Anomalous Intensity Reduction and Blueshift in the Raman Scattering Spectrum of Hole-Doped Soft-Anion-Coordinated Carbon Nanotubes

Carrier doping of single-walled carbon nanotubes changed their Raman activity. In this study, we employed separate processes, including chemical doping and subsequent ion replacement, to reveal that carrier doping and adsorption of soft ions with good affinity are essential for reducing the Raman scattering intensity of G⁺, G^–, D, and 2D bands and the radial breathing mode as well as the blueshift of the G⁺ band of nanotubes. These features cannot be realized via conventional electrochemical doping, wherein carrier injection and ion adsorption simultaneously occur. The good coordination of doped nanotubes with the soft ions is attributed to the suppressed graphitic wall vibrations, resulting in a reduction in intensity and hardening of Raman scattering peaks. Compared with the doped-only nanotubes, the doped and ion-replaced nanotubes exhibit improved stability of the doped states and further enhanced electrical conductivity. The improved carrier mobility due to the suppression of carrier scattering by lattice vibrations is ascribed to enhanced conductivity. Our proposed approach offers mobility enhancement as an “additional value of stable doping” beyond polarity tuning and carrier density modulation. Thus, soft-ion coordination achieved by using a facile wet process can be a novel strategy for developing nanoscale devices.