【作者】 刘艳玲；

【导师】 宋峰； 黄维；

【作者基本信息】 南开大学 ， 光子学与光子技术， 2013， 博士

【摘要】 本论文研究了在不同形状的金银纳米颗粒或金膜等金属纳米结构存在下Er3+/Yb3+共掺Y2Ti207发光薄膜、向列相液晶5CB、掺杂型EuEDTA和Eu(TTA)3稀土配合物液晶材料的发光特性,以及液晶对金属纳米结构的表面等离子体共振(SPR)或者局域表面等离子体共振(LSPR)的可调性研究。具体研究内容为：一、制备了球形、三角形的金银纳米颗粒,研究了在这些金属纳米颗粒存在下,Er3+/Yb3+共掺Y2Ti2O7发光薄膜上转换和近红外发光性质的变化。对比金银纳米球,三角形银纳米薄片对Er3＋的增强程度最大,在525nm,546nm,659nm处对应的最大增强因子为16.7,15.6,17.7,对于Er3+在1528nm处的近红外波段的最大增强因子为5.6；我们还具体分析了Er3+/Yb3+样品中上转换与近红外发光的能量传递过程,并提出了4113／2能级的粒子布居的另一种实现过程(能量反向转移),在此薄膜样品的上转换659nm处红光强度要大于上转换绿光强度,这与之前报道的Er3＋单掺的Y2Ti207薄膜文献结果不同。我们还分析了金属纳米颗粒增强Er3+发光的机理,提出两种机理：一方面是根据金属纳米颗粒的LSPR的辐射增强,当LSPR共振能量与发光材料的激发态分子的能量有重叠或者金属纳米颗粒的散射光与发光分子的辐射光在远场处的耦合,从而增强发光分子的辐射光强；另一方面是由于金属纳米颗粒对稀土离子样品中的羟基吸附,使得Er3+周围的羟基自由基增多,促进了Er3＋辐射跃迁,同时抑制Er3＋发生无辐射跃迁,进而提高Er3+辐射跃迁的效率,实现对Er3+发光的增强。二、金属纳米颗粒的LSPR特性与颗粒尺寸、形状以及周围介质环境有着密切的关系。而液晶具有各向异性的光学特性,我们将金属纳米结构与液晶相结合,通过液晶的光学性质来调控LSPR的共振。在理论上,根据麦克斯韦方程,在准静态近似条件下;通过调节金属种类、液晶壳层厚度、液晶的各向异性成分以及入射偏振方向来实现核金属纳米球颗粒的LSPR共振可调。研究发现液晶材料的各向异性成分对银纳米球的LSPR共振峰的半高宽(FWHM)的影响程度要大于液晶层厚度的变化；同时分析了X-Y平面内的局域场场增强分布,因此通过适当调节介质壳的各向异性成分,不仅可以增加电场强度,还可以改变局域场增强的区域分布,可见金属纳米球颗粒的各向异性对实现最大场增强有着重要的意义。在实验中,利用金属纳米颗粒的LSPR对周围介质的折射率的敏感性,对比了空气环境中与向列相液晶中的金纳米膜的LSPR,发现LSPR共振峰从在空气中的550nm红移到液晶中591nm处,此实验现象与准静态理论下的模拟结果符合一致。三、研究了在球形金银纳米颗粒存在下,LSPR对向列相液晶5CB的发光增强。在金纳米小球存在时,向列相液晶5CB在422nm处的发光明显增强,随着金纳米球颗粒质量浓度的增加,增强因子持续增加,当浓度为601μL时,达到最大增强,其后随着颗粒浓度的进一步增加,增强因子不再增加反而持续减小。在纳米颗粒与液晶相溶的过程中,相同浓度的银纳米小球在向列相液晶5CB中的溶解时间小于金纳米小球。在理论上,根据修正的Mie理论,将5CB发光分子近似为偶极子,研究了金属纳米颗粒对液晶发光分子的近场局域场增强和辐射衰减速率的影响。四、我们分别合成了EuEDTA和Eu(TTA)3两种配合物,然后将金属纳米颗粒溶入稀土离子掺杂液晶发光材料中,由于液晶分子的散射,Eu3＋发光相对比在盐化合物中强度增大,而又加入银三角形纳米颗粒时,根据LSPR,使得Eu3＋发光得到二次增强,最大增强因子为8,因此通过银三角形纳米颗粒的LSPR效应和液晶分子的散射作用,得到了高效发光EuEDTA、Eu(TTA)3掺杂的5CB液晶有机材料。为了分析Eu3＋辐射发光机理,我们还监测了615nm处Eu3＋的荧光寿命。另外,结合液晶分子的发光特性,在334nm激发下(5CB的激发波长),液晶5CB分子吸收激发光能量,然后将能量传递给Eu3＋,稀土离子Eu3＋在吸收能量后被激发到激发态,通过辐射跃迁,发出615nm处的红光,液晶的蓝紫光与红光耦合为淡粉色光,对应的CIE色坐标为(X=0.3823,Y=0.2332)。以上研究结果表明,利用金属纳米结构可实现稀土材料、液晶材料和掺杂型稀土液晶材料的发光增强,不同金属纳米结构对稀土离子或者液晶分子发光有不同程度的增强。液晶材料可以实现对金属纳米结构SPR或者LSPR的可调。更多还原

【Abstract】 In this work, we studied the metal nanostructures (different kinds of metal nanoparticles and Au films) on the luminescence of Er3+/Yb3+co-doped Y2Ti2O7films,5CB and EuEDTA (Eu(TTA)3) doped in liquid crystals, respectively. The tuning surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) of metal nanstructures by liquid crystal were also investigated. The main content of our work can be shown as follows:1. The photoluminescence of Er3+/Yb3+co-doped Y2Ti2O7films in the upconversion emission bands and near infrared emission when silver/gold (Ag/Au) nanoparticles (NPs) were doped. With the precipitation of the Ag nanoprisms, more intense green (525nm,546nm), red (659nm) upconversion emission bands and strong enhanced near infrared (NIR) emission were observed and the enhanced factors were up to16.7,15.6,17.7and5.6folds, respectively. The energy transfer were discussed in detail. The EBT process extra ordinarily contributes to the red UC emission. It is different from the results that the intensity of red emission is weaker than that of green emissions in Er3+single-doped Y2Ti2O7film. The accordance between the localized surface plasmon resonance (LSPR) of NPs and the UC emission band significantly increased the luminescence intensity. The efficiencies of the radiative emissions were enhanced also due to the hydroxyl groups primarily absorbed around the NPs.2. The localized surface plasmon (LSPR) was sensitive to the size and shape of the nanostructures, as well as the local surrounding environment. Liquid crystals (LCs) have a large optical anisotropy due to their anisotropic shape and alignment. By applying the anisotropy LCs, we achieve feasible tuning the LSPR. The far-field and near-field properties of a spherical nematic LCs coated metal NPs have been investigated in an external field, basing on the quasistatic theory. The resonant wavelength is tunable by varying metallic material of core, anisotropy extent and thickness of LCs. In addition, we find the extent of anisotropy of LCs have more significant effect on the full width at half maximum (FWHM) for the LSPR peak than the thickness. The field enhancement is along the incident polarization near the outer surface of the shell. The direction of field is reverse in the inner surface comparing with the one if outer shell. In contrast to isotropy shell, the LSPR shows an obvious red shift and field enhancement near outer surface of the shell always is stronger. Compared wit LSPR of gold film immersed in the air and LCs, LSPR redshift from550nm to591nm. It is in accordance with the simulated results based on the quasistatic theory.3. The enhanced photoluminescence in Ag/Au nanoparticles doped5CB LCs were observed. In presence of Au nanospheres, the enhancement of422nm was obvious, and the enhancement factor gradually increases until it reaches a maximum and then decreases. The optimum luminescence is obtained by the adding60μL Au nanosphere. The solubility of Ag nanospheres in LCs was easier than the Au nanospheres. In addition, we turned to the case of a5CB fluorophore, treated as an oscillating dipole, interacting with a Ag nanosphere in the near field. The effect on the radiative and nonradiative decay rate were further investigated, basing on the improved Mie theory.4. We prepared the europium complex [EuEDTA] and [Eu(TTA)3]. We showed LCs materials with narrow band red photoluminescence by doping a nematic host matrix with europium(Ⅲ) complexes. In the presence of Ag nanoprism, the luminescence of Eu3+ions was highly enhanced. The enhancement factor is8.The fluorescence lifetime of Eu3+at615nm was also observed, which further demonstrated the influence of LSPR effect.In addition, the europium complexes doped nematic LCs samples was excited by334nm (the excitation wavelength of5CB), considering the luminescence of5CB. The energy absorbed by5CB molecular was transferred to the Eu3+ions. The Eu3+ions then were excited. Finally, Eu3+ions return to the ground state from those levels through radiating615nm red light. The blue light radiated by5CB molecular coupled with the red light from Eu3+ions. The samples appeared light pink light, corresponding to the CIE chromaticity coordinates (X=0.3823, Y=0.2332) of the emission.Our work shows that the enhanced luminescence of rare earth ions and LCs was demonstrated in presence of the different metal nanostructures. The enhancement strongly depended on the nanostructures and the concentrations of nanoparticles. The tunable LSPR shift of metal NPs was achieved feasibly by liquid crystal. These would give us a better understanding of metal enhanced fluorescence by the metal nanostructures.更多还原