Fabrication And Luminescent Properties Of Eu～(3+), Tb～(3+) Ions Doped Core-Shell Structured Phosphors

Rare earth materials possess some special optical, acoustic, electronic, magnitic properties, once in nanometer scale, the properies will be more excellent. With the development of nanomaterials to nanocomposites, the composition of rare earth nanomaterials is an important issue in material science. By composition, people can synthesis controllably a lot of new nanostructural material and functional material with special morphology and size according to their willing.In this thesis, the core-shell structure of rare earth nanomaterials had been synthesized by coating, which resolved the problem of preparing directly the spherical rare earth nanocomposites. In addition, using the inorganic silica sphere as cores, the quantities of rare earth are economized and the cost of phosphor is decreased, which is valuable for saving the rare earth resource of our country.In this paper, we first employ the Pechini sol-gel method to prepare rare earth ions (Eu³⁺ and Tb³⁺) doped core-shell structured luminescent materials with good luminescent characteristics, including (1) oxides (SiO₂@Gd2O3:Eu³⁺); (2) titanates (SiO₂@Gd2Ti2O7:Eu³⁺); (3) vanadates (SiO₂@GdVO4:Eu³⁺); (4) tungstates (SiO₂@Gd2MoO6:Eu³⁺ and SiO₂@CaMoO4:Tb³⁺). Compared with other preparation methods, sol-gel method has the advantages as follows: simple procedure, easy operation, no complicated instruments, cheap and nontoxic precursors, lower annealing temperature, and the non-aggregated, uniform and monodispersed products.The photoluminescence (PL) and cathodeluminescence (CL) properties of rare earth ions (Eu³⁺ and Tb³⁺) as well as the energy transfer process were investigated. The morphology and the luminescence properties of the core-shell phosphors are largely dependent on the preparation conditions, such as solution concentrations, the annealing temperature, the thickness of coating layers and co-doping ions etc. We have found the optimal conditions for coating, and therefore core-shell phosphors with good transparence, color purity and luminescent intensity were achieved. Furthermore, uniform, smooth and crack-free phosphor films with thickness in the range of 50 to 70 nm were obtained. Therefore the sol-gel process is an excellent method to prepare luminescent core-shell phosphors. In the above-mentioned rare earth ions doped phosphor materials, upon excitation into their host band at their maximum values, the rare earth ions Eu³⁺ and Tb³⁺, both show their characteristic emissions due to an efficient energy transfer from the host lattices to them, i.e. Eu³⁺ [5D0-7FJ (J=0, 1, 2, 3, 4)], Tb³⁺[5D4-7FJ(J=3,4,5,6)] etc.For the luminescent materials investigated herein, core-shell particles all exhibit spherical morphology, submircometer size and narrow size distribution. The intensities of PL and CL as well as their luminescent lifetimes increased along with the elevated annealing temperature and increased numbers of coating layers. For Eu³⁺ in SiO₂@Gd2MoO6:Eu³⁺,SiO₂@GdVO4:Eu³⁺,and SiO₂@Gd2O3:Eu³⁺, it locates at sites without an inversion center and therefore red emission (5D0-7F2) originating from the electric-dipole transition dominates the emission spectrum. While for the SiO₂@Gd2Ti2O7:Eu³⁺, the situation is similar with the annealing temperature ranging from 600 to 800 oC, and orange emission (5D0-7F1) originating from the magnetic-dipole transition dominates the emission spectra due to the presence of an inversion center for samples annealed at 1000 oC. As for SiO₂@Gd2Ti2O7:Eu³⁺, it has been produced through the hydrolysis of metal-alkoxide and this pyrochlore-type material crystallizes well at 1000 oC much lower than the temperature those in the solid-state reaction. The as-prepared SiO₂@GdVO4:Eu3 and SiO₂@CaMoO4:Tb³⁺ core-shell structured luminescent materials can show luminescence even at the low-voltage cathode rays and thus have great potential in areas of field emission.