Is the intrinsic thermoelectric power of carbon nanotubes positive?

The thermoelectric power (TEP) of single-walled carbon nanotubes (SWNTs) is extremely sensitive to gas exposure history. Samples exposed to air or oxy gen have an always positive TEP, suggestive of holelike carriers. However, at fixed temperature the TEP crosses zero and becomes progressively more ne gative as the SWNTs are stripped of oxygen. The time constant for oxygen ad sorption/desorption is strongly temperature dependent and ranges from secon ds to many days, leading to apparently "variable" TEP for a given sample at a given temperature. The saturated TEP can be accounted for within a model of strong oxygen doping of the semiconducting nanotubes.