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

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journal contribution
posted on 07.03.2017 by Iko Hyppänen, Niina Höysniemi, Riikka Arppe, Michael Schäferling, Tero Soukka
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 4F9/2 energy state in both H2O and D2O.