Raman and excitonic photoluminescence characterizations of ZnO star-shaped nanocrystals

Optical properties of ZnO star-shaped nanostructures grown by wet chemical solution method were investigated by using Raman scattering and temperature-dependent photoluminescence. High intensity of Raman mode E₂^high with narrow FWHM of 9 cm^-1 appearing at 436 cm^-1 indicates the good quality of ZnO wurtzitic structure. Temperature-dependent ultraviolet photoluminescence was studied over the temperature range from 78 to 293 K. At low-temperatures and low-excitation intensities, the dominant spontaneous emissions are due to radiative recombination of excitons bound to donors and one longitudinal optical phonon assistant free exciton A. Finally, only the first-order longitudinal-optical phonon replica of the A free exciton recombination was observed at room temperature. The exciton emission behaviour, including excitonic energy, intensity and line width dependents on the temperature and excitation intensities are discussed. The deep-level emission band is barely observable both at room temperature and at cryogenic temperature measurements.