Showing papers on "Bilayer graphene published in 2013"
TL;DR: Band structure engineering in a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally aligned hexagonal boron nitride substrate is demonstrated, resulting in an unexpectedly large band gap at charge neutrality.
Abstract: van der Waals heterostructures constitute a new class of artificial materials formed by stacking atomically thin planar crystals. We demonstrated band structure engineering in a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally aligned hexagonal boron nitride substrate. The spatially varying interlayer atomic registry results in both a local breaking of the carbon sublattice symmetry and a long-range moire superlattice potential in the graphene. In our samples, this interplay between short- and long-wavelength effects resulted in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality. This picture is confirmed by our observation of fractional quantum Hall states at ± 5 3 filling and features associated with the Hofstadter butterfly at ultrahigh magnetic fields.
1,454 citations
TL;DR: It is demonstrated that moiré superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic modulation with ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum.
Abstract: Moire superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic potential modulation on a length scale ideally suited to studying the fractal features of the Hofstadter energy spectrum in large magnetic fields. In 1976 Douglas Hofstadter predicted that electrons in a lattice subjected to electrostatic and magnetic fields would show a characteristic energy spectrum determined by the interplay between two quantizing fields. The expected spectrum would feature a repeating butterfly-shaped motif, known as Hofstadter's butterfly. The experimental realization of the phenomenon has proved difficult because of the problem of producing a sufficiently disorder-free superlattice where the length scales for magnetic and electric field can truly compete with each other. Now that goal has been achieved — twice. Two groups working independently produced superlattices by placing ultraclean graphene (Ponomarenko et al.) or bilayer graphene (Kim et al.) on a hexagonal boron nitride substrate and crystallographically aligning the films at a precise angle to produce moire pattern superstructures. Electronic transport measurements on the moire superlattices provide clear evidence for Hofstadter's spectrum. The demonstrated experimental access to a fractal spectrum offers opportunities for the study of complex chaotic effects in a tunable quantum system. Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, two-dimensional systems of electrons exhibit a self-similar recursive energy spectrum1. Known as Hofstadter’s butterfly, this complex spectrum results from an interplay between the characteristic lengths associated with the two quantizing fields1,2,3,4,5,6,7,8,9,10, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical atomic lattices (with periodicities of less than one nanometre) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nanometres) the corresponding fields are too small to overcome disorder completely11,12,13,14,15,16,17. Here we demonstrate that moire superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodic modulation with ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.
1,438 citations
TL;DR: Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying their crystallographic alignment within van der Waals heterostuctures.
Abstract: Placing graphene on a boron nitride substrate and accurately aligning their crystallographic axes, to form a moire superlattice, leads to profound changes in the graphene’s electronic spectrum. In 1976 Douglas Hofstadter predicted that electrons in a lattice subjected to electrostatic and magnetic fields would show a characteristic energy spectrum determined by the interplay between two quantizing fields. The expected spectrum would feature a repeating butterfly-shaped motif, known as Hofstadter's butterfly. The experimental realization of the phenomenon has proved difficult because of the problem of producing a sufficiently disorder-free superlattice where the length scales for magnetic and electric field can truly compete with each other. Now that goal has been achieved — twice. Two groups working independently produced superlattices by placing ultraclean graphene (Ponomarenko et al.) or bilayer graphene (Kim et al.) on a hexagonal boron nitride substrate and crystallographically aligning the films at a precise angle to produce moire pattern superstructures. Electronic transport measurements on the moire superlattices provide clear evidence for Hofstadter's spectrum. The demonstrated experimental access to a fractal spectrum offers opportunities for the study of complex chaotic effects in a tunable quantum system. Superlattices have attracted great interest because their use may make it possible to modify the spectra of two-dimensional electron systems and, ultimately, create materials with tailored electronic properties1,2,3,4,5,6,7,8. In previous studies (see, for example, refs 1, 2, 3, 4, 5, 6, 7, 8), it proved difficult to realize superlattices with short periodicities and weak disorder, and most of their observed features could be explained in terms of cyclotron orbits commensurate with the superlattice1,2,3,4. Evidence for the formation of superlattice minibands (forming a fractal spectrum known as Hofstadter’s butterfly9) has been limited to the observation of new low-field oscillations5 and an internal structure within Landau levels6,7,8. Here we report transport properties of graphene placed on a boron nitride substrate and accurately aligned along its crystallographic directions. The substrate’s moire potential10,11,12 acts as a superlattice and leads to profound changes in the graphene’s electronic spectrum. Second-generation Dirac points13,14,15,16,17,18,19,20,21,22 appear as pronounced peaks in resistivity, accompanied by reversal of the Hall effect. The latter indicates that the effective sign of the charge carriers changes within graphene’s conduction and valence bands. Strong magnetic fields lead to Zak-type cloning23 of the third generation of Dirac points, which are observed as numerous neutrality points in fields where a unit fraction of the flux quantum pierces the superlattice unit cell. Graphene superlattices such as this one provide a way of studying the rich physics expected in incommensurable quantum systems7,8,9,22,23,24 and illustrate the possibility of controllably modifying the electronic spectra of two-dimensional atomic crystals by varying their crystallographic alignment within van der Waals heterostuctures25.
1,135 citations
TL;DR: In this article, the authors combine the unique electronic properties of semiconducting monolayer MoS2 with the high conductivity of graphene to build a 2D heterostructure capable of information storage.
Abstract: Memory cells are an important building block of digital electronics We combine here the unique electronic properties of semiconducting monolayer MoS2 with the high conductivity of graphene to build a 2D heterostructure capable of information storage MoS2 acts as a channel in an intimate contact with graphene electrodes in a field-effect transistor geometry Our prototypical all-2D transistor is further integrated with a multilayer graphene charge trapping layer into a device that can be operated as a nonvolatile memory cell Because of its band gap and 2D nature, monolayer MoS2 is highly sensitive to the presence of charges in the charge trapping layer, resulting in a factor of 10(4) difference between memory program and erase states The two-dimensional nature of both the contact and the channel can be harnessed for the fabrication of flexible nanoelectronic devices with large-scale integration
889 citations
TL;DR: The epitaxial growth of single-domain graphene on h-BN by a plasma-assisted deposition method and the synthesis method is potentially applicable on other flat surfaces could open new ways of graphene band engineering through epitaxy on different substrates.
Abstract: The epitaxial growth of large-area single-domain graphene on hexagonal boron nitride by plasma-assisted deposition is now reported. New sets of Dirac points are produced as a result of a trigonal superlattice potential, while Dirac fermion physics near the original Dirac point remain unperturbed. This growth approach could enable band engineering in graphene through epitaxy on different substrates.
870 citations
TL;DR: The tight-binding model is used to describe optical and transport properties including the integer quantum Hall effect, and the also discusses orbital magnetism, phonons and the influence of strain on electronic properties.
Abstract: We review the electronic properties of bilayer graphene, beginning with a description of the tight-binding model of bilayer graphene and the derivation of the effective Hamiltonian describing massive chiral quasiparticles in two parabolic bands at low energies. We take into account five tight-binding parameters of the Slonczewski–Weiss–McClure model of bulk graphite plus intra- and interlayer asymmetry between atomic sites which induce band gaps in the low-energy spectrum. The Hartree model of screening and band-gap opening due to interlayer asymmetry in the presence of external gates is presented. The tight-binding model is used to describe optical and transport properties including the integer quantum Hall effect, and we also discuss orbital magnetism, phonons and the influence of strain on electronic properties. We conclude with an overview of electronic interaction effects.
797 citations
TL;DR: The origin of the blue and green photoluminescence of GQDs and GOQDs is attributed to intrinsic and extrinsic energy states, respectively.
Abstract: Pristine graphene quantum dots and graphene oxide quantum dots are synthesized by chemical exfoliation from the graphite nanoparticles with high uniformity in terms of shape (circle), size (less than 4 nm), and thickness (monolayer). The origin of the blue and green photoluminescence of GQDs and GOQDs is attributed to intrinsic and extrinsic energy states, respectively.
553 citations
TL;DR: A new pathway is reported to greatly reduce the graphene nucleation density from ~10(6) to 4 nuclei cm(-2), enabling the growth of giant single crystals of monolayer graphene with a lateral size up to 5 mm and Bernal-stacked bilayers graphene with the lateral sizes up to 300 μm, both the largest reported to date.
Abstract: The growth of large-domain single crystalline graphene with the controllable number of layers is of central importance for large-scale integration of graphene devices. Here we report a new pathway to greatly reduce the graphene nucleation density from ~10(6) to 4 nuclei cm(-2), enabling the growth of giant single crystals of monolayer graphene with a lateral size up to 5 mm and Bernal-stacked bilayer graphene with the lateral size up to 300 μm, both the largest reported to date. The formation of the giant graphene single crystals eliminates the grain boundary scattering to ensure excellent device-to-device uniformity and remarkable electronic properties with the expected quantum Hall effect and the highest carrier mobility up to 16,000 cm(2) V(-1) s(-1). The availability of the ultra large graphene single crystals can allow for high-yield fabrication of integrated graphene devices, paving a pathway to scalable electronic and photonic devices based on graphene materials.
526 citations
TL;DR: In this paper, the authors used electron microscopy to measure the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene and found that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6-11 nm.
Abstract: Bilayer graphene has been a subject of intense study in recent years. The interlayer registry between the layers can have dramatic effects on the electronic properties: for example, in the presence of a perpendicular electric field, a band gap appears in the electronic spectrum of so-called Bernal-stacked graphene [Oostinga JB, et al. (2007) Nature Materials 7:151–157]. This band gap is intimately tied to a structural spontaneous symmetry breaking in bilayer graphene, where one of the graphene layers shifts by an atomic spacing with respect to the other. This shift can happen in multiple directions, resulting in multiple stacking domains with soliton-like structural boundaries between them. Theorists have recently proposed that novel electronic states exist at these boundaries [Vaezi A, et al. (2013) arXiv:1301.1690; Zhang F, et al. (2013) arXiv:1301.4205], but very little is known about their structural properties. Here we use electron microscopy to measure with nanoscale and atomic resolution the widths, motion, and topological structure of soliton boundaries and related topological defects in bilayer graphene. We find that each soliton consists of an atomic-scale registry shift between the two graphene layers occurring over 6–11 nm. We infer the minimal energy barrier to interlayer translation and observe soliton motion during in situ heating above 1,000 °C. The abundance of these structures across a variety of samples, as well as their unusual properties, suggests that they will have substantial effects on the electronic and mechanical properties of bilayer graphene.
440 citations
TL;DR: The graphene edge showed 4 orders of magnitude higher specific capacitance, much faster electron transfer rate and stronger electrocatalytic activity than those of graphene basal plane, making it an ideal electrode for electrocatalysis and for the storage of capacitive charges.
Abstract: Graphene has a unique atom-thick two-dimensional structure and excellent properties, making it attractive for a variety of electrochemical applications, including electrosynthesis, electrochemical sensors or electrocatalysis, and energy conversion and storage. However, the electrochemistry of single-layer graphene has not yet been well understood, possibly due to the technical difficulties in handling individual graphene sheet. Here, we report the electrochemical behavior at single-layer graphene-based electrodes, comparing the basal plane of graphene to its edge. The graphene edge showed 4 orders of magnitude higher specific capacitance, much faster electron transfer rate and stronger electrocatalytic activity than those of graphene basal plane. A convergent diffusion effect was observed at the sub-nanometer thick graphene edge-electrode to accelerate the electrochemical reactions. Coupling with the high conductivity of a high-quality graphene basal plane, graphene edge is an ideal electrode for electrocatalysis and for the storage of capacitive charges.
436 citations
TL;DR: Structural properties, syntheses, chemistry, stabilities, and electronic properties of fluorographene and other partially fluorinated, chlorinated, and brominated graphenes are discussed and patterned halogenation is presented as an interesting approach for generating materials with applications in the field of graphene-based electronic devices.
Abstract: Graphene derivatives containing covalently bound halogens (graphene halides) represent promising two-dimensional systems having interesting physical and chemical properties. The attachment of halogen atoms to sp2 carbons changes the hybridization state to sp3, which has a principal impact on electronic properties and local structure of the material. The fully fluorinated graphene derivative, fluorographene (graphene fluoride, C1F1), is the thinnest insulator and the only stable stoichiometric graphene halide (C1X1). In this review, we discuss structural properties, syntheses, chemistry, stabilities, and electronic properties of fluorographene and other partially fluorinated, chlorinated, and brominated graphenes. Remarkable optical, mechanical, vibrational, thermodynamic, and conductivity properties of graphene halides are also explored as well as the properties of rare structures including multilayered fluorinated graphenes, iodine-doped graphene, and mixed graphene halides. Finally, patterned halogenati...
TL;DR: In this paper, the authors theoretically study the optical absorption property of twisted bilayer graphenes with various stacking geometries and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers.
Abstract: We theoretically study the optical absorption property of twisted bilayer graphenes with various stacking geometries and demonstrate that the spectroscopic characteristics serve as a fingerprint to identify the rotation angle between two layers. We find that the absorption spectrum almost continuously evolves in changing the rotation angle, regardless of the lattice commensurability. The spectrum is characterized by series of peaks associated with the van Hove singularity, and the peak energies systematically shift with the rotation angle. We calculate the optical absorption in two frameworks: the tight-binding model and the effective continuum model based on the Dirac equation. For small rotation angles, less than ${10}^{\ensuremath{\circ}}$, the effective model well reproduces the low-energy band structure and the optical conductivity of the tight-binding model and, also, explains the optical selection rule analytically in terms of the symmetry of the effective Hamiltonian.
TL;DR: The important role played by intervalley coupling effects not directly captured by the continuum model is addressed using lattice calculations for specific domain wall structures.
Abstract: Electronic states at domain walls in bilayer graphene are studied by analyzing their four- and two-band continuum models, by performing numerical calculations on the lattice, and by using quantum geometric arguments. The continuum theories explain the distinct electronic properties of boundary modes localized near domain walls formed by interlayer electric field reversal, by interlayer stacking reversal, and by simultaneous reversal of both quantities. Boundary mode properties are related to topological transitions and gap closures, which occur in the bulk Hamiltonian parameter space. The important role played by intervalley coupling effects not directly captured by the continuum model is addressed using lattice calculations for specific domain wall structures.
TL;DR: It is shown that pyridinic and pyrrolic N impurities play an important role in determining the transport behavior of carriers in the authors' N-doped graphene sheets.
Abstract: A significant advance toward achieving practical applications of graphene as a two-dimensional material in nanoelectronics would be provided by successful synthesis of both n-type and p-type doped graphene. However, reliable doping and a thorough understanding of carrier transport in the presence of charged impurities governed by ionized donors or acceptors in the graphene lattice are still lacking. Here we report experimental realization of few-layer nitrogen-doped (N-doped) graphene sheets by chemical vapor deposition of organic molecule 1,3,5-triazine on Cu metal catalyst. When reducing the growth temperature, the atomic percentage of nitrogen doping is raised from 2.1% to 5.6%. With increasing doping concentration, N-doped graphene sheet exhibits a crossover from p-type to n-type behavior accompanied by a strong enhancement of electron-hole transport asymmetry, manifesting the influence of incorporated nitrogen impurities. In addition, by analyzing the data of X-ray photoelectron spectroscopy, Raman spectroscopy, and electrical measurements, we show that pyridinic and pyrrolic N impurities play an important role in determining the transport behavior of carriers in our N-doped graphene sheets.
TL;DR: Results suggest that adlayer growth proceeds by catalytic decomposition of methane trapped in a "nano-chemical vapor deposition" chamber between the first layer and the substrate, and submillimeter bilayer graphene was synthesized by applying a much lower growth rate.
Abstract: The growth of bilayer and multilayer graphene on copper foils was studied by isotopic labeling of the methane precursor. Isotope-labeled graphene films were characterized by micro-Raman mapping and time-of-flight secondary ion mass spectrometry. Our investigation shows that during growth at high temperature, the adlayers formed simultaneously and beneath the top, continuous layer of graphene and the Cu substrate. Additionally, the adlayers share the same nucleation center and all adlayers nucleating in one place have the same edge termination. These results suggest that adlayer growth proceeds by catalytic decomposition of methane (or CHx, x < 4) trapped in a “nano-chemical vapor deposition” chamber between the first layer and the substrate. On the basis of these results, submillimeter bilayer graphene was synthesized by applying a much lower growth rate.
TL;DR: In this paper, the properties of surface plasmons-polaritons in graphene and three possible ways of coupling electromagnetic radiation in the terahertz (THz) spectral range to this type of surface waves are discussed.
Abstract: We discuss the properties of surface plasmons-polaritons in graphene and describe three possible ways of coupling electromagnetic radiation in the terahertz (THz) spectral range to this type of surface waves. (i) the attenuated total reflection (ATR) method using a prism in the Otto configuration, (ii) graphene micro-ribbon arrays or monolayers with modulated conductivity, (iii) a metal stripe on top of the graphene layer, and (iv) graphene-based gratings. The text provides a number of original results along with their detailed derivation and discussion.
TL;DR: The synthesis of single-crystal graphene domains with tunable edges from zigzag to armchair via a growth–etching–regrowth process, providing a deep understanding of the fundamental problems that limit graphene growth by chemical vapor deposition.
Abstract: The controlled growth of large-area, high-quality, single-crystal graphene is highly desired for applications in electronics and optoelectronics; however, the production of this material remains challenging because the atomistic mechanism that governs graphene growth is not well understood. The edges of graphene, which are the sites at which carbon accumulates in the two-dimensional honeycomb lattice, influence many properties, including the electronic properties and chemical reactivity of graphene, and they are expected to significantly influence its growth. We demonstrate the growth of single-crystal graphene domains with controlled edges that range from zigzag to armchair orientations via growth–etching–regrowth in a chemical vapor deposition process. We have observed that both the growth and the etching rates of a single-crystal graphene domain increase linearly with the slanted angle of its edges from 0° to ∼19° and that the rates for an armchair edge are faster than those for a zigzag edge. Such edge-structure–dependent growth/etching kinetics of graphene can be well explained at the atomic level based on the concentrations of the kinks on various edges and allow the evolution and control of the edge and morphology in single-crystal graphene following the classical kinetic Wulff construction theory. Using these findings, we propose several strategies for the fabrication of wafer-sized, high-quality, single-crystal graphene.
TL;DR: In this paper, the authors studied the thermal stability of single layer graphene synthesized by either chemical vapor deposition or mechanical cleavage and found that the defects are initially sp3 type and become vacancy like at higher temperature.
Abstract: The thermal stability in air of graphene synthesized by either chemical vapor deposition or mechanical cleavage is studied. It is found that single layer graphene prepared by both methods starts to show defects at ~500 °C, indicated by the appearance of a disorder-induced Raman D peak. The defects are initially sp3 type and become vacancy like at higher temperature. On the other hand, bilayer graphene shows better thermal stability, and the D peak appears at ~600 °C. These results are quite different from those annealing in vacuum and controlled atmosphere. Raman images show that the defects in chemical vapor deposition graphene are not homogeneous, whereas those in mechanical cleavage graphene are uniformly distributed across the whole sample. The factors that affect the thermal stability of graphene are discussed. Our results could be important for guiding the future electronics process and chemical decoration of graphene. Copyright © 2013 John Wiley & Sons, Ltd.
TL;DR: Miré patterns are observed and the sensitivity of moiré interferometry proves that the graphene grains can align precisely with the underlying h-BN lattice within an error of less than 0.05°.
Abstract: Precisely aligned graphene grown on hexagonal boron nitride by catalyst free chemical vapor deposition
TL;DR: This work addresses the issue of the effective dielectric constant (ε) in N-layer graphene subjected to out-of-plane (E(ext)(⊥)) and in-plane(||) external electric fields and points to a promising way of understanding and controlling the screening properties of few- layer graphene through external electric Fields.
Abstract: The dielectric constant of a material is one of the fundamental features used to characterize its electrostatic properties such as capacitance, charge screening, and energy storage capability. Graphene is a material with unique behavior due to its gapless electronic structure and linear dispersion near the Fermi level, which can lead to a tunable band gap in bilayer and trilayer graphene, a superconducting-insulating transition in hybrid systems driven by electric fields, and gate-controlled surface plasmons. All of these results suggest a strong interplay between graphene properties and external electric fields. Here we address the issue of the effective dielectric constant (e) in N-layer graphene subjected to out-of-plane (E(ext)(⊥)) and in-plane (E(ext)(||)) external electric fields. The value of e has attracted interest due to contradictory reports from theoretical and experimental studies. Through extensive first-principles electronic structure calculations, including van der Waals interactions, we show that both the out-of-plane (e(⊥)) and the in-plane (e(||)) dielectric constants depend on the value of applied field. For example, e(⊥) and e(||) are nearly constant (~3 and ~1.8, respectively) at low fields (E(ext) < 0.01 V/A) but increase at higher fields to values that are dependent on the system size. The increase of the external field perpendicular to the graphene layers beyond a critical value can drive the system to a unstable state where the graphene layers are decoupled and can be easily separated. The observed dependence of e(⊥) and e(||) on the external field is due to charge polarization driven by the bias. Our results point to a promising way of understanding and controlling the screening properties of few-layer graphene through external electric fields.
TL;DR: In this paper, the authors implemented a generic method based on the $2n+1$ theorem within density functional perturbation theory to calculate the anharmonic scattering coefficients among three phonons with arbitrary wave vectors.
Abstract: We have implemented a generic method, based on the $2n+1$ theorem within density functional perturbation theory, to calculate the anharmonic scattering coefficients among three phonons with arbitrary wave vectors. The method is used to study the phonon broadening in graphite and graphene mono- and bilayers. The broadening of the high-energy optical branches is highly nonuniform and presents a series of sudden steps and spikes. At finite temperature, the two linearly dispersive acoustic branches TA and LA of graphene have nonzero broadening for small wave vectors. The broadening in graphite and bilayer graphene is, overall, very similar to the graphene one, the most remarkable feature being the broadening of the quasiacoustical Z-polarized branch. Finally, we study the intrinsic anharmonic contribution to the thermal conductivity of the three systems, within the single mode relaxation time approximation. We find the conductance to be in good agreement with experiments in the out-of-plane direction but underestimate by a factor 2 in-plane.
TL;DR: Density functional theory calculations indicate that boron dopants interact strongly with the underlying copper substrate while nitrogen dopants do not, and the distribution of dopants is observed to be completely random in the case of bor on, while nitrogen displays strong sublattice clustering.
Abstract: We use scanning tunneling microscopy and X-ray spectroscopy to characterize the atomic and electronic structure of boron-doped and nitrogen-doped graphene created by chemical vapor deposition on copper substrates. Microscopic measurements show that boron, like nitrogen, incorporates into the carbon lattice primarily in the graphitic form and contributes ∼0.5 carriers into the graphene sheet per dopant. Density functional theory calculations indicate that boron dopants interact strongly with the underlying copper substrate while nitrogen dopants do not. The local bonding differences between graphitic boron and nitrogen dopants lead to large scale differences in dopant distribution. The distribution of dopants is observed to be completely random in the case of boron, while nitrogen displays strong sublattice clustering. Structurally, nitrogen-doped graphene is relatively defect-free while boron-doped graphene films show a large number of Stone-Wales defects. These defects create local electronic resonances ...
TL;DR: It is found that creating well-defined contacts can suppress inelastic transport channels in graphene nanostructures and significantly influences the charge transport through the graphene nanoribbon but does not affect its electronic structure.
Abstract: Graphene nanostructures have a tremendous potential for electronic applications, although contacting them with atomic precision remains a challenge. Here, van der Lit and colleagues achieve contacting graphene nanoribbons via only a single atom, without affecting its electronic structure.
TL;DR: In this paper, strong third-harmonic generation in monolayer graphene grown by chemical vapor deposition (CVD) and transferred to an amorphous silica (glass) substrate was reported.
Abstract: We report strong third-harmonic generation (THG) in monolayer graphene grown by chemical vapor deposition (CVD) and transferred to an amorphous silica (glass) substrate; the photon energy is in three-photon resonance with the exciton-shifted van Hove singularity at the M-point of graphene. Our polarization- and azimuthal-rotation-dependent measurements confirm the expected symmetry properties for this nonlinear process. Since the third-harmonic signal exceeds that of bulk glass by more than two orders of magnitude, the signal contrast permits background-free scanning of graphene and provides insight into the structural properties of graphene.
TL;DR: This work demonstrates the use of an interfacial trapping technique to assemble laterally macroscopic films of pristine graphene that are up to 95% transparent and have long-range structure with conductivities on the order of 400 S/cm.
Abstract: Graphite’s insolubility in conventional solvents is a major obstacle to its utilization. This challenge is typically addressed by chemical modification such as oxidation, followed by reduction. However, pristine graphene possesses superior properties as oxidation and reduction lead to degradation of the graphene. Here we demonstrate the use of an interfacial trapping technique to assemble laterally macroscopic films of pristine graphene that are up to 95% transparent. This is accomplished by modest sonication of natural flake graphite in a water/heptane mixture to form continuous films at the interface between two immiscible liquids. Furthermore, the graphene sheets readily climb hydrophilic solid substrates, forming a homogeneous thin film one to four layers thick. These films are composed of a network of overlapping graphene sheets and shown to have long-range structure with conductivities on the order of 400 S/cm.
TL;DR: The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for systematic studies of interlayer coupling in misoriented few-layer graphene systems with well-defined geometry.
Abstract: Bilayer graphene is an intriguing material in that its electronic structure can be altered by changing the stacking order or the relative twist angle, yielding a new class of low-dimensional carbon system. Twisted bilayer graphene can be obtained by (i) thermal decomposition of SiC; (ii) chemical vapor deposition (CVD) on metal catalysts; (iii) folding graphene; or (iv) stacking graphene layers one atop the other, the latter of which suffers from interlayer contamination. Existing synthesis protocols, however, usually result in graphene with polycrystalline structures. The present study investigates bilayer graphene grown by ambient pressure CVD on polycrystalline Cu. Controlling the nucleation in early stage growth allows the constituent layers to form single hexagonal crystals. New Raman active modes are shown to result from the twist, with the angle determined by transmission electron microscopy. The successful growth of single-crystal bilayer graphene provides an attractive jumping-off point for syste...
TL;DR: Graphene QDs are synthesized through stepwise, well-controlled organic chemistry, achieving structures with an atomic precision that has not been possible for any other semiconductor materials, and extraordinarily slow "electron cooling"--the relaxation of electrons from high excited states to lower ones is observed.
Abstract: When the size of a semiconductor crystal is reduced to the nanometer scale, the crystal boundary significantly modifies electron distribution, making properties such as bandgap and energy relaxation dynamics size dependent. This phenomenon, known as quantum confinement, has been demonstrated in many semiconductor materials, leading to practical applications in areas such as bioimaging, photovoltaics, and light-emitting diodes.Graphene, a unique type of semiconductor, is a two-dimensional crystal with a zero bandgap and a zero effective mass of charge carriers. Consequently, we expect new phenomena from nanometer-sized graphene, or graphene quantum dots (QDs), because the energy of charge carriers in graphene follows size-scaling laws that differ from those in other semiconductors. From a chemistry point of view, graphene is made of carbon, an element for which researchers have developed a whole branch of chemistry. Thus, it is possible to synthesize graphene QDs through stepwise, well-controlled organic c...
TL;DR: In this paper, the authors theoretically investigate phonon dispersion in twisted bilayer graphene with various rotation angles and find that the stacking order affects the out-of-plane acoustic phonon modes the most.
Abstract: We theoretically investigate phonon dispersion in AA-stacked, AB-stacked, and twisted bilayer graphene with various rotation angles. The calculations are performed using the Born--von Karman model for the intralayer atomic interactions and the Lennard-Jones potential for the interlayer interactions. It is found that the stacking order affects the out-of-plane acoustic phonon modes the most. The difference in the phonon densities of states in the twisted bilayer graphene and in AA- or AB-stacked bilayer graphene appears in the phonon frequency range 90--110 cm${}^{\ensuremath{-}1}$. Twisting bilayer graphene leads to the emergence of different phonon branches---termed hybrid folded phonons---which originate from the mixing of phonon modes from different high-symmetry directions in the Brillouin zone. The frequencies of the hybrid folded phonons depend strongly on the rotation angle and can be used for noncontact identification of the twist angles in graphene samples. The obtained results and the tabulated frequencies of phonons in twisted bilayer graphene are important for the interpretation of experimental Raman data and in determining the thermal conductivity of these material systems.
TL;DR: By employing atomically thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium superfluidity of electron-hole pairs should be achievable for the first time and is suggested a new route toward realizing high-temperature superfluidity in existing quality graphene samples.
Abstract: Exciton bound states in solids between electrons and holes are predicted to form a superfluid at high temperatures. We show that by employing atomically thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium superfluidity of electron-hole pairs should be achievable for the first time. The transition temperatures are well above liquid helium temperatures. Because the sample parameters needed for the device have already been attained in similar graphene devices, our work suggests a new route toward realizing high-temperature superfluidity in existing quality graphene samples.
14 May 2013
TL;DR: Current developments in graphene devices utilizing boron-nitride dielectrics are reviewed and Field-effect transistor (FET) characteristics of these systems at high bias are examined.
Abstract: Two-dimensional atomic sheets of graphene represent a new class of nanoscale materials with potential applications in electronics. However, exploiting the intrinsic characteristics of graphene devices has been problematic due to impurities and disorder in the surrounding dielectric and graphene/dielectric interfaces. Recent advancements in fabricating graphene heterostructures by alternately layering graphene with crystalline hexagonal boron nitride (hBN), its insulating isomorph, have led to an order of magnitude improvement in graphene device quality. Here, recent developments in graphene devices utilizing boron-nitride dielectrics are reviewed. Field-effect transistor (FET) characteristics of these systems at high bias are examined. Additionally, existing challenges in material synthesis and fabrication and the potential of graphene/BN heterostructures for novel electronic applications are discussed.