Nonlinear Absorption Properties Of MoS₂/Graphene Nanocomposite Films

As typical two-dimensional nanomaterials,graphene and molybdenum disulfide?MoS₂?are expected to become a new generation of optoelectronic device.Graphene has many excellent physical and chemical properties,but the zero bandgap structure of graphene hinders its development in optical devices such as optical switches.Thus,it is necessary that exploring more two-dimensional materials to compensate for the shortage of graphene in applications.As a typical transition metal dichalcogenides,MoS₂ has a direct bandgap of single-layer,multilayers and bulks are indirect bandgap semiconductors,and it has a very high switching ratio?1×10⁸?in photovoltaics.Therefore,the characteristics of graphene and MoS₂ can complement each other.MoS₂/graphene heterostructure will also have better physical,chemical,and electro-optical properties than a single material.Among them,nonlinear optical responses are the basis for the application of materials in nonlinear optical devices.In this paper,MoS₂/graphene nanocomposite thin films with different thicknesses were prepared by liquid phase exfoliation and vacuum filtration technology,and their nonlinear absorption properties were studied by open-aperture Z-scan.The results show that the composite has stronger saturable absorption characteristics,and using the transient rate equation and the optical transmission equation to fit the experimental data shows the charge transfer between graphene and MoS₂.The mainly works of this thesis are as follows:?1?Build a Z-scan system.Based on the Z-scan theory and laboratory conditions,we independently designed and built a Z-scan system that can achieve high repetition rate,high intensity and broadband Z-scan.In addition,the Z-scanning system is optimized by collimating and testing the open-aperture Z scan based amplificated laser,and the system is calibrated using the standard sample CS₂.The high accuracy and high signal-to-noise ratio of Z-scan system provide a guarantee for results of testing samples.?2?Different thicknesses of MoS₂/graphene nanocomposite films were obtained by liquid phase exfoliation and vacuum filtration technology.For comparison,MoS₂ films and graphene films were prepared under the same conditions.Optical characterization of optical films such as atomic force microscopy,Raman spectroscopy and UV-visible absorption spectroscopy.This shows the successful preparation of the nanocomposite films.This simple and effective method for preparing nanocomposite films is sustainable and expandable for production and application of the films.?3?Samples were measured using an open-aperture Z scan based amplificated laser.The experimental results show that MoS₂/graphene nanocomposite film has enhanced saturable absorption characteristics under the same excitation conditions.We established a three-level model to analyze the enhanced nonlinear absorption.The fitting results shows that the mainly caused by the charge transfer between MoS₂ and graphene.The enhanced saturable absorption of MoS₂/graphene nanocomposite film for good performance of optical switches or mode lockers based on saturable absorbers.