Preparation And Properties Of TMDC Two-dimensional Materials Based On Chemical Vapor Deposition

Transition metal dichalcogenides(TMDC),such as molybdenum disulfide(MoS₂)and tungsten disulfide(WS₂),are well-studied two-dimensional materials with special lattice structure and nanoscale effects and have a large application prospect in the next generation optoelectronic devices because of their excellent electrical and optical properties.Chemical vapor deposition has been used to synthesis TMDC with high quality and large area.Furthermore,the van der Waals heterojunctions can be prepared by integrating TMDC grown by CVD via transfer method.In this paper,the process of synthesis MoS₂ and WS₂ by CVD was studied.Then the mechanism and process of transfer was studied.Van der Waals heterojunctions was further prepared and the optical properties as well as the modulation effect of twist angle on band gap were studied.The main research contents and achievements are as follows:Firstly,MoS₂ and WS₂ samples were prepared via CVD method.These samples were characterized carefully by optical microscope,atomic force microscope,scanning electron microscope,transmission electron microscope and Raman spectrometer,respectively.The size,thickness,layered property of these samples were carefully characterized and analyzed.Experiments of sulfurization on the surface of molybdenum metal and molybdenum oxide crystal were performed respectively and vertically aligned MoS₂ nanosheets with 5 nm thickness were synthesized.In the experiments of the reaction between Mo O₃ vapor and sulfur vapor,the single crystal MoS₂ and WS₂ samples with size of 20?m and thickness of 0.7nm were synthesized,which built a foundation for the further study of properties.Secondly,the mechanism of transfer was studied via interaction between MoS₂ and deionized water.Water droplets and water vapor were sprayed onto MoS₂,which was used to simulate the introduction of water during the process of transfer.The interaction process was recorded in real time under optical microscope.Through the detailed analysis of the interaction process,we found two modes of interaction between water and MoS₂ and proved the existence of liquid water film at the interface between MoS₂ and Si O₂ substrate is the reason why MoS₂can be exfoliated from growth substrate.In order to observe the interaction process between MoS₂ and water at nanometer scale,we used the environment-controlled atomic force microscope to scan MoS₂ in high relative humidity environment.We found existence of the super-wetting MoS₂-Si O₂interface and analyzed the reason and property of the super-wetting interface.A new transfer mechanism using super-wetting interface is proposed and the transfer of MoS₂ and WS₂ from growth substrate to flexible PDMS substrate or Si O₂ substrate was realized efficiently and cleanly.Finally,MoS₂/WS₂ van der Waals heterojunction and MoS₂/MoS₂ homojunction were prepared using transfer.We mainly focused on the optical property of MoS₂/WS₂ heterojunction and found the phenomenon of photoluminescence enhancement.The modulation effect of twist angle on heterojunction band gap was studied and we found that the coupling between layers is the key factor of modulation.