Study On The Preparation And Optoelectronic Properties Of Graphene/Ⅱ-Ⅵ Semiconductor Nanocrystal Composites

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.