Light Shed on Mystery of Raman Signal Enhancement

Investigations into the chemical basis of Raman signal enhancement have resulted in a path to complementary data from SERS and other Raman-based experiments.

SERS (surface-enhanced Raman spectroscopy) capitalizes on the enhancement of a Raman signal from a molecule placed on a rough metal surface that behaves like an array of antennas. The otherwise weak Raman signal is amplified billions of times by these structures, making it easier to detect. In recent years, this technique has been used to identify faded pigments in watercolor artist Winslow Homer’s colorless skies and been proposed as a nanoscale sensor in biological warfare. However, the chemical aspect of this enhancement has baffled researchers for decades.

By homing in on the distribution patterns of electrons around an atom, scientists have shown how certain vibrations from benzene thiol cause electrical charge to ‘slosh’ onto a gold surface (left), while others do not (right). The vibrations that cause this ‘sloshing’ behavior yield a strong SERS signal. (Image: Lawrence Berkeley National Laboratory)

“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 behavior,” said Alexey Zayak of Lawrence Berkeley National Laboratory. “Working with experimentalists and with a new theoretical approach, we were able to isolate the chemical contributions.”

Previous efforts to understand the change in Raman signal of molecules attached to a metal surface have struggled to separate chemical contributions to enhancement. In this study, Zayak and Jeff Neaton, director of the lab’s Theory of Nanostructured Materials Facility, performed quantum-mechanical calculations to show changes in Raman signal intensity caused by the chemical binding of benzene thiol to a gold surface.

Scientists (from left) Hyuck Choo, Jim Schuck, Jeff Neaton and Alexey Zayak have solved the chemical mystery behind the workings of SERS. (Image: Roy Kaltschmidt, Berkeley Lab Public Affairs)

By homing in on how electrons are distributed around an atom, the team showed how vibrations from this molecule cause electrical charge to “slosh” from benzene thiol to the gold and trigger signal enhancement. Pinpointing this change in behavior provided a straightforward understanding of chemical contributions to SERS.

“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 said. “The beauty of this work is our ability to quantitatively compare calculations with experimental results. This opened the door for a simple model that led to an unprecedented understanding of this elusive phenomenon.”

Now, using this model, other researchers can interpret chemical contributions to SERS in their own experimental data and, according to Zayak, can help in any situation where atomic vibrations are triggered at an interface between a molecule and a metal, such as in catalysis and the flow of electrical charge or heat through nanoscale interfaces and molecular junctions.

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