Gold adsorption on CeO₂ thin films grown on Ru(0001)

The adsorption of gold has been investigated on crystalline and defect-poor CeO₂(111) films by means of low-temperature scanning tunneling microscopy and spectroscopy. At low coverage, individual Au atoms are observed that homogenously distribute on the ceria surface and show no preference for binding at step edges. With increasing exposure, characteristic Au aggregates become visible, such as upright Au dimers and bilayer and trilayer pyramids. The ultrasmall clusters exhibit pronounced fluxionality; i.e., they easily modify their internal shape and binding position during the scanning process. This observation suggests the presence of various iso-energetic Au isomers on the surface and points to a relatively weak metal-oxide interaction. At higher Au exposure, tall 3D particles develop on the ceria surface. Conductance spectroscopy on these deposits reveals a set of unoccupied states localized in the energy region of the Au 6p levels. Neither the topographic nor the spectroscopic data indicate a charging of Au species on the defect-poor CeO ₂(111) surface, suggesting that Au mainly binds in the neutral charge state.