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Ultrafast Nonlinear Spectroscopy of Semiconducting Carbon Nanotubes

  • Author(s): Graham, Matthew Werden
  • Advisor(s): Fleming, Graham R
  • et al.

Single-walled carbon nanotubes (SWNTs) are often considered the prototypical nanomaterial with their characteristic large aspect ratios, high tensile strength and size tunable physical properties. While their remarkable electrical and mechanical properties have been well studied, the optical properties of semiconducting SWNTs are continuously emerging with recently reported developments such as greatly enhanced fluorescence yields and highly efficient nanotube photodiodes. The scope of optoelectronic applications for SWNTs depends critically on a detailed understanding of their highly congested absorption features consisting of strongly bound excitonic (E11, E22, etc.), vibronic and dark state transitions. Until recently, carbon nanotubes consisted of highly inhomogeneous distributions of tubes types that made advanced spectroscopic studies impractical. Enabled by samples highly enriched in specific tube types, we investigate carbon nanotubes photophysics in real-time by applying a wide range of techniques including transient absorption, photon echo and 2D Fourier transform spectroscopy. Through detailed analysis and simulation of these femtosecond nonlinear results, fundamental properties concerning excitonic structure, exciton-phonon interactions, and the dynamics of coherent excitons for these quasi one-dimensional nanomaterials are revealed.

Population relaxation dynamics in SWNTs are probed from the visible to mid-infrared regions using transient absorption or grating spectroscopy. Measurements in mid-IR region reveal a broad excited state absorption feature consistent with previously unobserved intra-excitonic transitions and bound vibrational states. Simulation of visible and near-IR transient absorption results suggest efficient exciton-exciton annihilation occurring by both one-dimensional diffusion or delocalized overlap of coherent excitons depending upon the timescale, state transitions, or temperature investigated (4.4 to 292 K). Throughout all the measurements reported, strong contributions from exciton-exciton scattering processes are evident.

The timescales for pure optical dephasing (and the corresponding homogeneous lineshape) were obtained through photon echo measurements. Surprisingly long room temperature dephasing times of 285 fs are reported, suggesting unusually weak exciton-phonon coupling and an inhomogeneously broadened absorption lineshape. At lower temperatures the dephasing induced by SWNT optical phonons is mitigated, and we show how acoustic phonon processes and environmental disorder determines the absorption line-shape properties.

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