Growth mode and electronic structure of Au nano-clusters on NiO(0 0 1) and TiO 2(1 1 0)

Abstract

The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO 2 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(0 0 1)-1 × 1 and TiO 2(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25–0.35 ML/min (1 ML = 1.39 × 10 15 atoms/cm 2:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] 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 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO 2 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/TiO 2 due to an electron charge transfer from Au to TiO 2. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO 2(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO 2 substrate.