Highly stable green and red up-conversion of LiYF4:Yb3+,Ho3+ for potential application in fluorescent labeling

Abstract

The development of novel phosphors with stable and efficient green and red up-conversion (UC) emission is important for many applications such as fluorescent labeling, color display and anticounterfeiting. In this work, we report a stable and efficient green and red up-converter of LiYF4:Yb3+,Ho3+ octahedrons obtained by modified hydrothermal method. The UC emission and luminescence decays of LiYF4:Yb3+,Ho3+ with various Yb3+ and Ho3+ ion concentrations were studied systematically under 976 nm LD excitation. The Yb3+ and Ho3+ ion concentrations were optimized to be 20 mol% and 1 mol% respectively, for efficient green and red UC emissions. LiYF4:Yb3+,Ho3+ presents not only controlled morphology of monodisperse octahedrons with length of ∼20 μm and width of ∼10 μm, but also performs efficient green and red UC emissions, which is about four times stronger than that of β-NaGdF4: 20 Yb3+,0.5Ho3+ (NaGdF4: 20 Yb3+,0.5Ho3+). More importantly, the stable UC emission with variation of green intensity ratio ΔG% (G% = I520-570nm/I500-700nm, ΔG%= (G%)max-(G%)min) and red intensity ratio ΔR% (R% = I625-685nm/I500-700nm, ΔR%= (R%)max-(R%)min) < 1% was found over 976 nm LD excitation power density (PD) ranging from 1.0 to 5.6 W/cm2. Whereas, their variations of ∼6% and 25% are observed at the same excitation PD for NaGdF4:20 Yb3+,0.5Ho3+ and commercial β-NaYF4:20 Yb3+,2Er3+ (NaYF4: 20 Yb3+,2Er3+), respectively. The mechanism of green and red UC emissions in LiYF4: Yb3+,Ho3+ as a consequence of energy transfer processes has been briefly discussed taking into account the UC emission kinetics. The highly stable intensity ratio of G% and R% combined with the efficient green UC color hints the potential application of LiYF4: Yb3+,Ho3+ in fluorescent labeling and color display.