Comparative study on the effects of different annealing conditions on the surface morphology, crystallinity, and optical properties of ZnO micro/nanoparticle-based discs

The effects of annealing parameters on the surface morphology, crystallinity, and optical properties of ZnO disc were investigated. Variations in the annealing temperatures and gas flow rates were found to have a profound impact; grain growth was enhanced even at the low annealing temperature of only 400 °C. SEM and AFM revealed smooth and uniform grain growth after annealing treatment, especially at 800 °C. A unique secondary growth of ZnO nanoparticles and multilayer grain growth that have not been reported elsewhere were also observed. The annealing treatment was also found to improve grain crystallinity as illustrated by the lowering of intrinsic compressive stress based on the XRD lattice constant and FWHM data. The PL spectra of the M-Disc showed a huge band edge emission at 371–376 nm. In contrast, the N-Disc exhibited a dominant and broad visible PL emission in the green band with peaks at 519–533 nm. These peaks were attributed to a very high concentration of structural defects (oxygen vacancies and zinc/oxygen interstitials). The annealing conditions had a significant effect on the properties of ZnO. Increased percentage of oxygen in the O/Ar from 50% to 100% did not change the M-Disc spectra. However, the XRD pattern of the N-Disc revealed that the (0 0 2) peak intensity decreased, the position of the (1 0 1) peak slightly shifted toward a higher angle, and the FWHM of the (1 0 1) peak improved. The experimental results showed that thermal annealing could enhance the different properties of ZnO discs.