Effect of particle size on photoluminescence emission intensity in ZnO

This study investigates the relationship between ZnO particle size and green emission photoluminescence intensity. Both commercially available ZnO powder and ZnO nanorods grown by solution processes are examined. Samples with different particle size distributions are prepared by centrifuging commercial powders or by modifying the growth conditions of nanorods. The green emission of powders and nanorods is measured as a function of particle size. A first-principles calculation is used to determine the density of states of the surface and bulk of the particles. This calculation strongly indicates that empty surface levels induce distortion of the energy band and inactivate luminescence centers near the surface. Assuming the existence of a non-luminescent surface layer on ZnO particles, equations relating particle size to emission intensity are constructed and used to describe the experimental data. The thickness of non-luminescent layers could be estimated using these modeling equations. The results from powder samples are similar to those from nanorod samples, supporting the validity of the models. Experimental results also show weak residual emission in the surface layer. The results are expected to provide valuable guidelines for future investigations into the mechanisms behind green emission in ZnO. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.