Enhancing blue luminescence from Ce-doped ZnO nanophosphor by Li doping

Undoped ZnO, Ce-doped ZnO, and (Li, Ce)-codoped ZnO nanophosphors were prepared by a sol-gel process. The effects of the additional doping with Li ions on the crystal structure, particle morphology, and luminescence properties of Ce-doped ZnO were investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and photoluminescence spectroscopy. The results indicate that the obtained samples are single phase, and a nanorod shaped morphology is observed for (Li, Ce)-codoping. Under excitation with 325 nm light, Ce-doped ZnO phosphors show an ultraviolet emission, a green emission, and a blue emission caused by Zn interstitials. The spectrum of the sample codoped with a proper Li concentration features two additional emissions that can be attributed to the Ce³⁺ ions. With the increase of the Li doping concentration, the Ce³⁺ blue luminescence of (Li, Ce)-codoped ZnO is obviously enhanced, which results not only from the increase of the Ce³⁺ ion concentration itself but also from the energy transfer from the ZnO host material to the Ce³⁺ ions. This enhancement reaches a maximum at a Li content of 0.02, and then decreases sharply due to the concentration quench. These nanophosphors may promise for application to the visible-light-emitting devices.