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

Abstract

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 wurtzitephase represents the energetically most favorable configuration. The width of the energybandgap of wurtzite ZnO NWs increases by reducing the NW diameter, which is inaccordance with the one-dimensional confinement effect. In contrast, this property fails inthe zinc blende and graphitic NWs. In the former it is due to the high density of surfacestates within the fundamental bandgap, while in the latter system the energybandgap becomes indirect and increases slowly by reducing the NW diameter.Our total energy results indicate that the hydrogen-passivated ZnO NWs aremore stable than the unpassivated ones. For thin hydrogen-passivated NWs, wefind that the graphitic phase becomes more stable than the wurtzite. For NWdiameters around 2 nm, the graphitic and wurtzite phases present similar formationenergies, while for larger diameters the wurtzite NWs become energetically morefavorable. Finally, comparing the behavior and the positions of the valence andconduction band edges for the unpassivated ZnO NWs, we proposed the formation oftype II band alignment for a hypothetical wurtzite/graphitic NW heterojunction.