Few-layer graphene (FLG) samples prepared by two methods (chemical vapour deposition on copper followed by transfer onto SiO2/Si substrate and mechanical exfoli-ation) are characterised by combined optical contrast and micro-Raman mapping experiments. We examine the be-haviour of the integrated intensity ratio of the 2D and G bands (A2D/AG) and of the 2D band width (2D) as a function of the number of layers (N). For our mechanically exfoliated FLG, A2D/AG decreases and 2D increases with N as expected for commensurately stacked FLG. For CVD grown FLG (2 to 6 layers), both similar and opposite behaviours are observed and are ascribed to dif-ferent stacking orders. For small (resp. large) relative ro-tation angle between adjacent layers (), the values of the A2D/AG ratio is smaller (resp. larger) and the 2D band is broader (resp. narrower) than for single layer graphene. Moreover, the A2D/AG ratio decreases (resp. increases) and, conversely, 2D increases (resp. decreases) as a function of N for small (resp. large) . An intermediate behaviour has also been found and is interpreted as the presence of both small and large  within the studied area. In terms of characterisation criteria, these results confirm that neither A2D/AG nor 2D are definitive criteria to iden-tify single layer graphene, or to count the number of layers in FLG.

Understanding and controlling growth of graphene on the carbon face (C-face) of SiC presents a significant challenge. In this work, we study the structural, vibrational, and dielectric function properties of graphene grown on the C-face of 4H-SiC by high-temperature sublimation in an argon atmosphere. The effect of growth temperature on the graphene number of layers and crystallite size is investigated and discussed in relation to graphene coverage and thickness homogeneity. An amorphous carbon layer at the interface between SiC and the graphene is identified, and its evolution with growth temperature is established. Atomic force microscopy, micro-Raman scattering spectroscopy, spectroscopic ellipsometry, and high-resolution cross-sectional transmission electron microscopy are combined to determine and correlate thickness, stacking order, dielectric function, and interface properties of graphene. The role of surface defects and growth temperature on the graphene growth mechanism and stacking is discussed, and a conclusion about the critical factors to achieve decoupled graphene layers is drawn.

In this communication, we will illustrate how Raman spectroscopy can be used tostudy the doping of graphene. We will first report data recorded by in situ Ramanexperiments on single-layer (SLG) graphene during exposure to rubidium vapor. Bythis way, we have been able to follow continuously the changes of the G and 2Dbands features over a broad doping range (up to about 1014 electrons/cm2). Previoustheoretical predictions have shown that the evolution of the G-mode in SLG resultsfrom the competition between adiabatic and non-adiabatic effects. We emphasizethat a possible substrate pinning effect, which inhibits the charge-induced latticeexpansion of graphene layer, can strongly influence the G band position [1].In the second part, we will show that the charge carrier density of grapheneexfoliated on a SiO2/Si substrate can be finely and reversibly tuned between electronand hole doping with visible photons. This photo-induced doping happens undermoderate laser power conditions but is significantly affected by the substratecleaning method. In particular, it requires hydrophilic substrates and vanishes forsuspended graphene. These findings also suggest that Raman spectroscopy is notalways as non-invasive as generally assumed [2].Finally, we will discuss how these results can be useful to extract information aboutgraphene doping and strain from Raman measurements.References[1] R. Parret, M. Paillet, J.-R. Huntzinger, D. Nakabayashi, T. Michel, A. Tiberj, J.-L.Sauvajol, A.-A. Zahab, ACS Nano, 7 (2013) 165.[2] A. Tiberj, M. Rubio-Roy, M. Paillet, J.-R. Huntzinger, P. Landois, M. Mikolasek, S.Contreras, J.-L. Sauvajol, E. Dujardin, A.-A. Zahab, Scientific Reports, 3 (2013)2355.

The ultimate surface exposure provided by graphene monolayer makes it the ideal sensor platform but also exposes its intrinsic properties to any environmental perturbations. In this work, we demonstrate that the charge carrier density of graphene exfoliated on a SiO2/Si substrate can be finely and reversibly tuned between hole and electron doping with visible photons. This photo-induced doping happens under moderate laser power conditions but is significantly affected by the substrate cleaning method. In particular, it requires hydrophilic substrates and vanishes for suspended graphene. These findings suggest that optically gated graphene devices operating with a sub-second time scale can be envisioned and that Raman spectroscopy is not always as non-invasive as generally assumed.

Abstract In this communication, we will illustrate how Raman spectroscopy can be used to study the doping of graphene. We will first report data recorded by in situ Raman experiments on single-layer (SLG) graphene during exposure to rubidium vapor. By this way, we have been able to follow continuously the changes of the G and 2D bands features over a broad doping range (up to about 1014 electrons/cm2). Previous theoretical predictions have shown that the evolution of the G-mode in SLG results from the competition between adiabatic and non-adiabatic effects. We emphasize that a possible substrate pinning effect, which inhibits the charge-induced lattice expansion of graphene layer, can strongly influence the G band position [1]. In the second part, we will show that the charge carrier density of graphene exfoliated on a SiO2/Si substrate can be finely and reversibly tuned between electron and hole doping with visible photons. This photo-induced doping happens under moderate laser power conditions but is significantly affected by the substrate cleaning method. In particular, it requires hydrophilic substrates and vanishes for suspended graphene. These findings also suggest that Raman spectroscopy is not always as non- invasive as generally assumed [2]. References [1] R. Parret, M. Paillet, J.-R. Huntzinger, D. Nakabayashi, T. Michel, A. Tiberj, J.-L. Sauvajol, A.-A. Zahab, ACS Nano, 7 (2013) 165. [2] A. Tiberj, M. Rubio-Roy, M. Paillet, J.-R. Huntzinger, P. Landois, M. Mikolasek, S. Contreras, J.-L. S!auvajol, E. Dujardin, A.-A. Zahab, Scientific Reports, 3 (2013) 2355.

The thermal expansion of α-Fe (ferrite) and γ -Fe (austenite) nanocrystals confined inside multiwalled carbon nanotubes (MWCNTs) is studied in situ, using synchrotron x-ray diffraction, as a function of temperature. While the thermal expansion of ferrite is similar to that of bulk material, a peculiar behavior is evidenced for austenite: The thermal expansion becomes abnormally high above 500 ◦C. A scenario involving progressive carbon uptake in γ -Fe nanocrystals gives a satisfactory understanding of the phenomenon, and allows one to propose a value of the carbon solubilization rate in γ -Fe particles confined inside MWCNTs.

The understanding of carbon nanotubes (CNT) growth is crucial for the control of their production. In particular, the identification of structural changes of carbon possibly occurring near the catalyst particle in the very early stages of their formation is of high interest. In this study, samples of nascent CNT obtained during nucleation step and samples of vertically aligned CNT obtained during growth step are analysed by combined spatially resolved Raman spectroscopy and X-Ray diffraction measurements. Spatially resolved Raman spectroscopy reveals that iron-based phases and carbon phases are co-localised at the same position, and indicates that sp2 carbon nucleates preferentially on iron-based particles during this nucleation step. Depth scan Raman spectroscopy analysis, performed on nascent CNT, highlights that carbon structural organisation is significantly changing from defective graphene layers surrounding the iron-based particles at their base up to multi-walled nanotube structures in the upper part of iron-based particles.