Communication: The reason why +c ZnO surface is less stable than -c ZnO surface: First-principles calculation

It has been experimentally shown that an O(-c)-polar ZnO surface is more stable than a Zn(+c)-polar surface in H(2) ambient. We applied first-principles calculations to investigating the polarity dependence on the stability at the electronic level. The calculations revealed that the -c surface terminated with H atom was stable maintaining a wurtzite structure, whereas the +c surface was unstable due to the change of coordination numbers of Zn at the topmost surface from four ( wurtzite) to six ( rock salt). This causes the generation of O(2) molecules, resulting in instability at the +c surface. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3675680]