The Raman enhancement effect is an extremely good way to identify molecules at very low concentrations. Among the numerous Raman enhancement techniques available today, SERS is the one that has been most studied by researchers. Indeed, the technique has been around for about 40 years. However, the problem is that the exact mechanisms behind Raman signal enhancement are still not well understood.

SERS works thanks to molecules adsorbed on rough metal surfaces or thanks to plasmonic nanostructures. The enhancement factor can be as high as 1010 to 1011, which means that the technique is able to detect single molecules.

EM and CM

The two mechanisms behind the SERS effect are currently thought to be: the enhancement of local electromagnetic fields around metallic structures, also known as the electromagnetic mechanism (EM); and the chemical interaction between a sample and the substrate it is placed on. This second, chemical mechanism (CM) is more difficult to identify because it is smaller than the EM and is usually masked by it.

Until now, scientists mainly used noble metals to study SERS because they greatly enhance Raman scattering through the EM effect. However, metals are far from ideal for such studies because of side reactions between a metal and the sample, and because of the strong spectral background produced as the metal degrades.

Enhancing Raman signals via the CM

Graphene (a sheet of carbon just one atom thick) is the ideal prototype 2D material for investigating SERS since its Raman spectrum is well known. Uniform, virtually defect-free graphene samples can now be produced quite easily and the height of these samples can be controlled on the atomic scale, something that helps yield clean, uniform Raman-enhanced signals. What is more, graphene (and indeed other 2D material sheets) strongly enhance Raman signals via the CM and not via the EM.

“Since graphene and related layered materials only enhance the Raman signal via the CM, analysing this effect on these 2D materials means that we can now understand the CM much better,” team member Xi Ling told nanotechweb.org. “Before, it was difficult for us to understand the CM clearly because it was always drowned out by the much stronger EM. 2D materials thus appear to be perfect systems for separating out the CM from the EM.”

CuPc probe

In this work, the team, led by Mildred Dresselhaus, discovered that the charge transfer interaction between graphene and the substrate it is grown on induces a strong Raman enhancement effect. The same applies for the hexagonal boron nitride system – except that here it is the strong dipole-dipole interactions between the material and the substrate that enhance Raman signals.

The researchers obtained their results by using copper phthalocyanine as a probe molecule. CuPc is a planar molecule that has a similar framework structure to graphene, h-BN and molybdenite (MoS2), explains Ling. It can lie flat on all of these 2D materials and strongly interact with them. “It also strongly scatters Raman light and is little affected by background photoluminescence,” she says.

Creating better SERS surfaces

To study how the 2D materials (which were placed on a 300 nm thick SiO2/Si substrate) enhanced Raman signals, the team deposited equal amounts of CuPc molecules on them. As a reference, it also deposited the CuPc molecules on a blank 300 nm thick SiO2/Si substrate and compared the Raman signals for all of the samples to identify the degree of light enhancement.

“Our results could help us better understand the SERS effect of various 2D materials, which are globally better than traditional SERS substrates in many ways – mainly because our substrates provide repeatable and quantitative signals,” added Ling. “In the future, we may even combine the advantages of 2D materials and metal substrates to create even better SERS surfaces.”

The current work is detailed in Nano Letters DOI: 10.1021/nl404610c.