Highly Uniform and Monodisperse β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, and Yb/Tm) Hexagonal Microprism Crystals: Hydrothermal Synthesis and Luminescent Properties

Abstract

β-NaYF₄:Ln³⁺ (Ln = Eu, Tb, Yb/Er, and Yb/Tm) hexagonal microprisms with remarkably uniform morphology and size have been synthesized via a facile hydrothermal route. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra as well as kinetic decays were used to characterize the samples. It is found that sodium citrate as a shape modifier introduced into the reaction system plays a critical role in the shape evolution of the final products. Furthermore, the shape and size of the products can be further manipulated by adjusting the molar ratio of citrate/RE³⁺ (RE represents the total amount of Y³⁺ and the doped rare earth elements such as Eu³⁺, Tb³⁺, Yb³⁺/Er³⁺, or Yb³⁺/Tm³⁺). Under the excitation of 397 nm ultraviolet light, NaYF₄:xEu³⁺ (x = 1.5, 5%) shows the emission lines of Eu³⁺ corresponding to ⁵D_0-3 → ⁷F_J (J = 0−4) transitions from 400 to 700 nm (whole visible spectral region) with different intensity, resulting in yellow and red down-conversion (DC) light emissions, respectively. When doped with 5% Tb³⁺ ions, the strong DC fluorescence corresponding to ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) transitions with ⁵D₄ → ⁷F₅ (green emission at 544 nm) being the most prominent group that has been observed. In addition, under 980 nm laser excitation, the Yb³⁺/Er³⁺- and Yb³⁺/Tm³⁺-codoped β-NaYF₄ samples exhibit bright green and whitish blue up-conversion (UC) luminescence, respectively. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.