Large and Flat Graphene Flakes Produced by Epoxy Bonding and Reverse Exfoliation of Highly Oriented Pyrolitic Graphite

We present a fabrication method producing large and flat graphene flakes that have a few layers down to a single layer based on substrate bonding of a thick sample of highly oriented pyrolytic graphite (HOPG), followed by its controlled exfoliation down to the few to single graphene atomic layers. As the graphite underlayer is intimately bonded to the substrate during the exfoliation process, the obtained graphene flakes are remarkably large and flat and present very few folds and pleats. The high occurrence of single layered graphene sheets having tens of micron wide in lateral dimensions is assessed by complementary probes including spatially resolved Micro-Raman Spectroscopy, Atomic Force Microscopy and Electrostatic Force Microscopy. This versatile method opens the way of deposition of graphene on any substrates including flexible ones.Comment: 15 pages 5 figure

Epitaxial graphene prepared by chemical vapor deposition on single crystal thin iridium films on sapphire

Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e. much higher than on metal thin films used so far.Comment: To appear in Appl. Phys. Let

Deviation from the normal mode expansion in a coupled graphene-nanomechanical system

We optomechanically measure the vibrations of a nanomechanical system made of a graphene membrane suspended on a silicon nitride nanoresonator. When probing the thermal noise of the coupled nanomechanical device, we observe a significant deviation from the normal mode expansion. It originates from the heterogeneous character of mechanical dissipation over the spatial extension of coupled eigenmodes, which violates one of the fundamental prerequisite for employing this commonly used description of the nanoresonators' thermal noise. We subsequently measure the local mechanical susceptibility and demonstrate that the fluctuation-dissipation theorem still holds and permits a proper evaluation of the thermal noise of the nanomechanical system. Since it naturally becomes delicate to ensure a good spatial homogeneity at the nanoscale, this approach is fundamental to correctly describe the thermal noise of nanomechanical systems which ultimately impact their sensing capacity

Convergent Fabrication of a Nanoporous Two-Dimensional Carbon Network from an Aldol Condensation on Metal Surfaces

We report a convergent surface polymerization reaction scheme on Au(111), based on a triple aldol condensation, yielding a carbon-rich, covalent nanoporous two-dimensional network. The reaction is not self-poisoning and proceeds up to a full surface coverage. The deposited precursor molecules 1,3,5-tri(4'-acetylphenyl) first form supramolecular assemblies that are converted to the porous covalent network upon heating. The formation and structure of the network and of the intermediate steps are studied with scanning tunneling microscopy, Raman spectroscopy and density functional theory.Comment: 1 Scheme, 5 Figure

Structure in Nascent Carbon Nanotubes Revealed by Spatially Resolved Raman Spectroscopy

The understanding of carbon nanotubes (CNT) growth is crucial for the control of their production. In particular, the identification of structural changes of carbon possibly occurring near the catalyst particle in the very early stages of their formation is of high interest. In this study, samples of nascent CNT obtained during nucleation step and samples of vertically aligned CNT obtained during growth step are analysed by combined spatially resolved Raman spectroscopy and X-Ray diffraction measurements. Spatially resolved Raman spectroscopy reveals that iron-based phases and carbon phases are co-localised at the same position, and indicates that sp2 carbon nucleates preferentially on iron-based particles during this nucleation step. Depth scan Raman spectroscopy analysis, performed on nascent CNT, highlights that carbon structural organisation is significantly changing from defective graphene layers surrounding the iron-based particles at their base up to multi-walled nanotube structures in the upper part of iron-based particles

Phonon dispersion and low energy anomaly in CaC6_6

We report measurements of phonon dispersion in CaC$_6$ using inelastic X-ray and neutron scattering. We find good overall agreement, particularly in the 50 meV energy region, between experimental data and first-principles density-functional-theory calculations. However, on the longitudinal dispersion along the $(1 1 1)$ axis of the rhombohedral representation, we find an unexpected anti-crossing with an additional longitudinal mode, at about 11 meV. At a comparable energy, we observe also unexpected intensity on the in-plane direction. These results resolve the previous incorrect assignment of a longitudinal phonon mode to a transverse mode in the same energy range. By calculating the electron susceptibility from first principles we show that this longitudinal excitation is unlikely to be due to a plasmon and consequently can probably be due to defects or vacancies present in the sample.Comment: Accepted for publication in Physical Review

Strain superlattices and macroscale suspension of Graphene induced by corrugated substrates

We investigate the organized formation of strain, ripples and suspended features in macroscopic CVD-prepared graphene sheets transferred onto a corrugated substrate made of an ordered arrays of silica pillars of variable geometries. Depending on the aspect ratio and sharpness of the corrugated array, graphene can conformally coat the surface, partially collapse, or lay, fakir-like, fully suspended between pillars over tens of micrometers. Upon increase of pillar density, ripples in collapsed films display a transition from random oriented pleats emerging from pillars to ripples linking nearest neighboring pillars organized in domains of given orientation. Spatially-resolved Raman spectroscopy, atomic force microscopy and electronic microscopy reveal uniaxial strain domains in the transferred graphene, which are induced and controlled by the geometry. We propose a simple theoretical model to explain the transition between suspended and collapsed graphene. For the arrays with high aspect ratio pillars, graphene membranes stays suspended over macroscopic distances with minimal interaction with pillars tip apex. It offers a platform to tailor stress in graphene layers and open perspectives for electron transport and nanomechanical applications

Anharmonicity in Raman-active phonon modes in atomically thin MoS2_2

Phonon-phonon anharmonic effects have a strong influence on the phonon spectrum; most prominent manifestation of these effects are the softening (shift in frequency) and broadening (change in FWHM) of the phonon modes at finite temperature. Using Raman spectroscopy, we studied the temperature dependence of the FWHM and Raman shift of $\mathrm{E_{2g}^1}$ and $\mathrm{A_{1g}}$ modes for single-layer and natural bilayer MoS$_2$ over a broad range of temperatures ($8 <$T$< 300$ K). Both the Raman shift and FWHM of these modes show linear temperature dependence for $T>100$ K, whereas they become independent of temperature for $T<100$ K. Using first-principles calculations, we show that three-phonon anharmonic effects intrinsic to the material can account for the observed temperature-dependence of the line-width of both the modes. It also plays an important role in determining the temperature-dependence of the frequency of the Raman modes. The observed evolution of the line-width of the A$_{1g}$ mode suggests that electron-phonon processes are additionally involved. From the analysis of the temperature-dependent Raman spectra of MoS$_2$ on two different substrates -- SiO$_2$ and hexagonal boron nitride, we disentangle the contributions of external stress and internal impurities to these phonon-related processes. We find that the renormalization of the phonon mode frequencies on different substrates is governed by strain and intrinsic doping. Our work establishes the role of intrinsic phonon anharmonic effects in deciding the Raman shift in MoS$_2$ irrespective of substrate and layer number

In-plane magnetic domains and N\'eel-like domain walls in thin flakes of the room temperature CrTe2_2 van der Waals ferromagnet

The recent discovery of magnetic van der Waals materials has triggered a wealth of investigations in materials science, and now offers genuinely new prospects for both fundamental and applied research. Although the catalogue of van der Waals ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective x-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domains walls in micron-sized flakes of the CrTe$_2$ van der Waals ferromagnet. Flux-closure magnetic patterns suggesting in-plane six-fold symmetry are observed. Upon annealing the material above its Curie point (315 K), the magnetic domains disappear. By cooling back down the sample, a different magnetic domain distribution is obtained, indicating material stability and lack of magnetic memory upon thermal cycling. The domain walls presumably have N\'eel texture, are preferentially oriented along directions separated by 120 degrees, and have a width of several tens of nanometers. Besides microscopic mapping of magnetic domains and domain walls, the coercivity of the material is found to be of a few mT only, showing that the CrTe$_2$ compound is magnetically soft. The coercivity is found to increase as the volume of the material decreases