Invited Presentation: One-Dimensional Plasmon Resonance in Single-Walled Carbon Nanotubes
Authors: Okazaki Toshiya 
Published: February 2020

Single-walled carbon nanotubes (SWCNTs) are attractive one-dimensional (1D) carbon materials. Confinement of electrons in the radial direction, which is provided by the monolayer thickness of SWCNTs, leads to the unusual electronic properties of these 1D structures. For instance, SWCNTs show very unique optical properties associating with their low-dimensionality over wide spectral ranges. In the ultraviolet (UV)-visible (vis)-near infrared (NIR) region, sharp transition peaks are observed, which is associated with the interband transition between 1D van Hove singularities [1]. In addition, a broad absorption peak is observed in the UV region, which can be attributed to the π-plasmon collective excitation of SWCNTs [2,3].
Even in the far infrared (FIR) and terahertz (THz) frequency ranges, there is a broad absorption peak around a few 100 cm-1 [4]. This feature was firstly reported more than 10 years ago and interpreted as the narrow gap of metallic SWCNTs. The finite radial curvature of SWCNTs induces the shift of the Fermi point from the original K point, resulting in the small energy gaps near the Fermi level of metallic SWCNTs. The induced narrow-gap was actually observed by the low-temperature scanning tunneling spectroscopy in the energy region of ~10 meV which corresponds to the FIR frequencies (a few 100 cm-1) [5]. Importantly, this narrow gap exhibits strong diameter dependence, i.e., the gap energy is inversely proportional to the square of the tube diameter.
An alternative interpretation of the optical response in the FIR-THz region is the plasmon excitation of SWCNTs with the finite tube length [6,7]. The confined free carriers in metallic SWCNTs interact with the low energy photons, resulting in the intense optical absorption. However, the detailed mechanisms of the phenomena have been unclear so far because of the luck of the systematic study on the FIR spectroscopy of well-prepared samples. Especially, because plasmonic motion in 1D nanostructures should be sensitive to the path length, it is highly desired that the spectroscopic measurements by using SWCNTs with the same tube diameter, but different tube length.
Here we report the FIR and THz study of various SWCNT samples having different tube length. Each SWCNT was cut by strong sonication with different duration time. Importantly the sonication processes did not affect the diameter distribution, which was confirmed by resonance Raman and UV-vis-NIR spectroscopy. The obtained FIR-THz absorption frequency show a linear correlation with the inverse of the tube length, which is consistent with the 1D dispersion relation. Moreover, the different doping effects on the FIR-THz spectra between metallic and semiconducting SWCNTs indicate that the motion of the photo-induced carriers is dominantly accounts for the optical response. The observed resonance peaks were very sensitive to the presence of the defects on the tube wall, and rationally correlated to the electric resistance of the specimens.

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