We present the Raman spectra of metallic carbon nanotubes in solution and deposited onto a Si/SiO2 substrate. In the different nanotube samples, we observe qualitatively similar behavior: an upshifted and narrowed metallic LO peak in solution compared to the spectrum on the substrate. This behavior is due to the Fermi energy shifting away from the intrinsic value in the solutions, which we confirm by in situ electrochemical Raman measurements. Our results have implications for all spectroscopic methods that probe nanotubes in aqueous solutions.
Raman spectra of metallic nanotubes in solution (orange) and on a Si/SiO2 substrate (black). The inset depicts the position of the Fermi energy in both situations.