Synthesis of Core-Shell ScF₃ Nanoparticles for Thermal Enhancement of Upconversion

Lanthanide-doped ScF₃ nanoparticles are synthesized and explored for thermal enhancement of upconversion. A hot-injection method is developed to control the formation of ScF₃ nanoparticles with a core-shell nanostructure of uniform size and morphology. Temperature dependence of the upconversion emission is investigated in the temperature range from 168 to 308 K. In contrast to typical NaYF₄:Yb/Er@NaYF₄ nanoparticles that display thermally quenched emissions, a 3.7-fold enhancement of the upconversion emission is recorded for the ScF₃:Yb/Er@ScF₃ nanoparticles as the temperature is increased from 168 to 248 K because of the negative thermal expansion of the ScF₃ host lattice. Besides, the emission profiles of the nanocrystals also display a strong temperature dependence, enabling convenient ratiometric thermometry with a relative sensitivity of 1.73%·K^-1 at 168 K. As the core-shell nanoparticles are minimally susceptible to surface quenching, our findings may inspire new ideas for developing thermally enhanced upconversion nanoparticles with high emission intensities.