Research Article

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2021, 14(3): 797–806

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https://doi.org/10.1007/s12274-020-3116-y

LiYF4:Yb/LiYF4 and LiYF4:Yb,Er/LiYF4 core/shell nanocrystals with luminescence decay times similar to YLF laser crystals and the upconversion quantum yield of the Yb,Er doped nanocrystals

Frederike Carl1, Leonie Birk1, Bettina Grauel2, Monica Pons2, Christian Würth2, Ute Resch-Genger2 (✉), and Markus Haase1 (✉)

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1 Institute of Chemistry of New Materials, Department Biology/Chemistry, University Osnabrueck, Barbarastr. 7, 49076 Osnabrueck, Germany
2 Federal Institute for Materials Research and Testing (BAM), Division 1.2 Biophotonics, Richard-Willstaetter-Str. 11, 12489 Berlin, Germany

Keywords: nanocrystal, decay time, luminescence, LiYF4, quantum yield, upconversion
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  • Abstract
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We developed a procedure to prepare luminescent LiYF4:Yb/LiYF4 and LiYF4:Yb,Er/LiYF4 core/shell nanocrystals with a size of approximately 40 nm revealing luminescence decay times of the dopant ions that approach those of high-quality laser crystals of LiYF4:Yb (Yb:YLF) and LiYF4:Yb,Er (Yb,Er:YLF) with identical doping concentrations. As the luminescence decay times of Yb3+ and Er3+ are known to be very sensitive to the presence of quenchers, the long decay times of the core/shell nanocrystals indicate a very low number of defects in the core particles and at the core/shell interfaces. This improvement in the performance was achieved by introducing two important modifications in the commonly used oleic acid based synthesis. First, the shell was prepared via a newly developed method characterized by a very low nucleation rate for particles of pure LiYF4 shell material. Second, anhydrous acetates were used as precursors and additional drying steps were applied to reduce the incorporation of OH in the crystal lattice, known to quench the emission of Yb3+ ions. Excitation power density (P)-dependent absolute measurements of the upconversion luminescence quantum yield (ΦUC) of LiYF4:Yb,Er/LiYF4 core/shell particles reveal a maximum value of 1.25% at P of 180 W·cm−2. Although lower than the values reported for NaYF4:18%Yb,2%Er core/shell nanocrystals with comparable sizes, these ΦUC values are the highest reported so far for LiYF4:18%Yb,2%Er/LiYF4 nanocrystals without additional dopants. Further improvements may nevertheless be possible by optimizing the dopant concentrations in the LiYF4 nanocrystals.
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LiYF4:Yb/LiYF4 and LiYF4:Yb,Er/LiYF4 core/shell nanocrystals with luminescence decay times similar to YLF laser crystals and the upconversion quantum yield of the Yb,Er doped nanocrystals. Nano Res. 2021, 14(3): 797–806 https://doi.org/10.1007/s12274-020-3116-y

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