【作者】 娄世云；

【导师】 周少敏；

【作者基本信息】 河南大学 ， 凝聚态物理， 2013， 博士

【摘要】 石墨烯，一个由单层碳原子堆积而成的二维蜂窝状结构的新材料，由于其非凡的电、热、机械性能，及高比表面积（2600m²/g）、易功能化等特点，目前已成为材料科学领域的一个研究热点，在纳电子学、能源、环境、生物医学等领域具有广泛的应用前景。然而，石墨烯是零带隙材料，在紫外到近红外光学吸收范围内呈现带间吸收主导的恒定的光电导现象，没有共振吸收峰，因而在光电转化应用中受到限制。半导体纳米晶，具有分立的电子能级，尺寸及组分依赖的能级间距，由于其优异的可调控的光学性能成为另一个引起人们研究兴趣的纳米体系，在生物荧光标记、电致发光、高效光电转化器件中具有潜在的应用。然而，半导体纳米晶的尺寸较小，稳定性差，需要利用有机配体对其进行包覆，而有机配体严重阻碍了纳米晶之间的耦合，影响纳米晶之间的电荷传输，导致半导体纳米晶体系具有极低电导率和光电导率，限制了它们在光电等方面应用。因此，将石墨烯和半导体纳米晶复合被视为一个制备高性能光电器件中潜在的材料的方法，因为它们结合了纳米晶的光电转换能力和石墨烯优异的导电能力，在光电器件、光催化和太阳能电池方面具有潜在的应用前景。本论文探索了几种制备II-VI族三元半导体纳米晶以及石墨烯半导体纳米晶复合物的方法，对它们的光学及光电性能进行了研究，并发展了提高石墨烯半导体纳米晶复合物及纳米晶薄膜光电性能的方法。主要研究内容如下：1.利用两相离子交换的方法，将预先制备的油溶性的硒化锌（ZnSe）纳米晶与水溶液中的镉离子在高压釜中进行阳离子交换和有机配体交换反应，成功制备了水溶性的Zn_xCd_1-xSe合金纳米晶。通过对反应时间和反应温度精确控制，得到了荧光发射波长从412到570nm的水溶性合金纳米晶。得到的纳米晶具有较高的量子产率，良好的稳定性及结晶性。这种新的方法使离子交换和配体交换同时进行，克服了水相制备纳米晶量子产率低、结晶性差等缺点。该方法可扩展到制备其它高质量的水溶性纳米晶材料。2.基于第一部分的两相法，在室温下，将水溶性的氧化石墨烯（GO）与油溶性的CdSe及CdSe/ZnS纳米晶混合，得到了高质量的GO-CdSe和GO-CdSe/ZnS复合物。该复合物中，3.0nm CdSe和6.5nm CdSe/ZnS纳米晶均匀的分散在氧化石墨烯的表面。由于该复合物中，氧化石墨烯上存在一定量的含氧基团，不利于电荷的传输，导致该复合物的光电流很小。利用水合肼还原后，复合物的光敏值可达5200%,这是由还原石墨烯的导电能力增强所致。为了进一步提高石墨烯半导体纳米晶的光敏性质，我们研究了煅烧对ITO基底上的CdSe纳米晶薄膜光响应性质的影响。结果表明，煅烧使纳米晶晶粒尺寸增大，纳米晶间距减小，纳米晶表面的有机物有效去除，致使相邻纳米晶之间的势垒高度降低，有利于光生载流子的传输，导致CdSe纳米晶薄膜的光响应速度提高。3.利用溶剂热法，分别在二甲基亚砜和乙二醇介质中，成功地制备了石墨烯-Zn_xCd_1-xS纳米晶复合物。在二甲基亚砜中，通过加入不同比例的锌源和镉源，一步将Zn_xCd_1-xS纳米晶成功地固定在石墨烯片上。纳米晶的尺寸约为9nm,均匀的分布在石墨烯的表面。在该反应中，二甲基亚砜起着溶剂和硫源的双重作用。而在乙二醇中，硫脲作为硫源，聚吡咯烷酮（PVP）为表面活性剂，得到了石墨烯-Zn_xCd_1-xS纳米晶复合物。该复合物中纳米晶的尺寸为3-5nm，它们自组装成20-30nm的多孔球。复合物中纳米晶的组分由加入锌和镉前驱体的比例决定。这两种介质中得到的石墨烯-纳米晶复合物均具有较高的光电流。这是因为复合物中石墨烯能够提高光生载流子从纳米晶到石墨烯上转移能力，使得这些复合材料在光电器件中具有广阔的应用前景。更多还原

【Abstract】 Graphene, a two-dimensional material, composed of monolayer carbon atoms packed into ahoneycomb network, due to its extraordinary electrical, thermal, and mechanical properties, extremely highspecific surface area （2600m2/g） and easy functionalization, has become a hot research topic on thehorizon of material science at present and it is highly attractive for nanoelectronics, energy, environment,and bio-medical applications. However, perfect graphene does not show evident advantages inoptoelectronics owing to its nature as a zero-bandgap semiconductor. Constant photoconductivephenomenon presents in the inter-band absorption of graphene in the range from ultraviolet to near-infrared,that is to say, graphene has not resonance absorption peak, and thus the application for the photoelectricconversion performance is limited.Semiconductor nanocrystals have a discrete electronic energy levels, size-and component-dependentenergy level spacing. They have been aroused research interests due to its excellent optical properties. andhave intensively investigated for their significant potential in biological fluorescent labels,electroluminescent, highly efficient photoelectric conversion devices. However, in order to make thesmall-size nanocrystalline stable, nanocrystals are usually coated with organic ligands to preventaggregation and chemical degradation. Neveretheless, the coating organic-ligand layer severely limits theconductivity and the photoconductivity of nanocrystal-array films because of the influence of the ligandson the interactions and the spatial distribution of nanopaticles. This severely encumbers the applications ofnanocrystals in optoelectronic devices.Thus, the graphene and semiconductor nanocrystals composites are regarded as a potential materialfor the preparation of high-performance optoelectronic devices, because it combines the photoelectricconversion ability of the nanocrystals and excellent electrical conductivity of the graphene. Thesecomposite materials will have potential applications in optoelectronic devices, photocatalysis and solar cell,and so on.In this paper, we explored several methods to prepare II-VI ternary semiconductor nanocrystals andgraphene semiconductor nanocrystal composites, and studied on their optical and photoelectric properties.Meanwhile, we also delveloped methods to improve photoelectric performance of graphene semiconductor nanocrystal composite and nanocrystal film. The main contents are as follows:1. A facile two-phase method was developed to synthesize water-soluble Zn_xCd_1-xSe nanocrystalsthrough cation exchange reaction of the pre-synthesized ZnSe nanocrystals （in organic phase） with Cd2+（inwater phase）. The obtained high-quality alloy nanocrystals with different desired emission wavelengths（ranging from412to570nm） can be made reproducibly and precisely by varying the reaction temperatureand time. The obtained alloy nanocrystals not only have high quantum yields （QYs） and stability, but alsohave good crystallinity. For the first time, this new strategy allows that Zn_xCd_1-xSe nanocrystals formationand3-mercaptopropionic acid （MPA） functionalization to be executed simultaneously. It overcomes thepoor quantum yields and crystalline shortcomings in the water-phase synthesis. It is expected that thereported simple synthetic strategy can be developed into a very practical approach to produce high-qualitywater-soluble nanocrystals.2. Based on the first part, at room temperature, high-quality graphene oxide–CdSe （GO–CdSe） andgraphene oxide–CdSe/ZnS （GO–CdSe/ZnS） nanocomposites were successfully synthesized by a two-phasemixing method. Many CdSe and CdSe/ZnS nanocrystals are uniformly distributed on surface of the GOsheets with a uniform size of around3.0and6.5nm, respectively. The as-synthesized GO–CdSe andGO–CdSe/ZnS composites show small photocurrent due to the presence of a certain amount ofoxygen-containing groups on the graphene oxide which blocked the transport of photo-generated carriers.After hydrazine reduction, the photosensitive of the composites can reach5200%. This behavior can beinterpreted graphene enhanced the efficient transfer of the photoinduced carriers. In order to furtherimprove the photosensitive properties of graphene semiconductor nanocrystal composites, we investigatedthe annealing effects on the photoresponse properties of CdSe nanocrystal thin films on ITO substrate.Results indicate that the increase of the crystallite sizes and necking the nanocrystals and the organiccapping agents removed effectively are observed after annealing. The photoresponse speed is improvedafter the annealing of the CdSe nanocrystal film due to lowering the height of the potential barriers existingbetween adjacent nanocrystals and enhances the transport speed of photogenerated carriers.3. Grapheng-Zn_xCd_1-xS nanocrystal composites were successfully obtained by solvothermal methodin dimethyl sulfoxide and ethylene glycol, respectively. In the dimethyl sulfoxide, by adding differentproportions of the zinc and cadmium source, Zn_xCd_1-xS nanocrystals s were anchored on the surface of graphene by a one-step reaction. Zn_xCd_1-xS nanocrystals are uniformly distributed on surface of thegraphene sheets with a size of around9nm. In the reaction, dimethyl sulfoxide plays the role of the solventand the sulfur source. In the ethylene glycol, the graphene-Zn_xCd_1-xS nanocrystal composites wereprepared by adding thiourea and polyethylene pyrrolidone （PVP）. Zn_xCd_1-xS nanocrystals of about3-5nmself-assemble into a porous sphere with a size of20-30nm. The composition of the sulfide can be changedby changing the Cd/Zn ratio in the precursor. The graphene-CdS composites prepared by the two methodshave a dramatically increased photocurrent. This behavior can be interpreted by the efficient transfer of thephotoinduced carriers from the CdS nanocrystals to the graphene. The ability to improve photoinducedcharge transfer makes these composites extraordinarily promising in optoelectronic device applications.更多还原