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 mm. 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 twoweaker 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.