BaYF₅:Yb³⁺,Tm³⁺ Upconverting Nanoparticles with Improved Population of the Visible and Near-Infrared Emitting States: Implications for Bioimaging

The necessity to enhance specific emissions in lanthanide-doped upconverting nanomaterials in view of a specific application is particularly challenging when Tm3+ is used as a dopant ion due to the plethora of possible cross-relaxation mechanisms. By exploring a host material with a greater Tm3+–Tm3+ interionic distance, it is possible to improve the population of the lower-energy 1G4, 3F2,3, and 3H4 emitting states. The longer Tm3+–Tm3+ interionic distance in cubic Ba1–xYxF2+x reduces cross-relaxation mechanisms between Tm3+ ions, which results in less probable population of the high-energy Tm3+ levels and weak UV luminescence. Using a thermal decomposition synthesis, nanoparticles are proven to crystallize in the cubic Ba1–xYxF2+x phase in a CaF2-related structure. Nanoparticles of best quality were obtained for the composition BaYF5 (when x = 0.5), with a cubic lattice parameter a = 5.9073(3) Å. The upconversion mechanisms of BaYF5: Yb3+, Tm3+ nanoparticles are investigated by varying the dopant concentration and characterized by luminescence spectra and decay time measurements. These results prove the absence of the population of the 1D2 level, thus favoring the lower-energy transitions. Quantum yield results also show improved efficiencies of the lower-energy emissions, compared to previously reported results on larger Tm3+ emitting nanoparticles. In summary, using BaYF5: Yb3+, Tm3+ represents a successful strategy to improve the emissions from the lower-energy states, which is advantageous for bioimaging applications that rely on these transitions.