Preparation And Third-Order Nonlinear Optical Properties Of Sb₂S₃ And Cs₂SnI₆/Graphene Composites

With the development of nonlinear optics,nonlinear optical materials with rich and colorful related characteristics have been applied to optoelectronics,optical switches and other devices as well as optical communication.At the same time,industrial production has led to increased demand for device applications.Therefore,it is necessary to constantly explore new nonlinear optical materials with excellent properties,which can provide reference for optimizing devices based on nonlinear optical materials.At present,nonlinear optical materials mainly include graphene,transition metal sulfide,metal-organic framework materials,perovskite materials and so on.Among these,graphene,transition metal sulfides have been extensively studied and applied in many fields.Additionally,halide perovskite has attracted much attention due to its structural diversity.It is widely used in up-conversion laser and ultrafast laser modulation,and is considered as a nonlinear optical material with great potential.Based on the excellent nonlinear optical properties of graphene,metal sulfide and halide perovskites,this paper proposes a method to compound graphene with Sb₂S₃ and Cs₂SnI₆,respectively,to construct graphene composite structures with different composite ratios,achieve the third-order nonlinear optical properties modulation and analyze the mechanism.The specific research content and results of this paper are discussed as follows:Firstly,Sb₂S₃ and Sb₂S₃/RGO composites with different composite ratios were prepared and their third-order nonlinear optical properties were investigated.Sb₂S₃ was successfully prepared by solvothermal method and in-situ growth of Sb₂S₃ on graphene was made use of oxygen-containing defect sites provided by GO.The morphology and structure analysis of Sb₂S₃/RGO with different composite ratios showed that its nanocrystals were spherical amorphous.The Z-scan test（λ=532 nm,hv=2.33 e V）showed that Sb₂S₃had two-photon absorption properties.Sb₂S₃/RGO exhibited nonlinear saturation absorption properties,the addition of graphene made Sb₂S₃/RGO exhibit nonlinear saturation absorption properties and the obvious nonlinear absorption response changes were attributed to the decrease of band gap and charge transfer.The nonlinear refraction of all samples showed self-focusing characteristics.The nonlinear polarizability of the composites increased first and then decreased with the increase of GO mass,and the maximum value was8.63×10^-12 esu,1.8 times that of Sb₂S₃,which successfully realized the third-order nonlinear optical property regulation.Secondly,the preparation and nonlinear optical properties of Cs₂SnI₆ nanocrystals with different morphologies were studied.In this paper,Cs₂SnI₆ nanocrystals with different morphologies were successfully prepared by changing the reaction time through the heat injection method commonly used in the preparation of nanocrystals.The morphologies（quantum dots,nano-cubes and nanosheets）of the samples were characterized by SEM,TEM and XRD and the environmental stability of the sample was explained.All the samples were tested by Z-scan technique（λ=1064 nm,hv=1.165 e V）at at high excitation energies.The results showed that all samples exhibit nonlinear absorption and self-focusing nonlinear refraction characteristics led by two-photon absorption（TPA）,and the variation of nonlinear polarizability was mainly affected by nonlinear absorption.The largest nonlinear absorption coefficient(1.53×10^-10 m/W)appeared in the nano-cubes,and quantum dots had the second largest nonlinear absorption coefficient(1.25×10^-10 m/W)next to the nano-cubes.The Z-scan results of Cs₂SnI₆ showed that the sample exhibited a gradual change from saturation absorption to two-photon absorption with the increase of excitation energy,which can be attributed to the existence of sub-band gap structure caused by defects in the sample.The environmental stability and excellent and controllable nonlinear optical properties of Cs₂SnI₆ show its potential application in photonic devices.Thirdly,the hybrid of Cs₂SnI₆ nano-cubes with graphene by thermal injection was an effective way to regulate the nonlinear optical properties.The results showed that the size of nanocrystals in the composite decreases,and the increase of graphene provided a larger growth area for the growth of Cs₂SnI₆,making the distribution of Cs₂SnI₆ more uniform.The addition of graphene had an impact on the linear optical properties.The absorption peak was redshifted and Cs₂SnI₆/G band gap gradually decreased compared with pure Cs₂SnI₆ nanocrystals.The comparison of X-ray photoelectron spectroscopy showed that the binding energy of the composite was shifted forward,which was due to the charge transfer process from Cs₂SnI₆ to G direction.The test results of Z-scan at 532 nm showed that Cs₂SnI₆/G has enhanced nonlinear polarizability.Combined with transient absorption spectrum analysis,the increase of the light-induced carrier concentration in the composite promoted the charge transfer rate and thus affected the nonlinear optical process.In addition,the large modulation depth（10.3%）and low saturation strength（0.265GW/cm²）of the composite reflect the stronger saturation absorption capacity,and the nonlinear polarization was increased compared with Cs₂SnI₆.The results of Z-scanning（λ=1064 nm）test showed that Cs₂SnI₆/G composites exhibited nonlinear optical properties of two-photon absorption and self-focusing.With the increase of the mass of GO,the nonlinear polarization increases first and then decreases.The nonlinear polarizability of Cs₂SnI₆/G appears at a maximum value of 7.03×10^-11 esu for a Cs₂SnI₆ to graphene mass ratio of 6:1,which was 8.5 and 2.5 times that of G and Cs₂SnI₆,respectively.The nonlinear optical effect can be attributed to the increase of exciton oscillation intensity and the change of charge distribution between components caused by the reduction of Cs₂SnI₆ nanocrystal size.