Effect of quantum confinement, surface states and exitonic energy levels on photoluminescence of zinc oxide nanostructures

Abstract

Nanostructuring of semiconductors is a novel means of developing new electronic and optoelectronic devices. The discovery of room-temperature visible photoluminescence (PL) from ZnO nanostructures has stimulated much interest in these particular kinds of nanoclusters and in small semiconductor particles. The possibility of tuning the optical response of ZnO nanomaterials by modifying their size has become one of the most challenging aspects of recent semiconductor research. It is established that quantum confinement (QC) can modify the energy gap that results visible luminescence as experimentally observed. Despite numerous proposed models, experiments and simulations to explain the luminescence, the mechanism of visible PL is far from being understood. The QC alone cannot interpret the essential features of the PL. The porous ZnO NWs and nanocrystalline with passivated surface has received extensive attention. Optical gain, observed in ZnO-NWs has given further impulse to this research. We develop a phenomenological model by combining the effects of surface states and QC. The size and shape dependent band gap and photoluminescence (PL) intensity for nanowires (NWs) with diameter ranging from 1.0 to 6.0 nm are calculated. By controlling a set of fitting parameters it is possible to tune the PL peak and intensity. It is found that the gap decreases for increasing NWs size. So both QC and surface effects in addition to exciton effects determine the optical and electronic properties of ZnO NWs. Visible luminescence is due to radiative recombination of electrons and holes in the quantum confined nanostructures. The role of surface states, exciton and QC effects on the gap energy and room temperature PL is understood. The results are in conformity with other model calculations and experimental observations. Our model can be extended to study the light emission from other nanostructures and may contribute towards the development of ZnO based optoelectronics.