| The properties and applications of two-dimensional (2D) layered materials are severely influenced by their atomic structures. Therefore, investigations of their structure evolution is of great significance to understand the correlations between their atomic structures and properties, which may further promote their diverse applications in many fields. In this thesis, we investigate the structure evolutions of 2D layered nanomaterials (including graphene, hexagonal boron nitride h-BN, molybdenum disulfide MoS2) by electron irradiation inside a transmission electron microscope (TEM). Base on irradiation mechanism analysis and the control of electron parameter, we realize the fabrication of low-dimensional structures on the atomic scale. We also attempt to realize the dynamic structural evolutions of 2D layered materials in a liquid environment based on the liquid-cell TEM. The main achievements are summarized as follows:1. Focused electron beam irradiation induced sputtering (EBIS) and deposition (EBID) on ultrasonically exfoliated graphene membranes have been studied inside the TEM. The relationship between EBIS and EBID as well as the influence of thermal excitation on EBIS and EBID are discussed. (1) It is found that EBIS is significant when the current density is larger than 300 A/cm~2, while EBID begins to dominate when the current density is less than 300 A/cm~2. (2) Thermal excitation can promote the reconstruction of defects in graphene, therefore the pores fabricated at high temperatures have better crystallinity. In addition, thermal excitation can also be adopted to modulate the size of as-fabricated pores. When the initial pore diameter is smaller than the graphene membrane thickness, the pore tends to shrink under thermal excitation, however when the initial pore diameter is larger than graphene membrane thickness, the pore tends to expand. The shrinkage and expansion of graphene nanopores can be explained by the thermal induced migration of uncombined carbon atoms, which energetically prefer to form a stable structure with low free surface energy.2. Structure evolution of h-BN sheets under an 80 keV electron irradiation has been investigated. (1) The observation demonstrates that B vacancies and triangle-shaped defects with N-terminated zigzag configurationare formed in h-BN monolayer due to high asymmetry between knock-on damage thresholds for B and N. (2) Squashed single-walled armchair BN nanotubes can be formed by irradiating AA1 stacked h-BN bilayer. The as-fabricated nanotubes could be thinned by removing atom row by row until the diameter being reduced to-0.45 nm. Further analysis demonstrates that the smallest freestanding single-walled BN nanotube is (3,3) tube, which is experimentally observed for the first time.3. Structure evolution of monolayer MoS2 under an 80 keV electron irradiation has been investigated. (1) The results indicate the density of sulphur vacancies (SVs) on the surface of MoS2 exfoliated by micromechanical cleavage is in the order of 1013 cm-2. In-situ irradiation experiments demonstrate that the cross-section for sputtering S atoms from perfect MoS2 monolayer is (1.7 ± 0.2) × 10-28 m~2. (2) Ultrafine molybdenum sulphur ribbons with a uniform width of 0.35 nm are formed in MoS2 monolayer under electron irradiation. The nano-ribbons have lattice constants distinct from those of MoS2. Further analysis demonstrates the nano-ribbons could be Mo5S4. These results confirm that electron beam irradiation can serve as a novel top-down technique to precisely fabricate unique nanostructures.4. Liquid cell TEM system has been set up to investigate structural evolution of 2D layered materials in a liquid environment. (1) 2D materials and liquid are encapsulated in a liquid cell, which is fabricated by traditional silicon-based nano-and micro-processing technology. (2) The morphology of graphene and MoS2 in water is observed. (3) Observation of structural evolution of some other materials in a liquid environment is attempted. |
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