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

Abstract

We developed a procedure to prepare luminescent LiYF4:Yb/LiYF(4)and LiYF4: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 LiYF4:Yb (Yb:YLF) and LiYF4: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 (phi(UC)) of LiYF4:Yb,Er/LiYF(4)core/shell particles reveal a maximum value of 1.25% atPof 180 Wcm(-2). Although lower than the values reported for NaYF4:18%Yb,2%Er core/shell nanocrystals with comparable sizes, these phi(UC)values are the highest reported so far for LiYF4:18%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.