Cu interactions with alpha-Al2O3(0001): effects of surface hydroxyl groupsversus dehydroxylation by Ar-ion sputtering

X-ray photoelectron spectroscopy studies and first principles calculations compare Cu adsorption on heavily hydroxylated alpha -Al2O3(0001) with dehyd roxylated surfaces produced by Ar+ sputtering followed by annealing in O-2. Annealing a cleaned sapphire sample with an O-2 partial pressure of simila r to 5 x 10(-6) Torr removes most contaminants, but leaves a surface with s imilar to0.4 ML (ML: monolayer) carbon and similar to0.4 ML OH. Subsequent light (6 min) Ar-ion sputtering at 1 keV reduces the carbon to undetectable levels but does not dehydroxylate the surface. Further sputtering at highe r Ar-ion excitation energies (>2 keV) partially dehydroxylates the surface, whereas 5 keV Ar-ion sputtering creates oxygen vacancies in the surface re gion. Further annealing in O-2 repairs the oxygen vacancies in the top laye rs, but those beneath the surface remain. Deposition of Cu on the hydroxyla ted surface at 300 K results in a maximum copper(I) coverage of similar to0 .35 ML, in agreement with theoretical predictions. Maximum copper(I) covera ge at 300 K decreases with decreasing surface hydroxylation. Exposure of a partially dehydroxylated sapphire(0001) surface to either 2 Torr H2O vapor or air results in recovery of surface hydroxylation, which in turn increase s the maximum copper(I) coverage. These results demonstrate that the hydrox yl surface coverage critically affects the ability of Cu to 'wet' sapphire( 0001) at 300 K. In addition, first principles density functional calculatio ns show that while an ad-OH species stabilizes copper(I) at room temperatur e, any nearby in-surface OH does the opposite. (C) 2000 Elsevier Science B. V. All rights reserved.