Investigation Of Controllable Synthesis Of Two-dimensional Transition Metal Dichalcogenide

TMDs materials have great potential value in electronic and optoelectronic devices due to their fascinating properties.But there are still some challenges in controllable preparation.In order to make these materials more versatile,further modification of their physical and chemical properties is requried.A possible way to fine-tune these properties is via chemical doping.Furthermore,these monolayer materials can be the basic building blocks for the preparation of heterojunctions,layer-by-layer stacking and in-plane seamlessly stitching can synthesize vertical and planar heterojunctions,respectively.Focused on the above,the main content are summarized as following:?1?We report a two step CVD method,the MoSe₂ monolayer flakes were prepared by CVD,then sulfurization happened in the presence of sulfur vapors.Using atomic resolution annular dark field scanning transmission electron microscopy?ADF-STEM?,we quantitatively characterized the distribution of S substitution atoms within the MoSe₂ lattice which were treated by different sulfurization time.We find the substitution of S atoms preferentially occurs in domain edges to be MoS₂-MoSe₂ heterostructures,and the interface of heterostructures is reaction center,which makes interface gradually moves into the inner domain of MoSe₂,finally forming monolayer MoS₂-MoSe₂planar heterostructures with atomically sharp interface.?2?The MoS₂ monolayer flakes were prepared on SiO₂/Si and graphene substrates,respectively.Then these flakes were sulfurized in the presence of Selenium vapors to get MoS_2xSe_2?1-x?alloys.The characterization results of atomic resolution ADF-STEM indicate that there is certain concentration of Se dopants in intra-domains of monolayer MoS₂,while it is much higher in grain boundaries and domain edges.Through the analyzing of strain,we demonstrated the reason of preferential substition of Se atoms in grain boundaries.Besides,we find that the interaction between monolayer MoS₂ and substrates,as well as the structure of substrates,has important effect on the preparation of MoS_2xSe_2?1-x?alloys.?3?Taking crystal violet as the seeding promoter,we successfully prepared monolayer MoS₂,and this method has a higher expermental repeatability than the conventional method.Then we use ultra-thin SiO₂ TEM grids as the substrates to directly grow MoS₂ flakes with the same method.Using ADF-STEM and elemental EDX mapping,we characterized the products,including layered MoS₂ and different nanoparticles,to research the micro-process of growth.The results allow us to propose a growth mechanism for monolayer TMDs.