Metallic character of the Al2O3(0001)-(√31 x √31)R ± 9° surface reconstruction

The basal (0001) surface plane of alpha -Al2O3 has been extensively studied both by experimental techniques and applications of theory. Although this surface is not easily created upon cleavage, it grows in during aluminum oxidation and has been shown to be one of the lowest energy surfaces of alumina. Upon heating under vacuum, this (0001) surface of alpha -Al2O3 undergoes a series of reconstructions. The final structure obtained at similar to 1700 K is an air-stable termination known as the (root 31 x root 31)R +/- 9 degrees reconstruction. Alternatively, this reconstructed surface can also be obtained at lower temperatures by aluminum deposition on a clean Al2O3 (1 x 1) basal plane surface. Applying density functional theory within the generalized gradient approximation to a model surface, we show that the transformation from the (1 x 1) termination to ( root 31 x root 31)R +/- 9 degrees is accompanied by a dramatic change in the band gap associated with the ceramic's surface. Specifically, the (1 x 1) termination is insulating for both surface and deeper bulk-like atoms, while the (root 31 x root 31)R +/- 9 degrees termination results in a metallic surface aluminum coating that behaves very much like an aluminum (111) surface. Although some experimental evidence for the (root 31 x root 31) R +/- 9 degrees metallic surface states exists, it has generally been ignored, and both the (1 x 1) and (root 31 x root 31)R +/- 9 degrees terminations are commonly treated as insulating. The metallic surface character of the reconstructed basal plane of alpha -Al2O3 may present interesting implications for future applications of heat-treated alumina surfaces.