PNNL

Abstract

The surface structure of MgO(111)-(1x1) bulk and thinned single crystals have been investigated by transmission and reflection high-energy electron diffraction (THEED and RHEED), low energy electron diffraction (LEED), and x-ray photoelectron and Auger electron diffraction (XPD). The (1x1) polar surface periodicity is observed both after 800°C annealing in air and also after oxygen plasma cleaning and annealing in ultrahigh vacuum. The x-ray photoelectron spectroscopy and diffraction (XPS and XPD) 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 2-D 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 0.98Å. The in-plane O and Mg positions fit regular rock-salt sites, the distance between the topmost O and Mg plane is 1.04 Å, contracted by ~14% with respect to bulk MgO distance of 1.21 Å, 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.