Two-dimensional materials under electron irradiation
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Atomic imaging of two-dimensional materials Two-dimensional (2D) systems such as graphene have been among the most studied materials in the past 10 years. Stable 2D materials are highly anisotropic crystals with strong covalent in-plane bonding and weak van der Waals interlayer bonding. The unusual properties of graphene, namely, its ballistic electron conductance and its high tensile strength, have made it one of the most studied materials of our time.1,2 However, the absence of a bandgap in graphene motivates the search for alternative 2D materials. Hexagonal boron nitride (h-BN) and the chalcogenides of molybdenum or tungsten are the most prominent examples that have been synthesized and characterized in detail.3–5 A common feature of many 2D materials is a hexagonal lattice in which the atoms are arranged in a honeycomb structure. Transmission electron microscopy (TEM) has turned out to be ideally suited for studying 2D materials. A large number of in situ experiments have been carried out in which the beam served not only for imaging but also for irradiation. The creation of point defects has been monitored in real time and with atomic resolution. The particular appeal of TEM studies of 2D materials is that the reconstruction of the lattice occurs in the 2D atomic plane and can, in many cases, be seen without any projection artifacts. Hence, a detailed picture of the atomic arrangement of defective materials becomes visible. Striking TEM images of point defects in graphene, recorded at a lateral
resolution smaller than the bond length of carbon, leave almost no room for uncertain interpretation. Novel mechanisms of lattice reconstruction have been discovered in in situ irradiation experiments, such as transformations between different polygons.6–8 In addition to a detailed understanding of radiation-induced defects and their influence on the properties of 2D sheets, the design of new structures has become feasible. Irradiationinduced structural changes can be achieved and characterized in real time and at the full lateral resolution of the instrument. On one hand, modern TEM instruments achieve an imaging resolution of better than 1 Å, which is less than the distance between atoms in solids. This provides the possibility of imaging individual atoms and their migration under electron irradiation. On the other hand, electron beams can be focused onto spots of less than 1 Å, which allows use of the beam as tweezers to manipulate materials at the single atom level.9–11 Detailed in situ electron irradiation experiments of 2D materials have been carried out,12–18 and the transmission electron microscope has become a useful tool for studying the structural stability of materials and creating new nanostructures.15,19–27 Not only the formation but also the annealing of defects can be controlled in such irradiation experiments if high-temperature specimen stages are used. The reconstruction of vacancies in carbon nanostructures, which has turned out to be the basis for many surprising phenomena, requires temperatures ab
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