The effect of annealing time and temperature on morphology and optical properties of ZnO nanostructures grown by a self-assembly method

Synthesis of ZnO nanostructures was achieved on glass substrate by the resistive evaporation of metallic zinc granules followed by dry oxidation process at 350, 450 and 550 °C. Characterization of the products with FESEM and TEM revealed that the structures obtained at 450 °C are mostly nanorods with diameters between 40 and 105 nm and lengths from 1 to 5 μm. One dimensional ZnO nanostructures such as ultra thin nanobelts, nanowires and nanorods were successfully synthesized at 550 °C, with lengths up to several micrometers. XRD patterns indicated that the structures are crystalline and that ZnO nanostructures grown at 550 °C are fully oxidized and are crystalline with wurtzite hexagonal structure. PL measurements were carried out at room temperature and they revealed that there are three band emissions; one sharp strong peak in the UV region and two weaker peaks in the visible region. A strong UV emission was observed in the spectra of the ZnO nanorods and nanowires which confirms the good optical properties of products. The PL spectra also exhibited a considerable shift toward the shorter wavelength in the UV region. It is clear that after the second annealing step photoluminescence of the 1D nanostructures, fabricated at 550 °C, have improved, especially in the UV region.