Study On The Photoluminescence Properties Of Sm³⁺, Eu³⁺ Doped Rare-earth Compound Nanocrystals

Nanosized rare-earth doped powders display many excellent functions based on photoluminescence, electric and magnetic duo to the unique electron structure of rare-earth, especially photoluminescence properties. Therefore, significant interest in investigation of nanosized rare-earth phosphor materials had been emerged. Among rare-earth ions as activators, europium (Eu³⁺) and Samarium (Sm³⁺) are very effective and show very strong luminescence under excitation. So, they are used in multicolor display widely. Though, nanosized rare-earth doped powders can improve the spatial resolution of display devices, the luminescent efficiency of phosphor is reduced with the decrease of the grain size due to a large contribution of the surface states to the nonradiative transition. Additionally, for Eu³⁺ ions, the purity of red emission depends on the symmetry of lattice sites of Eu³⁺ ions. In this thesis, we improved the PL properties successfully by co-doping other ions as co-activators, and we make a comprehensive study on effect of Bi³⁺ doping to increase PL intensity of La₂O₃:Sm³⁺, as well as effect of V⁵⁺ to improve the PL spectrum of Gd₂Ti₂O₇:Eu³⁺. The significant results are listed as follows:1. We prepared nanocrystalline Sm³⁺-doped La₂O₃ phosphors by combustion route. Under the excitation of 243 and 407 nm the phosphor can produce the three groups of emission lines centered at approximately 564, 608 and 651 nm corresponding to the intra 4f-4f transitions ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2 and ⁴G_5/2→⁶H_9/2 of Sm³⁺ ions, respectively. The luminescence intensity of the as-synthesized samples is associated with doping concentration of Sm³⁺ ions and the optimum doping concentration is determined as 1.0 mol%. Besides, the crystallinity of La₂O₃:Sm phosphors is greatly improved by increasing annealing temperature. As a result, the luminescence intensity is evidently enhanced.2. Sm³⁺/Bi³⁺ co-doped La₂O₃ phosphors were obtained by combustion route. According to the results of transmission electron microscopy, La_2(1-0.01-y)Sm_0.02Bi_2yO₃ nanoparticles are spherical-like with a broad size distribution of 40-60 nm. The emission spectra of all samples show similar spectral profiles with three groups of emission lines corresponding to the intra 4f-4f transitions ⁴G_5/2→⁶H_5/2, ⁴G_5/2→⁶H_7/2 and⁴G_5/2→⁶H_9/2 of Sm³⁺ ions, respectively. The PL emission intensity of Sm³⁺ ions is enhanced greatly by addition of various contents of Bi³⁺ ions, and the optimum doping concentration was found to be 0.8 mol% of Bi³⁺ ions. The mechanism was invested, the results show that the spectral overlap between the emission of Bi³⁺ ions and the excitation of Sm³⁺ ions leads to an efficient energy transfer from Bi³⁺ to Sm³⁺ ions.3. We prepared nanocrystalline Gd₂Ti₂O₇:Eu³⁺ phosphors with various Eu³⁺ doping concentration by sol-gel method. PL spectra of the samples were measured at room temperature. The results show that the PL intensity of Gd₂Ti₂O₇:Eu³⁺ is associated with the annealing temperature and doping concentration of Eu³⁺ ions. With increasing annealing temperature, the PL intensity of Gd₂Ti₂O₇:Eu³⁺ was enhanced greatly; With increasing the Eu³⁺ doping concentration, the PL intensity of Gd₂Ti₂O₇:Eu³⁺ was enhanced at first to the maximum and then decreases gradually, and the optimum doping concentration is determined as 4.0 mol%.4. Eu/V co-doped Gd₂Ti₂O₇ nanoparticles were obtained by sol-gel method. Size of the particles has a broad distribution from 60 to 80 nm. The present results show that the doping concentration of V⁵⁺ is one important factor in determining the intensity ratio of ⁵D₀-⁷F₂/⁵D₀-⁷F₁, which is ascribed to the strong influence of the defects, as well as the local chemical environment on the PL properties of Eu³⁺ emitter. Ultimately, modification on PL properties of Gd₂Ti₂O₇:Eu³⁺ was accomplished by introducing various amounts of V⁵⁺ ions into the host matrix.