Raman spectral imaging with resolution better than 1 nm has been achieved by a research team in China, delivering crisper details for chemists to analyze how single molecules behave. Until now, the best spatial resolution for the technique had been in the range of 3 to 15 nm.
“Chemical imaging resolution less than 1 nm is as good as it gets,” says Zachary D. Schultz, a spectroscopic imaging expert at the University of Notre Dame. Scanning tunneling microscopy (STM) has been able to map electronic density of samples at this level for a while, Schultz notes. “But the ability to also use the vibrational modes of parts of molecules is really pretty cool,” he says. That way, scientists can get information about chemical bonds in different parts of the molecule.
Zhenchao Dong, Jianguo Hou, and coworkers at the University of Science & Technology of China achieve the high resolution by carefully controlling the optical properties of the STM used for tip-enhanced Raman spectroscopy (TERS).
The researchers trap sample molecules in a cavity between a metal surface and the tip of the STM probe. Shining laser light on the sample results in Raman scattering from the molecules. It also creates waves of oscillating electrons known as a plasmon on the metal surface. The researchers tune the plasmon so that it is in resonance with both the laser light and the frequency of the Raman emission modes (Nature 2013, DOI: 10.1038/nature12151). That double resonance boosts the Raman signal to achieve spatial resolution better than 1 nm.
“Merging TERS with STM operated at ultrahigh vacuum and low temperature provides excellent control and tuning capability,” Dong says. The researchers tested the method by mapping molecules of an alkyl-substituted porphyrin on a silver surface. The molecule has a distinctive four-lobed shape discernible in the Raman images.
Dong expects the method to be useful in photonics to study the activity of biomolecules and cells. Other applications could include catalysis, molecular electronics, and other fields where single-molecule resolution is important, Dong says.