Investigations of transition-metal clusters deposited on solid substrates by photoelectron spectroscopy, bremsstrahlung isochromat spectroscopy and related techniques
The 2p core-level binding energies of Cu and Ni clusters increase while the kinetic energies of the LMM Auger lines decrease with decreasing cluster size. The shifts in the core-level binding energy and the Auger kinetic energy relative to the bulk metal are largest in the case of clusters deposited on an insulating substrate such as Al2O3 and least on amorphous carbon; shifts on TiO2 are intermediate between those on Al2O3 and carbon. The increase in core-level binding energies with decreasing cluster size signifies a metal to non-metal transition. The occurrence of such a transition is borne out more vividly by BIS studies. Accordingly, BIS studies on Pd clusters deposited on amorphous carbon clearly show that electron states are progressively introduced near the Fermi level with increase in cluster size. The chemical reactivity of transition-metal clusters varies with the size. Thus, the relative proportion of the atomic oxygen species increases with decreasing size of Ag clusters.