【作者】 李静；

【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， 2014， 博士

【摘要】 对稀土离子上转换现象的研究始于上世纪50年代，在过去的几十年里，人们对上转换发光机理进行了深入的分析，随着泵浦源、转换材料的发展，稀土上转换发光已经覆盖了整个可见光波段，而且在双频上转换三维立体显示、硅太阳能电池、防伪标记、固体激光器以及生物标记与医学成像等领域均显示出其广泛的应用前景。钪酸钙(CaSc2O4)由于具有较低的声子能量、较强的晶场、较近的原子距离以及较好的物理特性和化学稳定性，是一类非常优秀的上转换发光基质材料。本论文以稀土离子激活的新型氧化物材料CaSc2O4为研究对象，系统的研究了Tm3+/Yb3+及Ho3+/Yb3+在该基质材料中的上转换发光特性及其动力学原理，并发现了CaSc2O4: Tm3+/Yb3+中强的下转换发光现象，此外制备了有益于生物医学应用的亚微米级CaSc2O4和β-NaGdF4材料。具体研究内容及结果如下：1.研究新型氧化物材料CaSc2O4: Tm3+/Yb3+的上转换发光性质及其动力学原理。在CaSc2O4基质材料中，通过改变掺杂离子Tm3+及Yb3+的浓度，研究离子浓度对上转换发光强度、光谱分布以及能量传递速率的影响。在红外光谱区间（1000~1750nm），随着Yb3+浓度的增加，Yb3+离子980nm发射在Yb3+浓度为5%时达到最大值，而Tm3+离子1600nm发射在Yb3+浓度为15%时达到最大值。随着Yb3+浓度的增加，Yb-Yb和Yb-Tm对的距离减小，使得Yb3+离子之间的能量迁移速率增加，最终加快了Yb3+Tm3+能量传递。随着Tm3+浓度的增加，Yb3+980nm发射单调下降，表明材料中发生了有效的Yb3+Tm3+能量传递。光谱显示：Tm3+1600nm与Yb3+980nm发射的强度比正比于Yb3+浓度的平方，并且与Tm3+浓度成线性关系。结合稳态速率方程可知：第一步能量传递速率W1=C1n0(N0)2。在可见光谱区间，Tm3+离子480nm和800nm发射的最佳Yb3+浓度为10%，最佳Tm3+浓度为0.4%。随Yb3+和Tm3+浓度改变，Tm3+480nm和800nm发射的强度比的变化趋势与Yb3+离子980nm发射强度的变化趋势相同。实验测得光谱分布随浓度变化的规律与理论分析基本符合。此外，与浓度最优的典型氧化物材料Y2O3:0.004Tm3+,0.1Yb3+材料相比，浓度最优CaSc2O4:0.004Tm3+,0.1Yb3+样品展示了更强的上转换发光亮度，其中Tm3+近红外（800nm）和蓝光（480nm）发射分别增强了3.5和2.2倍。增强的上转换发光来源于CaSc2O4材料中Yb3+离子对980nm光子拥有更大的吸收截面（）以及Yb3+Tm3+第一步能量传递系数（C）更大、能量传递效率（ET）更高。利用光谱分布和寿命衰减曲线，计算得出：在CaSc2O4材料中，Yb3+的吸收截面（）是其在Y2O3材料中的3倍。在CaSc2O4材料中，初始能量传递系数（C）和能量传递效率（ET）分别为：C=9.29×10-17cm3s-1、 ET=0.7；在Y2O3中，初始能量传递系数（C）和能量传递效率（ET）仅为：C=2.87×10-17cm3s-1、 ET=0.47。2.研究新型氧化物材料CaSc2O4: Ho3+/Yb3+的上转换发光性质及其动力学原理。在CaSc2O4: xHo3+，yYb3+系列材料中，优化出最佳掺杂离子浓度分别为x=0.2%, y=10%。最佳样品CaSc2O4:0.2%Ho3+，10%Yb3+展现出良好的绿光单色性（Sgr=0.85），显示出其在多重荧光探针方面的应用前景。利用光谱分布、衰减曲线、泵浦能量的依赖性等方法深入探讨了CaSc2O4: Ho3+，Yb3+材料的上转换发光机制。Ho3+离子绿光和近红外发射的强度比不随激发波长和掺杂浓度的变化而变化，说明他们来自于同一能级（5F4+5S2），其泵浦路径为Yb3+:2F5/2+Ho3+:5I6→Yb3+:2F7/2+Ho3+:(5F4+5S2)；Ho3+红光上转换发射来自于5F5能级，该能级通过5I6→5I7的无辐射跃迁及随后的Yb3+:2F5/2+Ho3+:5I7→Yb3+:2F7/2+Ho3+:5F5的能量传递被布局。理论分析结果与实验数据基本吻合。另外，与浓度最优的典型氧化物材料Y2O3:0.2%Ho3+,10%Yb3+相比，浓度最优CaSc2O4:0.2%Ho3+,10%Yb3+样品展示了更强的上转换发光亮度，其中Ho3+离子绿光（545nm）和红光（660nm）发射分别增强了2.6和1.6倍。发光增强的原因为，CaSc2O4样品中Yb3+离子对980nm光子的吸收能力更强，并且存在更有效的Yb3+Ho3+能量传递。测量得到CaSc2O4:0.2%Ho3+，10%Yb3+样品的能量传递效率（ET）为50%。3.采用溶剂热和退火相结合的方法合成亚微米级棒状CaSc2O4: Tm3+/Yb3+材料。系统的研究了退火温度和时间对样品形貌和晶体结构的影响。与采用传统高温固相法合成的体材料相比，此方法制备的样品形貌规则、分散性好、尺寸小、发光性能良好，有利于其在生物荧光探针及生物成像等方面的应用。制备样品上转换发光更强，其中Tm3+的近红外发射(800nm)增强了2倍左右。4.采用简易的一步水热法制备具有六棱柱、六棱片、纺锤体以及球形等不同形貌的亚微米级纯相β-NaGdF4材料。系统的研究了前驱溶液的pH值以及采用螯合剂种类对形貌和晶体结构的影响。研究了Tm3+/Yb3+及Ho3+/Yb3+离子在不同形貌样品中的上转换发光性质，其中具有高结晶性的六棱形貌样品发光最强。利用测得的光谱分布和泵浦能量的依赖性，研究了其中的上转换发光机制。5.利用高温固相法制备CaSc2O4:0.2%Tm3+/xYb3+系列样品，研究其下转换发光性质。在Yb3+:2F5/2→2F7/2发射的激发谱中存在位于466nm处Tm3+:3H6→1G4吸收峰。用466nm激发Tm3+:1G4能级，除了Tm3+自身发射，还发现强的Yb3+:2F5/2→2F7/2发射带。随着Yb3+浓度的增加，Tm3+发射峰的强度逐渐下降；Yb3+离子发射峰的强度逐渐上升，在Yb3+=5%时达到最高点，随后由于浓度猝灭，强度开始下降。证明在CaSc2O4材料中，Tm3+:1G4→Yb3+:2F5/能级之间存在有效的能量传递。获得最高能量传递效率ETE=71%，理论最高量子效率TQE=171%。更多还原

【Abstract】 The study on upconversion luminescence (UCL) of rare earth (RE) ions dopedmaterials began in1950s. The UCL properties of RE doped materials have beenstudied extensively over the past few decades because of the interesting physicsresearch as well as the potential applications in solid-state lasers, high resolutiondisplay, biological labeling, infrared imaging, and diagnosis and therapy of diseasesand so on. CaSc2O4is a kind of promising oxide hosts because of the low cutoffphonon frequency, strong crystal feld, near atom distance, as well as stable chemicaland thermal properties.This thesis mainly focuses on the RE ions activated CaSc2O4oxide material. Weresearch upconversion properties and dynamics study in Tm3+/Yb3+and Ho3+/Yb3+codoped CaSc2O4oxide material in detail. The efficient quantum cutting process isalso found in CaSc2O4: Tm3+/Yb3+material. Furthermore, we have synthesized theCaSc2O4: Tm3+/Yb3+submicro-rods in favor of application in biological assays andmedical image. The major results obtained are as following:1. We research the doped concentration dependence of spectral distribution anddiscuss upconversion dynamics in Tm3+and Yb3+codoped CaSc2O4material. In theinfrared emission spectra from1000nm to1750nm, Yb3+emission reaches themaximum when Yb3+concentration reaches to5%, and Tm3+1600nm emissiongradually enhances when Yb3+concentration increases from1%to15%. The distancesof Yb-Yb and Yb-Tm pairs decrease with Yb3+concentration increasing, then theenhanced energy migration among Yb3+ions speeds up energy transfer from Yb3+toTm3+. With Tm3+concentration increasing, Yb3+emission has amonotonic decline. Itcan be considered as the indication of effcient Yb3+Tm3+energy transfer. Theintensity ratio of Tm3+1600nm emission to Yb3+980nm emission satisfies aproportional relationship with the Tm3+concentration and exhibits the quadratic dependence on Yb3+concentration. The first step energy transfer rate W1is written asW1=C1n0(N0)2. In the UCL spectrum, for480nm and800nm emissions of Tm3+ions,the optimized Yb3+concentration and Tm3+concentration is10%and0.4%,respectively. The480nm to800nm intensity ratio exhibits the same trend with Yb3+980nm emission intensity. Compared with that in dopant concentration optimizedY2O3:0.004Tm3+/0.1Yb3+, furthermore, the UCL around800nm and480nm inCaSc2O4:0.004Tm3+/0.1Yb3+is enhanced by a factor of3.5and2.2, respectively. TheUCL enhancement is attributed to a large absorption cross section (3times as that inY2O3) at980nm of Yb3+and Yb3+→Tm3+first step energy transfer coefficient(9.29×10-17cm3s-1in CaSc2O4vs2.87×10-17cm3s-1in Y2O3). The Yb3+→Tm3+energytransfer efficiency in CaSc2O4:0.004Tm3+/0.1Yb3+is0.70quite larger than0.47inY2O3:0.004Tm3+/0.1Yb3+.2. We research the doped concentration dependence of spectral distribution anddiscuss upconversion dynamics in Ho3+and Yb3+codoped CaSc2O4material. TheCaSc2O4:0.2%Ho3+,10%Yb3+sample has been optimized for the strongest greenUCL. And it presents the perfect green monochromaticity as Sgr=0.85, which favoursthe simultaneous tracking of multiple fluorescent probes. The studies of spectraldistribution, power dependence, and lifetime measurement reveal the UCLmechanism involved in CaSc2O4: Ho3+/Yb3+material. The intensity ratio of green toNIR emission does not vary with the excitation wavelengths and doped concentrations.Both emissions have the same time evolutions and pump power dependences under980nm excitation. We conclude the green and NIR emissions come from the sameupper levels (5F4+5S2). The evolution of the green intensity by experiment is in goodagreement with the theoretical calculation basing on infrared spectral distributions,illustrating it is populated via Yb3+:2F5/2+Ho3+:5I6→Yb3+:2F7/2+Ho3+:(5F4+5S2)pathway. The red emitting is from Ho3+:5F5level. The5F5state is populated through5I6→5I7nonradiative relaxation, subsequent Yb3+:2F5/2+Ho3+:5I7→Yb3+:2F7/2+Ho3+:5F5energy transfer. Furthermore, we found a large enhancement of UCL inconcentration optimized CaSc2O4:0.2%Ho3+/10%Yb3+. UCL intensities around545nm and660nm are enhanced by a factor of2.6and1.6by comparison with that inconcentration also optimized Y2O3:0.2%Ho3+/10%Yb3+, respectively. The largerabsorption cross section at980nm of Yb3+and Yb3+→Ho3+efficient energy transferplay an important role for achievement of intense UCL in CaSc2O4phosphor. Themeasured energy transfer efficiency reaches up to50%for CaSc2O4:0.2%Ho3+, 10%Yb3+.3. CaSc2O4: Tm3+/Yb3+submicro-rods were synthesized using the mildsolvothermal and annealing technique. The phase structures, morphologies, and UCLproperties of bulk and submicro-rod CaSc2O4: Tm3+/Yb3+samples were measured andinvestigated. The synthesized CaSc2O4: Tm3+/Yb3+sample possesses thecharacteristics of small size, remarkable monodispersity, and low synthesistemperature, compared with that prepared using conventional solid state reactionmethod. It also exhibits the stronger UCL than solid state reaction sample. Theenhancement factor approaches to1.5for800nm emission.4. The single phase β-NaGdF4submicron crystal phosphors were synthesizedusing the mild hydrothermal technique by one-step procedure. The pH value andchelators (EDTA and citric acid) have a crucial effect on the morphology of β-NaGdF4sample. The UCL properties for the Tm3+/Yb3+or Ho3+/Yb3+codoped samplessynthesized were researched under980nm excitation. The hexagonal prisms thatmeaning high degree crystallinity demonstrated the strong UCL in comparison withother morphologies such as spindles and spheres. By the spectral distribution andpower dependence, UCL mechanism is revealed in β-NaGdF4: Tm3+/Yb3+material.5. The series of CaSc2O4:0.2%Tm3+/x%Yb3+powder samples were synthesizedby a solid state reaction. An efficient near infrared quantum cutting has beendemonstrated. Upon excitation of Tm3+:1G4level with a blue photon at466nm, Yb3+:2F5/2level can emit two NIR photons around1000nm through cooperative ET fromTm3+to Yb3+. The estimated maximum energy transfer efciency (ηETE) from Tm3+toYb3+is71%. The theoretical quantum efciency (ηTQE) reaches up to171%. The Tm3+:3H6→1G4absorption around466nm is observed by monitoring Yb3+:2F5/2→2F7/2emission. Under466nm excitation corresponding to Tm3+:3H6→1G4transition, astrong emission band around1000nm assigned to the Yb3+:2F5/2→2F7/2transition isobserved. The emission intensities of Tm3+ion have a decline and NIR emissionintensity of Yb3+ion increases rapidly with Yb3+concentration increasing. For thehigher Yb3+concentration over5%, Yb3+emission is reduced due to concentrationquenching. It indicates the presence of Tm3+→Yb3+efficient energy transfer.更多还原