Research On Luminescent Properties Of Rare Earth Doped LED Phosphors And Barium Haloapatites

The aim of this thesis is to inverestgate tricolor phosphors which are suitable for near UV LED chips. During the studies of LED and lamp phosphors, the author found that Eu2+doped Ba5(PO4)3Cl could be a persistent phosphor or a photochromic material, depending on the raw materials used in the prepare process. Co-doping certain rare earth (R) elements in this compound can enhance their persistent luminescence or photochromism effects. Therefor, the last two chapters report the long afterglow and photochromism effect of rare earth doped Ba5(PO4)3Cl. Most of the materials were synthesized by conventional solid-state reaction method. Co-precipitation method was also used for samples which are not suitable for solid-state reaction method. The synthesized samples were characterized by XRD, diffuse reflectance spectra, photoluminescence emission and excitation spectra, and fluorescence decay curves of the synthesized phosphors were also investigated. The main results of the synthesized materials are listed in the following:1. A series Eu3+heavyly doped red phosphors, including A3R(PO4)2and Li6Y(BO3)3(A=Na, K, R=Y, Gd)were synthesized. The emission line of Eu3+doped phosphors have ideal quantum yields and spectral position, which can result in high luminous efficiency and color rendering index. By increasing the doping concentration of Eu3+, these phosphors can efficiently absorb～390nm near UV light.2. Bi3+, Eu3+codoped Y2O3and Ca4YO(BO3)3were synthesized. Y2O3:Eu3+is the best red lamp phosphor and its quantum efficiency is close to100%, while Y2O3:Bi3+can efficiently absorb near UV light. Due to the overlap between Bi3+emission and Eu2+absorption, energy transfer form Bi3+to Eu2+is expected. The results suggest that Y2O3:Bi3+,Eu3+is an efficient LED phosphor. By appropriate tuning of activator concentration and excitation wavelengths, blue, cyan, green, red and even white light can be obtained. Ca4YO(BO3)3is similar to Y2O3as a phosphor host. Althouth Bi3+or Eu3+can fluoresce efficiently in Ca4YO(BO3)3, the codoped samples are not. The luminescent propertyies of Y2O3:Bi3+, Ln3+(Ln=Sm, Eu, Dy, Er, Ho)are also investigated. The energy transfer efficiency of Bi3+â†'Ln3+and quantum efficiency of Ln3+were calculated.3. Blue-green phosphors SrMgAl10O17:Eu2+,Mn2+were synthesized. Upon excitation of UV light, two broad emission bands centered at470and515nm were observed, and they were assigned to Eu2+and Mn2+emissions, respectively. The emission color of the phosphors can be tuned from blue to cyan and finally to green by adjusting the concentration ratios of Eu2+and Mn2+. Effective energy transfer occurs from Eu2+to Mn2+in the host due to the spectral overlap between the emission band of Eu2+and the excitation bands of Mn2+. The energy transfer mechanism was demonstrated to be electric dipole-quadrupole interaction. The energy transfer efficiency and critical distance were also calculated.4. The persistent luminescence in Ba5(PO4)3Cl:Eu2+,R3+(R=Y, La, Ce, Gd, Tb and Lu) were investigated. Luminescence properties of the phosphors, including photoluminescence, luminescence decay, and thermoluminescence are systematically studied. A model of persistent luminescence was formulated on the basis of the experimental results. In the model, some Eu2+ions are oxidized to Eu3+under short UV excitation, and the released electrons are trapped at the positive defect. The persistent luminescence arises from the recombination of these trapped electrons with the photo-ionized Eu3+ions. The influence of auxiliary codopants was discussed in terms of ionic potential and ionic radius. Eventually, the concentration quenching of persistent luminescence was reported and discussed.5. A series white-amethyst photochromic materials, Ba5(PO4)3Cl:Eu2+,R3+(R=Y, La-Nd and Sm-Lu) are synthesized. These materials are sensitive to electromagnetic waves from short UV to X-ray. The materials change their color from white to amethyst by short UV or X-ray irradiation. The photo-colored samples can be bleached by green light irradiation or heat treatment. A model that involves ionization of the5d electron of Eu2+to conduction band states and subsequent trapped by a positive lattice defect is proposed for the photochromism in these compounds.