Intense Raman D Band without Disorder in Flattened Carbon Nanotubes.

Above a critical diameter, single- or few-walled carbon nanotubes spontaneously collapse as flattened carbon nanotubes. Raman spectra of isolated flattened and cylindrical carbon nanotubes have been recorded. The collapse provokes an intense and narrow D band, despite the absence of any lattice disorder. The curvature change near the edge cavities activates a D band, despite framework continuity. Theoretical calculations based on Placzek approximation fully corroborate this experimental finding. Usually used as a tool to quantify defect density in graphenic structures, the D band cannot be used as such in the presence of a graphene fold. This conclusion should serve as a basis to revisit materials comprising structural distortion where poor carbon organization was concluded on a Raman basis. Our finding also emphasizes the different visions of a defect between chemists and physicists, a possible source of confusion for researchers working in nanotechnologies.