Inductive coupled rf-plasma at 13.56 MHz was used to modify multiwalled carbon nanotubes (MWCNTs). This technique can be easily used to tailor the chemical composition of carbon nanotubes by attaching a wide variety of functional groups at their surface: oxygen-, nitrogen-, and fluorine-containing groups have been grafted. The influence of various plasma conditions (power, type of gas, treatment time, pressure, position of the CNT sample inside the chamber) on the functionalization of the MWCNT surface was analyzed by x-ray photoelectron spectroscopy. The results show that for too high oxygen plasma power, chemical etching occurs at the surface of the CNT, thus destroying its structure. On the other hand, for optimal values of the plasma parameters, functional groups (hydroxide, carbonyl, carboxyl, amine, fluorine, etc.) were found to bond to the CNT surface, suggesting that both the concentration and type of the functional groups are in close connection with the plasma conditions. These results were compared to interaction energies predicted by ab initio calculations for different functional groups under consideration, showing that functionalization by oxygen plasma produces mainly functional groups with lower interaction energy.

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We analyse the electronic structure and aromaticity of graphene nanoribbons and carbon nanotubes through a series of delocalisation and geometry analysis methods. In particular, the six-centre index (SCI) is found to be in good agreement with the mean bond length (MBL) and ring bond dispersion (RBD) geometry descriptors. Based on DFT periodic calculations, three distinct classes of aromaticity patterns have been found for armchair graphene nanoribbons, appearing periodically as the width of the ribbon is increased. The periodicity in the band gap is found to be related to these aromaticity patterns. Also, the appearance of such distinct aromaticity distribution is explained within the framework of the Clar's sextet theory. Both delocalisation and geometry analysis methods are shown to be very fast and reliable tools for easily analysing the aromaticity in carbon nanosystems.

We study the resonance behavior of the unusual Raman feature known as the coalescence-inducing mode ͑CIM͒, observed at ϳ1850 cm −1 , in samples of double-wall carbon nanotubes annealed at high temperatures. Resonance Raman spectra taken with different laser energies show that the intensity of the CIM band exhibits a maximum around 2.20 eV. By comparing the resonance Raman experimental results with first-principles calculations for the vibrational frequency and the energy gap, we propose that the CIM feature is associated with short carbon chains with an odd number of atoms, interconnecting the nanotube surfaces. DOI: 10.1103/PhysRevB.73.193408 PACS number͑s͒: 78.30.Na, 63.22.ϩm, 78.20.Bh, 78.66.Tr The Raman spectra of carbon nanotubes exhibit weak features in the spectral range between 1600 and 2000 cm −1 that are ascribed to a second-order Raman process involving the combination of two phonons.1,2 However, the observation of unusual and strong spectral features around 1850 cm −1 have been reported recently in the Raman spectra of carbon nanotube systems, [3][4][5] and have been ascribed to the vibration of one-dimensional ͑1D͒ chains of carbon atoms. In particular, a strong and sharp feature at ϳ1850 cm −1 is observed in the Raman spectra of double-wall nanotube ͑DWNT͒ samples heat treated at high temperatures and, since it appears at specific annealing temperatures ͑T htt ͒ that occur just below the T htt needed for full coalescence of DWNTs, it is named the coalescence-inducing mode ͑CIM͒.To characterize this unusual Raman feature at ϳ1850 cm −1 , we report a complete resonance Raman study of this phonon band, using many different laser excitation energies ͑E laser ͒. The intensity dependence of the CIM band on E laser is here determined, and is shown to exhibit a maximum at around 2.2 eV. The experimental results are compared with theoretical calculations, using either density functional theory ͑DFT͒ or Hartree-Fock ͑HF͒ approaches, for finite carbon chains containing a small number of carbon atoms for which the normal mode frequencies and energy gaps were obtained. According to the calculations presented here, the vibrational frequencies and the resonance behavior obtained experimentally are in close agreement with the formation of short linear carbon chains during the coalescence process, with the chains having an odd number of carbon atoms fixed to the carbon nanotube walls under different environments ͑deformed and twisted chains͒.Room-temperature Raman spectra were recorded in the backscattering configuration using a Dilor XY triple monochromator, equipped with a cooled charge-coupled device ͑CCD͒ and using several different laser line excitations from an Ar-Kr ion laser and a dye laser in the range 1.9-2.7 eV. A laser power Ͻ1 mW was focused on a ϳ2 m 2 spot during the measurements.The samples studied here consist of highly purified DWNT bundles synthesized by a catalytic chemical vapor deposition method. 6,7 The diameter distribution of the sample is 0.77Յ d t Յ 0.90 nm for the inner tubes and 1.4...

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