Electrostatic Doping-Induced Phonon Shift of Metallic Single-Wall Carbon Nanotubes

Confocal Raman microscopy is used to record the Raman spectra of a metallic single-wall carbon nanotube (SWNT) exposed to ambient air under electrostatic gating. Significant and irreversible changes in the carbon-carbon stretching modes (G band) Raman spectrum of the tube are observed. The gate dependent frequency shift and line broadening of the G band spectrum exhibit a much larger magnitude as well as a hysteresis behavior compared to the previous similar Raman scattering studies of individual nanotubes placed in vacuum. G band Raman spectra similar to those of typical semiconducting tubes are resolved when the metallic tube is highly doped, allowing for the observation of gradual evolution of the G band spectrum from a semiconducting type to the typical broad Fano type. Such observation reveals evidence of phonon interaction between the G band modes having same frequency. Raman spectroscopy is promising as a noninvasive tool to probe charge transfer doping in carbon nanotubes.