Stoichiometric reconstruction of the Al2O3(0001) surface

Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. We imaged the basal (0001) plane of alpha-aluminum oxide (alpha-Al2O3) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex (root 31 x root 31)R +/- 9 degrees reconstruction. The lateral positions of the individual oxygen and aluminum surface atoms come directly from experiment; we determined with computational modeling how these connect to the underlying crystal bulk. Before the restructuring, the surface Al atoms assume an unfavorable, threefold planar coordination; the reconstruction allows a rehybridization with subsurface O that leads to a substantial energy gain. The reconstructed surface remains stoichiometric, Al2O3.