Synthesis and optical properties of hierarchical pure ZnO nanostructures

We report the catalyst-free synthesis of hierarchical pure ZnO nanostructures with 6-fold structural symmetry by two-step thermal evaporation process. At the first step, the hexagonal-shaped nanowires consisting of a great deal of Zn and little oxide were prepared via the layer-by-layer growth mechanism; and at the second step, hierarchical pure ZnO nanostructures were synthesized by evaporating the Zn source on the basis of the step-one made substrate. Scanning electron microscopy, transmission electron microscope images, and the corresponding selected area electron diffraction pattern have been utilized to reveal the screw dislocation growth mechanism, through which the single crystal ZnO nanorods are epitaxially grown from the side-wall of central axial nanowires. Raman and photoluminescence spectra further indicate that, for the hierarchical ZnO nanostructures, the ultraviolet peak is related to the free exciton recombination, while the oxygen vacancies and high surface-to-volume ratio are responsible for the strong green peak emission.