SWNT Diameter Measurement

A so-called RBM (radial breathing mode) peak is observed around 300 to 100cm-1 in the Raman spectrum of single-wall carbon nanotubes (SWNT). This corresponds to vibrations due to expansions and contractions of the nanotube diameter. As the frequency is proportional to the reciprocal of the diameter, it can be used to estimate the nanotube diameter. In practice, the diameter (in nanometers) is calculated by dividing a coefficient (normally 248) by the RBM frequency. Fig.1 shows the Raman spectrum of SWNT created by laser vaporization. A nanotube diameter of 1.35nm was calculated from the RBM frequency. However, as resonance Raman scattering is dominant for carbon nanotubes (CNT), the diameter distribution cannot be determined from a single excitation wavelength.

The Scanning Probe Microscope (SPM) is second in resolution only to the Transmission Electron Microscope and creates images with good vertical resolution. The SPM can accurately measure the diameter of individual CNT in the height direction. Fig.2 shows SWNT measurement data. It indicates that the diameter is 1.3nm and 1.72nm at different measurement positions.