Study On The Influence Of PVP Surface Modification On The Green And Blue Emission Of ZnO Nanoparticles

The visible luminescence mechanism of ZnO is a very important problem. Although extensive researches have been made to explore the visible luminescence mechanism of ZnO, it is still lack of consensus. In this paper, to study the origin of blue and green emission, through adjusting preparation conditions, samples of blue and green emission ZnO nanoparticles are synthesized in the ethanol by the sol-gel method, respectively. To investigate the surface effect, the ZnO nanparticles are modified with different concentrations of PVP. The influence of PVP surfactants on the blue and green emission in ZnO nanoparticles is studied using fluorescence emission techniques. Taking into account that the particle size of fresh ZnO nanoparticles can increase with storage time within a certain period of time and has an effect on the luminescence properties, the solutions are stored at room temperature for 4 days to insure that the particle size do not change. Different concentrations of PVP are then added into the solutions. Photoluminescence(PL) spectra show that green luminescence is not sensitive to the PVP surface modification, with the increasing concentrations of PVP, the intensity of green emission band does not vary dramatically, indicating that the surface defects have an very small effect on green emission. However, passivation effect of PVP is very signficant for blue emission bands of ZnO, the intensity of blue emission bands decreases obviously, as the PVP concentration increases, showing that the blue emission of ZnO nanoparticles is related closely with the surface defects or traps.Studies have shown that the green emission peak has a more pronounced size dependence, its peak position shifted apparently to lower energies with particles size becoming larger; whereas the blue emission band appeared to be size independent. Other studies indicate that quantum confinement effect has an larger effect on the luminescence process within the nanoparticles. However, it has no effect on the the surface related luminescence of nanoparticles. Based on the above results and our experimental data, we can come to conclude that the green emission more likely originates from the deep defects within the nanoparticles, whereas the blue emission comes mainly from the defects on the surface.