Luminescence properties of MgO-sheathed and annealed ZnO nanowires

ZnO nanowires were synthesized by the thermal evaporation of a mixture of ZnO and graphite powders (ZnO:C = 1:1) in an oxidative atmosphere without using a metal catalyst. The nanowires were 80-200 nm in diameter and a few tens of micrometers in length. Also, ZnO-core/MgO-shell nanowires were prepared by sheathing the ZnO nanowires with MgO via a hydrothermal route. Transmission electron microscopy and X-ray diffraction analyses revealed that the cores and shells of the as-prepared core-shell nanowires were single crystal wurtzite-type ZnO and amorphous MgO, respectively. Sheathing ZnO nanowires with MgO resulted in a significant increase in the intensity of the near-band edge emission (NBE) and some increase in the intensity of the deep level (DL) emission. Consequently, the intensity ratio of the NBE emission to the DL emission of the MgO-sheathed ZnO nanowires was ∼3.5. The enhancement of the NBE emission may be attributed to two factors: the quantum confinement of the photo-generated carriers inside the ZnO cores, which is due to the larger band gap of the MgO shells; and the suppression of carrier capture by surface states, which is due to the passivation of the ZnO cores by the MgO shells. Post-annealing resulted in a decrease in the DL emission intensity, but a further decrease in the NBE emission intensity.