Nov 16, 2010
Can graphene ribbons be used for electronics?
Since a graphene sheet is a zero band gap material, there is great interest in narrow graphene ribbons which have been shown to have a band gap and can, therefore, show semiconducting behaviour. It is further known that the electronic structure depends strongly on both the width and the geometric structure of the edges of the graphene ribbons. It is known that narrow graphene ribbons are needed, but how narrow must they be?
Also, the electronic structures of armchair and zigzag ribbons, the two prototype varieties of graphene ribbons, are very different from one another. For this reason there is great interest in easy methods to distinguish between armchair and zigzag ribbons. Raman spectroscopy provides an easy way to distinguish between armchair and zigzag ribbons and to characterize the edge structure of such ribbons.
Saito et al. provide basic theoretical guidance on how this can be carried out based on the vibrational characteristics of graphene edges, showing how to distinguish between armchair and zigzag edges and what the relevant width would need to be for graphene ribbons to be of interest for use in electronics applications, such as in field effect transistors. In this article the authors show how the Raman spectra of graphene nanoribbons can be used for the characterization of the edge structure of ribbons and of ribbon width, the two key parameters determining the electronic structure. The Raman spectra of graphene nanoribbons are shown to have some unique features that provide special information about the ribbon edges.
Not only is there a radial breathing mode-like phonon, but there is also an edge-specific phonon mode that can be obtained as a function of the ribbon width and edge structure, respectively. Depending on the polarization direction, either the longitudinal optical or transverse optical phonon mode, which are both G-band phonon modes, can be Raman-active modes. In particular edge-specific phonon modes at 1450 cm–1 and 1530 cm–1, respectively, are predicted at zigzag and armchair edges. Although the calculation is carried out within bond-polarization theory, such calculations are expected to be useful for characterizing of graphene ribbons.
The researchers presented their work in the Journal of Physics: Condensed Matter.
About the author
This paper is the result of a collaboration between the Japanese research group of Professor Riichiro Saito of the Physics Department at Tohoku University, Sendai, Japan and Professors Mildred and Gene Dresselhaus' group at the Massachusetts Institute of Technology in Cambridge, Massachusetts, USA. These two research groups have collaborated in the carbon science field from about 1991 when they first suggested the existence of carbon nanotubes. They later wrote a book on the subject of carbon nanotubes which is still considered the standard text book in the field. These researchers have collaborated on at least 100 papers over the past two decades.