Stoichiometric reconstruction of the Al <sub>2</sub> O <sub>3</sub> (0001) surface
Johanna Hütner, Andrea Conti, David Kugler, Florian Mittendorfer, Georg Kresse, Michael Schmid, Ulrike Diebold, Jan Balajka
Abstract
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 α–aluminum oxide (α-Al 2 O 3 ) using noncontact atomic force microscopy with an atomically defined tip apex. The surface formed a complex ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msqrt> <mml:mrow> <mml:mn>31</mml:mn> </mml:mrow> </mml:msqrt> </mml:mrow> </mml:math> × <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msqrt> <mml:mrow> <mml:mn>31</mml:mn> </mml:mrow> </mml:msqrt> </mml:mrow> </mml:math> ) R ±9° 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, Al 2 O 3 .