Raman Spectroscopy and Optical Phonons of Graphene and Nanotubes
- Department of Engineering, University of Cambridge, UK
Lundi 16/06/2008, 14h00
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 .
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