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Electron-beam (e-beam) irradiation damage is often regarded as a severe limitation to atomic-scale study of two-dimensional (2D) materials using electron microscopy techniques. However, energy transferred from the e-beam can also provide a way to modify 2D materials via defect engineering when the interaction of the beam with the sample is precisely controlled. In this article, we discuss the atomic geometry, formation mechanism, and properties of several types of structural defects, ranging from zero-dimensional point defects to extended domains, induced by an e-beam in a few representative 2D materials, including graphene, hexagonal boron nitride, transition-metal dichalcogenides, and phosphorene. We show that atomic as well as line defects and even novel nanostructures can be created and manipulated in 2D materials by an e-beam in a controllable manner. Phase transitions can also be induced. The e-beam in a (scanning) transmission electron microscope not only resolves the intrinsic atomic structure of materials with defects, but also provides new opportunities to modify the structure with subnanometer precision.
We present molecular dynamics simulations using both empirical potentials (EP) and density functional theory (DFT) on ion irradiation of graphene. The comparison between the two methods shows that EP gives not only qualitatively but also quantitatively reasonable estimates of defect production during ion irradiation in carbon nanosystems. Ion irradiation is shown to give rise to a range of interesting defects e.g. single, double and triple vacancies, bond rotations, close-by Frenkel pairs and more complex defect structures. We show that the creation of these defects is related to the atomic processes upon the ion impact, and define the critical irradiation angles both for maximum damage and no penetration as a function of the ion mass.
Carbon nanotubes (CNTs) are one of the possible building blocks for electronic devices in the transition phase from traditional silicon-based microelectronics towards the few-nanometer regime. Remaining problems in integrating CNTs to the existing technology is the low reactivity of the CNT walls which leads to low conductance between CNTs and the other components. Because recent studies have shown that ion irradiation can be used to modify both the electrical and structural properties of CNTs, we propose that it could also be possible to use ion irradiation with low energies to enhance the conductance of these connections. We have used classical molecular dynamics simulations with empirically fitted potentials to examine this possibility by irradiating a single-walled carbon nanotube (SWCNT) on a silicon substrate at room temperature. The nanotube was deposited over a trench created to the silicon substrate so that the nanotube was partly suspended. Low irradiation doses and low energies (0.2 keV − 1.2 keV) were used to ensure that the irradiated CNT will not be destroyed. The simulations were carried out for silicon, carbon and neon ions. Our simulations indicate that ion irradiation will increase the number of covalent bonds between the CNT and the Si substrate. When the irradiation dose and energies are low, the damage caused to the SWCNT atomic network can be tolerable when compared to the improvement in the conductance of the contact regions. Furthermore, as the CNTs have high ability to heal the irradiation-induced damage, it is possible that the irradiation will not have a significant negative effect to the conductivity of the CNT in a system of this type.
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