Enhanced Raman microprobe imaging of single-wall carbon nanotubes

Abstract

We explore Raman microprobe capabilities of visualizing single-wall carbonnanotubes (SWCNTs). Although this technique is limited to the micron scale, wedemonstrate that images of individual SWCNTs, bundles, or their agglomerates canbe generated by mapping Raman active elementary excitations. We measuredthe Raman response from carbon vibrations in SWCNTs excited by confocalscanning of a focused laser beam. Carbon vibrations reveal key characteristics ofSWCNTs 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 orsemiconducting character of the tubes encoded in the lineshape of the G-modes at1520–1600 cm−1. In addition, SWCNTs are highly anisotropic scatterers. The Raman response from aSWCNT is maximal for incident light polarization parallel to the tube axis and vanishingfor perpendicular directions. We show that the SWCNT bundle shape or direction can bedetermined, with some limitations, from a set of Raman images taken for two orthogonaldirections of the incident light polarization.