Single-walled carbon nanotubes show stable emission and simple photoluminescence spectra with weak excitation sidebands at cryogenic temperatures

Low-temperature microphotoluminescence (PL) maps were measured over a broad range of emission and excitation wavelengths for various samples of individual single-walled carbon nanotubes (SWNTs). These included nanotubes produced by chemical vapor deposition which were probed either in direct contact with a sapphire substrate or suspended between supports (on top of a carbon nanotube ``forest''). Whereas surfactant-coated SWNTs manifest PL intermittency and spectral diffusion, the surfactant-free samples show stable PL over minute time scales at temperatures down to $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This indicates that PL intermittency and spectral diffusion are not intrinsic properties of SWNTs, but can be caused by interactions with a surfactant coating. We also demonstrate that interactions between spatially close (presumably contacting) nanotubes may have contributed to the complexity of previously reported low-temperature PL spectra. Apart from such multipeak PL maps characterized by intense excitation sidebands, we also observed simple single-peak PL maps which can be attributed to only one SWNT within the laser excitation spot. In addition, we compare typical PL intensity (quantum efficiency) and structural assignment of PL peaks for the various types of samples studied.