Raman Spectroscopy and Optical Phonons of Graphene and Nanotubes
Department of Engineering, University of Cambridge, UK
Mon, Jun. 16th 2008, 14:00
Bât. C5 P.421A, CEA-Grenoble
We review recent results on the optical phonons and Raman spectra of graphene and nanotubes. Graphene is the two-dimensional building block for carbon allotropes of every other dimensionality. Its recent discovery in free-state has finally provided the possibility to study experimentally its electronic and phonon properties. We present the Raman spectra of graphene and graphene layers [1-3]. The G and 2D Raman peaks change in shape, position and relative intensity with number of layers. This reflects the evolution of the electronic structure and electron-phonon interactions [1-3]. We give a set of simple empirical correlations between Raman fit parameters and number of layers. Transmission electron microscopy and electron diffraction validate the Raman layer count . We also consider the effects of doping on the Raman spectra of graphene .
We show that this induces a stiffening of the Raman G peak for both holes and electrons doping . This can only be explained including dynamic corrections to the Born-Oppenheimer approximation .
We then discuss the implications for the interpretation of the G peak of nanotubes . This can be fit with only two components, G+ and G- [4,5].
Metallic SWNTs have a broad downshifted G-. We assign the G+ and G- peaks of metallic SWNTs to TO and LO phonons [6-8]. We assign the broadening, downshift and diameter dependence of the G- peak to electron-phonon coupling effects [6-8]. We then present the experimental dependence of the
G+ and G- peaks of metallic and semiconducting SWNT on the electronic
temperature . Also, in this case static approaches do not reproduce experimental data and beyond Born-Oppenheimer corrections are necessary .
1. A. C. Ferrari et al. Phys. Rev. Lett. 97, 187401 (2006)
2. S. Pisana et al. Nature Mater. 6, 198 (2007)
3. S. Piscanec et al. Phys. Rev. Lett. 93, 185503 (2004)
4. A. Jorio et al. Phys. Rev. B 66, 115411 (2002).
5. M. S. Dresselhaus et al., Physica B 323, 15 (2002).
6. M. Lazzeri et al. Phys. Rev. Lett. 95, 236802 (2005)
7. M. Lazzeri et Phys. Rev. B 73, 155426 (2006)
8. S. Piscanec et al. Phys. Rev. B 75, 035427 (2007)