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Shedding Light on a Mystery of Raman Signal Enhancement

Unique new bilayer nanocatalyst system: single layers of metal and metal oxide nanocubes are deposited to create two distinct metal--metal oxide interfaces that allow for multiple, sequential catalytic reactions to be carried out selectively and in tandem.
By homing in on the distribution patterns of electrons around an atom, a team of scientists team at the Molecular Foundry showed how certain vibrations from benzene thiol, a ring-like organic molecule, cause electrical charge to 'slosh' onto a gold surface (left), while others do not (right). The vibrations that cause this 'sloshing' behavior yield a stronger SERS signal.

Led by the Molecular Foundry's Jeff Neaton, researchers have unraveled a mystery behind surface-enhanced Raman spectroscopy---a detection method useful for analyzing artwork and anthrax alike.

Raman spectroscopy uses light scattered by vibrations from atoms to provide scientists with a unique signal for a given molecule. For isolated molecules or nanoscale materials, this signal is usually too weak to be detected. Surface-enhanced Raman spectroscopy capitalizes on the enhancement of a Raman signal from a molecule placed on a rough metal surface. Like an array of antennas, the metal amplifies a signal billions of times or more, enabling single-molecule detection.

In this study, the team developed quantum-mechanical calculations to study changes in Raman signal intensity caused by the chemical binding of benzene thiol on a gold surface. Vibrations from this molecule cause electrical charge to "slosh" from a molecule to the metal, triggering signal enhancement.

Using this model, other researchers can interpret chemical contributions to signal enhancement in their own experimental data. This strategy can be extended to other interfaces involving a molecule and a metal, such the flow of electrical charge or heat through nanoscale interfaces and molecular junctions.

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A. T. Zayak, Y. S. Hu, H. Choo, J. Bokor, S. Cabrini, P. J. Schuck, and J. B. Neaton, "Chemical Raman enhancement of organic adsorbates on metal surfaces," Phys. Rev. Lett. 106 083003 (2011). DOI: 10.1103/PhysRevLett.106.083003