Carbon-nanotube photonics and optoelectronics

Carbon nanotubes (CNTs) are nearly ideal one-dimensional (1D) systems, with diameters of only 1-3 nm and lengths that can be on the scale of centimetres. Depending on the arrangement of the carbon-atom honeycomb structure with respect to their axis, CNTs can be direct bandgap semiconductors, or metals with nearly ballistic conduction. The excited states of semiconducting CNTs can be produced by either optical or electrical means and form strongly bound (with dissociation energies of around 0.5 eV), luminescent, 1D excitons. The single-atomic-layer structure makes the optical properties of CNTs especially sensitive to their environment and external fields, and this can be used to tune them. Here we review the nature and properties of CNT excited states, the optical and electrical mechanisms of their production, their radiative and non-radiative modes of decay, the role of external electric fields, and their possible technological use as nanometre-scale light sources, photodetectors and photovoltaic devices.