Growth mode and electronic structure of Au nano-clusters on NiO(001) and TiO2(110)

The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO2 were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(001)-1 x 1 and TiO2(110)-1 x 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 x 10(15) atoMS/cm(2):Au(1 1 1)). Initially two-dimensional (21)) islands with thickness of one Au-atom layer grow epitaxially on NiO(001) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the (I I I] direction. The critical size to form 3D-islands is estimated to be about 5 nm(2). The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 run, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO2 surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO2 due to an electron charge transfer from Au to TiO2. The work function of Au/NiO(001) gradually increases with increase in Au coverage from 4.4 eV (NiO(001)) to 5.36 eV (An(] 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO2(110) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO2 substrate. (c) 2006 Elsevier B.V. All rights reserved.