Chiral-Selective Protection of Single-Walled Carbon Nanotube Photoluminescence by Surfactant Selection

We study the effects of adding H ₂O ₂ to acid-purified and unpurified single-walled carbon nanotubes (SWNTs) in aqueous suspensions using photoluminescence (PL) and optical absorption spectroscopies. The addition of H ₂O ₂ to suspensions of unpurified SWNTs results in a rapid (1-2 h) quenching of the photoluminescence from all tubes, whereas H ₂O ₂ addition to acid-purified SWNTs causes the nanotube PL to grow in intensity over a period of several days before decaying in a tube-specific manner that depends on the binding strength of the surfactant sheath. With the appropriate choice of surfactants, the PL for specific acid-purified SWNTs can be protected such that novel mid-gap and phonon-assisted absorption and emission transitions can be observed without the obscuring effects associated with emission from other nanotubes. The H ₂O ₂ treatment also results in a reduction of the high-energy absorption background that has been associated with either carbonaceous impurities or the SWNT π-plasmon oscillation. An understanding of the related mechanisms leads to a new method for separating nanotubes by type based on selective oxidation followed by selective precipitation. These findings offer the possibility of efficiently separating large quantities of nanotubes by chirality.