Photoexcited electron relaxation processes in single-wall carbon nanotubes

The relaxation processes for photoexcited electrons in single-wall carbon nanotubes (SWNTs) are discussed in connection with recent photoluminescence excitation (PLE) spectroscopy experiments. The electron-phonon $\left(e\text{−}\mathrm{ph}\right)$ interaction of SWNTs is calculated for each phonon mode as a function of photoexcited electron energy. Because of the cylindrical surface of a SWNT, the twisting and radial breathing phonon modes are responsible for the $e\text{−}\mathrm{ph}$ interaction in the lower phonon energy region. Optic phonon modes are also responsible for the relaxation of photoexcited electrons. Compared with graphite, the relaxation of electrons is much faster in SWNTs because of their cylindrical shape. We identify and explain some aspects of both photoluminescent and Raman processes appearing in PLE plots for SWNTs.