Up-conversion luminescence, thermometry, and optical heating properties of Er3+- and Yb3+-doped K2LaNb5O15 submicro-particles synthesized by a simple molten salt method

Abstract

A series of Er³⁺- and Yb³⁺-doped K₂LaNb₅O₁₅ (KLN:xEr³⁺/Yb³⁺) up-conversion (UC) submicro-particles have been synthesized for the first time by a simple and low-cost molten salt (MS) approach. X-ray diffraction (XRD) was performed to analyze the phase and structure, and the prepared KLN:xEr³⁺/Yb³⁺ samples exhibited a single phase tetragonal tungsten bronze (TTB) structure. The morphologies were characterized by scanning electron microscopy (SEM), and submicro-rod-like particles were obtained for all samples. Under 980 nm excitation, KLN:xEr³⁺/Yb³⁺ emitted bright green and weak red emissions which arose from the intra-4f transitions of Er³⁺ ions. The UC emission intensities and R_R/G (the intensity ratio between red and green emissions) were disclosed to be tightly dependent on the Yb³⁺ ion concentration, and the involved UC luminescence mechanism was studied. Meanwhile, the slope of log I–log P plots displayed an evident reduction with a T_s (sintering temperature) increase, which was ascribed to the saturation effect coming from the competition between UC processes and linear decay. Furthermore, temperature-dependent UC behavior and temperature sensing properties of KLN:xEr³⁺/Yb³⁺ were probed based on the fluorescence intensity ratio (FIR) technique of UC green emission. The maximum sensor sensitivity (S) of KLN:0.04Er³⁺/Yb³⁺ (T_s = 900 °C) and KLN:0.16Er³⁺/Yb³⁺ (T_s = 900 °C) was determined to be as high as 10.90 × 10⁻³ and 12.27 × 10⁻³ K⁻¹, respectively. We also showed that the particle size has an evident influence on S, which can be qualitatively interpreted by J–O theory. Thermal-cycling measurements were conducted at different temperatures, and good reliability and repeatability were confirmed. In addition, an obvious optical heating effect was also realized and the variation of temperature induced with a laser was about 32 K. These results reveal that KLN:xEr³⁺/Yb³⁺ submicroparticles with high sensor sensitivity and an obvious laser-induced thermal effect are suitable for future optical thermometers and optical heaters.