| In recent years, two-dimensional ?2D? Trasition metal carbides ?TMCs? and nitrides ?TMNs? have attracted increasing interest because of their unique properties and many potential applications such as electrochemical energy storage,thermoelectric energy conversion, catalysis, and electromagnetic shielding. However,the structures of 2D TMCs and TMNs have not been fully understood. In this dissertation, we systematically studied the phase transformation and point defects of 2D ?-Mo2C and ?-MoN crystals grown by chemical vapor deposition ?CVD?,respectively,by transmission electron microscopy ?TEM?. The main results are shown as follows:It was found that the interstitial carbon atoms in 2D ?-Mo2C crystals migrate from ordered zigzag configuration to disordered random distribution in the molybdenum octahedrons under electron beam irradiation, resulting in phase transition of 2D M02C from orthorhombic ??? phase to hexagonal ??? phase. Based on first-principle calculations, carbon atom migration paths and the corresponding energy barriers were discussed. It was found that that the carbon atom migration parallel to electron beam incident direction has minimal energy barrier. Utilizing this unique phase transition property of 2D ?-Mo2C crystal and the advanced aberration-corrected scanning TEM, we fabricated the lateral heterostructure of 2D?/? Mo2C with well-controlled pattern and sharp interface, which provides a platform to study the vortex physics at the nanoscale and the synergistic catalytic effects of ? and ? phases at the interface.In addition, we found that CVD-grown 2D molybdenum nitride crystals are hexagonal ?-MoN. They are composed of a large-area ?1-MoN thin film and many?3-MoN triangles on its surface, forming a vertical heterostructure. Using aberration-corrected scanning TEM, we analyzed the structure of the large area?1-MoN thin film by atomic scale, and we found two types of point defect configurations and proposed their possible formation mechanisms... |
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