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

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+ (F-2(5/2)) + Ho3+ (I-5(7)) -> Yb3+ (F-2(7/2)) + Ho3+ (F-5(5)) ". While, "Yb3+ (F-2(5/2)) + Ho3+ (I-5(6)) -> Yb3+ (F-2(7/2)) + Ho3+ (F-5(4)/S-5(2)) " process accounts for the population of state F-5(4)/S-5(2) (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.