Adatoms and nanoengineering of carbon

We present a new and general mechanism for inter-conversion of carbon structures via a catalytic exchange process, which operates under conditions of Frenkel pair generation. The mechanism typically lowers reaction barriers by a factor of four compared to equivilent uncatalysed reactions. We examine the relevance of this mechanism for fullerene growth, carbon onions and nanotubes, and dislocations in irradiated graphite.Comment: 3 Figures, 5 Page letter accepted for publication in Chemical Physics Letter

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

Interlayer vacancy defects in AA-stacked bilayer graphene: Density functional theory predictions

© 2017 IOP Publishing Ltd.AA-stacked graphite and closely related structures, where carbon atoms are located in registry in adjacent graphene layers, are a feature of graphitic systems including twisted and folded bilayer graphene, and turbostratic graphite. We present the results of ab initio density functional theory calculations performed to investigate the complexes that are formed from the binding of vacancy defects across neighbouring layers in AA-stacked bilayers. As with AB stacking, the carbon atoms surrounding lattice vacancies can form interlayer structures with sp 2 bonding that are lower in energy than in-plane reconstructions. The sp 2 interlayer bonding of adjacent multivacancy defects in registry creates a type of stable sp 2 bonded 'wormhole' or tunnel defect between the layers. We also identify a new class of 'mezzanine' structure characterised by sp 3 interlayer bonding, resembling a prismatic vacancy loop. The V 6 hexavacancy variant, where six sp 3 carbon atoms sit midway between two carbon layers and bond to both, is substantially more stable than any other vacancy aggregate in AA-stacked layers. Our focus is on vacancy generation and aggregation in the absence of extreme temperatures or intense beams

Bistable N2–H complexes: The first proposed structure of a H-related colour-causing defect in diamond

International audienceWe examine the hydrogenated substitutional nitrogen pair defect, N2-H, in diamond, with different charge states using density functional theory calculations. Neutral N2-H is predicted to exhibit optical absorption in the 600 nm to near infrared range, hence contributing to colour. We suggest that it may be representative of a family of nitrogen-hydrogen complexes, which due to covalent nitrogen-hydrogen bonding could explain several observed absorptions. Changing the charge state leads to spontaneous geometric rearrangements, and we find the neutral charge state is thermodynamically metastable. The resultant negative-U behaviour suggests it is unstable as a donor. The structural changes with charge state may contribute to the thermochromic and photochromic behaviour of the so-called "chameleon" diamonds

Polyiodide structures in thin single-walled carbon nanotubes: A large-scale density-functional study

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Vacancy- and acceptor-H complexes in InP

It has been suggested that iron in InP is compensated by a donor, related to the 2316 cm-1 local vibrational mode and previously assigned to the fully hydrogenated indium vacancy, VInH4. Using AIMPRO, an ab initio local density functional cluster code, we find that VInH4 acts as a single shallow donor. It has a triplet vibrational mode at around this value, consistent with this assignment. We also analyse the other hydrogenated vacancies VInHn, n = 1,3, and determine their structure, vibrational modes, and charge states. Substitutional group II impurities also act as acceptors in InP, but can be passivated by hydrogen. We investigate the passivation of beryllium by hydrogen and find that the hydrogen sits at a bond-centred site and is bonded to its phosphorus neighbour. Its calculated vibrational modes are in good agreement with experiment.</p

Prismatic edge dislocations in graphite

Dislocations are a central concept in materials science, which dictate the plastic deformation and damage evolution in materials. Layered materials such as graphite admit two general types of interlayer dislocations: basal and prismatic dislocations, of which prismatic dislocations have been relatively less studied. Using density functional theory (DFT) calculations, we have examined different prismatic core structures in graphite and evaluated their structure, energetics and mobility. We find close energetic interplay between bonded and “free-standing” core structures in both zigzag and armchair directions, with a reconstructed stable zigzag core identified. We explore grain boundaries and prismatic dislocation pile-up, identifying metastable structures which may be important in energy storage. The role of interlayer stacking in core structure, dislocation glide and climb is also considered in-depth. Our calculations suggest that the prismatic dislocation core is stable up to high temperatures of approximately 1500 K in bulk graphite. Above this temperature, the breaking of bonds in the dislocation core can facilitate climb, grain-boundary motion, and the annealing of damage through prismatic dislocation glide. © 2021 Elsevier LtdANR-20-CE08-0026, TUBITAK-2219; Engineering and Physical Sciences Research Council, EPSRC: EP/P020232/1, EP/R005745/1This work was supported by the United Kingdom EPSRC grant EP/R005745/1 , Mechanisms of Retention and Transport of Fission Products in Virgin and Irradiated Nuclear Graphite. Kenny Jolley and Pavlos Mouratidis also gratefully acknowledge funds from EDF energy generation 2016–2021 . The authors gratefully acknowledge the use of Athena at HPC Midlands+, which was funded by the EPSRC grant EP/P020232/1 as part of the HPC Midlands + consortium. CE and AI acknowledge ANR-16-CE24-0008-01 “EdgeFiller” and ANR-20-CE08-0026 “OPIFCat” for funding. DE acknowledges support from the TUBITAK-2219 post-doctoral research abroad fund.This work was supported by the United Kingdom EPSRC grant EP/R005745/1, Mechanisms of Retention and Transport of Fission Products in Virgin and Irradiated Nuclear Graphite. Kenny Jolley and Pavlos Mouratidis also gratefully acknowledge funds from EDF energy generation 2016?2021. The authors gratefully acknowledge the use of Athena at HPC Midlands+, which was funded by the EPSRC grant EP/P020232/1 as part of the HPC Midlands + consortium. CE and AI acknowledge ANR-16-CE24-0008-01 ?EdgeFiller? and ANR-20-CE08-0026 ?OPIFCat? for funding. DE acknowledges support from the TUBITAK-2219 post-doctoral research abroad fund

Facile route to gold-graphene electrodes by exfoliation of natural graphite under electrochemical conditions

International audienceAn original and practical procedure for preparing graphene-modified gold electrodes has been developed by careful control of the electrochemical exfoliation process of natural graphite. Pre-exfoliated (intercalated) graphite is first mechanically deposited onto a gold electrode. A cathodic treatment is then performed in N,N-dimethylformamide containing tetraalkylammonium salts at potential lower than −2 V vs. Ag/AgCl. This erodes the carbon coating through exfoliation of graphite particles. The result of this simple process, as identified by Raman spectroscopy, is a highly stable carbon surface made of graphene sheets, that gives well reproducible voltammetric responses (both in potential and intensity). Confocal Raman microspectrometry demonstrates that the bonded graphene phase consists of either 1 or 3 layers over micron-sized areas of the gold substrate. These layers can then subsequently be used as a trap to fix different organic groups. This easy and highly reproducible process could greatly simplify gold-graphene electrode production in the field of electrochemistry of graphite and its parent compounds. © 2016 Elsevier Lt