Far-infrared gaps in single-wall carbon nanotubes

The infrared properties of single-wall carbon nanotubes have been measured over 15-5000 cm(-1). The sample is a freestanding film composed of purified nanotubes. The temperature dependence of the reflectance is quite small at all temperatures studied (7.8 to 298 K) and no vibrational features are ob served. The spectrum has a clear plasma edge, analyzed by a Drude model com bined with a low-frequency localized absorption at 135 cm(-1). The absorpti on band is assigned to a pseudogap in armchair nanotubes and/or to very sma ll gaps in zigzag or chiral nanotubes. [S0163-1829(99)50640-1].