The photo-switch effect and the energy-level population change of Li⁺ doping in Yb³⁺/Er³⁺ co-doped Y₂O₃ upconversion films

Li⁺-Er³⁺/Yb³⁺ co-doped Y₂O₃ upconversion films were prepared via a sol-gel method and spin coating. Li⁺ doping resulted in better crystallinity and larger crystal grains. All the films had purple, green and red upconversion emission bands. Li⁺ doping decreased the lattice symmetry of the matrix and enhanced energy transfer, and green and red emissions improved by more than 100 times. Meanwhile, Li⁺ doping also increased the lifetime of both the green and red emissions significantly via reducing the probability of non-radiation transition. Moreover, the ⁴S_3/2 energy level tends to be populated with increasing Li⁺, the matrix phonon energy change and the fluorescence quenching causing it. When the mole ratio of rare earth ions to Li⁺ was over 1:2, the purple emission shifted from 436 to 409 nm. Here, Li⁺ played a switching role in the conversion of these two channels. Li⁺ doping up to a critical concentration value changed the matrix phonon energy. The match of the phonon energy with the energy gap between ²G_7/2-²H_9/2 and that of ²G_7/2-⁴F_3/2 was the reason. (orig.)