Studies On Third-order Nonlinear Optical Properties Of CdSe Nanocrystals And CdSe Nano-composite Material

?-? semiconductor nanocrystals?NCs?have many potential applications such as all-optical switching,optical data storage and optical limiting device due to their unique nonlinear properties and large nonlinear optical response.CdSe NCs is one of the most widely used colloidal quantum dots in this field.A new method for the synthesis of colloidal CdSe NCs proposed by Stijn Flamee et al.have attracted a lot of attention,because of the reaction under the environment of air?none protective gas is used?,which allows us to carry the experiment easily and at a low cost.What is more,compared with Cd Se NCs synthesized by the traditional method,the obtained CdSe NCs have a different surface structure,smaller NC size and good monodispersity.Graphene as a hot spot in the field of material science has exhibit wide application in biology,physics,chemistry,medicine and other fields due to its high surface area,extraordinary optical,electrical,and mechanical properties.In order to make the unique properties of graphene applied in more fields,many different materials are used to composite with graphene.The graphene and semiconductor nanocrystals composites are regarded as a potential material,because it combines the good ability of the nanocrystals and excellent properties of the graphene.The existence of quantum dots can effectively reduce the agglomeration of graphene and improve the stability of the material.The thesis consists of five chapters,the first chapter briefly introduces the theory of nonlinear optics,and the basic characteristics and development of II-VI semiconductor materials?CdSe quantum dots?and graphene oxide are reviewed here;In the second chapter,we introduce the third-order nonlinear optical effect and the Z-Scan measurement,including the basic principle and calculation method;The preparation and the third-order nonlinear optical properties of different sizes of colloidal CdSe quantum dots are shown in chapter 3;In the fourth chapter,CdSe-GO composite is prepared and the third-order nonlinear optical properties of the composite material using the Z-Scan method are studied;Chapter 5 show the summary and prospect of the work.The main contents of this thesis are as follows:1.Different sizes of CdSe NCs were synthesized through the high temperature hot injection method under air?without any protective gas?by controlling the reaction time,and samples were tested by spectral characterization,structure characterization and morphology characterization.The absorption and fluorescence spectra of the samples showed that the absorption peaks and the fluorescence peaks appear obvious red shift with the increase of the size of NCs.XRD measurement results showed that the three peaks of 2?=25.3°,42°and49.7°are corresponding to the crystal planes?111?,?220?and?311?,which indicates that the obtained CdSe NCs are cubic zinc blende structure.It can be observed that the size of CdSe NCs with high purity is very small and uniform and the shape is nearly spherical.So it can be seen that the colloidal CdSe NCs prepared by this method is much better.2.The third-order nonlinear properties of Cd Se NCs with different sizes were measured by using the single beam Z-scan technique,and the results were analyzed and calculated.We obtained the open-aperture?OA?and closed-aperture?CA?curve at 532 nm.The OA curve shows symmetrical valley shape,while the CA curve exhibits a valley-peak distribution,which indicates that both the third-order nonlinear refractive index n₂ and nonlinear absorption coefficient?are positive.The third-order nonlinear absorption and refraction of the samples with different sizes are compared here,results show that the intensity of the absorption and refraction changes with size,and the nonlinear absorption is the main role.We calculated the measurement data and then put the third-order nonlinear optical susceptibility of all the samples into a figure.From the figure we can see clearly that,with the growth of CdSe NCs,the nonlinear susceptibility is increased initially and then reduced and the third-order nonlinear susceptibility of CdSe NCs reaches the maximum value of 1.81×10^-10esu with the size of 3.3 nm due to the resonance absorption?532 nm?.Compared with the previous research,it has been increased by an order of magnitude,which shows that the CdSe NCs have potential application value in the field of nonlinear optics.3.For further study,we prepared graphene oxide with high extent of oxidation by using the improved Hummers.Then the prepared CdSe quantum dots were synthesized on the graphene oxide?GO?by the two-phase mixing method,and the third-order nonlinearity of the composites was studied.The results of Z-scan showed that the third-order nonlinearity of GO was improved effectively.