Single-walled nanotubes have become the focus of much interest thanks to their potential for nanotechnological applications. Now they have demonstrated the effectiveness of a new technique in high-resolution microscopy. Measuring below 50 nm with optical techniques has been a challenge, but researchers at Rochester, Portland State and Harvard have achieved just that with images of single-walled carbon nanotubes with a resolution of around 25 nm.
The team has extended the concept of "near-field optical microscopy" to develop "near-field Raman spectroscopy" in which a small probe - in this case a sharp 10-15 nm radius silver tip - is placed very close to a surface to enhance the Raman effect (the inelastic scattering of photons by molecules in the sample). The technique thus overcomes the diffraction limit, which in normal optical methods prevents details smaller than half the light's wavelength being clearly seen.
In the near-field Raman technique the researchers focus laser light on the sample close to the tip, which is kept only a few nanometres away from the surface. The field established at the tip increases the Raman scattering, and the scattered light can be collected to form an image as the tip is raster-scanned across the sample. The frequencies of the light also provide information on chemical composition and molecular structure.
The team has successfully produced images of individual carbon nanotubes with walls only a single atom thick. They now plan to determine structural details of carbon nanotubes, and hope to develop the technique to produce detailed pictures of proteins in cell membranes. Such images could offer hints for designing better drugs.
Further reading
A Hartschuh et al. 2003 Phys. Rev. Lett. 90 095503-1.