Double-resonant Raman scattering in graphene, few-layer graphene, and carbon nanotubes

Orateur : F. HERZIGER (TU Berlin)

Although Raman spectroscopy is routinely used for the characterization of graphene and related carbon materials, the precise scattering processes of certain double-resonant Raman modes are still under discussion and not yet fully understood. Especially, the double-resonant 2D mode, which is one of the most discussed peaks in the Ram an spectrum of graphitic carbons, has been subject to controversial discussions in recent years. In single-layer graphene, this mode is well described by a nearly symmetric Lorentzian component, however, in bilayer graphene the 2D mode presents a lineshape with up to four different contributions. In this context, different models were proposed to explain the complex structure of this Raman mode observed in experiments. In this talk, I will present a detailed discussion of the different contributions to the 2D mode in bilayer graphene. Based on a two-dimensional calculation of the scattering cross-section and Raman measurements on freestanding bilayer graphene, I will analyze the different scattering processes, the influence of the TO phonon splitting on the 2D-mode lineshape, and quantum interference effects in the 2D-mode spectrum. These results finally explain the complex 2D-mode lineshape in bilayer graphene and the origin of its different contributions. In the following, I will present an universal, geometrical approach to analyze the doubleresonance process in carbon nanotubes with arbitrary chiral indices. This model is then applied to investigate the double-resonant D mode in carbon nanotubes and the dependence of its frequency on the CNT diameter. This particular question has been discussed by different authors in the past and different laser-energy dependent dispersion of the D-mode freque ncy were experimentally observed. I will demonstrate that in principle two different dispersion are expected, depending on the experimental conditions. The experimentally observed discontinuities between different optical transitions Eii is explained by curvature-induced renormalizations of the D-mode frequency.

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