Microstructure and upconversion luminescence in Ho3+ and Yb3+ co-doped ZnO naocrystalline powders

The Ho3+/Yb3+ co-doped ZnO nanocrytalline powders were fabricated by a facile hydrothermal route. The structure and morphology of the samples were studied by XRD and SEM, respectively. The upconversion luminescence properties of the Ho3+/Yb3+ co-doped ZnO samples were investigated under 980 nm excitation. The upconversion emission bands centered at 545 and 660 nm corresponding to 5F4+5S2→5I8 and 5F5→5I8 transitions of Ho3+ ion, respectively, were observed. The dependence of the upconversion emission intensity upon the Yb3+ ions concentration was also examined. Power studies revealed that a two-photon process was involved in the upconversion emissions and the possible upconversion mechanisms were discussed.