Nature 498, 82–86 (2013)

An optical technique for identifying lone molecules is now demonstrated, which could significantly improve molecular analysis and the search for dangerous materials.

Photons scattered inelastically (also called Raman-scattered) from a crystal lattice provide detailed information about the vibrational structure of the solid. Applying this same idea to just a single molecule is difficult because of the tiny amount of light and the subnanometre lengthscales involved. The first problem can be tackled by taking advantage of plasmonics: using metallic nanostructures to enhance the local optical field. However, for a tip used in scanning tunnelling microscopy (STM), for example, the spatial resolution remains limited to just a few nanometres.

Rui Zhang and colleagues have now combined low-temperature STM with tip-enhanced Raman spectroscopy to enhance the field surrounding a molecule near the end of the tip. Varying the potential difference between the point and the silver substrate enabled the researchers to tune the resonance of the plasmonic cavity to match the signature vibration modes of an organic molecule with submolecular spatial resolution.