Constructing a Self-Referenced NIR-II Thermometer with Energy Tuning of Coordinating Water Molecules by a Minimalist Method

Fluorescence thermometry based on metal halide perovskites is increasingly becoming a hotspot due to its advantages of high detection sensitivity, noninvasiveness, and fast response time. However, it still presents certain technical challenges in practical applications, such as complex synthesis methods, the use of toxic solvents, and being currently mainly based on the visible/first near-infrared light with poor penetration and severe autofluorescence. In this study, we synthesize the second near-infrared (NIR-II) luminescent crystals based on Yb³⁺/Nd³⁺-doped zero-dimensional Cs₂ScCl₅·H₂O by a simple "dissolve-dry" method. The whole synthesized process does not involve high temperatures or high pressures. Cs₂ScCl₅·H₂O/Yb³⁺,Nd³⁺ has an optimum fluorescence performance when the Yb³⁺/Nd³⁺ doping amount is 15%/20%. The emission intensity ratio attributed to Yb³⁺ and Nd³⁺ varies with temperature, and this variation is exacerbated due to the fact that the ²F_5/2 energy level of Yb³⁺ can be effectively aligned with the O-H bond stretching vibration energy of coordinating water molecules to facilitate energy transfer. Ultimately, the crystals can act as self-referenced ratiometric NIR-II luminescent thermometers with a maximum relative sensitivity of 1.66% K^-1 at 323 K. This work highlights the advantages of NIR-II luminescent materials for temperature sensing, which is significant for advancement in the field of noncontact thermometers.