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ABSTRACT: 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.
07/2012;
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ABSTRACT: 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.
07/2012;
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ABSTRACT: An attempt is presented to create a nomenclature for carbon nanoforms based on their morphol. differences and geometrical transformations with graphene as simplest structure, serving as a building block. The initial list of transformations include the following ones: stacked, cut, circularly wrapped, scrolled, coiled, screwed, and coned. By applying these transformation a family tree could be constructed which classifies the carbon nanoforms by morphol.: nanotubes (circularly wrapped 2D nanoform), nanoscroll (spirally wrapped 2D nanoform), nanotorois (circularly wrapped 1D nanoform), nanospiral (spirally wrapped 1D nanoform), etc. The description of the nanoforms can be refined by addnl. terms, not included in the family tree: single-wall, double-wall, multi-wall, double-core, multi-core, partitioned, corrugated, wavy, curved, etc. The proposed nomenclature includes the major groups: mol. forms, 0D (fullerenes), cylindrical nanoforms, 1D (nanotubes, stacked platelets) and layered nanoforms, 2D
Carbon 01/2012; 50(3):741-747. · 5.38 Impact Factor
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ABSTRACT: 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.
03/2011;
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ABSTRACT: We examine the behavior of hydrogen ions, atoms, and molecules in α-boron using density functional calculations. Hydrogen behaves as a negative-U center, with positive H ions preferring to sit off-center on interlayer 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 behavior in β-boron and compare with experimental muon-implantation data.
Phys. Rev. B. 01/2011; 83(2).
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ABSTRACT: X-ray photoelectron spectroscopy at 3.5 keV photon energy, in combination with high-resolution transmission electron microscopy, is used to follow the formation of the interface between rhodium and carbon nanotubes. Rh nucleates at defect sites, whether initially present or induced by oxygen-plasma treatment. More uniform Rh cluster dispersion is observed on plasma-treated CNTs. Experimental results are compared to DFT calculations of small Rh clusters on pristine and defective graphene. While Rh interacts as strongly with the carbon as Ti, it is less sensitive to the presence of oxygen, suggesting it as a good candidate for nanotube contacts.
ACS Nano 02/2010; 4(3):1680-6. · 10.77 Impact Factor
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ABSTRACT: We explore early stage oxygen addition to C60 buckminsterfullerene, and compare its oxygenation behavior to that of both pristine and defective metallic carbon nanotubes, using ab initio theoretical modeling. For fullerene oxygen addition up to C60O4, in general oxygenation preferentially occurs at the pentagon-hexagon bonds ([5,6] type addition), leading to open annulene structures, as opposed to the closed [6, 6] epoxide isomers. For carbon nanotubes the preference for annulene structures is significantly more pronounced as all epoxide addition is endothermic. Higher reaction enthalpies are found for oxidation in the proximity of defects as compared to the pristine sidewalls. In most cases higher reaction enthalpies are found for fullerene oxygenation as compared to carbon nanotubes.
Journal of Nanoscience and Nanotechnology 10/2009; 9(10):6113-9. · 1.56 Impact Factor
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ABSTRACT: We present a combined theoretical and experimental comparative study of the deposition of five different metals on perfect and defective graphene and multi-walled carbon nanotubes (MWNTs): Ti, Ni, Pd, Pt and Au. Atomistic modelling successfully provides a comprehensive picture of surface binding, diffusion and aggregation properties for these metals, highlighting some fundamental differences in their surface chemical and electronic behaviour. We correlate these theoretical results with experimental TEM images of metal deposited MWCNTs.
Journal of Nanoscience and Nanotechnology 10/2009; 9(10):6171-5. · 1.56 Impact Factor
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ABSTRACT: The interaction between carbon buckminsterfullerene (C60) and carbon nanohorns (also referred to as nanocones) with different tip angles is investigated theoretically. Attachment of C60 to both the interior and the exterior of the horns are considered. Calculations cover a range of cone angles from flat graphene, through 114 degrees, 84 degrees, 60 degrees, 39 degrees and 20 degrees to fullerene pair interaction. Full DFT/LDA calculations are performed and the influence of dispersion forces are considered independently using a numerical potential. Fullerenes bind weakly to the external nanocone wall with approximately 2.9 angstroms spacing (0.5-0.9 eV binding energy), showing no discernable trend with cone tip angle. Fullerene binding inside cones is significantly stronger (> 3 eV), primarily due to strong dispersion force interactions, with higher (approximately 3.1 angstroms) fullerene-nanohorn spacing. In this case, the binding energy increases with number of pentagons in the tip. In all cases the fullerenes will be freely rotating below liquid nitrogen temperatures. For pristine cones and fullerenes, the fullerenes will experience a driving force towards (away) from the nanohorn tip when inside (outside) the nanohorn.
Journal of Nanoscience and Nanotechnology 10/2009; 9(10):6144-8. · 1.56 Impact Factor
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ABSTRACT: 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 pre-treatment. 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.
ChemPhysChem 06/2009; 10(11):1799-804. · 3.41 Impact Factor
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ABSTRACT: Finely tuned: Carbon nanotubes are exposed to a CF(4) radio-frequency plasma (see picture). High-resolution photoelectron spectroscopy shows that the treatment effectively grafts fluorine atoms onto the MWCNTs, altering the valence electronic states. Fluorine surface concentration can be tuned by varying the exposure time.Multi-wall carbon nanotubes (MWCNTs) were exposed to a CF(4) radio-frequency (rf) plasma. High-resolution photoelectron spectroscopy shows that the treatment effectively grafts fluorine atoms onto the MWCNTs, altering the valence electronic states. Fluorine surface concentration can be tuned by varying the exposure time. Evaporation of gold onto MWCNTs is used to mark active site formation. High-resolution transmission electron microscopy coupled with density functional theory (DFT) modelling is used to characterise the surface defects formed, indicating that the plasma treatment does not etch the tube surface. We suggest that this combination of theory and microscopy of thermally evaporated gold atoms onto the CNT surface may be a powerful approach to characterise both surface defect density as well as defect type.
ChemPhysChem 05/2009; 10(6):920-5. · 3.41 Impact Factor
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ABSTRACT: We study the structure and vibrational modes of a wide range of oxohalides of vanadium (VOX(n)Y(m); X, Y = F, Cl, Br, I; n, m = 0-3, n + m < or = 3). The results agree well with experimental results for VOCl(3) and VOF(3) and suggest reassignment of the experimentally observed VOF to VOF(2). We provide new assignments for various experimental modes, identifying several intermediates (VOBr(2), VOBr) and mixed structures (e.g., VOCl(2)Br), and discuss formation trends and stabilities.
Inorganic Chemistry 04/2009; 48(8):3660-6. · 4.60 Impact Factor
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ABSTRACT: We study the interface between C nanotubes (CNTs) and surface deposited Ti using electron microscopy and photoemission spectroscopy, supported by d. functional calcns. Charge transfer from the Ti atoms to the nanotube and carbide formation is obsd. at the interface which indicates strong interaction. Nevertheless, the presence of O 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 O impurities are examd., namely O from any residual atm. and pre-existing O impurities on the nanotube surface, which we enhance through O plasma surface pre-treatment. Variation in literature data concerning Ohmic contacts between Ti and C nanotubes is explained via sample pretreatment and differing vacuum levels, and we suggest improved treatment routes for reliable Schottky barrier free Ti-nanotube contact formation. [on SciFinder(R)]
arXiv.org, e-Print Archive, Condensed Matter. 01/2009;
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ABSTRACT: Density functional theory (DFT) calculations suggest significantly different oxidation behaviour for phosphorus-doped heterofullerenes compared to their pure and nitrogen-doped counterparts, due to formation of a phosphene oxide. This oxide is not thermally labile, suggesting stable phosphofullerenes are likely to be C(59)POH and (C(59)PO)(2). In contrast, azafullerenes form stable epoxides when oxidised. We calculate the effect of oxidation on radical pairing and hydrogen passivation. Notably while the C(59)N radical behaves as a donor, C(59)PO will be an acceptor.
Physical Chemistry Chemical Physics 05/2008; 10(16):2145-8. · 3.57 Impact Factor
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ABSTRACT: We present a novel interpretation of defected tubes based on a dislocation model. A scroll (a sheet rolled without closure) is considered as an axial edge dislocation in a multiwall nanotube (MWNT). Screw dislocation type defects separate scroll from nested-tubes within the same nanotube. The glide of the screw dislocation causes the transformation between these two forms. In some cases, the mechanism of formation of an MWNT could, therefore, start with the formation of a scroll which, by gliding of a screw dislocation, is transformed into the more stable MWNT We compare the structure and energetics of prismatic screw and edge dislocations in graphite and carbon nanotubes. We present calculations for the Peierls barrier of the first kind for graphite and we discuss this result for glide motion of screw dislocations in nanotubes. There is no evidence for stable sp3 atoms in any of the studied nanostructures.
Journal of Nanoscience and Nanotechnology 11/2007; 7(10):3417-20. · 1.56 Impact Factor
