Controllable Growth And Optoelectronic Properties Of MoS₂/WSe₂ Vertical Heterostructures

Over the past few decades,2D materials have shown great promise in next-generation electronic and optoelectronic devices due to their unique physical properties.A few representative two-dimensional materials are graphene,black phosphorus,hexagonal boron nitride and transition metal dichalcogenides（TMDs）.TMDs have received extensive attention as a result of their rich material systems and variable band gaps.Furthermore,the researchers discovered that two or more TMDs materials may be stacked together to produce a TMDs vertical heterojunction,and that the benefits of several TMDs can be combined to create materials that fulfill a variety of applications.TMDs vertical heterojunctions outperform single-component materials in terms of interlayer coupling,variable energy band alignment,quick charge transfer at interfaces,and magnetism.The manufacture of high-quality TMDs vertical heterojunctions in a controlled manner is thought to provide the foundation for their device applications.Currently,the mechanical exfoliation method and vapor deposition approach are the most used ways for manufacturing vertical heterojunctions of TMDs.The samples prepared by the mechanical exfoliation method are suitable for basic research,and the vapor deposition method is currently a method for the controllable growth of vertical heterojunctions of TMDs.It is still challenging to make high-quality vertical TMDs heterojunctions with clean surfaces and no vapor deposition alloying.Based on this,this academic paper focuses on the controlled manufacture of high-quality vertical heterojunctions and their optoelectronic applications.The basic optical features of the MoS₂/WSe₂ vertical heterojunction and the performance of the phototransistor were explored,as well as the controlled synthesis and device design of the MoS₂/WSe₂ van der Waals vertical p-n heterojunction.The following are the particular research techniques and contents:（1）We have successfully used chemical vapor deposition to produce MoS₂ with a size range of 20-400μm.Atomic mechanical microscopy,photoluminescence,Raman spectroscopy,and high-resolution transmission electron microscopy were used to study the material’s crystal structure and thickness.The results reveal that we manufactured high-quality large-scale MoS₂ nanosheets with a thickness of 0.86 nm.A field effect transistor based on single-layer MoS₂ nanosheets was also built at the same time.The MoS₂ transistor shows conventional n-type semiconductor features and a high switching ratio of 10⁵,as well as a mobility of 3.87×10^-2 cm²V^-1s^-1.（2）Based on the base material MoS₂,we successfully synthesized the MoS₂/WSe₂van der Waals vertical heterojunction by controlling the deposition temperature and carrier gas flow rate by physical vapor deposition method.The test results of atomic mechanical microscopy,photoluminescence,Raman spectroscopy and X-ray photoelectron spectroscopy（XPS）are in one-to-one correspondence with the two materials,MoS₂ and WSe₂.The optical properties of MoS₂/WSe₂ heterojunctions were analyzed by second harmonic wave（SHG）test,and the results showed that different stacking modes of MoS₂ and WSe₂ have different optical properties.The polarization superposition of the SHG signal of the AA（0°）stacking MoS₂/WSe₂ heterojunction is enhanced,and the signal of the AB（60°）stacking heterojunction is weakened but not completely disappeared.The atomic arrangement of MoS₂/WSe₂ heterojunction is clear under high-resolution transmission electron microscope.Due to the interlayer coupling,obvious moiréfringes can be seen in the heterojunction area,which proves that our synthesized MoS₂/WSe₂ heterojunction has high crystalline quality.This lays a foundation for further exploration of optoelectronic applications based on MoS₂/WSe₂vertical heterojunctions.（3）After realizing the controllable fabrication of high-quality MoS₂/WSe₂ van der Waals vertical heterojunctions,we constructed MoS₂/WSe₂ electronic devices.In the electrical test,the working mode of the MoS₂/WSe₂ heterojunction field effect transistor can be regulated by adjusting the positive and negative gate voltage.The negative gate voltage is P-type,and the positive gate voltage is n-type.The carrier mobility of the MoS₂/WSe₂ transistor is as high as 9.01 cm²V^-1s^-1,which is two orders of magnitude higher than that of the underlying material MoS² transistor.Then,under the condition of adding light,the optical response under different laser power intensity,grid voltage and wavelength was tested.The photocurrent,responsivity and detectivity of MoS₂/WSe₂ vertical heterojunction photodetectors increase with the increase of gate voltage.With the increase of optical power and optical wavelength,the photoresponse of MoS₂/WSe₂ weakened,and the maximum photoresponse was 38.9 A/W under 450nm（blue）laser,with a detectivity of 5.86×10¹¹ Jones.Under the condition of gate voltage of 0 V and source-drain voltage of 1 V,the device exhibits excellent stability and high photoresponse speed.