Studies On The Third-order Nonlinear Optical Properties Of Bi₂Te₃/graphene Composite Materials

Since the advent of laser technology,nonlinear optical materials,which have nonlinear optical effect,large nonlinear coefficient and small optical loss,have been widely used in optical communication,optical limiter,optical switch,optical data storage and other optical devices.Nonlinear optical materials are divided into inorganic and organic nonlinear optical materials.At present,the research of inorganic nonlinear optical materials is relatively more,such as graphene,semiconductor nanomaterials and metallic materials.Graphene is a honeycomb structure consisting of two dimensional hexagonal carbon atoms.It is the basic unit of other graphite materials and it could change to be a zero dimensional fullerene,one dimensional carbon nanotube or three dimensional graphite through different array.In terms of thermal,electrical and mechanical properties,graphene has a stable and orderly two-dimensional plane structure,high specific surface area,ultra-fast electron transfer and so on,making graphene to become a good carrier of semiconductor nanomaterials.Graphene dirac cone with zero band gap,broadband absorption,controllable modulation depth,high damage threshold and other excellent properties are widely used in materials,energy,environment,medicine,biology and other fields.At the same time,many interesting nonlinear optical properties have been reported.There are six main methods of preparation of graphene,which can be divided into top-down and bottom-up methods.In this paper,the preparation method of chemical peeling graphite powder?reduced graphene oxide?was adopted,due to its short reaction time,simple reaction process,high safety in experimental process and less environmental pollution.In addition to graphene,another two-dimensional material,topological insulator,had attracted the great interest and wide attention of researchers because of its unique properties,including internal insulation,surface metal state,small band gap structure and inverse time symmetric spin orbit structure.This paper mainly dealt with the third-order nonlinear optical properties of bismuth telluride?Bi₂Te₃?nanoparticles/graphene composite materials,as well as Bi₂Te₃ nanosheets/graphene composites.Bi₂Te₃,one of the second generation topological insulators,was described from the aspects of morphology,structure,optical properties,charge transfer,nonlinear optical polarization and the strong photorefractive properties in detail.We discussed the synergistic enhancement mechanism of Bi₂Te₃ nanoparticles and graphene sheet composites,the interaction process between two kinds of hierarchical structure of Bi₂Te₃ nanosheets and graphene sheets.Meanwhile,the effects of reaction time,reaction concentration,material and sample morphology on the final experimental results were summarized.Finally,a reasonable comparison between the two samples was suggested.And it showed that Bi₂Te₃ nanoparticles were more conducive to improve the nonlinear optical properties of the composites.There were four chapters in this paper.The first chapter explained the basic properties and development of topological insulators Bi₂Te₃,graphene oxide and graphene,and introduced the third-order nonlinear optical theory,measurement method and calculation method.The second chapter presented the preparation methods of graphene oxide?GO?,reduction of graphene oxide?RGO?,Bi₂Te₃ nanoparticles,and graphene/Bi₂Te₃ nanoparticles composites.We characterized the samples and analyzed their third-order nonlinear optical properties.In the third chapter,Bi₂Te₃ nanosheets and graphene composites were prepared,and the third-order nonlinear optical properties of the materials were studied by Z-scan technology.The fourth chapter was a summary and prospect.The main contents of this paper were as follows:Firstly,the modified Hummers method was used to prepare graphene oxide with high purity in acidic environment.Then,on one hand,hydrazine hydrate was used to reduce the graphene oxide and prepare the pure Bi₂Te₃ nanoparticles.On the other hand,graphene/Bi₂Te₃ nanoparticles composites were prepared by one step synthesis method,at the same time,different samples could be obtained by controlling the reaction time.The samples were characterized by transmission electron microscopy,X-ray diffractometer,Raman spectroscopy and Fourier transform infrared spectroscopy.The nonlinear optical properties of the samples were tested with a wavelength of 532 nm picosecond laser.The results showed that,graphene,Bi₂Te₃nanoparticles and graphene/Bi₂Te₃ composites both had promising nonlinear optical properties and all them exhibited saturated absorption and self-focus.With the increased of the reaction time of the composites,the tendency of the refractive index was increased first and then decreased,and the third-order nonlinear polarizability changed followed the refraction.After the research,the enhancement was came from the synergistic effect of Bi₂Te₃ nanoparticles and graphene composites and the local field effect of small size Bi₂Te₃ nanoparticles anchored on graphene sheets.Meanwhile,the decrease tendency might be due to the excess of the Bi₂Te₃ nanoparticles falling off the graphene sheets,hindering the effective transfer of the charge and then influenced the nonlinear optical properties of the composites.Secondly,the nonlinear optical properties of graphene/Bi₂Te₃ nanosheet composites were studied.We prepared six corner lattice Bi₂Te₃ nanosheets by water bath heating.On the basis of this experiment,we could clearly see the improvement of the dispersivity of aggregated Bi₂Te₃ nanosheets after the addition of graphene oxide by transmission electron microscope.With the increased of the reaction time,it could be observed that the average diameter,thickness and size of six angle star shaped Bi₂Te₃ nanosheets were increased.This growth mechanism was similar to that of most nanomaterials.The structure and properties of Bi₂Te₃/graphene composites were characterized by X-ray diffraction,Fourier transform infrared spectroscopy,Raman spectra and UV-visible absorption spectra.From the Z-scan results of Nd-YAG mode-locked pulse laser,the third-order nonlinear polarizability of the composites had no obvious changes in the pure graphene and Bi₂Te₃.The Bi₂Te₃ nanosheets was closely attached with the graphene.Compared the close connection between the nanoparticles and the graphene layers,flake contact conjugate action was limited.However,after the reaction of 24 h,the third-order nonlinear polarizability of the Bi₂Te₃nanosheets and graphene composites was beyond the pure Bi₂Te₃.It showed that the higher the crystallinity and the more complete crystal type were,the stronger the synergistic effect between the Bi₂Te₃ nanosheets and graphene was.At the same time,we had given the possible growth process and mechanism of Bi₂Te₃nanosheets on the layers of graphene.It was found that the composites had the saturated absorption and self-focused properties.The results showed that the composite materials would have potential application in the saturated absorber and mode-locked pulse laser.Thirdly,we summarized the job content of this paper and looked forward to the future work.We also pointed out the problems and basic ideas hoped to solve in the future.