Nature 498, 82–86 (2013)

Tip-enhanced Raman spectroscopy (TERS) can capture chemical 'fingerprints' of samples with an impressive spatial resolution that approaches the nanometre regime. However, improvements are still needed for unveiling details on molecular and submolecular levels. Now, Rui Zhang, Yao Zhang and colleagues from China, Spain and Sweden claim to have broken the 1-nm-resolution barrier for TERS spatial mapping. This has enabled them to resolve the inner structure and surface configuration of a single meso-tetrakis(3,5-di-tertiarybutylphenyl)-porphyrin molecule on a silver substrate. As in conventional TERS, a strong Raman signal was obtained by using a small tip–substrate/sample distance to realize a high field strength. The researchers further improved the sensitivity by tuning the plasmon resonance to the frequency of the vibrational transition responsible for Raman scattering. A low temperature and a high vacuum were required to realize precise tuning. Tuning was performed by controlling the tip-to-substrate distance, the tunnelling current and other tip parameters. The team hopes that the technique can be used for chemical mapping of other single molecules and also for investigating the optical processes and photochemistry of single molecules.