Ultraviolet‑C Upconversion Luminescence of Pr³⁺-Doped Fluorophosphate Glass

The increasing demand for ultraviolet (UV) sterilization requires the development of new luminescent materials in the UVC region (100-280 nm). In this study, to obtain blue-to-UVC upconversion (UC) luminescence, fluorophosphate glasses doped with various Pr³⁺ concentrations were prepared by using the melt-quenching method, and their optical properties were evaluated. Absorption bands attributed to the 4f¹5d¹ → 4f² and 4f² → 4f² transitions of Pr³⁺ were observed at shorter than 230 nm and in the 430-490 nm range, respectively. The relatively high phonon energies promote strong ¹D₂ emission, which is highly sensitive to concentration quenching, leading to a decrease of the Pr³⁺ luminescence quantum yield at doping levels above 0.1%. Due to the small centroid shift and small crystal field splitting of the Pr³⁺ 4f¹5d¹ states caused by F^- coordination in glass matrices, the Pr³⁺-doped fluorophosphate glass samples exhibited broad luminescence bands attributed to the 4f¹5d¹ → 4f² transition in the UVC region under 180 nm excitation. UC luminescence, peaking at 276 nm, was observed under excitation with a 455 nm blue laser diode. When the Pr³⁺ concentration exceeded 0.1% in the fluorophosphate glass, UC luminescence was scarcely observed due to cross-relaxation between neighboring Pr³⁺ ions. The results of this study demonstrate that Pr³⁺-doped fluorophosphate glasses possess higher 4f¹5d¹ energy levels than Pr³⁺-doped oxide ceramics and glasses and exhibit luminescence in the UVC region, suggesting that they are promising blue-to-UVC upconverters for UV disinfection applications through compositional tuning.