LiYF 4 :Yb/LiYF 4 and LiYF 4 :Yb,Er/LiYF 4 core/shell nanocrystals with luminescence decay times similar to YLF laser cr
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titute of Chemistry of New Materials, Department Biology/Chemistry, University Osnabrueck, Barbarastr. 7, 49076 Osnabrueck, Germany Federal Institute for Materials Research and Testing (BAM), Division 1.2 Biophotonics, Richard-Willstaetter-Str. 11, 12489 Berlin, Germany
© The Author(s) 2020 Received: 18 June 2020 / Revised: 11 September 2020 / Accepted: 14 September 2020
ABSTRACT 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 ( UC) 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 UC 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.
KEYWORDS nanocrystal, decay time, luminescence, LiYF4, quantum yield, upconversion
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Introduction
LiYF4 and -NaYF4 are well-known host lattices for luminescent lanthanide ions owing to their large bandgap and low phonon frequencies, providing the necessary transparency for the luminescence of the dopant ions in the ultraviolet (UV), visible (vis) and near infrared (NIR) and minimizing internal luminescence quenching processes related to multiphonon relaxation (MPR) [1–9]. Growing large single crystals of high quality by the Czochralski process, however, is much easier for LiYF4 than for -NaYF4 [2, 10–12]. In contrast to -NaYF4, Czochralski-grown crystals of lanthanide-doped LiYF4 are commercially available from various suppliers and widely used as solid state laser materials. Well-known examples are LiYF4:Pr [13–20], LiYF4:Nd [21–29], LiYF4:Yb [30–35], LiYF4:Er [36–43], LiYF4:Ho [44–50] and LiYF4:Tm [51–56]. LiYF4 laser crystals are grown under strict exclusion of water and humidity to avoid the incorporation of OH− into the fluoride lat
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