Quenching Pathways in NaYF4:Er3+,Yb3+ Upconversion Nanocrystals

Lanthanide-doped upconversion (UC) phosphors absorb low energy infrared light and convert it into higher-energy visible light. Despite over 10 years of development, it has not been possible to synthesize nanocrystals (NCs) with UC efficiencies on a par with what can be achieved in bulk materials. To guide the design and realization of more efficient UC NCs, a better understanding is necessary of the loss pathways competing with UC. Here we study the excited-state dynamics of the workhorse UC material beta-NaYF4 co doped with Yb3+ and Er3+. For each of the energy levels involved in infrared-to visible UC, we measure and model the competition between spontaneous emission, energy transfer between lanthanide ions, and other decay processes. An important quenching pathway is energy transfer to high-energy vibrations of solvent and/or ligand molecules surrounding the NCs, as evidenced by the effect of energy resonances between electronic transitions of the lanthanide ions and vibrations of the solvent molecules. We present a microscopic quantitative model for the quenching dynamics in UC NCs. It takes into account cross-relaxation at high lanthanide-doping concentration as well as Forster resonance energy transfer from lanthanide excited states to vibrational modes of molecules surrounding the UC NCs. Our model thereby provides insight in the inert-shell thickness required to prevent solvent quenching in NCs. Overall, the strongest contribution to reduced UC efficiencies in core shell NCs comes from quenching of the near-infrared energy levels (Er3+: I-4(11/2). and Yb3+: F-2(5/2)), which is likely due to vibrational coupling to OW defects incorporated in the NCs during synthesis.