Trions detected in CNTs

An exciton is a bound pair of one electron and one hole, analogous to a hydrogen atom, but with much smaller binding energy. It can be excited in many semiconductors by light. An electron in valence band can be lifted by a photon to the conduction band and in the meanwhile a hole-quasiparticle- will be left behind. This electron shall attract with the hole due to electrostatic interaction and make an exciton. A trion is like an ionized hydrogen molecule, containing one electron and two holes. Such objects are of course less stable than an exciton, but have been observed. Now some scientists from Kyoto University observed these trions in carbon nanotubes. They seem much stable with binding energy of a tenth of an electron volt [Phys. Rev. Lett. 106, 037404 (2011) ]. A view point can be found here [Physics 4, 5 (2011)] :

The smoking gun of the trions in the experiment of Matsunaga et al. is the appearance of an additional peak in the optical spectra on the low-energy end of the exciton peak. Matsunaga et al. work hard to show that this peak indeed originates from trions and not, for example, from defects introduced through doping. They first investigate the influence of different dopants and doping concentrations and find that new peaks associated with trions in nanotubes appear at the same energy, regardless of the dopant species. They also become stronger with increasing doping concentration, along with a reduction of the exciton peak. Moreover, they find that excitons in nanotubes with different diameters and twist angles all come along with a corresponding trion partner and show clear “family patterns,” similar to those known for excitons in nanotubes. All this provides strong evidence for trions.