Atomic-scale Structural Modulation And Characterization Of Two-Dimensional Transition Metal Dichalcoqenide

The transition metal dichalcogenide(TMDs)has been studied in recent years because of its inherent atomic structure,excellent catalytic,electronic,optical,valley electronic properties and relative application potential in semiconductor devices and other fields,which makes it a new hotspot in low-dimensional materials after graphene.The performance of the material is determined by its structure,so the structural control of 2D TMDs is an important prerequisite for its wide application.In this thesis,the structure modulation on monolayer MoS2 was carried out by means of alloying and oxidation etching.And their structure evolution behaviours were analyzed with low-damaged aberration-corrected transmission electron microscopy combining with theoretical simulation,through which the atomic mechanism of the structure modulation is well studied.The first part of the thesis demonstrates the atomic structure characterization of the monolayer MoS2(1-x)Se2x alloy,and systematically studies the enrichment of Se atoms at MoS2 grain boundaries.Due to the grain boundaries which consist of the non-equilibrium dislocation core structures,the substitutional reaction barrier of Se atoms at the grain boundary is lower than that in the intra-domain,the Se atom is selectively enriched in the grain boundaries.And the distribution and concentration of Se atoms at the grain boundaries are closely related to the grain boundary angles,which is also concerned with the symmetry and the density difference of the dislocation cores in different structures.In the second part,the chemical vapor deposition method was used to obtain single layer MoS2(1-x)Te2x alloy with different Te doping concentration,and use the atomic resolution ADF-STEM image combined with the first principle calculation to reveal the type and distribution of Te dopants.The results show that the increase of doping temperature or the prolongation of doping time will significantly increase the Te concentration in the sample.Combined with the quantitative analysis on STEM images,disordered alloys are obtained under low temperature,in which the alloy degree is close to 100%.In high temperature doped samples,high concentration of Te will cause a large area of local distortion and a large number of S vacancies leading to a single Te atom chain structure.According to the DFT calculations,the stress release in the lattice may be the main cause of such distortion and the formation of defect chains.Afterwards,the coordinated TEM grids and aberration-corrected transmission electron microscopy were used to realize the "in situ" observation of the oxidation behavior of the monolayer MoS2 grown on the graphene substrate.After analysis on the statistics of the edge structures after etching,the origin and the atomic pathway of oxidation etching in single layer MoS2 were studied and deduced.The oxidation of monolayer MoS2 originates from the binding of O2 molecules to unsaturated S atoms,after the product dispersed in the form of gas phase,more unsaturated atoms are exposed,from which the oxidation process continues.Thus,the oxidative etching reaction happens preferentially at the grain boundaries,edges,and S vacancies in the domains which are rich in unsaturated bond S atoms.And the different edge structures leads to different etching pathways and residual structures.In addition,the zigzag-Mo termination edge is relatively passivated in the reaction with O2,so the zigzag-Mo edge dominates the edge structure after etching.Finally,the research works were summarized on the structural modulation of monolayer MoS2,and analyze the shortcomings in our work,and also prospects the scientific problems for the further TMDs research.