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.