Optical excitations of quasi-one-dimensional systems: carbon nanotubes versus polymers and semiconductor wires

We review the main characteristics of optical excitations of semiconductor nanotubes, as obtained from accurate ab-initio theories and model calculations as well as experimental evidence, and discuss them in light of the previous understanding of other quasi-one-dimensional semiconducting systems. We point out striking similarities of nanotubes with III-V quantum wires and conjugated polymers, especially (i) the clear excitonic nature of absorption, very far from the single-particle behaviour; (ii) its manifestations in optical spectra, where excitonic peaks are accompanied by a strong intensity reduction at the onset of the free-particle continuum; (iii) the strategies that allow experimental access to exciton binding energies. The recent theoretical and experimental evidence obtained on semiconducting single-walled nanotubes converges quantitatively to a picture of strongly bound excitons (about 0.3-1.0 eV for nanotubes with 0.4-1.0 nm diameter). We discuss its implications and list a few open issues of relevance to fundamental understanding and optoelectronic applications. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.