Electronic properties and charge state of gold monomers and chains adsorbed on alumina thin films on NiAl(110)

The adsorption of Au on a thin alumina film on NiAl(110) has been investigated with scanning tunneling microscopy (STM) and spectroscopy as well as density-functional theory (DFT). Gold monomers are identified as the main surface species at low coverage. According to the DFT, the adatoms are negatively charged and exhibit only a single occupied state in the alumina band gap. These results are compatible with the distinct bias-dependent contrast of Au monomers in the STM. With increasing exposure, Au chains of different length develop on the oxide surface. The adchains exhibit a number of maxima in conductance spectroscopy that are assigned with the help of DFT calculations to two sets of quantum well states with Au 6s-d(z2) and Au p character. By analyzing the node structure of the highest occupied states, the charge state of the adchains is determined. Chains containing three to seven Au atoms are found to host up to three excess electrons, in accordance with the charge-transfer model developed for Au adsorption on ultrathin oxide films.