Single-walled carbon nanotubes(1) (SWNTs) are predicted to be metallic
for certain diameters and pitches of the twisted graphene ribbons tha
t 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-d
onor (potassium, rubidium) and electron-acceptor (iodine, bromine) dop
ants. 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 dopin
g. These shifts provide evidence for charge transfer between the dopan
ts and the nanotubes, indicating an ionic character of the doped sampl
es. This, together with conductivity measurements(3), suggests that do
ping does increase the carrier concentration of the SWNT bundles.