【作者】 安利民；

【导师】 苏文辉； 杨延强；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2010， 博士

【摘要】 荧光纳米材料具有量子尺寸效应、表面效应及宏观量子隧道效应等多种新奇效应,在光、电、磁、热等方面呈现出优异的性能,近年来成为科技领域研究的热点。量子点(QDs、或称为半导体纳米晶体)的发射谱线窄、激发谱线宽、荧光波长随粒径变化连续可调、性能稳定,诸多理化性质方面的优点使其有望取代目前使用的有机荧光材料,在细胞成像、DNA测序、免疫检测、温度传感、白光LED等领域有广阔的应用前景。目前,量子点材料已经有较成熟的合成技术路线,但是仍需改进,例如油相合成应降低成本,水相合成需提高晶体质量。量子点受表面界面效应影响较大,很多光学特性产生的物理机制复杂,目前尚未完全研究清楚,如金属表面增强荧光、上转换荧光、量子点与有机分子间的相互作用等。与传统的油相合成方法不同,我们使用十八烯(ODE)作为反应溶剂,替代昂贵、有毒的三辛基氧化膦(TOPO),合成得到发光性能同样优异的不同颜色的CdSe量子点,降低了合成成本。在氯仿溶液中,获得了CdSe量子点与聚苯胺的复合体,实验发现量子点尺寸减小和聚苯胺浓度增加时复合体荧光强度会降低。研究表明,一方面是由于CdSe量子点向聚苯胺的共振能量传递,另一方面是由于聚苯胺可以有效截获CdSe量子点中的电荷传递,两者共同导致复合体的荧光淬灭现象。在水相以巯基羧酸为表面配体,合成得到不同尺寸的CdSe和CdTe量子点。CdSe量子点在金岛薄膜表面,与在玻璃表面相比较,荧光积分强度增加四倍,而荧光寿命减小接近一半。CdSe量子点和金之间的能带结构相匹配,所以金岛薄膜中激发态的电子隧穿注入CdSe核区,增加了量子点激发态的电子密度,从而导致荧光增强。荧光寿命减小则是由于金岛薄膜的存在,导致量子点中自由激子数目和辐射跃迁效率增加的结果。利用银纳米粒子表面配体(聚乙烯吡咯烷酮)与CdTe量子点之间的Cd—O键相互作用,将CdTe量子点自组装到银纳米粒子表面,观察到CdTe量子点的荧光增强、峰位红移和寿命缩短等现象,研究表明上述现象的出现与金属表面强局域电磁场和表面配体等有关。这种自组装方法简单、方便,对量子点在生命科学、化学、医学等领域应用中提高检测灵敏度具有重要的意义。制备出壳层厚度可以精确控制的水溶性CdTe/CdS核壳量子点,在该体系中存在不同于CdSe/CdS、CdTe/ZnS等核壳量子点的荧光峰展宽、大幅度红移以及荧光寿命增加现象。我们认为,随着CdS厚度的增加,量子点会从I型逐渐过渡到II型。对于II型CdTe/CdS核壳量子点,不仅有CdTe核区导带电子与价带空穴间的直接复合,还有CdS壳层导带电子与CdTe核价带空穴界面处的间接复合,这种发光机制的改变导致荧光光谱的上述变化。利用400nm和800nm不同波长的低强度飞秒激光,对CdTe和CdTe/CdS核壳量子点溶胶进行激发,获得其稳态和时间分辨荧光性质。800nm飞秒激光激发下,CdTe和CdTe/CdS量子点产生上转换发光现象,上转换荧光峰与400nm激发下的荧光峰相比发生蓝移,而且蓝移值与荧光量子产率有关,激发光功率与上转换荧光强度间满足平方关系。研究表明,CdTe和CdTe/CdS量子点荧光由带边激子态和诱捕态两种成分组成,两种成分峰位不同,带边激子态波长较短,诱捕态波长较长。800nm激发与400nm激发时相比,激子态相对比重增加,这导致荧光峰的蓝移。所以,表面态辅助的双光子吸收是低激发强度下量子点上转换发光的主要机制。更多还原

【Abstract】 Fluorescent nano-materials have many kinds of novel effects, such as the quantum size effect, the surface effect and the macroscopic quantum tunnel effect and so on. Fluorescent nano-materials become the research hotspot in recent years because of the outstanding performance in the optics, electricity, magnetism, thermology. Quantum dots (QDs or semiconductor nanometer crystals) have the narrow emission line, wide stimulation spectral line, stable property, continuously adjustable emission wave length along with particle size and much other excellent physical and chemical nature. It is hopeful to substitute present organic materials and obtains the widespread application in the domains of cell imaging, DNA sequencing, immunity examination, temperature sensing, white light LED and so on. At present, the synthesis of QDs has the mature technology route, but the oil phase synthesis needs to reduce the cost and the aqueous phase synthesis needs to raise the fluorescence efficiency. QDs have strong surface and interfacial effect. The physical mechanism of many optical characters is not completely clear, sucn as the metal surface enhancement fluorescence, up-coversion fluorescence, the interaction between QDs and other materials etc.We used the octadecene (ODE) to substitute three octyl oxidation phosphine (TOPO) as the resolver. It reduced the synthetic cost. The CdSe QDs with different color have been synthesized in the oil phase. The fluorescence performances of CdSe QDs are similar to the QDs obtained from the traditional oil phase synthetic method. In the chloroform solution, the polyaniline (PAni) were mixed with the different size CdSe QDs. Then CdSe QDs/PAni compound was obtained. The fluorescence intensity of CdSe QDs/PAni compound reduces when the size of CdSe QDs reduces. Simultaneously the polyaniline density increases can also reduce the fluorescence intensity of CdSe QDs/PAni compound effectively. The analysis indicates that the energy transfer and charge transfer occur between the CdSe QDs and the polyaniline. It causes the fluorescence quenching. On the one hand, resonance energy transmission has produced from CdSe QDs to the polyaniline. On the other hand the polyaniline may intercept the electric charge transmission in CdSe QDs effectively and cause the interruption of its radiative recombination process.The CdSe and CdTe QDs with different size were synthesized in the aqueous phase. The mercaptoacetic acid and thiohydracrylic acid were taken as the superficial ligands. Metal-Enhanced Fluorescence from CdSe and CdTe QDs were investigated. The spectrum change of the QDs on the metal surface is dependent on at least two factors: an enhanced local electromagnetic field and an increase in the intrinsic decay rate of the QDs. A significant enhancement (4-fold) in fluorescence intensity is observed from the CdSe QDs on the Au island thin film as compared to that on the glass. On the other hand, the fluorescence intensity decay of CdSe QDs on the Au island thin film shows a faster decay with nearly 2-fold decrease in average fluorescence lifetime. The energy band structure matches between the CdSe QDs and Au, therefore the excited electrons in the Au island thin film can pour into the CdSe nuclear area. It increases the excited state electron density in QDs, thus causes the fluorescence enhancement. The fluorescent lifetime reduction is a result of the free exciton number and the radiative transition efficiency increase.CdTe QDs were self-assembled on the surface of silver nanoparticles (NPs) capped with polyvinylpyrrolidone (PVP) through the ligand field effect of Cd—O. A significant 2-fold enhancement in the integrated fluorescence intensities, red shift of fluorescence peak and obvious decrease of lifetime were observed in the CdTe QDs assembled on the Ag NPs in comparison with the pure CdTe QDs. The fluorescence enhancement factor and red shift were found to depend on the Ag NPs concentration. Compared with previous reports, the occurrence of the self-assembly of CdTe QDs on the surface of PVP-capped Ag NPs is fairly simple and easy. From a practical point of view, the combination of CdTe QDs with Ag NPs may lead to the fluorescence enhancement, which could be utilized in a variety of biological, chemical and medical detection applications.Water-soluble CdTe/CdS core/shell quantum dots (QDs) with the different shell thickness capped were synthesized following the synthetic method of successive ion layer adsorption and reaction. The relationships of fluorescence quantum yield, the spectrum structure with CdS shell thickness were discussed. The CdTe/CdS core/shell QDs exhibite a significant red shift of emission peak, full width of half maximum (FWHM) increase and fluorescence lifetime lengthening which is different to CdSe/ZnS, CdTe/ZnS core/shell QDs. We hypothesize that CdTe QD will gradually evolve into a type-II core/shell QDs from a type-I CdTe core with increasing CdS shell thickness. For the type-II CdTe/CdS QDs, the fluorescence can come not only from the recombination between an electron in the conduction band of CdS shell and a hole in the valence band of CdTe core, resulting in a significant red shift and a long radiative lifetime, but also from the recombination from an electron in the conduction and valence bands of CdTe core. The two recombination processes will cause a broadening of the fluorescence band, as well as a significant red shift. The lack of fluorescence lifetime lengthening and quantum yield increasing was ascribed to the surface influence of the thick shell.We study the steady-state and time-resolved fluorescent properties of CdTe and CdTe/CdS core/shell QDs by one and two photons excitation with a femto-second laser under the low intensity. 800 nm laser excitation causes a blue shift of the emission peak compared with 400 nm laser excitation. The blue shift value is related to fluorescence quantum yield. The blue shift value is small when the fluorescence quantum yield is high. Near-quadratic laser power dependence of the up-conversion intensity and bi-exponential decay kinetics are observed. It is found that up-conversion fluorescence is composed of a photoinduced trapping and a band edge excitonic state. The blue shift of the emission peak is caused by the relative change in fluorescence intensity between excitonic and trapping states. Two-steps and two-photons absorption involving the surface as the intermediate states has been proposed for up-conversion fluorescence of CdTe/CdS quantum dots.更多还原