Concentration Quenching in Upconversion Nanocrystals

[Image: see text] Despite considerable effort to improve upconversion (UC) in lanthanide-doped nanocrystals (NCs), the maximum reported efficiencies remain below 10%. Recently, we reported on low Er(3+)- and Yb(3+)-doped NaYF(4) NCs giving insight into fundamental processes involved in quenching for isolated ions. In practice, high dopant concentrations are required and there is a trend toward bright UC in highly doped NCs. Here, additional quenching processes due to energy transfer and migration add to a reduction in UC efficiency. However, a fundamental understanding on how concentration quenching affects the quantum efficiency is lacking. Here, we report a systematic investigation on concentration-dependent decay dynamics for Er(3+) or Yb(3+) doped at various concentrations (1–100%) in core and core–shell NaYF(4) NCs. The qualitative and quantitative analyses of luminescence decay curves and emission spectra show strong concentration quenching for the green-emitting Er(3+ 4)S(3/2) and NIR-emitting (4)I(11/2) levels, whereas concentration quenching for the red-emitting (4)F(9/2) level and the IR-emitting (4)I(13/2) level is limited. The NIR emission of Yb(3+) remains efficient even at concentration as high as 60% Yb(3+), especially in core–shell NCs. Finally, the role of solvent quenching was investigated and reveals a much stronger quenching in aqueous media that can be explained by the high-energy O–H vibrations. The present study uncovers a more complete picture of quenching processes in highly doped UC NCs and serves to identify methods to further optimize the efficiency by careful tuning of lanthanide concentrations and core–shell design.