Photoluminescence and ZnO → Eu3+ energy transfer in Eu3+-doped ZnO nanospheres

ZnO and Eu3+ doped ZnO nanospheres were synthesized by a wet-chemical method. Scanning electron microscopy showed that all the samples have a regular spherical shape. D-5(1) -> F-7(2), D-5(0) -> F-7(0) and D-5(0) -> F-7(2) emissions originating from intra-atomic 4f-4f transition of Eu3+ were observed in the photoluminescence (PL) spectra. The PL excitation spectrum showed that two strong absorptions centred around 410 and 455 nm appeared besides a weak absorption at 374 nm and three absorption peaks belonging to Eu3+. According to the results of the Raman scattering spectrum and x-ray photoelectron spectroscopy, the two strong absorptions were attributed to substituted Eu3+ (Eu-Zn) and interstitial zinc (Zn-i) related defect bands, respectively. Moreover, it showed that Eu3+ ions might locate at different sites in the ZnO nanospheres and that energy transfer occurred from ZnO to Eu3+ ions. A possible energy transfer process was described and a corresponding schematic drawing labelling the energy levels of Eu3+ in the ZnO nanospheres was presented. In addition, a nearly white emission which was visible to the naked eye was achieved by introducing Eu3+ into the ZnO nanospheres and this might be significant for developing white light emitters by chemical methods.