Stability of the Surface Electron Accumulation Layers on the Nonpolar (10(1)over-bar0) and (11(2)over-bar0) Faces of ZnO

The stability of the hydroxyl termination and downward band bending on the m-plane (10 (1) over bar0) and a-plane (11 (2) over bar) faces of ZnO single crystals was investigated using synchrotron and real-time X-ray photoelectron spectroscopy. On these nonpolar surfaces, a strong correlation was found between the surface band bending and the surface OH coverage, both of which could be modified via heat treatment in ultra high vacuum (UHV). On the m-plane (10 (1) over bar0) face, in particular, a threshold temperature of similar to 400 degrees C was observed, after which there was a sudden increase in OH desorption and upward movement of the near-surface bands, resulting in a metallic-to-semiconductor transition in the electronic nature of the surface, with a change from surface electron accumulation to depletion. This loss of surface metallicity is associated with the disruption of a stable monolayer of chemisorbed hydroxyl groups that form a closed hydrogen-bonded network, across rows of ZnO dimers, on the m-plane (10 (1) over bar0) face. The surface electron accumulation layers on both the m-plane (10 (1) over bar0) and a-plane (11 (2) over bar) faces can be modified and eventually removed by simple UHV heat treatment, with important implications for the electrical properties of ZnO nanostructures and catalytic ZnO nanopowders, which contain a high proportion of these nonpolar surfaces.