INVESTIGATIONS OF PD CLUSTERS BY THE COMBINED USE OF HREM, STM, HIGH-ENERGY SPECTROSCOPIES AND TUNNELING CONDUCTANCE MEASUREMENTS

Palladium clusters deposited on carbon substrates by vacuum evaporatio n have been examined by high resolution electron microscopy and scanni ng tunneling microscopy to obtain the size distributions of the cluste rs. Pd(3d) binding energy of clusters of known size distributions have been measured to establish the variation of the binding energy with t he cluster size. Clusters of a mean diameter of less than or similar t o 1 nm which show the highest Pd(3d) binding energies relative to the bulk metal also show a conductance gap in tunneling spectroscopic meas urements. The results show the occurrence of a size-dependent metal-to -nonmetal transition in Pd clusters. This conclusion is corroborated b y valence band and bremsstrahlung isochromat spectroscopic measurement s which show the emergence of new states closer to E(F) with an increa se in the cluster size. ,