Optical properties of nanotube bundles by photoluminescence excitation and absorption spectroscopy

Photoluminescence excitation (PLE) and absorption spectroscopy are applied to investigate the impact of bundle size on the optical properties of single wall carbon nanotube (SWNT) bundles in aqueous suspensions. The existence and gradual formation of small bundles happens in aqueous suspensions even after ultrasonication and ultracentrifugation. With time, the emission and absorption spectra show weaker intensities and broader spectral profiles, confirming the formation of bigger SWNT bundles. We detect new PLE features assigned to energy transfer from excitons and excitonic phonon sidebands of donors to eh11 excitons of acceptors in such suspensions. In addition, the photoluminescence intensity from large gap nanotube donors weakens, while that from the smaller gap acceptors increases because of the exciton energy transfer. Our results caution the use of photoluminescence to determine the abundance of different SWNT species in conventionally prepared aqueous suspensions. On the contrary, absorption measurements represent a more reliable technique to reveal such information.