Cluster growth and core-electron binding energies in supported metal clusters

Examination of the coverage dependence of the core-electron photoemission intensity and binding energy for simple, noble, and transition-metal clusters supported on amorphous carbon reveals three distinct, cluster-size-dependent regimes. In very small clusters, containing fewer than {~}30 atoms, the final-state hole is screened by the polarization of the neighboring atoms, and the binding energy depends only weakly on cluster size. In larger, metallic clusters the hole is screened by conduction electrons, with the missing charge appearing at the surface of the cluster. In this regime, the core-electron binding-energy shifts vary inversely with the one-third power of the coverage, confirming that they are dominated by the Coulomb energy of the charged final-state cluster. In the third regime the clusters begin to coalesce, forming a contiguous metallic layer in which the binding energy rapidly approaches that of the bulk metal.