Optimized Energy Transfer and Enhanced Green Photoluminescence in Tb³⁺/Gd³⁺ Co-Doped BiF₃ Nanoparticles via Controlled Co-Precipitation

Abstract

Tb³⁺/Gd³⁺-codoped BiF₃ nanoparticles were synthesized via a controlled, room-temperature co-precipitation method to address the critical challenge of low photoluminescence (PL) quantum efficiency in singly Tb³⁺-doped fluoride phosphors. Structural analysis confirmed a stable cubic phase (Fm-3m) with successful substitutional doping on Bi³⁺ sites. At the optimal 5:5 mol% Tb:Gd ratio, a quantum yield of 13.5%—a fourfold enhancement over Tb³⁺-only systems—was achieved. The mechanism of enhancement operates through two complementary, excitation-wavelength-dependent pathways: (I) Under short-UV irradiation (230–260 nm), Gd³⁺ acts as a sensitizer—harvesting photons via its ⁸S₇/₂→⁶IJ/⁶PJ transitions and transferring energy to Tb³⁺ via resonance Gd³⁺→Tb³⁺ energy transfer, directly proven by excitation spectroscopy. (II) Under 365 nm excitation—where Tb³⁺ is the primary absorber and Gd³⁺ absorbs negligibly—Gd³⁺ enhances PL efficiency through a complementary crystal field modification mechanism: Gd³⁺ substitution modifies the local phonon environment around bulk Tb³⁺ sites, reducing the non-radiative depopulation rate of the ⁵D₄ excited state. This is directly evidenced by time-resolved PL decay showing +65% and +87% elongation of Tb³⁺ bulk lifetime components (τ₂ and τ₃) upon co-doping. CIE chromaticity analysis confirms high-purity green emission. These findings provide dual-mechanism characterization of Gd³⁺/Tb3+ co-doping in BiF₃, establishing a rational design framework for Gd³⁺-enhanced lanthanide phosphors.