Enhancement of UV emission in ZnO nanorods by growing additional ZnO layers on the surface

Ultraviolet (UV) photoluminescence emission from ZnO nanorod arrays was greatly enhanced by growing additional thin ZnO layers on the surface after thermal reduction of the nanorods. Appropriate selection of additives based on the shape of the original ZnO samples was found to be an important factor in designing the solution composition for growing the additional ZnO layers. This is because the additives modify the growth rates with respect to crystallographic planes. Adding ethylene glycol to the solution was effective for rod-shaped ZnO nanorods in enhancing the UV emission, whereas adding polyethylenimine was better for plate-like particles. These results can be explained by the presence of non-luminescent regions near the surface, where UV emission is thought to be suppressed by non-radiative surface centers. Growing additional layers on side planes increases the volume of the optically active region of ZnO nanorods, with a lower transmittance loss; thus, it effectively enhances the UV emission intensity.