Thermoelectric power of single-walled carbon nanotube films

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 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 T120 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 O₂^- 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 S0 state over the range 4 K<T<500 K. Model calculations of the TEP are presented which show that the small TEP is identified with the near mirror symmetry bands in metallic tubes and that the Fermi energy of the rope is determined by the balance of acceptor and donor states on the semiconducting tubes. A broad low-T feature in the TEP with maxima in the range 80 K<T<100 K is tentatively identified with phonon drag.