Direct Observation of Atomic Step-Assisted Stabilization of Polar Surface.

Polar surface is intrinsically unstable and thus highly reactive due to the uncompensated surface charges. The charge compensation is accompanied by various surface reconstructions, establishing novel functionality for its applications. Our in-situ atomic-scale electron microscopy study directly shows that the atomic step and step-assisted reconstruction play central roles in the charge compensation of polar oxide surface. The flat (LaO)+-terminated LaAlO3 (001) polar surface, when annealed at high temperature in vacuum, transits to the (015) vicinal surface via the dynamic motion and interaction of atomic steps. While the (015) vicinal surface possesses zero polarization along the surface normal, the thermodynamic ground state is achieved when the in-plane polarization is fully compensated via the reconstruction of step edge atoms; the step edge La atoms are displaced from their ordinary atomic sites toward the adjacent Al step edge site, resulting in the formation of negatively charged La vacancy at the corresponding step edges. As confirmed by first-principles calculations, the observed step reconstruction of (015) vicinal surface can completely cancel both out-of-plane and in-plane electric fields. This hitherto unknown mechanism reveals the central role of step reconstruction in stabilizing polar surfaces, providing valuable insights for understanding the novel charge compensation mechanism accompanied by the step reconstruction. This article is protected by copyright. All rights reserved.