Oxidation of Sn overlayers and the structure and stability of Sn oxide films on Pd(111)

The oxidation at 300 K of ultrathin Sn films has been studied by Auger and x-ray photoelectron spectroscopy. The process divides into two regimes: submonolayer Sn coverages are stable towards low (<10⁵L) oxygen exposures, becoming fully oxidized to stoichiometric SnO at higher exposures (~10¹¹L). Thicker Sn deposits oxidize more rapidly, and analysis of the Sn Auger parameter and substrate surface core-level shift indicates that oxidation is accompanied by relatively little change in the initial state charge density of the Sn atoms. This reflects strong preexisting Sn to Pd charge transfer and the low dimensionality of the oxidized Sn overlayers. Thermal stability of these films increases with thickness and they decompose by evolution of gaseous oxygen accompanied by Pd/Sn surface alloy formation. The strong Pd-Sn chemical bond exerts a controlling influence on both overlayer oxidation and oxide decomposition processes within the Sn/Pd(111) system.