Improvement of photoluminescence and lasing properties in ZnO submicron spheres by elimination of surface-trapped state.

We report on the enhancement of photoluminescence and laser action behaviors of ZnO submicron spheres fabricated by a sol-gel process. As annealing temperature increases, intense ultraviolet excitonic emission at room temperature can be observed. Upon pulsed excitations, the lasing threshold gradually decreases as annealed temperature increases, which is consistent with the results of the ratio of surface-trapped exciton to donor-bound exciton emissions at low temperature. X-ray photoelectron spectroscopy confirmed that adsorbed OH bonds played a critical role in trapping species of exciton at the ZnO surface. The superior optical properties of high-annealing ZnO mainly result from the elimination of a surface-trapped state.