Upconversion enhancement through engineering the local crystal field in Yb3+ and Er3+ codoped BaWO4 along with excellent temperature sensing performance

Abstract

High-efficiency upconversion (UC) luminescence has been a hot topic in the development of optical materials. In this study, an extremely strong green UC is successfully achieved in the BaWO₄:Yb³⁺/Er³⁺ phosphor through engineering the local environment around the luminescent centers, realized by Ca²⁺ doping. The green UC intensity of Ca²⁺ doped BaWO₄:Yb³⁺/Er³⁺ is 6.5 and 2.3 times stronger than that of BaWO₄:Yb³⁺/Er³⁺ and CaWO₄:Yb³⁺/Er³⁺, respectively. Eu³⁺ ions are employed as the structure probe to detect the evolution of the crystal field symmetry with the increase of Ca²⁺ doping concentration, the results of which match well with the corresponding variation of the UC spectra. Meanwhile, the BaWO₄:Yb³⁺/Er³⁺/Ca²⁺ exhibits excellent temperature sensing performance based on the fluorescence intensity ratio (FIR) between the thermally coupled ²H_11/2 and ⁴S_3/2 states of Er³⁺ ions. Its absolute sensitivity and relative sensitivity for optical thermometry can reach 1.21% K⁻¹ and 1.31% K⁻¹, outperforming the majority of the same type optical thermometers. The temperature resolution of the present thermometer remains lower than 0.1 K within the temperature range of 298 K to 573 K. The intense green UC luminescence along with the outstanding thermal properties makes the BaWO₄:Yb³⁺/Er³⁺ phosphor a promising candidate for optical thermometry.