Ultrafast Room‐Temperature Synthesis of Yb³⁺/Er³⁺ Codoped K₃ZrF₇ Nanocrystals for Thermal Enhancement of Upconversion

Abstract Lanthanide (Ln 3+ )‐doped upconversion nanocrystals (NCs) have attracted great attention due to their outstanding luminescent performance and wide applications in various fields. However, the preparation of most Ln 3+ ‐doped NCs requires rigorous conditions such as high reaction temperature, long reaction time, strict anhydrous and oxygen‐free environment, and tedious post‐treatment process. Herein, sub‐100 nm Yb 3+ /Er 3+ codoped K 3 ZrF 7 octahedral NCs are prepared by an extremely simple coprecipitation method (10 s only at room temperature). The resulting NCs show excellent upconversion luminescence (UCL), significantly superior to both their conventional high‐temperature‐synthesized counterparts and the room‐temperature‐synthesized NaBiF 4 :Yb/Er controls. More importantly, abnormal thermally enhanced UCL over a temperature range from room temperature to 473 K is found in these K 3 ZrF 7 :Yb/Er NCs, making them promising candidates for ratiometric thermometry applications. By employing the luminescence intensity ratio between two non‐thermally coupled energy levels of Er 3+ , namely 4 F 9/2 and 2 H 11/2 , these NCs display an extraordinarily high absolute sensitivity of 443.2% K −1 and a comparatively large relative sensitivity of 1.55% K −1 at room temperature. These results reveal that K 3 ZrF 7 :Yb/Er NCs can not only be facilely prepared via an ultrafast room‐temperature synthesis, but also hold great potential in various areas such as optical temperature sensing and biological applications.