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Quasi-2D crystals inside bilayer graphene have been observed in in-situ TEM experiments [Nature 564 (2018) 234]. It was also revealed that Li crystals have the FCC structure, nucleate at point defects in graphene and contain impurity atoms. Using first-principles calculations, we systematically study the interaction of isolated Li atoms and those assembled in FCC crystals with vacancy-type defects in graphene and show that quasi-2D Li crystals encapsulated between graphene sheets must indeed nucleate at the defects and that the interaction of not only isolated Li atoms but also Li crystals with the defects in graphene is strong. We further demonstrate that a moiré pattern develops at the graphene/Li interface. Finally, we investigate the behavior of impurities most likely to be found in the encapsulated Li crystals, such as O, N, S, and F and show that all impurity atoms take octahedral interstitial positions and strongly interact with atoms in Li crystals, thus impeding the de-lithiation process. Our theoretical work focused on the fundamental aspects of the behavior of Li inside bilayer graphene should help rationalize the results of in-situ TEM experiments and shed light on the role of impurities in the degradation of anode materials during Li-ion battery operation.

This article has an erratum: DOI 10.1016/j.carbon.2017.03.065 We examine swift heavy ion-induced defect production in suspended single layer graphene using Raman spectroscopy and a two temperature molecular dynamics model that couples the ionic and electronic subsystems. We show that an increase in the electronic stopping power of the ion results in an increase in the size of the pore-type defects, with a defect formation threshold at 1.22–1.48 keV/layer. We also report calculations of the specific electronic heat capacity of graphene with different chemical potentials and discuss the electronic thermal conductivity of graphene at high electronic temperatures, suggesting a value in the range of 1 Wm−1 K−1. These results indicate that swift heavy ions can create nanopores in graphene, and that their size can be tuned between 1 and 4 nm diameter by choosing a suitable stopping power.

Contrary to a wide-spread belief that alkali metal (AM) atoms intercalated into layered materials form single-layer structures only, recent experiments [Nature 564 (2018) 234] showed that multi-layer configurations of lithium are possible in bi-layer graphene. Using state-of-the-art first-principles calculations, we systematically study the intercalation energetics for various AMs (Li, Na, K, Rb, Cs) in bi-layer graphene and MoS2. We demonstrate that for bi-layer graphene as host the formation energy of multi-layer structures is negative for K, Rb and Cs and only slightly positive for both Li and Na. In view of the previous experimental data on lithium, a multi-layer of Na might therefore form, while it is well-known that single-layers of Na in graphitic hosts are energetically very unfavorable. In MoS2, multi-layer structures are considerably higher in energy than the single-layer ones, but the formation of the former can still occur, especially for the AMs with the lowest electro-negativity. To rationalize the results, we assess the charge transfer from the intercalants to the host material and analyze the interplay between the ionic and covalent bonding of AM and host atoms. While our theoretical effort primarily focuses on the fundamental aspects of AM intercalation, our findings may stimulate experimental work addressing multi-layer intercalation to maximize the capacity of anode materials in AM ion batteries.

The response of multi-walled carbon nanotubes encapsulating Co nanorods to ion irradiation was studied. The irradiation experiments with medium ion energies (40–500 keV) were carried out at high temperatures and combined with transmission electron microscopy and Raman characterization of the irradiated samples. Contrary to electron irradiation and high-energy (100 MeV) ion irradiation, we did not see accumulation of pressure inside irradiated nanotubes. We found that nanotubes with Co nanorods inside were transformed to amorphous carbon rods encapsulating Co clusters with typical diameters of 3–6 nm. As Co is magnetic, such one-dimensional composite systems could be used for various applications such as magnetic data storage or magnetic resonance imaging.

The traditional picture of a carbon nanotube as a rolled graphene sheet implies that the mechanisms of intra-layer atomic processes in the two systems should be qualitatively similar. Using density-functional theory and tight-binding methods we show that the mechanism of single vacancy migration in nanotubes is different from that in graphite, as the curvature of the nanotube atomic network breaks the trigonal symmetry of a perfect graphene sheet, making the diffusion anisotropic, and strongly influencing the migration barrier. We further demonstrate that the formation energy of a double vacancy in nanotubes is smaller than that for a single vacancy, a behavior different from most monatomic solids, including graphite.

We have studied the low energy irradiation of carbon nanotubes (CNT) with K ions using classical molecular dynamics simulations with analytical potentials. The studied CNTs had diameters of about 0.5–1.2 nm and single or multiple walls. The average penetration depth and probabilities to introduce an impurity atom into CNT were studied with simulations on irradiating the CNT with single K ion. The number of potassium clusters, their average sizes and the damage produced into the CNT due to the irradiation were studied using multiple K ion irradiations. We found that the K ions are mobile in CNTs right after the implantation event and that they cluster together. For CNTs with 1–3 coaxial tubes, the highest ratio of K atoms in clusters per total number of K ions was obtained by using an irradiation energy of about 100 eV. Also the least damage per K ion was found to be produced into the CNT with this energy when those energies high enough for the ion to penetrate the outermost wall of the CNT were considered.

Recent experiments have demonstrated that electron irradiation of bundles of single-walled carbon nanotubes resulted in dramatic increase of the bundle bending modulus at moderate irradiation doses, followed by a decrease in mechanical properties at higher doses. To understand such a behavior, we employ molecular dynamics simulations with empirical potentials and analytical approximations to calculate defect production rates and mechanical properties of the irradiated nanotubes. We show that the observed peak in the bending modulus originates from a trade-off between irradiation-induced bundle stiffening via inter-tube covalent bonds and a drop in the Youngs modulus of individual nanotubes due to vacancies.

Doping of carbon nanotubes with boron and nitrogen should provide more control over the nanotube electronic structure. In addition to the chemical substitution and arc-discharge methods used nowadays, we suggest using ion irradiation as an alternative way to introduce B/N impurities into nanotubes. Making use of molecular dynamics with analytical potentials we simulate irradiation of single-walled nanotubes with B and N ions and show that up to 40% of the impinging ions can occupy directly the sp 2 positions in the nanotube atomic network. We further estimate the optimum ion energies for the direct substitution. As annealing should further increase the number of sp 2 impurities due to dopant atom migration and interaction with vacancies, irradiation-mediated doping of nanotubes is a promising way to control the nanotube electronic structure.

Making use of molecular dynamics with analytical potentials we theoretically study channeling of heavy ions with keV energies through multi-walled carbon nanotubes. We show that under certain conditions on the tube alignment with respect to the ion beam and on ion energies, the ions can move through the empty cores of the MWNT with a very low probability of dechanneling. We further discuss how, by employing the experimental techniques routinely used nowadays for handling nanotubes on substrates, one can create a nanotube-based conduit for energetic ions, which should work as an aperture and allow one to manipulate the beam at the nanoscale.

As recent experimental and theoretical studies demonstrate, irradiation of carbon nanotubes with energetic particles can successfully be used for nano-engineering, e.g. for creating molecular junctions between the nanotubes, making nanotube-based quantum dots and composite materials with enhanced mechanical properties. We briefly review the recent progress in our understanding of ion-irradiation-induced phenomena in carbon nanotubes with a specific stress on theoretical results. We point out the issues which still lack complete comprehension and further outline the most promising ways of using beams of energetic particles for nanotube-related nano-engineering.

Using density-functional tight-binding and ab initio methods we study the adsorption and migration of carbon adatoms on surfaces of single-walled zigzag carbon nanotubes. We demonstrate that the adatoms form strong covalent bonds with the nanotubes and that the migration is strongly anisotropic. The adatom adsorption energy and migration barrier depend on the nanotube diameter and chirality. The migration barriers, being in the range of 0.6–1 eV, are in a good agreement with the experimental values (about 0.8 eV) reported in the literature. � 2003 Elsevier Ltd. All rights reserved.

Making use of empirical potential molecular dynamics, we simulated the bombardment of single-walled carbon nanotubes with various ions. We calculated the irradiation-induced damage which proved to be higher for heavy ions than for light ones due to higher values of the cross-section for the defect production in a nanotube. We demonstrated that chemical aspects of ion–nanotube interactions and finite temperatures are of minor importance when energetic ions penetrate through nanotubes. We finally estimated the ability of nanotubes to stop energetic ions.

Making use of empirical potential molecular dynamics, we study ion bombardment of crossed single-walled carbon nanotubes as a tool to join the nanotubes. We demonstrate that ion irradiation should result in welding of crossed nanotubes, both suspended and deposited on substrates. We further predict optimum ion doses and energies for ion-mediated nanotube welding which may potentially be used for developing complicated networks of joined nanotubes.

Tight-binding molecular dynamics simulations were used to obtain sticking cross-sections for CH 3 radical chemisorption on unsaturated carbon atom sites on carbon surfaces. Our results show that the chemisorption of a CH 3 radical is affected both by the angle of incidence of the radical and the local atomic neighborhood of the dangling bond. The sticking cross-sections are compared with experimental results and the implications of our modeling on C:T film growth in fusion devices are discussed.

Although the mechanisms of the erosion of carbon-based materials by low-energy hydrogen ions have recently been addressed from an atomistic point of view [see e.g. Phys. Rev. B 63 (2001) 195415], it is not clear yet whether the quantum-mechanical effects are important for an adequate description of the erosion. We now study the low-energy erosion mechanisms by tight-binding molecular dynamics, which accounts for the quantum-mechanical nature of the interactions between the impinging particle and surface. We simulate the ion-assisted bond breaking in simple model systems like carbon dimers and also model sputtering from realistic hydrogenated carbon surfaces. Our simulations confirm the empirical potential results on the low-energy erosion mechanism, and help one to distinguish between the different mechanisms of the surface erosion.

We calculated STM images of both metallic and semiconducting single-wall carbon nanotubes with atomic vacancies. In our simulations, we employed the tight-binding Green's function technique and the recursion method. We predict that vacancies should result in the formation of hillock-like features in STM images of metallic nanotubes, which are especially appreciable at small bias voltages (of about 0.1–0.4 V). An enhancement in the tunneling current is due to vacancy-induced states at the Fermi energy, and these states are spatially localized on the atoms surrounding the vacancies. Electronic superstructures analogous to those in graphite near point defects are observed near the vacancy. For semiconducting nanotubes, hillocks and superstructures are also visible at those bias voltages when band edges contribute to the tunneling current.

Making use of the tight-binding Green's function technique, we have calculated the STM images of graphite with surface and sub-surface defects, while taking into account the relaxation of the lattice due to defects. We have demonstrated that two different physical mechanisms may result in the formation of hillocks in the STM images: buckling of the graphite surface due to interstitials between the uppermost graphite layers and the enhancement of the electron density of states close to the Fermi energy on the carbon atoms in the vicinity of vacancies. Our results indicate that small hillocks may originate both from the interstitial clusters and from the vacancies. By contrast, however, large hillocks in excess of 10 A~ in diameter can be caused only by interstitial clusters.

The numerical solution of a system of non-linear kinetic equations for electron distribution functions and non-equilibrium phonons is obtained for quantum cascade intersubband lasers on quantum wells and wires. In the case of quantum wells the analytical solution of system of equations is found. The possibility of lowering the threshold current as a result of the accumulation and reabsorption of optical phonons and due to different effective masses of electrons in subbands is demonstrated. For lasers on quantum wires the lowering of threshold current takes place also, with the rate being greater as for lasers on wells.

On the basis of exact diagonalization of the Emery Hamiltonian the effect of the Anderson disorder on the density of states (DOS) for the two-dimensional cluster Cu 4 O 8 is numerically investigated. The results indicate the broadening of spikes in the DOS. The shrinkage of the charge-transfer gap takes place both for the undoped insulating state as well as for the electron- and hole-doped metallic states. The rapid decrease of the DOS near the Fermi energy occurs for the doped system giving rise to the metal-insulator transition at a certain critical value of the disorder parameter.