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 Carl, Leonie Birk, Bettina Grauel, Monica Pons, Christian Würth, Ute Resch‐Genger, Markus Haase
Abstract
Abstract We developed a procedure to prepare luminescent LiYF 4 :Yb/LiYF 4 and LiYF 4 :Yb,Er/LiYF 4 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 LiYF 4 :Yb (Yb:YLF) and LiYF 4 :Yb,Er (Yb,Er:YLF) with identical doping concentrations. As the luminescence decay times of Yb 3+ and Er 3+ 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 LiYF 4 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 Yb 3+ ions. Excitation power density ( P )-dependent absolute measurements of the upconversion luminescence quantum yield ( Φ UC ) of LiYF 4 :Yb,Er/LiYF 4 core/shell particles reveal a maximum value of 1.25% at P of 180 Wcm −2 . Although lower than the values reported for NaYF 4 :18%Yb,2%Er core/shell nanocrystals with comparable sizes, these Φ UC values are the highest reported so far for LiYF 4 :18%Yb,2%Er/LiYF 4 nanocrystals without additional dopants. Further improvements may nevertheless be possible by optimizing the dopant concentrations in the LiYF 4 nanocrystals.