Bi3+-sensitized up-conversion luminescence and non-contact optical thermometry in NaCaYV2O8:Er3+/Yb3+ phosphors

Abstract

Tri-doped NaCaYV₂O₈ phosphors incorporating Er³⁺, Yb³⁺, and variable Bi³⁺ concentrations (0–0.15 mol%) were synthesized via a sol–gel route. Phase purity and particle morphology were verified through X-ray diffraction (XRD) and Transmission electron microscopy (TEM). Optical spectroscopy revealed that Bi³⁺ incorporation mediates defect-state-induced band gap contraction (from 3.2 to 3.05 eV), facilitating enhanced energy transfer coupling within the Er³⁺/Yb³⁺ system. Under 980 nm excitation, pronounced green up conversion signals emerged at 525 and 552 nm, attributed to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺. Photoluminescence intensity scaled monotonically with Bi³⁺ content up to an optimum at 0.07 mol, beyond which non-radiative losses dominated. Temperature-dependent up conversion spectroscopy (298–513 K) demonstrated that fluorescence intensity ratios (FIR) derived from the thermally coupled ²H_11/2 and ⁴S_3/2 manifolds enabled reliable ratiometric thermometry independent of excitation fluctuations. The 7 mol% Bi³⁺-doped composition exhibited the highest absolute (S_a) and relative (S_r) thermal sensitivities, establishing this composition as optimal for simultaneous up conversion brightness and temperature-sensing performance. The findings underscore the strategic value of bismuth co-doping in vanadate hosts for synergistic enhancement of both luminescence efficiency and metrological precision.