Resonance Raman spectroscopy of single-walled carbon nanotubes

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2000.Includes bibliographical references (leaves 169-180).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 tangential G-band features attributable to metallic carbon nanotubes is presented. Only two components are needed to account for the entire G-band, both with predominantly A (Alg) symmetry, and the nanotube curvature callses the differences in their frequencies and gives rise to the Breit-Wigner-Fano coupling. Analysis of the second-order resonant Raman spectra of single-walled carbon nanotubes using different laser energies ill the range 1.58-2.71 eV is presented. Major emphasis is given to the overtones and combination modes associated with the two dominant features of the first-order spectra, the radial breathing mode and the tangential mode. Both of these modes, as well as their second-order counterparts, are associated with resonant enhancement phenomena arising from electron-phonon coupling to the unique one-dimensional density of electronic states for the single-wall carbon nanotubes. Overtones, combination modes, and the behavior of the D band and G' band in the Stokes and anti-Stokes spectra are also discussed briefly. Comparison between the Stokes and anti-Stokes spectra show that the resonance Raman process is stronger for metallic than for semiconducting nanotubes. The surface-enhanced resonant Raman scattering (SERRS) spectra of single-walled carbon nanotubes (SNWNTs) adsorbed on silver and gold metal island films and on colloidal silver cluster substrates were investigated using different laser excitation wavelengths. The observed enhancement in the SERRS signal of the SWNTs results from: (1) an "electromagnetic" SERS enhancement due to resonances between optical fields and the electronic excitations in the metallic nanostructures, (2) a "'chemical" SERS enhancement due to the interaction between the SNTs an(l the metal surfaces, and (3) a selective resonance Ramrran effect between the incidellt and scattered photons and electronic transitions between the D van Hove singularities in the electronic density of states of metallic and semiconducting nanotubes. We have observed changes in the relative intensities and shifts in the peak freqllencies of several vibrational modes of the SWNTs upon adsorption on a metal sulrface. which indicate a specific interaction of the nanotubes with the mnetals urf:ace. Chllangesi n the resonant Raman spectra due to interaction with the silver or gold surfaces are al)parent in the second-order Raman bands, especially in the (iislpersive features. such as the secondor( lder Raman G' band, which ulpshifts in the SERRS spectra relative to the resonant Raman scattering (RRS) spectra, providing evidence of a, significant perturbation of the elec(tronic levels for the adsorbed( nanotlbes. In addition, tlhe SER spectra show an additional enhancement of the Ramlan signal for slpecific featulres in the vibrational spectra of the metallic( nanotlbes, in contrast to the case for the sermlicon(lIictinInga notllbes for which the normal RRS an(l SERRS spectral profiles are very similar. These results can be rationalized in terms of a specific charge--transfer enhanc(ement effect for the metallic nanotllbes. The srface-enhanced Raman spectroscopy stlludies show that the coupling, which results in the Breit---Wigner- Fano lineslhape of soni of the R.anlma features associated with metallic nanotubes, is to a surface plasmon based electronic continuum.by Sandra Dawn Marie Brown.Ph.D