Red–orange to green tunable upconversion emission from HfO₂ ceramics embedded in polyester films

Up-conversion (UC) luminescence in hafnia powder (HfO₂) doped with Er³⁺, Yb³⁺ and Li⁺ was studied under continuous wave excitation at 980 nm. Powder samples were prepared by the simple evaporation technique and incorporated into polyester films (by spin coating technique). They were characterized using x-ray diffraction, electron microscopy, IR spectroscopy and UV–vis. A study of UC emission from Er³⁺ ions was performed including the analysis of the spectral and temporal behavior. The effect of Yb³⁺ co-doping concentration was found to change the intensity ratio of the Er³⁺ green and red characteristic emissions which allows for a color tuning of the UC emission. The mechanisms contributing to the UC luminescence are discussed as well.