Synthesis And Characterzation Of VS_2xSe_2（1-x） Alloy Nanosheets And VS_2xSe_2（1-x）/WSe₂ Heterostructures

Since graphene was discovered in 2004,two-dimensional（2D）materials have been extensively explored.Two-dimensional transition metal choriocarbides（2D-TMDs）are popular in the field of electronics and optoelectronic devices due to their excellent properties.Due to their atomic thickness and absence of suspended bonds on their surfaces,2D materials can be vertically stacked or laterally epitaxial to form heterojunctions or homojunctions,obtaining new properties.In traditional 2D material heterojunctions and 2D material devices,contact acts as a bridge between 2D materials,their devices and the 3D world.The quality of contact properties is usually limited to the intrinsic properties of the contact material（such as the work function of the metal）or the quality of the contact interface.The metal-semiconductor junction（MSJ）is a key component of electronic and optoelectronic devices,which largely determines device performance.There are strong Fermi level pinning phenomena in semiconductor materials due to interfacial gap states and defects in conventional evaporation electrodes.It is found that the formation of van der Waals metal-semiconductor junction（vd W MSJ）is an effective measure to prevent Fermi level pinling.But the metal electrodes with a constant work function already determine the contact barrier and limit the performance of devices.Therefore,adjusting the work function of contact metal of vd W MSJ can greatly reduce the contact barrier of vd W MSJ and improve the device performance.To this end,this paper mainly carried out the following work:（1）Metallic VS_2xSe_2（1-x）alloy nanosheets were synthesized by traditional chemical vapor deposition method.VS_2xSe_2（1-x）alloy nanosheets with different components could be obtained by adjusting the temperature of sulfur powder source.X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）,scanning electron microscopy（SEM）and atomic force microscopy（AFM）showed that VS_2xSe_2（1-x）alloy nanosheets were hexagonal and possessed high quality.The corresponding changes in the results of XRD,XPS,and TEM with the increasing S component are discussed.Kelvin probe force microscopy（KPFM）displays a continuous change of the work function from 4.79 e V（x=0）to4.62 e V（x=1）by continuous regulation of the S components x（0≤x≤1）of the VS_2xSe_2（1-x）alloy nanosheets.（2）Preparation of VS_2xSe_2（1-x）/WSe₂van der Waals heterojunctions（vdWH）.In this paper,the bilayer WSe₂large single crystal was successfully synthesized by improved chemical vapor deposition method,and its morphology and thickness were characterized by optical microscopy（OM）and atomic force microscopy（AFM）.Raman and fluorescence（PL）showed the extremely high crystal quality of WSe₂.The VS_2xSe_2（1-x）nanoplates with different components was grown on pre-growed bilayer WSe₂nanosheet,forming VS_2xSe_2（1-x）/WSe₂vdWH successfully through a two-step CVD process.（3）The electrical properties of VS_2xSe_2（1-x）/WSe₂vdWH field effect transistor（FET）.The VS_2xSe_2（1-x）/WSe₂vdWH FET devices were fabricated from VS_2xSe_2（1-x）/WSe₂vdWH with different S component by electronic exposure and thermal evaporation techniques.Consequently,we found that the performance of VS_2xSe_2（1-x）/WSe₂vdWH FET will be significantly better than the In/Au contacted WSe₂FET.And the vdWH FET performance is further improved by adjusting the work function of the contact metal VS_2xSe_2（1-x）.The VS_2xSe_2（1-x）/WSe₂（x=0.0）vdWH FET displays a excellent device performance with a high ON-current density of up to39μA/μm,a mobility of up to 72 cm²V^-1s^-1,and a on/off ratio of about 10⁷.