A detailed comparison of true epitaxial graphene monolayers grown on both faces of 6H SiC substrates (Si and C faces) is made by combining micro-Raman spectroscopy with transmission measurements. We have already shown that such combination allows to discriminate without any ambiguity between a graphene monolayer and a twisted or folded graphene bilayer (AA') grown on the C face of 6H SiC [1]. In this presentation, we will focus on the Raman spectra and the transmission measurements performed on graphene monolayers grown on the Si face of 6H SiC. The graphene growth was tuned to get a mixed surface at the early stage of graphitization with i) bare SiC, ii) buffer layer and iii) in some localized areas small monolayers flakes on top of the buffer layer. These unique samples enabled to measure properly the Raman spectrum of the buffer layer (carbon layer with a large number of sp3 bonds) and its optical extinction at 514.5 nm. The Raman spectrum of this buffer layer remains visible after the growth of one monolayer on top but the Raman intensity is strongly reduced (typically divided by a factor of 3). This cannot be attributed to the absorption coefficient of graphene which is relatively low (few percents). Finally, several Raman mapping reveal the uniformity of the graphene monolayers in terms of thickness and crystalline quality, but also that they are subjected to a non uniform compressive strain.

Graphite Intercalation Compounds have been the subject of intensive research. With the rise of graphene, new opportunities emerge in the field. On the one hand, adsorption and/or intercalation of hetero-atoms offer a gateway to tune graphene properties. On the other hand, investigations on a simpler system open the way towards increasing knowledge on the intercalant/host physical properties. New questions to be addressed also arise, such as, what happens when the number of layers is reduced? We have developed a setup derived from the two zone vapor transport method used to produce graphite intercalation compounds. It allows performing in situ Raman experiments during graphene doping by alkali vapor. The results obtained on the evolutions of the G and 2D bands during rubidium doping of graphene and bilayer graphene will be presented and discussed.

Raman results obtained on graphene edges will be presented. In a first study, the variation of the relative intensity of the D and G band as a function of the graphene ribbon width was investigated. In a second one, we developed an hyperspectral Raman method in order to determine the evolution of the ID/IG ratio as a function of excitation wavelength. Our results will be discussed in regards with published data on graphite nanocristallites and more recently on graphene.

We propose subharmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a subharmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the system's quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.

In this article, a multiscale investigation of few graphene layers grown on 6H-SiC(000-1) under ultrahigh vacuum (UHV) conditions is presented. At 100-mu m scale, the authors show that the UHV growth yields few layer graphene (FLG) with an average thickness given by Auger spectroscopy between 1 and 2 graphene planes. At the same scale, electron diffraction reveals a significant rotational disorder between the first graphene layer and the SiC surface, although well-defined preferred orientations exist. This is confirmed at the nanometer scale by scanning tunneling microscopy (STM). Finally, STM (at the nm scale) and Raman spectroscopy (at the mu m scale) show that the FLG stacking is turbostratic, and that the domain size of the crystallites ranges from 10 to 100 nm. The most striking result is that the FLGs experience a strong compressive stress that is seldom observed for graphene grown on the C face of SiC substrates.

Graphene has emerged recently as a new material with outstanding electronic properties1. This includes mass-less Dirac fermions, ballistic transport properties at room temperature and good compatibility with silicon planar technology2. Different techniques have been developed in the last six years to fabricate mono or bi-layer graphene. They range from exfoliated graphite, either mechanically1 or in a liquid-phase solution3 to chemical vapor deposition on a metal surface4, and, more recently, to substrate-free synthesis when passing ethanol into an argon plasma5. The method investigated in this work consists in a controlled sublimation of few atomic layers of Si from a single crystal SiC substrate6. Such epitaxial growth of graphene (EG) seems to be the most suitable option for industrial applications but, for easy control, it necessitates either a large and homogeneous sheet of monolayer graphene (MLG) or few layers graphene (FLG) covering the full wafer surface. Basically, on both the Si and C faces of any SiC substrate, graphene grows selectively on some reconstructed parts of the surface. Controlling the growth means then controlling locally the surface reconstruction. At low pressure conditions (below 10-6 Torr), it remains challenging to grow FLG with homogeneous domains larger than few hundred nanometers on both faces7. The homogeneity can be increased by lowering the sublimation rate. It has been demonstrated on the Si face by working at high pressure under a noble gas atmosphere such as argon8,9. In this work10, the surface reconstruction of the C face during the Si sublimation is modified by covering the SiC substrate with a graphite cap. It leads to a strongly step-bunched morphology with on few selected terraces the growth of long anisotropic graphene ribbons (5 μm wide and up to 600 μm long). Since the Raman fingerprint of Bernal stacked FLG depends strongly of the number of graphene layers11 and the absorbtance of FLG is almost independent of the wavelength and proportional to the number of graphene layers12, we combine micro-Raman spectroscopy with micro-transmission measurements to study the quality and thickness uniformity of these ribbons. We find that most of these ribbons are homogeneous monolayers or bilayers of graphene and that the thermal stress between the graphene layer and the 6H-SiC substrate is relaxed by the formation of wrinkles. This combination of techniques is especially useful to discriminate without any ambiguity between a monolayer graphene and a misoriented bilayer because their Raman fingerprint are identical. The spectra and extinction coefficient of a monolayer, a Bernal stacked bilayer noted AB, and a misoriented bilayer noted AA' are shown in Figure 1.

We have investigated graphene layers grown on 6H-SiC(000-1) under Ultra High Vacuum and under controlled atmosphere (600mb AR pressure, 2000°C). The UHV growth gives a thin system with an averaging thickness given by Auger spectroscopy between one and two graphene planes. The growth under AR pressure shows multilayer samples with an average thickness larger than 5 graphene planes. Already from the first layer, a significant rotational disorder is revealed by electron diffraction (LEED), although well defined preferred orientations exist. For the multilayer samples, STM studies show the presence of “ponds” (with lateral size <100 nm) separated by “ridges” of height 0.7 nm. Combined micro-differential transmission and micro-Raman spectroscopy confirm these results. The analysis of the so-called D, G and 2D bands give a good estimation of the thickness, the quality of the films and the stacking of the multilayer samples. Transport measurements have been performed using Van der Pauw design of 2.8 mm by 2.8 mm and hall bar structure of 1μm by 6μm. The mobility of the multilayer samples (around 0.8 m2V-1s-1) is larger than the one of the almost monolayer but seems to reveal a relative electronic coupling between the different graphene planes. At low temperature (4k), the low field magneto resistance is consistent with the weak localisation correction.