[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: 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.
Physical Review B 02/2014; 89(8):085422. DOI:10.1103/PhysRevB.89.085422 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The optical properties of single-wall carbon nanotubes are very promising for developing novel opto-electronic components and sensors with applications in many fields. Despite numerous studies performed using photoluminescence or Raman and Rayleigh scattering, knowledge of their optical response is still partial. Here we determine 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. These quantitative measurements permit determination of the oscillator strength of the different excitonic resonances and their dependencies on the excitonic transition and type of semiconducting nanotube. A non-resonant background is also identified and its cross-section comparable to the ideal graphene optical absorbance. Furthermore, investigation of the same single-wall nanotube either free standing or lying on a substrate shows large broadening of the excitonic resonances with increase of oscillator strength, as well as stark weakening of polarization-dependent antenna effects, due to nanotube-substrate interaction.
[Show abstract][Hide abstract] ABSTRACT: The ultimate surface exposure provided by graphene monolayer makes it the
ideal sensor platform but also exposes its intrinsic properties to any
environmental perturbations. In this work, we demonstrate that the charge
carrier density of graphene exfoliated on a SiO$_2$/Si substrate can be finely
and reversibly tuned between electron and hole doping with visible photons.
This photo-induced doping happens under moderate laser power conditions but is
significantly affected by the substrate cleaning method. In particular, it is
found to require hydrophilic substrates and to vanish in suspended graphene.
These findings suggest that optically gated graphene devices operating with a
sub-second time scale can be envisioned but also that Raman spectroscopy is not
always as non-invasive as generally assumed.
[Show abstract][Hide abstract] ABSTRACT: We review the information that we have obtained from combined Raman spectroscopy and electron diffraction experiments on individual free-standing single-walled carbon nanotubes (SWNT). From these data, we define Raman criteria allowing the indexing of SWNTs from their Raman features only and we show the efficiency of this approach in assigning the structure of individual carbon nanotubes. These results on index-identified individual SWNTs are compared with theoretical predictions.
[Show abstract][Hide abstract] ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
[Show abstract][Hide abstract] ABSTRACT: We present scanning tunnelling micrographs of C60 molecules deposited on polycrystalline platinum, a substrate which enables stable imaging in contrast to other investigated substrate materials. On a submicron scale, we observe layers of C60 molecules separated by monomolecular steps. Individual C60 molecules can be imaged with submolecular resolution. These submolecular features can possibly be attributed to carbon atoms or to bonds of the C60 molecule.
[Show abstract][Hide abstract] ABSTRACT: We report the measurement of the 13C NMR frequency in K3C60 under pressure. Comparing pressure dependences of the hyperfine spin lattice relaxation rate and of the resonance frequency provides the first determination of the 13C Knight shift ((61 ± 4) p.p.m.) and of the chemical shift of the C603- molecule (125 p.m.m./TMS). NMR data under pressure can be undestood with a weak-Coulomb-interaction model.
[Show abstract][Hide abstract] ABSTRACT: We report the pressure and temperature dependences of 13C nuclear spin-lattice relaxation time T1 in the normal state of K3C60. From the 1/T1T temperature and pressure profiles, a Fermi-liquid theory is used to investigate the variation of the density of states and the amplitude of electron-electron interaction. Finally the results obtained are shown to support a pairing mechanism for superconductivity that would involve high-frequency modes.
[Show abstract][Hide abstract] ABSTRACT: In order to decide definitely on the dependence of the intensity of the Breit-Wigner-Fano (BWF) component with the size of the bundle, we have measured the radial breathing modes and tangential modes (TMs) of well defined metallic individual single-wall carbon nanotubes (SWCNTs) and individual SWCNT bundles. In this aim, a complete procedure including the preparation of the substrates, the sample preparation, atomic-force-microscopy imaging and Raman spectroscopy has been developed. From this procedure, we show unambiguously that the BWF component vanishes in isolated metallic SWCNTs. In other words, the observation of a BWF component in the TM bunch is an intrinsic feature of the metallic SWCNT bundle.
[Show abstract][Hide abstract] ABSTRACT: We report an experimental study of static charge distribution in individual single-walled carbon nanotubes grown on a Si+115 nm SiO2 substrate. From these experiments, we conclude that charges are distributed uniformly along the nanotubes. We demonstrate that electrostatic force microscopy can accurately measure the amount of charges per unit length. We found that this amount is diameter dependent and in the range of 1 electron per nanometer for a 2.5 nm nanotube at a potential of -3.5 V.
[Show abstract][Hide abstract] ABSTRACT: While the behavior of solid C 60 upon doping has been extensively investigated, very little is known about the compounds based on C 70 , the next stable fullerene. The aim of this paper is to compare the potential capacities of both fullerenes to form intercalation compounds and to refer the recent achievements in the C 70 intercalation, both spontaneous (oxygen diffusion from the atmosphere) and induced (by pressure of alkali metal vapors or the applied electric field).
[Show abstract][Hide abstract] ABSTRACT: Li-doped films with controlled stoichiometries were prepared by performing redox reactions between single-wall carbon nanotubes thin films and solutions of organic radical anions. A monotonic upshift of the tangential modes is evidenced for LixC compounds (0<~x<~0.17). A similar doping dependence of the Raman profile is observed for vapor-phase Rb-doped samples. This universal behavior of the doping-induced upshift of the tangential modes is concomitant to a loss of absorption bands in the optical spectra and to a monotonic increase of the conductivity.
[Show abstract][Hide abstract] ABSTRACT: Phase transitions and staging in doped single-wall carbon nanotubes (SWNT’s) are controversial issues. Here, we report on combined in situ conductivity and Raman measurements of Rb-doped SWNT’s. Striking correlations between resistance, changes of resistance under laser irradiation, and frequency of the main Raman peak are observed. In the last steps of doping, two different Raman signatures, with peaks at 1596 and 1555 cm-1, respectively, are observed and assigned to two different stable doped phases. The two phases coexist in a specific range of doping with the latter growing progressively at the expense of the former.
Physical Review B 03/2001; 63(15). DOI:10.1103/PhysRevB.63.153407 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we report the influence of the water vapor on the
electrical resistance of a single-walled carbon nanotube mat (SWNTm).
Our results show that the sample exhibits a crossover from decreasing to
increasing conductance versus water concentration in the surrounding
atmosphere. We suggest that the water molecules act as a donor doping
the SWNTm which has p-type semiconductor character in the outgassed
state. The time dependence of the SWNTm resistance after exposition to
humid atmosphere is explained as an abnormal diffusion phenomenon.
[Show abstract][Hide abstract] ABSTRACT: Single wall nanotubes have been made by arc-discharge method. Residual impurities (fullerenes, amorphous carbon, catalyst metals...) have been removed by tangential filtration process followed by high temperature annealing under vacuum (1200 °C). In this work we present results on the influence of the surrounding gas nature (N 2 , H 2 , CO 2 , H 2 O…) on the electrical resistivity of carefully outgassed mat of such samples. In particular, we have observed that the sample resistance exhibits a strong dependence on water contamination during the transfer to the measurement reactor.
[Show abstract][Hide abstract] ABSTRACT: We present the observation of gold intercalation into the polycrystalline C60 and c70 thin films by a dry electrolysis of the electrode, detected by in situ and space resolved conductivity measurements at different temperatures and applied voltages. The time resistivity behavior is described on the basis of electric field stimulated diffusion of metal ions from the contact into the interstitials of crystal lattice. The resistivity minima correspond to a local formation of Au3C60 and respectively Au1C70 and Au4C70 phases. In contrast to AuxC60, the AuxC70 phases are stable only in the applied electric field.
[Show abstract][Hide abstract] ABSTRACT: The intercalation of metal M (M=Au, Ag, In) from the electrode into the polycrystalline C60 and C70 thin films is studied by time-dependent and space-resolved conductivity measurements under controlled oxygen-free atmosphere at different temperatures and electric fields. The time resistivity behavior is interpreted on the basis of electric-field-induced diffusion of metal ions from the electrodes to the interstitials of the fullerene crystal lattice and local formation of MxC60 and MxC70 phases. A comparison between the resistivity behaviors of C60 and C70 thin films provides evidence that the process is sensitive to the electronic level structure of the fullerenes molecules.