Study On The Optoelectronics Properties Of Two-Dimensional Tungsten And Molybdenum Chalcogenides And Their Heterojunctions

Two-dimensional transition metal chalcogenides（TMDCs）exhibit atomic-scale thickness,a bandgap in the range of 1～2 e V,strong spin-orbit coupling,and high bending rigidity,which make them for the next generation of spintronics and optoelectronics as well as provide a platform for the research of flexible electronic devices.In recent years,there has been a significant demand for broad-band photodetectors in the field of industrial production,infrared remote sensing detection and image sensing.Therefore,it is particularly important to develop photodetectors with high precision and wide spectral response range.However,photodetectors based on TMDSc materials face some serious challenges.The low absorption efficiency in the visible range due to the atomic-level thickness of the monolayer TMDCs material is not sufficient for practical applications in the field of photovoltaics.Moreover,the bandgap of TMDCs constrains the application of photodetectors in the wide wavelength range.This paper is based on the layer modulation of TMDCs and the epitaxial growth of van der Waals heterojunctions on TMDCs.Chemical vapor deposition（CVD）method is prepared to successfully multilayer 2H-and3R-stacked WSe₂,bilayer WS₂/Mo S₂ heterojunction,Sb₂O₃/single crystal TMDCs heterojunction as well as Sb₂O₃/polycrystalline W（Mo）S₂ heterojunction,further applied it to the field of high-performance photodetectors.The main research contents are as follows:（1）The multilayered 3R-and 2H-straked WSe₂ nanosheets were prepared by chemical vapour deposition.Using aberration-corrected scanning transmission electron microscopy（AC-STEM）,it was observed that the multilayer 2H-WSe₂ is composed of alternating W and Se atoms arranged with a hollow honeycomb lattice,while 3R-WSe₂ is a honeycomb lattice occupied in the middle.In addition,the layer number effect of 2H-and 3R-WSe₂are investigated by Raman spectroscopy（Raman）and fluorescence spectroscopy（PL）.And in the field of nonlinear optics,the second harmonic signal intensity of 2H-and 3R-phase WSe₂ crystals was compared as a function of the number of layers.Finally,photodetectors based on different thicknesses of 3R-WSe₂ exhibit excellent responsivity（R）,high specific detectivity（D*）,outstanding external quantum efficiency（EQE）and fast response times.（2）The lateral bilayer WS₂-Mo S₂ heterojunctions were prepared by a two-step chemical vapour deposition method.The optical properties and chemical structures of grown bilayer WS₂-Mo S₂ heterojunctions were investigated in detail using Raman spectroscopy,fluorescence spectroscopy,aberration-corrected scanning transmission electron microscopy,and second harmonic generation（SHG）.Then,the potential distribution between the bilayer WS₂ and Mo S₂ was investigated with the aid of Kelvin-probe force microscopy（k PFM）.Finally,the devices based on the bilayer WS₂-Mo S₂ heterojunction were constructed to exhibit typical semiconductor properties and excellent optoelectronic performance.（3）The triangular Sb₂O₃ microresonators grown on monolayer TMDCs were prepared by a two-step chemical vapour deposition method.The morphology and thickness distribution of the Sb₂O₃ microresonators/TMDCs were characterised by optical microscopy（OM）and atomic force microscopy（AFM）.The phase and crystal structure are investigated by using X-ray diffraction（XRD）,transmission electron microscopy（TEM）and selected area electron diffraction（SAED）.In addition,the different thickness of the Sb₂O₃ microresonators epitaxially grown on TMDCs can be modulating the Raman vibrational peak intensity and the PL peak intensity with the aid of confocal Raman microscope.Finally,the optoelectronic properties of the Sb₂O₃ microresonators coupled to monolayer TMDCs were investigated.（4）The epitaxially grown Sb₂O₃ nanosheets on monolayer polycrystalline TMDCs（Mo S₂ and WS₂）films were synthesized by a two-step chemical vapour deposition method.The epitaxially grown Mo（W）S₂/Sb₂O₃ was used as a template to construct two additional different stacked structures（Mo（W）S₂/Sb₂O₃/Sb₂O₃/Mo（W）S₂ and Sb₂O₃/Mo（W）S₂）by PMMA-assisted transfer methods.The optical properties of the different stacked structures were investigated by using optical microscopy,Raman spectroscopy,fluorescence spectroscopy and absorption spectroscopy.In addition,three different stacked devices were constructed to study their respective optoelectronic properties.The photodetectors based on various stacked structures（Mo（W）S₂/Sb₂O₃,Sb₂O₃/Mo（W）S₂,and Mo（W）S₂/Sb₂O₃/Sb₂O₃/Mo（W）S₂）are easily modulated by laser power density and gate voltage and also have a broad spectral optical response from the ultraviolet-visible-short wavelength infrared region.