Thermal Enhancement of Upconversion in Sub‐10 nm Yb³⁺/Er³⁺/Na⁺ Tridoped Cs₂ZrF₆ Nanocrystals for Ratiometric Temperature Sensing

Abstract Alkaline zirconium fluorides (A_xZr_yF_x+y, A = Li, Na, and K), featuring unique crystallographic structures, have recently emerged as a class of attractive hosts for fabricating lanthanide (Ln³⁺)‐doped upconversion nanocrystals (UCNCs) that exhibited distinct morphology, upconversion luminescence (UCL) performance, and physicochemical property. In this paper, for the first time the controlled preparation of Yb³⁺/Er³⁺‐doped UCNCs is reported based on the trigonal Cs₂ZrF₆ host, leading to tunable morphology and size of the resulting UCNCs by varying the reaction temperature and time. By further incorporating Na⁺ ions into the Cs₂ZrF₆ crystal lattice, sub‐10 nm Yb³⁺/Er³⁺/Na⁺ tridoped UCNCs with highly improved crystallinity and thus greatly enhanced UCL intensity are obtained. Moreover, these resulting UCNCs display abnormal thermal enhancement of UCL over a temperature range from 333 to 493 K, enabling the fabrication of supersensitive luminescent nanothermometers for temperature sensing. Based on the luminescence intensity ratio of two nonthermally coupled levels (i.e., ⁴F_9/2 and ²H_11/2) of Er³⁺, the as‐prepared Cs₂ZrF₆:Yb/Er/Na UCNCs exhibit an extremely large absolute sensitivity of 177.3% K⁻¹ and a considerably high relative sensitivity of 1.52% K⁻¹ at 333 K. These results unambiguously demonstrate that Cs₂ZrF₆ is a suitable host material for preparing small‐sized Ln³⁺‐doped UCNCs as nanothermometer for high‐performance ratiometric temperature sensing.