282 research outputs found
Nomenclature of sp2 carbon nanoforms
Carbonâs versatile bonding has resulted in the discovery of a bewildering variety of nanoforms which urgently need a systematic and standard nomenclature [1]. Besides fullerenes, nanotubes and graphene, research teams around the globe now produce a plethora of carbon-based nanoforms such as âbambooâ tubes, âherringboneâ and âbellâ structures. Each discovery duly gains a new, sometimes whimsical, name, often with its discoverer unaware that the same nanoform has already been reported several times but with different names (for example the nanoform in Fig. 1h is in different publications referred to as âbambooâ [2], âherringbone-bambooâ [3], âstacked-cupsâ [4] and âstacked-conesâ [5]). In addition, a single name is often used to refer to completely different carbon nanoforms (for example, the âbambooâ structure in [2] is notably different from âbambooâ in [6]). The result is a confusing overabundance of names which makes literature searches and an objective comparison of results extremely difficult, if not impossible
Supplementary report to the final report of the coral reef expert group: S4. Model to inform the design of a Reef Integrated Monitoring and Reporting Program
[Extract] This project developed a model to inform coral reef monitoring and management under the Reef 2050 Integrated Monitoring and Reporting Program (RIMReP) and the Reef 2050 Long-Term Sustainability Plan (Reef 2050 Plan). The model combines spatial statistical analyses with a mechanistic understanding of coral community dynamics. The purpose of the model is to analyse coral status and trend, and to guide the design of a coral monitoring program that most effectively captures these dynamics in space and time. This model uses per cent cover of hard corals and benthic composition as key indicators of reef state. Input variables include environmental data (e.g. temperature, salinity, sediment covers) and disturbance history (e.g. tropical cyclones, bleaching, water quality and outbreaks of the crown-of-thorns starfish). The model is calibrated against 20 years of in situ coral monitoring data and remotely sensed observations (1996-2015). A dual classification of all Great Barrier Reef (Reef) reefs was established based on (i) their benthic community composition and (ii) their coral cover trajectory over the 1996-2015 period, as a potential tool to stratify the future reef monitoring design. Both classifications, along with model outputs of coral cover, are available as a set of spatial layers (0.01 degree resolution).An accessible copy of this report is not yet available from this repository, please contact [email protected] for more information
Comment on âIncrease in specific heat and possible hindered rotation of interstitial C2 moleculesin neutron-irradiated graphiteâ
Iwata and Watanabeâs 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
Bromination of Graphene and Graphite
We present a density functional theory study of low density bromination of
graphene and graphite, finding significantly different behaviour in these two
materials. On 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 pi-cloud. Bromination opens a small (86meV) 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 stabilising normally unstable transient molecular states
Electron spectroscopy of carbon materials: Experiment and theory
We present a comparative spectroscopic study of carbon as graphite, diamond and C60 using C1s K-edge electron energy-loss spectroscopy (EELS), X-ray emission spectroscopy, and theoretical modelling. The first principles calculations of these spectra are obtained in the local density approximation using a self-consistent Gaussian basis pseudo-potential method. Calculated spectra show excellent agreement with experiment and are able to discriminate not only between various carbon hybridisations but also local variation in environment. Core-hole effects on the calculated spectra are also investigated. For the first time, the EEL spectrum of carbyne is calculated
Behavior of hydrogen ions, atoms, and molecules in a-boron studied using density functional calculations
We examine the behavior of hydrogen ions, atoms, and molecules in a-boron using density functionalcalculations. Hydrogen behaves as a negative-U center, with positive H ions preferring to sit off-center oninterlayer 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 -boron and compare with experimental muon-implantation data
First-Principles Study of Substitutional Metal Impurities in Graphene: Structural, Electronic and Magnetic Properties
We present a theoretical study using density functional calculations of the
structural, electronic and magnetic properties of 3d transition metal, noble
metal and Zn atoms interacting with carbon monovacancies in graphene. We pay
special attention to the electronic and magnetic properties of these
substitutional impurities and found that they can be fully understood using a
simple model based on the hybridization between the states of the metal atom,
particularly the d shell, and the defect levels associated with an
unreconstructed D3h carbon vacancy. We identify three different regimes
associated with the occupation of different carbon-metal hybridized electronic
levels:
(i) bonding states are completely filled for Sc and Ti, and these impurities
are non-magnetic;
(ii) the non-bonding d shell is partially occupied for V, Cr and Mn and,
correspondingly, these impurties present large and localized spin moments;
(iii) antibonding states with increasing carbon character are progressively
filled for Co, Ni, the noble metals and Zn. The spin moments of these
impurities oscillate between 0 and 1 Bohr magnetons and are increasingly
delocalized.
The substitutional Zn suffers a Jahn-Teller-like distortion from the C3v
symmetry and, as a consequence, has a zero spin moment. Fe occupies a distinct
position at the border between regimes (ii) and (iii) and shows a more complex
behavior: while is non-magnetic at the level of GGA calculations, its spin
moment can be switched on using GGA+U calculations with moderate values of the
U parameter.Comment: 13 figures, 4 tables. Submitted to Phys. Rev. B on September 26th,
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Atomic Configuration of Nitrogen Doped Single-Walled Carbon Nanotubes
Having access to the chemical environment at the atomic level of a dopant in
a nanostructure is crucial for the understanding of its properties. We have
performed atomically-resolved electron energy-loss spectroscopy to detect
individual nitrogen dopants in single-walled carbon nanotubes and compared with
first principles calculations. We demonstrate that nitrogen doping occurs as
single atoms in different bonding configurations: graphitic-like and
pyrrolic-like substitutional nitrogen neighbouring local lattice distortion
such as Stone-Thrower-Wales defects. The stability under the electron beam of
these nanotubes has been studied in two extreme cases of nitrogen incorporation
content and configuration. These findings provide key information for the
applications of these nanostructures.Comment: 25 pages, 13 figure
BN domains included into carbon nanotubes: role of interface
We present a density functional theory study on the shape and arrangement of
small BN domains embedded into single-walled carbon nanotubes. We show a strong
tendency for the BN hexagons formation at the simultaneous inclusion of B and N
atoms within the walls of carbon nanotubes. The work emphasizes the importance
of a correct description of the BN-C frontier. We suggest that BN-C interface
will be formed preferentially with the participation of N-C bonds. Thus, we
propose a new way of stabilizing the small BN inclusions through the formation
of nitrogen terminated borders. The comparison between the obtained results and
the available experimental data on formation of BN plackets within the single
walled carbon nanotubes is presented. The mirror situation of inclusion of
carbon plackets within single walled BN nanotubes is considered within the
proposed formalism. Finally, we show that the inclusion of small BN plackets
inside the CNTs strongly affects the electronic character of the initial
systems, opening a band gap. The nitrogen excess in the BN plackets introduces
donor states in the band gap and it might thus result in a promising way for
n-doping single walled carbon nanotubes
Tuning the Raman Resonance Behavior of Single-Walled Carbon Nanotubes via Covalent Functionalization
We present a systematic Raman study over a range of excitation energies of arc discharge single-walled carbon nanotubes (SWCNTs) covalently functionalized according to two processes, esterification and reductive alkylation. The SWCNTs are characterized by resonance Raman spectroscopy at each step of the functionalization process, showing changes in radial breathing mode frequencies and transition energies for both semiconducting and metallic tubes. Particular attention is given to a family of tubes clearly identified in the Kataura plot for which we continuously tune the excitation energy from 704 to 752 nm. This allows us to quantify the energy shift occurring in the spacing of the van Hove singularities. We demonstrate that, independently of the functionalization technique, the type of chain covalently bound to the tubes plays an important role, notably when oxygen atoms lie close to the tubes, inducing a larger shift in transition energy as compared to that of other carbonaceous chains. The study shows the complexity of interpreting Raman data and suggests many interpretations in the literature may need to be revisited
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