The role of heteroatoms during graphitisation: first principle calculations

Graphite is widely used in modern industry, particularly in nuclear power generation in the UK. Understanding its formation is important for economical and safety reasons. The process to turn carbon materials into graphite by heat treatment is called the graphitisation process. It is the transformation of amorphous carbon, through a 2D turbostratic carbon intermediate, into 3D ordered layers of graphite. While many manufacturing processes have been established and many authors have contributed to understanding the important stages of graphitisation, the chemistry involved is not fully understood. It appears that impurities found in precursors can have a direct impact on the final graphite obtained. The following work is an investigation of the role played by these heteroatoms during the graphitisation process. Using density functional theory (DFT), calculations on possible mechanisms involved in the graphitisation process are investigated. However, the initial stages contain complex and poorly defined chemistry, so we have chosen to avoid this area, even though factors such as the C:H:O ratios are clearly important. Instead, this work is focussed on the latter stages of graphitisation in order to better understand the ordering processes to obtain graphite (and their inverse disordering, insofar as it is relevant to radiation damage). In this way it is still possible to invoke standard concepts in the physics and chemistry of defects in crystals. If there is too much disorder, and the system is close to amorphous in nature, complexity would overwhelm the project. The descriptions of an amorphous material with a little extra order would be much more difficult than the descriptions of a crystal with some disorder. For this reason, we have focussed on the heteroatoms which endure until the later stages of graphitisation, boron and sulphur, and also on turbostratic graphite, where calculations of interlayer separation as a function of relative rotation of a layer and of its neighbours are described. We find for sulphur that it can open up folds in graphite, forming very stable sulphur decorated edges. In dislocation terms, this could be the beginning of the dissociation of a perfect prismatic edge dislocation. An edge dislocation is described as an added half plane. If the plane is a bilayer graphene terminating in a fold, the dislocation is perfect. If the plane is a single graphene the dislocation is ‘partial’. Importantly two partial dislocations have lower elastic energy than the perfect, so dissociation is important in stabilising the structure. For boron, we show how it can pin twist boundaries, preventing slip and suggest that radiation damage can achieve the same effect through vacancies. The mechanism does not appear to involve cross-linking bonds and provides a good explanation for the variations in C44 between different graphites and different methods of measurement. Furthermore, we show that B can aid in the removal of twist boundaries by pushing up their formation energy with respect to AB graphite

Stable hydrogenated graphene edge types: Normal and reconstructed Klein edges

Hydrogenated graphene edges are assumed to be either armchair, zigzag or a combination of the two. We show that the zigzag is not the most stable fully hydrogenated structure along the direction. Instead hydrogenated Klein and reconstructed Klein based edges are found to be energetically more favourable, with stabilities approaching that of armchair edges. These new structures "unify" graphene edge topology, the most stable flat hydrogenated graphene edges always consisting of pairwise bonded C2H4 edge groups, irrespective the edge orientation. When edge rippling is included, CH3 edge groups are most stable. These new fundamental hydrogen terminated edges have important implications for graphene edge imaging and spectroscopy, as well as mechanisms for graphene growth, nanotube cutting, and nanoribbon formation and behaviour.Fundação para a Ciência e a Tecnologia (FCT

Ab initio calculations of hybrid carbon-silicon nanostructures for new high performance lithium ion batteries

Graphite has been used as an anode in lithium ion batteries for a few decades now due to its coulombic efficiency and high charge/discharge cycling performance. However, with an increase in energy demand in newer electronic devices or future electric cars, and added to the low theoretical specific capacity of 372 mAh/g in graphite (one lithium atom for six carbon atom via intercalation), new kind of anodes has to be found. To satisfy this request, a lot of effort has already been made, from transition metal oxides to mesoporous carbon as candidates for anode materials. In recent years, research has been focused on silicon due to a theoretical specific capacity more than ten times higher than of graphite (4212 mAh/g, Li4.4Si). [5] Unfortunately the large volume change (> 300%) during lithiation and a poor cycle performance make bulk silicon unusable as a conventional anode material. Here, we present ab initio simulations for new nanostructures , as possible anode materials. We first investigated the structural and electronic properties of such structures. The electrical conductances of these hybrid nanostructures are also predicted for both pristine case and under lithiation

Ab initio infrared vibrational modes for neutral and charged small fullerenes (C20, C24, C26, C28, C30 and C60)

We calculate the infrared (IR) absorption spectra using DFT B3LYP(6–311G) for a range of small closed-cage fullerenes, Cn, n=20, 24, 26, 28, 30 and 60, in both neutral and multiple positive and negative charge states. The results are of use, notably, for direct comparison with observed IR absorption in the interstellar medium. Frequencies fall typically into two ranges, with C−C stretch modes around 1100–1500 cm−1 (6.7–9.1 μm) and fullerene-specific radial motion associated with under-coordinated carbon at pentagonal sites in the range 600–800 cm−1 (12.5–16.7 μm). Notably, negatively charged fullerenes show significantly stronger absorption intensities than neutral species. The results suggest that small cage fullerenes, and notably metallic endofullerenes, may be responsible for many of the unassigned interstellar IR spectral lines

Electrical transport through atomic carbon chains: The role of contacts

Chains of carbon atoms in the sp1 hybridization are the one-dimensional analog of graphene. The first experimental studies of electrical transport in atomic carbon chains have shown a much lower conductivity than the quantum conductance limit. Here we explain, experimentally and by ab-initio transport modeling, the limited conductivity by studying the influence of carbon contacts in different hybridization states on the electrical properties of carbon chains. In-situ measurements in an electron microscope allow the synthesis and electrical characterization of carbon chains. Current-voltage curves of carbon chains, spanning between carbon contacts with sp2-or sp3-hybridized contact atoms, are measured and calculated. Contact atoms in the sp2-hybridization allow up to two orders of magnitude higher current than through sp3 contacts. Another important factor is the electron distribution in the chain which is determined by an even or odd number of atoms. On the other hand, it is shown that the overall length of the chain and strain have only minor influence on the conductivity. A current carrying capacity of up to 6.5 mA at an applied voltage of 1.5 V is measured

Ab initio infrared vibrational modes for neutral and charged small fullerenes (C 20 , C 24 , C 26 , C 28 , C 30 and C 60 )

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DFT study of the chemistry of sulfur in graphite, including interactions with defects, edges and folds

Sulfur has several roles, desirable and undesirable, in graphitization. We perform density functional theory calculations within the local density approximation to define the structures and energetics of sulphur in graphite, including its interactions with point defects and edges, in order to understand its role in the later stages of graphitization. We find sulphur does not cross-link layers, except where there are defects. It reacts very strongly with vacancies in neighbouring layers to form a six coordinate split vacancy structure, analogous to that found in diamond. It is also highly stable at basal edge sites, where, as might be expected, the size and valency of sulfur can be easily accommodated. This suggests a role for sulphur in stabilizing graphene edges, and following from this, we show that sulfur dimers can open, i.e. unzip, folds in graphite rapidly and exothermically

Dirac Cones in two-dimensional conjugated polymer networks

Linear electronic band dispersion and the associated Dirac physics has to date been limited to special-case materials, notably graphene and the surfaces of three-dimensional (3D) topological insulators. Here we report that it is possible to create two-dimensional fully conjugated polymer networks with corresponding conical valence and conduction bands and linear energy dispersion at the Fermi level. This is possible for a wide range of polymer types and connectors, resulting in a versatile new family of experimentally realisable materials with unique tuneable electronic properties. We demonstrate their stability on substrates and possibilities for doping and Dirac cone distortion. Notably, the cones can be maintained in 3D-layered crystals. Resembling covalent organic frameworks, these materials represent a potentially exciting new field combining the unique Dirac physics of graphene with the structural flexibility and design opportunities of organic-conjugated polymer chemistry

The electronic and transport properties of two-dimensional conjugated polymer networks including disorder

Two-dimensional (2D) conjugated polymers exhibit electronic structures analogous to that of graphene with the peculiarity of π–π* bands which are fully symmetric and isolated. In the present letter, the suitability of these materials for electronic applications is analyzed and discussed. In particular, realistic 2D conjugated polymer networks with a structural disorder such as monomer vacancies are investigated. Indeed, during bottom-up synthesis, these irregularities are unavoidable and their impact on the electronic properties is investigated using both ab initio and tight-binding techniques. The tight-binding model is combined with a real space Kubo–Greenwood approach for the prediction of transport characteristics for monomer vacancy concentrations ranging from 0.5% to 2%. As expected, long mean free paths and high mobilities are predicted for low defect densities. At low temperatures and for high defect densities, strong localization phenomena originating from quantum interferences of multiple scattering paths are observed in the close vicinity of the Dirac energy region while the absence of localization effects is predicted away from this region suggesting a sharp mobility transition. These predictions show that 2D conjugated polymer networks are good candidates to pave the way for the ultimate scaling and performances of future molecular nanoelectronic devices

Adhesionless and near-ideal contact behavior of graphene on Cu thin film

Graphene coatings reduce surface adhesion owing to a low surface energy. In the present work, a single CVD-grown graphene layer on Cu is shown to modify the elastic contact behavior by eliminating adhesion. Nanoindentation load-displacement curves exhibit higher load bearing capacity for Cu/graphene in the elastic regime compared to bare Cu and a closer agreement with Hertz law. Molecular dynamics simulations confirm the quasi-absence of adhesion between graphene and indentor tip. These results open new opportunities regarding tribological issues related to coatings or MEMS applications