Environmental effects, intertube interactions and σ-π bond re-hybridization in bundles of double- and triple-walled carbon nanotubes

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In the literature, fine structuring and mixing of the radial breathing mode frequencies (ωRBM) due to intertube interactions (Ωint) for specific inner tubes in free-standing isolated double-walled nanotubes (DWNTs) have been addressed. Questions and arguments related to the Ωint nature and its dependence with the nanotube’s growth processes have also been addressed. Later, studies involving bundled (innermost tubes) and isolated (innermost and middle tubes) of triple-walled nanotubes (TWNTs) for diameters between 0.5 and 1.7 nm found no ωRBM fine structuring or mixing but suggested that the Ωint depends on the tube’s metallicity. Here, Raman Spectroscopy is combined with transmission electron microscopy (TEM) to show that for bundled DWNTs and TWNTs measured in a broad diameter range (0.5≤dt≤2.8 nm) three effects seem to be fundamental to explain the ωRBM behavior of their constituent tubes: interactions with the external environment, metallicity-independent Ωint and curvature effects due to σ-π bonds re-hybridization. Our results display no evidence of ωRBM mixing but show that Ωint has no apparent dependence with the growth process and that Ωint has the same nature of the tube interactions with the environment. The manuscript also discusses the thus far elusive TWNT outermost tubes ωRBM.