(41b) Solvent-Dependent Conductivity Enhancement of Carbon Nanotube Structures through Iodine Monobromide (IBr) Doping

Bulk carbon nanotube (CNT) electrical conductors are of interest for aerospace and terrestrial electrical transmission applications due to their low density, flexure tolerance, chemical stability, and high electrical and thermal conductivities. One research area of opportunity is the improvement of the electrical conductivity of bulk CNT structures towards the exceptionally high intrinsic conductivity of individual CNTs. To this end, the electrical conductivity of CNT structures can be modified by the incorporation of chemical dopants. Several methods of dopant delivery such as gaseous deposition and aqueous soaking have demonstrated success depending on the specific dopant species of interest. Recent work with interhalogen compounds, such as IBr and ICl, has shown the strong potential of these species to increase the conductivity of CNT thin films and wires, motivating the study and refinement of the delivery methodology for these dopants.

In this work, various CNT structures and types, including single- and multi-wall CNTs (SWCNTs and MWCNTs) in sheet and yarn formats, were doped with IBr, and the solvent-dependent conductivity enhancement was quantified. The solvatochromic shift of IBr in solutions of acetone, chloroform, dimethylacetamide (DMA), dimethyl sulfoxide (DMSO), ethanol, hexanes, isopropanol, methanol, or water was quantified using optical absorption spectroscopy. When CNT samples were exposed to equivalent concentrations of IBr in various solvents, differing levels of conductivity enhancement were observed. Top performing organic solvent systems included hexane and chloroform solutions of IBr, which caused a 5.7x increase over the purified CNT sheet conductivity, compared to an only 1.7x increase from doping solutions of IBr in acetone or DMSO. Conductivity enhancement was correlated with the solvatochromic shift, with a 6.7x increase for water being a notable outlier, indicating that dopant interaction with the CNTs in a bulk structure is dependent on the dopant-solvent and solvent-CNT interactions.

IBr adsorption onto the CNTs was also studied using time-dependent optical absorbance spectroscopy to gain further understanding of the dopant-CNT-solvent system, such as determining how much dopant was adsorbed by the CNTs as a function of time, and at what point the CNTs were saturated. This method allowed for the relative quantification of dopant interaction with CNTs and demonstrated that the doping solutions which showed greater increase in CNT conductivity are those that promoted greater dopant adsorption. Finally, differences in doping efficacy across CNT sample types is demonstrated to be an effect of varying dopant adsorption limitations, such as greater dopant adsorption on SWCNTs than on MWCNTs with a correspondingly larger increase in SWCNT conductivity than MWCNT conductivity.

These observations highlight the importance of dopant delivery methodology when increasing the electrical conductivity of bulk CNT structures. The trends depicted enable the selection of solvents that promote increased dopant interaction with CNTs and the determination of effective dopant exposure times on the order of half an hour at room temperature. The resulting increase in electrical conductivity for bulk CNT conductors can spur the adoption of multifunctional CNT materials for advanced wiring architectures.