Ref HAL: hal-00785831_v1
PMID 23194077
DOI: 10.1021/nn3048878
WoS: 000314082800020
Exporter : BibTex | endNote
29 Citations
Résumé:

We report in situ Raman scattering experiments on single-layer graphene (SLG) and Bernal bilayer graphene (BLG) during exposure to rubidium vapor. The G- and 2D-band evolutions with doping time are presented and analyzed. On SLG, the extended doping range scanned (up to about 1014 electrons/cm2) allows the observation of three regimes in the evolution of the G-band frequency: a continuous upshift followed by a plateau and a downshift. Overall the measured evolution is interpreted as the signature of the competition between dynamic and adiabatic effects upon n-doping. Comparison of the obtained results with theoretical predictions indicates however that a substrate pinning effect occurs and inhibits charge-induced lattice expansion of SLG. At low doping, a direct link between electrostatic gating and Rb doping results is presented. For BLG, the added electrons are shown to be first confined in the top layer, but the system evolves with time toward a more symmetric repartition of the added electrons in both layers. The results obtained on BLG also confirm that the slope of the phonon dispersion close to the K point tends to be slightly reduced at low doping but suggest the occurrence of an unexpected increase of the phonon dispersion slope at higher electron concentration.

Résumé:

The Raman bands intensities, frequencies, widths and profiles of few layer graphene (FLG) are extremely sensitive to doping, strain, stacking order, number of graphene sheets, crystalline quality and laser wavelength. For samples of epitaxial graphene grown on silicon carbide, the laser wavelength is basically the only factor known a priori. Furthermore, the concomitance of second order SiC Raman modes in the graphene G and D bands region complicates the extraction of the graphene signal. Developing a robust characterization tool enabling to disentangle all these effects and to provide reliable information is thus highly needed and challenging. We propose an approach where micro-Raman spectroscopy is combined with reflectivity and transmission measurements to discriminate between the different contributions in the measured Raman signatures and to access to the detailed characteristics of the samples. Here, we demonstrate the efficiency of this method in order to determine the number of graphene layers. We address then the question of using the 2D band width or ratio of the 2D and G bands intensities as a criteria to distinguish between mono and multi layer(s) graphene.

Ref HAL: hal-00785851_v1
DOI: 10.1021/jp307618h
WoS: 000311190800034
Exporter : BibTex | endNote
14 Citations
Résumé:

The thermal stability of catalytic precursors is of major importance for the synthesis of single-walled carbon nanotubes (SWNTs) with narrow diameter distributions by chemical vapor deposition at high temperature. We report a study of the thermal stability of iron (Fe) and iron oxide (Fe2O3) nanoparticles with narrow size distribution grown onto Si/SiO2 hydroxylated flat substrates. Whereas the Fe2O3 nanoparticles are thermally stable up to 900 °C, above their reducing temperature of ∼700 °C, the size distribution of Fe nanoparticles increases and broadens. An accurate analysis of atomic force microscopy data shows that this instability is due to a diffusion−coalescence mechanism, ruling out a possible Ostwald's ripening mechanism. The origin of this behavior is attributed to the dewetting of the nanoparticles. Using the thermally stable oxide nanoparticles as catalytic precursors, SWNTs can be successfully grown using a methane−hydrogen mixed gas. Statistical analysis of the SWNTs Raman radial breathing modesmeasured using four excitation wavelengths indicates that the diameter distribution of the nanotubes is centered at 1.3 ± 0.03 nm with a narrow full width at half-maximum of 0.3 ± 0.05 nm. The analysis of our results compared with other results recently published suggests that the metal−surface interaction could play a key role in the catalytic decomposition of methane and in the growth of SWNTs.

Ref HAL: hal-00718412_v1
DOI: 10.1557/opl.2012.464
Exporter : BibTex | endNote
Résumé:

In this paper, we discuss the low-frequency range of the Raman spectrum of individual suspended index-identified single-walled (SWCNTs) and double-walled carbon nanotubes (DWCNTs). In SWCNTs, the role of environment on the radial breathing mode (RBM) frequency is discussed. We show that the interaction between the surrounding air and the nanotube does not induce a RBM upshift. In several DWCNTs, we evidence that the low-frequency modes cannot be connected to the RBM of each related layer. We discuss this result in terms of mechanical coupling between the layers which results in collective radial breathing-like modes. The mechanical coupling qualitatively explains the observation of Raman lines of radial breathing-like modes, whenever only one of the layers is in resonance with the incident laser energy.

Ref HAL: hal-00711335_v3
Ref Arxiv: 1206.5464
DOI: 10.1016/j.carbon.2012.06.007
WoS: 000308898900001
Ref. & Cit.: NASA ADS
Exporter : BibTex | endNote
31 Citations
Résumé:

A spectroscopic study of the amino functionalization of double-walled carbon nanotube (DWCNT) is performed. Original experimental investigations by near edge X-ray absorption fine structure spectroscopy at the C and O K-edges allow one to follow the efficiency of the chemistry during the different steps of covalent functionalization. Combined with Raman spectroscopy, the characterization gives a direct evidence of the grafting of amino-terminated molecules on the structural defects of the DWCNT external wall, whereas the internal wall does not undergo any change. Structural and mechanical investigation of the amino functionalized DWCNT / epoxy composites show coupling between epoxy molecules and the DWCNTs. Functionalization improves the interface between amino-functionalized DWCNT and the epoxy molecules. The electrical transport measurements indicate a percolating network formed only by inner metallic tubes of the DWCNTs. The activation energy of the barriers between connected metallic tubes is determined around 20 meV.

Résumé:

Merging the fields of carbon nanotubes (NTs) and of organic semiconductors opens a wide range of fundamental investigations and potential applications. We developed a two step covalent functionalization method in order to graft complex Pi-conjugated systems on NT sidewalls. In a first step, iodoaniline or iododiphenylacetylene is anchored via the diazonium reaction. Several organometallic coupling reactions, as illustrated on figure 1, have been tested as a second step. The simpliest hybrid systems obtained are illustrated on figure 1 but several other much more complex have been successfully grafted. The hybrid systems obtained have been studied by resonant Raman spectroscopy, XPS, optical absorption and photoluminescence. The impact of functionalization on the properties of both moieties will be discussed and the different chemical reactions used will be compared.

Résumé:

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.