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Folded regions are commonly encountered in a number of hexagonal boron nitride (h-BN) based bulk and nanostructured materials. Two types of structural modifications occur in folded h-BN layers: local curvature at the folded edges and interlayer shear of the layers which changes the stacking of the overlapping flat regions. In this work we discuss, via density functional theory simulations, the impact of these structural modifications on the ground state electronic structure of the pristine monolayer. We show that, depending on the fold orientation, the overlapping region might present different stacking configurations with subsequent variations of the fundamental band gap; further gap changes occur at the folded regions. The overall electronic structure of a BN folded monolayer can finally be described as a type-II junction between two wide gap semiconductors located at the curved and flat overlapping zones.

Dislocations are a central concept in materials science, which dictate the plastic deformation and damage evolution in materials. Layered materials such as graphite admit two general types of interlayer dislocations: basal and prismatic dislocations, of which prismatic dislocations have been relatively less studied. Using density functional theory (DFT) calculations, we have examined different prismatic core structures in graphite and evaluated their structure, energetics and mobility. We find close energetic interplay between bonded and "free-standing" core structures in both zigzag and armchair directions, with a reconstructed stable zigzag core identified. We explore grain boundaries and prismatic dislocation pile-up, identifying metastable structures which may be important in energy storage. The role of interlayer stacking in core structure, dislocation glide and climb is also considered in-depth. Our calculations suggest that the prismatic dislocation core is stable up to high temperatures of approximately 1500K in bulk graphite. Above this temperature, the breaking of bonds in the dislocation core can facilitate climb, grain-boundary motion, and the annealing of damage through prismatic dislocation glide., Comment: 46 pages, 29 figures

Aqueous batteries, particularly those integrating organic active materials functioning in a neutral pH environment, stand out as highly promising contenders in the stationary electrochemical storage domain, owing to their unparalleled safety, sustainability and low-cost materials. Herein, a novel di-block oligomer (DNVBr), serving as the negative electrode of an all-organic aqueous battery, is shown to offer exceptional output capabilities. The battery's performance is further enhanced by a unique intermixed p/n-type storage mechanism, which is able to simultaneously exchange light and naturally abundant Na+, Mg2+ and Cl-. Reaching up to 105 mAh/g, this system shows remarkable capacity retention for several thousand cycles (6500 cycles, ~40 days) in various neutral electrolytes, including raw ocean water (~3000 cycles, ~75 days). The surprisingly fast kinetics of this di-block oligomer allow to attain an unmatched specific capacity of near to 60mAh/g electrode while entirely devoid of conducting additives, and more than 80mAh/g electrode with 10% carbon additive, as well as displaying an areal capacity as high as 3.4mAh/cm2 at C rate. Full cell validation was demonstrated over 1600 cycles by virtue of a commercial TEMPO molecule, which permitted an energy density of close to 40Wh/kgmaterials at C rate in a self-pH-buffered and inexpensive aqueous electrolyte., Comment: This is the pre-print (pre-submission) version, for final submitted version please visit DOI link

Herein, we report a improved method for the physical grafting of 1-nitropyrene (Pyr-NO2) onto highly graphitized carbon onion. This is achieved through a lowering of the onset potential of the pyrene polymerization via in situ reduction of the NO2 group. The additional redox activity pertaining to the reduced NO2 enables exceeding the faradaic capacity which is associated with the p-doping of the grafted pyrene backbone, as observed for pyrene, 1-aminopyrene, and unreduced Pyr- NO2. Theoretical calculations demonstrate the charge transfer and binding enthalpy capabilities of Pyr-NO2, which are significantly higher than those of the other two species, and which allow for improved p-stacking on the carbon surface. Upon 20 wt % grafting of Pyr-NO2, the capacity of the electrode jumps from 20 mAh g-1 electrode to 38 mAh g- electrode, which corresponds to 110 mAh g-1 per mass of Pyr-NO2 and the average potential is increased by 200 mV. Very interestingly, this high performance is also coupled with outstanding retention with respect to both the initial capacity for more than 4000 cycles, as well as the power characteristics, demonstrating the considerable advantages of employing the present in situ grafting technique.

The Electron Energy-Loss Spectroscopy (EELS) and X-ray Absorption Spectroscopy (XAS) database has been completely rewritten, with an improved design, user interface and a number of new tools. The database is accessible at https://eelsdb.eu/ and can now be used without registration. The submission process has been streamlined to encourage spectrum submissions and the new design gives greater emphasis on contributors original work by highlighting their papers. With numerous new filters and a powerful search function, it is now simple to explore the database of several hundred of EELS and XAS spectra. Interactive plots allow spectra to be overlaid, facilitating online comparison. An application-programming interface has been created, allowing external tools and software to easily access the information held within the database. In addition to the database itself, users can post and manage job adverts and read the latest news and events regarding the EELS and XAS communities. In accordance with the ongoing drive towards open access data increasingly demanded by funding bodies, the database will facilitate open access data sharing of EELS and XAS spectra., Comment: In Press in Microscopy and Microanalysis 2016

Endohedral metallofullerenes have been extensively studied, since the first experimental observation of La@C60 in a laser-vaporized supersonic beam in 1985. However, all of these studies have been carried out on metallofullerenes larger than C60 such as (metal)@C82, and there are no reported purified C60-based metallofullerenes except for [Li@C60]+(SbCl6)- salt. Pure (metal)@C60 has not been obtained because of their extremely high chemical reactivity. We report here the first isolation, structural determination and electromagnetic properties of crystalline C60-based metallofullerenes, Gd@C60(CF3)5 and La@C60(CF3)5. Synchrotron X-ray single-crystal diffraction reveals that La and Gd atoms are indeed encapsulated in the Ih-C60 fullerene. The HOMO-LUMO gaps of Gd@C60 and La@C60 are significantly widened by an order of magnitude with addition of CF3- groups. Magnetic measurements show the presense of a weak antiferromagnetic coupling in Gd@C60(CF3)3 crystals at low temperatures. Gd@C60 and La@C60 might exhibit superconductivity as the electronic structures resemble those of superconducting alkali-doped C60 fullerides., Comment: 15 pages, 4 figures

Having access to the chemical environment at the atomic level of a dopant in a nanostructure is crucial for the understanding of its properties. We have performed atomically-resolved electron energy-loss spectroscopy to detect individual nitrogen dopants in single-walled carbon nanotubes and compared with first principles calculations. We demonstrate that nitrogen doping occurs as single atoms in different bonding configurations: graphitic-like and pyrrolic-like substitutional nitrogen neighbouring local lattice distortion such as Stone-Thrower-Wales defects. The stability under the electron beam of these nanotubes has been studied in two extreme cases of nitrogen incorporation content and configuration. These findings provide key information for the applications of these nanostructures., Comment: 25 pages, 13 figures

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 <2-1-10> 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., Comment: Supplementary Materials pdf available in the source

Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS2) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown.

Conventional three-dimensional crystal lattices are terminated by surfaces, which can demonstrate complex rebonding and rehybridisation, localised strain and dislocation formation. Two dimensional crystal lattices, of which graphene is the archetype, are terminated by lines. The additional available dimension at such interfaces opens up a range of new topological interface possibilities. We show that graphene sheet edges can adopt a range of topological distortions depending on their nature. Rehybridisation, local bond reordering, chemical functionalisation with bulky, charged, or multi-functional groups can lead to edge buckling to relieve strain, folding, rolling and even tube formation. We discuss the topological possibilities at a 2D graphene edge, and under what circumstances we expect different edge topologies to occur. Density functional calculations are used to explore in more depth different graphene edge types., Comment: Additional figure in published version

The density functional tight binding approach (DFTB) is well adapted for the study of point and line defects in graphene based systems. After briefly reviewing the use of DFTB in this area, we present a comparative study of defect structures, energies and dynamics between DFTB results obtained using the dftb+ code, and density functional results using the localised Gaussian orbital code, AIMPRO. DFTB accurately reproduces structures and energies for a range of point defect structures such as vacancies and Stone-Wales defects in graphene, as well as various unfunctionalised and hydroxylated graphene sheet edges. Migration barriers for the vacancy and Stone-Wales defect formation barriers are accurately reproduced using a nudged elastic band approach. Finally we explore the potential for dynamic defect simulations using DFTB, taking as an example electron irradiation damage in graphene.

It is now possible to produce graphene nanoribbons (GNRs) with atomically defined widths. GNRs offer many opportunities for electronic devices and composites, if it is possible to establish the link between edge structure and functionalisation, and resultant GNR properties. Switching hydrogen edge termination to larger more complex functional groups such as hydroxyls or thiols induces strain at the ribbon edge. However we show that this strain is then relieved via the formation of static out-of-plane ripples. The resultant ribbons have a significantly reduced Young's Modulus which varies as a function of ribbon width, modified band gaps, as well as heterogeneous chemical reactivity along the edge. Rather than being the exception, such static edge ripples are likely on the majority of functionalized graphene ribbon edges., Comment: Supplementary Materials available

We demonstrate that free graphene sheet edges can curl back on themselves,reconstructing as nanotubes. This results in lower formation energies than any other non-functionalised edge structure reported to date in the literature. We determine the critical tube size and formation barrier and compare with density functional simulations of other edge terminations including a new reconstructed Klein edge. Simulated high resolution electron microscopy images show why such rolled edges may be difficult to detect. Rolled zigzag edges serve as metallic conduction channels, separated from the neighbouring bulk graphene by a chain of insulating sp$^3$-carbon atoms, and introduce Van Hove singularities into the graphene density of states., Comment: To appear in Phys. Rev. Lett

We present a density functional theory study of low density bromination of graphene and graphite, finding significantly different behaviour in these two materials. On graphene we find a new Br2 form where the molecule sits perpendicular to the graphene sheet with an extremely strong molecular dipole. The resultant Br+-Br- has an empty pz-orbital located in the graphene electronic pi-cloud. Bromination opens a small (86meV) band gap and strongly dopes the graphene. In contrast, in graphite we find Br2 is most stable parallel to the carbon layers with a slightly weaker associated charge transfer and no molecular dipole. We identify a minimum stable Br2 concentration in graphite, finding low density bromination to be endothermic. Graphene may be a useful substrate for stabilising normally unstable transient molecular states.

We examine the behaviour of hydrogen ions, atoms and molecules in alpha-boron using density functional calculations. Hydrogen behaves as a negative-U centre, with positive H ions preferring to sit off-center on inter-layer bonds and negative H ions sitting preferably at in-plane sites between three B12 icosahedra. Hydrogen atoms inside B12 icosahedral cages are unstable, drifting off-center and leaving the cage with only a 0.09 eV barrier. While H0 is extremely mobile (diffusion barrier 0.25 eV), H+ and H- have higher diffusion barriers of 0.9 eV. Once mobile these defects will combine, forming H2 in the interstitial void space, which will remain trapped in the lattice until high temperatures. Based on these results we discuss potential differences for hydrogen behaviour in beta-boron, and compare with experimental muon-implantation data.

We study the interface between carbon nanotubes (CNTs) and surface-deposited titanium using electron microscopy and photoemission spectroscopy, supported by density functional calculations. Charge transfer from the Ti atoms to the nanotube and carbide formation is observed at the interface which indicates strong interaction. Nevertheless, the presence of oxygen between the Ti and the CNTs significantly weakens the Ti?CNT interaction. Ti atoms at the surface will preferentially bond to oxygenated sites. Potential sources of oxygen impurities are examined, namely oxygen from any residual atmosphere and pre-existing oxygen impurities on the nanotube surface, which we enhance through oxygen plasma surface pretreatment. Variation in literature data concerning Ohmic contacts between Ti and carbon nanotubes is explained via sample pre-treatment and differing vacuum levels, and we suggest improved treatment routes for reliable Schottky barrier-free Ti?nanotube contact formation.

We show that electron irradiation in a dedicated scanning transmission microscope can be used as a nano-electron-lithography technique allowing the controlled reshaping of single walled carbon and boron nitride nanotubes. The required irradiation conditions have been optimised on the basis of total knock-on cross sections calculated within density functional based methods. It is then possible to induce morphological modifications, such as a change of the tube chirality, by removing severals tens of atoms with a nanometrical spatial resolution. These irradiation techniques could open new opportunities for nano-engeneering a large variety of nanostructured materials., Comment: article + supporting materials

We present a new and general mechanism for inter-conversion of carbon structures via a catalytic exchange process, which operates under conditions of Frenkel pair generation. The mechanism typically lowers reaction barriers by a factor of four compared to equivilent uncatalysed reactions. We examine the relevance of this mechanism for fullerene growth, carbon onions and nanotubes, and dislocations in irradiated graphite., Comment: 3 Figures, 5 Page letter accepted for publication in Chemical Physics Letters