand s, have been studied using Au-197 Mossbauer spectroscopy. Emission Mos sbauer measurements were also performed on the larger Pt-309 cluster, in which Au-197 was produced by neutron activation. There is a large i nfluence of the ligands on the charge densities at the surface atoms o f 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 m etal, and it depends also slightly on the type of ligands on it. For t he Pt-309 compound, the inner core atoms have the same charge density at the nucleus as in the corresponding bulk metal. We, therefore, conc lude that the inner core of the Pt-309 cluster is metallic with respec t to the electronic charge densities, while this is not yet the case f or the Au-55 cluster.