Raman spectroscopy of metallic carbon nanotubes

Abstract

Metallic carbon nanotubes are one dimensional conductors that are both technologically promising for electronic applications, and scientifically interesting for studying the physics of low dimensional materials. In this thesis, we present a detailed study of the inelastic light scattering (Raman) spectrum of individual metallic carbon nanotubes, with a focus on the influence of electronic excitations and charged carriers. We have demonstrated that the frequency and linewidth of certain phonon modes of metallic carbon nanotubes depend strongly the Fermi energy, because they couple strongly to low lying electron hole pairs. Next, we report the first experimental observation of electronic Raman scattering in carbon nanotubes. This observation demonstrates that the same electron-hole pairs that participate in damping the optical phonons of metallic carbon nanotubes, may themselves scatter light, thus giving rise to an electronic Raman spectrum. An analysis of the Fermi level and laser energy dependence of the electronic Raman and phonon Raman contributions allows us to explain the asymmetric lineshape of the G-band phonon modes in terms of a Fano interference. In another experiment, we have shown that the charge-induced expansion and contraction of the the graphitic C-C bond length is different for metallic and semiconducting nanotubes. Finally, we have measured the Stokes and antiStokes intensities of the Raman modes in electrically contacted metallic nanotubes in order to determine their phonon populations during high-field electrical transport. The experiments reported here, have helped to clarify the origin of several features in the Raman spectra of metallic carbon nanotubes that have been heavily debated in recent years. These result also shed light on the way electronic excitations and charged carriers affect the physical properties of metallic carbon nanotubes.