Electronic charging of non-metallic clusters: Size-selected Mo _xS _y clusters supported on an ultrathin alumina film on NiAl(110)

Two photon photoemission was used to investigate the interfacial charge transfer for size-selected Mo _xS _y (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 Mo _xS _y clusters. Electron tunneling is favored by the large electron affinities of the Mo _xS _y 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 Mo _xS _y 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.