Graphene: The rising star in Raman spectroscopy

Marie Freebody, marie.freebody@photonics.com

With its numerous appealing qualities, including biocompatibility, chemical inertness and abundance, graphene has long been a popular metal among chemists and physicists alike. Thanks to these properties, it has found its way into new applications ranging from chemical sensors to transistors. Until now, however, its potential as a substrate for Raman enhancement had not been investigated.

The challenge was taken up by Zhang Jin and colleagues at Beijing National Laboratory for Molecular Sciences. They found that graphene-based probes did indeed enhance the Raman signal compared with conventional noble metal-based probes. The discovery could expand the application of graphene to microanalysis and help to better explain the basic properties of both graphene and surface-enhanced Raman scattering (SERS).

Raman spectroscopy is an important and powerful tool used for characterizing the structure of materials. However, the weak intensity of Raman signals results in low sensitivity and prompts many scientists to seek out alternatives. Although SERS provides better results, thanks to surface plasmon-related enhancements that boost the Raman signal, it is not without its difficulties.

For SERS, the key to strong enhancement is the substrate, traditionally a rough surface of a noble metal such as silver, gold or copper. However, fabricating a rough metal surface is not easy, and the biological incompatibility of the metals makes them difficult to work with.

This illustration of the experimental flow and typical result shows stronger Raman signals from phthalocyanine on graphene than on the silicon dioxide/silicon substrate. Courtesy of Zhang Jin.

Graphene, on the other hand, poses none of these issues and is cheap and easy to obtain and can be used directly. In the Beijing team’s experiments, detailed online in the December 2009 issue of Nano Letters, some common molecules used for Raman probes were deposited equally on graphene and on a conventional silicon dioxide/silicon substrate. The Raman signals of the molecule on graphene and the silicon dioxide/silicon substrate were then compared.

“We found the Raman enhancement effect exists on graphene and is, in fact, much stronger than on the silicon dioxide/silicon substrate,” Jin said. “While the precise origin of this enhancement is not clear, we believe this discovery will expand the application of graphene and will also help us to understand the charge transfer of graphene as well as chemical enhancement mechanisms.”

More research is needed to determine the origin of the enhancement and to prepare the path toward using graphene-based Raman probes in real applications.

“The most important thing is to investigate the phenomenon and optimize it,” he said. “We then plan to adjust the enhancement efficiency using various methods, such as combining our system with the classical SERS system.”