Tailoring the Green, Yellow and Red defect emission bands in ZnO nanowires via the growth parameters

The optical properties of ZnO nanowires, grown by the vapor-transport method using Au as catalyst over quartz, sapphire and SiO2/Si substrates, have been characterized as a function of the growth parameters such as temperature and time, and the thickness of the Au-catalyst film. The diameter and the length of the nanowires range from 40 to 200 nm and 0.2 to 2 mu m, respectively. Room-temperature absorption and emission spectra show a well-defined exciton peak in the ultraviolet. Moreover, photoluminescence measurements show a very broad emission band in the visible range from 420 to 800 nm, which exhibits three distinct peak-like contributions in the Green, Yellow and Red at around 520, 590 and 720 nm (2.38, 2.10 and 1.72 eV), respectively. We observed that the intensity of the broad emission band in the visible increases relative to that of the exciton emission as the wire diameter decreases due to a surface effect. The origin of these visible contributions change as a function of the growth parameters and correspond to different defect-related recombination processes, which will be useful in the future applications of ZnO for optoelectronics.