Charge transport mechanisms in macro-scale CNT films

Carbon nanotubes (CNT) attract considerable attention due to their unique physical properties and potential application in optoelectronics. Despite of intensive studies there is still a lack of agreement in experimental data on electrical properties of the material. Here we report on extremely broad-band conductivity and dielectric permittivity spectra of macro-scale thin films composed of large number of randomly distributed pristine and p-doped CNTs of different length, measured in the frequency range 5-24 000 cm-1 and at temperatures from 5 to 300 K. We show that terahertz-infrared spectra of the films are determined by response of delocalized charge carriers. Controversially to the existing experimental results we did not clearly observe the so-called terahertz conductivity peak. Yet, a weak bump-like feature in conductivity spectra around 30 cm-1 showed no signs of tube length dependence. We associate its origin with plasmonic excitation due to reflections of charge carrier plasma at the CNT intersections. Applying the Drude-model to describe the low frequency conductivity and dielectric permittivity spectra of CNT films we obtained effective values of carries parameters. Our results can shed light on electromagnetic waves absorption mechanisms and will be useful while designing new CNT-based devices.