Abstract
Raman spectroscopy is a powerful tool for the characterization of sp2 carbon materials. The 2D-band Raman mode, especially, can provide physical information such as the number of layers, strain, defects, and so on, as has been extensively demonstrated on graphene with confocal Raman spectroscopic techniques. On carbon nanotubes (CNTs), however, such technique cannot be applied, owing to its one dimensional structure, which is beyond the optical diffraction limit. To overcome the above problem, we utilize tip-enhanced Raman spectroscopy (STM-TERS), in which the spatial resolution is typically in the range of a few nanometers or less. Using this technique, we investigated Raman spectra from various locations within the same CNT, and we found a significant correlation between the 2D- and G-peaks intensity-ratio {I(2D)/I(G)} and the number of layers of an MWNT. It isalso found that at the crossing point of two CNTs, the I(2D)/I(G) increases, indicating that this result is due to interaction between the two CNTs, in analogy to interaction between sheets of graphene. The peak shift observed at the crossing point can be ascribed to mechanical stress on the CNTs.
