Transport properties of single wall carbon nanotubes
In this thesis, we study the properties of macroscopic samples of single walled carbon nanotubes (SWNT) in the form of buckypapers and fibers, and how these properties are affected by doping, alignment and C60 filling. A metal-insulator (MI) transition in macroscopic SWNT conductors is revealed by systematic measurements of resistivity, ρ(T), and transverse magnetoresistance, MR(B), as a function of p-type redox doping. In the insulating regime, the ρ( T) can be described by variable-range hopping (VRH) mechanism. The MR(B) and ρ(T) of strongly doped samples are best described by a weak localization (WL) model. The 1D character of the electronic and phonon spectra becomes apparent in the thermopower (TEP) of strongly doped samples. In contrast to weakly doped samples, TEP is small at the lowest temperatures, and then increases at a characteristic temperature which depends on the position of the Fermi energy. We attribute this unusual behavior to 1D phonon drag, in which the electron-phonon interaction is restricted to phonons with the appropriate wave-vector. This idea is supported by a model calculation in which the low T behavior of phonon drag is specifically related to the one-dimensional character of the electronic spectrum. TEP data are correlated with optical reflectivity, Raman spectroscopy, mass loss and resistivity measurements. We performed the careful dedoping of H2SO4 doped samples and in situ measurements of the temperature dependence of TEP. It is found that the effect of H2SO4 doping is to change the Fermi energy and not to introduce new features into DOS of SWNTs. A model is developed which can explain some of the features observed in the diffusion TEP. Filling SWNTs with C60 molecules does not have a considerable effect on p and TEP of macroscopic samples and only very weak effect on heat capacity and thermal conductivity is expected. A possibility of observing cluster dynamics using diffraction methods is discussed. We show that the conduction mechanism in bulk SWNT samples is not affected by a partial nanotube alignment. The effect of alignment on ρ can be described by a proportionality factor, which depends on the number of rope-rope contacts along the most probable conduction path.
Vavro, Juraj, "Transport properties of single wall carbon nanotubes" (2004). Dissertations available from ProQuest. AAI3138084.