The effects of high energy probe sonication on the thermoelectric power of large diameter multiwalled carbon nanotubes synthesized by chemical vapor deposition

The thermoelectric properties of large diameter multiwalled carbon nanotubes synthesized by chemical vapor deposition have been measured after successively longer periods of high energy probe sonication. Electrical conductivity decreases with increased sonication time due to shortening of the nanotubes. The initial Seebeck coefficient before sonication is −2.5 μV/K, but increases to 12 μV/K for sonication periods longer than 12 h. This effect is attributed to the introduction of defects and shortening of the nanotubes during sonication which exposes previously shielded carbon from internal walls to oxygen doping from exposure to atmosphere which results in increased p-type doping and a positive thermoelectric power. Temperature dependent measurements indicate that this oxygen doping of the exposed nanotube ends has a thermally activated charge transfer energy since low temperature thermoelectric power measurements of the sonicated samples approach that of the raw n-type nanotubes.