Study On The Optoelectrical Property Of 2D Tungsten Disulfide Selenium Doping And Its Heterojunction

Recently,two-dimensional(2D)layered transition metal dichalcogenides(TMDCs)have drawn widespread research interest as a new class of atomically thin semiconductors due to the underlying physics and promising applications in photonics,optoelectronics and the development of valleytronics.In particular,as one type of significant 2D materials,tungsten disulfide(WS₂)have a thickness dependent band gap,moreover,monolyer WS₂ have a direct band gap and could emit light at room temperature,which makes people have high expectations for its application in optoelectronic devices.The band gap of WS₂ is fixed,but in order to obtain a device with ideal photoelectric response,a wider band gap range is required,besides the band gap and electronic structure of WS₂ need to be adjusted in a controllable way.Therefore,on the basis of WS₂,we synthesized large area W(S_xSe_1-x)₂ alloy nanosheets with various sulfur composition and constructed WS₂/Pd Se₂heterojunction to expand the band gap range,and systematically studies their optoelectronic properties and device performance.Our findings provide a universal,stable,effective and industrially scalable avenue toward future adjustment for their physical properties and expand the potential application scenarios of bulk or thin film semiconductors,helping to expand their future applications in functional electronic/optoelectronic devices.Specific research contents are as follows:(1)High quality monolayer W(S_xSe_1-x)₂ nanosheets with different ratios of sulfur were synthesized by a one-step chemical vapor deposition(CVD)method.The bandgap and exciton emission behavior of single layer W(S_xSe_1-x)₂nanosheets(0?x?1)can be remarkably tuned by changing the sulfur content.The increase of sulfur content enlarges the optical band gap of monolayer W(S_xSe_1-x)₂ films and promotes the conversion of excitons to trions,because the number of extra electrons in the alloy increases,and they synthesize trions with excitons.The exciton energy of W(S_xSe_1-x)₂ decreases with the increase of temperature,and the exciton-phonon coupling is enhanced due to the increase of temperature,which leads to the decrease of the exciton energy.In addition,the intensity of excitons and trions generally decrease with the increase of temperature,but there are great differences of W(S_xSe_1-x)₂ monolayers with different components.This is related to the non-radiative recombination of electron hole pairs in excitons.With the increase of temperature,the non-radiative recombination lifetime decreases,and the increase of radiative recombination efficiency of electron hole pairs leads to the decrease of exciton intensity.The value of I_T/I_X increases with increasing sulfur molar content,as the phonon energy in the out-of-plane A_1g(S-W-Se)mode increases with sulfur,and the coupling effect of exciton and trion regulated by the phonon is strengthened,and the exciton intensity ratio I_T/I_X tends to rise.These highlight the importance of further detailed characterization on exciton features in ternary alloy nanosheets,and can provide a strong physical basis for the application of this two-dimensional material in the field of optoelectronic devices.(2)The ultrathin W(S_xSe_1-x)₂ nanosheets with different sulfur mole content are successfully prepared via a Na Cl assisted CVD method.The effects of different sulfur and selenium ratios and temperature on the electrical transport characteristics of W(S_xSe_1-x)₂ alloy FET fabricated by electron beam lithography were systematically studied.The W(S_xSe_1-x)₂ FETs exhibit a high on/off ratio over 10⁵,but because of the existence of non-ohmic contact,two back-to-back Schottky barrier formed,coupled with the ionization impurity scattering and the effect of alloy scattering,the carrier mobility of single-layer W(S_xSe_1-x)₂ alloy transistor is low.In addition,the charge carrier types of alloy nanosheet transistors can be systematically tuned by adjusting the alloy composition.The electrical transport characteristics of W(S_xSe_1-x)₂ alloy FET with different sulfur contents show a systematic shift of carrier types from p-type semiconductors in WSe₂-rich samples to n-type semiconductors WS₂-rich samples,while the intermediate component of the W(S_xSe_1-x)₂ alloy transistor show bipolar characteristics,which lays the foundation for their logic devices and analog applications.Moreover,we have also studied the photoelectric properties of transistors,the high photoresponsivity of ultrathin W(S_xSe_1-x)₂ phototransistors imply their potential applications in image sensors and communication devices.These researches could contribute to expanding diverse applications in future functional electronic/optoelectronic devices.(3)Two dimensional vertically stacked WS₂/Pd Se₂heterostructure is successfully obtained by mechanical exfoliation method.The variation of Raman phonon vibration,exciton transition and surface potential in WS₂/Pd Se₂heterostructures were systematically investigated,and the influence of different temperatures on their characteristics was also explored.The Raman phonon vibration frequency shift in WS₂/Pd Se₂ heterostructure was more obvious than that in pure WS₂or Pd Se₂only,which is due to the lattice mismatch and interlayer coupling between the heterojunctions.The maximum values of A_g and B_1g modes cannot appear at the same time under the condition of parallel polarization scattering and cross polarization scattering by using angular polarization Raman spectroscopy,and the interlayer coupling of heterojunction will not have an essential effect on the four Raman vibration properties related to WS₂.This can be used to identify which vibration mode the unknown Raman peaks belong to.By studying the surface potential of WS₂/Pd Se₂ heterostructure,it is proved that the heterojunction can be used to optimize the related charge transfer performance of WS₂ photodetector,and Pd Se₂ can be used as a transition layer between the metal electrode and WS₂,effectively reducing the adverse effect of Fermi level pinning effect on the charge transfer efficiency of the device.The synthesis of WS₂/Pd Se₂ heterostructure can be used to regulate the band gap of WS₂,which lays a solid foundation for expanding the application scope of the two-dimensional heterojunction,optimizing the electrical and photoelectric properties of related transistor devices,and giving full play to their related role in the development of two-dimensional electronic devices.