Strained graphene: tight-binding and density functional calculations

We determine the band structure of graphene under strain using density functional calculations. The ab-initio band strucure is then used to extract the best fit to the tight-binding hopping parameters used in a recent microscopic model of strained graphene. It is found that the hopping parameters may increase or decrease upon increasing strain, depending on the orientation of the applied stress. The fitted values are compared with an available parametrization for the dependence of the orbital overlap on the distance separating the two carbon atoms. It is also found that strain does not induce a gap in graphene, at least for deformations up to 10%

Predicting experimentally stable allotropes: Instability of penta-graphene

International audienceIn recent years, a plethora of theoretical carbon allotropes have been proposed, none of which has been experimentally isolated. We discuss here criteria that should be met for a new phase to be potentially experimentally viable. We take as examples Haeckelites, 2D networks of sp2-carbon–containing pentagons and heptagons, and “penta-graphene,” consisting of a layer of pentagons constructed from a mixture of sp2- and sp3-coordinated carbon atoms. In 2D projection appearing as the “Cairo pattern,” penta-graphene is elegant and aesthetically pleasing. However, we dispute the author’s claims of its potential stability and experimental relevanc

Molecular characterization and phylogenetic analysis of betasatellite molecules associated with okra yellow vein mosaic disease in Sri Lanka

Okra production in Sri Lanka has been severely affected by okra yellow vein mosaic disease (OYVMD), which is caused by begomoviruses and associated betasatellites. These betasatellite molecules commonly determine the development and severity of the disease. Therefore, knowledge about the genetic variability of betasatellites associated with OYVMD could assist okra breeding programs in the selection of resistant varieties. The present study aimed to characterize the betasatellite DNA sequences associated with OYVMD in Sri Lanka and to determine their phylogenetic relationships. Betasatellite DNA of six virus isolates from widely separated geographical locations were sequenced and compared with already reported begomovirus betasatellites. The betasatellite molecules have features common to other betasatellite DNAs: a conserved nonanucleotide TAATATTAC, a coding sequence for the protein βC1, an adenine rich region and a satellite conserved region. Nucleotide diversity among the isolates was relatively low (π = 0.034). A recombination event was detected at a specific region in the genome of all isolates. The isolates shared >96% sequence identity with bhendi yellow vein betasatellites reported from India and phylogenetic analysis confirmed their genetic relationship

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

Metastable Frenkel pair defect in graphite: source of Wigner energy?

The atomic processes associated with energy storage and release in irradiated graphite have long been subject to untested speculation. We examine structures and recombination routes for interstitial-vacancy (I-V) pairs in graphite. Interaction results in the formation of a new metastable defect (an intimate I-V pair) or a Stone-Wales defect. The intimate I-V pair, although 2.9 eV more stable than its isolated constituents, still has a formation energy of 10.8 eV. The barrier to recombination to perfect graphite is calculated to be 1.3 eV, consistent with the experimental first Wigner energy release peak at 1.38 eV. We expect similar defects to form in carbon nanostructures such as nanotubes, nested fullerenes, and onions under irradiation

Doubly charged silicon vacancy center, Si-N complexes, and photochromism in N and Si codoped diamond

Diamond samples containing silicon and nitrogen are shown to be heavily photochromic, with the dominant visible changes due to simultaneous change in total SiV0/− concentration. The photochromism treatment is not capable of creating or destroying SiV defects, and thus we infer the presence of the optically inactive SiV2− . We measure spectroscopic signatures we attribute to substitutional silicon in diamond, and identify a silicon-vacancy complex decorated with a nearest-neighbor nitrogen SiVN, supported by theoretical calculations

Hydrogen Interaction with Dislocations in Si

An H plasma has a remarkable effect on dislocation mobility in silicon, reducing its activation energy to 1.2 eV. Applying density functional theory to the interactions of H and H? With the core of the 90 degrees partial dislocation in Si, we have identified a path for motion involving kink formation and migration at hydrogenated core bonds which conforms exactly to the experimentally measured activation energ