Magnesium oxide (110) surface ordering

The study of the polarity of oxide surfaces has recently received a renewed interest. MgO, in particular, has become a model standard due to its rock-salt structure typical for ionically bonded solids. Its lowest energy neutral (nonpolar) termination in the (001) direction closely matches the bulk atomic structure. In contrast, the polar (111) termination was predicted to have a diverging bulk-like surface energy, and therefore be highly unstable. The MgO (110) surface, a non-polar but higher energy surface termination, provides an ideal middle point in examining the difference between the neutral and polar oxide surface regimes.; In this research the issue of faceting is more carefully examined, with observation upon each successive specimen preparation step. MgO (110) is examined with a host of electron and scanning probe microscopy methods, along with electron diffraction, while undergoing chemical etching and vacuum annealing. Atomic force microscopy (AFM) and tilt-resolved scanning electron microscopy (SEM) are used to resolve the morphology. Faceting is observed as a result of chemical etching, and is found to be composed of shallow, vicinal angles rather than the 45° MgO (001) planes. Different morphology was observed based upon chemical etchant and etching duration. The faceting was found to not be promoted or erased upon annealing to a temperature of 1000°C.; Upon high temperature annealing (1600°C), a MgO (110) c(2x2) reconstruction is found. Profile edges, which show different crystallography than the previous faceting, are compared for low index planes. The c(2x2) reconstruction spots are characterized and compared to a multislice atomic models, with a preference for Mg.; Still higher (1700°C) is found to erase the reconstruction, and promote another profile edge crystallography. At this higher temperature visual (110) striping and diffraction splitting is observed. Profile MgO (110) edges are observed to be composed of MgO (111) steps. The MgO (111) surface termination shows a different surface periodicity, which is consistent with previous reports of high temperature MgO (111) reconstructions stabilizing the surface.