Studies On Third-order Nonlinear Optical Properties Of Bi₂S₃/MoSe₂ And Graphene Composite Materials

In the 21st century,the laser industry has developed rapidly.With the appear of lasers,the investigates of the nonlinear absorption and refraction of materials under laser have been a hot topic.The study of the properties of third-order nonlinear optics is an important part of the study of nonlinear optics.It is an important way to explore new materials to achieve breakthroughs in the performance of nonlinear optical devices.Nonlinear optical devices can be widely used in all-optical switches,optical frequency conversion,optical limiters and optical information communications.Semiconductor nanomaterials with good nonlinear optical properties usually have unique advantages such as larger carrier mobility,faster response speed,smaller threshold power and higher nonlinear polarizability.Bismuth sulfide（Bi₂S₃）,one of the metal chalcogenides,is an inorganic semiconductor material with direct band gap of 1.3 eV,which is environmentally friendly and has excellent photoelectric properties.Molybdenum selenide（MoSe₂）belongs to transition metal disulfide（TMDs）,which is a direct band gap semiconductor of 1-2 eV.It has a unique thin atomic layer structure and optical absorption properties.It can be used in photocatalysis,electrodes,lubrication and other fields.Two-dimensional（2D）graphene is composed of hexagonal lattice sp²hybridized carbon atoms.It has special two-dimensionalπ-electron conjugate structure and novel properties,such as Dirac tapered band structure,excellent thermal conductivity,mechanical stability,carrier mobility and chemical stability.In this paper,we selected two typical semiconductor nanomaterials to investigate their composites with graphene of the third-order nonlinear optical properties and enhancement mechanism.On this basis,a series of problems about charge transfer process and model construction between semiconductor quantum dots and graphene were proposed.The synergistic polarization effect between Bi₂S₃ nanoparticles and graphene nanosheets,and the effect of layer growth mechanism between MoSe₂ nanosheets and graphene nanosheets on the nonlinear optical properties were discussed from the aspects of morphology,structure,linear optical properties,charge transfer mechanism and nonlinear optical polarizability.At the same time,in order to explore the influence of time,concentration and light intensity on the nonlinear optical properties,the third-order nonlinear optical properties of composites under different reaction time,doping concentration of graphene and radiation intensity were studied and analyzed in detail.In introduction,we introduces the development of semiconductor quantum dots,graphene and nonlinear optics.The second chapter mainly introduces two important properties of nonlinear optical absorption and nonlinear optical refraction,and the basic principle and theoretical calculation of Z-scan technology for measuring nonlinear optical parameters.The third chapter is about the third-order nonlinear optical properties of Bi₂S₃ and graphene composites.The fourth chapter is about the third-order nonlinear optical properties of MoSe₂ and graphene composites.The fifth chapter is summary and prospect.The main contents of this paper are as follows:Firstly,high purity graphene oxide（GO）and reduced graphene oxide（RGO）were prepared by wet chemical method.The structure and performance of graphene oxide before and after reduction were compared by X-ray diffraction,Fourier transform infrared spectroscopy and ultraviolet-visible absorption spectroscopy.Finally,the nonlinear optical properties of reduced graphene oxide were measured by Z-scan technology.Secondly,Bi₂S₃/RGO composite materials were prepared by the mean of a facile liquid-phase synthesis method.The morphology and structure characterizations of the Bi₂S₃/RGO composite materials indicated that the Bi₂S₃ nanoparticles were well-dispersed on the surface of the graphene nanosheets.The third-order nonlinear optical properties of all materials were measured by using Z-scan technique with a picosecond pulse laser at 532 nm.The Z-scan results showed that both Bi₂S₃ and Bi₂S₃/RGO composite materials had reverse saturable absorption and positive nonlinear refraction properties.The enhancement of nonlinear optical properties could be affected by quantum size effect,enhanced charge transfer property and synergistic effect.Our results not only deepen the understanding of Bi₂S₃/RGO composite materials,but also widen their potential applications in the areas of the optical limiting and optical switches.Thirdly,we prepared the MoSe₂/RGO composite materials via a facile solvothermal method and investigated their third-order nonlinear optical properties.Structure and morphology characterizations of the MoSe₂/RGO composite materials showed that MoSe₂nanosheets formed a network structure on the surface of graphene nanosheets.The third-order nonlinear optical properties of all materials were measured by using Z-scan technique with 30ps pulse laser at 532 nm.The results showed that both MoSe₂ and MoSe₂/RGO composite materials had saturable absorption and positive nonlinear refraction properties,indicating the potential candidates for application in saturable absorber and optical switches.Fourthly,the work of this paper was summarized,and the problems of charge transfer and structural defects between graphene and semiconductor composites were prospected.