Preparation And Spectroscopic Characterization Of Silicates And Gadolinium Oxychoride Powders Doped With Rare Earth Ions

This dissertation consists of three chapters. Chapter 1 introduces the background of rare earth luminescent materials. Chapter 2 deals with the preparation, characterization and luminescent properties of Ln₂Si₂O₇ (Ln=Lu,Gd,Y,) powders doped with rare earth ions (Ce³⁺,Tb³⁺,Eu³⁺). And upconversion in rare earth ions doped GdOCl powders is presented in Chapter 3.In chapter 1, an introduction of rare earth luminescent materials is given at first. Then the basic principles of luminescence and properties of rare-earth ions are stated. At last, several preparation methods of rare earth doped luminescent compounds are presented.In chapter 2, the recent research progress of rare earth doped silicate were given, then preparation, characterization and luminescent properties of the rare earth ions (Ce³⁺,Tb³⁺,Eu³⁺) doped in Lu₂Si₂O₇,Y₂Si₂O₇,Gd₂Si₂O₇ samples are presentedThe rare earth ions (Ce³⁺,Tb³⁺,Eu³⁺) doped in Lu₂Si₂O₇,Y₂Si₂O₇,Gd₂Si₂O₇ nanocrystals were prepared by sol-gel method. And their structural properties were studied by X-ray diffraction (XRD), and thermal-gravimetry analysis and differential thermal analysis (TG-DTA). The effects of different concentrations and synthetic temperature on the structure of the nanopowders were studied. According to the XRD patterns, the lutetium pyrosilicate (Lu₂Si₂O₇) crystallized in the two structural types depend on synthetic temperature and doping concentration. The samples sintered at 1100°C with low doping concentration crystallized in typicalβ-Lu₂Si₂O₇ structure with partcle size about 40nm, while the samples sintered at 1100°C with high concentration or sintered at 1000°C crystallized inα-Lu₂Si₂O₇ structure. Gd₂Si₂O₇ and Y₂Si₂O₇ sintered at 1000°C crystallized in a structure, and the doping concentration had no effects in structure. The excitation and emission spectra of Ln₂Si₂O₇ (Ln=Lu,Gd,Y,) powders doped with rare earth ions (Ce³⁺,Tb³⁺,Eu³⁺) at room temperature by using synchrotron radiation as the excitation source. The typical components of Ce³⁺, Tb³⁺ and Eu³⁺ excitation and emission spectra appear. The host absorption and the f-d transition bands all exist in the excitation spectra of Ce³⁺ and/or Tb³⁺ doped samples. And the excitation spectra of Eu³⁺ doped samples consist of the host absorption, charge transfer band of Eu³⁺-O^2- and the f-f transition bands. The fluorescence decay times of Gd₂Si₂O₇:Eu³⁺ were analyzed. A distinct decreasing of lifetime with increasing Eu³⁺ contents was observed, and the luminescence lifetimes of samples increased with increasing synthesis temperatures.The energy transfer from Ce³⁺ to Tb³⁺ were all observed in Ce³⁺ and Tb³⁺ co-doped samples. The reverse energy transfer phenomenon of Tb³⁺ to Ce³⁺ via Gd³⁺ sublattice in Gd₂Si_O7 sample was also observed. There were another energy transfer such as Gd³⁺→Tb³⁺, Gd³⁺→Ce³⁺ in Ce³⁺,Tb³⁺ co-doped Gd₂Si₂O₇ system.In chapter 3, the upconversion in rare earth ions doped GdOCl powders were investigated. An introduction about upconversion phenomenon, its application, history and research progress were given firstly, and then chosen of matix, doping ions and the influence of upconversion efficiency were discussed.The rare earth doped GdOCl powders were prepared by solid state reaction. The structural properties of GdOCl powders were studied by XRD, scanning electron microscope (SEM) and Raman spectroscopy. XRD and SEM show that the samples sintered at 1000°C crystallized in typical GdOCl structure with particle size about 1um. Raman spectra show that the phonon cutoff of GdOCl is smaller than 510cm^-1.The upconversion luminescence properties of Er³⁺ doped GdOCl powders with exicitation of 980nm and 514.5nm were investigated, respectively. Under 980nm laser excitation, green and red upconversion were observed in GdOCl:Er samples. Laser power dependence on upconverted emissions confirmed that two-photon absorption upconversion process was involved for the green and red upconversion emissions. The effect of doping concentrations was also discussed. The enhancement of the ratio of red intensity to green intensity with the increasing of Er³⁺ was observed and analyzed. Under 514.5nm laser excitation, blue upconversion was observed in GdOCl:Er³⁺ samples. Laser power and doping concentration dependence of the upconversion were studied to understand the upconversion mechanisms. Excited state absorption and energy-transfer processes are the possible mechanisms of the visible emissions.Yb³⁺ codoping enhanced the upconversion emission intensities of Er³⁺ doped GdOCl powders. The enhancement of the ratio of red intensity to green intensity with the increasing of Yb³⁺ was observed and upconversion mechanisms were analyzed. The upconversion of Tm³⁺-Yb³⁺, Ho³⁺-Yb³⁺, Tb³⁺-Yb³⁺, Pr³⁺-Yb³⁺ co-doped GdOCl powders under 980nm excitation were also observed, and their upconversion mechanisms were also analyzed, respectively.