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

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 (similar to 1540 nm) was improved by 20 % in Th4+ co-doped YF3 : Yb3+,Er3+ nanocrystalline particles. The same is ascribed to lowering of symmetry via formation of asymmetric Er-F bonds and less symmetric Y3+ sites due to distortions on Th4+ co-doping. Benefiting from the distinct temperature response of stark sublevels of the S-4(3/2) and F-4(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 (similar to -193 degrees 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 S-4(3/2 (1))/S-4(3/2 (2))-> I-4(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 F-4(9/2 (1))/F-4(9/2 (2))-> I-4(15/2) transitions. The high sensitivity values were obtained than that reported for the YF3 host. We believe this work with improved UC luminescence on Th4+ addition will be boon to cryogenic temperature-sensing.