Structure of the hydrogen-stabilized MgO(111)-(1x1) polar surface: Integrated experimental and theoretical studies

The surface structure of MgO(111)-(1x1) bulk and thinned single crystals have been investigated by transmission and reflection high-energy electron diffraction, low-energy electron diffraction (LEED), and x-ray photoelectron and Auger electron diffraction. The (1x1) polar surface periodicity is observed both after 800 degrees C annealing in air and also after oxygen plasma cleaning and annealing in ultrahigh vacuum. The x-ray photoelectron spectroscopy and diffraction results were analyzed by simulations based on path-reversed LEED theory and by first-principles calculations to help distinguish between different mechanisms for the stabilization of this extremely polar oxide surface: (1) stabilization by adsorption of a hydrogen monolayer; maintaining the insulating nature of the surface and (2) stabilization of the clean O or Mg terminated 1x1 surface by interlayer relaxations and two-dimensional surface metallization. The analysis favors stabilization by a single OH layer, where hydrogen sits on top of the O ions with O-H bond distance of 0.98 angstrom. The in-plane O and Mg positions fit regular rocksalt sites, the distance between the topmost O and Mg plane is 1.04 angstrom, contracted by similar to 14% with respect to bulk MgO distance of 1.21 angstrom, while the interlayer separation of the deeper layers is close to that of bulk, contracted by less than 1%. The presence of a monolayer of H associated with the terminal layer of oxygen reduces significantly the surface dipole and stabilizes the surface.