Synthesis of NaYF4 nanocrystals doped with Yb/Er and influence of citric acid on the green and red luminescence

In this paper, NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺ were synthesized in a medium containing polyethylene glycol and citric acid. This nanocrystal presents up-converting green and red emission bands which were simultaneously observed under the excitation of a 980 infrared diode laser. Mainly, the green to red ratio (GRR) of the up-conversion emission of the hexagonal NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺ can be finely tuned by changing the content of citric acid to be nearly an arithmetic progression, i.e. 6/4, 5/4, 4/4, and 3/4. The further analysis revealed that citric acid plays a key role in improving the surface crystallinity of NaYF₄ nanocrystals doped with Yb³⁺ and Er³⁺, to which the achievement of fine controlling on GRR is ascribed.