Evidence for charge transfer in doped carbon nanotube bundles from Raman scattering

Single-walled carbon nanotubes(1) (SWNTs) are predicted to be metallic for certain diameters and pitches of the twisted graphene ribbons that make up their walls(2). Chemical doping is expected to substantially increase the density of free charge carriers and thereby enhance the electrical (and thermal) conductivity. Here we use Raman spectroscopy to study the effects of exposing SWNT bundles(1) to typical electron-donor (potassium, rubidium) and electron-acceptor (iodine, bromine) dopants. We find that the high-frequency tangential vibrational modes of the carbon atoms in the SWNTs shift substantially to lower (for K, Rb) or higher (for Br-2) frequencies. Little change is seen for I-2 doping. These shifts provide evidence for charge transfer between the dopants and the nanotubes, indicating an ionic character of the doped samples. This, together with conductivity measurements(3), suggests that doping does increase the carrier concentration of the SWNT bundles.