We examine the excitonic nature of the E33 optical transition of the individual free-standing index-identified(23,7) single-walled carbon nanotube by means of the measurements of its radial-breathing-mode and G-modeRaman excitation profiles. We confirm that it is impossible to determine unambiguously the nature of its E33optical transition (excitonic vs band to band) based only on the excitation profiles. Nevertheless, by combiningRaman scattering, Rayleigh scattering, and optical absorption measurements on strictly the same individual(23,7) single-walled carbon nanotube, we show that the absorption, Rayleigh spectra, and Raman excitationprofiles of the longitudinal and transverse G modes are best fitted by considering the nature of the E33 transitionas excitonic. The fit of the three sets of data gives close values of the transition energy E33 and damping parameter. This comparison shows that the fit of the Raman excitation profiles provides with good accuracy the energy and damping parameter of the excitonic optical transitions in single-walled carbon nanotubes.

Counting number of graphene layers on copper ? (OK for SiO2/Si, SiC, glass)Graphene source in Montpellier by CVD on copper and on SiC and by SiC sublimation (development of prototype furnace)

Raman spectroscopy (RS) of graphene-related materials (GRM) is being considered as a fast, versatile, powerful and non-destructive characterization technique. Indeed, RS is sensitive to the number of layers, their stacking order, the nature and density of defects, the charge carrier density and in-plane strain variations. However, the positions, linewidths, profiles, intensities of the graphene/MLG Raman bands are not only affected by all these perturbations but also depends on the uniformity across the probed area (laser spot size and field depth) and on the substrate (through optical interference effects, dielectric screening…). An accurate interpretation of Raman spectra becomes then extremely complex and deserves the combined use of complementary diagnosis. We recently developed a unique expertise in terms graphene/MLG characterization. This know-how is based on instrumental developments (including laser power, transmission, reflection and Raman signal simultaneous monitoring), as well as the development of data treatment (including specific algorithms to subtract the substrate Raman background), interpretation tools and a detailed modeling of the Raman scattering and optical properties of graphene/MLG on a wide set of substrates (including SiC, Cu, SiO2/Si,…). Special care was paid to define protocols that ensure a high reproducibility and repeatability of calibrated measurements. In this contribution, we apply this tool for counting the number of layers of any kind of graphene samples. We define and validate standard procedures for GRM characterization on different substrates.

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.

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.