【作者】 韦先涛；

【导师】 许武； 尹民；

【作者基本信息】 中国科学技术大学 ， 凝聚态物理， 2010， 博士

【摘要】 本论文的研究内容主要包含两个部分,第一部分是关于红外下转换材料的研究,主要的研究内容和研究结果在第2章到第4章中进行论述。第二部分是关于LED荧光粉材料的研究,主要的研究内容和研究结果在第5章中进行论述。论文的第1章为绪论部分,论述了本文研究内容的背景以及一些基本知识。首先给出了稀土发光材料研究的一些热点,然后讨论了光致发光的基本原理和稀土离子发光原理,最后简单介绍了量子剪裁现象和溶胶-凝胶制备方法。在关于红外下转换材料的研究部分中,首先在第2章介绍了红外下转换材料的研究背景和研究现状。然后在第3章和第4章中详细讨论了Y2O3:Bi3+,Yb3+、Y2O3:Eu3+,Yb3+和YVO4:Yb3+的制备、表征和发光性质。主要研究内容和结果如下:1.利用溶胶凝胶法制备了Bi3+和Yb3+共掺的Y2O3粉末。在紫外光的激发下观测到了980nm左右来自Yb3+的强烈近红外发射。监测Yb3+的发射,在320nm到380nm波段出现了一个宽带激发峰,该激发峰源于Bi3+的6s2→6s6p跃迁的吸收,这表明样品中存在从Bi3+到Yb3+的能量传递。研究了Bi3+和Yb3+的发光强度与Yb3+掺杂浓度的关系,发现在高掺杂浓度下Yb3+发光具有浓度猝灭现象。为了研究从Bi3+到Yb3+的能量传递过程,在355nm脉冲激光器的激发下,测量了Bi3+的荧光衰减曲线。实验结果表明Bi3+可以通过共合作能量传递过程有效的将能量传递给临近的一对Yb3+离子。该材料可以将硅太阳能电池不能有效利用的紫外光子转换为两个位于硅太阳能电池响应峰值附近的980nm近红外光子,因此在提高硅太阳能电池效率方面具有巨大的应用前景。2.利用溶胶凝胶法制备了不同掺杂浓度的Eu3+和Yb3+共掺的Y2O3粉末980nm红外半导体激光激发下,在共掺样品中观察到了Eu3+(5D0-7FJ(J=0,1,2))的红色上转换发光。测量了上转换发光的强度与掺杂浓度和激发光强度的关系,认为两光子过程的共合作能量传递是上转换发光的主要机理。激发Eu3+时观察到了来自Yb3+(2F5/2-2F7／2)的发光,其发射波长位于1μm附近。为了研究从Eu3+到Yb3+的能量传递过程,在266nm脉冲激光器的激发下,测量了Eu3+的5DJ(J=0,2)能级的荧光衰减曲线。结果表明共合作能量传递和交叉弛豫过程都对从Eu3+到Yb3+的能量传递有贡献。3.利用高温固相法制备了YVO4:Yb3+粉末。在紫外光的激发下,观察到了Yb3+的强烈近红外发射,其发射波长在980nm左右,对应于Yb3+的2F5／2→2F7／2跃迁。监测Yb3+的发射,观察到了一个从250nm到350nm的宽带激发峰,该激发峰源自于基质的吸收,这表明样品中存在从基质到Yb3+的能量传递过程。研究了基质和Yb3+的发光强度与Yb3+掺杂浓度的关系,发现在高掺杂浓度下Yb3+发光具有浓度猝灭现象。为了研究从基质到Yb3+的能量传递过程,在266nm脉冲激光器的激发下,测量了基质发光的荧光衰减曲线。结果表明,在掺入Yb3+之后,基质的荧光衰减加快,这更进一步说明了存在从基质到Yb3+的能量传递过程。研究认为共合作能量传递过程是其中可能的能量传递过程。YVO4:Yb3+可以通过共合作能量传递过程将一个紫外光子转换为两个近红外光子,这对于提高硅太阳能电池的效率非常有利。在Yb3+掺杂浓度为16%时,估算得到材料的量子转换效率为185.7%。然而这一估算的量子效率比实际值偏大,这是由于存在以下三个方面的因素：(1)Yb3+发光的浓度猝灭；(2)存在从基质到的电荷迁移态的直接能量传递；(3)存在从基质到猝灭中心的直接能量传递。在关于LED荧光粉的研究部分中,首先对白光LED和LED荧光粉进行了简要的介绍。然后详细讨论了Gd2O3:Bi3+,Eu3+的制备、表征和发光性质。主要研究内容和结果如下：用溶胶凝胶法制备了Gd2O3:Bi3+,Eu3+粉末。研究了该材料在紫外光激发下的发光性质。Bi3+的掺入使得Eu3+的激发带变宽,在320-380nm之间出现了一个宽激发带,这一激发带来源于Bi3+的6s2→6s6p跃迁的吸收,这说明材料中存在从Bi3+到Eu3+的能量传递。掺入Bi3+后,在325nm和355nm光的激发下样品中Eu3+的发光强度大大增加。这是由于Bi3+的发射和Eu3+的吸收之间的能量交叠很大使得从Bi3+到Eu3+的共振能量传递非常有效。共掺样品中Eu3+的发光强度可以达到单掺样品中的十倍,因此该材料是一种潜在的白光LED用红色荧光粉材料。然而,在266nm紫外光的激发下,共掺样品中Eu3+的发光非常明显地猝灭。Bi3+和Eu3+在吸收266nm光子上的竞争和可能存在的从Eu3+的电荷迁移态到Bi3+的1P1态的能量传递这两个过程是造成Eu3+发光猝灭的主要因素。更多还原

【Abstract】 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.更多还原