Well-aligned macroscopic fibers composed solely of single-walled carbon nanotubes (SWNTs) were produced by conventional spinning. Fuming sulfuric acid charges SWNTs and promotes their ordering into an aligned phase of individual mobile SWNTs surrounded by acid anions. This ordered dispersion was extruded via solution spinning into continuous lengths of macroscopic neat SWNT fibers. Such fibers possess interesting structural composition and physical properties.

The electronic properties of p-doped single-walled carbon nanotube ͑SWNT͒ bulk samples were studied by temperature-dependent resistivity and thermopower, optical reflectivity, and Raman spectroscopy. These all give consistent results for the Fermi level downshift ͑⌬E F ͒ induced by doping. We find ⌬E F Ϸ 0.35 eV and 0.50 eV for concentrated nitric and sulfuric acid doping respectively. With these values, the evolution of Raman spectra can be explained by variations in the resonance condition as E F moves down into the valence band. Furthermore, we find no evidence for diameter-selective doping, nor any distinction between doping responses of metallic and semiconducting tubes. DOI: 10.1103/PhysRevB.71.205423 PACS number͑s͒: 71.23.Ϫk, 61.48.ϩc, 73.63.Fg, 81.07.De The electronic spectra of single-wall carbon nanotubes ͑SWNTs͒ are dominated by van Hove singularities, manifestations of the 1-D structure. The location of the Fermi energy E F with respect to these singularities can be tuned by chemical ͑alkali metals, acids, halogens, …͒ 1 or electrochemical doping. 2 Doping response in bulk samples is complicated by the presence of metallic and semiconducting tubes and by diameter and chirality dispersion, both of which imply a distribution of initial work functions. 3,4 Further complications arise from tube-tube interactions in bundles or ropes. 5,6 Here we report a systematic study of chemically p-doped SWNTs combining resistivity, thermopower, reflectivity, and Raman spectroscopy. Experimental results from each of the above techniques have been reported before, but quantitative analysis of the Fermi level shift has not been routinely performed. Also, the consistency of experimental results from different techniques has never been carefully addressed. In this work, we compare data obtained for relatively weak and strong protonic acids, HNO 3 and H 2 SO 4 , respectively, in order to test consistency of results from different measurements. We discuss the results in terms of a rigid band model 7 whereby doping shifts E F without affecting the band structure. We assume all tubes in the undoped bulk sample have the same work function, and that E F is initially near the middle of the gap or pseudogap of semiconducting or metallic tubes, respectively. We also assume that doping is spatially uniform, with no energy barriers between metallic and semiconducting tubes. We find that this simplest of models gives consistent results for ⌬E F , the Fermi level shift upon doping. Using the experimentally determined ⌬E F values as input, the evolution of Raman spectra with doping can be simply explained by the variation of resonance conditions with E F , with no evidence for diameter-selective doping as recently proposed. [8][9][10] Samples were prepared from pulsed laser vaporization ͑PLV͒ 11 and HiPco SWNT. 12 The former have a narrow distribution of relatively large diameters, 13 1.36± 0.09 nm, while HiPco tubes have smaller average diameters extending over a broad range, 14 0.8 to 1.4 nm. Starting materials for the doping ex...

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 Tesla fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5-9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both x-ray and Raman data are required for a complete texture analysis of oriented SWNT materials. Comments Postprint version. Published in Journal of ABSTRACTThick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 Tesla fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field.Anisotropy in both transport properties is modest, with ratios in the range 5 -9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both x-ray and Raman data are required for a complete texture analysis of oriented SWNT materials.

No abstract

We find strong evidence for a metal-insulator (MI) transition in macroscopic single wall carbon nanotube conductors. This is revealed by systematic measurements of resistivity and transverse magnetoresistance (MR) in the ranges 1.9-300 K and 0-9 Tesla, as a function of p-type redox doping. Strongly H 2 SO 4 -doped samples exhibit small negative MR, and the resistivity is low and only weakly temperature dependent. Stepwise de-doping by annealing in vacuum induces a MI transition. Critical behavior is observed near the transition, with ρ(T) obeying power-law temperature dependence, ρ(T) ∝ T -β . In the insulating regime (high annealing temperatures) the ρ(T) behavior ranges from Mott-like 3-dimensional (3D) variable-range hopping