Controllable Preparation And Bandgap Modulation Of Tungsten-based Two-dimensional Transition Metal Dichalcogenide

In the past decade,the attention paid to two-dimensional nanostructures has been greatly enhanced.In addition to the traditional nanostructures,the newly discovered two-dimensional nanomaterials have attracted great attention due to their unique properties.The new size effect of the two-dimensional nanostructure will provide great potential for the future practical applications.Graphene is a typical example,because electrons are limited to two-dimensional environments,which exhibits unique extraordinary physical,electronic,and chemical properties,with the electrons limited to the two-dimensional environments.A new application of graphene is in the nanoscale tunable optical modulators of communication functions as well as other electronic instruments.However,graphene is not the semiconductor and has a zero gap value,which largely hinders its application,especially in optoelectronic devices.Although there has been many ways to open the gap of graphene,the results are not very good and the cost is high.Therefore,it is very necessary to find new alternative2D materials.The transition metal dichalcogenides?TMDCs?have very similar structure to that of graphene,which attract significant attention due to their large energy gap values and fast carrier mobility.Among them,two-dimensional semiconductor materials such as MoS₂,WS₂,MoSe₂,and WSe₂ have excellent field effect properties,and can be used in the construction of high-performance field effect transistors and their optoelectronic devices.Such materials also have good flexibility and transparency,with a commendable application prospect in the field of flexible devices.Besides,TMDCs provide a unique class of nanomaterial systems for studying new phenomena in condensed matter physics.In a word,the research on two-dimensional TMDCs has both scientific significance and broad application prospects.The bandgap determines the operating wavelength of the semiconductor device and is an important parameter of the semiconductor.In order to study the potential applications of TMDCs in functional optoelectronic devices,it is particularly important to obtain high quality,different morphology,and gap-adjustable two-dimensional nanomaterials.In view of this,this paper successfully synthesized a variety of monolayer and bilayer TMDCs nanosheets by traditional chemical vapor deposition method,and the bilayers are AA and AB stacked structure.On this basis,the lateral and vertical WS₂/WSe₂ heterostructures can be synthesized through careful design.The traditional thermal evaporation two-dimensional nanomaterial growth technology is improved to synthesize WS_2?1-x?Se_2xx alloy nanosheets with adjustable composition and WS₂/WS_2?1-x?Se_2xx nanosheets with different morphology.The main representative research results are:1.Under the adjustment of temperature,a variety of monolayer and bilayer structure TMDCs?WSe₂,WS₂,MoS₂?nanosheets were successfully synthesized by conventional vapor deposition method.The bilayer nanosheets obtained in the experiment were AA and AB stacking.The size of the first layer adjacent to SiO₂ was about 35 microns,and the area of the second layer was about 15-20 microns.Under the excitation of argon ion laser,there is only A exciton luminescence observed in the single layer region,and a weak B exciton peak appears in the bilayer region except for the A exciton luminescence peak,while the double layer WSe₂ nanosheet also has the A'exciton peak.Under the SHG characterization,as the symmetry is further broken,the SHG signal intensity of the AA stack is about 4 times stronger than that of the monolayer,and since the upper and lower layers are completely symmetrical,the SHG signal intensity of the AB stack nanosheet is zero.The systematic study of the optical properties of the pure material nanosheet lays a good theoretical foundation for the synthesis and research of corresponding heterojunctions2.By effectively controlling the evaporation temperature of WSe₂,the lateral and vertical WS₂/WSe₂ heterostructures can be successfully synthesized in two steps.In the experiments,when the evaporation temperatures of WSe₂ are 1100-1130°C,the lateral heterostructure with a size of about 20 microns can be successfully synthesized.The optical images show that the middle and edge colors of the triangular nanosheetsare inconsistent,confirming the synthesis of the lateral heterostructure.The lateral WS₂/WSe₂ heterojunction can be further confirmed by corresponding optical characterization.When the evaporation temperatures of WSe₂ are1130-1170°C,the vretical WS₂/WSe₂ heterostructures can be successfully synthesized.The microstructure characterizations indicated that the vertical heterostructures are highly crystalline.The SHG signal study proved that the vertical WS₂/WSe₂ heterostructures are formed by AA and AB stacking,and the optical properties of the vertical WS₂/WSe₂ heterostructures are stacking depending.At the same time,we can control the synthesis of vertical heterojunctions with AA and AB stacking by effectively controlling the deposition temperature.thecontrollable synthesis of the lateral and vertical WS₂/WSe₂ heterostructures and stacking method of vertical heterostructures provide multiple options for the application of heterostructures,which are of great significance in the application of functional optoelectronic devices with two-dimensional layered nanomaterials.3.By effectively controlling the evaporation temperature of Se source,we successfully synthesized the WS_2?1-x?Se_2xx nanosheets with single-way and both-way composition graded structure and WS₂-WS_2?1-x?Se_2xx lateral heterostructures by a developed one-step chemical vapor deposition?CVD?approach.The composition graded nanosheets obtained in the experiment were approximately 30 microns in size.At 532 nm laser excitation,all the compodition graded nanosheets have only a single photoluminescence peak.Microstructural characterization confirmed the high crystallinity of the nanosheets.By effectively controlling the evaporation temperature of Se powder,we can synthesize arsenic nanosheets with luminescence peaks between650-760 nm and gradients ranging from 30-40 nm on a single nanosheet.At the same time,the WSe₂ powder can replace the Se source to obtain the full-composition graded WS_2?1-x?Se_2xx nanosheets.The photoluminescence on the single nanosheet can be continuously adjusted from 650 nm to 760 nm.These WS_2?1-x?Se_2xx alloy nanostructures with adjustable components will be of great significance in two-dimensional basic research and functional device construction.4.By adjusting the ratio of WS₂ and WO₃ sources,we successfully synthesized hexagonal WS₂ nanosheets.On this basis,the WS₂/WS_2?1-x?Se_2xx lateral heterostructures of hexagonal and triangular morphology were successfully synthesized by effective control of the reaction concentration of Se powder.The simulation results of growth mechanism show that the synthesis of hexagonal WS₂/WS_2?1-x?Se_2xx nanosheets is controlled by thermodynamic growth at lower Se concentration.At higher Se concentration,The process of the synthetic triangle WS₂/WS_2?1-x?Se_2xx nanosheets is a non-equilibrium process controlled by growth kinetics.Photoluminescence and Raman spatial distribution clearly shown the spatial distribution of the composition on the nanosheets.The synthesis of WS₂/WSe_2xx S_2?1-x?nanosheets with different morphologies is of great significance in the application of functional optoelectronic devices for two-dimensional layered nanomaterials,and theoretical calculations have given us a deeper understanding of material synthesis.