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 evaporation have been examined by high resolution electron microscopy and scanning tunneling microscopy to obtain the size distributions of the clusters. Pd(3d) binding energy of clusters of known size distributions have been measured to establish the variation of the binding energy with the cluster size. Clusters of a mean diameter of ≲ 1 nm which show the highest Pd(3d) binding energies relative to the bulk metal also show a conductance gap in tunneling spectroscopic measurements. The results show the occurrence of a size-dependent metal-to-nonmetal transition in Pd clusters. This conclusion is corroborated by valence band and bremsstrahlung isochromat spectroscopic measurements which show the emergence of new states closer to E F with an increase in the cluster size.