Nano-faceted stabilization of polar-oxide thin films: The case of MgO(111) and NiO(111) surfaces

Molecular beam epitaxy growth of polar MgO(111) and NiO(111) films demonstrates that surface stabilization of the films is achieved via the formation of neutral nano-faceted surfaces. First-principles modeling of the growth of polar MgO(111) films reveals that the growth does not proceed layer-by-layer. Instead, the Mg or O layers grow up to a critical sub-monolayer coverage, beyond which the growth of the next layer becomes energetically favorable. This non-layer-by-layer growth is accompanied by complex relaxations of atoms both at the surface and in the sub-surface, and leads to the experimentally observed surface nano-faceting of MgO and NiO (111) films through formation of neutral nano-pyramids terminated by {100} facets. These facets are limited in size by an asymptotical surface energy relation to their height; with the reconstruction being much more stable than previously reported surface terminations across a wide range of growth conditions. The termination offers access to lower coordinated atoms at the intersection of the neutral {100} planes whilst also increasing the surface area of the film. The unique electronic structures of these surfaces can be utilized in catalysis, as well for forming unique heterostructures for electronic and spintronic applications.