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.
Investigations on Raman bands from carbon linear chains in multiwalled carbon nanotubes
Castriota M;
2008
Abstract
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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
