Multicolor Emission in Chemically and Structurally Tunable Er:Yb:SrFX (X = Cl, Br) Upconverting Nanocrystals

The synthesis, structural analysis, and NIR-to-visible upconversion luminescence of color-tunable Er:Yb:SrFX (X = Cl, Br) nanocrystals are reported. Green, orange, and red-emitting upconverters were achieved by employing chemically and structurally tunable alkaline-earth fluorohalides as hosts for Er³⁺–Yb³⁺ activator–sensitizer pairs. Solution-phase thermolysis of metal trifluoroacetates and of the corresponding trihaloacetic acid was utilized to synthesize Er:Yb:SrFCl and Er:Yb:SrFBr nanocrystals with average sizes of ≈15 and 50 nm, respectively. X-ray total scattering and Sr K edge absorption spectroscopy showed that a bulk-type crystallographic model provided an adequate description of the average and local structures of Er:Yb:SrFBr but failed to describe the local structure of Er:Yb:SrFCl nanocrystals, as deviations from the model were observed in the second coordination shell of strontium. Steady-state and time-resolved spectrofluorometric analyses showed that the red-to-green ratio of the upconverted emission could be compositionally tuned between 1 (green emitter) and 50 (red emitter). Single-band, red-emitting Er:Yb:SrFCl nanocrystals with Er³⁺ excited state lifetimes in the 100–160 μs range were thus achieved.