Structure and vibrational properties of single wail carbon nanotubes

J. L. Sauvajol, E. Anglaret, S. Rols, C. Joumct, C. Goze, P. Bemier, Groupc de Dynamique des Phases Condenstes (UMR 5581), Universite de Montpcllier II Place E. Bataillon, case courrier 026,34095 Montpellier cedex 05, France W.K. Maser, E. Muiioz, A.M. B&to, M.T. Martinez Institute de Carboquimica, CSIC, Zaragoza 50015, Spain G. Coddens Laboratoire Leon Brillouin (CEA/CNRS), CEN Saclay, 9 119 1 Gif sur Yvette Cedex, France A. J. Dianoux bstitut Laue-Langevin, 38042 Grenoble Cedex, France Abstract: We report on some Raman measurements of single wall carbon nanotubes (SWNT) prepared by laser ablation (LA) or electric arc discharge (EA). We correlate the Raman features with structural information and discuss the resonance behaviour of the Raman signal in terms of a distribution of tubes diameters and symmetries. Keywords: Single wall carbon nanotubes, Neutron diffraction, Raman scattering. 1. IhTRODUCTION The original and exciting properties (transport, mechanical, optical...) of single wall carbon nanotubes (SWNT) are known to be very dependent on both the structure of the tubes themselves and that of the crystalline-like bundles they usually form [ 11. The former has been investigated with microscopic [Z-4] or spectroscopic techniques 1.5-91. The structure of the bundle has been studied both by TEM [S] and by diffraction techniques (X- rays [4,8,10] or neutrons [l 11). Jn this paper, we review recent Raman results. We point out the correlations between the Raman profiles in the frequency range of the Ar, breathing modes and the structural information derived from the analysis of the neutron diffraction data [ 11,121. We discuss the resonance behaviour of the Raman spectra in terms of a distribution of tubes diameters and symmetries. A detailed presentation of this work is published elsewhere [9]. 2. EXPERIMENTAL SWNT were prepared by the laser ablation technique (LA) in Zaragoza [3] and by the electric arc discharge (EA) [4] in Montpellier. Some EA samples were ptitied by filtering a suspension of SWNT in a water/surfactant solution, following a process described in this issue [ 131. Micro Raman spectra were recorded in a back scattering configuration using a “Jobin-Yvon T64000” spectrometer (nitrogen-cooled charge couple device detector). The 5 14.5 run line of an Argon ion laser and the 647.1 run line of an ArgonlKripton laser were used as as the light sources. The intrumental linewidth was fixed at G cm-l. Several spectra recorded on different spots were summed to give a macroscopic Raman picture of the samples, 3. RAMAN RESULTS In fig 1, we compare the Raman spectra of the LA, EA and purified EA samples in the frequency range of Al, mode with two incident wavelengths. Significant differences can be observed between the different samples. The LA sample displays a broad bunch including numerous peaks. At least 9 peaks are required to fit the bunch. As the frequency of the peak depends (only) on the diameter of the tubes, this Raman profile should be assigned to a rather polydisperse sample. The relation between the tube diameters and the frequency of the Al, mode was estimated from calculations [5,6,14]. This allows to estimate the diameter distribution from the Raman and for LA sample, we find that it ranges from 1.2 to 1.7 nm. The (unusual) presence of large tubes in a significant amount is indicated by the lower frequency peaks in the bunch. This is confirmed by the unusual profile of the neutron diffraction spectrum for the same sample that can only be accounted for by considering a large distribution of tube diameters [ 11,12]. By contrast, the EA sample, and in a larger extend the purified EA sample display much more narrow Raman profiles in this frequency range. 7 and 5 lines, respectively, are sufficient to fit the data, This leads diameters distributions from 1.17 to 1.51 A and 1.12 1.32 for the EA purified EA, respectively. This is also agreement with our analysis of the neutron diffraction data for the EA sample that leads to a narrow distribution of tube diameters centered on the diameter of (10,lO) tubes, i.e. 13.8 A. One also observes significant differences in the spectra excited at 514.5 nm and at 647.1 run. This illustrates the well-known resonance behaviour of Raman scattering by SWNT [5,6,9]. The Ratnan signal is enhanced for a given tube when the incident