Hall effect in carbon nanotube thin films

We have investigated Hall coefficient and magnetoresistance in thin films of single-walled carbon nanotubes, prepared in four different ways. Hall voltages are linear for all samples in magnetic fields up to 6 T, and the measured carrier density lies in ∼1021–1022 cm−3. Whereas earlier Hall-effect experiments reported ∼1018–1019 cm−3 for the carrier density, our results are consistent with the theoretically predicted value of ∼1022 cm−3, calculated for the aligned metallic CNTs. The signs of the Hall coefficients are positive in general, indicating that majority carriers are holes in these films. In a nanotube film with the lowest conductivity, however, we find the Hall coefficient reverses the sign at low temperature around T = 15 K. The origin of the sign change is not clear. In strongly localized regime, the Hall effect can be anomalous.