Effects of post-heat-treatment temperature for seed layers on the properties of ZnO nanostructures grown by using the hydrothermal method

Zinc oxide (ZnO) nanostructures were grown on sol-gel spin-coated ZnO seed layers by using the hydrothermal method. The seed layers were post-heated at various temperatures ranging from 350 to 500 °C. The effects of the post-heat-treatment temperature for the seed layers on the structural and the optical properties of the ZnO nanostructures were investigated by using scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The seed layers postheated at temperatures above 450 °C exhibited mountain-chain-like structures with voids while the surfaces of the ones post-heated at relatively lower temperatures showed a smooth morphology with high density. The full width at half maximum (FWHM) of the ZnO (002) diffraction peak of the seed layers decreased with increasing post-heat temperature, which indicates that the crystal quality of the seed layers was enhanced as the post-heat temperature was increased. The uniformity and the density of the ZnO nanostructures were affected by the uniformity and the morphology of the seed layers. In addition, the highest c-axis preferred orientation, the narrowest FWHM of the ZnO (002) diffraction peak, and the largest average grain size were shown in the ZnO nanostructures grown on the seed layers post-heated at a temperature of 400 °C. Moreover, the intensity of the deep-level emission peak in the PL spectra increased with increasing post-heat temperature.