This article reviews the basic theoretical aspects of graphene, a one-atom-thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. The Dirac electrons behave in unusual ways in tunneling, confinement, and the integer quantum Hall effect. The electronic properties of graphene stacks are discussed and vary with stacking order and number of layers. Edge (surface) states in graphene depend on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. Different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.

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The electronic properties of graphene

Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene research and applications, and attempts to identify future directions in which the field is likely to develop.

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Graphene: Status and Prospects

Although graphite is known as one of the most chemically inert materials, we have found that graphene, a single atomic plane of graphite, can react with atomic hydrogen, which transforms this highly conductive zero-overlap semimetal into an insulator. Transmission electron microscopy reveals that the obtained graphene derivative (graphane) is crystalline and retains the hexagonal lattice, but its period becomes markedly shorter than that of graphene. The reaction with hydrogen is reversible, so that the original metallic state, the lattice spacing, and even the quantum Hall effect can be restored by annealing. Our work illustrates the concept of graphene as a robust atomic-scale scaffold on the basis of which new two-dimensional crystals with designed electronic and other properties can be created by attaching other atoms and molecules.

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Control of graphene's properties by reversible hydrogenation: Evidence for graphane

Every few years, a new material with unique properties emerges and fascinates the scientific community, typical recent examples being high-temperature superconductors and carbon nanotubes. Graphene is the latest sensation with unusual properties, such as half-integer quantum Hall effect and ballistic electron transport. This two-dimensional material which is the parent of all graphitic carbon forms is strictly expected to comprise a single layer, but there is considerable interest in investigating two-layer and few-layer graphenes as well. Synthesis and characterization of graphenes pose challenges, but there has been considerable progress in the last year or so. Herein, we present the status of graphene research which includes aspects related to synthesis, characterization, structure, and properties.

Graphene: The New Two-Dimensional Nanomaterial

Graphene based materials: Past, present and future

Granular CoCrPt–SiO2 films with perpendicular magnetic anisotropy were deposited onto arrays of SiO2 nanoparticles with diameters down to 10 nm. Columnar CoCrPt grains with their c-axis pointing perpendicular to the particle surface were formed, creating a unique hedgehog-like cap structure. This peculiar structure induced by the curvature of the particles substantially modifies the magnetic properties. Underneath the CoCrPt pillars a continuous Co-rich layer was observed, which gives rise to enhanced intergranular exchange coupling resulting in single domain states. The temperature dependence of the coercivity and the angular dependence of the switching field were extracted and correlated with the microstructure.

Magnetic hedgehog-like nanostructures

CrC with the fcc NaCl (B1) structure is a metastable phase that can be obtained under the non-equilibrium conditions of high ion irradiation. A nano-composite coating consisting of amorphous carbon embedded in a CrC matrix was prepared via the unbalanced magnetron sputtering of graphite and Cr metal targets in Ar gas with a high ionized flux (ion-to-neutral ratio Ji/Jn = 6). The nanoscale amorphous carbon clusters self-assembled into layers alternated by CrC, giving the composite a multilayer structure. The phase, microstructure, and composition of the coating were characterized using x-ray diffraction, transmission electron microscopy, and aberration corrected scanning transmission electron microscopy coupled with electron energy loss spectroscopy. The interpretation of the true coating structure, in particular the carbide type, is discussed.

C/CrC nanocomposite coating deposited by magnetron sputtering at high ion irradiation conditions

We have used atomic number, Z-contrast, imaging, performed using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) to study bimetallic ruthenium-platinum nanoparticles anchored within silica nanopores, a system that shows great promise as a catalyst for single-step hydrogenation reactions. By exploiting the ultra-high spatial resolution obtained with aberration-corrected STEM, it is possible to study the internal structure of individual nanoparticles within the support, and to analyze their size distribution. Specimens examined before catalysis showed evidence for coalescence of the precursor units (12-atom clusters) during loading into the silica support. An indication of the stability of the system during catalysis is given by the similarity of pre- and post-catalysis size distributions, whilst the multimodal nature of each distribution suggests certain preferred ('magic') particle sizes. Some larger particles appear facetted, and exhibit image periodicity characteristic of crystalline structures.

Nano-metrology of platinum-ruthenium bimetallic catalysts and the cluster-to-crystal transformation

Structure and electronic properties of epitaxial fluorite-type IrSi 2 on Si(001)

The SuperSTEM Laboratory is a user facility to enable access to the advantages of aberration corrected scanning transmission electron microscopy (STEM) and analysis for a wide range of scientists [1]. An essential motivation for aberration correction in STEM is the ability to perform electron energy loss spectroscopy (EELS) with the highest spatial resolution. It has taken some time for the first results to become available due, in large part, to the additional complexity involved in coping with the larger convergence angles required to form a 0.1nm probe. When the aberration corrector (fabricated by Nion Co.) has, as in our case, been retrofitted to an existing column (VG HB501), these larger angles necessitate additional optics to increase the post specimen angular compression so that good collection efficiency and resolution are maintained. In practice, this additional optics is made more complex because its physical size displaces the spectrometer so that the virtual source is now further from the entrance plane. The, so-called, quadrupole coupling module (QCM) therefore performs the dual function of compressing the angular distribution and moving the virtual source for the spectrometer to the correct distance.

Andrew Bleloch

Papers

Magnetic hedgehog-like nanostructures

C/CrC nanocomposite coating deposited by magnetron sputtering at high ion irradiation conditions

Nano-metrology of platinum-ruthenium bimetallic catalysts and the cluster-to-crystal transformation

Structure and electronic properties of epitaxial fluorite-type IrSi 2 on Si(001)

High spatial resolution electron energy loss spectroscopy and imaging in an aberration corrected STEM