Two-dimensional quantum transport in highly conductive carbon nanotube fibers

Measurements of the electrical resistivity, from 1.5 to 300 K, and of the low temperature magnetoresistance of highly conductive carbon nanotube (CNT) fibers, obtained by wet-spinning from liquid crystalline phase (LCP), are reported. At high temperature the results obtained on the raw CNT fibers show a typical metallic behavior and the resistivity levels without postdoping process were found to be only one order of magnitude higher than the best electrical conductors, with the specific conductivity (conductivity per unit weight) comparable to that of pure copper. At low temperature a logarithmic dependence of the resistivity and the temperature dependence of the negative magnetoresistance are consistent with a two-dimensional quantum charge transport—weak localization and Coulomb interaction—in the few-walled CNT fibers. The temperature dependence of the phase-breaking scattering rate has also been determined from magnetoresistance measurements. In the temperature range $T<100\mathrm{K}$, electron-electron scattering is found to be the dominant source of dephasing in these highly conductive CNT fibers. While quantum effects demonstrate the two-dimensional aspect of conduction in the fibers, the fact that it was found that their resistance is mainly determined by the intrinsic resistivity of the CNTs—and not by intertube resistances—suggests that better practical conductors could be obtained by improving the quality of the CNTs and the fiber morphology.

Figure 1

High resolution SEM picture of CNT fiber spun from chlorosulphonic acid. The morphology consists of highly branched, Y-shape, and very long ropes of densely packed and well-aligned CNTs with diameters in the range 10–100 nm.

Figure 2

Temperature dependence of the resistivity for the various CNT fiber samples using (a) a linear and (c) a logarithmic temperature scale. For the logarithmic plots, the measured resistance is normalized to the resistivity minimum. In (b), the temperature-dependent resistivity component for sample A is reported in the higher temperature range as a log-log plot.

Figure 3

Magnetoresistance vs magnetic field for the various CNT fibers (a) sample A, (b) sample B, and (c) sample C at various temperatures, as indicated. The solid curves are fits to the experimental data using Eqs. (2) and (4). In (d), a schematic drawing of a few-walled CNT is shown. The fitting parameters are summarized in Table 1.

Figure 4

Temperature variation of the characteristic magnetic field $H_{\varphi }$ and length scale $L_{\varphi }$ for phase-breaking scattering mechanisms for each CNT fiber sample.

Figure 5

Relative differences in slope of the logarithmic temperature dependence of the resistivity with magnetic field for sample A ($\mathrm{\Delta }R/R_{\text{decade}}\sim 13\%$ at zero field). The blue dots are the experimental points obtained at various transverse magnetic fields to the fiber axis. The continuous red line is the numerical calculations using Eq. (2) for a fully perpendicular magnetic field to the graphene layers.