The structural properties of the multi-layer graphene/4H-SiC(000-1) system as determined by Surface X-ray Diffraction

We present a structural analysis of the multi-layer graphene-4HSiC(000-1}) system using Surface X-Ray Reflectivity. We show for the first time that graphene films grown on the C-terminated (000-1}) surface have a graphene-substrate bond length that is very short (0.162nm). The measured distance rules out a weak Van der Waals interaction to the substrate and instead indicates a strong bond between the first graphene layer and the bulk as predicted by ab-initio calculations. The measurements also indicate that multi-layer graphene grows in a near turbostratic mode on this surface. This result may explain the lack of a broken graphene symmetry inferred from conduction measurements on this system [C. Berger et al., Science 312, 1191 (2006)].Comment: 9 pages with 6 figure

Raman Topography and Strain Uniformity of Large-Area Epitaxial Graphene

We report results from two-dimensional Raman spectroscopy studies of large-area epitaxial graphene grown on SiC. Our work reveals unexpectedly large variation in Raman peak position across the sample resulting from inhomogeneity in the strain of the graphene film, which we show to be correlated with physical topography by coupling Raman spectroscopy with atomic force microscopy. We report that essentially strain free graphene is possible even for epitaxial graphene.Comment: 10 pages, 3 figure

Raman spectra of epitaxial graphene on SiC and of epitaxial graphene transferred to SiO2

Raman spectra were measured for mono-, bi- and trilayer graphene grown on SiC by solid state graphitization, whereby the number of layers was pre-assigned by angle-resolved ultraviolet photoemission spectroscopy. It was found that the only unambiguous fingerprint in Raman spectroscopy to identify the number of layers for graphene on SiC(0001) is the linewidth of the 2D (or D*) peak. The Raman spectra of epitaxial graphene show significant differences as compared to micromechanically cleaved graphene obtained from highly oriented pyrolytic graphite crystals. The G peak is found to be blue-shifted. The 2D peak does not exhibit any obvious shoulder structures but it is much broader and almost resembles a single-peak even for multilayers. Flakes of epitaxial graphene were transferred from SiC onto SiO2 for further Raman studies. A comparison of the Raman data obtained for graphene on SiC with data for epitaxial graphene transferred to SiO2 reveals that the G peak blue-shift is clearly due to the SiC substrate. The broadened 2D peak however stems from the graphene structure itself and not from the substrate.Comment: 27 pages, 8 figure

Rayleigh Imaging of Graphene and Graphene Layers

We investigate graphene and graphene layers on different substrates by monochromatic and white-light confocal Rayleigh scattering microscopy. The image contrast depends sensitively on the dielectric properties of the sample as well as the substrate geometry and can be described quantitatively using the complex refractive index of bulk graphite. For few layers (<6) the monochromatic contrast increases linearly with thickness: the samples behave as a superposition of single sheets which act as independent two dimensional electron gases. Thus, Rayleigh imaging is a general, simple and quick tool to identify graphene layers, that is readily combined with Raman scattering, which provides structural identification.Comment: 8 pages, 9 figure

Probing Mechanical Properties of Graphene with Raman Spectroscopy

The use of Raman scattering techniques to study the mechanical properties of graphene films is reviewed here. The determination of Gruneisen 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.Comment: To appear in the Journal of Materials Scienc

Technique for the Dry Transfer of Epitaxial Graphene onto Arbitrary Substrates

In order to make graphene technologically viable, the transfer of graphene films to substrates appropriate for specific applications is required. We demonstrate the dry transfer of epitaxial graphene (EG) from the C-face of 4H-SiC onto SiO2, GaN and Al2O3 substrates using a thermal release tape. We further report on the impact of this process on the electrical properties of the EG films. This process enables EG films to be used in flexible electronic devices or as optically transparent contacts.Comment: 8 pages, 4 figures and supplementary info regarding procedure for transfe

In Situ SR-XPS Observation of Ni-Assisted Low-Temperature Formation of Epitaxial Graphene on 3C-SiC/Si

Low-temperature (~1073 K) formation of graphene was performed on Si substrates by using an ultrathin (2 nm) Ni layer deposited on a 3C-SiC thin film heteroepitaxially grown on a Si substrate. Angle-resolved, synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS) results show that the stacking order is, from the surface to the bulk, Ni carbides(Ni(3)C/NiC(x))/graphene/Ni/Ni silicides (Ni(2)Si/NiSi)/3C-SiC/Si. In situ SR-XPS during the graphitization annealing clarified that graphene is formed during the cooling stage. We conclude that Ni silicide and Ni carbide formation play an essential role in the formation of graphene

Unoccupied surface state on the (√3 × √3) R30° of 6H-SiC(0001)

Applying k-resolved inverse photoemission (KRIPES) to the $\sqrt 3\times \sqrt3$ $\rm R30^\circ$-reconstructed 6H-SiC(0001) face, we have observed a sharp surface state U located at $1.10 \pm 0.05\;{\rm eV}$ above the Fermi level at the centre of the surface Brillouin zone. Its bandwidth of $0.34 \pm 0.05\;{\rm eV}$ is in good agreement with the 0.35 eV predicted by first-principle calculations based on a Si-adatom model. However, LDA calculations predict a half-filled {\mit\Sigma}_1 state and a metallic character for this reconstruction. Together with recent ARUPS data, our results reveal that the one-electron band {\mit\Sigma}_1 is split into two bands, giving a semiconducting surface with a reduced indirect bandgap around 2.0 eV at the $\bar K'$ point. Many-body correlation effects may give rise, in the limit of strong localization, to this bandgap opening

Solid-state graphitization mechanisms of silicon carbide 6H-SiC polar faces

International audienceDue to the higher vapour pressure of silicon, silicon carbide surfaces annealed at high temperature under vacuum tend to graphitize. The comparison of graphite formation on the silicon and carbon terminations of 6H-SiC reveals significant differences in the graphitization mechanisms involved. The conduction-band structure of these interfaces has been Ž. determined by angle-resolved inverse photoemission spectroscopy KRIPES. Although the graphite layers grown on the C face are essentially polycrystalline, a small fraction of the film keeps a preferred orientation, where the graphite lattice basis vectors are rotated by 308 with respect to the basis vectors of the SiC lattice as in the case of the Si face. This in-plane disorder is in contrast with the growth of graphite on the Si face that takes place on a ''passivated'' adatom-terminated surface, leading to single-crystalline, heteroepitaxial graphite growth. The observation of unshifted p) states indicates a very small interaction of the first graphite monolayer with the Si face. In contrast, KRIPES reveals that the first graphite layer is strongly bound to the C face. A rehybridization of the graphite p) states with occupied orbitals of the substrate is inferred from an observed increase in the density of states in the vicinity of the Fermi level.