Hydrothermal synthesis and photoluminescent properties of ZnO sub-micrometer and micrometer rods

ZnO sub-micrometer and micrometer rods with hexagonal structure were synthesized by the hydrothermal method and characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM) images and Raman spectra. Their luminescent properties including room-temperature emission spectra, fluorescent dynamics and temperature dependence of luminescence were studied. The results indicated that, as Zn(NO3)2 · 6H2O and NaOH satisfied the stoichiometric ratio (1:2) in precursor solution, the ZnO rods were with an average diameter of 100 nm and length of 1 μm. As NaOH was insufficient or excessive, both diameter and length of the rods increased significantly. And more, a large number of defect states were involved, which greatly affected the photoluminescent properties of ZnO rods. The ultraviolet (UV) emission of excitons (3.2 eV) and visible emission (2.0 eV) from deep levels (DLs) of defects were observed. As NaOH was insufficient or excessive, the relative intensity of the defect emission increased, while that of the exciton emission decreased. The lifetimes of the excitons decreased with the increase of DLs. Two decay components were observed for the fluorescent dynamics of DLs. According to temperature-dependent luminescence, some important parameters were obtained and compared, such as the Einstein temperature for the excitons, the thermal activation energy of excitons or DLs etc.