Single-walled carbon nanotubes (SWCNTs) are promising for thermoelectric generators (TEGs) in IoT applications; however, the long-term instability of the n-type properties remains a significant challenge. This study systematically investigates the factors governing n-type stability in SWCNT films combined with cationic surfactants featuring different halide counterions: DODMAC (Cl–), DODMAB (Br–), and DODMAI (I–). Through a combination of experimental characterization and molecular dynamics (MD) simulations, we demonstrate that all SWCNT films mixed with the surfactant films remain stable n-type for more than 280 days, with Seebeck coefficients of approximately −50 μV/K. MD simulations and XPS depth profiling reveal that the cationic surfactants form a dense adsorption layer that acts as a robust physical barrier─a “bulk encapsulation”─effectively shielding the SWCNTs from oxygen infiltration. While Seebeck stability was consistent across all surfactants, electrical conductivity was found to depend on the counterion species, with DODMAC exhibiting the highest performance due to its dense bundle packing. These findings elucidate the synergistic mechanism of electronic doping and physical shielding, providing crucial design guidelines for the development of highly stable n-type SWCNT thermoelectric materials.
Sunaga et al. (Tue,) studied this question.