  We performed detailed photoluminescence (PL) spectroscopy studies of three
different types of single-walled carbon nanotubes (SWNTs) by using samples that
contain essentially only one chiral type of SWNT, (6,5), (7,5), or (10,5). The
observed PL spectra unambiguously show the existence of an emission sideband at
~ 145 meV below the lowest singlet excitonic (E11) level, whose identity and
origin are now under debate. We find that the energy separation between the E11
level and the sideband is almost independent of the SWNT diameter. Based on
this, we ascribe the origin of the observed sideband to coupling between
K-point LO phonons and dipole-forbidden dark excitons, as recently suggested
based on the measurement of (6,5) SWNTs.
