[Show abstract][Hide abstract] ABSTRACT: Molecular dynamics simulation is used to study radiation damage cascades in graphite. High statistical precision is obtained by sampling a wide energy range (100–2500 eV) and a large number of initial directions of the primary knock-on atom. Chemical bonding is described using the Environment Dependent Interaction Potential for carbon. Graphite is found to exhibit a radiation response distinct from metals and oxides primarily due to the absence of a thermal spike which results in point defects and disconnected regions of damage. Other unique attributes include exceedingly short cascade lifetimes and fractal-like atomic trajectories. Unusually for a solid, the binary collision approximation is useful across a wide energy range, and as a consequence residual damage is consistent with the Kinchin–Pease model. The simulations are in agreement with known experimental data and help to clarify substantial uncertainty in the literature regarding the extent of the cascade and the associated damage.
[Show abstract][Hide abstract] ABSTRACT: The response of pristine, nitrogen and boron doped carbon nanotube (CNT) sensors to NO2, CO, C2H4 and H2O at ppm concentrations was investigated at both room temperature and 150 °C. N-doped CNTs show the best sensitivity to nitrogen dioxide and carbon monoxide, while B-doped CNTs show the best sensitivity to ethylene. All tubes (including undoped) show strong humidity response. Sensing mechanisms are determined via comparison with density functional calculations of gas molecule absorption onto representative defect structures in N and B-doped graphene. N-CNTs show decreased sensitivity with temperature, and detection appears to occur via gas physisorption. B-CNTs appear to react chemically with many of the absorbed species as shown by their poor baseline recovery and increasing sensitivity with temperature. This limits their cyclability. Overall gas sensitivity is as good or better than post-growth functionalised nanotubes, and used in combination, CNTs, N-CNTs and B-CNTs appear highly promising candidates for cheap, low power, room temperature gas sensing applications.
[Show abstract][Hide abstract] ABSTRACT: Pristine and oxygen plasma functionalised carbon nanotubes (CNTs) were studied after the evaporation of Pt and Pd atoms. High resolution transmission electron microscopy shows the formation of metal nanoparticles at the CNT surface. Oxygen functional groups grafted by the plasma functionalization act as nucleation sites for metal nanoparticles. Analysis of the C1s core level spectra reveals that there is no covalent bonding between the Pt or Pd atoms and the CNT surface. Unlike other transition metals such as titanium and copper, neither Pd nor Pt show strong oxygen interaction or surface oxygen scavenging behaviour.
Chemical Physics Letters 05/2013; 571:44–48. · 1.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Using a combination of transmission electron microscopy and density functional modeling we examine covalent bridging between carbon nanoforms, focusing on fullerene attachment to carbon nanocones (nanohorns). We show that oxygen mediates covalent cross-linking between carbon nanoforms, analogously to oxygen-mediated fullerene dimerisation (C120O). We confirm this theoretically and experimentally for fullerenes bonded to nanocone tips. Oxygen bridging only occurs in systems with relatively localized double bond character, i.e., in the case of nanocones, bridging only occurs between fullerenes and high angle nanocone tips.
[Show abstract][Hide abstract] ABSTRACT: A practical numerical method for the rapid construction of nanotube caps
is proposed. Founded upon the notion of lattice duality, the algorithm
considers the face dual representation of a given nanotube which is used
to solve an energy minimization problem analogous to The Thomson
Problem. Not only does this produce caps for nanotubes of arbitrary
chirality, but the caps generated will be physically sensible and in
most cases the lowest energy structure. To demonstrate the applicability
of the technique, caps of the (5,5) and the (10,0) nanotubes are
investigated by means of density-functional tight binding (DFTB). The
calculation of cap energies highlights the ability of the algorithm to
produce lowest energy caps. Due to the preferential construction of
spherical caps, the technique is particularly well suited for the
construction of capped multiwall nanotubes (MWNTs). To validate this
proposal and the overall robustness of the algorithm, a MWNT is
constructed containing the chiralities (9,2)@(15,6)@(16,16). The
algorithm presented paves the way for future computational
investigations into the physics and chemistry of capped nanotubes.
[Show abstract][Hide abstract] ABSTRACT: The transformation of amorphous carbon nanorods into multi-wall nanotubes is studied using molecular dynamics. The effect of the density, width and shape of the initial nanorod is investigated. High-temperature annealing simulations show that the transformation is a robust process which occurs at all densities, regardless of the nanorod shape. The least-defective nanotubes arise from tetrahedral amorphous carbon precursors with an initial density of 3 g/cc. By excising selected regions of the nanorod, we show that the perimeter in cross-section determines the tube diameter, with the number of walls being primarily controlled by the density of the rod. This transformation suggests an alternative approach for generating carbon nanotube networks in the laboratory using ion-beam deposition in combination with lithography.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
physica status solidi (b) 07/2012; 249(2). · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The breakdown of the diamond lattice is explored by ion implantation and molecular dynamics simulations. We show that lattice breakdown is strain-driven, rather than damage-driven, and that the lattice persists until 16% of the atoms have been removed from their lattice sites. The figure shows the transition between amorphous carbon and diamond, with the interfaces highlighted with dashed lines.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] ABSTRACT: Vertically aligned multiwalled carbon nanotubes (v-MWCNTs) are functionalized using atomic oxygen generated in a microwave plasma. X-ray photoelectron spectroscopy depth profile analysis shows that the plasma treatment effectively grafts oxygen exclusively at the v-MWCNT tips. Electron microscopy shows that neither the vertical alignment nor the structure of v-MWCNTs were affected by the plasma treatment. Density functional calculations suggest assignment of XPS C 1s peaks at 286.6 and 287.5 eV, to epoxy and carbonyl functional groups, respectively.
The Journal of Physical Chemistry C 10/2011; 115(42):20412-20418. · 4.84 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Computational methods are used to control the degree of structural order in a variety of carbon materials containing primarily sp2 bonding. Room-temperature thermal conductivities are computed using non-equilibrium molecular dynamics. Our results reproduce experimental data for amorphous and glassy carbons and confirm previously proposed structural models for vitreous carbons. An atomistic model is developed for highly oriented thin films seen experimentally, with a maximum computed thermal conductivity of 35 W m−1 K−1. This value is much higher than that of the amorphous and glassy structures, demonstrating that the microstructure influences the thermal conductivity more strongly than the density.
[Show abstract][Hide abstract] ABSTRACT: 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°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.
Journal of Nuclear Materials 06/2011; 413(3):150-155. · 2.02 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a study of purifn. of single-walled carbon nanotubes (SWCNTs) using different oxidn. temps. and chem. treatments. We have developed a simple two annealing-steps procedure resulting in high nanotube purity with minimal sample loss. The process involves annealing the SWCNTs at 300° for 2 h with subsequent reflux in 6 M HCl at 130°, followed by further annealing at 350° for 1 h with reflux in 6 M HCl at 130°. The process results in effective removal of carbon impurities and metal particles which are assocd. with SWCNTs prodn. The process is less time consuming (complete in 4.5 h) than conventional acid purifn. methods which require over 5 h, and less destructive than conventional methods with a yield of 26%. SWCNT purity was assessed using Raman spectroscopy, thermogravimetry and SEM coupled with energy-dispersive X-ray spectroscopy. [on SciFinder(R)]
The European Physical Journal Applied Physics 04/2011; 54(1):10401/1-10401/7. · 0.79 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a density-functional theory study of low-density bromination of graphene and graphite, finding significantly different behavior in these two materials. In 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 π cloud. Bromination opens a small (86-meV) 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 stabilizing normally unstable transient molecular states.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: The transformation of carbon peapods (encapsulated fullerenes in nanotubes) into double-walled nanotubes was studied using molecular dynamics simulation. The simulations reproduce the two main trends known experimentally: the production of low-defect nanotubes and the templating effect of the outer tube. The process involves a low-temperature polymerization of the fullerenes followed by higher temperature self-assembly into a tube. Modelling of this second stage is made possible by the use of the Environment-Dependent Interaction Potential, a large number of atoms and long-time annealing. Analysis shows that the outer tube acts as a container for the self-assembly process, analogous to previous simulations and experiments in which free surfaces, either external or internal, template the formation of highly ordered sp2 phases.
[Show abstract][Hide abstract] ABSTRACT: 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.
Physical Review B 08/2010; 82(5):056101. · 3.66 Impact Factor
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] ABSTRACT: 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 nano(2)hybrids is quantified for the detection of toxic species like NO(2), CO, C(2)H(5)OH and C(2)H(4).