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 SiO2/Si substrate can be finely and reversibly tuned between hole and electron 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 requires hydrophilic substrates and vanishes for suspended graphene. These findings suggest that optically gated graphene devices operating with a sub-second time scale can be envisioned and that Raman spectroscopy is not always as non-invasive as generally assumed.

Computation of the infrared active modes in single-walled boron nitride nanotube bundles.

In this work, the infrared active modes are computed for homogeneous bundles of single-walled boron nitride nanotubes (BBNNTs), using the so-called spectral moments method. The dependence of the wavenumber on these modes in terms of diameters, lengths, and numbers of tubes, is investigated. To this end, use is made of a Lennard-Jones potential for describing the van der Waals interactions between tubes in a bundle. We find that, for a finite homogeneous bundle, additional modes appear as a specific signature. Finally, these results are useful for the interpretation of the experimental infrared spectra of BBNNTs.

Experimental evidence of a mechanical coupling between layers in an individual double-walled carbon nanotube.

We perform transmission electron microscopy, electron diffraction, and Raman scattering experiments on an individual suspended double-walled carbon nanotube (DWCNT). The first two techniques allow the unambiguous determination of the DWCNT structure: (12,8)@(16,14). However, the low-frequency features in the Raman spectra cannot be connected to the derived layer diameters d by means of the 1/d power law, widely used for the diameter dependence of the radial-breathing mode of single-walled nanotubes. We discuss this disagreement in terms of mechanical coupling between the layers of the DWCNT, which results in collective vibrational modes. Theoretical predictions for the breathing-like modes of the DWCNT, originating from the radial-breathing modes of the layers, are in a very good agreement with the observed Raman spectra. Moreover, the mechanical coupling qualitatively explains the observation of Raman lines of breathing-like modes, whenever only one of the layers is in resonance with the laser energy.

Raman spectra of C60 dimer and C60 polymer confined inside a (10, 10) single-walled carbon nanotube.

A new set of C-C interball force constant was developed in order to reproduce the low wavenumber density of states measured by neutron scattering and the Raman spectra of the C(60) dimer and C(60) polymer chain. The nonresonant Raman spectra of the C(60) dimer and C(60) polymer confined inside a (10, 10) single-walled carbon nanotube were calculated in the framework of the bond-polarization theory by using the spectral moments method. The main changes of the Raman spectrum as a function of the organization of the C(60) molecules inside the nanotubes were identified. We found that the radial breathing modes of a (10, 10) single-walled carbon nanotube are more sensitive on the structure of the C(60) molecules than the G-modes. These predictions are useful to interpret the experimental Raman spectrum of fullerene peapods.

Lattice dynamics of oligo(phenylenethienylene)s: a far-infrared and inelastic neutron scattering study.

New promising oligo(phenylenethienylene)s have been synthesized to realize suitable materials for improving electronic transport properties, particularly in organic field effect transistors (OFETs). Far-infrared and incoherent neutron scattering measurements have been performed to assign their phonon modes. The assignment of the main low-frequency phonon modes of these materials has been performed experimentally by using a filiation procedure. Assuming a small frequency dispersion of the high-frequency modes, the main intramolecular phonon modes of a model oligomer have been assigned by using first-principles calculations on its isolated molecule. These assignments constitute the preliminary work for a better understanding of these new promising materials in electronic and opto-electronics applications.

Raman-active modes in homogeneous and inhomogeneous bundles of single-walled carbon nanotubes.

In the present work, the non-resonant Raman-active modes are calculated for several diameters, chiralities and sizes for homogeneous and inhomogeneous bundles of single-walled carbon nanotubes (BWCNTs), using the spectral moment's method (SMM). Additional intense Raman-active modes are present in the breathing-like modes (BLM) spectra of these systems in comparison with a single fully symmetric A(1g) mode characteristic of isolated nanotubes (SWCNTs). The dependence of the wavenumber of these modes in terms of diameters, lengths and number of tubes was investigated. We found that, for a finite (in)homogeneous bundle, additional breathing-like modes appear as a specific signature.

How confinement affects the dynamics of c60 in carbon nanopeapods.

The dynamics of confined systems is of major concern for both fundamental physics and applications. In this Letter, the dynamics of C60 fullerene molecules inside single walled carbon nanotubes is studied using inelastic neutron scattering. We identify the C60 vibrations and highlight their sensitivity to temperature. Moreover, a clear signature of rotational diffusion of the C60 is evidenced, which persists at lower temperature than in 3D bulk C60. It is discussed in terms of confinement and of reduced dimensionality of the C60 chain.

Correlations between structure and far-infrared active modes in polythiophenes.

We have investigated the experimental X-ray and far-infrared responses of three polythiophenes synthesized from a thiophene, alpha-bithiophene, and alpha-quaterthiophene monomer. The X-ray data show that the crystallinity of the different polythiophene samples depends on the synthesis conditions. An excellent correlation between the crystallinity of polythiophenes and their far-infrared signatures is demonstrated. In addition, the assignment of the far-infrared phonon modes in polythiophenes is given by using both an experimental filiation procedure and first-principles calculations. In particular, the ring libration inside the polymeric chain, directly involved in the electron-phonon coupling, is assigned.

Raman active phonons of identified semiconducting single-walled carbon nanotubes.

The resonant Raman spectra of (n, m) semiconducting single-walled carbon nanotubes, unambiguously identified from their electron diffraction patterns, have been measured. The diameter dependence of the frequency of the tangential modes with A symmetry has been obtained in the diameter range from 1.4 to 2.5 nm. The comparison between the excitation energies and the calculated transition energies allowed us to determine precisely the values of the Es33 and Es44 transition energies. Finally, in the debate concerning the dominant process at the origin of the first-order Raman scattering in single-walled carbon nanotubes (single resonance process or double resonance process), our results are well understood in the framework of a single resonance process.

Infrared spectroscopy of single-walled carbon nanotubes.

By using the spectral moments method, we calculate the infrared spectra of chiral and achiral single-walled carbon nanotubes (SWCNTs) of different diameters and lengths. We show that the number of the infrared modes, their frequencies, and intensities depend on the length and chirality of the nanotubes. Furthermore, the dependence of the infrared spectrum as a function of the size of the SWCNT bundle is analyzed. These predictions are useful to interpret the experimental infrared spectra of SWCNTs.

Raman spectroscopy of free-standing individual semiconducting single-wall carbon nanotubes.

The radial breathing modes and tangential modes have been systematically measured on a large number of individual semiconducting single-wall carbon nanotubes (thin bundles) suspended between plots (free-standing single-wall carbon nanotubes). The strong intensity of the Raman spectra ensures the precision of the experimentally determined line shapes and frequencies of these modes. The diameter dependence of the frequencies of the tangential modes was measured. This dependence is discussed in relation with recent calculations. The present data confirm/contradict some previous interpretations.

Vanishing of the Breit-Wigner-Fano component in individual single-wall carbon nanotubes.

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.

Far- and mid-infrared of crystalline 2,2'-bithiophene: ab initio analysis and comparison with infrared response.

Infrared intramolecular vibrations and lattice modes in the crystalline phase of 2,2'-bithiophene (2T) are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. For the first time, the far- and mid-infrared responses have been calculated from the Gamma-point modes and the Born effective charge tensors of the 2T crystalline phase. The relative good agreement between the calculated and experimental infrared spectra allows us to assign the origin of the main features of the experimental spectra, which is of particular interest in the far-infrared domain. These assignments are useful for understanding all the properties of the 2T crystalline phase in which phonon-phonon and electron-phonon interactions play an important role.

Polymorphism of crystalline alpha-quaterthiophene and alpha-sexithiophene: ab initio analysis and comparison with inelastic neutron scattering response.

Phonons in the alpha-quaterthiophene (4T) and alpha-sexithiophene (6T) polymorph phases are investigated using the direct method combined with density functional theory (DFT)-based total energy calculations. The simulation of inelastic neutron scattering spectra (INS) on the LT and HT polymorph phases of 4T and 6T enable the corresponding spectral signatures of these materials to be identified. In particular, there are two fingerprints: (i) the low-frequency vibrational modes (frequencies lower than 200 cm(-1)) and (ii) the vibrational modes in the 600-900 cm(-1) frequency range. The good agreement with the INS experimental data allows us to assign unambiguously the origin of all features (first-order and high-order processes) of these spectra and to predict that the LT phase is the phase measured experimentally both on the 4T and 6T materials. Moreover, the broad background in the 600-1400 cm(-1) frequency range and the well-defined features which appear around 940 cm(-1) in the calculated INS spectra of 4T/HT and 6T/HT are assigned to multiphonon contributions. This multiphonon contribution at 940 cm(-1), which is absent in the 4T/LT and 6T/LT INS spectra, also constitutes a fingerprint of the HT phases. Finally, the calculated dispersion curves of the two polymorph phases of 4T and 6T are given.

Solid state polymorphism of liquid crystals in confined geometries.

Solid polymorphism of 4-alkyl-4'-cyanobiphenyl (nCB) was studied so far as a function of thermal history. In this paper we show that metastable solid phases of 4-octyl-4'-cyanobiphenyl (8CB) are also formed when the mesogens are confined in porous silica matrices and we study their structure by neutron diffraction and by Raman spectroscopy. Three metastable solid states are identified: one crystalline phase K', two frozen-in smectic-like phases K(s) and K'(s). We discuss the relation between the structure of the metastable solid phases and that of the mesomorph phases.

Switching behavior and electro-optical properties of liquid crystals in nematic gels

Anisotropic nematic gels are prepared via in situ polymerization of diacrylate monomers in an orientated nematic liquid crystal (LC) matrix. The switching behavior of the LC molecules under electric field is probed in polarized Raman spectroscopy and straight theta-2straight theta elastic light scattering experiments. The electro-optical characteristics of the gels are directly related to the electric field dependence of the fraction of switched molecules. The electro-optical contrast relates to the coexistence of switched LC domains and LC domains anchored to the polymer network.

Phonon density of states of single-wall carbon nanotubes

The vibrational density of states of single-wall carbon nanotubes (SWNT) was obtained from inelastic neutron scattering data from 0 to 225 meV. The spectrum is similar to that of graphite above 40 meV, while intratube features are clearly observed at 22 and 36 meV. An unusual energy dependence below 10 meV is assigned to contributions from intertube modes in the 2D triangular lattice of SWNT bundles, and from intertube coupling to intratube excitations. Good agreement between experiment and a calculated density of states for the SWNT lattice is found over the entire energy range.