23 research outputs found
Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition
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
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-carbonâcontaining pentagons and heptagons, and âpenta-graphene,â consisting of a layer of pentagons constructed from a mixture of sp2- and sp3-coordinated carbon atoms. In 2D projection appearing as the âCairo pattern,â penta-graphene is elegant and aesthetically pleasing. However, we dispute the authorâs claims of its potential stability and experimental relevanc
Ripple edge engineering of graphene nanoribbons
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 availabl
Low energy graphene edge termination via small diameter nanotube formation
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-carbon atoms, and
introduce Van Hove singularities into the graphene density of states.Comment: To appear in Phys. Rev. Let
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 CiĂȘncia e a Tecnologia (FCT
Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition
Abstract 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 MoS 2 ) 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
Pattern formation on carbon nanotube surfaces
Calculations of fluorine binding and migration on carbon nanotube surfaces show that fluorine forms varying surface superlattices at increasing temperatures. The ordering transition is controlled by the surface migration barrier for fluorine atoms to pass through next neighbor sites on the nanotube, explaining the transition from semi-ionic low coverage to covalent high coverage fluorination observed experimentally for gas phase fluorination between 200 and 250°C. The effect of solvents on fluorine binding and surface diffusion is explored
Radiation-induced defect reactions in tin-doped Ge crystals
We have recently shown that Sn impurity atoms are effective traps for vacancies (V) in Ge:Sn crystals irradiated with MeV electrons at room temperature [V.P. Markevich etal., J. Appl. Phys. 109 (2011) 083705]. A hole trap with 0.19 eV activation energy for hole emission to the valence band (Eh) has been assigned to an acceptor level of the Sn-V complex. In the present work electrically active defects introduced into Ge:Sn+P crystals by irradiation with 6 MeV electrons and subsequent isochronal annealing in the temperature range 50-300 °C have been studied by means of transient capacitance techniques and ab-initio density functional modeling. It is found that the Sn-V complex anneals out upon heat-treatments in the temperature range 50-100 °C. Its disappearance is accompanied by the formation of vacancy-phosphorus (VP) centers. The disappearance of the VP defect upon thermal annealing in irradiated Sn-doped Ge crystals is accompanied by the effective formation of a defect which gives rise to a hole trap with Eh = 0.21 eV and is more thermally stable than other secondary radiation-induced defects in Ge:P samples. This defect is identified as tinvacancy- phosphorus (SnVP) complex. It is suggested that the effective interaction of the VP centers with tin atoms and high thermal stability of the SnVP complex can result in suppression of transient enhanced diffusion of phosphorus atoms in Ge. © (2011) Trans Tech Publications, Switzerland
The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite
Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. Migration of isolated monovacancies is estimated to have an activation energy Ea â 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.Validerad; 2013; Bibliografisk uppgift: Article no 135403; 20130305 (latham