Jessica L. McChesney

Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide

Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties, high carrier mobility and ballistic transport up to room temperature. It has the potential for technological applications as a successor of silicon in the post Moores law era, as a single-molecule gas sensor, in spintronics, in quantum computing or as a terahertz oscillator. For such applications, uniform ordered growth of graphene on an insulating substrate is necessary. The growth of graphene on insulating silicon carbide (SiC) surfaces by high-temperature annealing in vacuum was previously proposed to open a route for large-scale production of graphene-based devices. However, vacuum decomposition of SiC yields graphene layers with small grains (30-200 nm; refs 14-16). Here, we show that the ex situ graphitization of Si-terminated SiC(0001) in an argon atmosphere of about 1 bar produces monolayer graphene films with much larger domain sizes than previously attainable. Raman spectroscopy and Hall measurements confirm the improved quality of the films thus obtained. High electronic mobilities were found, which reach mu=2,000 cm (2) V(-1) s(-1) at T=27 K. The new growth process introduced here establishes a method for the synthesis of graphene films on a technologically viable basis.

Scanning tunneling spectroscopy of inhomogeneous electronic structure in monolayer and bilayer graphene on SiC

The authors present a scanning tunneling spectroscopy (STS) study of the local electronic structure of single and bilayer graphene grown epitaxially on a SiC(0001) surface. Low voltage topographic images reveal fine, atomic-scale carbon networks, whereas higher bias images are dominated by emergent spatially inhomogeneous large-scale structure similar to a carbon-rich reconstruction of SiC(0001). STS spectroscopy shows an ∼100meV gaplike feature around zero bias for both monolayer and bilayer graphene/SiC, as well as significant spatial inhomogeneity in electronic structure above the gap edge. Nanoscale structure at the SiC/graphene interface is seen to correlate with observed electronic spatial inhomogeneity. These results are relevant for potential devices involving electronic transport or tunneling in graphene/SiC.

Morphology of graphene thin film growth on SiC(0001)

Epitaxial films of graphene on SiC(0001) are interesting from a basic physics as well as an applications-oriented point of view. Here, we study the emerging morphology of in vacuo prepared graphene films using low-energy electron microscopy (LEEM) and angle-resolved photoemission spectroscopy (ARPES). We obtain an identification of single-layer and bilayer graphene films by comparing the characteristic features in electron reflectivity spectra in LEEM to the ?-band structure as revealed by ARPES. We demonstrate that LEEM serves as a tool to accurately determine the local extent of graphene layers as well as the layer thickness.

Symmetry breaking in few layer graphene films

Recently, it was demonstrated that the quasiparticle dynamics, the layer-dependent charge and potential, and the c-axis screening coefficient could be extracted from measurements of the spectral function of few layer graphene films grown epitaxially on SiC using angle-resolved photoemission spectroscopy (ARPES). In this paper we review these findings, and present detailed methodology for extracting such parameters from ARPES. We also present detailed arguments against the possibility of an energy gap at the Dirac crossing ED.

Van Hove singularity and apparent anisotropy in the electron-phonon interaction in graphene

We show that the electron-phonon coupling strength obtained from the slopes of the electronic energy vs wave vector dispersion relations, as often done in analyzing angle-resolved photoemission data, can differ substantially from the actual electron-phonon coupling strength due to the curvature of the bare electronic bands. This effect becomes particularly important when the Fermi level is close to a van Hove singularity. By performing ab initio calculations on doped graphene, we demonstrate that, while the apparent strength obtained from the slopes of experimental photoemission data is highly anisotropic, the angular dependence of the actual electron-phonon coupling strength in this material is negligible.

Photoemission studies of graphene on SiC: growth, interface, and electronic structure

The possibility to grow well ordered graphitic films on SiC(0001) surfaces with thicknesses down to a single graphene layer is promising for future applications. Photoelectron spectroscopy (PES) is a versatile technique for investigating a variety of fundamentals and technologically relevant properties of this system. We survey results from recent PES studies with a focus on the growth of graphene and few layer graphene, the electronic and structural properties of the interface to the SiC substrate, and the electronic structure of the films.

Strictly one-dimensional electron system in Au chains on Ge(001) revealed by photoelectron k -space mapping

Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley 94720, California, USA (Received: Day Month 2010) Atomic nanowires formed by Au on Ge(001) are scrutinized for the band topology of the conduc-tion electron system by k-resolved photoemission. Two metallic electron pockets are observed. Their Fermi surface sheets form straight lines without undulations perpendicular to the chains within ex-perimental uncertainty. The electrons hence emerge as strictly confined to one dimension. Moreover, the system is stable against a Peierls distortion down to 10 K, lending itself for studies of the spectral function. Indications for unusually low spectral weight at the chemical potential are discussed. PACS numbers: 73.20.At, 68.37.Ef, 71.10.Pm, 73.20.Mf

The interaction of quasi-particles in graphene with chemical dopants

We review recent developments on the electronic properties of graphene under the influence of adsorbates. Potassium and hydrogen adsorbed on graphene induce very different effects on the graphene electron gas because of the different types—ionic versus covalent—of chemical bonds formed. Potassium readily donates electrons to graphene, and the resulting Fermi sea shows strong electron-plasmon scattering but weak defect scattering. In contrast, hydrogen adsorption saturates a carbon bond, leading to the removal of electrons from the graphene. Such hydrogen bonds act as a lattice defect, leading to a sharp reduction in conductivity and an insulating temperature dependence of the resistivity. The marked contrast in the behaviour of these adsorbates derives from the different symmetry classes of their defect geometries.

Surface Floating 2D Bands in Layered Nonsymmorphic Semimetals: ZrSiS and Related Compounds

In this work, we present a model of the surface states of nonsymmorphic semimetals. These are derived from surface mass terms that lift the high degeneracy imposed in the band structure by the nonsymmorphic bulk symmetries. Reflecting the reduced symmetry at the surface, the bulk bands are strongly modified. This leads to the creation of two-dimensional floating bands, which are distinct from Shockley states, quantum well states or topologically protected surface states. We focus on the layered semimetal ZrSiS to clarify the origin of its surface states. We demonstrate an excellent agreement between DFT calculations and ARPES measurements and present an effective four-band model in which similar surface bands appear. Finally, we emphasize the role of the surface chemical potential by comparing the surface density of states in samples with and without potassium coating. Our findings can be extended to related compounds and generalized to other crystals with nonsymmorphic symmetries.