By computing the double-resonant Raman scattering cross section completely from first principles and including the electron-electron interaction at the GW level, we unravel the dominant contributions for the double-resonant 2D mode in bilayer graphene. We show that, in contrast to previous works, the so-called inner processes are dominant and that the 2D-mode line shape is described by three dominant resonances around the K point. We show that the splitting of the transversal optical (TO) phonon branch in the Γ-K direction, as large as 12  cm(-1) in the GW approximation, is of great importance for a thorough description of the 2D-mode line shape. Finally, we present a method to extract the TO phonon splitting and the splitting of the electronic bands from experimental data.

Graphene shows many outstanding properties [1], which make it suitable for a wide range of applications [2]. However, the lack of a band gap strongly affects the development of graphene-based field effect transistors. A simple method to open a gap is to use quantum confinement, i.e. to cut graphene into small stripes, called graphene nanoribbons (GNRs). The electronic properties of GNRs strongly depend on the width and on the atomic arrangement at the edges [3]. In particular, applications require ultra-narrow and well defined GNRs with perfect crystallographic edges. This is very difficult to achieve with the traditional top-down technology. Recent advances in bottom-up synthesis allowed production of GNRs of defined structure simply by sculpting the molecular precursor [4-6]. This opens a way to study narrow atomically-precise GNRs by optical means such as Raman spectroscopy. In this work we present a detailed multi-wavelength Raman analysis of bottom-up synthesized ultranarrow armchair GNRs (1-2nm in width) in powder. We also measured the precursors and the molecules synthesized from the original precursor during the GNRs synthesis. The first-order Raman spectrum of GNRs is dominated by two prominent peaks, namely the D and G peak. The G peak is blue-shifted and broadened as compared to graphene, due to quantum confinement. The intense D peak is activated by confinement of π-electrons into a finite-size graphitic domain, similar to the band at around 1300 cm -1 observed in Polycyclic Aromatic Hydrocarbons (PAHs) [7]. The D peak shows an interesting feature: depending on the excitation energy and power used, it splits into two components. Both the components follow the same vibration symmetry of the D peak. The Raman spectrum of GNRs also shows low energy modes similar to the Radial-Breathing Modes (RBMs) of CNTs. The measured frequencies of those peaks depend on the widths of the GNRs in good agreement with theoretical predictions [8]. This makes Raman spectroscopy a powerful technique to characterize GNRs in analogy to CNTs. References [1] K.S. Novoselov, Rev. Mod. Phys. 83 (2011) 837. [2] K.S. Novoselov et al., Nature 490 (2012) 192. [3] J.-C. Charlier et al., Top. Appl. Phys. 111 (2008) 673. [4] J. Cai et al., Nature 466 (2010) 470. [5] M. Schwab et al., J. Am. Chem. Soc. 134 (2012) 18169. [6] A. Narita et al., submitted. [7] C. Casitglioni et al., J. Chem. Phys. 114 (2001) 963. [8] R. Gillen et al., Phys. Rev. B 81 (2010) 205426.

In this article we unravel the origin of the two-phonon D + D �� peak and its asymmetric line shape by combining experimental data of single-layer graphene with a full twodimensional calculation of the double-resonant Raman process based on fourth-order perturbation theory. We show that the main peak originates from phonons along the K highsymmetry line and that the asymmetry is due to phonons from the two-dimensional Brillouin zone. The analysis of the asymmetric line shape in experiment provides a direct probe of the two-dimensional phonon dispersion. We further show how the D + D �� peak evolves with the number of graphene layers.

We present a double-resonant Raman mode in few-layer graphene, which is able to probe the number of graphene layers reliably. This so-called N mode on the low-frequency side of the G mode results from a double-resonant Stokes/anti-Stokes process combining a LO and a ZO' phonon. Simulations of the double-resonant Raman spectra in bilayer graphene show very good agreement with the experiments. The investigation of the out-of-plane ZO' phonon for layer number determination is expected to be transferable to other layered materials like boron nitride.

Excitonic effects of metallic single-walled carbon nanotubes are measured with temperature-dependent resonant Raman spectroscopy. By changing the temperature in the range of 300-870 K, we observe variations in the optical transition energy E(ii) as well as in the maximum Raman intensity. We find both dependences to be different for semiconducting and metallic single-walled carbon nanotubes. We suggest an interpretation in terms of excitons dissociated into free electron-hole pairs at temperatures related to the exciton binding energy. We furthermore discuss how the oscillator strength is influenced by temperature.

Resonant Raman spectroscopy is used to investigate the temperature dependence of the optical transitions of metallic and semiconducting nanotubes. While the semiconducting nanotubes show an approximately linear temperature dependence as known for bulk semiconductors, the metallic nanotubes show a different temperature dependent behavior with a non-monotonic dependence of the transition energy on the temperature. This result can be attributed to dissociation of bound electron hole pairs (excitons), leading to a quasi band-to-band transition.[GRAPHICS]First optical transition E-11(M) of metallic nanotubes and sketch of a metallic (13,1) tube.

Increasing evidence suggests a central role for oxidative stress in the pathology of prion diseases, a group of fatal neurodegenerative disorders associated with structural conversion of the prion protein (PrP). Because UV-light-induced protein damage is mediated by direct photo-oxidation and radical reactions, we investigated the structural consequences of UVB radiation on recombinant murine and human prion proteins at pH 7.4 and pH 5.0. As revealed by circular dichroism and dynamic light scattering measurements, the observed PrP aggregation follows two independent pathways: (i) complete unfolding of the protein structure associated with rapid precipitation or (ii) specific structural conversion into distinct soluble β-oligomers. The choice of pathway was directly attributed to the chromophoric properties of the PrP species and the susceptibility to oxidation. Regarding size, the oligomers characterized in this study share a high degree of identity with oligomeric species formed after structural destabilization induced by other triggers, which significantly strengthens the theory that partly unfolded intermediates represent initial precursor molecules directing the pathway of PrP aggregation. Moreover, we identified the first suitable photo-trigger capable of inducing refolding of PrP, which has an important biotechnological impact in terms of analyzing the conversion process on small time scales.