(209d) Mechanism of Chemical Doping in Electronic Type Separated Single Wall Carbon Nanotubes Towards High Electrical Conductivity

Enhanced electrical conductivity of carbon nanotubes (CNTs) can enable their implementation in a variety of wire and cable applications traditionally employed by metals. Electronic-type separated single wall carbon nanotubes (SWCNTs) offer an optimum platform to quantify the unique physiochemical interactions from different chemical dopants and their stability. In this work, a comprehensive study of chemical doping with purified commercial CNT sheets shows that alkali gold halides, gold halides, iodine-based halides and acid solutions are the most effective at increasing the conductivity of CNT films by factors between 3x and 8x. The best performing dopants (i.e. I₂, IBr, HSO₃Cl (CSA) and KAuBr₄) were used to further investigate changes in optical absorption, Raman spectra, and electrical conductivity on type-separated SWCNT thin films. The thin-films were fabricated by dispersing commercially available electronic-type-separated NanoIntegris SWCNT materials in chlorosulphonic acid (CSA), and vacuum filtration on to an Anodisc^TM alumina membrane. The thickness of the SWCNT thin-films, which is critical when quantifying electrical conductivity, was rapidly and accurately measured using non-contact optical interferometry. The results indicate that all four dopants enhance the electrical conductivity of semiconducting SWCNT thin-films in a similar manner, increasing from 6-8×10⁴ S/m to 2-4.5×10⁵ S/m (3x to 6x increase) while resulting in quenching of the S₁₁ absorbance peak and a red shift of 8-10 cm^-1 of the Raman Gʹ peak, when compared to a purified SWCNT thin-film.  On the other hand, there is a marked difference in how the different dopants interact with metallic SWCNT thin-films. The increase in electrical conductivity of metallic SWCNT thin-films is gradual and depends on the dopant. With an average conductivity value of 9.0×10⁴ S/m for the purified metallic SWCNT thin-films, I₂ doping increases the electrical conductivity to 1.0×10⁵ S/m (1.1x increase), IBr to 1.5×10⁴ S/m (1.7x), KAuBr₄ to 2.4×10⁵ S/m (2.6x), and CSA to 3.2×10⁵ S/m (3.5x). Although there is no appreciable quenching of the optical absorbance spectra of metallic SWCNT thin-films, shifts of the Raman Gʹ-peak follow with I₂ blue shifting by 1 cm^-1, and IBr, KAuBr₄ and CSA red shifting  by 1cm^-1, 4 cm^-1, and 7 cm^-1, respectively, when compared to a purified control sample. This difference in doping behavior between semiconducting and metallic SWCNT thin films is explained by the redox potential of the doping species and how they compare to the position of the SWCNT metallic and semiconducting transition energies. The results of this work present an improved understanding of the doping mechanism of electronic-type-separated SWCNTs as a function of the redox potential of doping species, and provide a path towards the realization of carbon nanotube based conductors in connectivity applications.