Vibrational Signatures in the Infrared Spectra of Single- and Double-Walled Carbon Nanotubes and Their Diameter Dependence

While the vibrational features detected by Raman spectroscopy constitute an essential fingerprint of carbon nanotubes, the same is not true for infrared spectroscopy: studies are few, and agreement is scarce. In this paper, we study wide-range transmission spectra of a series of single-walled nanotube samples, differing in their diameter distribution. In all samples, distinct vibrational features could be detected in the mid-infrared region. We found a correlation between the frequency of the S11 electronic transitions and the 860 cm–1 vibrational band, leading to the conclusion that this vibrational mode hardens with increasing nanotube diameter. Our results are in accordance with density functional theory calculations. The vibrational features in double-walled nanotubes correspond to those of the isolated single-walled tubes of similar size.