Selective synthesis of LaF₃ and NaLaF₄ nanocrystals: Via lanthanide ion doping

The capability to selectively obtain expected nanocrystals from a series of possibilities with similar chemical compositions has been a long-term challenge. β-LaF₃ and β-NaLaF₄ nanocrystals are a kind of host material with excellent physical and chemical stability as well as low phonon energy. Since β-NaLaF₄ nanocrystals are difficult to form due to the very strong La³⁺-F^- ionic bond, investigations of the doping effect of different lanthanide ions on the formation of the final product were carried out. The experimental results have indicated that Ce³⁺ and Pr³⁺ doping has no obvious effect on the product compared to pure β-LaF₃ nanocrystals without doping, and doping of Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, and Tb³⁺ leads to the formation of pure β-NaLaF₄ nanocrystals while doping of Dy³⁺, Ho³⁺, Er³⁺, Tm³⁺, Yb³⁺, and Lu³⁺ only results in a mixture of both. What is more, proper lanthanide ion doping can also help in obtaining enhanced up- and downconversion luminescence. A theoretical model from the nanocrystal growth free energy point of view has been developed to explain the above phenomenon.