Raman spectroscopy is one of the most widely used techniques to characterize carbon materials. The Raman spectrum of single-layer graphene and few-layer graphene consists of two sets of peaks. These are: the G and D peaks, which come from in-plane molecular vibrations and dominate the optical region of graphene and other sp2 bonded materials; and low-energy peaks resulting from the relative motion of the carbon planes, such as shear (C) and the layer-breathing modes (LBMs). These can be used to directly probe the number of layers in graphene.

The low-energy region, where the C and LBMs are located, is particularly interesting since it is here that carbon allotropes have specific spectral fingerprints. In the case of carbon nanotubes (CNTs), for example, a characteristic peak at lower energy (associated with the so-called radial breathing mode, RBM, of all the atoms of the structure) can be used to determine tube diameter. Graphene nanoribbons (GNRs), which are promising materials for next-generation digital nanoelectronics, optoelectronics and spintronics, should also have characteristic Raman features in this spectral region thanks to their finite width and low dimensionality.

Researchers at the MPI of Mainz have recently succeeded in making well-defined GNRs (less than 10 nm wide) with defined edges, functionalized by alkyl chains to improve the GNRs’ solubility. These nanostructures are ideal for Raman spectroscopy studies says a team led by Cinzia Casiraghi from the Free University Berlin and the University of Manchester, Deborah Prezzi of the Nanoscience Institute of CNR in Modena and Andrea Ferrari from the University of Cambridge. The researchers found that the radial-like breathing mode (RLBM) is influenced by the ribbon edges and not just their width. They also observed a characteristic D peak dispersion, which appears to be a further fingerprint of these materials and which differentiates them from other sp2 carbon systems, such as graphene or CNTs.

Raman spectroscopy could fully characterize GNRs

“From a practical point view, our results show that we could use Raman spectroscopy to fully characterize GNRs – as was previously done with graphene and CNTs,” says Casiraghi. “From a fundamental viewpoint, our work provides further insights into how the vibrational and electronic properties of nanostructures change with their dimensionality. We compared, for instance, the Raman spectrum of 1D GNRs with those of 2D graphene and OD polyaromatic hydrocarbons, looking for characteristic fingerprints associated with 1D GNRs.”

The team says that it is now busy studying a larger set of GNRs, in which every structural parameter (length, width, type of functional group and type of edge) is systematically changed. “This work will help us to unearth a more general relation between structure and Raman features,” Casiraghi tells nanotechweb.org.

The researchers, reporting their work in Nano Letters DOI: 10.1021/acs.nanolett.5b04183, say they used different Raman spectrometers in their study, to avoid any artefacts. Three labs were involved: one in the Physics Department at the Free University Berlin, another at the Graphene Centre at the University of Cambridge and a third in the School of Chemistry of the University of Manchester. The CNR-NANO group in Modena also performed atomistic simulations that backed up the experimental observations thanks to the support of the CINECA Supercomputing Centre (PRACE and ISCRA programmes).