Defect Engineering in ZnO Nanocrystals: Photoluminescence and Raman Spectroscopy Study

Zinc oxide (ZnO) is a direct, wide bandgap semiconductor material with many promising properties for a variety of electronic/optoelectronic and biomedical applications. In this study, low temperature photoluminescence (PL) and Raman spectroscopy were applied to study defects in nanocrystalline ZnO. The nanocrystals of varied sizes were synthesized using the forced hydrolysis of zinc acetate dihydrate, where various amounts of nanopure water were added to control the size of the nanoparticles. Portions of each sample were annealed in air or nitrogen atmosphere at various temperatures from 350-550°C. The samples were characterized by x-ray diffraction, transmission electron microscopy, confocal optical microscopy, x-ray photoelectron spectroscopy. Raman and PL spectra measured at 10 K were excited by ultraviolet light (325 nm). The spectra of samples subject to variable synthesis parameters (nanocrystal size, annealing temperature and atmosphere, surfractant/surface capping agent) showed that defects leading to the appearance of visible PL bands can be introduced in the nanocrystals in a controllable way. Raman spectra show a defect-induced shift of the ZnO phonon peak and the appearance of a defect-related band which correlates with the corresponding PL spectra. The appearance of visible light emission from the ZnO nanocrystals has potential for applications in biomedical imaging and optoelectronics.