Correlative Study of Enhanced Excitonic Emission in ZnO Coated with Al Nanoparticles using Electron and Laser Excitation

Metal nanoparticle (NP) surface coatings are known to significantly enhance the ultra-violet luminescence intensity of ZnO. Although there is general agreement that resonantly excited Localized Surface Plasmons (LSPs) in metal NPs can directly couple to excitons in the semiconductor increasing their spontaneous emission rate, the exact mechanisms involved in this phenomenon are currently not fully understood. In this work, LSP-exciton coupling in a ZnO single crystal and ZnO nanorods coated with a 2 nm Al layer has been investigated using correlative photoluminescence and depth-resolved cathodoluminescence and time-resolved photoluminescence spectroscopy. Temperature-resolved cathodoluminescence and photoluminescence measurements from 10 K to 250 K show enhancement factors up to 12 times of the free exciton (FX) emission at 80 K. The FX couple more efficiently to the LSPs in Al compared to the localized donor-bound excitons. Furthermore, a strong polarization dependence of the LSPs with respect to the FX was observed with higher enhanced FX transitions polarized in the same direction as the electric field of the incident excitation. These results indicate that selective enhancement of the ultra-violet excitonic PL in ZnO can be achieved by careful alignment of the crystallographic axes of the ZnO relative to the electric vector of the excitation source.