Synthesis and Luminescence Properties of Water Soluble α-NaGdF₄/β-NaYF₄:Yb,Er Core–Shell Nanoparticles

Hexagonal phase (β) sodium rare earth tetrafluorides (NaREF₄, RE = Y, Gd, Lu, et al.) are considered the ideal matrices for lanthanide (Ln) ions doped upconversion (UC) luminescence materials, because they can provide favorable crystal lattice structures for the doped luminescent Ln ions to make intensive emissions. However, the cubic phase (α) NaREF₄ always preferentially forms at low reaction temperature in short time as it is dynamically stable. Therefore, it is hard to obtain small sized β-NaREF₄ via the traditional solvothermal method. In this paper, small sized β-NaYF₄:Yb,Er nanoparticles were synthesized by a heterogeneous-core-induced method via the solvothermal reaction. The heterogeneous α-NaGdF₄/β-NaYF₄: Yb, Er core–shell structure was confirmed by the local elemental mapping. The formation mechanism of β-NaYF₄:Yb,Er shell on the surface of α-NaGdF₄ core was explained in detail. We reasoned that a hetero interface with a lower lattice symmetric structure was produced by cation exchanges between the core and shell, which caused the preferential growth of anisotropic hexagonal phase shell. The existence of this hetero interface has also been proven by observation of Gd³⁺ UC emission.