Electrical and thermal transport in carbon nanotubes: Improved nanotube-based materials and thermoelectric power in the Coulomb blockade regime

We have studied the electrical and thermal properties of single wall carbon nanotubes (SWNTs) both at the bulk and single molecule level. The first group of experiments aims to verify the zone-folding model for the thermal conductivity κ(T) of SWNTs. Because of the geometry of SWNTs, the phonon (as well as electronic) states are quantized. This quantization leads to an observed linear behavior of κ(T) below 40 K. By varying the average diameters of bulk samples, the temperature at which higher phonon subbands contribute shifts, further evidence for phonon quantization. The second set of experiments are aimed at studying the transport properties of nanotube based materials. These include highly aligned SWNT films; SWNT-epoxy composites and nanotube fibers. We observed improvement in κ of these materials compared to disordered films and pristine epoxy. We have also characterized the electrical and thermal transport in C60-filled SWNTs and have found that at high temperatures, their thermoelectric power (TEP) does not become negative compared to unfilled material. This behavior is proposed to be due to the C60 balls preventing oxygen from leaving the interior of the tubes. The last set of experiments focuses on the first measurements of the TEP of SWNT bundles and individual SWNTs. We present a novel device we have developed that allows us to measure the TEP directly of single molecules. Our first experiments on bundles reveal p-type behavior and a temperature dependence that is very similar to air-exposed bulk samples. We surmise that inter-rope contacts are not important in the total TEP of SWNT films. At the single molecule level, the TEP of a semiconducting nanotube has been measured in the Coulomb blockade regime. We observe TEP oscillations, the amplitude and period of which are consistent with the theory of the TEP of quantum dots. Also, as the tube is depleted of carriers, the oscillations grow larger and are centered around an offset value of the TEP. We attribute the offset to defects on the tube and the Schottky barriers at the contacts which contribute a negative thermopower. From the TEP offset, we are also able to estimate the size of the depleted regions in the nanotube