Cu-doped ZnO nanoneedles and nanonails: Morphological evolution and physical properties

Controlling novel morphologies and developing effective doping strategies are two important tasks for advancing ZnO-based nanomaterials. We have grown vertically aligned Cu-doped ZnO nanonails and nanoneedles and observed a continuous evolution between various morphologies. Selecting source compositions and regulating vapor and gas pressures modify the Ehrlich-Schwoebel energy barrier for the surface diffusion and determine the morphologies. X-ray diffraction study indicates a decrease in the lattice parameter after the Cu doping. Photoluminescence measurements taken on both doped and undoped samples show that, in the Cu-doped ZnO nanostructures, the band-edge UV emission and the broad green emission are red-shifted by ~7 and 20 nm, respectively. X-ray photoelectron spectroscopy study revealed a higher level of oxygen vacancies in nanoneedles, which was found to enhance the green emission. Room-temperature ferromagnetism was also observed in Cu-doped ZnO nanomaterials. On the basis of the strong correlations between structures and properties, we demonstrate that the morphologies and the optical and magnetic characteristics can be tailored to a large degree in transition-metal-doped ZnO nanostructures.