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

ABSTRACT: 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 ?-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⁺: ⁴I_11/2 and Yb³⁺: ²F_5/2), which is likely due to vibrational coupling to OH^- defects incorporated in the NCs during synthesis.

SUBMITTER: Rabouw FT 

PROVIDER: S-EPMC5968434 | biostudies-literature | 2018 May

REPOSITORIES: biostudies-literature