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

Raman-modes of index-identified free-standing single-walled carbon nanotubes

Using electron diffraction on free-standing single-walled carbon nanotubes we have determined the structural indices (n,m) of tubes in the diameter range from 1.4 to 3nm. On the same free-standing tubes we have recorded Raman spectra of the tangential modes and the radial breathing mode. For the smaller diameters (1.4-1.7nm) these measurements confirm previously established radial breathing mode frequency versus diameter relations, and would be consistent with the theoretically predicted proportionality to the inverse diameter. However, for extending the relation to larger diameters, either a yet unexplained environmental constant has to be assumed, or the linear relation has to be abandoned.Comment: 4 pages, 4 figures, +additional materials (select PostScript to obtain it

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$_2$/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

Vulnerability of optical detection systems to megajoule class laser radiative environment

The Laser MegaJoule (LMJ) facility will host inertial confinement fusion experiments in order to achieve ignition by imploding a Deuterium-Tritium filled microballoon [1]. In this context an X-ray imaging system is necessary to diagnose the core size and the shape of the target in the 10-100 keV band. Such a diagnostic will be composed of two parts: an X-ray optical system and a detection assembly. The survivability of each element of this diagnostic has to be ensured within the mixed pulse consisting of X-rays, gamma rays and 14 MeV neutrons created by fusion reactions. The design of this diagnostic will take into account optics and detectors vulnerability to neutron yield of at least 1016. In this work, we will present the main results of our vulnerability studies and of our hardening-by-system and hardening-by-design studies

Buffer layers inhomogeneity and coupling with epitaxial graphene unravelled by Raman scattering and graphene peeling

The so-called buffer layer (BL) is a carbon rich reconstructed layer formed during the sublimation of SiC (0001). The existence of covalent bonds between some of the carbon atoms in this layer and the underlying silicon atoms makes it different from epitaxial graphene. We report a systematical and statistical investigation of the BL signature and its coupling with epitaxial graphene by Raman spectroscopy. Three different kinds of BLs are studied: bare buffer layer obtained by direct growth (BL 0), interfacial buffer layer situated between graphene and SiC (c-BL 1) and the interfacial buffer layer without graphene above (u-BL 1). To obtain the latter, we develop a mechanical exfoliation of graphene by depositing and subsequently removing an epoxy-based resin or nickel layer. The observed BLs are ordered-like on the whole BL growth temperature range. BL 0 Raman signature may vary from sample to sample but also forms patches on the same terrace. u-BL 1 share similar properties with BL 0 , albeit with more variability. These BLs have a strikingly larger overall intensity than BL with graphene on top. The signal onset on the high frequency side upshifts upon graphene coverage, that cannot be explained by a simple strain effect. Two fine peaks situated at 1235 and 1360 cm-1 are present for epitaxial monolayer while absent for BL and transferred graphene. These findings point to a coupling between graphene and BL

Substrate transfer and ex situ characterization of on-surface synthesized graphene nanoribbons

Recent progress in the on-surface synthesis of graphene nanoribbons (GNRs) has given access to atomically precise narrow GNRs with tunable electronic band gaps that makes them excellent candidates for room-temperature switching devices such as field-effect transistors (FET). However, in spite of their exceptional properties, significant challenges remain for GNR processing and characterization. This contribution addresses some of the most important challenges, including GNR fabrication scalability, substrate transfer, long-term stability under ambient conditions and ex situ characterization. We focus on 7- and 9-atom wide armchair graphene nanoribbons (i.e, 7-AGNR; and 9-AGNR) grown on 200 nm Au(111)/mica substrates using a high throughput system. Transfer of both, 7- and 9-AGNRs from their Au growth sub-strate onto various target substrates for additional characterization is accomplished utilizing a polymer-free method that avoids residual contamination. This results in a homogeneous GNR film morphology with very few tears and wrinkles, as examined by atomic force microscopy. Raman spectroscopy indicates no significant degradation of GNR quality upon substrate transfer, and reveals that GNRs have remarkable stability under ambient conditions over a 24-month period. The transferred GNRs are analyzed using multi-wavelength Raman spectroscopy, which provides detailed insight into the wavelength dependence of the width-specific vibrational modes. Finally, we characterize the optical properties of 7- and 9-AGNRs via ultra-violet-visible (UV-Vis) spectroscopyComment: 30 pages, 14 figure

Synthèse et propriétés physiques de nanotubes de carbone monofeuillets individuels

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Graphene and related 2D materials: An overview of the Raman studies

International audienceA After a brief overview of the discovery and the Raman study of new forms of carbons (intercalated graphite, carbon fiber, fullerenes, carbon nanotubes), the invaluable contribution of late Professor M. Dresselhaus is noted and the 10 reviews and 10 contributions collected to present a picture of the present Raman investigations of graphene and related 2D materials (such as black phosphorus, MoS2) are presented. Methods for numbering the graphene layers, the effects of external perturbations (temperature, pressure, doping and magnetic field) on the phonons of graphene, characterization of the chemical and structural properties of graphene at the nanoscale level by tip-enhanced Raman spectroscopy (TERS), surface enhanced Raman spectroscopy (SERS) and hyperspectral imaging, and applications combining graphene and Raman spectroscopy are addressed