Resonance Raman Spectroscopy of Single-Walled Carbon Nanotubes

The unusual one-dimensional properties of phonons in crystalline arrays of carbon nanotubes is presented. The main technique for probing the phonon spectra is Raman spectroscopy and the many unique and unusual features of the Raman spectra of carbon nanotubes are highlighted. Various features of the first-order Raman spectra are emphasized, with regard to their 1D behavior and special characteristics, such as the radial breathing mode, and the tangential G-band (1600cm-') associated with carbon atom displacements on the cylindrical shell of the nanotube (C-C stretching motion of the atoms). The strong coupling between electrons and phonons in this one-dimensional system furthermore gives rise to highly unusual resonance Raman spectra, and unique features in the Stokes and anti-Stokes Raman spectra. The Raman tangential G-band feature associated with semiconducting nanotubes have a different characteristic lineshape than those associated with metallic carbon nanotubes. The differences in the electronic density of states of metallic nanotubes relative to semiconducting nanotubes leads to differing resonance behaviors, thus resulting in differing lineshapes in the tangential G-band region of the Raman spectrum. A diameter selective resonance process allows resonant enhancement of the Raman tangential G-band for the metallic nanotubes in a narrow range of laser excitation energies for a sample of nanotubes with a narrow distribution of diameters. The anti-Stokes Raman spectra of single-wall carbon nanotubes (SWNTs) are unique relative to other crystalline systems, especially in exhibiting large asymmetries with regard to their corresponding Stokes spectra. This asymmetry is due to the unique resonant enhancement phenomena arising from their one-dimensional electronic (1D) density of states. The anti-Stokes spectra are therefore selective of specific carbon nanotubes, as previously reported for the Stokes spectra, but the anti-Stokes spectra are selective of different single wall nanotubes than for the corresponding Stokes spectra at a given laser excitation energy Elaser. The unique behavior of the anti-Stokes spectra for the first-order tangential modes, which allow accurate identification of the range of Easer where metallic nanotubes contribute to the resonant Raman spectra, is discussed. A detailed Breit-Wigner-Fano lineshape analysis of the