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 β-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 Förster 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+: 4I11/2 and Yb3+: 2F5/2), which is likely due to vibrational coupling to OH- defects incorporated in the NCs during synthesis.
Keywords: excited-state dynamics; lanthanide ions; nanocrystals; shell growth; surface quenching; upconversion luminescence.