Stoichiometric reconstruction of the Al$_{2}$O$_{3}$(0001) surface

TL;DR

This study combines atomic force microscopy and computational modeling to reveal the detailed atomic structure of the Al₂O₃(0001) surface, showing a complex reconstruction that maintains stoichiometry.

Contribution

It provides the first direct atomic-scale imaging of the Al₂O₃(0001) surface and links experimental data with modeling to elucidate its reconstruction.

Findings

Reconstructed surface exhibits a complex (√31×√31)R±9° pattern.

Surface Al atoms rehybridize with subsurface O, stabilizing the structure.

The surface remains stoichiometric Al₂O₃ after reconstruction.

Abstract

Macroscopic properties of materials stem from fundamental atomic-scale details, yet for insulators, resolving surface structures remains a challenge. The basal (0001) plane of ${\alpha}$-Al$_{2}$O$_{3}$ was imaged with noncontact atomic force microscopy with an atomically-defined tip apex. The surface forms a complex $({\sqrt31} {\times} {\sqrt31})R{\pm}9{\deg}$ reconstruction. The lateral positions of the individual O and Al surface atoms come directly from experiment; how these connect to the underlying crystal bulk was determined based on computational modeling. 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$_{3}$.