Time-dependent conductance of Pd-dosed SnO2 (110)

Time-dependent phenomena related to the reaction of ultrathin films of Pd d eposited on SnO2 (110) surfaces of differing oxygen content have been inves tigated. Conductance measurements and X-ray photoemission spectroscopy (XPS ) were used to characterize changes occurring on the surface over a range o f metal coverage. Effects on conductance included a rapid change immediatel y following a deposition and a much slower relaxation over a time scale of 1000 s. On the oxygen-rich surface, prepared by oxygen plasma treatment, on ly one relaxation process is observed for all coverages of Pd below 5 monol ayers-equivalent (MLE). On the nearly stoichiometric surface, prepared by t hermal oxidation, the conductance exhibits a conversion from one type of re laxation to another of opposite sign near 1 MLE coverage of Pd. XPS results indicated a slow oxidation of Pd. The results are interpreted using a mode l that includes oxidation of metal species, formation and charging of surfa ce states, and the effects of overlapping depletion regions that result fro m the Schottky barriers formed around each metal cluster. (C) 1998 Publishe d by Elsevier Science B.V. All rights reserved.