Stabilization mechanisms of polar surfaces: ZnO surfaces

First-principles calculations reveal two mechanisms that compete to determine the structure of ZnO polar surfaces. One is the electron-counting rule, which favors semiconducting surfaces. The other is the large ZnO cohesive energy, which favors unreconstructed metallic surfaces with 1×1 periodicity. Their close competition results in crossovers in the preferred surface structure as the oxygen chemical potential is varied, consistent with a variety of surface morphologies observed under different experimental conditions.