Unlocking advanced thermometric capabilities: BiVO ₄ : Er ³⁺ /Yb ³⁺ nanophosphors with dual-mode up-conversion and down-shifting features

Luminescent materials doped with rare-earth (RE) ions have emerged as powerful tools in thermometry, offering high sensitivity and accuracy. However, challenges remain, particularly in maintaining efficient luminescence at elevated temperatures. This study investigates the thermometric properties of BiVO₄: Yb³⁺/Er³⁺ (BVO: Er/Yb) nanophosphors synthesized <i>via</i> the sol-gel method. Structural, morphological, and optical analyses confirm the high purity and monoclinic crystal structure of the materials. Dual-mode luminescence under UV and near-infrared (NIR) excitation is explored, revealing complex thermal dynamics. The distinct performances of these luminescent thermometers, in terms of thermal sensitivity and temperature uncertainty, were evaluated in the non-saturation regime in both down-shifting (DS) and up-conversion (UC) processes. Utilizing fluorescence intensity ratio (LIR) measurements, we quantified absolute and relative sensitivities, as well as temperature uncertainties, over a temperature range of 300-450 K. Temperature sensing was based on the LIR of green emission bands arising from the thermally coupled ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺. The maximum absolute sensitivity (<i>S</i> _a) reached 60 × 10^-4 K^-1 at 388 K under 975 nm excitation (UC) and 56 × 10^-4 K^-1 at 400 K under 325 nm excitation (DS). Notably, for both excitation modes, the relative sensitivity (<i>S</i> _r) decreased consistently with increasing temperature, peaking at 0.908% K^-1 and 0.87% K^-1 at 300 K, and gradually declining to 0.4% K^-1 and 0.39% K^-1 at 450 K for the DS and UC processes, respectively. Temperature resolution (δ<i>T</i>) also varied with temperature, increasing from 0.55 K to 1.23 K as the temperature rose from 300 to 450 K under 325 nm excitation. A comparable trend was observed for δ<i>T</i> under 975 nm excitation. These findings underscore the potential of BVO: Er/Yb nanophosphors as versatile and effective luminescent thermometers for a broad range of applications.