Semiconducting Graphene from Highly Ordered Substrate Interactions.

While numerous methods have been proposed to produce semiconducting graphene, a significant band gap has never been demonstrated. The reason is that, regardless of the theoretical gap formation mechanism, subnanometer disorder prevents the required symmetry breaking necessary to make graphene semiconducting. In this work, we show for the first time that semiconducting graphene can be made by epitaxial growth. Using improved growth methods, we show by direct band measurements that a band gap greater than 0.5 eV can be produced in the first graphene layer grown on the SiC(0001) surface. This work demonstrates that order, a property that remains lacking in other graphene systems, is key to producing electronically viable semiconducting graphene.

The bottom-up growth of edge specific graphene nanoribbons.

The discovery of ballistic transport in graphene grown on SiC(0001) sidewall trenches has sparked an intense effort to uncover the origin of this exceptional conductivity. How a ribbon's edge termination, width, and topography influence its transport is not yet understood. This work presents the first structural and electronic comparison of sidewall graphene grown with different edge terminations. We show that armchair and zigzag terminated ribbons, grown from SiC, have very different topographies and interact differently with the substrate, properties that are critical to device architecture in sidewall ribbon electronics.

Wide-gap semiconducting graphene from nitrogen-seeded SiC.

All carbon electronics based on graphene have been an elusive goal. For more than a decade, the inability to produce significant band-gaps in this material has prevented the development of graphene electronics. We demonstrate a new approach to produce semiconducting graphene that uses a submonolayer concentration of nitrogen on SiC sufficient to pin epitaxial graphene to the SiC interface as it grows. The resulting buckled graphene opens a band gap greater than 0.7 eV in the otherwise continuous metallic graphene sheet.

High island densities and long range repulsive interactions: Fe on epitaxial graphene.

The understanding of metal nucleation on graphene is essential for promising future applications, especially of magnetic metals which can be used in spintronics or computer storage media. A common method to study the grown morphology is to measure the nucleated island density n as a function of growth parameters. Surprisingly, the growth of Fe on graphene is found to be unusual because it does not follow classical nucleation: n is unexpectedtly high, it increases continuously with the deposited amount θ and shows no temperature dependence. These unusual results indicate the presence of long range repulsive interactions. Kinetic Monte Carlo simulations and density functional theory calculations support this conclusion. In addition to answering an outstanding question in epitaxial growth, i.e., to find systems where long range interactions are present, the high density of magnetic islands, tunable with θ, is of interest for nanomagnetism applications.

High Layer Uniformity of Two-Dimensional Materials Demonstrated Surprisingly from Broad Features in Surface Electron Diffraction.

Paradoxically, a very broad diffraction background, named the bell-shaped-component (BSC), has been established as a feature of graphene growth. Recent diffraction studies as a function of electron energy have shown that the BSC is not related to scattering interference. Here, additional experiments are carried out as a function of temperature over the range in which single-layer graphene (SLG) grows. Quantitative fitting of the profiles shows that the BSC follows the increase of the Gr(10) spot, proving directly that the BSC indicates high-quality graphene. Additional metal deposition experiments provide more information about the BSC. The BSC is insensitive to metal deposition, and it increases with metal intercalation, because a more uniform interface forms between graphene and SiC. These experiments support the conclusion that the BSC originates from electron confinement within SLG, and surprisingly, it is an excellent measure of graphene uniformity.

First direct observation of a nearly ideal graphene band structure.

Angle-resolved photoemission and x-ray diffraction experiments show that multilayer epitaxial graphene grown on the SiC(0001) surface is a new form of carbon that is composed of effectively isolated graphene sheets. The unique rotational stacking of these films causes adjacent graphene layers to electronically decouple leading to a set of nearly independent linearly dispersing bands (Dirac cones) at the graphene K point. Each cone corresponds to an individual macroscale graphene sheet in a multilayer stack where AB-stacked sheets can be considered as low density faults.

Revealing interfacial disorder at the growth-front of thick many-layer epitaxial graphene on SiC: a complementary neutron and X-ray scattering investigation.

Epitaxial graphene on SiC provides both an excellent source of high-quality graphene as well as an architecture to support its application. Although single-layer graphene on Si-face SiC has garnered extensive interest, many-layer graphene produced on C-face SiC could be significantly more robust for enabling applications. Little is known, however, about the structural properties related to the growth evolution at the buried interface for thick many-layer graphene. Using complementary X-ray scattering and neutron reflectivity as well as electron microscopy, we demonstrate that thick many-layer epitaxial graphene exhibits two vastly different length-scales of the buried interface roughness as a consequence of the Si sublimation that produces the graphene. Over long lateral length-scales the roughness is extremely large (hundreds of Å) and it varies proportionally to the number of graphene layers. In contrast, over much shorter lateral length-scales we observe an atomically abrupt interface with SiC terraces. Graphene near the buried interface exhibits a slightly expanded interlayer spacing (∼1%) and fluctuations of this spacing, indicating a tendency for disorder near the growth front. Nevertheless, Dirac cones are observed from the graphene while its domain size routinely reaches micron length-scales, indicating the persistence of high-quality graphene beginning just a short distance away from the buried interface. Discovering and reconciling the different length-scales of roughness by reflectivity was complicated by strong diffuse scattering and we provide a detailed discussion of how these difficulties were resolved. The insight from this analysis will be useful for other highly rough interfaces among broad classes of thin-film materials.

Laboratory-based real and reciprocal space imaging of the electronic structure of few layer graphene on SiC(0001¯) using photoelectron emission microscopy.

We present real and reciprocal space photoelectron emission microscopy (PEEM) results on few layer graphene using laboratory based He I and II radiation. The combination of a focused high-intensity source and high transmission PEEM electron optics provides good signal to noise ratios for the different modes of acquisition. We demonstrate work function mapping and secondary electron analysis, related to the graphene layer thickness, band structure imaging from micron scale regions by wave vector resolved PEEM (k-PEEM) and local secondary electron spectroscopy, giving information on the valence and conduction band states and the dispersion relations of the π bands. Dark field PEEM is done by selecting the Dirac cone corresponding to the specific rotation of each graphene layer and allows spatial mapping of the commensurate rotation angles. The use of He II radiation increases the volume of reciprocal space accessible to k-PEEM and improves signal to background. The preferential linear polarization of the light source is used to investigate aspects of the electronic chirality near the Dirac cone. Recent developments in sample manipulation and cooling are presented.

Why multilayer graphene on 4H-SiC(0001[over ]) behaves like a single sheet of graphene.

We show experimentally that multilayer graphene grown on the carbon terminated SiC(0001[over ]) surface contains rotational stacking faults related to the epitaxial condition at the graphene-SiC interface. Via first-principles calculation, we demonstrate that such faults produce an electronic structure indistinguishable from an isolated single graphene sheet in the vicinity of the Dirac point. This explains prior experimental results that showed single-layer electronic properties, even for epitaxial graphene films tens of layers thick.

Electronic structure of epitaxial graphene layers on SiC: effect of the substrate.

A strong substrate-graphite bond is found in the first all-carbon layer by density functional theory calculations and x-ray diffraction for few graphene layers grown epitaxially on SiC. This first layer is devoid of graphene electronic properties and acts as a buffer layer. The graphene nature of the film is recovered by the second carbon layer grown on both the (0001) and (0001[over]) 4H-SiC surfaces. We also present evidence of a charge transfer that depends on the interface geometry. Hence the graphene is doped and a gap opens at the Dirac point after three Bernal stacked carbon layers are formed.

Influence of quantum size effects on island coarsening.

Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on Si(111)-(7 x 7). It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times tau are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux F (i.e., tau is approximately 1/F). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures.