Studies On Third-order Nonlinear Optical Properties Of SnS₂/PANI/Reduced Graphene Oxide Composites

The appearance and development of laser has greatly broadened people's understanding of the optical phenomenon of matter.At that time,a large number of emerging nonlinear optical phenomena first entered people's field of vision,which aroused their strong interest in inquiry.Consequently,nonlinear optics as a small branch of physics came into being.Nonlinear optics mainly explore the nonlinear optical phenomena and its practical applications that arising from the interaction between intense light and matter.Not only do people carry out research on the nonlinear optical properties of substances in detail,but also screen out high-quality materials for application to all-optical switching,optical storage,light limit,optical communication and other applications.Reduced graphene oxide and tin disulfide?SnS₂?nano-layered materials have excellent nonlinear optical absorption properties due to their good electronic band structure characteristics.Combining the two may optimize the nonlinear optical properties of the monomer.SnS₂ nanosheet is an n-type thin layered semiconductor materials with an indirect band gap of 2.1 e V and a direct band-gap of 2.8 e V.It has non-toxic,convenient row materials extraction,relatively stable chemical properties and nonlinear saturated absorption,which makes it a high-quality material.Reduced graphene oxide is a two-dimensional carbon nano-material with a hexagonal lattice that is composed of carbon atoms with sp2 hybrid orbitals.It has the optical properties of broadband ultra-fast saturation absorption,which can be explained by valence band loss and conduction band filling.This saturated absorption can be used in laser locking and other applications.What's more,Polyphenyl amine?PANI?is a quite outstanding third-order nonlinear conductive polymer material with delocalized large conjugate electronic system.In view of its wide range of row sources,easy preparation and superb thermal and optical stability,as well as different electronic structural states and unique doping mechanisms,PANI is doped to the SnS₂/reduced graphene oxide complexes which have exceptional optical properties.On these grounds,we can probe into whether it can improve the third-order nonlinear optical properties of the SnS₂/reduced graphene oxide composites.We focused on the third-order nonlinear properties of SnS₂/reduced graphene oxide compounds and SnS₂/PANI/reduced graphene oxide complexes,respectively.First of all,we prepared the SnS₂/reduced graphene oxide nanocomposites by a simple hydrothermal method.Second,a series of optical tests were carried out on the samples.The structure and morphology of the samples were determined by using spectral test results such as X-ray diffraction spectroscopy?XRD?and transmission electron microscope test?TEM?et al.And on one hand,the linear absorption of the samples were analyzed by using the UV-Vis spectroscopy.On the other hand,the nonlinear optical properties were finally characterized by the Z-scan test.Moreover,we compared the differences of the optical coefficients of the samples such as the third-order nonlinear refractive coefficient,nonlinear absorption coefficient and nonlinear polarization rates,and put forward the theory of electron transfer to explain the results.The general content of the paper is as follows:Firstly,the higher oxidation of graphene oxide?GO?was prepared by using the improved Hammer method,and the reduction of graphene oxide?RGO?was obtained by using a suitable reducing agent.SnS₂ nanosheet was prepared by the hydrothermal method.And then,through the structural and morphological analysis of the two kinds of samples,it was shown that the samples are of high quality,and the SnS₂ nanosheet appeared a flake shape with a size of 40-60 nm.In order to obtain the optical properties of above,the Z-scan optical test was performed.Both of them exhibit nonlinear saturated absorption.Reduced graphene oxide presented a positive nonlinear refractive indexe,but SnS₂ nanocrystals exhibited a negative nonlinear refractive index.Besides,the third-order nonlinear susceptibility of reduced graphene oxide was one order of magnitude greater than SnS₂ nanosheets.Secondly,SnS₂/reduced graphene oxide nanocomposites were prepared by a simple hydrothermal method.All samples were tested by Z-scan technique to determine their nonlinear optical properties.The closed-aperture/open-aperture image of reduced graphene oxide showed a valley-peak shape,which indicates that it has a positive nonlinear refractive index.While SnS₂ nanosheets and SnS₂/reduced graphene oxide composites both exhibited a peak-valley shape,which indicated they all have negative nonlinear refractive indices.The opening aperture curves of the above three samples all exhibited peak shapes symmetrical about the focal point,which indicated that they all exhibit nonlinear saturation absorption properties.Bsides,in order to explore the nonlinear optical properties of the SnS₂/reduced graphene oxide complexes,we took the mass ratio of SnS₂ to reduced graphene oxide in the composite as a variable quantity.We reduced the mass ratio of SnS₂ to reduced graphene oxide from 22 to 6,and the third-order nonlinear susceptibility of SnS₂/reduced graphene oxide?S-G?composite samples showed an overall trend of increasing first and then decreasing,that is to say,the S-G3 sample had the largest third-order nonlinear susceptibility of 3.31×10-12 esu..Compared it to two monomers,it was one order of magnitude larger than SnS₂ nanocrystals and it was at the same order as reduced graphene oxide.We proposed the theory of electron transfer to explain this result.Thirdly,we first prepared SnS₂ and PANI?S-P?complexes with different mass ratios of PANI to SnS₂.With the increase of the mass ratio,the third-order nonlinear susceptibility tended to increase and then decrease.Besides,the best sample was the S-P2 sample whose mass ratio is 0.7,and the third-order nonlinear susceptibility of 1.90×10-12 esu.Subsequently,S-P2 composites of different qualities and reduced graphene oxide were ternary composites.As the mass of S-P2 composites increases,its third-order nonlinear susceptibility tended to increase first and then decrease.Among them,S-P-G3 composites was the best composite materials.And it's third-order nonlinear susceptibility was 5.66×10^-13 esu.Moreover,the S-P2 sample and the PANI and S-P-G composite samples all exhibited negative nonlinear refractive index and optical properties of saturation absorption.We proposed the theory of electron transfer to explain this result at last.