Abstract:
Single walled carbon nanotubes (SWNTs) are single-graphite-layer helix cylinders with diameters of nanometer size and lengths up to few centimeters. With such a high ratio of length to diameter, SWNTs are generally regarded as quasi-one-dimensional (1D) system. Distinguished from other quantum wires, SWNTs present no surface states since all the carbon bonds are saturated and no dangling bonds are present. So SWNTs offer an ideal platform for studying physics in 1D system. Coulomb screening effect has been a widely discussed topic and is generally weak in 1D system like SWNTs due to the reduced dimensionality. Consequently excitons, bound states of electrons and holes due to strong Coulomb interaction, play an important role in optical transitions of SWNTs. It has been revealed by a number of theoretical work and independent experiments that the excitonic component is dominant in the optical transitions of SWNTs and the exciton binding energy could be as large as a fraction of the band gap, ranging from several tens meV to almost one eV, depending on their diameters, chiralities and environmental dielectric constants.
Excitonic nature of optical transitions in individual SWNTs were investigated by Rayleigh-scattering based reflectance spectroscopy in this study. Back scattering spectroscopic measurements, including both Raman- and Rayleigh-scattering based reflectance spectra, on individual single walled carbon nanotubes were carried out. Individual SWNTs were identified by the resonant Raman spectroscopy (RRS) combined with the Rayleigh-scattering based reflectance spectra. The well-defined single peaks with nearly symmetrical line-shape were observed in reflectance spectrum, indicating the excitonic origins of the optical transitions in SWNTs other than the free band-to-band transition. With the help of high intensity of the supercontinuum white light source, a phonon sideband separated by 0.19 eV from the first optical transition peak was observed. This provides further evidence of the existence of excitons in SWNTs, and also offers an opportunity to study the exciton-phonon coupling strength in 1D. By fitting to the reflectance spectrum, a static dielectric constant of 10 for the SWNT was estimated.