Growth And Properties Of Twodimensional Semiconductor Molybdenum Diselenide And Its Heterostructures

Two-dimensional（2D）layered functional materials have excellent physical and chemical properties,which is a kind of new fashioned energy catalytic materials and considered to be ideal construction materials of next-generation electronic and optoelectronic devices.Structure of materials determines the properties of materials and their application.Guided by the function,design and preparation nanostructure materials with specific performance have always been a hot research focus.In 2D materials field,especially 2D layered TMDC,the research on functional structure materials with distinctive architecture and unique performance is almost a blank.The thesis starts out from 2D layered MoSe₂ functional material.MoSe₂ and its related heterojunction materials with novel structures are designed and prepared.Moreover,the structure-function relationship between functional structure and the correlative performance is also revealed.Several important progresses have been obtained in optoelectrical devices and electrocatalytic HER application territory.Large-area monolayer MoSe₂ hollow sphere ordered structure arrays are designed and prepared in this thesis.Firstly,the regularly arranged SiO₂ solid sphere ordered structure arrays template is received by self-assembly process.The MoSe₂ is grown on the surface of SiO₂ to form shell-core MoSe₂/SiO₂ structure products by CVD technique.Subsequently,inner SiO₂ template is removed to obtain large-area monolayer MoSe₂ hollow sphere ordered structure arrays,which is applied in photodetector field.Hollow sphere architecture can effectively increase light trapping effect of the photodetector by multiple reflection and scattering process within the hollow sphere interior to improve photoabsorption efficiency under light illumination.The research shows monolayer MoSe₂ hollow sphere ordered structure arrays have wide spectrum responsescope,ranging from ultraviolet region to visible region.This photodetector displays higher photoresponse value,which is tenfold greater than that for MoSe₂ compact film in the same condition.Large area mosaic MoS₂/MoSe₂ lateral and MoO_x/MoSe₂ vertical heterojunctions on SiO₂/Si substrate are directly grown by CVD method.During the CVD process,MoSe₂ is grown along triangular MoS₂ edges and occupies the vacant region of the substrate,ultimately generates the large area mosaic MoS₂/MoSe₂ lateral heterojunctions.MoS₂/MoSe₂ heterojunction has typical type-Ⅱ energy band structure.This structure can encourage effective separation of photogenerated electrons and holes,which not only leads to long photogenerated carrier longevity but also suppresses the interfacial electron-hole recombination.This heterojunction displays the best photoresponse characteristic compared with individual MoS₂ and MoSe₂ under the same condition,which is triple and double higher than that for MoS₂ and MoSe₂.MoO_x/MoSe₂ vertical heterojunction nanolayers have hexagonal structure with the average edge dimension of 8 μm.This heterojunction device demonstrates distinctly current-rectifying speciality,and it can be adjusted by gate voltage.Moreover,the device shows excellent photoresponse characteristic.Different component 3D ternary vertical MoS_2（1-x）Se_2x（x = 0,0.21,0.47,0.72 and 1）alloy nanosheets are controllably prepared on conductive carbon cloth substrate by adjusting the precursors ratio of S and Se sources through one step CVD process.Morphological evolution process of the products is obtained by changing different reaction time.The catalyst with three-dimensional vertical structure and alloy constituent can effectively expose edge active sites of TMDC and activate inert base plane.Furthermore,the catalyst is directly grown on the electrode,which can facilitate the contact of catalytic active materials and electrode to accelerate electron transfer process.The electrocatalytic HER testing results demonstrate that Mo（S0.53Se0.47）2 exhibits the optimally catalytic activity in MoS_2（1-x）Se_2x alloy nanosheets.Moreover,density functional theory calculation results further prove that alloy nanosheets have the most suitable hydrogen adsorbed Gibbs free-energies and S,Se vacancies formation energies,when x is ⁰.5.Transitional metal oxide MoO₂ has metallicity combined with catalytic active MoSe₂ functional material,which can effectively reduce the resistance of catalytic active materials and sharply improve electrocatalytic HER performance of the products.MoO₂ and MoSe₂ two substances possess higher lattice mismatch,which can generate a mass of structure defect in core-shell MoO₂/MoSe₂ nanosheets.The defect serves as catalytic active sites can increase electrocatalytic HER performance.Therefore,3D vertical core-shell MoO₂/MoSe₂ nanosheets are successfully grown in this thesis.The products exhibit superexcellent electrocatalytic HER performance with very small Tafel slope（49.1 m V dec-1）.Co Mo Se/MoSe₂ lateral and vertical heterojunction nanolayer products possess hexagonal structure with the average edge size of 15 μm.The maximum edge measurement of the products is 100 μm.The edge of vertically multilayer heterojunctions presents clearly terrace structure.This catalyst can be comparable to the optimal electrocatalyst in the other 2D TMDC atomic layer type electrocatalysts.The work in this thesis can enrich the controllably preparative technique and optoelectronic and electrocatalytic HER application investigation of TMDC and the related heterojunction functional nanomaterials with a variety of unique structure.These results establish the firm foundation for MoSe₂ application in high performance microelectronic device,electrocatalytic HER,optoelectronic device and so on,and the research and application in the correlative fields.