Electronic charging of non-metallic clusters: size-selected MoxSy clusters supported on an ultrathin alumina film on NiAl(110)

Two photon photoemission was used to investigate the interfacial charge transfer for size-selected MoxSy (x/y: 2/6, 4/6, 6/8, 7/10) clusters deposited on an ultrathin alumina film prepared on a NiAl(110) surface. The local work function of the surface increases with increasing cluster coverage, which is unexpected for charge transfer resulting from the formation of Mo-O bonds between the clusters and the alumina surface. By analogy with Au atoms and clusters on metal-supported ultrathin oxide films, we invoke electron tunneling from the NiAl substrate to explain the charge transfer to the MoxSy clusters. Electron tunneling is favored by the large electron affinities of the MoxSy clusters and the relatively low work function induced by the presence of the alumina film. The interfacial dipole moments derived from coverage-dependent measurements are cluster dependent and reflect differences in MoxSy cluster structure and surface bonding. These results extend previous observations of electronic charging to non-metallic clusters, specifically, metal sulfides, and suggest a novel way to modify the electronic structure and reactivity of nanocatalysts for heterogeneous chemistry.