The Photoelectric Properties Of Mixed-dimensional Heterojunction Based On 2D WS₂

Since graphene was obtained through exfoliation in 2004,a variety of semiconducting two-dimensional materials,such as black phosphorus,transition metal dichalcogenides（TMDs）,have been discovered and studied.Among them,transition metal dichalcogenides have been widely studied and reported due to their wide variety.As a member of TMDs,tungsten disulfide（WS₂）has a lot of excellent physical and chemical properties,such as better etching resistance,high carrier mobility,high quantum yield and large optional orbital coupling,and direct bandgap semiconductors in the case of monolayers,etc.These characteristics make it very suitable for the preparation of high-performance filed effect transistors,which have broad application prospects in next generation of optoelectronic devices.However,due to the lack of built-in electric filed and photoconductive gain effects,the performance of a single material for photoelectric device is not ideal.At this time,taking advantage of the fact that two-dimensional materials have no surface dangling bonds,a van der Waals heterojunction can be formed by combining and stacking different low-dimensional materials.Weak van der waals forces connect the materials together,and the energy bands appear inⅠ,ⅡandⅢtype arrangements,thus exhibiting special optoelectronic decice properties.In this paper,the large-scale preparation method of two-dimensional WS₂ is discussed,and the physical properties of two-dimensional WS₂ in the construction of heterojunction with one-dimensional semiconductor materials were studied,and the application value in photodetectors was explored.The main research and conclusions of this paper are as follows:In the first part,by using an improved two-way aeration firing physical vapor deposition（PVD）method,the high-purity WS₂ powder as precursor,successfully grown large-size（～300μm）high-quality 2D WS₂ nanosheets on Si/SiO₂ substrate.Before reaching the growth temperature of 1150°C,nitrogen gas was introduced in the reverse direction to inhibit the deposition of impurity powder on the substrate.After reaching the growth temperature,nitrogen gas was introduced in the forward direction to ensure the deposition of WS₂.We can control the thickness and size of WS₂ nanosheets by adjusting the growth temperature and growth time.By atomic force microscopy,transmission electron microscopy,Raman spectroscopy and PL Mapping characterization,it was proved that the prepared WS₂nanosheets were of uniform texture and high quality,which provides a method reference for the preparation of large-scale thin layers of WS₂ by controllable PVD.In the second part,with the help of a mechanical transfer platform,polyvinyl alcohol（PVA）was used for dry transfer to wrap one-dimensional CdS microwires between the PVD-prepared thin layer WS₂ and the mechanically exfoliated multilayer WS₂,thereby realizing Mixed-dimension photodetectors in sandwich stacks.The multilayered WS₂ and CdS layers in the device structure were used to provide efficient light absorption,while the underlying thin layer of WS₂ acts to passivate the substrate and reduce impurity scattering.In addition,due to the appropriate energy band positions between the three components,three type II energy band arrangements were formed,which can effectively separate the photogenerated carriers and enhance the photocurrent under the action of the built-in electric field.In particular,by confining the photo-generated electrons in CdS,the electron-hole recombination is reduced,thereby increasing the number of cycles of the photo-generated holes in the external circuit,thereby achieving a high optical gain effect.Based on the above co-action,the fabricated photodetector achieves a responsivity of～4.7 A/W,a detectivity D*of 3.4×10¹² Jones,and a simultaneous response and recovery time of 13.7 and 15.8 ms,respectively.The performance indicators of the device belong to the excellent category in the same type of device.In the third part,inspired by the anisotropy of the lattice structure of one-dimensional materials,we prepared a mixed-dimensional photodetector combining one-dimensional Teand multi-layer WS₂,which can achieve a great improvement in optoelectronic properties while also polarizing detection of light.In the designed device,the introduction of one-dimensional Temakes the two-dimensional WS₂ bend in-plane,adjust the local electronic structure of WS₂,and realize the continuous type II band arrangement in the horizontal direction.In addition,the heterojunction structure of Teand WS₂ in the vertical direction achieves high optical gain.Under the combined effect of these two effects,the transfer of photogenerated carriers is optimized and verified by theoretical calculations.Under the 405nm wavelength laser light source at room temperature,the final device has a turn-on ratio of more than 3.5×10₄,which was many orders of magnitude larger than that of a single WS₂ or Tedevice.The responsivity R and detection rate D*were～27.7A/W and9.5×10¹²Jones,respectively.In addition,for polarized light detection,the photocurrent didirectional ratio can reach 2.1.This hybrid-dimensional structure heterojunction device breaks through the limitation of a single material,gives full play to the characteristics of each component,and points out an effective way to construct new multifunctional optoelectronic devices.