Plasma-induced enhancement of UV photoluminescence in ZnO nanowires

The photoluminescence emission of ZnO nanowires was modified by means of plasma etching, inducing a strong increase in the Near Band Edge emission (NBE) in the UV region (3.26 eV). In contrast, annealing treatment at 600 degrees C quenched the NBE peak. This opens the possibility to modify the emission properties of ZnO nanowires by means of surface treatment, and is of great importance in view of the application of ZnO as an emitting material. ZnO nanowires were prepared by the vapour phase growth technique in a tubular furnace. Morphological and structural characterization was carried out by scanning and transmission electron microscopy, showing no modifications to the morphology and crystalline arrangements after plasma etching. The nanowires are single crystalline with a high aspect ratio and diameters in the 50-100 nm range. The optical properties of the ZnO nanowires were studied by continuous wave photoluminescence (PL). Ar and O-2 plasma etching treatment caused modification of the PL spectrum, inducing a strong enhancement of the NBE peak, and an increased ratio of NBE over Deep Level Emission (DLE), thus indicating a better sample quality after the treatment. The effect of the Ar treatment is higher than the effect of O-2 treatment, since the ratio of NBE : DLE obtained for O-2 plasma etching is smaller than the ratio observed for Ar. Conversely, annealing at 600 degrees C in an O-2 or Ar atmosphere induced the quenching of the NBE and an increase of the DLE in the PL spectrum of the ZnO nanowires. The variations in the spectroscopic features are discussed in terms of hydrogen diffusion and the passivation of states responsible for non-radiative recombination.