Effect of the Ce3+ions co-doping on the emission color of the up-converting NaYbF4 doped with Ho3+ ions

NaYbF4 luminophores doped with Ho3+ and increasing amount of Ce3+ were synthesized by the solvothermal method in a DAB-2 reactor. The structure and morphology of the obtained materials were determined by X-ray diffraction and a scanning electron microscope. It was confirmed that the phase structure and morphology can be changed by modifying the concentration of Ce3+ ions and synthesis conditions. Analysis of the elemental composition of each sample, based on energy-dispersive X-ray spectroscopy measurements, showed that the desired concentration of the Ce3+ ions is different than expected. Moreover, spectroscopic properties were carefully examined. Up-conversion emission spectra were measured under 975 nm excitation and two characteristic bands of Ho3+ were assigned to the transitions: S-5(2), F-5(4)-> I-5(8) and F-5(5)-> I-5(8) . These bands had the strongest influence on the emission colour of the luminophores. Furthermore, an additional transition band I-5(6)-> I-5(8) appeared in the near-infrared range. Noteworthy, the increase in Ce3+ concentration from 0 to 3.6% allowed for tuning the emission colour from green to orange. Interestingly, the emission above these values is again greener. Finally, the up-conversion mechanism was shown and verified.