Abstract
Based on high-resolution noncontact atomic force microscopy (NC-AFM) experiments we reveal a detailed structural model of the polar (111) surface of the insulating ternary metal oxide, MgAl2O4 (spinel). NC-AFM images reveal a 6√3×6√3R30° superstructure on the surface consisting of patches with the original oxygen-terminated MgAl2O4(111) surface interrupted by oxygen-deficient areas. These observations are in accordance with previous theoretical studies, which predict that the polarity of the surface can be compensated by removal of a certain fraction of oxygen atoms. However, instead of isolated O vacancies, it is observed that O is removed in a distinct pattern of line vacancies reflected by the underlying lattice structure. Consequently, by the creation of triangular patches in a 6√3×6√3R30° superstructure, the polar-stabilization requirements are met.
Highlights
While the application of metal oxides in, e.g., catalysis, gas sensors, fuel cells, high-k dielectrics and corrosion protection has seen a very strong development, fundamental research on the surface properties of metal oxides has been a topic of growing interest [1]
A few step edges are resolved on the surface, forming 60° angles, which reflects the hexagonal symmetry of surface atoms on the (111) surface
Under the assumption that the surface becomes terminated by an oxygen layer when synthesized in an oxygen atmosphere, a structural model is proposed that complies with both the electrostatic stabilization criterion for this polar surface, which requires the removal of ~42% of the surface oxygen, and fits with the size and triangular symmetry of the observed 6√3×6√3R30° superstructure
Summary
While the application of metal oxides in, e.g., catalysis, gas sensors, fuel cells, high-k dielectrics and corrosion protection has seen a very strong development, fundamental research on the surface properties of metal oxides has been a topic of growing interest [1]. The NC-AFM data presented in this work reveal the MgAl2O4(111) surface to have a characteristic surface morphology consisting of triangular patches, the orientation and coverage of which are in agreement with the theoretical predictions for an oxygen-terminated surface with a certain percentage of the surface-layer atoms removed.
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