Bimolybdate single crystals codoping with trivalent ytterbium and erbium ions for high-stable and high-sensitive optical thermometry

Replacing conventional powder polycrystalline materials with crystalline materials is an effective strategy to improve the sensing sensitivity, responsiveness, and anti-interference capabilities of the fluorescence intensity ratio (FIR) based optical temperature sensing techniques. In this work, Yb ³⁺/Er ³⁺ codoped bimolybdate single crystals, NaBi(MoO ₄) ₂ (NBM) with different Er ³⁺ ion doping concentrations, were grown by the Czochralski method, in which is expected to be used for optical temperature sensing applications. The structure of the crystals was fully characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and transmission electron microscopy (TEM). Under 980 nm laser excitation, the crystals demonstrated notable green and slight red upconversion (UC) emissions, and the mechanism of UC luminescence was thoroughly investigated. The luminescence intensity was found to be contingent upon the doping concentration, which gradually decayed with increase of Er ³⁺ ions. The optical temperature sensing performance of Yb ³⁺/Er ³⁺ codoped NBM single crystals was evaluated by the FIR technique based on the thermally coupled level of Er ³⁺ ions ( ²H _11/2/ ⁴S _3/2). The experimental findings revealed that the 10 at% Yb ³⁺, 10 at% Er ³⁺: NBM crystal exhibited highest sensor sensitivity among the synthesized crystals, with a maximum absolute sensitivity of approximately 1.22 % K ⁻¹ at 510 K. Furthermore, the temperature sensing characteristics of the crystals appeared to be dependent on the doping concentration of Er ³⁺ ions. In summary, the Yb ³⁺/Er ³⁺ codoped NBM single crystal showcased excellent sensor sensitivity and held considerable promise as materials for optical temperature sensing applications.