The effect of surface modification on the electronic properties of ZnO nanoparticles by Tb

Structural and optical properties of the Tb doped ZnO nanoparticles are systematically studied as a function of the Tb mole-fraction. Our study suggests that the Tb incorporates mostly on the surface and affects the optical properties of the ZnO nanoparticles by influencing the attachment of certain adsorbed groups, which are found to be responsible for the appearance of a broad green luminescence (GL) band in the photoluminescence spectra recorded for these nanoparticles. It has been found that the accumulation of Tb on the surface of the nanoparticles not only enhances the band edge UV luminescence by suppressing the GL under the vacuum condition but also increases the band gap energy by introducing a hydrostatic compressive strain in individual nanoparticles, which provides a unique opportunity to study the pressure dependence of the optical properties of nanoparticles without applying any external pressure. The hydrostatic compressive strain is explained in terms of the increase of the surface strain energy as a result of the Tb accumulation on the surface of the nanoparticles. The average value of the surface energy density for the particles has been estimated as a function of Tb mole-fraction. The pressure coefficient of the band gap, which is obtained from the variation of the band gap energy with the hydrostatic strain, has been found to decrease significantly with the particle size for the ZnO nanoparticles.