Metal-Insulator Transition in Doped Single-Wall Carbon Nanotubes

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 H2SO4-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 (VRH), ρ(T) ∝ exp[(-T0/T)-1/4], to Coulomb-gap (CGVRH) behavior, ρ(T) ∝ exp[(-T0/T)-1/2]. Concurrently, MR(B) becomes positive for large B, exhibiting a minimum at magnetic field Bmin. The temperature dependence of Bmin can be characterized by Bmin(T) = Bc(1 - T/Tc) for a large number of samples prepared by different methods. Below a sample-dependent crossover temperature Tc, MR(B) is positive for all B. The observed changes in transport properties are explained by the effect of doping on semiconducting SWNTs and tube-tube coupling

An Exact Solution for the Lattice Gas Model in One Dimension

A simple method to obtain a canonical partition function for one dimensional lattice gas model is presented. The simplification is based upon rewriting a sum over all possible configurations to a sum over numbers of clusters in the system.Comment: 6 pages, LaTe

Charge transfer and Fermi level shift in p-doped single-walled carbon nanotubes

The electronic properties of p-doped single-walled carbon nanotube (SWNT) 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 (δ EF) induced by doping. We find δ EF ≈ 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 EF 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

Correlation of properties with preferred orientation in coagulated and stretch-aligned single-wall carbon nanotubes

We report structure-property correlations in single wall carbon nanotube (SWNT) fibers, among electrical, thermal and chemical parameters with respect to stretch-induced preferential SWNT alignment along the fiber axis. Purified HiPco tubes are dispersed with the aid of an anionic surfactant and coagulated in the co-flowing stream of an adsorbing polymer. The fibers are then dried, rewetted under tensile load and redried to improve the alignment. Complete removal of the polymer was assured by annealing in hydrogen at 1000oC. The degree of alignment was determined by x-ray scattering from individual fibers using a 2-dimensional detector. The half width at half maximum (HWHM) describing the axially symmetric distribution of SWNT axes decreases linearly from 27.5o in the initial extruded fiber to 14.5o after stretching by 80%. The electrical resistivity ρ at 300 K decreases overall by a factor ~4 with stretching, for both as-spun composite and polymer-free annealed fibers. However, the temperature dependence ρ(T) is markedly different for the two, implying different electron transport mechanisms with and without the polymer. Thermal conductivity also improves with increasing alignment, while the absolute values are limited by the disordered network of finite length tubes and bundles. Comparisons are made with results from similar fibers spun from oleum, and with magnetically aligned buckypapers

Electrical and thermal properties of C\u3csub\u3e60\u3c/sub\u3e-filled single-wall carbon nanotubes

We report measurements of electrical resistivity, thermopower, and thermal conductivity of highly C60-filled single-wall carbon nanotubes and unfilled controls, from 1.5 to 300 K. The data suggest that the C60 chains provide additional conductive paths for charge carriers, increase the rate of phonon scattering, and block interior sites from sorbing other gas molecules

Magnetically aligned single wall carbon nanotube films: preferred orientation and anisotropic transport properties

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

Thermoelectric Power of p-Doped Single-Wall Carbon Nanotubes and the Role of Phonon Drag

We measured thermoelectric power S of bulk single-wall carbon nanotube (SWNT) materials p-doped with acids. In contrast to oxygen-exposed or degassed samples, S is very small at the lowest temperatures, increases super-linearly above a characteristic and sample-dependent T, and then levels off. We attribute this unusual behavior to 1-D phonon drag, in which the depression of the Fermi energy cuts off electron-phonon scattering at temperatures below a characteristic T0. This idea is supported by a model calculation in which the low temperature behavior of phonon drag is specifically related to the one-dimensional character of the electronic spectrum

Single Wall Carbon Nanotube Fibers Extruded from Super-Acid Suspensions: Preferred Orientation, Electrical and Thermal Transport

Fibers of single wall carbon nanotubes extruded from super-acid suspensions exhibit preferred orientation along their axes. We characterize the alignment by x-ray fiber diagrams and polarized Raman scattering, using a model which allows for a completely unaligned fraction. This fraction ranges from 0.17 to 0.05±0.02 for three fibers extruded under different conditions, with corresponding Gaussian full widths at half-maximum (FWHM) from 64o to 44o±2o. FWHM, aligned fraction, electrical and thermal transport all improve with decreasing extrusion orifice diameter. Resistivity, thermoelectric power and resonant-enhanced Raman scattering indicate that the neat fibers are strongly p-doped; the lowest observed ρ is 0.25mΩcm at 300 K. High temperature annealing increases ρ by more than 1 order of magnitude and restores the Raman resonance associated with low-energy van Hove transitions, without affecting the nanotube alignment

Macroscopic Neat Single-Walled Carbon Nanotubes Fibers

The first-ever well-aligned continuous macroscopic neat single-walled carbon nanotube (SWNT) fibers were produced using conventional spinning techniques. Neat SWNT fibers, containing no surfactant or polymer, were made by spinning dispersions of SWNTs in 102% sulfuric acid into different coagulants. The critical role of sulfuric acid in dispersing and aligning SWNTs during fiber formation has been explored. Characterization shows alignment greater than any other macroscopic neat SWNT material reported to-date while providing insight into the fundamental hierarchy and nature of SWNT fiber formation. Electrical, thermal, and mechanical measurements indicate that neat SWNT fibers hold tremendous potential for future applications