Role of Surface Plasmon Decay Mediated Hot Carriers toward the Photoluminescence Tuning of Metal-Coated ZnO Nanorods

Photoluminescence spectra of metal (Al and Au)-coated ZnO nanorods synthesized by a facile low-temperature hydrothermal method with in situ addition of KMnO4 has been investigated. Further, dependence of defect density prior to metal coating on enhancement/suppression of UV and defect-related emissions have been investigated. The UV emission from metal-coated ZnO nanorods was greatly enhanced whereas the visible emission was significantly suppressed compared with the case of bare ZnO nanorods. Here, we have proposed a new mechanism elucidating the effect of Al and Au coating, incorporating the fact that nonradiative decay of surface plasmons to hot electrons and hot holes (generated through interband transitions) can be assigned for UV-emission enhancement and defect-related emission passivation, respectively. The recombination of electrons present at the defect level of ZnO to the hot holes generated with d-sp transition can be attributed for the suppression of deep level emission rather than the transfer to the Al Fermi level, whereas electron transfer from the defect states to the Fermi level and transfer of hot holes to the ZnO valence band level is responsible for UV emission enhancement in Au coated ZnO nanorods. Moreover, we have also discussed the interaction of charge carriers present at various defects states, viz., neutral, singly ionized, and doubly ionized oxygen vacancies with metals. The observed results were further verified using the Kubelka-Munk absorption technique. This work provides a plausible explanation behind the emission tuning of the metal-coated ZnO nanorods.