Facile synthesis of β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, Yb/Tm) microcrystals with down- and up-conversion luminescence

β-NaYF4:Ln3+ (Ln = Eu, Tb, Yb/Er, Yb/Tm) hexagonal microrods have been successfully synthesized through a facile molten salt method without any surfactant. X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, high-resolution transmission electron microscopy, and photoluminescence spectra were used to characterize the samples. It is found that at a preferred reaction temperature of 400 °C, the structure of β-NaYF4 can gradually transform from microtubes to microrods as reaction time extends from 0.5 to 4 h. Furthermore, as the molar ratio of NaF:RE3+ (RE represents the total amount of Y3+ and the doped rare earth elements such as Eu3+, Tb3+, Yb3+/Er3+, or Yb3+/Tm3+) increased, the phase of sample transforms from YF3 into NaYF4. Under the excitation of 395 nm ultraviolet light, β-NaYF4:5 %Eu3+ shows the emission lines of Eu3+ corresponding to 5D0-3 → 7F J (J = 1–4) transitions from 400 to 700 nm, resulting in red down-conversion (DC) light emission. When doped with 5 % Tb3+ ions, the strong DC fluorescence corresponding to 5D4 → 7F J (J = 6, 5, 4, 3) transitions with 5D4 → 7F J (green emission at 544 nm) being the most prominent group that has been observed. Moreover, upon 980 nm laser diode excitation, the Yb3+/Er3+- and Yb3+,Tm3+- co-doped β-NaYF4 samples exhibit bright yellow and blue upconversion (UC) luminescence, respectively, by two- or three-photon UC process. The luminescence mechanisms for the doped lanthanide ions were thoroughly analyzed.