Doping-dependent G-mode shifts of small diameter semiconducting single-walled carbon nanotubes

We investigate the impact of electrochemical doping on the frequency of the G+-mode of a selection of polymer-sorted, nearly monochiral and semiconducting single-walled carbon nanotubes (SWCNTs) via in-situ Raman spectroscopy of ambipolar electrolyte-gated SWCNT network transistors. The purified semiconducting nanotubes have small diameters from 0.76 nm to 1.39 nm. In contrast to previous experimental data on large diameter nanotubes and current electron-phonon coupling theories, we find a pronounced red-shift (phonon softening) of the G+-mode at low to medium hole concentrations before an upshift is observed for larger hole concentrations. The effect is largest for those nanotubes with the smallest diameters. The pronounced dependence of these frequency shifts on the diameter of the semiconducting SWCNTs suggests that curvature effects start to play a significant role and should not be neglected.

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