Graphene recently hit the headlines as a futuristic molecular material from which almost atomic scale transistors might be carved. Interest has thus been piqued in studying all things graphitic. Now, an international team has turned the Raman spotlight on graphene and its chemical cousins to figure out a bit more about why these materials display their unique properties.

Mildred Dresselhaus of Massachusetts Institute of Technology, USA, and colleagues there and at the Federal University of Minas Gerais, Brazil, Tohoku University and CREST, Sendai, Japan, point out that Raman spectroscopy has played an important historical role in understanding graphitic materials. Most usefully, Raman can reveal information about defects and stacking of graphene sheets. The team has now used Raman to look at the modern counterparts of these materials, nanographites and individual graphene molecules.

Chemists and materials scientists suspected that on moving from macro scales to micro- and nano-metre sized graphitic materials, there would be significant amorphization as the proportion of carbon atoms in the sp³ (aliphatic) state as opposed to the aromatic sp² state would be present. Such a switch would of course have implications for the materials' Raman profiles. However, little information had been obtained until now.

The international team has carried out a systematic study of the Raman spectra associated with specific defects and consequently developed a method for calculating the D-band Raman spectra, including, the researchers explain, optical matrix elements, the electron phonon-interaction and the calculated D-band intensities versus laser energy and crystallite size. "Results of the disorder-induced bands on both armchair and zig-zag edges show that the intensity of the D-band is higher at the armchair edges, showing that Raman spectra can provide information about the atomic structure at graphite edges," the team explains.

Near-field Raman spectroscopy might provide even greater insights, the team says. Its higher spatial resolution could be used to complement tunnelling microscopy techniques to allow dynamic defect studies to be carried out. Such studies could provide useful information just as they have done for phenomena seen in carbon nanotubes and so accelerate the technological development of graphemes and related materials.

Related links:

• Phys. Chem. Chem. Phys., 2007, 9, 1276-1290
• Dresselhaus Page
• Graphene News
   

Article by David Bradley