Large Enhancement and Its Mechanism of Ultraviolet Emission from ZnO Nanorod Arrays at Room and Low Temperatures by Covering with Ti Coatings

ZnO has attracted considerable attention as an ultraviolet (UV) light emitting material. Many efforts have been made to promote efficiency for UV emission. We report large enhancement in UV emission from ZnO nanorod (NR) arrays by covering with Ti coatings. Together with morphology and structure characterization, the capability of UV emission from the Ti-coating-covered ZnO (Ti/ZnO) NR arrays was evaluated by systematically examining the photoluminescence at room and low temperatures. At room temperature, the bare ZnO NR arrays are capable of emitting a photoluminescence composed of a strong UV near-band-edge (NBE) emission of ZnO and a weak visible emission associated with deep-level defects in ZnO. The UV NBE emission is enhanced by about 24 times after covering the ZnO NR arrays with Ti coatings. The UV NBE emission increases drastically as the temperature drops, with the increase of the UV NBE emission of the Ti-covered ZnO NR arrays being much faster than that of the bare ZnO NR arrays. The UV NBE emission of the Ti/ZnO NR array is increased by about 175 times when the temperature drops from room temperature to 10 K. Compared to the UV NBE emission of the bare ZnO NR arrays, a 70-fold enhancement in the UV NBE emission of the Ti-covered ZnO NR arrays is obtained at 10 K.