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.

The ultimate surface exposure provided by graphene makes it the ideal sensor platformbut also exposes its intrinsic properties to any environmental perturbations. Ramanspectroscopy has been used here to demonstrate the influence of the laser power and thesurface chemistry of the SiO2/Si substrate on the charge carrier density of exfoliatedgraphene in air. It is shown that the charge carrier density of graphene exfoliated on aSiO2/Si substrate can be finely and reversibly tuned between hole (p-type) and electron(n-type) doping, across neutral state (i-type) with visible photons. This effect issignificantly affected by the substrate cleaning method and completely suppressed insuspended graphene. The observed phenomenon has a subsecond characteristic time anddoes not involve the chemical modification of graphene.One technical implication of our study for the entire scientific community using Ramanspectroscopy of graphene as a routine characterization technique is that it should beconsidered as potentially invasive as far as electronic properties are concerned. On theother hand, the ability to tune the charge carrier density with visible photons opens awide set of opportunities to develop optically gated graphene electronic devices and anew approach to graphene optoelectronics. Finally, this effect should allow studying theinterplay between graphene properties and the environment and triggering laser-assistedfunctionalization of graphene leading to more advanced devices.[1] 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, 2355(2013).

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.

Recently, we determined using spatial modulation spectroscopy, over a broad optical spectralrange, the spectrum and amplitude of the absorption cross-section of individual semiconductingsingle-wall carbon nanotubes.[1] Here, we will present resonant Raman excitation profiles (REP)of radial breathing modes and G modes measured on the same SWNTs. The transition energiesand full width at half-maximum (FWHM) obtained using both measurements techniques will becompared to discuss the different models commonly used to fit REP [2] as illustrated on Figure1. Furthermore, interference effects on Raman intensity [3] will also be evidenced in the case oftwo energetically close optical transitions. Finally, some recent results obtained on other systems(SWNTs homogeneous dimer and double-walled carbon nanotubes) will be briefly reported.[1] J.-C. Blancon et al., Nat. Comm. 4 (2013), 2542.[2] C. Thomsen and S. Reich, M. Cardona, R. Merlin (Eds.): Light Scattering in Solid IX,Topics Appl. 3.Physics 108 (2007), pp. 116-232.[3] V. N. Popov and Ph. Lambin, Phys. Rev. B 73 (2006), 165425.

This study is devoted to elucidate the interplay of catalyst thickness and growth conditions in the activation and selectivity of single-walled carbon nanotube growth using cobalt deposited on Si/SiO2 as a model system. In situ Raman studies reveal that thin catalyst layers require a higher pressure of carbon precursor to initiate nanotube growth. However, if the catalysts are pre-reduced, all catalyst thicknesses display the same low threshold pressure and a higher yield of single-walled carbon nanotubes. To explain these results, catalysts formed from a gradient of cobalt thickness are studied. Surface analyses show that during the catalyst preparation, catalyst atoms at the interface with silica form small and hard-to-reduce silicate nanoparticles while the catalyst in excess leads to the formation of large oxide particles. Weakly-reducing conditions of pretreatment or synthesis are sufficient to reduce the large oxide particles and to lead to the growth of large-diameter multi-walled carbon nanostructures. However, highly-reducing conditions are required to reduce the small silicate domains into small cobalt particles able to grow single-walled carbon nanotubes. These results show that reaction of the catalyst with the support to form more refractory compounds greatly impact the nucleation yield and the growth selectivity of single-walled carbon nanotubes.

The structural, optical, and transport properties of graphene grown by chemical vapor deposition (CVD) of propane under hydrogen on the Si face of SiC substrates have been investigated. We show that little changes in temperature during the growth can trigger the passivation of the SiC surface by hydrogen. Depending on the growth condition, hole or electron doping can be achieved, down to a few 1011 cm−2. When the growth temperature is high (T≈1500–1550∘C), we obtain electron-doped graphene monolayers lying on a buffer layer. When the growth temperature is slightly lowered (T≈1450–1500∘C), hole-doped graphene layers are obtained, lying on a hydrogen-passivated SiC surface, as confirmed by the enhancement of the mobility (of the order of 4500 cm2/Vs) and the persistence of weak localization almost up to room temperature (250 K). The high homogeneity of this graphene allows the observation of the half-integer quantum Hall effect, typical of graphene, at the centimeter scale in the best cases. The influence of the SiC steps on the transport properties is discussed.

Recently, we determined using spatial modulation spectroscopy, over a broad optical spectral range, the spectrum and amplitude of the absorption cross-section of individual semiconducting single-wall carbon nanotubes. Here, we will present resonant Raman excitation profiles (REP) of radial breathing modes and G modes measured on the same SWNTs. The transition energies and full width at half-maximum (FWHM) obtained using both measurements techniques will be compared to discuss the different models commonly used to fit REP as illustrated on Figure 1. Furthermore, interference effects on Raman intensity will also be evidenced in the case of two energetically close optical transitions. Finally, some recent results obtained on other systems (SWNTs homogeneous dimer and double-walled carbon nanotubes) will be briefly reported.