Electronic transition pathways in energy transfer processes for upconversion photoluminescence of Yb³⁺/Ho³⁺ co-doped NaLa(MoO₄)₂ microcrystals

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In recent years, upconversion photoluminescent micro/nanocrystals with unique anti-Stokes luminescence properties have shown high application value in many relevant fields. In this work, pure tetragonal phase NaLa(MoO₄)₂:Yb³⁺/Ho³⁺ microspheres have been synthesized by a solvothermal method in the presence of oleic acid and 1-octadecene. Under the excitation of 980 nm laser, NaLa(MoO₄)₂:Yb³⁺/Ho³⁺ microcrystals emit red, green and near infrared upconversion luminescence with peak wavelengths at 660/643 nm, 541 nm, and 756 nm. Based on the analysis of excitation power-dependent upconversion luminescence intensities and energy level diagram of Yb³⁺/Ho³⁺, electronic transition pathways in the energy transfer processes for upconversion luminescence of NaLa(MoO₄)₂:Yb³⁺/Ho³⁺ microcrystals are studied. For red upconversion luminescence at 660/643 nm, the predominant population pathway to state ⁵F₅ (Ho³⁺) is “Yb³⁺ (²F_5/2) + Ho³⁺ (⁵I₇) → Yb³⁺ (²F_7/2) + Ho³⁺ (⁵F₅)”. While, “Yb³⁺ (²F_5/2) + Ho³⁺ (⁵I₆) → Yb³⁺ (²F_7/2) + Ho³⁺ (⁵F₄/⁵S₂)” process accounts for the population of state ⁵F₄/⁵S₂ (Ho³⁺) for radiative transitions of upconversion luminescence at 541 and 756 nm. Energy back transfer contributes to the intense red upconversion luminescence at 660/643 nm. These electronic transition pathways in energy transfer processes are further confirmed by the analyses based on energy level mismatch and luminescent lifetimes.