Controllable Growth And Optoelectronic Properties Of Vertical MoS₂/WS_2（1-x）Se_2x Alloy Heterostructures

The discovery of graphene has brought new research and application opportunities to other two-dimensional materials with a layered structure.After the unremitting efforts and continuous exploration of experts and scholars,the members of the two-dimensional material family are increasing day by day,and are used in communications,optoelectronic integration,Information transmission and sensing and other fields play a important role.Among them,two-dimensional materials with semiconductor properties have sprung up like mushrooms under the guidance of Moore’s Law,playing a key role in the field of optoelectronic information integration.These 2D materials include black phosphorus（BP）,boron nitride（h-BN）,and 2D transition metal dichalcogenides（TMDs）.Among them,TMDs have many unique properties that other materials do not have.For example,the energy band structure changes with the number of layers.The band gap structure in bulk is an indirect band gap,and when thinned to a single layer,it becomes a direct band gap.The luminous efficiency of the material is greatly improved,and this characteristic promotes the application of TMDs in photoresponsive devices;its band gap value is between 1 e V and 2.5 e V,covering the range from visible light to infrared light,and it can also pass through alloys.It can be regulated by methods such as chemistry,doping,and external stress,thereby broadening the application of TMDs in other optoelectronic devices,etc.However,with the continuous innovation and breakthrough of technology,no matter how excellent a single two-dimensional semiconductor material is facing the development trend of high speed,high integration and small volume,it is not enough.With the in-depth development of research,scholars have found that the formation of van der Waals heterojunctions by connecting two or more different materials with weak van der Waals forces by vapor deposition or mechanical exfoliation can not only retain their respective advantages,but also showed other novel properties that single materials do not have.Since then,the study of heterojunctions has gradually come to the center of the stage.Another advantage of van der Waals heterojunctions is that they are not constrained by lattice matching.A single two-dimensional semiconductor can be combined with one-dimensional quantum dots,two-dimensional nanosheets,and three-dimensional bulk materials to form heterojunctions,which will greatly improve enriching the existing material system will pave the way for the further application of two-dimensional semiconductor materials in the field of optoelectronic information,and also build a good research platform for exploring other physical properties of heterojunctions.In view of this,in order to solve the problem that a single two-dimensional semiconductor material is difficult to meet the new performance requirements,three TMDs,molybdenum disulfide（MoS₂）,tungsten disulfide（WS₂）,and tungsten diselenide（WSe₂）are used as source materials.designed a series of vertical bilayer MoS₂/WS_2（1-x）Se_2x（0≤x≤1）alloy van der Waals heterojunctions with different mole fractions x of Se.The controllable construction of heterojunctions,the relationship between the optical properties of heterojunctions and the mole fraction x of Se,and the performance of field effect transistors are systematically explored.The details are as follows:1.Large-area MoS₂nanosheets were successfully obtained by chemical vapor deposition.Atomic force microscope needles show that the obtained MoS₂nanosheets have very low surface roughness and a thickness of only 0.8 nm;transmission electron microscopy results show that the obtained MoS₂nanosheets have neatly arranged atoms,no obvious defects,and a set of clear and bright selected area electron diffraction spots.At the same time,we used confocal Raman microscopy to test the optical properties of the obtained MoS₂nanosheets,and a narrow and strong luminescence peak was collected at 680 nm,indicating that the obtained MoS₂nanosheets had good crystal quality.2.By changing the mass ratio of the source materials WS₂and WSe₂for physical vapor deposition,a series of large-area WS_2（1-x）Se_2x（0≤x≤1）alloy nanosheets with different compositions were successfully prepared.Its thickness was characterized by microscopy,and the results showed that the thickness of the alloy nanosheet was 0.82 nm,corresponding to a single layer.The photoluminescence spectrum shows that the luminescence peak of WS_2（1-x）Se_2xalloy can be continuously tuned from 680 nm to 770 nm with the increase of the mole fraction x of Se.exhibits strong composition-dependent band-edge emission.3.Using the combination of Chemical Vapor Deposition（CVD）and Physical Vapor Deposition（PVD）,the composition-dependent vertical bilayer MoS₂/WS_2（1-x）Se_2x（0≤x≤1）controllable preparation of alloy heterojunction,photoluminescence and Raman spectroscopy results show strong composition-dependent luminescence phenomenon and lattice vibration characteristics.By integrating the Se atomic mole fraction,the PL spectral extraction of the heterojunction,and the energy band calculation,the correlation between the emission wavelength,the energy band value and the Se atomic mole fraction x in the sample is elucidated.Field effect transistor devices fabricated based on heterojunctions also exhibit composition-dependent transfer properties.