111 research outputs found
Raman spectra of misoriented bilayer graphene
We compare the main feature of the measured Raman scattering spectra from
single layer graphene with a bilayer in which the two layers are arbitrarily
misoriented. The profiles of the 2D bands are very similar having only one
component, contrary to the four found for commensurate Bernal bilayers. These
results agree with recent theoretical calculations and point to the similarity
of the electronic structures of single layer graphene and misoriented bilayer
graphene. Another new aspect is that the dependance of the 2D frequency on the
laser excitation energy is different in these two latter systems
Infrared-active phonons in carbon nanotubes
The aim of the present paper is to identify the main infrared vibrational features of carbon nanotubes. In this goal, infrared experiments have been performed on different well-characterized single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) as well as graphite and carbon aerogel. The comparison between the experimental spectra measured on these different samples allows us to identify the infrared-active modes of carbon nanotubes. In SWCNTs, the tangential modes are located around 1590 cm−1 and the radial mode around 860 cm−1. This latter mode vanishes in the infrared spectrum of DWCNTs. Finally, in the infrared spectra of all the carbon nanotubes investigated, a band around 1200 cm−1 is evidenced and assigned to the D-band (disorder-induced band)
Structural selective charge transfer in iodine-doped carbon nanotubes
We have investigated iodine intercalated carbon nanostructures by extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. We discuss here the charge transfer and the iodine–carbon interaction as a function of the carbon nanostructures (graphite, multi-walled, double-walled and single walled nanotubes). The results show that iodine is weakly adsorbed on the surface of all multi-walled nanotubes. By contrast, a charge transfer between iodine and single walled nanotubes is evidenced
Use of resonance Raman spectroscopy to study the phase diagram of PbZr0.52Ti0.48O3
Evidence is presented for the first time that the sharp and continuous
spectral changes observed in PbZr0.52Ti0.48O3 (PZT) between 350 and 10 K with
the 647.1 nm wavelength are due to a resonance Raman effect. Such a phenomenon
can be explained by means of a self-trapped exciton emission oxygen deficient
complex (TiTi' - VO-) of PZT powder whose energy is close to the radiation line
of the laser. This kind of approach should also be very useful to distinguish
the phase transition sequence for other related ferro/ piezoelectric systems
EXAFS investigations of iodine-doped carbon nanotubes
International audienceWe report an x-ray absorption fine structure study at the iodine-K edge of the local structure in iodine-doped carbon nanotubes. The iodine-carbon host interaction is shown to be weaker in multiwalled carbon nanotubes (MWNTs) than in single-walled carbon nanotubes (SWNTs). Iodine species are only localized at the surface of the external tube for MWNTs, whereas iodine species enter inside SWNTs. For doped SWNTs, both the experimental and the theoretical EXAFS spectra allow us to establish the structure of the iodine chain as disordered pentaiodide at the saturation level
EXAFS study of rubidium-doped single-wall carbon nanotube bundles
International audienceThe local structure around the rubidium ions inserted in single-wall carbon nanotube bundles (Rb-doped SWCNT) is studied by Rb K-edge extended x-ray-absorption fine structure (EXAFS). The dependence of the local order around the rubidium ions is investigated as a function of the time of doping (i.e., as a function of the stoichiometry of the sample). The first coordination shell of the rubidium ions, related to the distance between rubidium and the first nearest-neighboring carbon atoms, has a clear time doping dependence. Comparison between ab initio simulations of the EXAFS spectra and experimental data questions the interstitial site (between three tubes) as the preferential insertion site in SWCNT bundles. The results indicate that the rubidium ions are mainly located inside the tubes and around the bundles. The results are in good agreement with combined x-ray and neutron diffraction experiments performed on the same samples
Computational study of the shift of the G band of double-walled carbon nanotubes due to interlayer interactions
The interactions between the layers of double-walled carbon nanotubes induce
measurable shift of the G bands relative to the isolated layers. While
experimental data on this shift in free-standing double-walled carbon nanotubes
has been reported in the past several years, comprehensive theoretical
description of the observed shift is still lacking. The prediction of this
shift is important for supporting the assignment of the measured double-walled
nanotubes to particular nanotube types. Here, we report a computational study
of the G-band shift as a function of the semiconducting inner layer radius and
interlayer separation. We find that with increasing interlayer separation, the
G band shift decreases, passes through zero and becomes negative, and further
increases in absolute value for the wide range of considered inner layer radii.
The theoretical predictions are shown to agree with the available experimental
data within the experimental uncertainty
High-pressure behavior of polyiodides confined into single-walled carbon nanotubes: A Raman study
International audienceThe high-pressure behavior of polyiodides confined into the hollow core of single-walled carbon nanotubes organized into bundles has been studied by means of Raman spectroscopy. Several regimes of the structural properties are observed for the nanotubes and the polyiodides under pressure. Raman responses of both compounds exhibit correlations over the whole pressure range (0–17 GPa). Modifications, in particular, take place, respectively, between 1 and 2.3 GPa for polyiodides and between 7 and 9 GPa for nanotubes, depending on the experiment. Differences between one experiment to another are discussed in terms of nanotube filling homogeneity. These transitions can be presumably assigned to the tube ovalization pressure and to the tube collapse pressure. A nonreversibility of several polyiodide mode modifications is evidenced and interpreted in terms of a progressive linearization of the iodine polyanions and a reduction in the charged species on pressure release. Furthermore, the significant change in the mode intensities could be associated to an enhancement of lattice modes, suggesting the formation of a new structure inside the nanotube. Changes in the nanotube mode positions after pressure release point out a decrease in the charge transfer in the hybrid system consistent with the observed evolution of the charged species
High-pressure behavior of polyiodides confined into single-walled carbon nanotubes: A Raman study
International audienceThe high-pressure behavior of polyiodides confined into the hollow core of single-walled carbon nanotubes organized into bundles has been studied by means of Raman spectroscopy. Several regimes of the structural properties are observed for the nanotubes and the polyiodides under pressure. Raman responses of both compounds exhibit correlations over the whole pressure range (0–17 GPa). Modifications, in particular, take place, respectively, between 1 and 2.3 GPa for polyiodides and between 7 and 9 GPa for nanotubes, depending on the experiment. Differences between one experiment to another are discussed in terms of nanotube filling homogeneity. These transitions can be presumably assigned to the tube ovalization pressure and to the tube collapse pressure. A nonreversibility of several polyiodide mode modifications is evidenced and interpreted in terms of a progressive linearization of the iodine polyanions and a reduction in the charged species on pressure release. Furthermore, the significant change in the mode intensities could be associated to an enhancement of lattice modes, suggesting the formation of a new structure inside the nanotube. Changes in the nanotube mode positions after pressure release point out a decrease in the charge transfer in the hybrid system consistent with the observed evolution of the charged species
Reversible optical doping of graphene
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/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
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