Despite considerable effort to improve upconversion (UC) in lanthanide-doped nanocrystals (NCs), the maximum reported efficiencies remain below 10%. Recently, we reported on low Er3+- and Yb3+-doped NaYF4 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 Er3+ or Yb3+ doped at various concentrations (1–100%) in core and core–shell NaYF4 NCs. The qualitative and quantitative analyses of luminescence decay curves and emission spectra show strong concentration quenching for the green-emitting Er3+ 4S3/2 and NIR-emitting 4I11/2 levels, whereas concentration quenching for the red-emitting 4F9/2 level and the IR-emitting 4I13/2 level is limited. The NIR emission of Yb3+ remains efficient even at concentration as high as 60% Yb3+, 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.

Surface chemistry of nanocrystals determines distinct thermal behaviours of upconversion emission.

Splitting one high-energy photon into two lower energy photons through downconversion has been demonstrated for a variety of combinations of rare earth (RE) ions. However, the low absorption cross section of RE3+ 4f–4f transitions hampers practical application. Therefore, enhancing the absorption by sensitization is crucial. We demonstrate efficient dye-sensitized downconversion using a strong blue/UV absorbing Coumarin dye to sensitize downconversion of the Pr3+–Yb3+ couple in NaYF4 nanocrystals (NCs). Luminescence spectra and lifetime measurements reveal Förster resonant energy transfer (FRET) from Coumarin to Pr3+ in NaYF4:Pr3+Yb3+ NCs, followed by downconversion, resulting in Yb3+ IR emission with ∼30 times enhancement. The present study demonstrates the feasibility of dye-sensitized downconversion as a promising strategy to engineer strongly absorbing downconversion NCs to enhance the efficiency of photovoltaic cells.

The morphology of NaY(WO4)2:Er,Yb nanoparticles and the thermal sensitive upconversion luminous (UCL) mechanism correspond to temperature sensing behavior.