Dual-mode photoluminescence of Ba2GdF7: Yb3+, Er3+, Eu3+ spheroidal nanoparticles for optical anticounterfeiting

A series of Yb³⁺, Er³⁺ and Eu³⁺ doped Ba₂GdF₇ spheroidal nanoparticles are prepared by hydrothermal procedure. Upconversion (UC) and downconversion (DC) dual-mode luminescence are obtained in Yb³⁺-Er³⁺-Eu³⁺ tri-doped Ba₂GdF₇ and energy transfer processes are investigated in detail. Under 980 nm near-infrared (NIR) excitation, the UC photoluminescence of Ba₂GdF₇: Yb³⁺, Er³⁺, Eu³⁺ is dominated by intense green (521& 540 nm, ²H_11/2/⁴S_3/2 → ⁴I_15/2) and red emission (651 nm, ⁴F_9/2 → ⁴I_15/2) from Er³⁺ due to efficient energy transfer (ET) of Yb³⁺ → Er³⁺, the continuous ET of Yb³⁺ → Er³⁺ → Eu³⁺ leads to weak red UC emission of Eu³⁺ (591 nm, ⁵D₀ → ⁷F₁). Under 273 nm ultraviolet (UV) excitation, characteristic 525, 555, 591, 619, 650, and 699 nm emissions of Eu³⁺ from ⁵D₂ → ⁷F₃, ⁵D₁ → ⁷F_J (J = 1, 2) and ⁵D₀ → ⁷F_J (J = 1, 2, 3, 4) transitions are obtained through predominant Gd³⁺ → Eu³⁺ ET process. Tunable DC emission color from yellow to orange-red are achieved by adjusting Eu³⁺ content in Ba₂GdF₇: Yb³⁺, Er³⁺, Eu³⁺. The concentration optimized Ba₂GdF₇: 25 mol% Yb³⁺, 6 mol% Er³⁺, 15 mol% Eu³⁺ nanophosphor exhibits green and orange-red light under NIR and UV excitation, respectively, indicating the application prospect in optical anticounterfeiting and information encryption.