The anomalous resonant behavior of the tangential Raman modes of carbon nanotubes has been studied in the critical region of laser energies 1.7-2.2 eV. The special enhancement of the Raman modes is explained by a model that takes into account the transition between the singularities in the one-dimensional density of electronic states for the metallic nanotubes and the distribution of diameters in the sample. The results agree with direct measurements of the electronic density of states for the metallic nanotubes and establish their association with the specially enhanced high frequency, first-order Raman modes. ͓S0163-1829͑98͒50848-X͔ Resonant Raman spectroscopy is a very useful tool for the characterization of the one-dimensional ͑1D͒ properties of carbon nanotubes. It has been used to study multiwall nanotubes ͑MWNT͒, 1 single-wall nanotubes ͑SWNT͒, 2-5 and was recently examined theoretically. 6 We show here evidence that special tangential phonon modes of metallic carbon nanotubes are enhanced in a narrow range of laser energies between 1.7 and 2.2 eV by electronic transitions between the first singularities in the 1D electronic density of states ͑DOS͒ in the valence and conduction bands v 1 →c 1 . This result establishes the association of the specially enhanced highfrequency, tangential modes with the metallic carbon nanotubes.The unique resonant behavior of the Raman spectra in SWNTs was first reported by Rao et al., 2 who showed that the shape and position of the Raman bands associated with the radial breathing mode ͑RBM͒, around 180 cm Ϫ1 were strongly dependent on the energy of the exciting laser E laser . This result was identified 2 with the tube diameter dependence of both the RBM frequency and the separation between the singularities in the valence and conduction bands of the 1D electronic DOS. It was also reported in this work that the shape of the Raman band associated with the tangential modes ͑between 1500-1600 cm Ϫ1 ͒ obtained with the laser energy E laser ϭ1.92 eV was qualitatively different from those recorded with either higher or lower E laser . Kasuya et al. 3 recently reported a similar result for a SWNT sample prepared by the arc discharge method. They observed anomalies in the intensity of the Raman bands and a dip in the optical transmittance spectrum near 1.8 eV, and discussed their results in terms of the existence of critical points ͑singularities͒ in the electronic DOS of carbon nanotubes.In a previous resonant Raman study of SWNTs, 4 we suggested that the change in the shape of the tangential Raman band obtained for E laser ϭ1.92 eV could be related to the optical transitions of the metallic nanotubes. We present here a detailed experimental study and a consistent explanation for the special resonance Raman scattering behavior of the highfrequency tangential displacement modes of SWNTs in the critical region between 1.7 and 2.2 eV. It is shown here how the spectra change from the relatively sharp bands observed for low and high energy laser lines to the broad bands observed for 1...

Surface enhancement factors of at least 10(12) for the Raman scattering of single-walled carbon nanotubes in contact with fractal silver colloidal clusters result in measuring very narrow Raman bands corresponding to the homogeneous linewidth of the tangential C-C stretching mode in semiconducting nanotubes. Normal and surface-enhanced Stokes and anti-Stokes Raman spectra are discussed in the framework of selective resonant Raman contributions of semiconducting or metallic nanotubes to the Stokes or anti-Stokes spectra, respectively, of the population of vibrational levels due to the extremely strong surface-enhanced Raman process, and of phonon-phonon interactions.

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An analysis of the Stokes and anti-Stokes resonant Raman spectra of single-walled carbon nanotubes ͑SWNT's͒ using different laser energies in the range 1.49рE laser р2.19 eV shows major differences in the tangential band spectra for the Stokes and anti-Stokes processes for selected E laser values. These differences arise from the differences in the one-dimensional density of electronic states for metallic and semiconducting nanotubes. This observation allows selection of E laser to excite only semiconducting or only metallic nanotubes in the Stokes and anti-Stokes spectra. RAPID COMMUNICATIONS R5140PRB 61 S. D. M. BROWN et al.

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