【作者】 郭琳娜；

【导师】 王育华；

【作者基本信息】 兰州大学 ， 材料物理与化学， 2014， 博士

【摘要】 无机纳/微米稀土上转换/下转换发光材料有重要的应用前景,如固体激光器、激光防伪、生物荧光探针、三维立体显示、发光二极管背光源和太阳能电池等方面。而它们的成分、尺寸、形貌和结构等对其发光性能有十分重要的影响,探索它们之间的关系以及按人们的意愿设计和组装具有独特性能和结构的上转换/下转换发光材料具有重要的意义。虽然各国在稀土上转换发光材料方面的研究投入了巨大的人力、财力和物力,但是研究结果表明,上转换发光效率太低以至于真正可以实际应用的材料仍然很少。因此上转换研究的重点之一就是进一步加强基础研究,寻找一些新的发光机理,新的制备方法和新的材料组合来提高上转换发光效率。与上转换发光相反,下转换就是将一个高能量光子转变为两个或者以上低能量光子的发光过程。近几年来,如何提高太阳能电池的能量转换效率成为人们研究的热点。近红外量子剪裁下转换发光材料,可以将一个高能量的光子转化为光谱响应性好的两个或多个近红外光子,从而提高了电池的光谱响应性并减少了载流子热能化损耗。但这些研究主要集中在高温固相法合成的块体材料中,且量子效率有待进一步提高。本论文我们集上转换和下转换发光等性能于一体,制备并研究了不同形貌、不同尺寸和不同稀土离子掺杂的含氧化合物Y2Si05、氟化物GdF3/BaGdF5/ErF3以及氟氧化物Lu6O5Fg的.发光性质。主要内容及结果如下：1.考虑到含氧化合物的化学稳定性好,首先选择Y2SiO5作为上转换发光的基质材料。研究了在980nm激发下Y1.98-2xYb2x Er0.02SiO5(0.00≤x<0.15)上转换发光的过程。发现红光猝灭浓度比绿光的高,这是因为Er3+离子向Yb3+离子的反向能量传递起主导作用。此外,Yb3+/Tm3+和Yb3+/Ho3+掺杂的Y2Si05样品分别发射蓝光和黄光,通过调节掺杂离子Yb3+/Ho3+/Tm3+勺浓度和激发功率进而改变蓝光和黄光发射强度比,从而在Y2SiO5:0.4%Tm3+,5%Yb3+,0.1%Ho3+样品中实现了上转换白光发射。2.考虑到氟化物声子能量低,可以降低无辐射弛豫跃迁几率,从而提高发光效率,所以选择了氟化物同时作为上转换发光及近红外量子剪裁下转换发光的基质。Gd3+本身在常温下具有磁性,所以本着磁光多功能材料的目标选择含有Gd3+的GdF3和BaGdF5作为基质。首先利用第一性原理分析了GdF3的能带结构,研究结果表明GdF3是直接半导体,其带隙为7.344eV。采用水热法合成了分散性良好且均匀的稀土离子掺杂的GdF3纳米棒。发现GdF3:0.15Yb3+/0.002Ho3+/0.008/Tm3+样品经过退火处理后,上转换发光的色坐标由偏红光移向白光区。还分析了稀土离子对Yb3+/Tb3+; Yb3+/Ho3+; Yb3+/Tm3+; Yb3+/Pr3+; Yb3+/Er3+掺杂在GdF3中的近红外量子剪裁机理和量子效率。其中GdF3:10%Yb3+,0.5%Pr3+的量子效率高达166%,GdF3:10%Yb3+,0.5%Er3+的量子效率高达150%,然而,在GdF3:x%Yb3+,0.5Ho3+(Tm3+,Tb3+)样品中并没有发现近红外量子剪裁的现象。故GdF3:10%Yb3+,0.5%Pr3+, GdF3:10%Yb3+,0.5%Er3+纳米材料在涂覆太阳能电池且提高太阳能电池光电转换效率方面有潜在的应用。对于BaGd0.9-x%Yb0.1Hox%F5(0≤x<1.1)样品,首先给出了BaGdF5晶体的结构数据,其中Gd3+在BaGdF5晶格中的位置相当于Gd3+随机取代BaF2中Ba2+的格位。采用水热法合成了具有纳米颗粒、微米级胶囊状、花生状形貌的BaGdF5:Yb3+/Ho3+样品,进一步分析了胶囊状、花生状形貌的形成机理为自组装生长机理。在980nm激发下,具有花生状形貌的BaGdo892Yb0.1Ho0.008F5样品有最强的上转换发射强度,对应的Yb3+→Ho3+能量传递效率为0.348%。BaGdF5在常温下的磁性大小为0.7emug-1/10kOe,这与生物分离方法中需要的磁性值1.0emug-1/10KOe相当。最后还研究了BaGdF5:Yb3+/Ho3+的近红外量子剪裁的性质,以反向能量传递的机理解释了近红外发射光谱特征,得到最佳的量子效率为192%。分别通过水热法和共沉淀法制备了具有不同形貌(薄片,八面体,花状和米粒状)的ErF3样品。由于ErF3中Er-Er之间的交叉弛豫能量传递占主导,所以在980nm光源的激发下,ErF3样品发射红光。通过适当地掺杂Li+增强了ErF3上转换／阴极射线发光强度。所以ErF3在生物学和场致发射显示器等行业具有潜在的应用价值。3.结合氧化物和氟化物的优点,选择氟氧化物作为上转换发光基质。Lu6O5F8作为一种新的氟氧化物,具有化学稳定性好,声子能量低的优点,但是至今没有关于LU6O5F8的合成方法、结构和发光性质的报道。本论文研究表明Lu6O5F8是正交晶系,一个晶胞中有76个原子,能带结构为间接半导体,带隙为4.13eV。为了提高上转换发射强度,本文采用共沉淀法制备了Li+离子和稀土离子Yb3+/Er3+(Ho3+,Tm3+)共掺的Lu6O5F8纳米晶,研究了它的上转换、下转换及阴极射线发光性质。结果表明适当Li+浓度的引入可以提高这三种发光形式的发光强度,但同一个样品的光致发光和阴极射线发光光谱不一样,归因于激发发射机理不一样。值得指出的是通过调节Li+的掺杂浓度最终在Lu6O5F8:6%Yb3+,0.3%Er3+,0.4%Tm3+,5%Li+样品中实现了上转换白光发射,同时Lu6O5F8:20%Yb3+,1%Er3+,3%Li+的上转换发射强度达到商用粉NaYF4:20%Yb3+/2%Er3+的5倍。此外利用添加剂辅助的水热法合成了具有多种大小均匀且形貌规整的Lu6O5F8:Eu3+/Tb3+/Ce3+/Dy3+纳/微米晶,包括纳米颗粒、球形、六棱柱形、花簇形、玫瑰花状等多种形貌。研究了Eu3+/Tb3+/Ce3+/Dy3+单掺的前驱体(NH4Lu2F7/Lu(OH)1.57F1.43)和最终产物Lu6O5F8在真空紫外、紫外和电子束激发下的发光性质。总之,集上转换,下转换,阴极射线发光,磁性于一体的多功能稀土掺杂材料在固态激光器,太阳能电池,场发射显示器,生物成像标记等方面有潜在的应用。更多还原

【Abstract】 Inorganic nano/micro-rare earth doped upconversion (UC) and downconversion (DC) materials have potential applications prospect, such as solid-state laser, laser anti-counterfeiting brand, fluorescence probe, three dimensional display, light-emitting diode black light and solar cells, etc. It is common sense that the properties of inorganic nano/micro-materials are dependent on their composition, size, shape and crystalline structure. Investigation on the ralationships among these features, and then designing and assembling the structure with unique properties have great significance. Researchers have invested substantive manpower, material resources and funds, but the study suggests that the low efficiency of UC would limit their practical application. Therefore, the selection of new luminescence mechanism, new preparation method and new matrix in order to obtain high luminescence efficiency is a hot spot in this field. In contrary to UC process, DC process is defined as that absorbing one high-energy photon and emitting two or more low-energy photons. In recent years, how to enhance the energy conversion efficiency of solar cells has become an important issue of concern. Near infrared quantum cutting (NIR QC) DC emission process, converting one high-energy photon (UV/VIS) into two or more low-energy infrared photons which can be better absorbed by silicon crystal, is a promising approach to enhance the energy conversion efficiency of Si solar cells. The energy loss due to the thermalization in c-Si solar cells could be minimized. However, intensive studies on the NIR QC materials are focused on high temperature solid phase synthesis of bulk materials and the quantum efficiency should be further enhanced. We combined UC and DC in one multi, prepared and investigated effects of different morphologies, sizes and dopants on the luminescence properties of Y2SiO5GdF3/BaGdF5/ErF3and Lu6O5F8. The main points are listed below:(1) Considering oxy-compound possesses high chemical stability, we select Y2Si05as the host of UC emission firstly.Under excitation at980nm, the UC emission processes of Y1.98-2xYb2x Er0.02Si05(0.00≤x≤0.15) were studied. The concentration quenching of the red light emission is higher than that of green, due to the back energy transfer between Yb3+and Er3+ions (4F7/2(Er3+)+2F7/2(Yb)-→4I11/2(Er)+2F5/2(Yb)); Furthermore, Yb3+/Tm3+and Yb3+/Ho3+codoped Y2Si05samples emit strong blue and yellow light, respectively. Bright white luminescence upon980nm near-infrared excitation was obtained in Y2SiO5:0.4%Tm3+,5%Yb3+,0.1%Ho3+by tuning the doping concentrations and pump power.(2) Fluoride has very low vibrational energy, and its nonradiative loss could be suppressed and a high quantum efficiency of the desired luminescence could be obtained. So we select fluoride as not only UC luminescence but also NIR QC host at the same time.It is well known that Gd3+ions have a4f7electronic configuration, so we choose GdF3and BaGdF5as matrices, which are expected to be efficient magnetic-optical multifunctional materials. The band structures and densities of state of GdF3were studied with the help of first principles calculations, and the direct band gap of GdF3was estimated to be7.443eV wide. A series of GdF3:Yb3+/Ho3+/Tm3+nanorods were prepared by a simple and green hydrothermal method. After heat treatment, the emission color coordinates of GdF3:0.15Yb3+/0.002Ho3+/0.008/Tm3+moved from red region to the central white region of the chromaticity diagram. In addition, NIR QC mechanisms and quantum efficiencies of GdF3:Yb3+/Ln3+(Ln=Ho, Tm, Er, Pr, Tb) nanorods were investigated. The results indicated that the quantum efficiency of GdF3:10%Yb3+,0.5Er3+and GdF3:10%Yb3+/0.5Pr3+are150%and166%, respectively. However, there is not any NIR QC phenomenon on the GdF3:x%Yb3+/0.5Ho3+(Tm3+,Tb3+) samples. Therefore, GdF3:10%Yb3+/0.5Er3+and GdF3:10%Yb3+/0.5Pr3+nanomaterials are convenient for practical applications in the coating of solar cells.For BaGd0.9-x%Yb0.1Hox%F5(0≤x≤1.1) samples, the structure information of BaGdF5solid-solution is obtained as the BaF2cubic system with Gd3+ions occupying Ba2+sites with the same possibility. Nanoparticles, capsule-like and peanut-like BaGdF5:Yb3+/Ho3+nano/microcrystals were obtained by hydrothermal method, and possible mechanism for the formation of capsule-like and peanut-like morphologies was regarded as self-assemble process. Under excitation at980nm, BaGd0.891Yb0.1Ho0.008F5with the peanut morphology has the highest UC emission intensity, and the energy transfer efficiency of Yb3+→Ho3+was calculated to be0.348%. In addition, the measured field dependence of magnetization of the BaGdFs nanoparticles shows excellent paramagnetism. At last, NIR QC properties of BaGdF5:Yb3+/Ho3+were investigated, and the back energy transfer Yb3+→Ho3+process was proposed to analyze the characteristic of the NIR emission spectra. The optimal quantum efficiency is192%.ErF3samples with different morphologies (flakes, truncated octahedral, flower-like and rice-like submicrocrystals) were prepared by aqueous-based hydrothermal and co-precipitation route, respectively. ErF3shows almost bright red UC emission under excitation at980nm, because cross-relaxation process of Er3+ions is the main one populating the4F9/2level of Er3+. It is worthwhile to note that UC and cathodoluminescence (CL) intensities are enhanced via further doping with Li+. As a conclusion, ErF3has potential applications in the biological and field emission display devices and so forth.(3) Combining the merits of oxide and fluoride, oxyfluoride was also chosen as the UC and DC luminescence host.Lu6O5F8, as a novel oxyfluoride matrix, has good chemical stability and low vibrational energy advantages. However, Lu6O5F8has not been investigated up to now on the synthetic method, detailed crystal structure as well as luminescence properties. The result indicated that Lu6O5F8belongs to orthorhombic system, and seventy-six atoms are presented in one unit cell, and the indirect band gap of Lu6O5F8is estimated to be4.13eV wide.Li+-doped Lu6O5F8:20%Yb3+,1%Er3+(Tm3+) samples were prepared by coprecipitaion method and fabricated on the basis of the luminescence efficiency and intensity increment by Li+doping, and their UC, DC and CL properties in one compound were studied. It was found that doping of Li+could enhance all the intensities of UC, DC and CL. The differences between PL and CL spectra are due to the different excitation mechanisms. It is worthwhile to point out that according to the effects of Li+on emission intensity ratio, white UC emission was achieved in the Lu6O5F8:6%Yb3+,0.3%Er3+,0.4%Tm3+5%Li+compared to Li+free sample with the same activator concentration. The integrated UC emission intensity of Lu6O5F8:20%Yb3+,1%Er3+,3%Li+is5times as strong as that of commercial UC phosphor (NaYF4:20%Yb3+,2%Er3+).In addition, Lu6O5F8:Eu3+/Tb3+/Ce3+/Dy3+with diverse controlled morphologies were prepared by additive-assisted hydrothennal method。 Furthermore, excellent luminescence properties of Eu3+/Tb3+/Ce3+/Dy3+single doped NH4Lu2F7/Lu(OH)1.57F1.43precursors and final product Lu6O5F8under excitation at VUV/UV/electron beam were investigated. In a word, combining UC, DC, CL and magnetism in one multi, would find potential applications in solid-state laser, solar cell, field emission display and biological imaging.更多还原