CsRe2F7@glass nanocomposites with efficient up-/down-conversion luminescence: from in situ nanocrystallization synthesis to multi-functional applications.

Recently, lanthanide-doped luminescent materials have been widely studied and most investigations have been limited to rare-earth-containing fluorides formed with lighter alkali metals (Li, Na and K). Hence, it is important to understand the luminescence properties of cesium rare-earth fluorides. Herein, a novel type of multi-functional luminescent material, hexagonal β-CsRe2F7 (Re = La-Lu, Y, Sc) nanocrystals, is successfully prepared via in situ crystallization inside glass. Specifically, Yb/Er:β-CsLu2F7@glass exhibits a much higher upconversion quantum yield than Yb/Er:β-NaYF4@glass (about 6 times), which is believed to be one of the most efficient upconversion materials so far. Impressively, Er:CsYb2F7@glass shows a significant photothermal effect, which can produce variable upconversion emission colors induced by an incident 980 nm laser diode, enabling it to find practical application in novel/high-precision anti-counterfeiting. In addition, Ce:CsLu2F7@glass with a maximal photoluminescence quantum yield reaching 67% can yield intense X-ray excitable radioluminescence, which is even higher than that of a commercial Bi4Ge3O12 scintillator. Benefitting from the effective protection of robust oxide glass, lanthanide-doped CsRe2F7 nanocrystals show long-term stability in harsh environments, retaining near 100% luminescence after directly immersing them in water/oil for 30 days. It is expected that the present nanocomposites have potential applications in the fields of high-end upconversion anti-counterfeiting and high-energy radiation detection.