  We have measured the electroluminescence and photoluminescence of (9,7)
semiconducting carbon nanotube devices and demonstrate that the
electroluminescence wavelength is determined by the nanotube's chiral index
(n,m). The devices were fabricated on Si3N4 membranes by dielectrophoretic
assembly of tubes from monochiral dispersion. Electrically driven (9,7) devices
exhibit a single Lorentzian shaped emission peak at 825 nm in the visible part
of the spectrum. The emission could be assigned to the excitonic E22 interband
transition by comparison of the electroluminescence spectra with corresponding
photoluminescence excitation maps. We show a linear dependence of the EL peak
width on the electrical current, and provide evidence for the inertness of
Si3N4 surfaces with respect to the nanotubes optical properties.
