Energy transfer from Bi3+ to Eu3+ triggers exceptional long-wavelength excitation band in ZnWO4:Bi3+, Eu3+ phosphors

Abstract

Color-tunable phosphors of ZnWO₄:x mol% Bi³⁺, 3 mol% Eu³⁺ (x = 0.5, 1, 2, 3, 4, 5) were prepared through a precipitation method and their luminescence properties were investigated as a function of Bi³⁺ concentration. The most intense 616 nm emission indicates that Eu³⁺ occupies the Zn²⁺ sites without inversion symmetry. The spectral overlap between the emission band of Bi³⁺ and the excitation band of Eu³⁺ supports the energy transfer from Bi³⁺ to Eu³⁺, which has been demonstrated to be of a resonant type via a dipole–dipole mechanism. Energy transfer from Bi³⁺ to Eu³⁺ triggers a long-wavelength excitation band at 340 nm originating from the Bi^{3+ 1}S₀ → ³P₁ transition, which makes the phosphors fit for long-wavelength radiation. The decay curves of Bi³⁺ and Eu³⁺ emission were measured to understand the energy transfer processes. Interestingly, the critical concentration of Bi³⁺ for Eu³⁺ 616 nm emission in ZnWO₄:Bi³⁺, Eu³⁺ was greatly increased by 400 times compared with that for Bi³⁺ 560 nm emission in ZnWO₄:Bi³⁺. Color-tunable emission in ZnWO₄:Bi³⁺, Eu³⁺ phosphors can be obtained by the modulation of the excitation wavelength and the ratio of Bi³⁺ and Eu³⁺. Our work provides a novel approach to develop phosphors which can be excited effectively under long-wavelength radiation.