Enhancement of the UV emission from gold/ZnO nanorods exhibiting no green luminescence

Abstract

Large reflection losses at interfaces in light-emitting semiconductor devices cause a significant reduction in their light emission and energy efficiencies. Metal nanoparticle (NP) surface coatings have been demonstrated to increase the light extraction efficiency from planar high refractive index semiconductor surfaces. This emission enhancement in Au NP-coated ZnO is widely attributed to involvement of a green (∼ 2.5 eV) deep level ZnO defect exciting localized surface plasmons in the NPs. In this work, we achieve a 6 times enhancement of the ultra-violet excitonic emission in ZnO nanorods coated with 5 nm Au NPs without the aid of ZnO defects. Cathodoluminescence (CL) and photoluminescence (PL) spectroscopy revealed that the increased UV emission is due to the formation of an additional fast excitonic relaxation pathway. Concurrent CL-PL measurements ruled out the presence of charge transfer mechanism in the emission enhancement process. While time-resolved PL confirmed the existence of a new excitonic recombination channel that is attributed to exciton relaxation via the excitation of rapid non-radiative Au interband transitions that increases the UV spontaneous emission rate. Our results establish that ZnO defect levels ∼ 2.5 eV are not required to facilitate Au NP induced enhancement of the ZnO UV emission.