Th4+ Co‐doped YF3 : Yb3+, Er3+ Nanostructures for Enhanced Visible and NIR‐II Emissions and Potential Application as Cryogenic Thermometer

Abstract Remarkable enhancement in green and red upconversion (UC) emissions by 124 and 88 %, respectively was achieved whereas down‐conversion (DC) luminescence in NIR‐II region (~1540 nm) was improved by 20 % in Th 4+ co‐doped YF 3 : Yb 3+ ,Er 3+ nanocrystalline particles. The same is ascribed to lowering of symmetry via formation of asymmetric Er−F bonds and less symmetric Y 3+ sites due to distortions on Th 4+ co‐doping. Benefiting from the distinct temperature response of stark sublevels of the 4 S 3/2 and 4 F 9/2 states, we have proposed a strategy for luminescence intensity ratios (LIR) based thermometry involving thermally coupled (TCL) stark sub‐levels. This can be promising in more accurate temperature read‐out at cryogenic temperatures (~ −193 °C). Using the same, high S a and S r values of 0.0139 K −1 (80 K) and 0.682 % K −1 (80 K) were obtained for 4 S 3/2 (1) / 4 S 3/2 (2) → 4 I 15/2 transitions. Higher S a and S r values of 0.226 K −1 (80 K) and 2.627 %K −1 (80 K) were obtained for 4 F 9/2 (1) / 4 F 9/2 (2) → 4 I 15/2 transitions. The high sensitivity values were obtained than that reported for the YF 3 host. We believe this work with improved UC luminescence on Th 4+ addition will be boon to cryogenic temperature‐sensing.