International audienceno abstrac

We present epitaxial graphene (EG) growth on nonpolar 6H-SiC-faces by solid-state decomposition of the SiC substrate in the Knudsen flow regime in vacuum. The material characteristics are compared with those known for EG grown on polar SiC-faces under similar growth conditions. X-ray photoelectron spectroscopy (XPS) measurements indicate that nonpolar faces have thicker layers than polar faces. Among nonpolar faces, the m-plane (11̅ 00) has thicker layers than the aplane (112̅ 0). Atomic force microscopy (AFM) shows nanocrystalline graphite features for nonpolar faces, consistent with the small grain size measured by Raman spectroscopy. This is attributed to the lack of a hexagonal template, unlike on the polar Si-and C-faces. These nonpolar face EG films exhibited stress decreasing with increasing growth temperature. These variations are interpreted on the basis of different growth mechanisms on the various faces, as expected from the large differences in surface energy and step dynamics on the various SiC surfaces. Surfaces with smaller grain sizes systematically exhibited thicker layers. Using this observation, we argue that multilayer EG growth, after the nucleation of the first layers, is determined primarily by Si diffusion through grain boundaries and defects, as Si cannot diffuse through a perfect graphene lattice. A greater density of grain boundaries allows more Si to escape during growth, allowing thicker layers of carbon to be grown.

We show ~10x polariton-enhanced infrared reflectivity of epitaxial graphene on 4H-SiC, in SiC's restrahlen band (8-10µm). By fitting measurements to theory, we extract the thickness, N, in monolayers (ML), momentum scattering time (), Fermi level position (E F ) of graphene and estimate carrier mobility. By showing that 1/n s , the carrier concentration/ML, we argue that scattering is dominated by short-range interactions at the SiC/graphene interface. Polariton formation finds application in near-field optical devices such as superlenses.Graphene, a two-dimensional (2D) form of carbon in a honeycomb crystal structure, is the basic building block of other sp 2 carbon nanomaterials, such as carbon nanotubes. It exhibits unusual electronic and optical properties [1][2][3][4][5][6]. Graphene has a dispersionless linear electronic band structure as opposed to the quadratic form observed for most semiconductors. This leads to "massless" Dirac-fermion behavior, and consequently, high electron mobility, as opposed to the usual Schrodinger behavior exhibited by most semiconductors [6][7]. Furthermore, the recent development of epitaxial graphene (EG) formed by the solid-state decomposition of a SiC surface has enabled the systematic production of large area graphene films on a commercial substrate platform. This has prompted the investigation of many high performance electronic devices, such as field effect transistors and p-n junction diodes, photonic devices such as terahertz oscillators, as well as low noise sensors [8][9][10][11]. For all of these applications, knowledge of the optical properties of graphene is important, as it gives insight into the interaction of graphene with external electromagnetic fields.

Electrochemical functionalization and possible hydrogenation of treated epitaxial graphene samples on 6H-SiC are presented. To attract H+ ions to react with the exposed working cathode, a 10% sulfuric acid electrolyte was used with a Pt counter anode. Functionalization was determined using Raman spectroscopy and measured by a marked increase in I(D)/I(G) ratio and introduction of C-H bond peak at ∼2930 cm−1. There was also a marked increase in fluorescence background, which clearly differentiates functionalization from lattice damage in the graphene. Quantifying the fluorescence, we estimate that H-incorporation as high as 50% was achieved based on results on hydrocarbons, although other functional groups cannot be excluded. We further distinguished these functionalization signatures from lattice damage through measurements on nanocrystalline graphene on a and m plane SiC, which displayed very different surface morphologies and no measureable fluorescence. Finally, we show that the extent of functionalization is strongly substrate dependent by using samples cut from three semi-insulating 6H-SiC substrates with similar resistivity but orientations varying from on-axis (∼0.02°), 0.5° to 1.0° off-axis. This functionalization was found to be thermally reversible at ∼1000 °C. Scanning tunneling spectroscopy indicates the presence of sp3-like localized states not present in the starting graphene, further supporting the assertion that functionalization has occurred.

Q1 5 6 a b s t r a c t a r t i c l e i n f o 7