Enlightening the ultrahigh electrical conductivities of doped double-wall carbon nanotube fibers by Raman spectroscopy and first-principles calculations

Abstract

Highly aligned, packed, and doped carbon nanotube (CNT) fibers with electrical conductivities approaching that of copper have recently become available. These fibers are promising for high-power electrical applications that require light-weight, high current-carrying capacity cables. However, a microscopic understanding of how doping affects the electrical conductance of such CNT fibers in a quantitative manner has been lacking. Here, we performed Raman spectroscopy measurements combined with first-principles calculations to determine the position of the average Fermi energy and to obtain the temperature of chlorosulfonic-acid-doped double-wall CNT fibers under high current. Due to the unique way in which double-wall CNT Raman spectra depend on doping, it is possible to use Raman data to determine the doping level quantitatively. The correspondence between the Fermi level shift and the carbon charge transfer is derived from a tight-binding model and validated by several calculations. For the doped fiber, we were able to associate an average Fermi energy shift of ∼-0.7 eV with a conductance increase by a factor of ∼5. Furthermore, since current induces heating, local temperature determination is possible. Through the Stokes-to-anti-Stokes intensity ratio of the G-band peaks, we estimated a temperature rise at the fiber surface of ∼135 K at a current density of 2.27 × 10 A m identical to that from the G-band shift, suggesting that thermalization between CNTs is well achieved.

Charged iodide in chains behind the highly efficient iodine doping in carbon nanotubes

“…This value corresponds to the shift of the G + o band between the doped and dedoped fibers. Therefore, we can conclude that no adiabatic effect is present based on the formulation in our previous work [32]. From this shift, it is possible to estimate the charge transfer factor per carbon atom: f C = 10/350 = 0.029 and, then, the average Fermi level shift by E F N = 6.04 √ f C = 1 eV, a relation that is only valid for very large doping (f C > 0.007) [32].…”

“…Since no significant changes were observed, with adsorption energy difference lower than 0.01 eV, we performed all our energy studies on metallic systems. In a previous work of Tristant et al [32] (main text and electronic supplemental information), we also analyzed the case of highly doped CNTs with large diameter and found a similar Fermi level shift for both semiconducting and metallic CNTs.…”

“…Iodine-doped fibers had a conductivity of 6.5 ± 0.1 × 10 6 S m −1 , which is higher than that of chlorosulfonic acid-doped fibers [32]. From Fig.…”

“…Figure 6 shows the Raman G bands of (a) an iodine-doped DWCNT fiber and (b) a dedoped DWCNT fiber, fit with four Lorentzians. We previously described how this decomposition analysis provides information on doping [32]. These four peaks represent the upper (G + ) and lower (G − ) branches of the inner (G i ) and outer (G o ) tube characteristics of the G band of DWCNTs.…”

“…To estimate the number of conducting channels, a full calculation could be done as in our previous work [32], but a rough estimation is also possible using [37] (metallic i = 0, 3, 6, . .…”