Preparation And Optical Properties Of Rare Earth Doped GdNbTiO₆ Red Phosphors For LED

Rare earth doped phosphors have been widely applied in various fields,such as solid-state lighting,liquid crystal display,solar cells and so on.In the field of solid-state lighting,rare earth doped phosphors were applied in white light emitting diodes（w-LEDs）.Phosphor-conversion method is an important way to fabricate w-LEDs,which can be achieved by mixing blue,green and red phosphors with near-ultraviolet（NUV）or blue light LEDs.Red phosphors have significant effects on the color rendering performance of w-LEDs,however,the luminescence performance and the excitation efficiency of them is lower than those of blue and green phosphors when excited by NUV light.Hence,it is necessary to develop high efficient red phosphors which can be effectively excited by NUV light.In this thesis,GdNbTiO₆ with good physical and chemical stabilities was selected as host matrix,Bi³⁺was chosen as sensitizer,Eu³⁺and Sm³⁺doped red phosphors were synthesized by a high temperature solid-state reaction method.The luminescence performance,optical transition properties and temperature-dependent luminescence characteristics of those as-prepared samples were investigated.The main contents are as follows:1.Eu³⁺single-doped and Eu³⁺,Bi³⁺co-doped GdNbTiO₆ phosphors were synthesized.The crystal structure of the samples was analyzed by means of X-ray diffraction（XRD）and the as-prepared samples were confirmed to be orthorhombic phase GdNbTiO₆.The results of photoluminescence spectra indicated the samples can be effectively excited by NUV light and emit strong red emission at about 615 nm.The introduction of Bi³⁺effectively broadened the excitation band of Eu³⁺in NUV light region,and there was an effective energy transfer from Bi³⁺to Eu³⁺.It was confirmed that exchange interaction between Eu³⁺ions was mainly responsible for the concentration dependent luminescence quenching of Eu³⁺by using Van Uitert model.Luminescence thermal quenching behavior was observed in both Eu³⁺single-doped and Eu³⁺,Bi³⁺co-doped GdNbTiO₆ phosphors,which was ascribed to the crossover process.The activation energy of the quenching process was obtained to be about0.154 e V according to Arrhenius model.2.GdNbTiO₆:Sm³⁺phosphors with various Sm³⁺concentrations were prepared.The crystal structure of the samples was characterized by means of XRD and the as-prepared samples were pure phase GdNbTiO₆.Photoluminescence properties were investigated by measuring the concentration-and temperature-dependent photoluminescence spectra.Concentration-dependent luminescence quenching and luminescence thermal quenching behaviors were observed and they were respectively ascribed to the electric dipole-dipole interaction between Sm³⁺ions and the cooperation of energy transfer and crossover process.In addition,temperature-induced redshift of charge transfer band（CTB）of GdNbTiO₆ host was found in temperature-dependent excitation spectra and the opposite variations of different excitation peaks were utilized for optical thermometry.Finally,the optical transition property of Sm³⁺was studied on the basis of the diffuse reflectance spectra and Judd-Ofelt（J-O）theory,meanwhile,its accuracy was evaluated by the result of emission spectra.3.GdNbTiO₆:Eu³⁺,Sm³⁺phosphors were synthesized.The results of XRD confirmed the as-synthesized samples were pure phase GdNbTiO₆.From photoluminescence spectra,it was confirmed that there was an effective energy transfer from Sm³⁺to Eu³⁺,but the energy transfer from Eu³⁺to Sm³⁺was invalid.The introduction of Sm³⁺can also extend the excitation band of Eu³⁺in NUV light region.According to J-O theory and emission spectra,the J-O intensity parameters of Eu³⁺in Eu³⁺single-doped and Eu³⁺,Sm³⁺co-doped GdNbTiO₆samples were both calculated,and it was confirmed that the introduction of Sm³⁺did not have obvious effect on the optical transition property of Eu³⁺.