Spectral management and energy-transfer mechanism of Eu³⁺-doped β-NaGdF₄:Yb³⁺,Er³⁺ microcrystals

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β-NaGdF₄:Yb³⁺,Er³⁺ upconversion (UC) microcrystals were prepared by a facile hydrothermal process with the assistance of ethylene diamine tertraacetic acid (EDTA). The β-NaGdF₄ UC microcrystal morphology was controlled by changing the doses of EDTA and NaF. Uniform hexagonal structure can be obtained at the 2 mmol EDTA and 9-10 mmol NaF. The UC emissions of β-NaGdF₄:Yb³⁺,Er³⁺ microcrystals were tuned by the variation of Eu³⁺ doping level (0%-5%), where the red/green intensity ratio decreased with the Eu³⁺ concentration increase. It was found on the base of rate equations that with the Eu³⁺ doping, the energy back transfer process ²H_11/2/⁴S_3/2 (Er³⁺) → ⁴I_13/2 (Er³⁺) decreased. In addition, an energy-transfer process from ⁴F_7/2 (Er³⁺) to ⁵D₁ (Eu³⁺) and a cross relaxation process of ⁷H_9/2 (Er³⁺) + ⁵D₀ (Eu³⁺) → ⁴F_7/2 (Er³⁺) + ⁵D₂ (Eu³⁺) were proposed and verified by rate equations, which dominated the energy-transfer mechanism between Er³⁺ and Eu³⁺, resulted in the spectra tuning of β-NaGdF₄:Yb³⁺,Er³⁺. The results suggested that the color tuning of β-NaGdF₄:Yb³⁺,Er³⁺,Eu³⁺ UC microcrystals would have potential applications in such fields as flat-panel displays, solid-state lasers, and photovoltaics.