Enhanced Raman microprobe imaging of single-wall carbon nanotubes

We explore Raman microprobe capabilities of visualizing single-wall carbon nanotubes (SWCNTs). Although this technique is limited to the micron scale, we demonstrate that images of individual SWCNTs, bundles, or their agglomerates can be generated by mapping Raman active elementary excitations. We measured the Raman response from carbon vibrations in SWCNTs excited by confocal scanning of a focused laser beam. Carbon vibrations reveal key characteristics of SWCNTs such as the nanotube diameter distribution (radial breathing modes (RBM), 100-300 cm(-1)), the presence of defects and functional groups (D-mode, 1300-1350 cm(-1)), strain and oxidation states of SWCNTs, as well as the metallic or semiconducting character of the tubes encoded in the lineshape of the G-modes at 1520-1600 cm(-1). In addition, SWCNTs are highly anisotropic scatterers. The Raman response from a SWCNT is maximal for incident light polarization parallel to the tube axis and vanishing for perpendicular directions. We show that the SWCNT bundle shape or direction can be determined, with some limitations, from a set of Raman images taken for two orthogonal directions of the incident light polarization.