Abstract
Well-defined tetragonal LiYF4:Yb3+/Ln3+ (Ln = Er, Tm, Ho) microcrystals were successfully prepared by a surfactant-free molten salt method for the first time. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and photoluminescence (PL) spectra were used to characterize the samples. It is found that with prolonged time, the phase of product transforms from orthorhombic YF3 through hexagonal NaYF4 to tetragonal LiYF4, while the morphology changes from nanoparticles via microtubes to micro-sized bi-pyramids. By changing the dopant species (Ln3+), multicolor (yellow, green, and blue) upconversion (UC) emissions can be obtained in LiYF4:Yb3+/Ln3+ under 980 nm laser diode (LD) excitation. The UC mechanisms in co-doped LiYF4 samples were analyzed in detail based on the emission spectra. Importantly, this synthetic methodology may offer a new alternative in the fabrication of high quality rare earth fluorides and open the possibility to meet the increasing commercial demand.
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