An ab initio study of energetic stability and electronic confinement for different structural phases of ZnO nanowires

We performed an ab initio total energy investigation of hexagonal (wurtzite and graphitic) and zinc blende ZnO nanowires (NWs) aligned along the [0001] and [111] directions, respectively, as a function of the NW diameter. We have considered unpassivated and (hydrogen) passivated NW surfaces. For the unpassivated system, we find that the wurtzite phase represents the energetically most favorable configuration. The width of the energy bandgap of wurtzite ZnO NWs increases by reducing the NW diameter, which is in accordance with the one-dimensional confinement effect. In contrast, this property fails in the zinc blende and graphitic NWs. In the former it is due to the high density of surface states within the fundamental bandgap, while in the latter system the energy bandgap becomes indirect and increases slowly by reducing the NW diameter. Our total energy results indicate that the hydrogen-passivated ZnO NWs are more stable than the unpassivated ones. For thin hydrogen-passivated NWs, we find that the graphitic phase becomes more stable than the wurtzite. For NW diameters around 2 nm, the graphitic and wurtzite phases present similar formation energies, while for larger diameters the wurtzite NWs become energetically more favorable. Finally, comparing the behavior and the positions of the valence and conduction band edges for the unpassivated ZnO NWs, we proposed the formation of type II band alignment for a hypothetical wurtzite/graphitic NW heterojunction.