Regularly arranged ZnO/TiO2, HfO2, and ZrO2 core/shell hybrid nanostructures - towards selection of the optimal shell material for efficient ZnO-based UV light emitters

Luminescent properties of ZnO/TiO2, ZnO/HfO2, and ZnO/ZrO2 core/shell hybrid nanotubes (NTs) with the shell thickness varying between 9 and 40 nm were studied. The hybrid nano-ceramics demonstrated distinct differences in their luminescence performance. The highest UV/VIS ratio and the longest fluorescence lifetime were observed for the ZnO/TiO2 NTs. The behavior was ascribed to resonance energy/charge transfer between TiO2 and ZnO owing to the similar position of conduction and valence band edges, and comparable bandgap energies (Eg) which allowed for a simultaneous excitation of electron-hole pairs in both semiconductors. The difference between the other two core/shell NTs was attributed to the larger bond energy of HfO2 as compared to that of ZrO2 and smaller refractive index of HfO2 as compared to that of ZnO. The results obtained in this work strongly indicate that in the optimal core/shell heterostructure, not only the shell material should form a type-I heterojunction with the ZnO nanostructure but also the excitation energy should be comparable to or larger than the Eg of the coating material. Moreover, the shell material with a high negative formation enthalpy and lower refractive index than that of ZnO would assure an efficient surface passivation and better photon extraction from the emitter.