【作者】 彭洪尚；

【导师】 黄世华；

【作者基本信息】 北京交通大学 ， 光学， 2007， 博士

【摘要】 对于三价稀土离子掺杂的无机纳米材料而言，表面效应是影响其发光特性的主要因素。但限于材料体系和制备方法的差异，目前人们对表面效应的认识尚有一些模糊和矛盾的地方。本论文的前半部分工作以高对称性的La₂O₂S：Eu³⁺和YVO₄：Eu³⁺为对象，对表面效应作用下纳米晶发光特性进行了深入的研究，主要内容如下：1．利用干胶热释分解法制备了La₂O₂S：Eu³⁺纳米晶（平均粒径～18nm），通过激光光谱对表面Eu³⁺进行了探测。在时间分辨光谱上，表面Eu³⁺的发光表现为短延迟时间下原本简并⁵D₀→⁷F₁（E）谱线在高能侧的劈裂。⁵D₀→⁷F₂区域的激光选择激发的结果表明，表面发光中心局域对称性的退化呈现出突变行为。结合纳米晶的材料体系和生长过程，认为所制备的硫氧化物纳米晶可能具有La₂O₂S／La₂O_2+xS_1-x结构。2．通过点电荷模拟YVO₄：Eu³⁺纳米晶中的表面缺陷，利用修正的点电荷模型计算了不同局域环境中Eu³⁺的⁷F₂晶场能级。由计算结果可清晰看到，随着模拟电荷微扰的增强，⁷F₂晶场能级逐渐发生劈裂和移动。基于理论计算的⁷F₂能级，对⁵D₀→⁷F₂区域选择激发光谱中的表面发光中心进行了归属，认为纳米晶中主要的表面缺陷为Eu-O²断键。此外，发射谱线归属结果也表明表面效应对选择定则的放松作用是不严格的。表面效应作用下，某些原本禁戒的光谱跃迁在选择定则上变得允许，但实际的跃迁可能只存在于表面Eu³⁺，对近表面Eu³⁺来说并不发生。稀土配合物是进行生物荧光标记的一类重要材料，虽然具有诸多优点，但在稳定性、量子效率等方面仍存在不足。若将稀土配合物包埋于基质材料形成的纳米微粒中，颗粒表层对外界环境的隔离作用可大大提高其稳定性和量子发光效率，从而克服了单个稀土配合物分子的自身缺陷。本论文的后半部分围绕稀土配合物Eu（DBM）₃TPPO(DT-Eu³⁺)荧光纳米颗粒的制备及发光性质，进行了一系列的研究工作。首先利用再沉淀法制备了纯相DT-Eu³⁺的荧光纳米颗粒（直径～10 nm）。对纳米颗粒进行AFM、时间相关吸收光谱以及荧光光谱等析，发现纯相纳米颗粒容易团聚，且水分子的荧光猝灭现象比较严重。通过引入适量疏水性硅烷，制备了具有较强荧光、均匀尺寸和良好分散性的杂相DT-Eu³⁺@OTS纳米颗粒。微粒表面形成的亲水性二氧化硅薄层，不仅防止了纳米颗粒的疏水性聚集，也在一定程度上隔断了配合物与水分子的联系。DT-Eu³⁺@OTS纳米颗粒具有与DT-Eu³⁺单分子相同的发光，但其量子发光效率要比在THF溶液中提高了近两成。被溶剂无辐射弛豫掉的振动能在纳米颗粒得以保存被认为是量子效率提高的主要原因。通过分析新制备纳米颗粒悬浊液的吸收光谱和荧光光谱随时间的演化过程，对DT-Eu³⁺配合物纳米颗粒的形成机制进行了探讨。认为超声作用形成的均匀分布注入液的微液滴是纳米颗粒的前驱体，随着THF的渗出这些疏水性分子在疏水作用下相互聚集而形成纳米颗粒。更多还原

【Abstract】 For trivalent rare earth doped inorganic nano-materials, surface effect is the key factor affecting their luminescent properties. However, restricted by the differences of material system and preparation method, knowledge on surface effect is still somewhat discrepant and hazy. To further study the luminescent properties of nanocrystals under surface effect, systems with high symmetry such as La₂O₂S: Eu³⁺ and YVO₄: Eu³⁺ are studied. The main research is as follows:1. La₂O₂S: Eu³⁺ nanocrystals with a mean size of 18 nm are prepared by gel thermolysis. The surface Eu³⁺ ions are first detected by time-resolved spectra in the ⁵D₀→⁷F₁ region. Because the symmetry of the sites occupied by surface Eu³⁺ ions is lower, the ⁵D₀→⁷F₁ line, which is doubly degenerate in the bulk crystal, is split, and the fluorescence lifetime becomes shorter. The results of the laser-selective excitation indicate that the degradation of the site symmetry of Eu³⁺ seems to be abrupt, which means the as-synthesized La₂O₂S: Eu³⁺ nanocrystals might be of the La₂O₂S /La₂O_2-xS_1+x core-shell structure and the shell is not in a disordered state but a rather pure one.2. By simulating surface defects in YVO₄: Eu³⁺ nanocrystals with point charges, ⁷F₂ crystal field levels of Eu³⁺ under different local microstructures are calculated employing a modified point charge model. From the calculation results, the gradual splitting and shift of ⁷F₂ energy levels can be seen clearly with the strengthening of disturbation of simulated charge. Based on the theoretical ⁷F₂ levels, surface Eu³⁺ indicated by the ⁵D₀→ ⁷F₂ selective excitation spectra are attributed, and Eu-O² broken bond is assumed to be responsible for the surface defects in YVO₄: Eu³⁺ nanocrystals. In addition, Furthermore, it is proposed from the attribution that relaxation of selection rules by surface effect is feeble for near surface Eu³⁺, whereas it is more intense for surface Eu³⁺.Lanthanide chelates are widely used as fluorescent labels in bioassay. Though several advantages are favorable, their shortcomings in stability and lower quantum yield hinder their further application. If lanthanide chelates are doped into nanoparticles formed with inert material, particle layer of particle would separate chelate molecules from surroundings. Hence, stability and luminescent yield would be greatly improved. The second part of this paper is about the preparation of fluorescent nanoparticles from Eu³⁺ chelate -Eu（DBM）₃TPPO (DT-Eu³⁺), and studies of fluorescent properties.Firstly, small sized DT-Eu³⁺ nanoparticle （10 nm） suspension is prepared by a novel reprecipitation-encapsulation method. It is found that the pure nanoparticles are easy to aggregate in aqueous, and their fluorescent are severely quenched by surrounding water molecules. Then an alkyl alkoxysilane encapsulation agent is included during the nanoparticle formation process, and the resultant DT-Eu³⁺@OTS nanoparticles are well dispersed, along with even size and intense luminescence. The developed encapsulation layer around nanoparticle inhibits aggregation and quenching from water. The emission spectrum of hybrid nanoparticle is the same as that of DT-Eu³⁺ molecule. However, their luminescent yield is nearly 20 % higher than that in THF solution.The formation mechanism of nanoparticles is studied with the time-correlated UV-vis spectroscopy and timebased luminescence. The evenly distributed microjets of stock solution formed in the mixture by sonication are believed to be the pre-nanoparticles, which nucleate and grow into nanoparticles under hydrophobic interaction with the diffusion of THF.更多还原