Preparation, Characterization, And Upconversion Properties Of Rare Earth Ions Doped Y₂O₃ Microspheres And Films

This thesis consists of five chapters.In Chapter 1,the research progress of upconversion materials was reviewed.The host materials and doping ions reported were presented and the upconversion mechanisms were involved.Some synthesis methods of upconversion materials were also introduced.In Chapter 2,rare earth ions doped Y₂O₃ microspheres were prepared by a simple solyothermal method followed by further heat treatment.The structural properties of samples were investigated by X-ray diffraction,Field emission scanning electron microscopy,Raman spectroscopy and Fourier transform infrared spectroscopy.XRD patterns show that the samples crystallized in typical cubic Y₂O₃ structure with particle size about 50 nm for samples sintered at 900℃.FE-SEM images indicate that monodisperse Y₂O₃ microspheres with diameter of 1.5-3μm composed of nanoparticles with size of about 50 nm were prepared.Raman and FT-IR spectra show that the phonon off of Y₂O₃ is smaller than 600 cm^-1 and there is no organic residue(such as OH and CO group) in Y₂O₃ sintered at 900℃,which indicating that rare earth ions doped Y₂O₃ microspheres sintered at 900℃may present higher upconversion efficiency.Especially, the effect of CH₃COONa on the formation of microspheres was investigated also.Under 980 nm laser excitation,strong green(518-570 nm),strong red(645-686 nm) and weak violet(405-420 nm) upconversion emissions have been observed in Y₂O₃:Er and Y₂O₃:Er,Yb microspheres.Laser power dependence of the upconverted emissions revealed that two-photon and three-photon process lead to the red,green and violet emissions, respectively.With increasing Er³⁺ and(or) Yb³⁺ concentration,the upconverted emissions of Y₂O₃:Er³⁺ and Y₂O₃:Er³⁺,Yb³⁺ samples have been observed andanalyzed.Results indicate that Y₂O₃:Er and Y₂O₃:Er,Yb microspheres may be used in upconversion phosphors.Under 785 nm laser excitation,visible green(²H_11/2,⁴S_3/2→⁴I_15/2) and red(⁴F_9/2→⁴I_15/2) upconversion emissions of Er³⁺ in Y₂O₃:Er microspheres were investigated.Laser power and Er³⁺ concentration dependence of upconversion emissions were carried out to understand the upconversion mechanisms.Rome temperature bright white upconversion emission was obtained in Y₂O₃:,Tm³⁺,Er³⁺,Yb³⁺ microspheres under single-wavelength diode laser(980 nm) excitation.The upconversion mechanisms were proposed based on the pump power dependence.In addition,the luminescent properties of Y₂O₃:Eu³⁺ microspheres with different solvothermal treatment time and different annealing temperature were also investigated. And the results obtained were used to elaborate Y₂O₃:Re³⁺ upconversion phosphors.In Chapter 3,the preparation and characterization of Y₂O₃:Er³⁺ films were reported. The Y₂O₃:Er³⁺ films were prepared by a simple sol-gel process and dip-coating technique by using yttrium acetate as precursors.X-ray diffraction,Fourier transform infrared spectroscopy and field emission scanning electron microscopy were used to characterize the structural properties of Y₂O₃:Er³⁺ films.The results show that the Y₂O₃:Er³⁺ films crystallized in typical cubic Y₂O₃ structure and indicate that Y₂O₃:Er³⁺ may have high upconversion efficiency because of their low vibration energy and no OH and CO group residue.Under 785 um and 980 nm laser excitation,the samples show green(²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2) and red(⁴F_9/2→⁴I_15/2) upconversion emissions.The upconversion mechanisms were studied in detail through laser power dependence.Excited state absorption and energy transfer process are discussed as possible upconversion mechanisms. The Y₂O₃:Er³⁺ films prepared by sol-gel method may be applied in upconversion layers.In Chapter 4,the investigation on Sr₂SiO₄ based white LED phosphors is introduced briefly.Sr₂SiO₄:Eu³⁺ phosphors were synthesized by a conventional solid-state reaction technique.Sr₂SiO₄:Eu²⁺ phosphors were obtained by annealing Sr₂SiO₄:Eu³⁺ phosphors in a reduction atmosphere.The structural and luminescent properties of Eu³⁺ and Eu²⁺ doped Sr₂SiO₄ phosphors were investigated.The X-ray diffraction results show that the obtained phosphors are composed of orthorhombicα'-Sr₂SiO₄ and monoclinicβ-Sr₂SiO₄ phase. Sr₂SiO₄:Eu³⁺ phosphors show intense emission in the red region when excited by 256 nm light.Sr₂SiO₄:Eu²⁺ phosphors give white emissions(x=0.30,y=0.40,Tc=6500 K) ranging from 425 to 650 nm when it was excited by near-ultraviolet(near-UV) light,indicating that Sr₂SiO₄:Eu²⁺ is a good light-conversion candidate phosphor for near-UV chip.In Chapter 5,the conclusions of the thesis were summarized.