CVD Preparation Method And Photoelectric And Gas-sensing Performance Study Of Vertical Molybdenum Disulfide Nanosheets

Molybdenum disulfide（MoS₂）,as a typical representative of two-dimensional transition metal dichalcogenides（TMDCs）,has received increasing attention in recent years due to its unique physical and chemical properties and rich nanostructure features.Due to its unique three-dimensional structure,large surface area,multiple exposed boundaries,abundant active sites,good mechanical stability,and optical absorption ability,vertical MoS₂ nanosheets show greater application value than two-dimensional thin-layer MoS₂,has been widely used in the fields of microelectronics,optoelectronics,sensors,energy storage and catalysis.With the in-depth research of vertical MoS₂ nanosheets in various fields,more stringent requirements have been put forward for their preparation methods,structure,and morphology regulation.Chemical vapor deposition（CVD）is the most common method for preparing high-quality MoS₂.In recent years,a variety of synthetic strategies for the preparation of vertical MoS₂nanosheets by CVD have been developed,but these strategies still have the disadvantages of high cost,complicated process,poor uniformity,and low height of nanosheets.The development of simple,controllable,low-cost,large-area vertical nanosheet CVD preparation methods to meet the needs of different application fields is an urgent problem to be solved,which is of great significance for its performance research and application development.This dissertation aims to develop a low-cost,large-area,high-quality controllable vertical MoS₂ nanosheet CVD preparation method and to study the properties of the prepared vertical MoS₂ nanosheets.The large-area fabrication,patterned fabrication,optoelectronic properties,and gas-sensing properties of vertical MoS₂ nanosheets were studied in detail.The specific research contents of this paper are as follows:（1）Micron-height vertical MoS₂ nanosheets were prepared by titanium dioxide（Ti O₂）induced layer-assisted CVD method.The prepared films were characterized in detail and the optoelectronic properties of the vertical MoS₂ nanosheets were studied.The growth mechanism of vertical MoS₂ nanosheets induced by Ti O₂ induced layer and the growth model of vertical MoS₂ nanosheets based on boundary diffusion and surface diffusion are proposed.The vertical MoS₂ nanosheet photodetector exhibits better photoelectric response and faster response time than the monolayer and few-layer MoS₂ thin film photodetectors,which proves that the vertical MoS₂ nanosheet is a potential photoelectric detection material.（2）To meet the application needs of MoS₂ nanosheets,we developed two patterning strategies for vertical MoS₂ nanosheets.One uses a patterned Ti O₂ induction layer to directly grow vertical MoS₂ nanosheet patterns.The patterned Ti O₂-induced layer locally changes the surface properties of the substrate,successfully realizing the vertical growth of MoS₂nanosheets and the preferential growth on the Ti O₂-induced layer.The other is to prepare vertical MoS₂ nanosheet patterns by metal-assisted anomalous exfoliation method.The large surface area of the vertical MoS₂ nanosheets increases the interaction force between the MoS₂nanosheets and the upper metal capping layer,making it larger than that between the MoS₂nanosheets and the underlying Ti O₂ induced layer.Thus,patterned anomalous exfoliation of MoS₂ nanosheets is achieved.（3）In order to improve the responsivity of MoS₂ photoelectric sensor and expand the response wavelength range,we prepared lead sulfide quantum dots（Pb S QDs）/vertical MoS₂nanosheet composite broadband photodetector for the first time by pulling coating method.Benefiting from the excellent light absorption capability of Pb S QDs and the large specific surface area and excellent carrier mobility of vertical MoS₂ nanosheets,the composite sensor exhibits excellent photodetection capability.Compared with before spin coating,the responsivity of the composite detector was increased by 195 times to 58.6 A/W under 635 nm wavelength illumination.The response wavelength of the composite detector extends from the visible band to the near-infrared band,and the farthest response wavelength is 1550 nm.Its responsivity can reach 1.46 A/W under 1450 nm illumination.The composite structure of vertical nanosheet-like 2D materials and quantum dots provides a novel structure for next-generation optoelectronic devices.（4）We fabricated a vertical MoS₂ nanosheet/Ti O₂ thin film composite gas sensor,and tested the gas sensing performance of the composite gas sensor.The prepared composite gas sensor exhibits ultra-high gas-sensing response,good stability,and excellent selectivity to NO₂gas.The prepared composite sensor has a response of 56.8%to 200 ppb NO₂ gas under UV light and room temperature.The ultra-high gas-sensing response mainly originates from the large specific surface area of vertical MoS₂ nanosheets,more S and Mo edges,and more abundant high-energy active sites and junction regions formed at the interface with the Ti O₂film.