[en] The standard model for interpretation of radiation damage of graphite invokes self-interstitials and vacancies and their aggregation to explain observed dimensional changes. Vacancies aggregate into lines which heal and contract the basal planes, interstitials aggregate into interlayer disks which expand the dimension perpendicular to the layers. Small clusters of interstitials (C_n, n = 4±2) appear which expand the interlayer distance, d₀₀₂, but for an unknown reason can disappear and not evolve into disks. First principles calculations show that these aggregations are improbable at low temperatures (below 250^oC) and that the observed annealing at these temperatures would be impossible. Our calculations show the inadequacies and motivate a new, robust and atomistically based model. At low temperatures, point defects are immobile, but form sturdy links between planes which cause buckling. Since the planes are effectively 'flexible but inextensible', the crystal expands perpendicular and contracts parallel to the basal planes. At higher temperatures, radiation damage causes collisions with planes, which then fold. At all stages, point defects do form, but their aggregation effects are minority effects at all but the highest temperatures. This new model has major implications for the interpretation of Wigner energy. (author)

[en] Full text of publication follows. The story behind radiation damage in graphite is framed in terms of interstitials and vacancies. First principles calculations all show that interstitials are immobile at low (e.g. liquid N₂) temperatures. We have performed the first serious and first principles calculations on dislocations in graphite and found some remarkable results, which give clear explanations for the dimensional change and creep in graphite under neutron irradiation. Dimensional change can be substantial (sometimes exceeding 100%) and creep can be a linear (i.e. non-saturating) function of dose. We find that basal dislocations lie at the heart of nearly all these effects and not, as was originally thought, the Frenkel pairs formed from irradiation. The explanation lies in prismatic loops, screw dislocations and interactions with basal dislocations, so the structure and energetics of all of these will be discussed. The physical effects they give are buckling and forming folds ruck and tuck defects. The findings are expected to be general to layered materials. (authors)

[en] Boron strongly modifies electronic and diffusion properties of graphite. We report the first ab initio study of boron interaction with the point defects in graphite, which includes structures, thermodynamics, and diffusion. A number of possible diffusion mechanisms of boron in graphite are suggested. We conclude that boron diffuses in graphite by a kick-out mechanism. This mechanism explains the common activation energy, but large magnitude difference, for the rate of boron diffusion parallel and perpendicular to the basal plane

[en] The diffraction behaviour of stacked layers of graphene and hexagonal boron nitride are studied computationally by direct calculation of the diffraction pattern using the Debye scattering equation. Analysis of the position and profile of the diffraction peaks show that while single-layer graphene is unambiguously a 2D material, ordered stacks of three or more layers diffract as bulk material. Following the Scherrer equation, we correlate the known crystallite size with the diffraction peak parameters and observe strong affine relationships which exist separately for single-layer, bi-layer and three or more layer (bulk) structures. We determine a series of expressions to calculate the crystallite size which do not suffer the well-known size-dependence or rely on assumptions about the shape. We present a detailed workflow showing how these expressions can be applied to experimental data. (paper)

[en] Full text: Graphite is one of the critical materials required to enable the energy transition. It is the anode material used in all lithium-ion battery chemistries and is where the lithium is safely stored when the battery is charged. However, graphite is highly energy intensive to produce due to the high temperatures required. Atomistic understandings are needed for determining the critical defects preventing graphite formation and how to remove them at lower temperatures and over shorter timespans. In this work, microscopy and computational approaches provide an atomistic mechanism for carbonised mesophase transforming into graphite. High resolution transmission electron microscopy reveals the presence of screw dislocations in partially graphitising PVC. Molecular dynamics simulations provide insights into how an aligned mesophase gives rise to screws. The importance of alignment is explored for producing a graphite precursor instead of disordered carbon. The removal of screw dislocations are also explored and found to be due to edge dislocation gliding within a dislocation loop. The self assembled simulations are explored using a new virtual reality game engine that allows for an atom’s eye view of graphite forming. The visualiser runs within the Meta Quest virtual reality headsets and allows continuous exploration in a periodic simulation. These tools reveal further insights into edge reconstructions and defect geometries. While outstanding questions remain, the role of screw dislocations and their removal are critical for explaining graphite formation and provide new targets for producing graphite more cheaply. The virtual reality headset will be demonstrated after the presentation and during the break. (author)

[en] Irradiation damage in graphite has been the subject of intensive research in the latter half of the twentieth century, driven by nuclear applications. Nevertheless, even parameters as fundamental as the interstitial and vacancy migration energies are still controversial. In general the reason for this is inherent in the usually poor quality of graphite crystals and its delocalized Π system which frustrates many characterization techniques common in semiconductors. In particular, it is because indirect measures of irradiation damage (changes in crystal dimensions and in electrical and thermal resistance) have been directly attributed to point defects or small clusters thereof. First principles calculations of the structure and energetics of irradiation damage defects afford a direct interrogation of point defects and their clusters. In previous work, we have shown that the ground state of the self - interstitial is a spiro form bonding between graphene sheets, forming a core of five carbon atoms analogous to the spiro-pentane (apex sharing triangles) structure. A basal shear of approximately 0.7 Angstroms along a (11-20) direction is required for this, and this provides the strong interaction with basal dislocations. The vacancy migration energy has been found to be 1.7 eV, much lower than the experimental value (3 eV), but it was proposed this could be reconciled if inter-vacancy binding is taken into account (either across planes or in-plane third neighbour interactions). The interlayer binding can also give rise to shearing forces between planes. We apply density functional pseudopotential supercell calculations (AIMPRO code) where the wavefunctions are expanded in a Gaussian basis, the charge density in a plane wave basis and the pseudopotential of Bachelet, Hamann and Schluter-type for carbon and Troullier-Martins type for boron. Geometry optimization is performed by the conjugate gradient method. We find an interstitial formation energy of 5.5 eV and vacancy formation energy of 8.3 eV, giving a Frenkel pair energy of 13.8 eV (in good agreement with 14 eV obtained from experiment). Our initial calculations on the migration of an interstitial give a ca. 1 eV barrier, which leads us to suspect that the experiments giving apparent migration energies in the range 0.02 to 0.5 eV are complex, secondary effects of interstitials such as shear and buckling of graphene planes. Preliminary geometry optimization on basal dislocations demonstrate that shear and buckling can give rise to substantial dimensional changes. Dislocations involve mismatch between lattice planes, which means that planes must either stretch or compress to accommodate the mismatch. For graphene planes the easy deformation is compression by buckling. The direct evidence comes from electron microscope observations of the growth of interstitial prismatic dislocation loops and reveals an activation energy of ca. 1.2 eV. Ref. 4 suggests that boron traps self-interstitials (figure 1), giving the large apparent migration energy - we have confirmed that such a trap exists with a trap energy of 1.1 eV, rather close to the experimental activation energy, but release rate from this trap could involve an activation barrier, which is the subject of further investigation. Similarly, the vacancy is trapped by boron with a trap energy of 2.7 eV, providing yet another possible cause for the elevated vacancy migration energy observed in experiment. The activation barrier for release from this trap is also under investigation. (authors)

[en] The default theory of radiation damage in graphite invokes Frenkel pair formation as the principal cause of physical property changes. We set out its inadequacies and present two new mechanisms that contribute to a better account for changes in dimension and stored energy. Damage depends on the substrate temperature, undergoing a change at approximately 250 deg. C. Below this temperature particle radiation imparts a permanent, nano-buckling to the layers. Above it, layers fold, forming what we describe as a ruck and tuck defect. We present first principles and molecular mechanics calculations of energies and structures to support these claims. Necessarily we extend the dislocation theory of layered materials. We cite good experimental evidence for these features from the literature on radiation damage in graphite.

[en] Iwata and Watanabe's model for the observed low-temperature specific heat of neutron-irradiated graphite [T. Iwata and M. Watanabe, Phys. Rev. B 81, 014105 (2010)] assumes that self-interstitial atoms exist as clusters of nearly free C₂ molecules. We suggest that their hypothesis is not supported by other experiments and theory, including our own calculations. Not only is it inconsistent with the long-known kinetics of interstitial prismatic dislocation loop formation, density-functional theory shows that the di-interstitial is covalently bonded to the host crystal. In such calculations no prior assumptions are made about the nature of the bonding, covalent or otherwise.

[en] Di-interstitial defects appear to play a key role in the microscopic understanding of radiation-induced damage in graphite. Their formation has been invoked as both one of the main causes of dimensional change and as an energy releasing step in annealing cryogenic radiation-induced damage. In the present work, first principles calculations are employed to examine several models for these defects. Two of the structures possess nearly equal energy, yet take very different forms. The results suggest that di-interstitial defects cannot play the principal role in radiation damage that has been assigned to them. The possibility that one of the structures may exhibit ferromagnetism is also investigated

[en] Carbon nanotube surfaces, activated and randomly decorated with metal nanoclusters, have been studied in uniquely combined theoretical and experimental approaches as prototypes for molecular recognition. The key concept is to shape metallic clusters that donate or accept a fractional charge upon adsorption of a target molecule, and modify the electron transport in the nanotube. The present work focuses on a simple system, carbon nanotubes with gold clusters. The nature of the gold-nanotube interaction is studied using first-principles techniques. The numerical simulations predict the binding and diffusion energies of gold atoms at the tube surface, including realistic atomic models for defects potentially present at the nanotube surface. The atomic structure of the gold nanoclusters and their effect on the intrinsic electronic quantum transport properties of the nanotube are also predicted. Experimentally, multi-wall CNTs are decorated with gold clusters using (1) vacuum evaporation, after activation with an RF oxygen plasma and (2) colloid solution injected into an RF atmospheric plasma; the hybrid systems are accurately characterized using XPS and TEM techniques. The response of gas sensors based on these nano²hybrids is quantified for the detection of toxic species like NO₂, CO, C₂H₅OH and C₂H₄.