Growth, microstructure, UV and orange pink emission from ZnO nanocones

ZnO nanocones are synthesized as freestanding powder as well as thin film by precipitation in a highly alkaline aqueous solution. X-ray diffraction demonstrated well-formed wurtzite crystal structure. The microstructure and morphology of individual nanocone were studied by transmission electron microscopy. Photoluminescence studies show strong UV emission in both powder and thin film form. Powder samples also show weak orange pink emission under UV excitation. High UV to visible emission ratio implies purity of the crystallites. However, the ZnO crystallites are also excitable by purple/blue light to emit in orange/red region and act as down conversion phosphor in the visible range indicating role of some intragap states. Fluorescence lifetime spectroscopy indicates fast recombination in hundreds of picoseconds range and complex charge transfer process in the microsecond range suggesting role of excitons and intragap states in the photophysical process.