Sloshing charge: the origin of SERS

SERS calculations

New quantum calculations show how electrical charge can slosh from a target molecule to the metal being used to enhance Raman signals in Surface-enhanced Raman spectroscopy (SERS). The finding could finally explain the mechanism by which this technique boosts Raman spectra in terms of the chemical contribution to the effect.

SERS is a sensitive way to enhance the Raman scattering by molecules adsorbed on a non-smooth metal surface and allow details of bonding and symmetry in sample molecules available in nanoscopic quantities to be discerned. The enhancement can be as high as 10 to 100 billion-fold allowing it to detect single molecules. SERS has come to the fore as a powerful, but essentially non-invasive analytical technique during the last few years and has been applied to identifying faded pigments in historical artefacts and the paintings of American artist Winslow Homer, for instance, and in nanosensors for detecting biological and chemical weapons.

The precise details of all the enhancements and how they boost the SERS signal has remained something of a mystery. Researchers at the Lawrence Berkeley National Laboratory (Berkeley Lab) explain that SERS exploits the roughness of a metal surface to amplifying the signal with the metal somehow behaving as an antenna array.

"The role of surface chemistry in SERS has been unexplained for more than thirty years," says postdoctoral researcher Alexey Zayak, who is working with Jeff Neaton, Director of the Theory of Nanostructured Materials Facility at Berkeley Lab's Molecular Foundry. He points out that, "Many theories have been proposed for why it occurs, but the chemical contributions to SERS were never established enough to draw a simple, systematic picture of this behaviour. Working with experimentalists and with a new theoretical approach, we were able to isolate the chemical contributions."

Many research teams interested in explaining SERS have previously tried to separate the chemical contribution to Raman signal enhancement. However, Zayak and Neaton have now performed quantum-mechanical calculations that show how changes in Raman signal intensity occur on a gold surface. They focused their calculations on the chemical binding of a well-known organosulfur molecule, benzene thiol, to the A{111} surface.

They homed in on the way in which electrons are distributed in this system to show how molecular vibrations can cause charge to "slosh" from the organosulfur compound to the gold surface and so initiate signal enhancement. The research team, which also includes Y.S. Hu, Hyuck Choo, Jeff Bokor, Stefano Cabrini and Jim Schuck, suggests that pinpointing this change in behaviour provides a way to model the chemical contribution to SERS in a relatively simple and straightforward manner.

We now understand the chemical contributions to SERS, Neaton told SpectroscopyNOW. "SERS is dominated by electromagnetic (EM) contributions, and the EM enhancement is largely responsible for the billion-fold increase in signal. We [have now] shows that upon attachment to a metallic substrate, a molecule's spectral fingerprint will be altered, and a smaller, additional (chemical) enhancement can be gained. We uncovered the details of the origin of this 'chemical enhancement', i.e. the 'charge sloshing' between the metal and molecule - induced by the excited Raman-active vibrational mode."

Open-door science

"We wanted to find a model to isolate how the fingerprint of a molecule changes or becomes enhanced as it interacts with a surface," says Neaton. "Here, we did a rigorous, controlled set of calculations that were validated by quantitative agreement with experiment. Our calculations provide a complementary dimension to experimental results, and helped us identify which of the existing models best explained both our calculations and the experimental data. Neaton is pleased that they could quantitatively compare calculations with experimental results and so "open the door" for a simple model of SERS.

Zayak adds that the new model could be more widely applicable to surface science than understanding SERS. It could be extended to "any situation where atomic vibrations are triggered at an interface between a molecule and a metal," he says. Surface catalysis studies and research into the flow of electrical charge or heat through nanoscale interfaces and molecular junctions could also benefit.

"We also took the step of validating our theoretical explanation, comparing our unique, state-of-the-art calculations with a new approach for isolating 'chemical contributions' from experiment," Neaton told us. "The quantitative agreement confirms our understanding, and the intuition that comes out of this can be applied to understand SERS in other molecules chemically bound to other substrates, and potentially also to other situations where metal-molecule interactions are paramount, such as heterogeneous catalysis."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.