Probing The Green Luminescence Mechanism Of ZnO Nanoparticles By Band-edge Modulation

Zinc oxide (ZnO) is an important II-VI wide band gap (Eg=3.37 eV) semiconductor material with wutzite structure. Due to the large exciton binding energy of 60 meV, which ensures the high efficient excitonic emission at room temperature, it is regarded as one of the most promising material for UV light-emitting diodes (LED), laser diodes (LD), gas sensors, flat panel displays, etc. In order to improve the UV emisson efficiency and solve the p-doping problem, it is very important to pay much attention on the visible emission mechanisms of ZnO. In this paper, the visible luminescence mechanism of ZnO has been studied by the band modulation approach.ZnO nanoparticles were synthesized by the sol-gel method, and the green luminescence mechanism was studied by changing the growth time of ZnO particles. It can be seen from absorption and emission spectra, absorption onset,the UV and the visible emission bands shift to lower energies with increasing aging time. It is indicated that the band gap of ZnO decrease and quantum confinement effect decrease remarkably. From the energetic maximum of the green emission band was plotted versus that of the UV emission band, we conclude that the transition of an electron from deep trap to a valance band lead to the green luminescence.ZnO: Mg nanoparticles were synthesized by the sol-gel method. The XRD and absorption spectra showed that Mg ions replace Zn ions in the lattice and push the bottom of the conduction band of ZnO to higher energy and enlarged the band gap. The UV emission band thus shifted to higher energies with Mg incorporation. However, the green emission shows no distinguishable shift with the increase of Mg concentration. This is because that the bottom of conduction band in ZnO is determined by the 4s state of Zn, the top of valence band is determined by the 2p state of O. Mg ion in ZnO substitute for zinc and enter a slightly distorted tetrahedral site, which leads to the bottom of conduction band of ZnO has a shift to higher energy. But Mg incorporation has little if not no influence on deep levels and the top of the valence band. Therefore, we conclude that the green emission is due to the transition of electrons from deep level to valence band.