Laboratoire Charles Coulomb UMR 5221 CNRS/UM2 (L2C)


Accueil > La Recherche > Axes & Equipes > Nanostructures & Spectroscopies > Equipe : Nanomatériaux > Thème : Propriétés intrinsèques des nanotubes individuels et du graphène


par Sébastien LAYSSAC - publié le , mis à jour le

Involved researchers : D. Nakabayashi, T. Michel, M. Paillet, J.-L. Sauvajol
Collaborations : P. Poncharal, A. Ayari (LPMCN, Lyon), J.-R. Huntzinger, A. Tiberj, J. Camassel (GES, Montpellier)

Graphene is a two-dimensional honeycomb lattice of carbon atoms. With carbon nanotubes, graphene is another promising nanomaterial for a carbon-based nano-electronic. Since graphene is uniquely composed of surface atoms, its physical properties are drastically affected by the environment. Part of the interest in this material is related to its electronic band structure which permits carriers to behave as massless Dirac fermions with vanishing density of states at the Fermi level. However, if two graphene layers are stacked in Bernal AB configuration, this property is destroyed since the electronic dispersion curve is no longer linear. Because some Raman active modes originate from a double resonance process, Raman spectroscopy can be used to probe the electronic structure of graphene and stacked graphene layers.

To this aim, our expertise in studying vibrational properties of carbon nanotubes was recently extended to graphene. In particular, we use the Raman spectroscopy and Atomic Force Microscopy (AFM) to characterize graphene samples and investigate the effect of stacking (Bernal and/or misoriented) on the electronic band structure.
The samples are prepared by mechanical exfoliation of graphite and transferred to Si/SiO2 substrate. Naturally, some graphene flakes are folded or overlapped, as showed in Figure 1a. In this configuration, AFM is used to measure the number of graphene layers. Figure 1b compares the second order 2D peak of graphene, overlapped (misoriented) bi-layer graphene and Bernal bi-layer graphene. The unique 2D peak for a misoriented graphene bi-layer compares well with graphene signature although its frequency is different. Moreover, the Raman fingerprint of the AB stacked Bernal (oriented) bi-layer is significantly different with respect to the others systems. In agreement with theoretical calculations, these measurements indicate that the interaction between layers in the misoriented sample is weak and doesn’t result in an electronic band splitting contrary to the case of a Bernal bi-layer for which four components are observed.

Figure 1 : AFM image of a folded graphene layer on Si/SiO2 substrate. Scale bar 1µm.Second order Raman spectra of graphene, Bernal bilayer and misoriented graphene bilayer - © L2C

To go further in the understanding of the electronic structure and the phonon dispersion curves a current project is devoted to determine the dispersion of the 2D peak with the laser excitation energy (Figure 3). For instance, monolayer and misoriented bi-layer samples display parallel dispersion indicating similar Fermi velocity for both systems. The measurement of samples on other substrates (e.g. SiC, in collaboration with the group of Jean Camassel, GES) is also in progress.

Figure 3 : Dispersion of the 2D Raman peak of different monolayer and misoriented bi-layer samples as a function of the laser excitation energy - © L2C

P. Poncharal et al., PRB 78, 113407 (2008), P. Poncharal et al., PRB 79, 237402 (2009), P. Poncharal et al., PRB 79, 195417 (2009).