Structures and electrical properties of pure and vacancy-included ZnO NWs of different sizes

The structures and electronic properties of ZnO nanowires (NWs) of different diameters are investigated by employing the first-principles density functional theory. The results indicate that the oxygen vacancy (V_O) exerts a more evident influence on the band gap of the ZnO NWs. However, the effect will be weakened with the increase of the diameter. In addition, the energy band shifts downward due to the existence of V_O and the offset decreases with the reduction of the V_O concentration. As the concentration of surface Zn atoms decreases, the conduction band shifts downward, while 2p electrons are lost in the oxygen vacancy, resulting in the split of valence band and the formation of an impurity level. Our findings agree well with the previous observations and will be of great importance for theoretical research based on ZnO NWs.