Excitation-dependent luminescence behavior of solvothermally synthesized LaF3:Ce3+ nanophosphors

Ce 3+ -doped LaF 3 nanophosphors synthesized by a solvothermal method are promising candidates for X-ray-induced photodynamic therapy owing to their efficient ultraviolet emission and biocompatibility. While the synthesis conditions of LaF 3 :Ce 3+ nanoparticles have been optimized in previous studies, the excitation-dependent luminescence processes remain insufficiently understood. In this work, the luminescence mechanisms of solvothermally synthesized LaF 3 :Ce 3+ nanophosphors were investigated using vacuum-ultraviolet (70 nm), ultraviolet (255 nm), and X-ray excitation. Photoluminescence measurements revealed characteristic Ce 3+ emission bands centered at approximately 290 and 305 nm, corresponding to the 5d–4f transitions. Distinct differences in the relative emission intensities were observed depending on the excitation source. Under direct excitation at 255 nm, the emission associated with the 2 F 7/2 terminal state was dominant, whereas host-lattice excitation at 70 nm preferentially enhanced the 2 F 5/2 -related emission. These results indicate that the relaxation pathway of excited carriers strongly depends on the excitation mechanism. Temperature-dependent measurements performed between 8 K and 300 K showed that the discrete features observed in the excitation spectra were preserved over the entire temperature range, suggesting that their origin is not related to low-temperature effects. Furthermore, X-ray-excited luminescence exhibited spectral characteristics similar to those obtained under direct Ce 3+ excitation, suggesting that secondary electronic processes generated by X-ray irradiation may contribute to the excitation of Ce 3+ luminescent centers. The present results demonstrate that the emission characteristics of LaF 3 :Ce 3+ nanophosphors are strongly governed by excitation pathways and associated energy-transfer processes. These findings provide fundamental insight into the design of highly efficient fluoride-based scintillating nanophosphors for future X-ray-induced photodynamic therapy applications.