Raman spectroscopy (RS) of graphene-related materials (GRM) is being considered as a fast, versatile, powerful and non-destructive characterization technique. 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/multilayer graphene (MLG) Raman bands are not only affected by all these perturbations but also depends on the uniformity across the probed area 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 RS set-up allowing to monitor simultaneously the laser power, transmission, reflection and Raman signal.In this contribution, we discuss the application of this tool for counting the number of layers of different kind of graphene/MLG samples with the aim to define standard procedures for GRM characterization on different substrates.

Raman spectroscopy of graphene-related materials (GRM) is considered as a fast, versatile, powerful and non-destructive characterization technique. Raman signal 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/few-layer graphene (FLG) Raman bands are not only affected by all these perturbations but also depends on the uniformity across the probed area 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.In this work, we present studies carried out with a home-made set-up combining optical contrast and micro-Raman mapping experiments. Typical sample maps consist of about 70k spectra and can be obtained with up to 5 different laser wavelengths covering the visible range, allowing us to obtain statistically robust results.FLG samples prepared by two methods (chemical vapor deposition (CVD) followed by transfer onto SiO2/Si substrate and mechanical exfoliation) have been analyzed to discuss the relevant parameters for standardization concerns. In particular, we will discuss, for different stacking orders, the validity domain and the behavior of the integrated intensity ratio of the 2D and G bands (A2D/AG), of the 2D band width and of the normalized G-band integrated intensity (AG/HOPG) as a function of the number of layers (N) which was determined independently (Figure 1).[1] The current status of measurements of the AG/HOPG and optical contrast in various experimental configurations (wavelengths, numerical aperture…), for FLG produced by different methods (mechanical exfoliation, CVD, SiC sublimation…) and different substrates (SiO2/Si, glass, SiC, Cu…) will be presented and compared to theoretical models. Finally, new insights on the behavior of Raman modes in 2LG as a function of the twist angle between the layers will be discussed. [1] Bayle, M., Reckinger, N., Huntzinger, J.-R., Felten, A., Bakaraki, A., Landois, P., Colomer, J.-F., Henrard, L., Zahab, A.-A., Sauvajol, J.-L. and Paillet, M. (2015), Phys. Status Solidi B. doi: 10.1002/pssb.201552204

The rise of efficient extraction techniques triggered a renewal of interest in semiconducting carbon nanotubes (s-SWNT) research. It represents a great interest for optoelectronics, with outstanding properties in field-effect transistor, and s-SWNT ability to efficiently emit light in the near-IR range. In particular, polyfluorene (PFO) wrapped s‑SWNT (s-SWNT@PFO) display strong photoluminescence, and could be coupled with photonic devices such as microring resonators [1,2] to control photoluminescence linewidth and enhance photoluminescence intensity. The main challenge for using s-SWNT@PFO in optoelectronics lies in the difficulty to establish good electrical contact with a PFO embedded carbon nanotube, and existing studies only focused on optical pumping of carbon nanotubes networks, without addressing issues of electrical driving. We propose to investigate these issues using s-SWNT@PFO networks deposited on patterned substrate (Figure). The network formation process allow to control nanotube density, while the amount of remaining metallic nanotube in the network could be adjusted at the extraction phase. A selective annealing process under low pressure is used to tune PFO wrapping around the nanotubes. The resulting s-SWNT@PFO networks are then probed by AFM, Raman spectroscopy, absorption, photoluminescence and electrical experiments.

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.

A double-walled carbon nanotube (DWNT), a coaxial composite of two single walled carbon nanotubes (SWNT), provides a unique model to study interactions between thetwo constituent SWNTs. Combining high resolution transmission electron microscopy (HRTEM), electron diffraction (ED), and resonant Raman scattering (RRS) experiments on the same individual suspended DWNT is the ultimate way to relate unambiguously its atomicstructure, defined by the chiral indices of the coaxial outer/inner SWNTs, and its Raman-active vibration modes. This approach is used to investigate the intertube distance dependence of theG-modes of individual index-identified DWNTs composed of two semiconducting SWNTs.We state the main features of the dependence of the G-mode frequencies on the distance between the inner and outer layers: (i) When the interlayer distance is larger than the nominal van der Waals distance (close to 0.34 nm), a downshift of the inner-layer G-modes with respectto the G-modes in the equivalent SWNTs is measured. (ii) The amplitude of the downshiftdepends on the interlayer distance, or in other words, on the negative pressure felt by the innerlayer in DWNT. (iii) No shift is observed for an intertube distance close to 0.34 nm.