Spontaneous Oxidation of Mg Atoms at Defect Sites in an MgO Surface

Scanning tunnelling microscopy and X-ray photoelectron spectroscopy have been used to study the adsorption of Mg on 12 ML thick MgO films grown either on Ag(001) or Mo(001) supports. The initially arriving Mg atoms spontaneously oxidize on the MgO surface, whereas metallic aggregates only form at higher exposure. The total amount of cationic Mg is considerably larger on the Mo- compared to the Ag-supported oxide films. The charge transfer is suggested to be induced by Mg attachment to MgO line defects with high electron-trapping potential. The density of those defect lines and hence the oxidation power of the film is governed by the lattice mismatch with the underlying metal support.