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

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 LiYF 4 :Yb 3+ ,Ho 3+ octahedrons obtained by modified hydrothermal method . The UC emission and luminescence decays of LiYF 4 :Yb 3+ ,Ho 3+ with various Yb 3+ and Ho 3+ ion concentrations were studied systematically under 976 nm LD excitation. The Yb 3+ and Ho 3+ ion concentrations were optimized to be 20 mol% and 1 mol% respectively, for efficient green and red UC emissions. LiYF 4 :Yb 3+ ,Ho 3+ 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-NaGdF 4 : 20 Yb 3+ ,0.5Ho 3+ (NaGdF 4 : 20 Yb 3+ ,0.5Ho 3+ ). More importantly, the stable UC emission with variation of green intensity ratio ΔG% (G% = I 520-570nm /I 500-700nm , ΔG%= (G%) max -(G%) min ) and red intensity ratio ΔR% (R% = I 625-685nm /I 500-700nm , ΔR%= (R%) max -(R%) min ) < 1% was found over 976 nm LD excitation power density (PD) ranging from 1.0 to 5.6 W/cm 2 . Whereas, their variations of ∼6% and 25% are observed at the same excitation PD for NaGdF 4 :20 Yb 3+ ,0.5Ho 3+ and commercial β-NaYF 4 :20 Yb 3+ ,2Er 3+ (NaYF 4 : 20 Yb 3+ ,2Er 3+ ), respectively. The mechanism of green and red UC emissions in LiYF 4 : Yb 3+ ,Ho 3+ 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 LiYF 4 : Yb 3+ ,Ho 3+ in fluorescent labeling and color display.