| Summary: | Many novel physical phenomena and fascinating properties emerge when materials are thinned down to monolayers. For instance, graphene, the monolayer form of graphite, has zero gap, ultra-high electron mobility, massless chiral Dirac fermions, and an anomalous quantum Hall effect that is not observed in graphite. However, monolayer materials, i.e., 2D materials, are not necessarily flat, and may have defects and interfaces that have profound influence on their properties and their performance in devices. With the recent development of single-atom imaging in aberration corrected scanning transmission electron microscope (STEM) and the combination of first-principles atomic-scale calculations such as density functional theory (DFT) and molecular dynamics (MD), the atomic structure of the defects and interfaces and their properties can be analyzed and predicted on single-atom scale.
In this dissertation, the synergistic combination of STEM imaging and atomic-scale calculations are used to understand complicated defect structures and their effects on the physical properties of various 2D materials. Specific projects using this combined technique are detailed as follows: (1) Stacking boundaries in bilayer graphene are revealed to be not atomically sharp but continuously strained channels that extend over several nanometers in the form of ripples; (2) The atomic structure of an atomically abrupt lateral interface between WS2 and MoS2 monolayers is directly visualized at the atomic scale with a prediction of type-II band alignment at the sharp interface; (3) The formation of inversion domains in monolayer MoSe2 is found to be driven by the collective evolution of Se vacancies excited by the electron beam, where the formation mechanism is explored by DFT calculations; (4) Controllable fabrication of three-atom-wide metallic nanowires within semiconducting transition-metal dichalcogenide monolayers is developed using electron irradiation, where the mechanical and electronic properties of the nanowires are studied by DFT. At the end of this dissertation, perspectives of future research in 2D materials are outlined and possible experiments are proposed.
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