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Multiwalled carbon nanotubes (MWCNT) have been synthesized on graphite cathodes by arc discharge in He atmosphere, either by using a catalytic Ni-Cr mixture or without catalysts. A preliminary scanning electron microscopy (SEM) investigation evidences the presence of nanotubes. Raman peaks due to longitudinal optic (LO) modes of linear carbon chains with sp hybridization (carbyne), encapsulated inside the nanotubes, occur in the wavenumber range 1780-1870 cm(-1), with variable intensity and shapes, in various regions of the analysed samples, together with the typical bands of MWCNT. In some sample zones the Raman band of carbyne results very strong with respect to G band of the host MWCNT, about one order of magnitude higher than in other studies dealing with carbon chains inside nanotubes. The second order Raman scattering is also observed, with frequency values of 2LO overtones slightly lower than the exact doubling of one phonon peak. No evidence of two-phonon density of state is found for higher frequency, as expected on the basis of existing theoretical predictions. Finally, the temperature dependence of intensity and frequency of carbyne LO modes is studied up to about 1000 K, and their behaviours are consistent with a reversible change of the bond configuration from polyynic to cumulenic character. (c) 2008 Elsevier B.V. All rights reserved.

The present investigation concerns multiwalled carbon nanotubes synthesized on graphite cathodes by arc discharge in a He atmosphere, either with the insertion of a catalytic Ni-Cr mixture or without catalysts. The morphology of the deposited cathodes was investigated by SEM, while the amount of carbon chains inside multiwalled carbon nanotubes (C@MWCNT) in various regions of the deposited cathodes was revealed by a parallel micro-Raman study, through the analysis of the signal from the Raman bands generated by C@MWCNT in the range 1780-1870 cm(-1) (L bands). In samples obtained by using the catalyst, a very high concentration of linear carbon chains is found in some zones, as indicated by the intensity of the L band, which turns out remarkably stronger than the G band of the host nanotubes. In these zones, the second-order Raman scattering is clearly observed, too. The experimental wavenumber values of the 2 L overtone are slightly lower than the exact doubling of the one-phonon peak wavenumber, and this fact is discussed in terms of the existing theoretical predictions for the chain-mode dispersion curve. Copyright (C) 2008 John Wiley & Sons, Ltd.

The vibrational and electronic properties of hydrogen molecules adsorbed on single-wall carbon nanotubes were investigated by high-resolution electron-energy-loss spectroscopy. We found that the adsorption of H2 molecules is dissociative at room temperature and 2.6×10-2Pa. The C-H bond formed upon hydrogen adsorption has a vibrational energy equal to 360 meV, in agreement with the expected sp2-sp3 H-induced local rehybridization. The hydrogenation of carbon nanotubes also gives rise to significant changes for both the line shape and the excitation energy of the ? plasmon. Upon H adsorption, the plasmon shifts by 0.8 eV towards lower energies and, moreover, a weak feature rises up at 3.5 eV.