METALLIC BEHAVIOR IN A PT(309) CLUSTER REVEALED BY AU-197 MOSSBAUER-SPECTROSCOPY

SMALL Metal clusters, containing of the order of 10 to 10,000 atoms, a re of interest both for exploring the fundamental question of how atom ic-scale properties develop into bulk properties and because such clus ters are expected to have interesting optical, electronic and magnetic properties1. The expectation is that bulk-like metallic behaviour wil l become increasingly evident as the cluster size increases, but at wh at stage bulk properties appear is not yet clear. Here we present resu lts of a study of the platinum cluster compound Pt309(Phen)36O30+/-10 (refs 2-4), in which we use Mossbauer spectroscopy to investigate the bonding environment of the Pt atoms. Because Pt lacks a good Mossbaue r isotope, we transform a fraction of the Pt-196 atoms in the clusters into Mossbauer nuclei Au-197 by neutron irradiation. The resulting sp ectra show that the inner (147-atom) core of the Pt cluster exhibits a metallic character like that in the bulk metal, whereas the surface a toms are not bulk-like. These bulk-like metallic properties may be acq uired by metal-cluster cores as small as about 150 atoms.