LAUSR

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.