Enhancing the blue and NIR emissions of NaBiF4:Er3+,Yb3+ phosphors by co-doping with Mg2+ or Mn2+

This work reports the morphological, structural, and photoluminescent properties of NaBiF4:Er3+/Yb3+ phosphors with and without Mg2+ or Mn2+ co-dopants. The XRD patterns showed that NaBiF4 (NBF) host and NaBiF4:Er3+/Yb3+ phosphors have a mixture of hexagonal and cubic phases, while the phosphors co-doped with Mg2+ or Mn2+ ions only presented a cubic phase. The analysis by microscopy revealed that the NaBiF4:Er3+/Yb3+ phosphors are composed of particles with quasi-spherical morphology, and adding the Mg or Mn co-dopants increased the particle size from 218-397 nm to 346–447 nm. The downconversion (DC) photoluminescence (PL) spectrum (λexc = 240 or 305 nm) of the undoped NBF host showed intense green emission (at 507 nm). After doping with the NBF host with Er3+/Yb3+ ions, violet-blue (403–472 nm) and NIR (804–852 nm) emissions were observed under UV excitation. Later, Mg2+ and Mn2+ co-dopants were incorporated into the NaBiF4:Er3+/Yb3+ system and enhanced the intensity of the NIR emission by 3–4.1 times. Furthermore, the NaBiF4:Er3+/Yb3+ phosphor doped with 5 mol% Yb3+ was annealed at 600 °C and excited with NIR light (980 nm). Such phosphor exhibited intense green (545 nm) and red (658 nm) upconversion (UC) emissions, which were not observed in the phosphors synthesized without annealing treatment. According to the CIE maps, the phosphors excited with UV and NIR light had their color coordinates in the blue and yellow regions, respectively. Overall, the phosphors studied here are promising lighting sources for biomedical applications such as bioimaging or biomarkers due to their intense NIR emission at 852 nm (not previously reported in the literature for the NaBiF4:Er3+/Yb3+ system).