Synthesis And Optical Properties Of Rare-earth Ions Doped NaYF₄Nanocrysstals

Rare-earth doped upconversion luminescent materials have a great deal of applications in many fields,including phosphors, lighting and display, biological luminescent labeling and infrared inspection, et al.The upconversion luminescence property is directly related to the structure and morphology of the matrix.The main objective of this work is to synthesize rare-earth doped NaYF₄phosphors controllably.Hydrothermal and solvent thermal were used to prepare upconversion luminescent materials withdifferent morphologies and sizes. The samples were characterized using X-ray powder diffraction （XRD）,scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, Fourier transform infraredSpectrum （FT-IR） and luminescence spectrometer with a PMTH-S1-CR131photomultiplier detector and a980nm laser as excitation source. Different reaction mechanisms of preparation methods are primarilydiscussed. Main research contents and results of this thesis are as follows:1. Synthesis of cubic NaYF34:Yb+/Er³⁺nanocrystals via solvent thermal approach. The morphologyand crystal structure of the products were investigated by means of powder X-ray diffraction （XRD） andtransmission electron microscopy （TEM）. The results show the products with spherical-like morphologies.The particles possess excellent monodispersity with size about50nm. The microspheres with size about500nm were synthesized through regulating fluoride sources and complexant in the same system. The sizesand morphologies of the NaYF₄nanocrystals are controllably synthesized by changing fluoride sources andcomplexant. The up-conversion luminescence properties of the products were investigated under980nmexcitation. The up-conversion emission intensity of the crystals is closely related to size and morphology.2. Synthesis of hexagonal NaYF₄:Yb³⁺/Er³⁺with micro/nano particle size via solvent thermalapproach. The effects of synthesis conditions, including reaction time, temperature, solvent constitution onthe phase, size and morphology of the luminescent materials were systematacially investigated. The resultsindicate that the constitution ratio of water to eth of the solvent plays important role in controlling thelength to diameter ratio of the products. Hexagonal flakes with the lowest length to diameter ratio weresynthesized in pure water system. High crystalline degree of the product can be obtained under hightemperature and long reaction temperature. Controlled synthesis of series of hexagonal NaYF+4:Yb³⁺/Er3 crystals with different sizes and morphologies were achieved through this method. The upconversionluminescence intensity of the materials is strongly connected with morphology and crystallization degree ofthe crystals. The greater of the crystal size, the higher of the crystallinity degree, the stronger of theluminescence intensity.3. Hexagonal NaYF+4:Yb3/Er³⁺microtubes were successfully synthesized via a facile hydrothermalroute. The effects of reaction temperature and Ce doping on the morphology and luminescence of thecrystals were investigated. Results indicate that the microtubes present excellent up-conversionluminescence properties. And the crown-like crystals present strongest emission intensity. Theup-conversion emission intensity of the materials can be regulated through adjusting matrix crystalstructure and morphology by doping Ce³⁺. This facile way in adjusting emission intensity of theup-conversion materials is simple and effective compared with changing temperature and time. The Eu³⁺doped down-conversion luminescence properties of the hexagonal NaYF₄microtubes were also investigated.The doping Ce³⁺also plays a role in regulating the crystal structure and morphology of the matrix of theEu³⁺doped down-conversion materials. The emission intensity of the Eu³⁺doped down-conversionmaterials varied mainly through the energy transfer from Ce³⁺to Eu³⁺. The down-conversion emissionintensity of the NaYF₄: Eu³⁺microtube sharply enhanced after doping Ce³⁺.