Study On The Preparation And Properties Of Two-dimensional TMDCs

Since researchers isolated graphene in bulk graphite,the concept of two-dimensional materials began to be widely used.However,unlike blocks,graphene is a semi-metallic material with zero band gap and high mobility.Because graphene’s zero band gap limits its applications in digital and optoelectronics,it needs to be replaced with band-gap materials such as Transition Metal Dichalcogenides（TMDCs）.TMDCs is a new type of layered semiconductor material,whose band gap size is greatly affected by the number of layers,so it is considered as an ideal material for the next generation of electronic devices.Chemical vapor deposition（CVD）is a more feasible way to prepare TMDCs than other methods because of the higher crystal quality and more efficient properties of the prepared materials.However,at present,the synthesis of high-quality two-dimensional TMDCs materials still cannot meet the application in the field of optoelectronic devices,and researchers still need to challenge the synthesis of two-dimensional materials with higher performance.Therefore,in this thesis,three kinds of two-dimensional TMDCs with different band gaps were grown on silica substrate by atmospheric chemical vapor deposition（APCVD）method.It was found that appropriate precursor concentration and carrier gas flow rate could promote the growth of large and regular two-dimensional materials in CVD system.Experiments show that the growth torsion double layer WS₂can be controlled by adjusting the growth time and temperature,and the effects of different torsion angles on the interlayer coupling are systematically studied.The main contents of this thesis are as follows:（1）In the CVD system,the S concentration gradient is formed by changing the distance between the substrate and the S source,and the correlation between the S concentration and the size and shape of Mo S₂ is explored.The results show that the concentration of S is the most abundant on the substrate nearest to the source of S,and continuous Mo S₂ films of more than 200μm are grown on the substrate.As the distance between the substrate and the source of S increases,the concentration of S decreases gradually.On the remaining three substrates,triangles and hexagons of about 20μm or even truncated triangles of about 10μm are grown.Continuous Mo S₂ thin films and triangular and hexagonal Mo S₂thin films were characterized by SEM,XRD,Raman and PL spectra.The results show that Mo S₂ thin films have good uniformity and excellent crystal quality.Our work provides a simple and controllable strategy for the synthesis of high quality monolayer Mo S₂ films.（2）Single crystal VS₂ nanosheets were grown by a space-constrained APCVD method.Moreover,the single-crystal VS₂ nanosheets were characterized by SEM,AFM,Raman and PL spectra.The results show that the grown single-crystal VS₂ nanosheets have better optical properties and excellent crystal quality.In addition,during our exploration,we found the effect of H₂ gas on the growth of VS₂ nanosheets by CVD.Therefore,we conducted experiments on the introduction time of H₂ and the proportion of H₂ in argon-hydrogen mixture respectively,and characterized and analyzed the residual substances in the quartz boat after growth by XRD and XPS spectra.Through the growth results and residue characterization analysis results,it is concluded that:A moderate amount of H₂will reduce vanadium to the required valence state for growth,but excess H₂ will reduce enough vanadium and etch growing VS₂.The optimal introduction time of H₂ and the optimal proportion of H₂ in argon-hydrogen mixture were obtained through research and exploration,so as to achieve the controlled growth of single crystal VS₂ nanosheets.（3）WS₂ nanosheets were grown by CVD method,and the influence of different growth conditions on WS₂ nanosheets during the growth process was explored.The results showed that the growth of torsional bilayer WS₂ could be promoted by regulating the growth temperature and growth time.The effects of different torsion angles on interlayer coupling are systematically studied.At 0°and 60°,the interlayer coupling strength is strongest due to the minimum interlayer distance.However,between0°and 60°,the interlayer coupling strength becomes weaker due to the increasing interlayer distance.Different interlayer coupling determines its different optical,mechanical and electrical properties,the twisted double layer WS₂ homojunction is prepared by adjusting the growth parameters,so that it can be better applied to TMDCs-based devices.