Oxygen uptake by thin lead deposits on polycrystalline gold

Abstract

The oxidation of thin lead overlayers on a predominantly (111) polycrystalline gold surface at room temperature and oxygen pressure of 8×10 −6 Torr was investigated using Auger electron spectroscopy (AES). Oxidation was monitored using the oxide lead peak obtained by spectral decomposition. This peak experiences a shift from pure lead of 2.3±0.1 eV for deposits up to 3 monolayer-equivalents (ME) and 2.7±0.1 eV for deposits greater than about 4 ME. The oxygen signal for a 1 ME deposit at saturation exposure is about 20% of the value expected for a complete oxygen monolayer. This is explained in terms of clusters of about 5 Pb atoms for each oxygen atom. This differs from the PbO stoichiometry which has been observed under similar conditions on bulk lead and also claimed for the oxidation of thin lead deposits on Ag and Cu. Also contrary to results on Ag and Cu is the rate of oxygen uptake on Au which increases with lead coverage in the submonolayer region; this is explained by the absence of adsorption and surface diffusion of oxygen on gold, and the need for clusters of about 5 lead atoms for oxidation to occur. Deposition of more than 1 ME of lead results in lead-gold compound formation as observed by previous authors. Exposure of this compound to oxygen causes oxidation of the surface lead with inward migration of gold from the surface. The oxide layer is about 2 monolayers thick and greatly reduces the rate of further oxidation. The large decrease in the gold substrate signal on oxidation is a sensitive method for differentiating between compound formation and altermative growth modes for the bimetallic surface.