Environmental Impact on the Excitation Path of the Red Upconversion Emission of Nanocrystalline NaYF₄: Yb³⁺,Er³⁺

The mechanism for red upconversion luminescence of Yb-Er codoped materials is not generally agreed on in the literature. Both two-photon and three-photon processes have been suggested as the main path for red upconversion emission. We have studied β-NaYF₄:Yb³⁺,Er³⁺ nanoparticles in H₂O and D₂O, and we propose that the nanoparticle environment is a major factor in the selection of the preferred red upconversion excitation pathway. In H₂O, efficient multiphonon relaxation (MPR) promotes the two-photon mechanism through green emitting states, while, in D₂O, MPR is less effective and the three-photon path involving back energy transfer to Yb³⁺ is the dominant mechanism. For the green upconversion emission, our results suggest the common two-photon path through the ⁴F_9/2 energy state in both H₂O and D₂O. (Figure Presented).