A microcalorimetric study of the heat of adsorption of copper on well-defined oxide thin film surfaces: MgO(100), p(2×1) oxide on Mo(100) and disordered W oxide

Abstract

The heats of adsorption as a function of coverage have been determined for copper adsorption onto several well-defined oxide thin film surfaces at room temperature by microcalorimetric measurements. The heats of adsorption are accurately determined as a function of coverage with resolution of 2% of a monolayer. For all three oxide surfaces investigated, MgO(100), a p(2×1) molybdenum oxide film on Mo(100) and disordered W oxide, the initial heat of copper adsorption is much lower than the heat of sublimation for Cu (337.4 kJ mol-1). On MgO(100) the initial Cu heat of adsorption in the first 2–4% of a monolayer is 240 kJ mol-1 and increases rapidly to the heat of Cu sublimation. Auger spectroscopy shows that Cu grows on MgO(100) as two-dimensional (2-D) islands until ≈0.3 monolayers where it switches to the growth of 3-D islands, at which point the heat of adsorption of Cu reaches ≈92% of its heat of sublimation. The room temperature sticking probability of Cu on MgO was also investigated as a function of coverage and determined to be >0.99. On the ordered p(2×1) oxide of molybdenum on Mo(100), the initial Cu heat of adsorption is 287 kJ mol-1. The heat of adsorption then decreases slightly to 278 kJ mol-1 in the first 15% of a monolayer, after which it rapidly increases to the heat of sublimation. Similarly, on the disorder W oxide surface the initial heat of Cu adsorption was 280 kJ mol-1 at 300 K. These results are compared to Pb adsorption on the same oxide thin films and are discussed in the context of important factors influencing metal island growth.