Upconversion luminescence and temperature-sensing properties of Ho3+/Yb3+-codoped ZnWO4 phosphors based on fluorescence intensity ratios

Abstract

Ho³⁺/Yb³⁺-codoped ZnWO₄ phosphors were synthesized using a solid state reaction method and their structures, upconversion (UC) luminescence, and temperature-sensing properties were investigated. The obtained ZnWO₄:0.01Ho³⁺/xYb³⁺ phosphors crystallized in the monoclinic phase with space group P2/c. Under 980 nm excitation, bright green [(⁵F₄, ⁵S₂) → ⁵I₈], weak red (⁵F₅ → ⁵I₈), and near-infrared emissions [(⁵F₄, ⁵S₂) → ⁵I₇] were observed. The optimal Ho³⁺ and Yb³⁺ doping concentrations in ZnWO₄ were 0.01 and 0.15, respectively. The near-infrared-green (I₇₅₇/I₅₄₀) and red-green (I_641,665/I_540,549) fluorescence intensity ratios (FIRs) were studied as a function of temperature at 83–503 K. The sensitivity of the ZnWO₄:0.01Ho³⁺/0.15Yb³⁺ phosphors was also discussed and their potential application as thermal sensors in luminescence thermometry was analyzed using a four-level system and the intensity ratio of the red and green emissions. ZnWO₄:0.01Ho³⁺/0.15Yb³⁺ phosphors could potentially be applied as optical temperature-sensing materials.