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.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.