Currently, atomic force microscopy (AFM) provides nanometre resolution images of the topography of nanostructured materials, but no chemical information about the materials. Raman scattering gives specific chemical information, but only at the micrometre scale. However, by merging the two techniques together it is possible to obtain the best of both worlds and achieve chemical imaging of materials at the nanometre scale.

Label-free chemical imaging

This combined approach is called tip-enhanced Raman scattering (TERS), which is based on the localized surface enhanced Raman scattering effect at the apex of a gold coated AFM tip. The apex of the AFM tip acts as a single gold nanoparticle, providing localized enhancement of the Raman signal, which is then scanned across the sample to build chemical images with spatial resolution well beyond the diffraction limit of visible light.

This method was applied to look at a mixture of bundles of single-walled carbon nanotubes with different diameters and chirality. Due to the small difference in the size of the carbon nanotubes and their close proximity to each other, it was not possible to localize the distribution of the various types and diameters using standard confocal Raman microscopy or AFM alone.

Using the scanning TERS method, which provides a spatial resolution of 14 nm, the team revealed that the prepared mixture of carbon nanotubes obtained from different manufacturers does not form bundles after the mixing process. In addition, the group found that within the bundle of carbon nanotubes from the one manufacturer were carbon nanotubes of different diameters. Nanosized amorphous carbon contaminant was also detected using this method.

The researchers presented their work in the journal Nanotechnology.