Optical properties of ZnO nanocrystals synthesized by using sol-gel method

ZnO nanocrystals were synthesized by using a sol-gel method and their optical properties were investigated as functions of the ZnO nanoparticle size. The photoluminescence spectrum showed two peaks: one was an ultraviolet (UV) exciton emission at around 363 nm, and the other was a visible emission at around 500 nm that originated from deep-band traps. As the particle size was increased, the intensity of the visible emission decreased, while that of the UV emission increased. Both the UV and the visible emissions shifted to a lower energy with increasing particle size. The band-edge emission exhibited a red-shift in relation to the absorption edge due to the Stokes shift. Exciton emission was observed at a slightly red-shifted wavelength with an energy of 180 meV in respect to the absorption onset, as observed in other II-VI semiconductor nanocrystals.