【作者】 刘永福；

【导师】 张家骅；

【作者基本信息】 中国科学院研究生院（长春光学精密机械与物理研究所） ， 凝聚态物理， 2012， 博士

【摘要】 白光发光二极管（white LED）具有全固态、体积小、寿命长、高效、环保、节能等诸多优点，被认为是新一代照明光源，是国际照明领域研究的热点。目前产生白光LED的主流方案是在蓝光GaN基LED芯片上涂敷传统的黄色荧光粉Y₃Al₅O₁₂:Ce³⁺(YAG:Ce³⁺)。YAG:Ce³⁺的发射光谱主要为黄绿光，红光成分较少，封装的白光LED显色指数低（<80），不及传统的白炽灯和荧光灯。为解决这一问题，人们提出绿色和红色荧光粉混合的方案替代黄色荧光粉。这种方案存在着不同荧光粉之间的再吸收问题，影响白光LED的流明效率。同时，不同荧光粉还存在发光温度特性和老化特性不一致的问题，导致白光LED的色彩随温度和使用时间漂移。因此，获得光谱成分均衡的全色LED荧光粉是人们追求的更高目标。本论文主要针对目前蓝光基白光LED中缺少单一基质全光谱高显色性荧光粉的问题，选择新型的钪硅酸盐为基质材料开展研究。首先，研究了具有石榴石结构的Ca₃Sc₂Si₃O₁₂（CSS）体系。稀土离子Ce³⁺激活的绿色荧光粉Ca₃Sc₂Si₃O₁₂:Ce³⁺(CSS:Ce³⁺)由于较高的发光效率和优越的热稳定性而引起人们的广泛关注。该荧光粉在蓝光激发下发射出峰值505nm的绿光，光谱中缺少长波成分，不能合成单一白光。针对这一问题，本论文工作采用调控基质阳离子组分、氮化技术、掺杂技术等方法，在CSS中成功引入长波发射中心，实现了全光谱发射，获得了具有高显色性的白光LED荧光粉，主要研究结果如下：（1）通过调控基质组分，在(Ca_2.94-xLu_xCe_0.06)(Sc_2-yMg_y)Si₃O₁₂中改变Lu³⁺-Mg²⁺的含量，Ce³⁺的发射光谱可从505nm的绿光连续红移至565nm的黄光。固定Mg的含量为y=1，研究了不同x值时Lu含量对晶体结构的形成、发光特性以及温度特性的改善作用。研究结果表明，Lu的引入提高了Ce³⁺的发光亮度。这是由Ce³⁺吸收能力增强引起而不是内量子效率增加引起。通过调节Lu的含量获得较纯的石榴石晶体，实现了Ce³⁺的较强宽带发射。x=0.54时，Ce³⁺的发光最强，为不含Lu，即x=0时的156%。此时含Lu荧光粉的激活能（0.2eV）大于不含Lu的（0.18eV），因而表现出较好的温度特性。该单一荧光粉与蓝光芯片结合封装制作成白光LED，其显色指数高达86、发光效率高达86lm/W。（2）研究了电荷平衡对石榴石晶体形成的作用，分析了Lu³⁺和Mg²⁺分别取代Ca²⁺和Sc³⁺引起的键长与共价性的变化。结果表明Ce³⁺激发、发射光谱的移动是由Ce³⁺的5d轨道晶场劈裂和质心位移共同作用的结果。（3）在(Ca₂Lu_1-xCe_x)（ScMg）Si₃O₁₂中，研究了不同Ce³⁺浓度荧光粉的发光特性，观察到了Ce³⁺的多个发光中心以及Ce³⁺-Ce³⁺之间的能量传递。当x从0.01增加至0.15时，Ce³⁺的发射位置由530nm红移至575nm。Ce³⁺的最佳浓度为x=0.09。采用此浓度的单一荧光粉，我们获得了显色指数高达87.9、色温5814K，效率77lm/W的白光LED；x=0.15时，还获得了显色指数大于80，色温低于4000K的暖白光LED。不同Ce³⁺浓度下荧光粉的温度特性和Ce³⁺离子之间的能量传递机理做了研究。（4）采用氮化技术，研究了Si₃N₄对绿色荧光粉CSS:Ce³⁺结构和发光特性的影响。将N引入Ce³⁺的配位环境，不仅保持了Ce³⁺在505nm的绿光发射，同时还增加了Ce³⁺在610nm的红光发射。监测610nm红光中心，其激发谱为一个属于红光中心510nm的激发带和一个属于绿光中心450nm的激发带，表明Ce³⁺的绿光中心向红光中心有能量传递，测量Ce³⁺的荧光衰减曲线得出能量传递效率为38%。通过调节N的含量，绿色荧光粉CSS:Ce³⁺逐渐转变为橙黄色荧光粉CSS-N:Ce³⁺。由绿光中心向红光中心的能量传递，在CSS-N:Ce³⁺中实现了红、绿全光谱发射。将氮化之后的CSS-N:Ce³⁺单一荧光粉应用于白光LED时，获得了较高显色指数为86、较低色温为4700K的白光。（5）采用掺杂技术，将长波发射中心Mn²⁺引入CSS:Ce³⁺中，构建了基于能量传递机制的CSS:Ce³⁺, Mn²⁺体系。Mn²⁺在CSS中可以产生两个发光中心，分别为Mn²⁺占据Ca²⁺位置时产生的574nm黄光中心(Mn²⁺（I）)和Mn²⁺占据Sc³⁺位置时产生的680nm红光中心(Mn²⁺（II）)。Ce³⁺-Mn²⁺共掺时，Ce³⁺向Mn²⁺的两个发光中心同时存在有效能量传递。测量Ce³⁺、Mn²⁺荧光衰减曲线研究了Ce³⁺→Mn²⁺能量传递的动力学过程，能量传递的效率为45%，传递速率为14.01×106s-1。通过Ce³⁺→Mn²⁺能量传递，实现了Ce³⁺的505nm绿光、Mn²⁺的574nm黄光和680nm红光的全光谱发射。三价稀土离子Ln³⁺（Ln=La、Lu、Gd、Y）占据Ca²⁺格位可以平衡Mn²⁺占据Sc³⁺格位产生的电荷差，进而调节CSS:Ce³⁺, Mn²⁺体系中Mn²⁺的红光发射。采用Y³⁺增加Mn²⁺的红光发射，在CSS:Ce³⁺, Mn3+单一荧光粉中，我们获得了显色指数高达91⁹2白光。其次，研究了具有菱形结构的Ba₉Sc₂（SiO₄）₆:Ce³⁺, Mn²⁺(BSS:Ce³⁺, Mn²⁺)体系。主要研究结果如下：（6）通过X射线精细扫描，Ce³⁺、Mn²⁺在BSS晶体中分别倾向占据Sc³⁺、Ba²⁺的格位。BSS:Ce³⁺发射出峰值383nm很强的紫外光，有望用于紫外杀菌、消毒、光动力治疗以及光复印等领域。BSS:Mn²⁺发射出峰值615nm较弱的红光。BSS:Ce³⁺, Mn²⁺中，通过Ce³⁺→Mn²⁺的能量传递，Mn²⁺的红光发射显著增强，此时该荧光粉有望为紫外基白光LED提供红色光源。更多还原

【Abstract】 Phosphor-converted white light emitting diodes （pcWLEDs） are regarded as anew lighting source for the next generation due to their high efficiency, long lifetime,and environment friendly. The most current pcWLEDs employ the yellow emittingY₃Al₅O₁₂:Ce³⁺(YAG:Ce³⁺) garnet phosphor and blue InGaN LEDs. Due to deficientemission in the red spectral region, YAG:Ce³⁺phosphor limits the color renderingindex （CRI） of pcWLEDs below80, in comparison to CRIs of100for incandescentlamps and82⁸5for fluorescent lamps. To achieve high CRIs （>80）, a phosphorblend of yellow or green emitting phosphors with a red emitting phosphor isgenerally applied. The phosphor mixture gives fluorescence reabsorption andnon-uniformity of luminescent properties, resulting in a loss of luminous efficiencyand time-dependent shift of the color point. To achieve a single phase phosphor withfull color emissions is therefore desirable.The main purpose of this work is to achieve a full-color-emitting phosphor forwhite LEDs with high color rendering. Therefore, studies on the novel scandiumsilicate host matrix are carried out.First, we investigate the novel scandium silicate Ca₃Sc₂Si₃O₁₂（CSS） withgarnet structure. Ce³⁺doped green-emitting CSS:Ce³⁺phosphor has attracted muchattention for its high emission intensity and high thermal stability superior to YAG:Ce³⁺. However, under blue excitation, CSS:Ce³⁺exhibits green emissions atabout505nm, which lacks yellow and red emissive components and cannot generatewhite light by this single phosphor. In this work, to enrich the longer wavelengthvisible emission and achieve white light in the single CSS host, the approaches ofmodification by cations, nitriding, and codoping are employed. The main resultsobtained are listed as follow:For the Lu-Mg modified Ce³⁺:Ce³⁺(CSS:Ce³⁺) garnet phosphor.（1） In (Ca_2.94-xLu_xCe_0.06)(Sc_2-yMg_y)Si₃O₁₂（x=0⁰.94, y=0¹）, controllableluminescent intensity and emitting color of the Ce³⁺are studied as a function of Luand Mg contents. With increasing value of x and y, the Ce³⁺emission shift from505to565nm, correspondingly resulting in the luminescence color of the phosphorchanges from green to yellow. Fixing the Mg content to be1, the effects of Lu ongarnet crystal structure formation, luminescence properties, and temperaturecharacteristics are discussed. It is revealed that the Lu-induced luminescentenhancement is the result of an increase in absorbance of Ce³⁺rather than theinternal quantum efficiency. Intense and broadband emission is realized bycontrolling the Lu content to obtain a pure garnet phase. The maximumluminescence intensity is obtained at x=0.54, which is as high as156%of theLu-free phosphor. The Lu-containing phosphor also exhibits better temperaturecharacteristics for its bigger activation energy （0.20eV） than the Lu-free one （0.18eV）. Combining the present phosphor with a blue LED chip, a white LED with anexcellent color rendering index of86and a high luminous efficiency of86lm/W isobtained.（2） The effect of charge balance on the garnet crystal structure formation isstudied. The changes of bond length and covalence caused by the replacement ofLu3+and Mg²⁺ for Ca²⁺ and Sc3+are analyzed. The shift of the Ce³⁺emission andexcitation can be attributed to the combined results from crystal field splitting effectand centroid shift of Ce³⁺5d levels.（3） In (Ca₂Lu_1-xCe_x)（ScMg）Si₃O₁₂(CLSM:xCe³⁺x=0.01⁰.15), luminescence properties for various Ce³⁺concentrations are investigated. Different Ce³⁺emissionsite and energy transfers between them are observed, resulting in a red shift of theemission spectra from530to575nm with increasing x from0.01to0.15. Theoptimum concentration is x=0.09. Combining with blue （460nm） LEDs,CLSM:Ce³⁺shows excellent performances for pcWLEDs with higher colorrendering index of87.4⁸7.9and lower color temperature of5034⁵814K,especially for warm pcWLEDs with a high color rendering （CRI>80） and a lowcolor temperature （CCT <4000K）. Energy transfers and thermal quenchingbehaviors depending on Ce³⁺concentrations are discussed.For the N^3- incorporated CSS:Ce³⁺phosphor.（4） Adding Si3N4into green emitting CSS:Ce³⁺garnet phosphor generates anadditionally red emission band peaking around610nm that are assigned to Ce³⁺ionshaving N3-in their local coordination. The excitation spectrum of the red bandconsists of not only a distinct band at510nm of itself but also an intense blue bandat450nm that belongs to the typical Ce³⁺ions with green emission, indicating anotable energy transfer from the green emitting Ce³⁺ions to the red ones. The energytransfer significantly enables the achievement of a broad emission spectrum coveringa red and green spectral region suitable for generating white light upon a blue LEDexcitation. The decay patterns of the red and green fluorescence are discussed inrelation to the effect of energy transfer. A white LED with a high color rendering of86and a low color temperature of4700K is fabricated using the present singlephosphor.For the energy transfer modified CSS:Ce³⁺, Mn²⁺phosphor.（5） Mn²⁺ may not only occupy Ca2+sites to generate a yellow emission (Mn²⁺（I）)at574nm but also Sc3+sites to generate a red emission (Mn²⁺（II）) at680nm.Remarkable energy transfers from the green emitting Ce³⁺to both Mn²⁺（I） andMn²⁺（II） occur upon blue excitation into Ce³⁺. Concentration dependence of Mn²⁺emission is analyzed based on Ce³⁺→Mn²⁺energy transfer, steady state rateequations, and fluorescence lifetimes. Energy transfer efficiency （ηT） and rate （W） are calculated with values as high as45%and14.01×106s-1, respectively.Considerable Mn²⁺substitution for Sc3+can be performed through balancing theircharge difference by introducing trivalent rare earth ions Ln³⁺（Ln=La, Gd, Lu, Y）to replace Ca²⁺, making tunable full color emission available in CSS:Ce³⁺, Mn²⁺.White LEDs with excellent color rendering index of91⁹2are achieved by usingthe present single phosphor.Secondly, the investigations of the novel scandium silicate Ba9Sc2（SiO4）6:Ce³⁺,Mn²⁺(BSS:Ce³⁺, Mn²⁺) with rhombohedra structure are carried out. The main resultsare listed as follow:（6） Analysis of XRD patterns indicates that Ce³⁺and Mn²⁺in BSS are located atSc3+and Ba2+sites, respectively. BSS:Ce³⁺exhibits an intense UV emission bandpeaking at383nm, indicating this phosphor could be used for phototherapy andphotocopying. Codoping Mn²⁺into this material can generates a red emission bandat615nm of Mn²⁺through Ce³⁺→Mn²⁺energy transfer. BSS:Ce³⁺, Mn²⁺could beused for UV-based white LEDs as the red light source.更多还原