Controllable Synthesis Of WSe₂/WS₂ Vertical Heterostructures And The Application Of Optoelectronics Devices

In recent years,novel electronic and optoelectronic devices based on two-dimensional（2D）TMDs materials have been widely studied and are considered as one of the best candidate materials for future optoelectronic devices.2D vertical heterostructures can not only retain the excellent performance of their single component materials,but also have new physical phenomena such as ultra-fast charge transfer,tunable band arrangement and steep interface,showing great application potential in photovoltaic devices.At present,there have been a lot of work on the preparation of 2D vertical heterostructures.However,due to problems such as thermal decomposition,atomic substitution and uneven nucleation in the process of preparation,the obtained heterostructures are of poor crystal quality.It is still a great challenge to design and fabricate application-oriented electronic and optoelectronic devices with high mobility and high responsivity.This paper focuses on the key scientific and technical problems existing in the preparation of the above 2D vertical heterostructures.The WS e₂/WO₃ heterobilayer were prepared.The highly reactive WO₃ monolayer nanosheets were used as the precursor to grow WS₂ nanosheets in the second step.The WSe₂/WS₂ heterostructures with high quality were synthesized at low temperature（～550℃）.This method overcomes the problem of high growth temperature second-step caused by low chemical reactivity and high melting point of WO ₃ powder precursors in traditional chemical vapor deposition（CVD）.Further,based on the obtained high-quality WSe₂/WS₂ heterostructures,we systematically studies the application of WSe ₂/WS₂ in optoelectronic devices.The specific research contents are described as follows:（1）Firstly,controllable synthesis of WSe₂ single crystal was achieved by physical vapor deposition（PVD）.The synthesized samples were characterized by scanning electron microscopy（SEM）,atomic force microscopy（AFM）,transmission electron microscopy（TEM）,optical and electrical measurements.The results show that the WSe₂ monolayer has a clean and uniform surface with a thickness of 0.9 nm and good crystal quality.The WSe₂ monolayer exhibits strong single-peak photoluminescence（PL）emission and excitons are the quasiparticles that dominate PL emission.The electrical properties of the WSe₂ monolayer show p-type characteristics,indicating the transport behavior of the holes.The successful preparation of WSe ₂ single crystal has laid a foundation for the preparation of WSe ₂/WO₃ heterostructure.（2）In order to prepare high-quality WSe₂/WS₂ heterostructures,we achieved controllable synthesis of WSe₂/WO₃ heterostructures by controlling deposition temperature using a universal PVD method.Meanwhile,we also used SEM,AFM and X-ray photoelectron spectroscopy（XPS）to conduct basic characterization of the samples.The results show that the thickness of WO ₃ monolayer is 1 nm.Compared with WO₃ powder,monolayer WO₃ nanosheets have higher chemical reactivity and lower melting point because the surface effect and small size effect of nanomaterials are significantly enhanced with the decrease of size.Therefore,the successful preparation of monolayer WO₃ nanosheets lays a foundation for obtaining high-quality WSe₂/WS₂ heterostructures.（3）Finally,based on the WSe₂/WO₃ heterostructures obtained above,high-quality WSe₂/WS₂ heterostructures were prepared by two-step CVD.WSe₂ was used as the substrate for the second step growth,and WO ₃ nanosheets were used as the precursor for the second step growth of WS₂.Therefore,high-quality WSe₂/WS₂heterostructures were successfully prepared by optimized two-step CVD at low temperature（～550℃）.The samples were characterized by SEM,AFM,TEM,optical and electrical methods.Specific research results and innovations are described as follows:（a）High-quality WSe₂/WS₂ heterostructures were prepared by two-step CVD;（b）Interlayer exciton emission was found in the WSe ₂/WS₂ heterostructures,and the quenching of the intralayer excitons was very obvious,indicating that the interlayer coupling of the WSe₂/WS₂ heterostructures was strong.（c）The WSe₂/WS₂heterostructures were used to construct the dual-channel electronic devices.It was found that the WS₂ dominated the current,presenting an n-type characteristic with electron mobility of 8.29 cm²/（V?s）.（d）Under the light condition,the device exhibit good photoresponsivity（3 A/W）and fast photoresponse speed（<1 ms）.