Intersubband Plasmon Observation in Electrochemically Gated Carbon Nanotube Films

Intersubband plasmon excited in heavily doped carbon nanotubes by the external electromagnetic field in the near-infrared range is known to be sensitive to both the nanotube diameter and Fermi level shift. This work provides experimental results on the intersubband plasmon frequency measured as a function of gating voltage in single-walled carbon nanotube (SWCNT) thin films. We have explored SWCNTs with three mean diameters of 1.0, 1.4, and 2.0 nm being electrochemically gated in the ionic liquid. It is demonstrated that the plasmon frequency could be tuned in the range from 0.8 to 1.5 eV while the Fermi level was shifted up to 1.2 eV by using the electrochemical gating. Indeed, we have observed the theoretically predicted intersubband plasmon and discussed the influence of tubes bundling on the plasmon frequency shift with respect to the theoretical prediction for isolated SWCNTs. The increase of the bundle size red-shifts the plasmon frequency similar to the increase of the nanotube diameter.