Facile Synthesis, Structural And Luminescent Properties Of Fluoride Nanocrystals By Ionic Liquids Based Hydrothermal Process

The synthesis and luminescent characterization of rare earth ions (REI) doped fluoride nanocrystals have aroused material scientists' great interest due to their potential application in phosphors, high resolution displays, electroluminescent devices, optical amplifiers, and fluorescent labels for biomolecules. Among the fluorides, LaF3 nanocrystals have received particular attention because of the outstanding physical and chemical properties of this material:excellent thermal and environ-mental stability, low phonon energy (as low as 350 cm-1), the capability of being easily doped with rare earth ions, and so on.Tradition synthetic methods for fluoride nanocrystals are complicated and organic solvent or template always be used in process. Therefore, scientists expect to find green, facile and general method to prepare fluoride nanocrystals. Recently, ionic liquids (ILs) have attracted considerable interest owing to their exceptional features such as wide liquid range, thermal stability, non-coordinating properties, electrochemical stability, and adjustable solvent polarity. On the other hand, hydrothermal synthesis is considered as one of the useful and powerful pathways to prepare novel nanocrystals owing to high productivity, high purity, good dispersity, and so on.In this thesis, the main contents are as follows:1. Combining the advantages of Ionic liquids and hydrothermal method, a novel and easy synthesis-ionic liquids assisted hydrothermal process is reported. Here. Ionic liquid ([bmim]BF4) acts as both a co-solvent and a reactant. When [bmim]BF4 exceeds its boiling temperature as a result of superheating, it decomposes. The BF4-anions undergo fast hydrolysis at temperature higher than 120℃. The fluoride ion reacts with the REI and REI-doped fluoride nanocrystals formed.2. Highly luminescent, water-soluble LaF3:RE are prepared by ionic liquids assisted hydrothermal process. XRD, TEM and FE-SEM are used to characterize the structural properties of the LaF3:RE nanocrystals. The crystallite sizes of LaF3 nanocrystals are about 60 nm. Excited by 980 nm laser, intense green upconversion emissions are observed for LaF3:Er(1%) samples in the solid state and dispersion in solution. The quantum efficiency of LaF3:Ce(15%),Tb(5%) nanocrystals is about 34 %. The photoluminescent properties indicate that rare-earth ion-doped LaF3 nanocrystals may be applied as phosphors for high-resolution displays.3. A novel and easy synthesis of highly luminescent, water-soluble LaF3:Ce,Tb nanodiskettes by an ionic liquids-based hydrothermal process is reported. XRD patterns and TEM images show that highly monodisperse LaF3:Ce,Tb nanodiskettes with mean diameter of about 25nm and mean thickness of about 5 nm are elaborated. EDTA-2Na is found to play dual roles as achelating ligand and shape modifier for the formation of green LaF3:Ce,Tb nanodiskettes. The photoluminescent properties indicate that LaF3:Ce,Tb nanocrystals may be applied as phosphors for high-resolution displays and fluorescent labels for biomolecules.4. RE3+ ions doped CaF2 nanocrystals have been successfully synthesized by ionic liquids assisted hydrothermal process. The structural properties of samples are investigated by XRD, FE-SEM, TEM respectively. The luminescent properties of CaF2:Eu and CaF2:Ce,Tb are evaluated under ultraviolet excition.(397nm for Eu3+and 254nm for Ce3+, Tb3+). Under 980 nm laser excitation, weak blue (404-420 nm), green (513-564 nm) and red (643-676 nm) upconversion emissions have been observed in CaF2:Er,Yb. The laser power dependence of the upconverted emissions confirms that three-photon process for blue emission and two-photon process for green and red emission.