KYb2F7:Er3+ based nanothermometers: controlled synthesis, enhanced red emission, and improved sensitivities via crystal-site engineering

Abstract

KYb₂F₇ nanocrystals have stimulated extensive research interest due to their unique structure and tunable upconversion (UC) properties for the development of different technological applications. Herein, the controlled synthesis of KYb₂F₇:Er³⁺ nanocrystals for precise control over the crystal phases and morphologies was realized by varying the KF dose and reaction time via a facile hydrothermal method. Upon 365 nm excitation, the downconverted red emission of the KYb₂F₇:Er³⁺ nanocrystals pre-excited with the 980 nm laser was significantly enhanced. By manipulating the KYb₂F₇:Er³⁺ host structure via substitution of cations such as Ca²⁺, Ti⁴⁺, Si⁴⁺, Ge⁴⁺, Y³⁺, and Nd³⁺ for Yb³⁺ with optimized concentrations, the thermometric properties of KYb₂F₇:Er³⁺ were greatly improved. The substitution of Ti⁴⁺, Ge⁴⁺, and Y³⁺ has significantly improved the S_R (for Ti⁴⁺ both S_A and S_R are improved). The highest S_A (0.0028 K⁻¹) and S_R (1158/T² K⁻¹) were obtained for KYb_1.8Ti_0.15F₇:Er³⁺ and KYb_1.8Ge_0.15F₇:Er³⁺, respectively. The mechanistic investigations revealed that the green and red UC emissions resulted from the three- and two-photon process, respectively. We believe that the substitution strategy could be a powerful tool for enhancing the thermometric properties of luminescent materials.