Carbon Nanotube Photophysics
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Carbon Nanotube Photophysics
A. Jorio, R. Saito, T. Hertel, R.B. Weisman, G. Dresselhaus, and M.S. Dresselhaus Abstract In single-walled carbon nanotubes (SWNTs), their electronic and vibrational structure as well as their charge-carrier dynamics are crucial for potential ultrasmall optical device applications. SWNT properties have now been obtained from optical absorption and time-resolved photoemission and, at the single-nanotube level, by resonance Raman scattering and photoluminescence studies. This article presents an overview of SWNT photophysics, discussing important findings for the characterization of carbon nanotube properties and directions for future research and potential applications. The unique optical properties observed in SWNTs are due to the one-dimensional confinement of electronic states, resulting in van Hove singularities in the nanotube density of states. Optical measurements of phonons, charge-carrier dynamics, and the electronic transition energy van Hove singularities are discussed. Keywords: carbon nanotubes, fast optics, infrared spectroscopy, photoluminescence, Raman spectroscopy.
Introduction Single-walled carbon nanotubes (SWNTs) are tubules with diameters dt on the nanometer scale and lengths reaching the micrometer range; they are the best-known prototype for one-dimensional materials. Photons, having no mass and no charge, are valuable probes for the study and characterization of the structure of SWNTs. This article presents an overview of the photophysics of carbon nanotubes, discussing how the one-dimensionality gives rise to unique optical properties, making SWNTs promising for ultrasmall optical device applications. For example, electrochemically induced bandgap luminescence1 and photoconductivity2 have been observed, the optical effects being observable at the single isolated nanotube level. The unique optical properties of SWNTs are due to the one-dimensional (1D) confinement of electronic states, resulting in so-called van Hove singularities in the nanotube density of states (DOS).3–4 These singularities in the DOS, and correspondingly in the joint density of states (JDOS), are of great relevance for a variety of optical phenomena. Whenever the energy of incident photons matches a van Hove singularity in the JDOS of the valence and conduction bands (subject to selection 276
rules for optical transitions), one expects to find resonant enhancement of the corresponding photophysical process. Owing to the diverging character of van Hove singularities in these 1D systems, such enhancement can be extremely confined in energy, appearing almost as if transitions in a molecular system are excited. In combination with this unique 1D electronic structure, strong electron– phonon coupling in resonance Raman scattering experiments allows one to obtain detailed information about vibrational properties of nanotubes, even at the isolated SWNT level.5 A variety of optical techniques, such as absorption measurements, resonance Raman and infrared spectroscopy, fast optics, and photolumines
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