Th<sup>4+</sup> Co‐doped YF<sub>3</sub> : Yb<sup>3+</sup>, Er<sup>3+</sup> Nanostructures for Enhanced Visible and NIR‐II Emissions and Potential Application as Cryogenic Thermometer
Annu Balhara, Santosh K. Gupta, Brindaban Modak, Sushri Sangita Nanda, S. Dash, K. Sudarshan, R. Acharya
Abstract
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.