Photophysics of carbon nanotubes

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (leaves 213-233).This thesis reviews the recent advances made in optical studies of single-wall carbon nanotubes. Studying the electronic and vibrational properties of carbon nanotubes, we find that carbon nanotubes less than 1 nm in diameter exhibit dramatic changes in their electron and phonon dispersion relations due to the curvature of the nanotube sidewall and the enhanced electron correlation effects associated with one dimensionality. The optical transition energies in small-diameter carbon nanotubes show a strong dependence on their geometrical structure, as was first observed in the photoluminescence experiments. The frequencies of the Raman-active phonon modes also become very sensitive to the geometrical structure of small-diameter carbon nanotubes. In particular, certain phonon modes exhibit anomalous behavior that significantly affects resonance Raman spectra of small-diameter carbon nanotubes. We have developed the extended tight-binding and advanced force-constant models that properly take into account the curvature effects in the small-diameter limit. The many-body corrections are fitted to the photoluminescence and resonance Raman spectroscopy data.(cont.) The resulting extended tight-binding model with semiempirical many-body corrections shows a good agreement with the experimental results. The electron-photon and electron-phonon transition matrix elements are calculated within the framework of the extended tight-binding model. Finally, the photoluminescence and Raman intensities in the graphene sheet and carbon nanotubes are calculated. The calculated intensities show a reasonable agreement with the experimental results and allow structural characterization of carbon nanotubes by their spectroscopic signatures.by Georgii G. Samsonidze.Ph.D

Fermi-Energy-Dependent Structural Deformation of Chiral Single-Wall Carbon Nanotubes

In this work, we use an extended tight-binding approach for calculating the Fermi-energy dependence of the structural deformation of chiral single-wall carbon nanotubes (SWNTs). We show that, in general, nanotube strains occur in such a way as to avoid a net charge from being accumulated on the nanotube. We also investigate the effect of the Fermi-energy-induced strains on the electronic structure of SWNTs, showing that the optical transition energies change by up to 0.5 eV due to the induced strains and that this change is nearly independent of how the nanotube is deformed. Finally, we also consider the contribution of the electron-electron Coulomb repulsion to the total energy by using an effective regularized potential energy model. We show that the inclusion of the Coulomb repulsion leads to larger strains and smaller net charges transferred to the nanotube.National Science Foundation (U.S.) (Grant DMR-1004147

Selection Rules for One- and Two-Photon Absorption by Excitons in Carbon Nanotubes

Recent optical absorption/emission experiments showed that the lower energy optical transitions in carbon nanotubes are excitonic in nature, as predicted by theory. These experiments were based on the symmetry aspects of free electron-hole states and bound excitonic states. The present work shows, however, that group theory does not predict the selection rules needed to explain the two photon experiments. We obtain the symmetries and selection rules for the optical transitions of excitons in single-wall carbon nanotubes within the approach of the group of the wavevector, thus providing important information for the interpretation of theoretical and experimental optical spectra of these materials.Comment: 4 pages, 1 figure, 1 tabl