Curvature-induced Effects on the Phonon Modes in Sub-nanometer Diameter Single-walled Carbon Nanotubes

Sub-nanometer diameter single-walled carbon nanotubes (sub-nm SWNTs) are of great interest for fundamental studies due to the effect of large curvature on their properties. We have recently synthesized high quality, narrow diameter distribution sub-SWNTs using CoMn catalysts supported on MCM-41 silica templates in a thermal chemical vapor deposition process [1]. The high curvature in the sub-nm SWNTs leads an unusual S-like dispersion of the G-band frequency due to the strong electron--phonon coupling. In addition, we observe diameter-selective intermediate frequency modes (IFMs) that are as intense as the low frequency radial breathing modes (RBMs). The effect of large curvature in the sub-nm SWNTs is also evident in the lower phonon dispersion of the double resonant Raman features compared to SWNTs with larger diameters. The origin of previously unidentified IFM features (600-1100 cm$^{-1})$ and the dispersion of high frequency phonons (1650 -- 2300 cm$^{-1})$ will be discussed.\\[0pt] [1] C. Z. Loebick \textit{etal.}, \textit{J. Am. Chem. Soc.},132, 11125 (2010)