Thermoelectric power of single-walled carbon nanotube films

Abstract

We report measurements of the thermoelectric power (TEP) S and electrical resistance R of thin films of tangled ropes of single-walled carbon nanotubes. Experiments at 300 K under applied force normal to the plane of the film revealed a $\ensuremath{\sim}10--20 %$ decrease in the four-probe resistance, indicating that the contact resistance between ropes had improved. The TEP, however, was not affected by the applied force and the concomitant decrease in the contact resistance between ropes. A quasilinear, diffusion TEP is observed for temperatures $T\ensuremath{\gtrsim}120 \mathrm{K},$ indicating that the metallic tubes provide the dominant contribution. As the sample is heated in a vacuum (degassed), the TEP is observed to decrease with time and changes sign, retaining its metallic character. This behavior is explained as a consequence of the balance between charge ${\mathrm{O}}_{2}^{\ensuremath{-}\ensuremath{\delta}}$ species which can be removed by degassing and an unidentified donor state. In fact, by stopping the degassing process at the appropriate time, one can achieve a $S\ensuremath{\sim}0$ state over the range $4 \mathrm{K}