【作者】 牛金钟；

【导师】 李林松；

【作者基本信息】 河南大学 ， 凝聚态物理， 2010， 博士

【摘要】 近年来,发光半导体纳米晶由于具有独特的光学性质和光电特性等,所以其在材料科学领域引起了人们的广泛的研究兴趣,制备各种高质量的纳米晶材料的方法手段也得以快速发展。在现有的各种制备方法中,利用有机相条件高温合成的纳米晶具有更好的结晶性、较窄的尺寸分布以及更强的光稳定性和发光效率。Bawendi小组在1993年介绍了有机金属方法,制备得到了高质量的硫族化合物半导体纳米晶。2000年后,Peng小组发展了一种比较廉价的、毒性相对比较弱的有机相合成纳米晶的方法,即用氧化镉(CdO)代替甲基镉(Cd(CH3)2)合成镉-硫族元素纳米晶以及用硬脂酸锌代替甲基锌或者乙基锌合成Zn-硫族元素纳米晶。他们又使用十八烯代替三辛基氧化膦( trioctylphosphine oxide,TOPO)作为溶剂,使得合成纳米晶的成本进一步降低。但是,半导体纳米晶的合成在各个方面还存在着一些问题。针对现有方法在制备纳米晶时的缺陷我们主要解决了三个方面的问题。首先,制备硒化物纳米晶的时候,合成硒的前驱体时大家一般使用的是三丁基膦(tributylphosphine, TBP)或三辛基膦(trioctylphosphine,TOP)溶解硒作为硒的前驱体,我们把这种方法称为含膦方法,而三丁基膦和三辛基膦都是有毒易燃易爆并且比较昂贵的药品。在这里,我们使用十八烯直接作为硒粉的溶剂,大大降低了反应物的毒性,使反应可以在通风橱中进行,而不需要在手套箱中操作。并且由于十八烯的价钱便宜,我们降低了合成纳米晶的成本,只有原来的50%左右。其次,在实验室制备纳米晶的实验中,一次反应制备的纳米晶的数量很少,一般只有几毫克。在本论文中,我们提出了反向注入方法,一次可以制备多达5克的高质量的半导体纳米晶。再次,常规制备的半导体纳米晶,由于镉元素的存在,使得纳米晶具有较大的毒性,在一些方面,尤其是生物标记方面的应用受到了限制,在这里,我们利用II型核壳结构半导体纳米晶的概念,用ZnSe和ZnS作为壳层,把很小的CdS核限制在纳米晶的最里面。我们制备的CdS/ZnSe/ZnS core/shell1/shell2核壳结构纳米晶不但量子产率高,而且镉的含量被降低到只有1%的原子比,从而大大提高了纳米晶产物的绿色环保性质,使得制备的半导体纳米晶具有更加广阔的应用前景。本论文主要工作包括以下几个部分:在第二章中,我们制备了高质量的CdSe纳米晶以及CdSe/ZnS核壳结构纳米晶。在本章中,我们制备硒的前驱体时,是不需要使用易燃易爆且昂贵的TBP/TOP这样的含膦化学药品的。另外,与以往的反应方法不同,我们介绍了反向注入方法,可以大大提高一次反应制备的半导体纳米晶的数量,从而使得我们的方法更加适用于工业化的大规模生产。制备核壳结构纳米晶时,我们采用宽带隙半导体包覆窄带隙半导体,不但提高了核壳结构纳米晶的稳定性,而且提高了纳米晶的发光量子产率,量子产率可达50-80%,荧光峰的半高宽可控制在30 nm以内,在质量方面达到了已知的质量最好的CdSe纳米晶的水准。我们这种方法不但降低了合成成本,而且反应物更加绿色,使整个反应过程更加环保,尤其是一次反应制备的纳米晶的数量大大提高,为纳米晶在生物标记,太阳能电池,以及LED等方面的应用打下了良好的基础。在第三章中,我们使用十二烷基硫醇作为生成CdS纳米晶和ZnS纳米晶的硫源兼表面活性剂。通过调节反应温度,前驱体比例,我们可以优化实验条件,制备得到高质量的CdS和ZnS纳米晶。由于十二烷基硫醇的特殊的性质,这个方法非常适合于制备小尺寸的硫化物纳米晶。而由于十二烷基硫醇是一种高效的空穴受体,会强烈淬灭半导体纳米晶的本征荧光,所以我们在CdS纳米晶的外面包裹了ZnS壳层,形成CdS/ZnS核壳结构纳米晶,得到了CdS纳米晶的本征带隙发光。在第四章中,利用II型核壳结构半导体纳米晶的概念,我们制备得到了CdS/ZnSe/ZnS core/shell1/shell2核壳结构纳米晶,其发光范围覆盖了大部分的可见光区。我们选择三种不同尺寸的CdS作为核,包覆ZnSe作为壳层,形成II型核壳结构半导体纳米晶。II型核壳结构纳米晶的发光带隙可以通过材料的选择,核的大小,以及壳层的厚度来调节。通过这些调节,我们的CdS/ZnSe/ZnS纳米晶的发光可以从500 nm调节到630 nm,半高宽限制在50 nm内。在最外层包裹了很厚的ZnS壳层以后,纳米晶的发光量子产率从30%提高到50-60%。根据XRD测量结果以及纳米晶的高发光量子产率,我们认为整个纳米晶的生长过程是外延生长的。我们利用水相转移方法把制备的CdS/ZnSe/ZnS纳米晶溶解在水中,使之适合于生物方面的应用。根据长期的测量结果,我们制备的CdS/ZnSe/ZnS纳米晶具有很好的发光稳定性。在第五章中,我们研究了包裹ZnS壳层对纳米晶的影响。通过一系列的测量,我们发现,厚的ZnS壳层可以提高半导体纳米晶的发光量子产率,并且可以提高纳米晶的发光稳定性。利用我们合成的纳米晶,我们组装了模型光电器件,并对其性能进行了一些初步的测量。更多还原

【Abstract】 Since the introduction of quantum size effect by L. Brus, inorganic nanocrystals have attracted more and more attention of researchers. Especially for semiconductor nanocrystals, since their band gap can be tuned by their size, so the size of semiconductor nanocrystals will decide the photoluminescence positions. Recently, because of the unique optical and optoelectronic properties of semiconductor nanocrystals, much effort was devoted on the synthesis of high quality semiconductor nanocrystals.Generally, nanocrystals synthesized in organic solutions are better than the hydrothermo method, with better crystallinity, narrower size distribution, stronger stabilization, and higher quantum yields. In the synthesis of high quality nanocrystals, Bawendi’s group introduced organometallic method in 1993 and high quality cadmium chalcogenide nanocrystals were synthesized. After 2000, Peng’s group introduced a cheap, low toxic method to synthesize semiconductor nanocrystals. CdO was used to replac(eCd(CH3)2), reacted with oleic acid as the Cd precursor. Octadecene was used as noncoordinating, replace traditional trioctylphosphine oxide. These improvements made synthesis of nanocrystals greener and cheaper. But, many problems were still existed in the synthesis of high quality nanocrystals.Here, we made improvements mainly in three aspects. Firstly, for the preparation of metal selenide nanocrystals, selenium powder was directly dissolved in octadecene at elevated temperature as the Se precursor, without the use of hazardous and expensive tributylphosphine (TBP) and trioctylphosphine (TOP). Since octadecene was used as solvent for selenium, so the toxicity was largely reduced, and the reaction can be conducted without the use of glove box. And the cost of nanocrystals was reduced as much as 50%. Secondly, with the traditional methods, only few nanocrystals could be prepared in one reaction, generally some milli-grams of nanocrystals. Here we report the large scale synthesis of high quality semiconductor nanocrystals, about 5 grams of semiconductor nanocrystals was synthesized in one reaction. Thirdly, traditional cadmium chalcogenide nanocrystals are toxic because the existence of cadmium, so the application in in vivo biolabeling was limited. Here we use the type-II concept of core/shell nanocrystals to prepare green nanocrystals. With CdS as the innermost core, ZnSe as the middle shell, and ZnS as the outest shell, we got type-II/type-I nanocrystals. Cd content was reduced to about 1% in atomic ratio, and physically in the innermost core, so the resulted CdS/ZnSe/ZnS nanocrystals were much greener and have broad applications in future.In chapter 2, high quality CdSe nanocrystals and CdSe/ZnS nanocrystals were synthesized with green starting materials. For the preparation of Se precursor, we used octadecene, instead of hazardous and expensive TBP/TOP. And we use inverse injection method, using Cd precursor as the injection solution, and the volume of nanocrystals could be largely improved. This large scale synthesis of CdSe nanocrystals is more suitable for industrial applications. For the synthesis of CdSe/ZnS core/shell nanocrystals, the wide band gap material ZnS eliminated surface defects on CdSe, so the photoluminescence quantum yields were improved to 50-80%, and the full width at half maximum (FWHM) was below 30 nm. With our method, the synthesis cost was largely lowered, the starting materials were green, the synthesis scale was largely increased.In chapter 3, using dodecanethiol as the S source and surface ligands, CdS and ZnS nanocrystals were synthesized. Different reaction temperatures and precursor ratios were tested to optimize synthesis conditions. With this method, ultra small CdS nanocrystals were synthesized easily. As an effective hole acceptor, thiol groups will quench the band gap emissions of CdS and ZnS nanocrystals, so ZnS shells were grown on the CdS cores to recover the band gap emission of CdS.In chapter 4, high quality CdS/ZnSe/ZnS core/shell1/shell2 nanocrystals were synthesized. Using the type-II concept, emission of CdS/ZnSe nanocrystals could be tuned from 500 nm to 630 nm, by the control of core size of CdS and shell thickness of ZnSe. After the grown of outest ZnS shell, quantum yields were improved from 30% to 50-60%, and the Cd content was reduced to about 1% in atomic ratio. According to the X-ray diffraction results and the high quantum yields, we think the growth of shells was epitaxial. By the phase transfer experiment, original hydrophobic nanocrystals were transferred into water phase, and the superior optical properties were retained.In chapter 5, thick ZnS shells were grown on the core nanocrystals. We investigated the influence of thick shell on the optical properties of nanocrystals. Thick shells not only improved the emission quantum yields, but also improved the luminescent stability. We assembled elementary devices with our nanocrystals and measured the current-voltage curves. And some promising results were presented here.更多还原