Hydrothermal Synthesis and Luminescence Properties of Monodisperse Spherical CaF<sub>2</sub> and CaF<sub>2</sub>:Ln<sup>3+</sup> (Ln = Eu, Tb, Ce/Tb) Microcrystals

Spherical CaF₂ and CaF₂:Ln³⁺ (Ln = Eu, Tb, Ce/Tb) with tunable particle size (about 2.5 µm) have been synthesized by one-step facile and effective hydrothermal method. The spherical structure was highly uniform and well-dispersed. It was found that reaction time, pH value, and reaction temperature have important effects on the controlled synthesis of spherical CaF₂. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), photo-luminescence (PL) and luminescence decay curve. Under UV excitation, the CaF₂:Eu³⁺ showed the red emission (⁵D₀→⁷F_{J = 0, 1, 2, 3}) and the CaF₂:Tb³⁺ presented the green emission (⁵D₄→⁷F_{J = 6, 5, 4, 3}), respectively. Furthermore, Ce³⁺/Tb³⁺ co-doped CaF₂ showed efficient energy transfer from Ce³⁺ to Tb³⁺, which presented strong green photo-luminescence of Tb³⁺. Due to excellent luminescent properties, the obtained samples can be used in many fields, such as light display systems, optoelectronic devices and biological imaging.