Graphene and AlN are promising materials, interesting to combine together. In this study, we will present first results for direct growth of graphene on bulk AlN and on AlN templates using chemical vapor deposition, including the annealing of these substrates at high temperatures. Atomic force microscopy (AFM) enabled us to study the evolution of the AlN surface morphology after annealing and growth. Few-layer graphene deposition is demonstrated on the basis of X-ray photoemission and Raman spectroscopy

The effects of quantum interferences on the excitation dependence of the intensity of G modes have beeninvestigated on single-walled carbon nanotubes [Duque et al., Phys. Rev. Lett. 108, 117404 (2012)]. In this work,by combining optical absorption spectroscopy and Raman scattering on individual index identified double-walledcarbon nanotubes, we examine the experimental excitation dependence of the intensity of longitudinal opticaland transverse optical G modes of the constituent inner and outer single-walled carbon nanotubes. The observedstriking dependencies are understood in terms of quantum interference effects. Considering such effects, the excitation dependence of the different components of the G modes permits us to unambiguously assign each ofthem as originating from the longitudinal or transverse G modes of inner and outer tubes.

Silicon carbide (SiC) sublimation is the most promising option to achieve transfer-free graphene at the wafer-scale. We investigated the initial growth stages from the buffer layer to monolayer graphene on SiC(0001) as a function of annealing temperature at low argon pressure (10 mbar). A buffer layer, fully covering the SiC substrate, forms when the substrate is annealed at 1600°C. Graphene formation starts from the step edges of the SiC substrate at higher temperature (1700°C). The spatial homogeneity of the monolayer graphene was observed at 1750°C, as characterized by Raman spectroscopy and magneto-transport. Raman spectroscopy mapping indicated an AG-graphene/AG-HOPG ratio of around 3.3%, which is very close to the experimental value reported for a graphene monolayer. Transport measurements fromroom temperature down to 1.7 K indicated slightly p-doped samples (p~10^10cm-2) and confirmed both continuity and thickness of the monolayer graphene film. Successive growth processes have confirmed the reproducibility and homogeneity of these monolayer films.

The experimental approach combining high resolution transmission electron microscopy (HRTEM), electron diffraction (ED) and resonant Raman spectroscopy (RRS) on the same free-standing individual carbon nanotubes (CNT) is the most efficient method to determine unambiguously the intrinsic features of the Raman-active phonons. In this paper, we review the main results obtained by the approach regarding the intrinsic features of the phonons of single-walled (SWNT) and double-walled carbon nanotubes (DWNT). First, we detail the different methods to identify the structure of SWNTs and DWNTs from the analysis of their electron diffraction patterns (EDP). In the following, we remind the principal features of the Raman response of SWNTs, unambiguously index-identified by ED. A special attention is devoted to the effect of the inter-layer interaction on the frequencies of the Raman-active phonons in index-identified DWNTs. The information obtained on index-identified SWNT and DWNT allows us to propose Raman criteria, which help identifying CNT when the ED fails to propose a single assignment. The efficiency of the Raman criteria as the complement to the ED information for the index-assignment of a few SWNTs and DWNTs is shown. The same approach to index-assign a triple-walled carbon nanotube (TWNT), by combining ED and RRS information, is reported.

La Spectroscopie Raman du Graphène: Une approche expérimentale

Counting graphene layers: from lab to standard

Raman spectroscopy (RS) of graphene and related materials (GRM) is being considered as a fast, versatile, powerful and non-destructive characterization technique. RS is indeed sensitive to the number of layers, their stacking order, the defects nature and density, doping and strain. This wealth of information should make RS the ideal “turnkey” experiment for routine characterization purposes.[1] However, graphene/multi-layer graphene Raman bands lineshapes also depend on the uniformity across the probed area (strain, doping, numer of layers…) and on the substrate (optical interference effects, dielectric screening…). An accurate interpretation of Raman spectra becomes then very complex and deserves the combined use of complementary diagnosis. In this talk, we focus only on single-layer graphene and present how RS can be used to separate and estimate strain and doping in graphene. Indeed, it was shown that the correlation analysis of the Raman G and 2D bands of graphene can be used to optically separate the effects of strain and charges and quantify them.[2] Here, using an optical doping technique,[3] we extract and discuss the relative dependencies of the G, 2D and 2D’ bands frequencies as a function of strain and at zero doping. We further highlight the advantages of using the 2D’ instead of the 2D. Finally, based on an example where a sharp gradient of charges concentration is present in the sample, we discuss the impact on Raman signatures of charges heterogeneities with a characteristic length scale smaller than the laser spot size.[1] A.C. Ferrari, D.M. Basko, Raman spectroscopy as a versatile tool for studying the properties of graphene, Nat. Nano., 8, 235 (2013).[2] Ji Eun Lee, Gwanghyun Ahn, Jihye Shim, Young Sik Lee, Sunmin Ryu, Optical separation of mechanical strain from charge doping in graphene, Nat. Comm. 3, 1024 (2012).[3] A. Tiberj, M. Rubio-Roy, M. Paillet, J.-R. Huntzinger, P. Landois, M. Mikolasek, S. Contreras, J.-L. Sauvajol, E. Dujardin, A.-A. Zahab Reversible optical doping of graphene Scientific Reports 3, 2355 (2013).