ZnO nanoparticles: Effect of surface on the Structural and Optical properties

In order to enhance and control the optical properties, we have incorporated Tb in ZnO nanoparticles, which have a tremendous potential for various optoelectronic applications. Our systematic study of the structural and optical properties reveals that Tb accumulates mostly on the surface of the ZnO nanoparticles for the Tb mole-fraction below  4%, above which Tb incorporates in the core. It has been found that the surface accumulation results in a substantial enhancement of the band-edge related ultra-violet luminescence(UVL) as compared to defect related (broad) green-luminescence(GL) by influencing the attachment of the hydroxyl groups on the surface, which is shown to be the primary cause for GL emission. The study furthermore suggests that in the core-incorporation regime, the broad GL band originates mainly from certain point defects generated due to Tb incorporation, resulting in a high GL to UVL ratio. The luminescence characteristics thus can be controlled through Tb doping. It has also been observed that the accumulation of Tb on the surface of the nanoparticles increases the band gap energy by introducing a hydrostatic compressive strain in the lattice. Interestingly, the equivalent hydrostatic pressure has been estimated to be as high as 13 GPs even when only a few percent of the surface atoms are replaced by Tb. The technique, therefore, provides a unique way to introduce hydrostatic strain on individual particles without applying pressure through any external means, opening up a new route to study the influence of pressure on the electronic properties of these nanoparticles. Our study shows that the pressure coefficient of the band gap for ZnO nanoparticles decreases with the particle size. More interestingly, the exciton-phonon coupling strength is found to decrease significantly with the increase of pressure, implying that the quantum efficiency for the UVL can also be controlled through Tb incorporation.