Copper wetting of α-Al2O3(0001): theory and experiment

X-ray photoelectron spectroscopy (XPS) studies have been carried out on sputter deposited copper on a substantially hydroxylated α-Al2O3(0001) (sapphire) surface under ultra-high vacuum (UHV) conditions. XPS-derived Cu uptake curves show a sharp change in slope at a coverage of 0.35 ML (on a Cu/O atomic basis), indicative of initial layer-by-layer growth. Cu(LMM) lineshape data indicate that, prior to the first break in the curve, Cu is oxidized to Cu(I). At higher coverages, metallic Cu(0) is observed. These data agree with first principles theoretical calculations, indicating that the presence of adhydroxyl groups greatly enhances the binding of Cu to bulk sapphire surfaces, stabilizing Cu(I) adatoms over two-dimensional metallic islands. In the absence of hydroxylation, calculations indicate significantly weaker Cu binding to the bulk sapphire substrate and non-wetting. Calculations also predict that at Cu coverages above 1/3 ML, CuCu interactions predominate, leading to Cu(0) formation. These results are in excellent agreement with experiment. The ability of surface hydroxyl groups to enhance binding to alumina substrates suggests a reason for contradictory experimental results reported in the literature for Cu wetting of alumina.