Influence of Tb doping on the luminescence characteristics of ZnO nanoparticles

Structural and optical properties of the Tb-doped ZnO nanoparticles with average diameter approximate to 4 nm have been systematically investigated. Our X-ray diffraction studies show a contraction of the ZnO lattice with the increase of the Tb mole-fraction x for x <= 0.02 and an expansion beyond x approximate to 0.02. The photoluminescence spectra are found to be comprised of a near band edge ultra violet luminescence (UVL) and a broad green luminescence (GL) band. Under the atmospheric condition, the intensity of the GL band is found to increase with the Tb mole-fraction over the entire doping range. On the other hand, under the vacuum condition, it has been observed that the GL intensity decreases with the increase of x up to x approximate to 0.02 but further increase of x leads to a gradual revival of the GL emission. Our study suggests that for x > 0.02, GL results due to the physisorption of certain groups on the surface of the nanoparticles (GL-groups). It is also found that in this Tb mole-fraction regime, Tb incorporates mostly on the surface of the nanoparticles and affects the UVL to GL intensity ratio by influencing the attachment of the GL-groups. However, for x > 0.02, GL originates not only from the GL-groups but also from certain point defects, which are likely to be generated due to the incorporation of Tb in the core of the nanoparticles. A simple rate equation model is introduced to get a quantitative understanding about the variation of the density of the centers responsible for the GL emission as a function of x under the atmospheric and the vacuum conditions.