Preparation And Optical Properties Study Of Two-dimensional Material Tungsten Diselenide And Graphene

Two-dimensional（2D）materials are a type of thin film materials with only one or a few layers of monoatomic thickness,which often exhibit significantly different physical and chemical properties from their corresponding three-dimensional bulk materials.In the last decade,the researches and applications of 2D materials have attracted much attention from scientists and engineers around the world.Among all the 2D materials,semiconductor tungsten diselenide（WSe₂）and semi-metal graphene are two typical materials.Multilayer WSe₂is an indirect bandgap semiconductor which becomes a direct bandgap semiconductor when thinned to monolayer.Compared with other 2D transition metal dichalcogenides（TMDs）,WSe₂has a larger band splitting（～400 me V）caused by the spin-orbit coupling and is also one of the few TMDs materials that can be modulated into n-p dual semiconductors,making it a promising material in the fields of electronics and optoelectronics.Although there have been many studies and applications on monolayer WSe₂at present,the electronic devices prepared with multilayer WSe₂usually have higher carrier mobility and lower contact resistance due to the interfacial reflection effect between the material and the substrate.Therefore,to solve the problem of weak photoluminescence（PL）intensity of multilayer WSe₂and to exploit its advantages in optoelectronics and device preparation has great significance for the application of WSe₂.In the study on optical properties of multilayer WSe₂bubbles,multilayer WSe₂films and bubbles are prepared by mechanical exfoliation method,and are characterized by Atomic Force Microscope（AFM）,PL spectra,Raman spectra,second harmonic generation（SHG）spectra,etc.By comparing the differences in optical properties between bubbles and flat film regions,the effect of strain on the optical properties of WSe₂is deeply understood with the help of the multilayer WSe₂band structure theoretical calculations.The ultra-high carrier mobility,electrical conductivity,thermal conductivity,light transmission,mechanical properties of graphene make it have great application potential in the field of transparent conductive films,radio-frequency transistors,sensors,etc.Among the various preparation methods of graphene,chemical vapor deposition（CVD）technology is an effective method for preparing large-area and high-quality graphene.However,graphene films prepared by CVD methods often appear as polycrystalline films with high-density grain boundaries,and their physical properties are reduced to some extent on the grain boundaries.Therefore,it is of great research significance to fully understand the growth mechanism of CVD graphene and to prepare high-quality large single crystal graphene,bilayer and multilayer graphene films with large-size crystal domains.In the preparation of high-quality graphene,the effects of substrate pretreatment,substrate geometry and thickness on the growth of CVD graphene are studied,and high-quality single crystals and bilayer graphene with large-size domains are prepared,which is of great significance for the application of graphene in large scale integrated circuit..The main achievements are summarized as follows:1.The shape of the multilayer WSe₂bubbles prepared by the mechanical exfoliation method conforms to the spherical cap model.Based on this model,the average tensile strain of a bubble is about 0.2%,which can tune the band structure of multilayer WSe₂,such as leading to an enhancement of direct transition（K→K）and a decrease of direct bandgap.And experimentally,the tuned band structure is manifested by the enhanced PL intensity of direct A excitons and the redshift of the PL peak position in the bubble region.In addition,it was observed in the temperature-dependent PL test that the intensity of PL peak at 1.4-1.6 e V in the bubble region increases with increasing temperature.In view of the bright and dark states in WSe₂,it can be proved that there is a dark state and a bright state（direct A exciton）in this 1.4-1.6e V energy range.The other PL peak whose energy is close to the K→K direct transition energy is an indirect bandgap transition.Moreover,due to the interference of light,the PL intensity distribution on the bubble shows oscillatory intensity rings,which greatly enhances the PL intensity in the local area.The PL enhancement in the bubble region makes the multilayer WSe₂films have great application potential in the field of optoelectronics.2.The increase of the lattice constant caused by tensile strain also leads to the redshift of the Raman characteristic peaks and low-frequency Raman peaks for multilayer WSe₂.And the intensity mapping image of Raman peaks also exhibits the appearance of enhanced and weakened intensity rings.However,the interference is between the incident laser and the reflected light in the Raman test,and is between the photoluminescence and the reflected light in the PL test,so the oscillation period of these two is different.Besides,the tensile strain in the bubble is too small to significantly change the center inversion symmetry of the material,so the overall SHG intensity in the bubble area does not change much.The SHG intensity oscillation behaviour caused by the optical interference can also be observed on the bubble.3.In the study of high-quality graphene preparation,a series of pretreatments such as annealing,electrochemical polishing,and thermal oxidation are performed on the copper（Cu）substrate,which effectively improve the smoothness of the substrate and passivate Cu surface active sites.And the graphene nucleation density is greatly decreased on the pretreated Cu substrate.Subsequently,the pretreated Cu substrate is prepared into the geometric configuration of a pocket with a Cu foil inside,creating two different growth environments between the inner and outer side of the pocket.The enclosed environment in the pocket leads to less Cu evaporation loss,so the inner surface of the pocket is smoother than the outer surface,facilitating a much lower nucleation density and much larger graphene domains on inner side.Moreover,carbon atoms diffuse from the inner surface to the outer surface driven by the difference of carbon atom concentration inside and outside the pocket and participate in the multilayer graphene growth on the outer surface.We also control the energy barrier of carbon atom diffusion by controlling the thickness of the substrate,and obtain different growth results on the inner and outer surfaces of the pocket and the Cu foil surface in the pocket.Finally,single-crystal graphene domains up to～2 mm and bilayer graphene domains up to～300μm across along the diagonal are obtained.And almost no defect-related D band is detected by Raman spectroscopy,which indicates that the grown graphene has high quality and high applicaiton value.