Highly stable and efficient pure green up-conversion emission of rod-like β-NaGdF4:Yb3+,Ho3+ submicro-crystals via ion-exchange for fluorescent labeling

High purity green up-conversion (UC) materials have wide applications in fluorescent labeling, plastic recycling and displays, etc. However, Yb3+/Er3+-based UC materials exhibit poor color purity of green light, and their output color is normally unstable under a variable excitation power density, which hinders their application in the aforementioned areas. Herein, we report the highly stable and efficient pure green UC of rod-like β-NaGdF4:Yb3+,Ho3+ submicro-crystals synthesized via a newly developed ion-exchange modified (IEM) method. IEM-derived NaGdF4:Yb3+,Ho3+ monodisperse submicro-crystals present not only a high-purity green output color (due to the large intensity ratio of green to red, IGreen/IRed ≈ 5) but also excellent color stability under varying conditions, such as excitation power density and doping concentration of Yb3+/Ho3+. More importantly, IEM arouses an enormously enhanced UC emission intensity of ∼3700-fold that of the precursor crystals (PC). The UC emission intensity of IEM β-NaGdF4:Yb3+,Ho3+ is ∼4 times stronger than that of its high temperature annealing (HTA) counterpart. The emission intensity of the single green band is ∼40% stronger than that of the IEM β-NaGdF4:20%Yb3+,2%Er3+ submicro-crystals. A proof-of-concept fluorescent labeling of clover patterns (15 × 12 mm) printed from NaGdF4:Yb3+,Ho3+ ink with stable and high purity green fluorescence is demonstrated under excitation with various power densities.