Tip-Enhanced Raman Spectroscopy of Multiwalled Carbon Nanotubes through D‑Band Imaging: Implications for Nanoscale Analysis of Interwall Interactions

The interwall interactions in multiwalled carbon nanotubes (MWNTs), which are associated with the number of walls and may vary even at nanoscale, are important to investigate as they strongly influence the optical properties of MWNTs. Interlayer interactions in a closely related material, multilayered graphene, have been studied by Raman spectroscopy, where the 2D-band Raman mode was used as the investigation tool. However, this mode cannot be reliably utilized for MWNTs in a similar fashion due to the structural difference between the two materials. Here we demonstrate that unlike graphene, another Raman mode, the D-band, which is conventionally known to represent localized structural defects, can be activated by the interwall interactions in a MWNT and thus can be used as an effective tool to investigate interwall interactions in MWNTs. To study the interwall interactions at nanoscale, we employed tip-enhanced Raman spectroscopy (TERS) and experimentally confirmed that the D-band was actually strongly correlated to the number of walls. We also interpreted the origin of this correlation by numerical calculation, which takes into account both the exponentially decaying intensity of the near-field light at the tip apex and the nonlinear increase of sample volume as the number of walls increases. Our findings pave ways to analyze interwall interactions in MWNTs through TERS and can be applied for spectroscopic analysis of the correlation between the optical signal and the topography for nanomaterials.