Intense Raman D Band without Disorder in Flattened Carbon Nanotubes

Area of Science:

• Materials Science
• Nanotechnology
• Physical Chemistry

Background:

• Single- or few-walled carbon nanotubes (CNTs) can spontaneously collapse into flattened structures above a critical diameter.
• Raman spectroscopy is commonly used to assess the structural integrity and defect density of carbon-based nanomaterials.

Purpose of the Study:

• To investigate the origin of the D band in Raman spectra of collapsed CNTs.
• To determine if the D band in flattened CNTs is indicative of lattice disorder or other structural phenomena.

Main Methods:

• Recording and analyzing Raman spectra of isolated, flattened, and cylindrical CNTs.
• Performing theoretical calculations using the Placzek approximation to model the observed spectral features.

Main Results:

• The collapse of CNTs induces an intense and narrow D band in their Raman spectra.
• This D band appears despite the absence of significant lattice disorder or framework discontinuities.
• Theoretical calculations confirm that curvature changes near edge cavities activate the D band.

Conclusions:

• The D band in flattened CNTs arises from curvature changes, not lattice defects.
• The D band is not a reliable indicator of defect density in structures with graphene folds.
• This finding necessitates a re-evaluation of Raman-based conclusions regarding carbon organization in distorted materials.