Trivalent rare earth ions (RE³⁺) activated upconversion (UC) materials have been widely studied due to their potential applications in lasers, fingerprint, fluorescent markers, flat panel displays and solar energy harvesting. Yb³⁺/Er³⁺ co-doped UC materials have been reported for their efficient green and red UC emissions in different excitation power density (PD). However, the fluorescent ratio of green and red UC emissions varies as the PD increase, which limits its applications in fluorescent labeling and full color display. Yb³⁺/Ho³⁺ activated UC materials exhibits similar luminescence behavior and yield high purity green UC emission output color, but the stability of these two individual UC emissions in the combination of Yb³⁺/Ho³⁺ was rarely reported.

Recently, a research group led by Prof. HU Lili from Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences have developed LiYF₄:Yb³⁺,Ho³⁺ micro-crystals prepared by a two-step modified hydrothermal method. The study was published in Journal of Alloys and Compounds. In the experiment, they found that this method could control morphology of monodisperse octahedrons with length of ~20 μm and width of ~10 μm without surface modifier such as EDTA participated.

They demonstrated that, unlike other UC combination, the LiYF₄:20Yb³⁺,1Ho³⁺ microparticles synthesized by this method exhibited stable green and red UC emission with variation of green intensity ratio ΔG% and red intensity ratio ΔR%＜1% under 976 nm LD excitation PD ranging from 1.0 to 5.6 W/cm². Whereas, the variations of ~6% and ~25% were observed at the same excitation PD for NaGdF₄:20Yb³⁺,0.5Ho³⁺ and commercial β-NaYF₄:20Yb³⁺,2Er³⁺, respectively.

The fluorescent labeling experiment presented strong green color by LiYF₄:20Yb³⁺,1Ho³⁺ while the commercial NaYF₄:20Yb³⁺,2Er³⁺ gradually changed from pure green to yellow with increasing excitation power density from ~1.0 W/cm2 to ~5.6 W/cm².

The stable and efficient green and red UC emissions of LiYF₄:20Yb³⁺,1Ho³⁺ promote and confirm that the octahedron LiYF₄:Yb³⁺,Ho³⁺ microcrystals have potential applications in fluorescent labeling, color display and bio-probes.

Normalized UC emission spectra of LiYF₄:Yb³⁺,Ho³⁺ under 976 nm LD excitation PD ranging from 1.0 to 5.6 W/cm² and fluorescent labeling images of LiYF₄:20Yb³⁺,1Ho³⁺ and commercial NaYF₄:20Yb³⁺,2Er³⁺. (Image by SIOM)

Article website:
https://doi.org/10.1016/j.jallcom.2020.155820

Contact:
Mr. CAO Yong
General Administrative Office
Shanghai Institute of Optics and Fine Mechanics, CAS
Email: caoyong@siom.ac.cn