A group of scientists working on surface-enhanced Raman spectroscopy (SERS) has made a nanoruler to provide insight into the longitudinal plasmonic fields in nanocavities, according to a research effort published June 19 in Journal of the American Chemical Society.

SERS is a highly sensitive and powerful spectral analysis technique applicable in various fields. In contrast to weak Raman scattering, SERS achieves a dramatically enhanced Raman signal of up to 10^10-15, allowing the analysis of single molecules. 

"How we develop the technology depends, to a large extent, on what we know about plasmonic fields. In the experiments, we observed an uneven distribution in plasmonic field at nano-scale. But it lacks theoretic and experimental support. So we decided to figure it out," said YANG Liangbao, who leads the team at the Hefei Institutes of Physical Science of the Chinese Academy of Sciences.

"Powerful tools are needed," said YANG. When recapped on the very beginning of the study, YANG and his team had to find some way to plasmonic field exploration. "So we designed and fabricated the nanoruler to look into it in high spatial resolution."

They made a unique nanoruler with a spatial resolution of about 7 x 10^-10 m, which was actually a plasmonic nanocavity fabricated by combining ultra-smooth gold films and single gold nanoparticles. 

In addition, they designed a special and innovative structure, the spacer layer, which is a five-layer two-dimensional atomic crystal, in which they inserted a monolayer of WS₂ as A SERS probe and the remaining four layers of WS₂ as reference layers.

This special design generated a strong enough quantitative SERS intensity, which was able to quantitatively and directly detect the longitudinal plasmonic field distribution. 

Beside the fabrication and direct experiments, the team supplemented and validated their research with theoretical derivations, calculations, and spectral measurements. Their results show that longitudinal plasmonic field in an individual nanocavity is heterogeneously distributed with an unexpected large intensity gradient.

The fabrication offers a way to study plasmonic field in nanocavity, and casts a new idea for understanding plasmonics and SERS technology.

The left shows the different position of monolayer, WS₂ in the Nanocavity. The right presents distribution of plasmonic field intensity. (Image by CHEN Siyu)