Ultrathin alumina film Al-sublattice structure, metal island nucleation at terrace point defects, and how hydroxylation affects wetting

Abstract

First principles density functional slab calculations have produced the following results: (1) for 5 Å (two O-layer) alumina films on Al(111) and Ru(0001), with larger unit cells than in recent work, the lowest energy stable film was found to have an even mix of tetrahedral (t) and octahedral (o) Al ions arranged in alternating zig-zag rows. This most closely resembles the κ-phase of bulk alumina, where this pattern results in a greater average lateral separation of Al-ions than with pure t or o. A second structure with an even mix was also found, consisting of alternating stripes. These patterns can exist in any of three equivalent directions on close packed substrates. (2) Because of numerical problems associated with the very large relaxations in alumina surfaces, MgO(100) was used to investigate metal island nucleation. Common point defects (vacancies, pairs of vacancies, and water by-products) were placed in supercells and their effects on Pt adatom and ad-dimer binding computed. Unexpectedly, single vacancies were found to destabilize metal dimers, and only the mixed (F_s-V_s) divacancy increases stability. Among the water-by-products, in-surface OH (produced by H⁺ reaction with O^2-) was uninteresting, but ad-OH was found to both increase adatom binding and significantly stabilize dimers on the surface, suggesting the latter defect nucleates metal islands even at elevated temperatures. We believe these results apply to all highly ionic oxides. (3) Finally, the effect of a substantial coverage of hydroxy on Cu deposition and growth on α-Al₂O₃(0001) was investigated. While Born–Haber calculations show wetting is not thermodynamically preferred on the clean surface, at experimentally relevant ad-OH coverages the strength of the Cu–oxide bond is more than doubled and wetting is strongly favored. This causes a very stable ≈1/3 monolayer (ML) coverage of Cu⁺¹, which then induces layer-by-layer Cu growth, in agreement with recent experiments. Hydroxy coverage can thus control deposited metal morphology across a wide spectrum.