Electronic States Of Graphene Research Articles

Электрон-электронное и электрон-фононное взаимодействия в графене на полупроводниковой подложке: простые оценки

AbstractThe problem of epitaxial graphene formed on a semiconductor substrate is considered in the context of the extended Hubbard and Holstein–Hubbard models for electron–electron and electron–phonon interactions. The Haldane–Anderson model is chosen for the density of states of the substrate. Three regions of the phase diagram, specifically, spin- and charge-density waves and a spin- and charge-homogeneous paramagnetic state are considered. For a number of special cases used as examples, the similarities and differences of the electron states of graphene on semiconductor and metal substrates are demonstrated. It is shown that the main difference arises, if the Dirac point of graphene lies within the band gap of the semiconductor. Numerical estimations are performed for a SiC substrate.

Interface and Doping Effects on Li Ion Storage Behavior of Graphene/Li2O

Graphene/metal oxide nanocomposites have been widely used as the anode materials for Li ion batteries, which exhibit much higher Li storage capacity beyond their theoretical capacity. In order to make clear the Li storage mechanism in graphene/metal oxide, we systematically investigated the interface and (B, N, O, S) doping effects on Li ion storage behavior in graphene/Li2O using first-principles total energy calculations. It is revealed that the doping elements increase the van der Waals interface interaction of graphene/Li2O by changing the electronic structure of graphene through different mechanisms. The Li storage at the graphene/Li2O interface exhibits the synergistic effect resulting from the enhanced interface interaction by the Li insertion at the interface. The p-type and n-type doping induced by B and N dopants in graphene enhance and reduce the Li storage capability of graphene/Li2O, respectively. O and S doping result in the localization of the electronic states in graphene which benefits th...

Erratum to: Mutual influence of uniaxial tensile strain and point defect pattern on electronic states in graphene

Erratum to: Mutual influence of uniaxial tensile strain and point defect pattern on electronic states in graphene

Effective Hamiltonian for protected edge states in graphene

Edge states in topological insulators (TIs) disperse symmetrically about one of the time-reversal invariant momenta $\Lambda$ in the Brillouin zone (BZ) with protected degeneracies at $\Lambda$. Commonly TIs are distinguished from trivial insulators by the values of one or multiple topological invariants that require an analysis of the bulk band structure across the BZ. We propose an effective two-band Hamiltonian for the electronic states in graphene based on a Taylor expansion of the tight-binding Hamiltonian about the time-reversal invariant $M$ point at the edge of the BZ. This Hamiltonian provides a faithful description of the protected edge states for both zigzag and armchair ribbons though the concept of a BZ is not part of such an effective model. We show that the edge states are determined by a band inversion in both reciprocal and real space, which allows one to select $\Lambda$ for the edge states without affecting the bulk spectrum.

Mutual influence of uniaxial tensile strain and point defect pattern on electronic states in graphene

The study deals with electronic properties of uniaxially stressed mono- and multi-layer graphene sheets with various kinds of imperfection: point defects modelled as resonant (neutral) adsorbed atoms or molecules, vacancies, charged impurities, and local distortions. The presence of randomly distributed defects in a strained graphene counteract the band-gap opening and even can suppress the gap occurs when they are absent. However, impurity ordering contributes to the band gap appearance and thereby re-opens the gap being suppressed by random dopants in graphene stretched along zigzag-edge direction. The band gap is found to be non-monotonic with strain in case of mutual action of defect ordering and zigzag deformation. Herewith, the minimal tensile strain required for the band-gap opening (≈12.5%) is smaller than that for defect-free graphene (≈23%), and band gap energy reaches the value predicted for maximal nondestructive strains in the pristine graphene. Effective manipulating the band gap in graphene requires balanced content of ordered dopants: their concentration should be sufficient for a significant sublattice asymmetry effect, but not so much that they may suppress the band gap or transform it into the “quasi- (or pseudo-) gap”.

Influence of Pd/Pd2 decoration on the structural, electronic and sensing properties of monolayer graphene in the presence of methane molecule: A dispersion-corrected DFT study

This paper is devoted to study on the methane (CH4) adsorption ability of graphene decorated by a single palladium (Pd) atom and its dimer (Pd2), using dispersion-corrected density functional theory (DFT-D2). The adsorption energy, optimum geometry and electronic structure in terms of density of states, band structure, and charge transfer are calculated. Our results show that van der Waals interactions lead to physisorption of CH4 on the prisitine graphene with the adsorption energy of −112 meV, which is close to its experimental value. The calculations also show that hybridization of Pd and graphene electronic states strongly binds Pd atoms to graphene surface, inducing chemisorption of the adsorbate. The adsorption energies of Pd atom and its dimer have been found to be −1.43eV and −2.27eV, respectively. Furthermore symmetry breaking of the graphene structure, due to the Pd/Pd2-decoration, leads to opening a band gap, bringing graphene from semimetalic to semiconducting state. In the presence of methane molecule, the electronic band gap of Pd-decorated graphene is increased from 73 meV to 90 meV, while for Pd dimer it is decreased from 185 meV to 55 meV. As a result, compared to Pd-decorated graphene, Pd2-decorated graphene has a stronger interaction with the methane molecule and may provide a more sensitive signal for methane gas detection.

Mass inversion in graphene by proximity to dichalcogenide monolayer

Proximity effects resulting from depositing a graphene layer on a TMD substrate layer change the dynamics of the electronic states in graphene, inducing spin orbit coupling (SOC) and staggered potential effects. An effective Hamiltonian that describes different symmetry breaking terms in graphene, while preserving time reversal invariance, shows that an inverted mass band gap regime is possible. The competition of different perturbation terms causes a transition from an inverted mass phase to a staggered gap in the bilayer heterostructure, as seen in its phase diagram. A tight-binding calculation of the bilayer validates the effective model parameters. A relative gate voltage between the layers may produce such phase transition in experimentally accessible systems. The phases are characterized in terms of Berry curvature and valley Chern numbers, demonstrating that the system may exhibit quantum spin Hall and valley Hall effects.

Electronic states of aryl radical functionalized graphenes: Density functional theory study

Functionalized graphenes are known as a high-performance molecular device. In the present study, the structures and electronic states of the aryl radical functionalized graphene have been investigated by the density functional theory (DFT) method to elucidate the effects of functionalization on the electronic states of graphene (GR). Also, the mechanism of aryl radical reaction with GR was investigated. The benzene, biphenyl, p-terphenyl, and p-quaterphenyl radicals [denoted by (Bz)n (n = 1–4), where n means numbers of benzene rings in aryl radical] were examined as aryl radicals. The DFT calculation of GR–(Bz)n (n = 1–4) showed that the aryl radical binds to the carbon atom of GR, and a C–C single bond was formed. The binding energies of aryl radicals to GR were calculated to be ca. 6.0 kcal mol−1 at the CAM-B3LYP/6-311G(d,p) level. It was found that the activation barrier exists in the aryl radical addition: the barrier heights were calculated to be 10.0 kcal mol−1. The electronic states of GR–(Bz)n were examined on the basis of theoretical results.

Spatially resolved edge currents and guided-wave electronic states in graphene

A far-reaching goal of graphene research is exploiting the unique properties of carriers to realize extreme nonclassical electronic transport. Of particular interest is harnessing wavelike carriers to guide and direct them on submicron scales, similar to light in optical fibers. Such modes, while long anticipated, have never been demonstrated experimentally. In order to explore this behavior, we employ superconducting interferometry in a graphene Josephson junction to reconstruct the real-space supercurrent density using Fourier methods. Our measurements reveal charge flow guided along crystal boundaries close to charge neutrality. We interpret the observed edge currents in terms of guided-wave states, confined to the edge by band bending and transmitted as plane waves. As a direct analog of refraction-based confinement of light in optical fibers, such nonclassical states afford new means for information transduction and processing at the nanoscale.

Synthesis of graphene through the carbidization of Gd on pyrolytic graphite

The formation of graphene on the surface of a Gd film on a highly oriented pyrolytic graphite substrate has been studied by photoelectron spectroscopy using synchrotron radiation. It has been demonstrated that the formation of graphene passes through the phase of gadolinium carbidization, which is transformed with increasing annealing temperature. It has been established that, at a temperature of 1300 K, gadolinium carbide with the Gd2C3 stoichiometry is transformed into the carbide with the GdC2 stoichiometry. The analysis of all transient phase processes has been performed on the basis of the fine structure of photoelectron lines and dispersion of electron states. It has been shown that the Dirac cone of electron states of graphene is retained.

Tunable Hybridization Between Electronic States of Graphene and Physisorbed Hexacene

Noncovalent functionalization via physisorption of organic molecules provides a scalable approach for modifying the electronic structure of graphene while preserving its excellent carrier mobilities. Here we investigated the physisorption of long-chain acenes, namely, hexacene and its fluorinated derivative perfluorohexacene, on bilayer graphene for tunable graphene devices using first-principles methods. We find that the adsorption of these molecules leads to the formation of localized states in the electronic structure of graphene close to its Fermi level, which could be readily tuned by an external electric field in the range of ±3 eV/nm. The electric field not only creates a variable band gap as large as 250 meV in bilayer graphene, but also strongly influences the charge redistribution within the molecule–graphene system. This charge redistribution is found to be weak enough not to induce strong surface doping, but strong enough to help preserve the electronic states near the Dirac point of graphene. Our results further highlight graphene’s potential for selective chemical sensing of physisorbed molecules under the external electric fields.

Tight-binding approximation for bulk and edge electronic states in graphene

The tight-binding approximation is employed here to investigate electronic bulk and edge (“surface”) states in semi-infinite graphene sheets and graphene monolayer ribbons with various edge terminations (zigzag, horseshoe, and armchair edges). It is shown that edge states do not exist for a uniform hopping (transfer) matrix. The problem is generalized to include edge elements of the hopping matrix distinct from the infinite-sheet (“bulk”) ones. In this case, semi-infinite graphene sheets with zigzag or horseshoe edges exhibit edge states, while semi-infinite graphene sheets with armchair edges do not. The energy of the edge states lies above the (zero) Fermi level. Similarly, symmetric graphene ribbons with zigzag or horseshoe edges exhibit edge states, while ribbons with asymmetric edges (zigzag and horseshoe) do not. It is also shown how to construct the “reflected” solutions (bulk states) for the intervening equations with finite differences both for semi-infinite sheets and ribbons.

The structure and properties of graphene on gold nanoparticles.

Graphene covered metal nanoparticles constitute a novel type of hybrid material, which provides a unique platform to study plasmonic effects, surface-enhanced Raman scattering (SERS), and metal-graphene interactions at the nanoscale. Such a hybrid material is fabricated by transferring graphene grown by chemical vapor deposition onto closely spaced gold nanoparticles produced on a silica wafer. The morphology and physical properties of nanoparticle-supported graphene are investigated by atomic force microscopy, optical reflectance spectroscopy, scanning tunneling microscopy and spectroscopy (STM/STS), and confocal Raman spectroscopy. This study shows that the graphene Raman peaks are enhanced by a factor which depends on the excitation wavelength, in accordance with the surface plasmon resonance of the gold nanoparticles, and also on the graphene-nanoparticle distance which is tuned by annealing at moderate temperatures. The observed SERS activity is correlated with the nanoscale corrugation of graphene. STM and STS measurements show that the local density of electronic states in graphene is modulated by the underlying gold nanoparticles.

Quantitative correlation between defect density and heterogeneous electron transfer rate of single layer graphene.

Improving electrochemical activity of graphene is crucial for its various applications, which requires delicate control over its geometric and electronic structures. We demonstrate that precise control of the density of vacancy defects, introduced by Ar(+) irradiation, can improve and finely tune the heterogeneous electron transfer (HET) rate of graphene. For reliable comparisons, we made patterns with different defect densities on a same single layer graphene sheet, which allows us to correlate defect density (via Raman spectroscopy) with HET rate (via scanning electrochemical microscopy) of graphene quantitatively, under exactly the same experimental conditions. By balancing the defect induced increase of density of states (DOS) and decrease of conductivity, the optimal HET rate is attained at a moderate defect density, which is in a critical state; that is, the whole graphene sheet becomes electronically activated and, meanwhile, maintains structural integrity. The improved electrochemical activity can be understood by a high DOS near the Fermi level of defective graphene, as revealed by ab initio simulation, which enlarges the overlap between the electronic states of graphene and the redox couple. The results are valuable to promote the performance of graphene-based electrochemical devices. Furthermore, our findings may serve as a guide to tailor the structure and properties of graphene and other ultrathin two-dimensional materials through defect density engineering.

Valley polarization induced second harmonic generation in graphene

The valley degeneracy of electron states in graphene stimulates intensive research of valley-related optical and transport phenomena. While many proposals on how to manipulate valley states have been put forward, experimental access to the valley polarization in graphene is still a challenge. Here, we develop a theory of the second optical harmonic generation in graphene and show that this effect can be used to measure the degree and sign of the valley polarization. We show that, at the normal incidence of radiation, the second harmonic generation stems from imbalance of carrier populations in the valleys. The effect has a specific polarization dependence reflecting the trigonal symmetry of electron valley and is resonantly enhanced if the energy of incident photons is close to the Fermi energy.

Periodic grain boundaries formed by thermal reconstruction of polycrystalline graphene film.

Grain boundaries consisting of dislocation cores arranged in a periodic manner have well-defined structures and peculiar properties and can be potentially applied as conducting circuits, plasmon reflectors and phase retarders. Pentagon-heptagon (5-7) pairs or pentagon-octagon-pentagon (5-8-5) carbon rings are known to exist in graphene grain boundaries. However, there are few systematic experimental studies on the formation, structure and distribution of periodic grain boundaries in graphene. Herein, scanning tunneling microscopy (STM) was applied to study periodic grain boundaries in monolayer graphene grown on a weakly interacting Cu(111) crystal. The periodic grain boundaries are formed after the thermal reconstruction of aperiodic boundaries, their structures agree well with the prediction of the coincident-site-lattice (CSL) theory. Periodic grain boundaries in quasi-freestanding graphene give sharp local density of states (LDOS) peaks in the tunneling spectra as opposed to the broad peaks of the aperiodic boundaries. This suggests that grain boundaries with high structural quality can introduce well-defined electronic states in graphene and modify its electronic properties.

Electronic transport properties of graphene nanostructures

The electronic states of graphene near the Fermi energy are well described by massless Dirac Fermion. The presence of edges, however, makes strong implications for the spectrum of the electrons. In graphene nanoribbons with zigzag edges, localized states appear at the edge with energies close to the Fermi level. In contrast, edge states are absent for ribbons with armchair edges. In my talk, we focus on edge and nanoscale effect on the electronic properties of graphene nanoribbons. We discuss the following aspects of graphene nanostructured systems. (1) In zigzag nanoribbons, for nonmagnetic long-ranged disorder, a single perfectly conducting channel emerges associated with a chiral mode due to the edge state, i.e., the absence of the localization in this class. (2) We show the electronic transport properties of graphene nanojunctions crucially depend on the peripheral lattice structures. The condition for electron confinement is discussed. (3) We will discuss the effect of edge chemical modification on magnetic properties of nanographene systems. Also, we discuss the hole doping effect on the spin-polarized states appearing along the graphene zigzag edges. Our studies reveal that the peculiar electronic, magnetic and transport properties of graphene nanostructured systems. In addition, we present our recent work on graphene double layer structure (GDLS), where two graphene layers are separated by a thin dielectric. We will discuss the dielectric environment effect on the charged-impurity-limited carrier mobility of the GDLS on the basis of the Boltzmann transport theory. It is found that carrier mobility strongly depends on the dielectric constant of the barrier layer if the interlayer distance becomes larger than the inverse of the Fermi wave vector. Our results suggest effective use of ultra-thin dielectric barriers and a practical design strategy to improve the charged-impurity-limited mobility of the GDLS.

Anomalous confined electron states in graphene superlattices

We show that periodic scalar potentials can induce the localization of some electronic states in graphene. Particularly, localized states are found at energies outside the potential variation range and embedded in the continuum spectrum of delocalized ones. The picture of the connection of wave functions with typical symmetries defined in relevant-edge nanoribbons is employed to explain the formation of the electronic structure and to characterize/classify eigen-states in graphene superlattices.

Effect of high-frequency electric field on the electron magnetotransport in graphene

The effective spectrum of electron states in graphene in quantizing magnetic and high-frequency electric fields is calculated. The presence of the high-frequency field is shown to lead to the Landau levels splitting in graphene. The magnitude of this splitting is calculated. The influence of the high-frequency electric field on the magnetic oscillations of graphene conductivity is investigated. The possibility of controlling of conductivity oscillations by change of electric field amplitude is shown.

Graphene quasi-energy spectrum under high-frequency electromagnetic radiation

The effective spectrum of electron states in graphene exposed to a circularly polarized electromagnetic wave is calculated. The bandgap in the graphene spectrum is shown to increase under exposure to a high-frequency electromagnetic wave. The effect of the change in the bandgap on the graphene magnetoconductivity is studied.