【作者】 余启钰；

【导师】 刘春艳；

【作者基本信息】 中国科学院研究生院（理化技术研究所） ， 物理化学， 2009， 博士

【摘要】 胶态半导体量子点由于其优异的随尺寸变化的光学性质和灵活的溶液处理化学在基础研究和应用研究中都具有重要的意义。本论文的主要工作是发展II–VI族半导体量子点材料的“绿色”低成本合成方法,探讨纳米晶的成核与生长机理,并研究量子点的光学及其他性质。(一)在多相体系中低温合成油溶性的CdS和CdSe量子点。利用吸收光谱,荧光光谱,透射电镜,X射线衍射的方法表征了所合成的CdS和CdSe量子点。另外,通过光谱分析,研究了两种吸收位置分别在359 nm和379 nm的CdS幻数团簇的光学性质。(二)在十八烯/甘油双液相体系中合成均匀的,发射强荧光的CdS量子点。此体系比通常用的甲苯/水双液相体系的优势在于其低毒性,而且在常压下还可用于较高温度(一般100~200°C)下的合成反应。另外,此体系也可用于Ag纳米晶的合成。所得的CdS和Ag纳米晶由于表面的憎水包覆层而分散在十八烯相。在同属的液体石蜡/甘油体系中也得到了类似的结果。(三)用紫外—可见吸收光谱方法研究了CdS幻数团簇的性质。近年来,半导体幻数团簇由于其独特的结构和稳定性而受到很多重视。一种吸收在约323 nm的幻数团簇作为中间产物,在合成CdS纳米晶的反应前期出现。他们具有极小的颗粒尺寸,但这种团簇胶体溶液的聚沉稳定性非常低。在醇分子的作用下,他们还可以转换成其他两种CdS颗粒,一种吸收在约309 nm,另外一种吸收在约348 nm。这两种CdS颗粒非常不稳定;当醇分子从体系中移除后,他们转化成另外一种吸收在约312 nm的幻数团簇。(四)研究了石蜡/甘油体系中CdS纳米晶的生长情况,提出了一种幻数团簇参与的CdS纳米晶成核与生长机理。我们跟踪了不同温度时,CdS纳米晶的紫外—可见吸收光谱、荧光光谱及浓度的变化情况。发现温度较低时,纳米晶的成核与生长交迭在一起,得到的纳米晶尺寸分布较宽;而在较高温度下,可以使成核在很短时间内完成,得到均匀的纳米晶。结合反应中间产物——幻数团簇的性质,我们提出了CdS纳米晶可能的成核与生长机理。(五)研究了醇分子对CdS量子点的荧光猝灭作用,提出了可能的猝灭机理。影响醇分子对CdS量子点荧光猝灭的主要因素为醇分子的空间位阻和浓度:醇分子的位阻越小,浓度越高,则其猝灭作用越强。这种荧光猝灭作用是可逆的;当醇分子从体系中移除后,CdS量子点的荧光可以完全恢复。醇分子对CdS量子点的带边荧光的猝灭能力远大于其对缺陷荧光的猝灭能力。我们提出了可能的静态猝灭机理。另外,我们还研究了醇加入前后的时间分辨荧光光谱。更多还原

【Abstract】 Colloidal semiconductor quantum dots (QDs) are important in fundamental and application research, due to their size-dependent optical properties and excellent solution processing chemistry. In this dissertation, we mainly developed“greener”and cheaper synthesis methods for II–VI semiconductor quantum dots, discussed the nanocrystal (NC) nucleation and growth mechanism, and studied the optical and other properties of the QDs.(1) Oil-soluble CdS and CdSe QDs were prepared in multiphase systems at low temperatures. As-prepared CdS and CdSe QDs were characterized with ultraviolet–visible (UV–vis) absorption and photoluminescence (PL) spectra, transmission electron microscopy (TEM) and X-ray diffraction (XRD). In addition, the optical properties of two magic-size cluster (MSC) species, absorbing at 359 and 379 nm respectively, were studied by using spectral analysis methods.(2) Nearly monodisperse and fluorescent CdS QDs were prepared in a 1-octadecene (ODE)/glycerol biphasic system. Compared to toluene/water, the system is environmentally friendlier, and can be used for higher temperature (generally 100~200°C) synthesis. Besides, Ag NCs can also be prepared in this system. As-prepared CdS and Ag NCs were dispersed in the ODE phase due to their hydrophobic capping layers. In a congeneric system of liquid-paraffin/glycerol, similar experimental results were obtained.(3) The properties of a CdS MSC species were studied using a UV–vis spectroscopic technique. In recent years, semiconductor MSCs have been receiving much attention due to their special structure and stability. A 323-nm-absorbing MSC species, as intermediate, developed at an early stage of the synthesis of regular CdS NCs. The colloid solution of the MSCs was unstable with respect to aggregation in spite of the extremely small size. Induced by alcohol molecules, the MSCs would transform into other two CdS species, absorbing at ~309 and ~348 nm respectively. The two species were highly unstable; they would transform into a 312-nm-absorbing MSCs after the alcohol molecules were removed from the system.(4) The NC growth in liquid-paraffin/glycerol was studied, and a MSC-mediated nucleation–growth mechanism of CdS NCs was proposed. We followed the temporal evolution of the UV–vis absorption and PL spectra, and the particle concentration of CdS NCs at different synthesis temperatures. At low synthesis temperatures, the nucleation and growth stages were overlapped, which resulted in NC ensembles with broad size-distributions. At relatively high temperatures, the nucleation stage finished in a short time, which resulted in monodisperse NCs. Taking into consideration of the properties of the cluster intermediate, we proposed a plausible mechanism for CdS NC development.(5) Alcohol-induced PL quenching of CdS QDs was studied, and a plausible quenching mechanism was proposed. The quenching effect was mainly dependent on the steric hindrance and the concentration of the alcohol. Weaker hindrance and higher concentration of the alcohol made a stronger quenching effect. The quenching effect was reversible; the PL was totally recovered after the alcohol was removed from the system. The quenching effect was significantly stronger to the band-edge emission than the trap-emission. A possible static quenching mechanism was proposed. Besides, the time-resolved PL spectra before and after alcohol addition were studied.更多还原