Photoluminescence of X‑ray-Induced Divalent Tm Ions in BaLiF3:Tm3+ Nanocrystals

A mechanochemical preparation route for nanocrystalline BaLiF3 doped with trivalent Tm3+ is reported, and the photoluminescence properties of X-irradiation-generated divalent Tm2+ in this system are demonstrated. To quantify the X-ray storage capability, the photoluminescence intensities of Tm2+ (2F5/2 → 2F7/2) were measured as a function of increasing X-ray dose. Reverse photobleaching of Tm2+ was also investigated as a function of excitation power density indicating a single-photon ionization process and an interatomic spacing of 17 Å between the Tm2+ ions and the electron acceptors. The X-ray-induced Tm2+ was found to be relatively stable in the dark but was found to bleach rapidly under exposure to sunlight. The response of nanocrystalline BaLiF3:Tm3+ (Tm3+ → Tm2+) to X-irradiation was compared with those of BaLiF3:Eu3+ and BaLiF3:Sm3+ nanocrystals prepared by ball milling. While BaLiF3:Tm3+ exhibited infrared Tm2+ (2F5/2 → 2F7/2) emission at 1136 nm upon X-ray exposure, BaLiF3:Eu3+ and BaLiF3:Sm3+ exhibited Eu2+ (4f65d → 8S7/2 (4f7)) and Sm2+ (5D0 → 7F1) emission at 421 and 694 nm, respectively. These results point to the potential of BaLiF3 being a promising host matrix, which can enable luminescence within a broad optical range by selecting an appropriate activator such as Tm, Eu, or Sm.