| The research content of this thesis consists of two parts:the research on near-infrared dwonconversion materials, which is discussed from Chapter 2 to Chapter 4, and the research on LED phosphor, which is discussed in Chapter 5.Chapter 1 is the introduction to the study. It discusses briefly the research background and the basic knowledge. First, the research spot of rare-earth luminescent materials is given. Then, the basic principles of photoluminescence and luminescence properties of rare-earth ions are discussed. At last, an introduction of quantum cutting and so-gel process is presented.In the part of near-infrared downconversion materials, Chapter 2 discusses the research background and the research progress on this topic at first. Then, in Chapter 3 and Chapter 4, the preparation, characterization and luminescent properties of Y2O3:Bi3+,Yb3+, Y2O3:Eu3+,Yb3+, YVO4:Yb3+are discussed in detail. The research contents and conclusions are listed below:1. Bi3+and Yb3+codoped cubic Y2O3 phosphors were prepared by pechini sol-gel method. Strong near-infrared (NIR) emission around 980 nm from Yb3+(2F5/2→2F7/2) was observed under ultraviolet light excitation. A broad excitation band ranging from 320 to 360 nm owing to the 6s2→6s6p transition of Bi3+ions was recorded when the Yb3+emission was monitored, which suggests a very efficient energy transfer from Bi3+ions to Yb3+ions. The Yb3+concentration dependence of both the Bi3+and the Yb3+emission was investigated. The decay curve of Bi3+emission under the excitation of 355 nm pulse laser was used to explore the Bi3+→Yb3+energy transfer process. It has been demonstrated that Bi3+ion can efficiently transfer their energy to two neighbouring Yb3+through the CET process. The results indicate that this material has potential application in the high efficiency silicon-based solar cells by downconversion of one UV photon which is almost useless in the silicon solar cell to two NIR photons around 980 nm where the Si solar Cell exhibits the greatest spectral response.2. Yb3+and Eu3+codoped Y2O3 phosphors with different doping concentration were synthesized by a pechini sol-gel method. Under 980 nm laser excitation, red emission (5Do-7FJ (J=0,1,2)) of Eu3+is observed in cubic Y2O3 codoped with Eu3+ and Yb3+. The doping concentration and laser power dependence of the upconverted emission were studied. The two photon process (cooperative energy transfer process) is discussed as the possible mechanisms for the red UC luminescence. Yb3+emission around 1000 nm (2F5/2-2F7/2) is reported upon excitation of Eu3+ions. The decay curves of 5DJ (J=0,2) emission of Eu3+under excitation of 266 nm pulse laser were used to investigate the Eu3+→Yb3+energy transfer process. Two energy transfer processes, cooperative energy transfer process and cross-relaxation process, are proved to make a contribution to the Eu3+→Yb3+energy transfer.3. Upon ultraviolet (UV) light excitation, an intense near-infrared (NIR) emission of Yb3+(2F5/2→2F7/2) around 980 nm was observed in YVO4:Yb3+phosphors. Owing to host absorption of YVO4, a broad excitation band ranging from 250 to 350 nm was recorded when the Yb3+emission was monitored, which suggests an efficient energy transfer from host to Yb3+ions. The Yb3+concentration dependence of the visible vanadate emission as well as the Yb3+emission were investigated. The decay curve of vanadate emission was measured under the excitation of a 266 nm pulsed laser. The decay time of the vanadate emission at 500 nm was remarkably reduced by introducing Yb3+ions, further verifying that the energy transfer from the vanadate host to the Yb3+ions is very efficient. Cooperative energy transfer (CET) is discussed as a possible mechanism for the NIR emission. The YVO4:Yb3+phosphor can convert each UV photon into two NIR photons via cooperative energy transfer, which has potential application in the high efficiency silicon-based solar cells. The calculated quantum efficiency can reach as high as 185.7%for the sample doped with 16 mol% Yb3+. But, this value may be over estimated for three reasons:(1) the concentration quenching effect of Yb3+; (2) the direct energy transfer from the excited VO43- to charge transfer state of Yb3+; (3) the quenching effect caused by the direct energy transfer from VO43- to quenching centers.In the part of LED phosphor, an introduction of white LED and LED phosphor is given at first. Then, the preparation, characterization and luminescent properties of Gd2O3:Bi3+, Eu3+are discussed. The research contents and conclusions are listed below:Bi3+and Eu3+codoped cubic Gd2O3 nanocrystals were prepared by pechini sol-gel method. Their photoluminescent properties were investigated under ultraviolet light excitation The introduction of Bi3+ ions broadened the excitation band of Eu3+ emission, of which a new strong band occurred ranging from 320 to 380 nm due to the 6s2→6s6p transition of Bi3+ions, implying a very efficient energy transfer from Bi3+ions to Eu3+ions. Upon 325 and 355 nm light excitation, the luminescent intensity of Eu3+ions was remarkably improved by the incorporation of Bi3+ions. The significant energy overlap between the emission band of Bi3+ions and the excitation band of Eu3+ions makes the efficient energy transfer from Bi3+ions to Eu3+ions possible The emission intensity of codoped sample can be as much as ten times that of the Eu3+singly doped sample, therefore Gd2O3:.Eu3+, Bi3+is a promising candidate for the application in white LEDs. But a significant quenching of Eu3+emission was observed under 266 nm light excitation when Bi3+was codoped. The competition from Bi3+ions on the absorption of 266 nm light and the energy transfer from excited Eu3+ions to the1P1 state of Bi3+ions, from which most of the energy is released as heat to the lattice by nonradiative process, are proposed to be the main reasons for the quenching of Eu3+emission under 266 nm excitation.
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