Resonant Raman Effect in Single-wall Carbon Nanotubes
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Resonant Raman effect in single-wall carbon nanotubes M. A. Pimenta,a) A. Marucci, S. D. M. Brown, and M. J. Matthews Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
A. M. Rao and P. C. Eklund Department of Physics and Astronomy and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506
R. E. Smalley Department of Chemistry and Center for Nanoscale Technology & Science, Rice University, Houston, Texas 77005
G. Dresselhaus Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
M. S. Dresselhaus Department of Electrical Engineering and Computer Science and Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 3 March 1998; accepted 7 May 1998)
A resonant Raman study of single-wall carbon nanotubes (SWNT) using several laser lines between 0.94 and 3.05 eV is presented. A detailed lineshape analysis shows that the bands associated with the nanotube radial breathing mode are composed of a sum of individual peaks whose relative intensities depend strongly on the laser energy, in agreement with prior work. On the other hand, the shape of the Raman bands associated with the tangential C–C stretching motions in the 1500–1600 cm21 range does not depend significantly on the laser energy for laser excitation energies in the ranges 0.94–1.59 eV and 2.41–3.05 eV. However, new C –C stretching modes are observed in the spectra collected using laser excitations with energies close to 1.9 eV. The new results are discussed in terms of the difference between the 1D electronic density of states for the semiconducting and metallic carbon nanotubes.
I. INTRODUCTION
Since the discovery of carbon nanotubes in the early 1990s,1 theoretical work has shown that various physical properties should be strongly dependent on the nanotube diameter and chirality.2–4 This dependence is ultimately related to the way that the two-dimensional Brillouin zone of the graphene sheet is folded into the onedimensional Brillouin zone of the carbon nanotubes.5 Due to the large length-to-diameter ratio for the nanotubes, the electron motion is normally confined to the tube axis, and therefore, for small diameter nanotubes, the one-dimensional (1D) density of electronic states exhibits sharp singularities below and above the Fermi level EF as shown in Fig. 1. The carbon nanotube can be metallic or semiconducting, depending on the magnitude and direction of its chiral vector. As shown in Fig. 1, the structures of the electronic density of states (DOS) for semiconducting and metallic nanotubes are significantly
a)
Permanent address: Departamento de Fisica, Universidade Federal de Minas Gerais, Belo Horizonte, 30123-970 Brazil. e-mail: [email protected]
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J. Mater. Res., Vol. 13, No. 9, Sep 1998
Downloaded: 16 Jul 2014
different. Within each category, there are singularities in the 1D density of sta
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