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 a homogeneous 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 I2, IBr, HSO3Cl (CSA) and KAuBr4 are the most effective at increasing the electrical conductivity of CNT films by factors between 3× and 8×. These dopants are used with electronic-type-separated SWCNT thin-films to further investigate changes in SWCNT optical absorption, Raman spectra, and electrical conductivity. The dopant effects with semiconducting SWCNTs result in quenching of the S11 and S22 transitions, and a red shift of 8–10 cm−1 of the Raman G′ peak, when compared to a purified SWCNT thin-film. The average electrical conductivity of purified semiconducting SWCNT thin-films is 7.3 × 104 S m−1. Doping increases this conductivity to 1.9 × 105 S m−1 for CSA (2.6× increase), 2.2 × 105 S m−1 for IBr (3.1×), to 2.4 × 105 for I2 (3.3×), and to 4.3 × 105 for KAuBr4 (5.9×). In comparison, metallic SWCNT thin-films exhibit only slight quenching of the optical absorbance spectra for the M11 transition, and shifts in the Raman G′-peak of less than 1 cm−1 for I2 and IBr, whereas KAuBr4 and CSA promote red shifting by 4 cm−1, and 7 cm−1, respectively, when compared to a purified control sample. The increase in electrical conductivity of metallic SWCNT thin-films is gradual and depends on the dopant. With an average value of 9.0 × 104 S m−1 for the purified metallic SWCNT thin-films, I2 doping increases the electrical conductivity to 1.0 × 105 (1.1× increase), IBr to 1.5 × 105 S m−1 (1.7×), KAuBr4 to 2.4 × 105 S m−1 (2.6×), and CSA to 3.2 × 105 S m−1 (3.5×). The time-dependent stability of the chemical dopants with SWCNTs is highest for KAuBr4, which remains in effect after 70 days in ambient conditions. The doping-enhanced electrical conductivity is attributed to the relative potential difference between the SWCNT electronic transitions and the redox potential of the chemical species to promote charge transfer. The results of this work reinforce the chemical doping mechanism for electronic-type-separated SWCNTs and provide a path forward to advance SWCNT conductors.