Expanding the synthetic toolbox to access pristine and rare-earth-doped BaFBr nanocrystals

A new synthetic route to access pristine and rare-earth-doped BaFBr nanocrystals is described. Central to this route is an organic-inorganic hybrid precursor of formula Ba₅(CF₂BrCOO)₁₀(H₂O)₇ that serves as a dual-halogen source. Thermolysis of this precursor in a mixture of high-boiling point organic solvents yields spherical BaFBr nanocrystals (≈20 nm in diameter). Yb:Er:BaFBr nanocuboids (≈26 nm in length) are obtained following the same route. Rare-earth-doped nanocrystals display NIR-to-visible photon upconversion under 980 nm excitation. The temperature-dependence of the green emission from Er³⁺ may be exploited for optical temperature sensing between 150 and 450 K, achieving a sensitivity of 1.1 × 10^-2 K^-1 and a mean calculated temperature of 300.9 ± 1.5 K at 300 K. The synthetic route presented herein not only enables access to unexplored upconverting materials but also, and more importantly, creates the opportunity to develop solution-processable photostimulable phosphors based on BaFBr.