We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

The physics and applications of a broad class of artificial electromagnetic materials composed of lattices of aligned metal rods embedded in a dielectric matrix are reviewed. Such structures are here termed wire metamaterials. They appear in various settings and can operate from microwaves to THz and optical frequencies. An important group of these metamaterials is a wire medium possessing extreme optical anisotropy. The study of wire metamaterials has a long history, however, most of their important and useful properties have been revealed and understood only recently, especially in the THz and optical frequency ranges where the wire media correspond to the lattices of microwires and nanowires, respectively. Another group of wire metamaterials are arrays and lattices of nanorods of noble metals whose unusual properties are driven by plasmonic resonances.

Wire metamaterials: physics and applications.

We have produced ultrathin epitaxial graphite films which show remarkable 2D electron gas (2DEG) behavior. The films, composed of typically 3 graphene sheets, were grown by thermal decomposition on the (0001) surface of 6H-SiC, and characterized by surface-science techniques. The low-temperature conductance spans a range of localization regimes according to the structural state (square resistance 1.5 kOhm to 225 kOhm at 4 K, with positive magnetoconductance). Low resistance samples show characteristics of weak-localization in two dimensions, from which we estimate elastic and inelastic mean free paths. At low field, the Hall resistance is linear up to 4.5 T, which is well-explained by n-type carriers of density 10^{12} cm^{-2} per graphene sheet. The most highly-ordered sample exhibits Shubnikov - de Haas oscillations which correspond to nonlinearities observed in the Hall resistance, indicating a potential new quantum Hall system. We show that the high-mobility films can be patterned via conventional lithographic techniques, and we demonstrate modulation of the film conductance using a top-gate electrode. These key elements suggest electronic device applications based on nano-patterned epitaxial graphene (NPEG), with the potential for large-scale integration.

Ultrathin epitaxial graphite: 2D electron gas properties and a route toward graphene-based nanoelectronics

Intensive interest in graphene has centered on its unique 2D crystal lattice and remarkable properties that offer unique opportunities to address ever-increasing global energy demands. The past years have witnessed considerable advances in the fabrication of graphene-based materials and significant breakthroughs in advanced energy applications. In this Review, two methodologies for graphene production, namely, the bottom-up growth from hydrocarbon precursors and the top-down exfoliation of graphite (to graphene) and graphite oxide (to graphene oxide followed by reduction) are first summarized. The advantages and disadvantages of these methods regarding their accessibility, scalability, graphene quality, and inherent properties are compared. Particular attention is concentrated on tailored nanostructures, electronic properties, and surface activities of these intriguing materials. The preparation of graphene-based composites containing a wide range of active constituents (e.g., transition metals, metal oxides, and conducting polymers) by in-situ hybridization and ex-situ recombination is also discussed with an emphasis on their microstructures and hybrid architectures. This Review is devoted largely to current developments of graphene and its derivatives and composites in energy conversion (i.e., polymer solar cells, dye-sensitized solar cells, perovskite solar cells, and fuel cells) and energy storage (i.e., lithium-ion batteries and supercapacitors) on the basis of their intrinsic attributes in improving photovoltaic and electrochemical performance. By critically evaluating the relationship between the nanostructures and the device performance, we intend to provide general guidelines for the design of advanced graphene-based materials with structure-to-property tailored toward specific requirements for targeted energy applications. Lastly, the potential issues and the perspective for future research in graphene-based materials for energy applications are also presented. By summarizing the current state-of-the-art as well as the exciting achievements from laboratory research, this Review aims to demonstrate that real industrial applications of graphene-based materials are to be expected in the near future. (1346 references).

Graphene-based materials with tailored nanostructures for energy conversion and storage

The wave nature of electrons in low-dimensional structures manifests itself in conventional electrical measurements as a quantum correction to the classical conductance. This correction comes from the interference of scattered electrons which results in electron localisation and therefore a decrease of the conductance. In graphene, where the charge carriers are chiral and have an additional (Berry) phase of \pi, the quantum interference is expected to lead to anti-localisation: an increase of the conductance accompanied by negative magnetoconductance (a decrease of conductance in magnetic field). Here we observe such negative magnetoconductance which is a direct consequence of the chirality of electrons in graphene. We show that graphene is a unique two-dimensional material in that, depending on experimental conditions, it can demonstrate both localisation and anti-localisation effects. We also show that quantum interference in graphene can survive at unusually high temperatures, up to T~200 K.

Transition between electron localisation and antilocalisation in graphene

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges.

Fabrication of hexagonally ordered nanopores in anodic alumina: An alternative pretreatment

Strong magnetoresistance effects are often observed in ferromagnet–nonmagnet multilayers, which are exploited in state-of-the-art magnetic field sensing and data storage technologies. In this work we report a novel current-perpendicular-to-plane magnetoresistance effect in multilayer graphene as grown on a catalytic nickel surface by chemical vapor deposition. A negative magnetoresistance effect of ∼104% has been observed, which persists even at room temperature. This effect is correlated with the shape of the 2D peak as well as with the occurrence of D peak in the Raman spectrum of the as-grown multilayer graphene. The observed magnetoresistance is extremely high as compared to other known materials systems for similar temperature and field range and can be qualitatively explained within the framework of “interlayer magnetoresistance” (ILMR).

Giant Current-Perpendicular-to-Plane Magnetoresistance in Multilayer Graphene as Grown on Nickel

Graphene charge-injection photodetectors

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-003977.pdf

S. C. Bodepudi

Papers

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges.

Fabrication of hexagonally ordered nanopores in anodic alumina: An alternative pretreatment

Giant Current-Perpendicular-to-Plane Magnetoresistance in Multilayer Graphene as Grown on Nickel

Graphene charge-injection photodetectors

Giant Current-Perpendicular-to-Plane Magnetoresistance in Multilayer-Graphene Grown on Nickel