A Versatile Method for Synthesizing Colloidal Cr ³⁺ -Based Fluoride Nanocrystals: Near-IR-Emitting Cs ₂ NaCrF ₆ , Na ₃ CrF ₆ , and Yb ³⁺ -Doped Cs ₂ NaCrF ₆

Colloidal fluoride nanocrystals containing luminescent rare-earth ions are powerful nanophosphors for bioimaging, optical sensing, and other photonic functions. The utility of luminescent fluoride nanocrystals could be broadened if a greater composition space could be accessed by the development of new synthetic capabilities. Here, we report a general solution-phase fluoride-salt synthesis method that allows preparation of colloidal fluoro-elpasolite and -cryolite nanocrystals, two phases that have received little attention at the nanoscale. We demonstrate that this synthetic method is compatible with various trivalent (e.g., Cr3+, Al3+, Ga3+) and monovalent (e.g., Cs+, Na+, NH4+) cations, providing access to a rich portfolio of ternary and quaternary fluoride nanocrystals. In particular, this method is used to prepare the Cr3+-based fluoro-elpasolite Cs2NaCrF6 on the nanoscale. Broadband near-infrared Cr3+ 4T2g → 4A2g emission is observed at room temperature from these nanocrystals. Under related conditions, analogous Cr3+-based cryolite nanocrystals (Na3CrF6) could also be prepared. With this method, Yb3+ was successfully doped into Cs2NaCrF6 nanocrystals at various concentrations. Cr3+ d–d transitions are found to sensitize the Yb3+ f–f luminescence at room temperature, and broad tunability of the relative photoluminescence intensities of Cr3+ and Yb3+ was achieved via composition control. The utility of this synthesis method for preparing these ternary and quaternary nanocrystals with complex and tunable compositions suggests opportunities for the development of other challenging fluoride lattices on the nanoscale using this approach.