Non-hexagonal Rings Research Articles

A grossly warped nanographene and the consequences of multiple odd-membered-ring defects

Graphite, the most stable form of elemental carbon, consists of pure carbon sheets stacked upon one another like reams of paper. Individual sheets, known as graphene, prefer planar geometries as a consequence of the hexagonal honeycomb-like arrangements of trigonal carbon atoms that comprise their two-dimensional networks. Defects in the form of non-hexagonal rings in such networks cause distortions away from planarity. Herein we report an extreme example of this phenomenon. A 26-ring C80H30 nanographene that incorporates five seven-membered rings and one five-membered ring embedded in a hexagonal lattice was synthesized by stepwise chemical methods, isolated, purified and fully characterized spectroscopically. Its grossly warped structure was revealed by single-crystal X-ray crystallography. An independent synthetic route to a freely soluble derivative of this new type of 'nanocarbon' is also reported. Experimental data reveal how the properties of such a large graphene subunit are affected by multiple odd-membered-ring defects.

Grain boundaries in graphene grown by chemical vapor deposition

The scientific literature on grain boundaries (GBs) in graphene was reviewed. The review focuses mainly on the experimental findings on graphene grown by chemical vapor deposition (CVD) under a very wide range of experimental conditions (temperature, pressure hydrogen/hydrocarbon ratio, gas flow velocity and substrates). Differences were found in the GBs depending on the origin of graphene: in micro-mechanically cleaved graphene (produced using graphite originating from high-temperature, high-pressure synthesis), rows of non-hexagonal rings separating two perfect graphene crystallites are found more frequently, while in graphene produced by CVD—despite the very wide range of growth conditions used in different laboratories—GBs with more pronounced disorder are more frequent. In connection with the observed disorder, the stability of two-dimensional amorphous carbon is discussed and the growth conditions that may impact on the structure of the GBs are reviewed. The most frequently used methods for the atomic scale characterization of the GB structures, their possibilities and limitations and the alterations of the GBs in CVD graphene during the investigation (e.g. under e-beam irradiation) are discussed. The effects of GB disorder on electric and thermal transport are reviewed and the relatively scarce data available on the chemical properties of the GBs are summarized. GBs are complex enough nanoobjects so that it may be unlikely that two experimentally produced GBs of several microns in length could be completely identical in all of their atomic scale details. Despite this, certain generalized conclusions may be formulated, which may be helpful for experimentalists in interpreting the results and in planning new experiments, leading to a more systematic picture of GBs in CVD graphene.

Formation and development of dislocation in graphene

The formation and development processes of dislocation in graphene are investigated by performing tight-binding molecular dynamics (TBMD) simulation and ab initio total energy calculation. It is found that the coalescence of pentagon-heptagon (5-7) pairs with vacancy defects induces the formation of dislocation due to the separation of two 5-7 pairs. In TBMD simulations, adatoms are ejected and evaporated from graphene surface so that the dislocation is developed. It is observed that diffusing carbon atoms nearby dangling bonds help non-hexagonal rings change into stable hexagonal rings. These results might give some ideas for the control of structural properties by inducing defect structures.

Non-hexagonal-ring defects and structures induced by healing and strain in graphene and functionalized graphene

We perform ab initio calculations for the strain-induced formation of non-hexagonal-ring defects in graphene, graphane (hydrogen-functionalized graphene) and graphenol (hydroxyl-functionalized graphene). We find that the simplest of such topological defects, the Stone–Wales defect, acts as a seed for strain-induced dissociation and multiplication of topological defects. Through the application of inhomogeneous deformations to graphene, graphane and graphenol with varying initial concentrations of pentagonal and heptagonal rings and small-sized voids, we obtain several novel stable structures that possess, at the same time, large concentrations of non-hexagonal rings (from fourfold to elevenfold) and small formation energies.

Nanostructuration of carbonaceous dust as seen through the positions of the 6.2 and 7.7μm AIBs

Context. Carbonaceous cosmic dust is observed through infrared spectroscopy either in absorption or in emission. The details of the spectral features are believed to shed some light on its structure and finally enable the study of its life cycle. Aims. The goal is to combine several analytical tools in order to decipher the intimate nanostructure of some soot samples. Such materials provide interesting laboratory analogues of cosmic dust. In particular, spectroscopic and structural characteristics that help to describe the polyaromatic units embedded into the soot, including their size, morphology, and organisation are explored. Methods. Laboratory analogues of the carbonaceous interstellar and circumstellar dust were produced in fuel-rich low-pressure, premixed and flat flames. The soot particles were investigated by infrared absorption spectroscopy in the 2−15 μm spectral region. Raman spectroscopic measurements and high-resolution transmission electron microscopy were performed, which offered complementary information to better delineate the intimate structure of the analogues. Results. These laboratory analogues appeared to be mainly composed of sp 2 carbon, with a low sp 3 carbon content. A cross relation between the positions of the aromatic C=C bands at about 6.2 micron and the band at about 8 micron is shown to trace differences in shapes and structures of the polyaromatic units in the soot. Such effects are due to the defects of the polyaromatic structures in the form of non-hexagonal rings and/or aliphatic bridges. The role of these defects is thus observed through the 6.2 and 7.7 μm aromatic infrared band positions, and a distinction between carriers composed of curved aromatic sheets and more planar ones can be inferred. Based on these nanostructural differences, a scenario of nanograin growth and evolution is proposed.

Graphene-like structure of activated anthracites

The structure of a series of activated carbons prepared from anthracite by chemical activation has been studied using wide-angle x-ray scattering, molecular dynamics and Raman spectroscopy. The BET surface areas of the investigated samples are in the range 1500–3430 m2 g−1 and the average pore sizes vary from 0.75 to 1.35 nm. The diffraction measurements were carried out to a maximum value of the scattering vector Kmax = 22 Å−1. The obtained diffraction data have been converted to a real space representation in the form of the pair correlation function. The structure of the studied samples consists of one or two graphite-like layers, stacked without spatial correlations. The size of the ordered layer region is approximately 24 Å. The atomic arrangement within an individual layer has been described in terms of paracrystalline ordering, in which lattice distortions are propagated proportionally to the square root of inter-atomic distances. The paracrystalline structure has been simulated by introducing the Stone–Thrower–Wales, mono-vacancy and di-vacancy defects, randomly distributed in the network. These defects lead to the formation of a defected network with the presence of non-hexagonal rings in which distortion of the structure extends outside of a defect region. Computer generated structural models have been relaxed at room temperature using the reactive empirical bond order potential for intra-layer interactions and the Lennard-Jones potential for inter-layer interactions. For such generated models the structure factors and the pair correlation functions were computed. A good agreement between the simulation results and the experimental data in both reciprocal and real space provides evidence for the correctness of the proposed models. The Raman data support the validity of these models. Porosity of the activated anthracites is discussed in relation to their defective structure.

Theory and hierarchical calculations of the structure and energetics of [0001] tilt grain boundaries in graphene

Several experiments have revealed the presence of grain boundaries in graphene that may change its electronic and elastic properties. Here, we present a general theory for the structure of [0001] tilt grain boundaries in graphene based on the coincidence site lattice (CSL) theory. We show that the CSL theory uniquely classifies the grain boundaries in terms of the misorientation angle $\ensuremath{\theta}$ and periodicity $d$ using two grain-boundary indices $(m,n)$, similar to the nanotube indices. The structure and formation energy of a large set of grain boundaries generated by the CSL theory for ${0}^{\ensuremath{\circ}}<\ensuremath{\theta}<{60}^{\ensuremath{\circ}}$ (up to 15 608 atoms) were optimized by a hierarchical methodology and validated by density functional calculations. We find that low-energy grain boundaries in graphene can be identified as dislocation arrays. The dislocations form hillocks like those observed by scanning tunneling microscopy in graphene grown on Ir(111) for small $\ensuremath{\theta}$ that flatten out at larger misorientation angles. We find that, in contrast to three-dimensional materials, the strain created by the grain boundary can be released via out-of-plane distortions that lead to an effective attractive interaction between dislocation cores. Therefore, the dependence on $\ensuremath{\theta}$ of the formation energy parallels that of the out-of-plane distortions, with a secondary minimum at $\ensuremath{\theta}=32.{2}^{\ensuremath{\circ}}$ where the grain boundary is made of a flat zigzag array of only 5 and 7 rings. For $\ensuremath{\theta}>32.{2}^{\ensuremath{\circ}}$, other nonhexagonal rings are also possible. We discuss the importance of these findings for the interpretation of recent experimental results.

The role of pentagon–heptagon pair defect in carbon nanotube: The center of vacancy reconstruction

We show that pentagon–heptagon (5–7) pair defects in carbon nanotube play an important role as the center of vacancy reconstruction using tight-binding molecular dynamics simulations and ab initio total energy calculations. Single vacancy defect diffuses toward and coalesces with 5–7 pair defects and the coalescence structure is reconstructed into a new and more stable 5–7 pair defect plus an adatom by an exchange mechanism. In the case of four single vacancy defects, the vacancy defects coalesce with 5–7 pair defects and form defect structures with nonhexagonal rings. Finally, these defective structures reconstruct into two new 5–7 pair defects.

Defective BN Nanotubes: A Density Functional Theory Study of B-11 and N-14 NQR Parameters

Abstract A density functional theory (DFT) study is performed to investigate the influence of structural defects on the electronic structure properties of perfect boron nitride nanotubes (BNNTs). To this aim, as representative models, the single-walled (6,0) BNNT consisting of 36 boron, 36 nitrogen, and 12 hydrogen atoms and the single-walled (4,4) BNNT consisting of 36 boron, 36 nitrogen, and 16 hydrogen atoms are considered. The nuclear quadrupole resonance (NQR) parameters are calculated and compared in two perfect and defective models of the considered BNNTs. The results indicate that due to formation of non-hexagonal rings in the defective model because of removing a B-N bond, the NQR parameters at the sites of first neighbouring nuclei are significantly influenced by imposed perturbation, however, the sites of other nuclei, farther from perturbation, remain almost unchanged. The calculations are performed at the level of the BLYP method and 6-31G* standard basis set using the GAUSSIAN 98 package

Electron transport properties of ordered networks using carbon nanotubes

The electronic transport properties of ordered networks using carbonnanotubes as building blocks (ON-CNTs) are investigated within theframework of a multiterminal Landauer–Buttiker formalism using ans,px,py,pz parameterization of the tight-binding Hamiltonian for carbon. The networks exhibitelectron pathway selectiveness, which is shown to depend on the atomic structure of thenetwork nodes imposed by the specific architecture of the network and the distribution ofits defects (non-hexagonal rings). This work represents the first understandings towardsleading current through well-defined trajectories along an organic nanocircuit.

A mimetic porous carbon model by quench molecular dynamics simulation

A mimetic porous carbon model is generated using quench molecular dynamics simulations that reproduces experimental radial distribution functions of activated carbon. The resulting structure is composed of curved and defected graphene sheets. The curvature is induced by nonhexagonal rings. The quench conditions are systematically varied and the final porous structure is scrutinized in terms of its pore size distribution, pore connectivity, and fractal dimension. It is found that the initial carbon density affects the fractal dimension but only causes a minor shift in the pore size distribution. On the other hand, the quench rate affects the pore size distribution but only causes a minor shift in the fractal dimension.

Self-healing in defective carbon nanotubes: a molecular dynamics study

The self-healing phenomenon of defective single-walled carbon nanotubes (SWCNTs) isobserved at the atomic level from a molecular dynamics (MD) simulation test. The idealnetwork of carbon nanotubes is unable to avoid damage under destabilizing loads at hightemperature, leading to unforeseen patterns in bond breakages and vacancy defects on thewall. We observe that (10, 10) and (17, 0) carbon nanotubes containing such vacancies areenergetically unstable. In the situation of unloading or increasing temperature, the localstructures around the vacancies reconstruct through dangling bond saturation,forming non-hexagonal rings, 5–7–7–5 defects or an ideal graphite network. We findthat a defective carbon nanotube with large vacancies is re-mendable, and theStone–Wales (SW) construction is energetically preferred in self-healing processes.

STM imaging of carbon nanotube point defects

In this work the STM investigation of nanotube point defects created by ion irradiation (e.g. vacancies) is presented. The defects appeared as hillock-like features on the STM images. These defects were compared with similar features observed on the STM images of as-grown coiled carbon nanotubes. In this case the observed hillock-like features were attributed to the non-hexagonal carbon rings, which are responsible for the growth of bent and coiled nanotube structures. For irradiation, multi-walled carbon nanotubes produced by the arc-discharge method were dispersed on highly oriented pyrolitic graphite (HOPG) surface and irradiated with Ar + ions of 30 keV, using a low dose ofD = 5 x 10 11 ions/cm 2 .

Imaging active topological defects in carbon nanotubes

A single-walled carbon nanotube (SWNT) is a wrapped single graphene layer, and its plastic deformation should require active topological defects--non-hexagonal carbon rings that can migrate along the nanotube wall. Although in situ transmission electron microscopy (TEM) has been used to examine the deformation of SWNTs, these studies deal only with diameter changes and no atomistic mechanism has been elucidated experimentally. Theory predicts that some topological defects can form through the Stone-Wales transformation in SWNTs under tension at 2,000 K, and could act as a dislocation core. We demonstrate here, by means of high-resolution (HR)-TEM with atomic sensitivity, the first direct imaging of pentagon-heptagon pair defects found in an SWNT that was heated at 2,273 K. Moreover, our in situ HR-TEM observation reveals an accumulation of topological defects near the kink of a deformed nanotube. This result suggests that dislocation motions or active topological defects are indeed responsible for the plastic deformation of SWNTs.

Nanoporous carbon membranes for separation of nitrogen and oxygen: Insight from molecular simulations

Here we summarize some of our recent work on using molecular simulations to understand the key mechanism that result in large differences in the reported permeation rates of O 2 and N 2 in nanoporous carbon membranes. Two different representation of the amorphous nanoporous membrane structure are used; the hypothetical C 168 Schwarzite and a single wall carbon nanotube with a constriction. By comparing the results obtained from empirical planar graphite potential and an ab initio-based potential, the effect of carbon curvature and the presence of non-hexagonal carbon rings in C 168 Schwarzite is also investigated. It is found using either force field, that the energetic effect alone cannot explain the experimental observations. However, simulations performed using carbon nanotube with a constriction show that the size or entropic effect can be dominant. In particular, it is shown that an appropriate size constriction can result in large transport resistance to nitrogen while letting oxygen to pass through at a much higher rate, even though these gases have very similar molecular sizes and interaction energetics.

Magnetic properties of carbon nanostructures

We investigate the magnetic response and the ring currents induced by the presence of an external magnetic field in different carbon nanostructures using a π-electron tight binding model in conjunction with the London approximation. We consider fullerenes, corrugated and non-corrugated carbon nanotori, and finite graphene sheets. For corrugated carbon nanotori, we have constructed the structures in two different ways: by joining the ends of carbon nanotubes containing pentagonal, hexagonal and heptagonal carbon rings (rectangular and hexagonal Haeckelites); and by coalescing C60 molecules along the three different axes of symmetry (C2, C3 and C5). The non-corrugated carbon nanotori have been constructed by joining the ends of standard carbon nanotubes containing hexagonal rings only. For the graphene ribbons, we considered those exhibiting armchair or zig-zag edges. Our results reveal strong paramagnetic signals in structures containing non-hexagonal carbon rings. For carbon nanotori, the presence of strong paramagnetism causes the electrons to flow around the torus. We also observed that the ring currents of graphene ribbons are small and localised at the edges. Finally, the magnetic moment of nanotori constructed from Haeckelite structures is also described as a function of doping (adding additional electrons or holes).

Necklace-like Hollow Carbon Nanospheres from the Pentagon-Including Reactants: Synthesis and Electrochemical Properties

Necklace-like hollow carbon nanospheres (CNSs) have been successfully synthesized from the pentagon-including reactants, which provide an auxiliary example for the theoretical prediction that necklace-like hollow CNSs are assumed to be composed of the regular occurrence of nonhexagonal rings at the atomic level. Benefits of the as-obtained hollow CNSs also arise from the high Brunauer-Emmett-Teller value of 594.32 m(2)/g and a narrow pore distribution at 5 nm. The electrochemical hydrogen storage experiments for the as-obtained necklace-like hollow CNSs exhibit a capacity of 242 mAh/g at the current density of 200 mA/g, corresponding to a hydrogen storage of 0.89 wt %, which is higher than the previously reported electrochemical capacities for the multiwalled carbon nanotubes (MWCNTs). Furthermore, the as-obtained necklace-like hollow CNSs show a lithium capacity advantage compared with the carbon solid particles for application in lithium batteries. These results indicate that the necklace-like hollow CNSs provide a new candidate for the application in hydrogen storage and lithium batteries.

Irradiation of carbon nanotubes with a focused electron beam in the electron microscope

The paper reviews in-situ electron irradiation studies of carbon nanotubes in electron microscopes. It is shown that electron irradiation at high specimen temperature can lead to a variety of structural modifications and new morphologies of nanotubes. Radiation defects such as vacancies and interstitials are created under irradiation, but the cylindrically closed graphene layers reconstruct locally and remain coherent. The generation of curvature in graphene layers with non-hexagonal rings allows us to alter the topology of nanotubes. Several examples of irradiation-induced modifications of single- and multi-wall nanotubes are shown. Conclusions about the mobility of interstitials and vacancies are drawn which are important to explain the behaviour and the properties of nanotubes with an atomic arrangement deviating from the hexagonal network of graphene.

Magnetism in Fe-based and carbon nanostructures: Theory and applications

We demonstrate that it is possible to encapsulate ferromagnetic nanowires of Fe, FeCo and FeNi inside carbon nanotubes via chemical vapor deposition methods. These wires exhibit extremely high coercive fields when compared with the bulk phases. We review the state-of-the art characterization carried out on these novel wires and discuss the importance of having aligned arrays of carbon nanotubes filled with ferromagnetic materials, towards the development of novel magnetic storage devices. In this context, we will show from the experimental and theoretical stand points, that the wire shape, aspect ratio and inter-wire distances play a crucial role in the fabrication of novel storage components. In addition, we theoretically show that pure carbon nanostructures such as carbon nanotori, perforated fullerenes and nanoporous graphitic structures, exhibiting negative Gaussian curvature introduced by the presence of non-hexagonal rings, behave as strong paramagnets experiencing large magnetic moments when an external magnetic field is applied. The latter results could explain some of the magnetic properties observed experimentally in carbon nanofoams and polymerized C 60 phases. We envisage that magnetism in different families of nanostructures will be playing a key role in the development of emerging technologies in the present century.

Energy relaxation and pulsed neutrons diffraction studies of carbon nanotubes

Carbon nanotubes containing non-hexagonal rings have been computer generated and relaxed by minimization of the energy using the conjugate–gradient method. Then the powder diffraction patterns have been computed and converted to a real-space representation by the Fourier transform, yielding pair correlation functions which are compared with the pulsed neutrons experimental data for the single-wall carbon nanotubes produced by catalytic chemical vapour deposition. Effects of defective graphene cylinders on the interatomic distances and the resulting structures are discussed. The obtained results are also discussed in relation to previously reported paracrystalline behaviour of the nearest-neighbours distance fluctuations.