ABSTRACT
In a PL map, one can generally find some peaks other than bright PL peaks assigned to E22 absorption and E11 emission of particular nanotube species allowed for incident light polarized parallel to the nanotube axis. Figure 1.2 shows the typical spectral features of (7,5) SWNTs. First, one can find a broad absorption peak in the shorter excitation wavelength side. Second, one can also find a small absorption peak in the longer emission wavelength side. In addition, for a fixed excitation wavelength, there is a weak emission peak (or peaks) in the longer emission wavelength side. The physical origins of these peaks have been recently demonstrated as, respectively, excitonic phonon sideband in absorption10-13, absorption peak for perpendicularly polarized light to the nanotube axis14,15, excitonic phonon sideband in emission16,17, and/or emission from triplet exciton states.18,19 Recently, there also have been reports on the direct observation of nominally dark excitons for cross-polarized excitons to the nanotube axis (transverse excitons).20 Details of these important features will be discussed in the following sections.
