Growth and electronic properties of Au nanoclusters on thin-film Al2O3/NiAl(1 0 0) studied by scanning tunnelling microscopy and photoelectron spectroscopy with synchrotron radiation

With scanning tunnelling microscopy and synchrotron radiation photoelectron spectroscopy, we studied Au nanoclusters, grown by vapour deposition on ordered Al2O3 thin-film on NiAl(1 0 0), as functions of coverage and substrate temperature. The microscope images reveal that two-dimensional (2D) clusters form preferentially at small coverages (<0.1 ML); with greater coverage both diameter and height of the clusters increase evidently. Unlike other metal clusters, Au clusters formed at elevated substrate temperatures exhibit bimodal size distributions comprising smaller quasi-2D (with height of 1–2 atomic layers) and larger three-dimensional clusters. Photoelectron spectra indicate that the electronic properties of clusters vary with their size. At a mean diameter 2.3 nm and height 0.6 nm the Au 4f core-levels begin shifting to greater binding energies; with decreasing size the shift increases about 1.4 eV from bulk value, corresponding to 2D clusters. The quasi-2D clusters show a non-metallic electronic property, indicated by a positive shift of Au 4f core-level energies and by the I–V curves from measurements of scanning tunnelling spectra.