The early stages of epitaxial graphene layer growth on the Si-terminated 6H-SiC(0001) are investigated by Auger electron spectroscopy (AES) and depolarized Raman spectroscopy. The selection of the depolarized component of the scattered light results in a significant increase in the C-C bond signal over the second order SiC Raman signal, which allows to resolve submonolayer growth, including individual, localized C=C dimers in a diamond-like carbon matrix for AES C/Si ratio of ∼3, and a strained graphene layer with delocalized electrons and Dirac single-band dispersion for AES C/Si ratio >6. The linear strain, measured at room temperature, is found to be compressive, which can be attributed to the large difference between the coefficients of thermal expansion of graphene and SiC. The magnitude of the compressive strain can be varied by adjusting the growth time at fixed annealing temperature.PACS numbers: 81.05. Uw, 65.40.De, Graphene has been shown recently to possess many of the remarkable electronic properties of carbon nanotubes [1], while lending itself more readily to the planar paradigm of integrated-circuit fabrication processes. Epitaxial graphene (epigraphene) on silicon carbide (SiC) surfaces is emerging as an attractive process alternative to the painstaking layer-by-layer exfoliation of graphite crystals [1,2,3,4,5]. Epigraphene shares the key electron transport properties of free-standing exfoliated films [1,2,3]. However, in contrast to the exfoliated films, new features in the electronic structures appear in epitaxial graphene grown on the (0001) surface of 6H-SiC, whose origins remain controversial [6,7,8,9]. The novel electronic properties and the choice of the type of substrate are significant for the design of nonlinear devices and underscores the importance of substrate interactions in epitaxial films. This Letter is focussed on Si-terminated 6H-SiC(0001).The interaction of epigraphene with SiC substrate is mediated by a monolayer of C atoms, so called "buffer layer", arranged in a honeycomb lattice, like graphene, but bonded in sp 3 configuration, with each atom forming a covalent bond to a Si atom beneath [5]. This buffer layer evolves from C-rich, high temperature surface reconstructions of 6H-SiC (0001) upon thermal desorption of Si atoms around T=1100• C. Annealing to higher temperature (1250• C) results in further desorption of Si, which promotes the formation of a second carbon layer, and deprives the original (topmost) carbon atoms of their covalent bonds to Si atoms, inducing sp 2 bonding configuration, i.e., into that of a graphene layer [2,10]. This paper presents a detailed investigation of the processes by which Si loss occurs, and its effect on the bonding, electronic and mechanical properties of the resulting carbon film. Our observations are made possible by the polarization analysis of the Raman signal from the sample surface. Although unpolarized Raman spectroscopy has been widely used in characterizing graphene layers [11,12,13,14,15,16,17,18,19], this is, to the best our k...

Large-scale utilization of solar-energy resources will require considerable advances in energy-storage technologies to meet ever-increasing global energy demands. Other than liquid fuels, existing energy-storage materials do not provide the requisite combination of high energy density, high stability, easy handling, transportability and low cost. New hybrid solar thermal fuels, composed of photoswitchable molecules on rigid, low-mass nanostructures, transcend the physical limitations of molecular solar thermal fuels by introducing local sterically constrained environments in which interactions between chromophores can be tuned. We demonstrate this principle of a hybrid solar thermal fuel using azobenzene-functionalized carbon nanotubes. We show that, on composite bundling, the amount of energy stored per azobenzene more than doubles from 58 to 120 kJ mol(-1), and the material also maintains robust cyclability and stability. Our results demonstrate that solar thermal fuels composed of molecule-nanostructure hybrids can exhibit significantly enhanced energy-storage capabilities through the generation of template-enforced steric strain.

Metal-catalyzed crystallization of amorphous carbon to graphene by thermal annealing is demonstrated. In this “limited source” process scheme, the thickness of the precipitated graphene is directly controlled by the thickness of the initial amorphous carbon layer. This is in contrast to chemical vapor deposition processes, where the carbon source is virtually unlimited and controlling the number of graphene layers depends on the tight control over a number of deposition parameters. Based on the Raman analysis, the quality of graphene is comparable to other synthesis methods found in the literature, such as chemical vapor deposition. The ability to synthesize graphene sheets with tunable thickness over large areas presents an important progress toward their eventual integration for various technological applications.

The use of Raman scattering techniques to study the mechanical properties of graphene films is reviewed here. The determination of Grüneisen parameters of suspended graphene sheets under uni-and bi-axial strain is discussed, and the values are compared to theoretical predictions. The effects of the graphene-substrate interaction on strain and to the temperature evolution of the graphene Raman spectra are discussed. Finally, the relation between mechanical and thermal properties is presented along with the characterization of thermal properties of graphene with Raman spectroscopy.

Carbon materials are excellent candidates for photovoltaic solar cells: they are Earth-abundant, possess high optical absorption, and maintain superior thermal and photostability. Here we report on solar cells with active layers made solely of carbon nanomaterials that present the same advantages of conjugated polymer-based solar cells, namely, solution processable, potentially flexible, and chemically tunable, but with increased photostability and the possibility to revert photodegradation. The device active layer composition is optimized using ab initio density functional theory calculations to predict type-II band alignment and Schottky barrier formation. The best device fabricated is composed of PC(70)BM fullerene, semiconducting single-walled carbon nanotubes, and reduced graphene oxide. This active-layer composition achieves a power conversion efficiency of 1.3%-a record for solar cells based on carbon as the active material-and we calculate efficiency limits of up to 13% for the devices fabricated in this work, comparable to those predicted for polymer solar cells employing PCBM as the acceptor. There is great promise for improving carbon-based solar cells considering the novelty of this type of device, the high photostability, and the availability of a large number of carbon materials with yet untapped potential for photovoltaics. Our results indicate a new strategy for efficient carbon-based, solution-processable, thin film, photostable solar cells.

Raman spectroscopy is widely used to evaluate the nature and potential origins of carbonaceous matter in Earth's oldest rocks and minerals. It is also the tool that will be used for organic detection on the next vehicles to remotely explore the surface of Mars.Here we present, for the first time, a novel quantitative method in which previously neglected Raman spectral features are correlated directly, linearly, and with excellent accuracy, to the microchemistry of carbonaceous materials through the elemental H:C ratio, regardless of contamination. We show applicability and predictive capabilities of this methodology in evaluating H:C ratios between 0.01 and 0.65 in Archean and type III kerogens. We demonstrate its application to chemical microRaman mapping by statistical analysis of a 750Ma microfossil and its encompassing sediments. Raman-derived H:C data can also be used to estimate the degree to which kerogen C-isotopic data has been shifted from its original values due to the effects of metamorphism. The new methodology directly and non-invasively affords spatially resolved assessments of organic matter preservation and microscale chemical diversity within any geologically 2 preserved terrestrial or extraterrestrial sample, including in the use of organic matter in technological applications.

During the past six years, the adsorption geometries of several rare gases in structures having several different symmetries on a variety of substrates were determined using low-energy electron diffraction (LEED). In most of these studies, a preference is found for the rare gas atoms to adsorb in the low-coordination sites. Only in the case of adsorption on graphite has a clear preference for a high-coordination site for a rare gas atom been found. This unexpected behaviour is not yet completely understood, although recent density functional theory (DFT) calculations for these and similar surfaces suggest that this is a general phenomenon. This paper reviews the early studies that were presages of the discovery of top site adsorption for rare gases, the discovery itself, and the present state of understanding of this curiosity. It also details some of the features of the LEED experiments and analysis that are specific to the case of rare gas adsorption.

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