Thermal Enhancement of Upconversion in Sub‐10 nm Yb<sup>3+</sup>/Er<sup>3+</sup>/Na<sup>+</sup> Tridoped Cs<sub>2</sub>ZrF<sub>6</sub> Nanocrystals for Ratiometric Temperature Sensing
Changhe Hu, X.G. Li, Wenfeng Xie, Jie Liu, Huhui Fu, Yaning Zhang, Jingbing Lu, Qi Xiong, Yuanqi Qian, M. Liu
Abstract
Abstract Alkaline zirconium fluorides (A x Zr y F x+y , A = Li, Na, and K), featuring unique crystallographic structures, have recently emerged as a class of attractive hosts for fabricating lanthanide (Ln 3+ )‐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 Yb 3+ /Er 3+ ‐doped UCNCs is reported based on the trigonal Cs 2 ZrF 6 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 Cs 2 ZrF 6 crystal lattice, sub‐10 nm Yb 3+ /Er 3+ /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., 4 F 9/2 and 2 H 11/2 ) of Er 3+ , the as‐prepared Cs 2 ZrF 6 :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 Cs 2 ZrF 6 is a suitable host material for preparing small‐sized Ln 3+ ‐doped UCNCs as nanothermometer for high‐performance ratiometric temperature sensing.