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The electronic properties of graphene nano-flakes (GNFs) with different edge passivation are investigated by using density functional theory. Passivation with F and H atoms is considered: CNc XNx (X = F or H). We studied GNFs with 10 < NNc < 56 and limit ourselves to the lowest energy configurations. We found that: (i) the energy difference ▵ between the highest occupied molecular orbital and the lowest unoccupied molecular orbital decreases with NNc, (ii) topological defects (pentagon and heptagon) break the symmetry of the GNFs and enhance the electric polarization, (iii) the mutual interaction of bilayer GNFs can be understood by dipole-dipole interaction which were found sensitive to the relative orientation of the GNFs, (iv) the permanent dipoles depend on the edge terminated atom, while the energy gap is independent of it, and (v) the presence of heptagon and pentagon defects in the GNFs results in the largest difference between the energy of the spin-up and spin-down electrons which is larger for the H-passivated GNFs as compared to F-passivated GNFs. Our study shows clearly the effect of geometry, size, termination, and bilayer on the electronic properties of small GNFs. This study reveals important features of graphene nano-flakes which can be detected using Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Based on the first-principles calculations, we investigate the magnetic and electronic properties of α-graphyne nanoribbons (NRs). We show that all the armchair α-graphyne NRs are nonmagnetic semiconductors with band gaps as a function of ribbon widths. The zigzag α-graphyne NRs are found to have magnetic semiconducting ground state with ferromagnetic ordering at each edge and opposite spin orientation between the two edges. Under the application of transverse electric field, we further predict the existence of half-metallicity in the zigzag NRs which strongly depends on the width of the ribbon. [ABSTRACT FROM AUTHOR]

The effects of a propagating sinusoidal out‐of‐plane flexural deformation in the electronic properties of a tense membrane of graphene are considered within a non‐perturbative approach, leading to an electron–ripple coupling. The deformation is taken into account by introducing its corresponding pseudo‐vector and pseudo‐scalar potentials in the Dirac equation. By using a transformation to the time‐cone of the strain wave, the Dirac equation is reduced to an ordinary second‐order differential Mathieu equation, i.e., to a parametric pendulum, giving a spectrum of bands and gaps determined by resonance conditions between the electron and the ripple wave‐vector (G), and their incidence angles. The location of the nth gap is thus determined by E ≈nvFħG, where vF is the Fermi velocity. Physically, gaps are produced by diffraction of electrons in phase with the wave. The propagation is mainly in the direction of the ripple. In the case of a pure pseudoelectric field and for energies lower than a certain threshold, we find a different kind of equation. Its analytical solutions are in excellent agreement with the numerical solutions. The wavefunctions can be expressed in terms of the Mathieu cosine and sine functions, and for the case of a pure pseudo‐electric potential, as a combination of Bessel functions. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) is used to investigate the spin-dependent quantum transport through bended graphene. Bending results in reduced bandgap in graphene and affects the spin transport by increasing current in parallel configuration (PC) resulting in an increase in magnetoresistance (MR). In antiparallel configuration (APC), bending limits the spin-down current, which results in higher magnetoresistance at all biases. In bended graphene, the magnetoresistance obtained is higher than the MR obtained in pristine and twisted graphene-based structure. High spin filtration for PC and APC is observed in the case of bended graphene as compared with pristine and twisted graphene. However, pristine graphene gives better spin filtration compared with twisted graphene at low voltages. [ABSTRACT FROM AUTHOR]

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'. [ABSTRACT FROM AUTHOR]

Despite its superb electronic properties, the semi-metallic nature of graphene with no band gap (Eg) at the Dirac point has been a stumbling block for its industrial application. We report an improved means of producing a tunable band gap over other schemes by doping low energy (10 eV) potassium ions (K+) on single layer graphene formed on 6H-SiC(0001) surface, where the noble Dirac nature of the π-band remains almost unaltered. The changes in the π-band induced by K+ ions reveal that the band gap increases gradually with increasing dose (θ) of the ions up to Eg=0.65 eV at θ=1.10 monolayers, demonstrating the tunable character of the band gap. Our core level data for C 1s, Si 2p, and K 2p suggest that the K+-induced asymmetry in charge distribution among carbon atoms drives the opening of band gap, which is in sharp contrast with no band gap when neutral K atoms are adsorbed on graphene. This tunable K+-induced band gap in graphene illustrates its potential application in graphene-based nano-electronics. [ABSTRACT FROM AUTHOR]

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Various techniques, like doping, vacancy creation, strain engineering are tried to open a gap in the bandstructure of graphene and in some cases the gap has opened up. However, when the gap opens up the Dirac cones disappear. Without Dirac cones, graphene loses all its novelty. So opening a gap in graphene, retaining Dirac cones has become a challenging task. We, through first principles study using Density Functional theory, have done band gap tuning investigations. We have succeeded in opening the band gap, retaining the Dirac cones. Surface doping (adsorption) of various elements are tried and finally surface doping of sulfur is found to induce band gap opening in graphene. The Dirac cones are retained and the graphene is now a semiconductor with fast moving massless Dirac Fermions. We are reporting this type of calculations for the first time. [ABSTRACT FROM AUTHOR]

Adsorption of tetracyanoethylene molecule on graphene and graphyne was investigated in view of finding an effective way to control their electronic properties. The electronic properties of graphene, α-, β- and γ- graphyne were studied by using the density functional theory. The binding energy, equilibrium adsorption distance, amount of charge transfer, electronic band structure, and density of states were calculated. The small adsorption energy and large adsorption distance identified physisorption of tetracyanoethylene molecule on graphene and graphyne sheets. It was found that charge is transferred from sheets to tetracyanoethylene molecule. In the presence of this acceptor molecule, the graphene and α-, β- and γ-graphynes, with semimetallic properties, show semiconducting behaviour. The tetracyanoethylene molecule has no considerable effect on the semiconducting property of γ-graphyne. Our results reveal that adsorption of organic molecules, such as tetracyanoethylene, is a proper method to open a band gap in graphene and graphyne. [ABSTRACT FROM AUTHOR]

We study the effect of mechanical deformations on the electronic properties of hexagonal flakes of bilayer graphENe. The behavior of electrons induced by triaxial strain can be described by an effective pseudo ‐ magnetic field which is homogENeous in the cENter of the flake. We find that in ‐ plane strain, applied to both layers equally, can break the layer symmetry leading to differENt behavior in the top and bottom layers of graphENe. At low ENergy, just one of the layers feels the pseudo ‐ magnetic field: the zero ‐ ENergy pseudo ‐ Landau level is missing in the second layer, thus creating a gap betweEN the lowest non ‐ zero levels. While the layer asymmetry is most significant at zero ENergy, interaction with the edges of the flake extENds the effect to higher pseudo ‐ Landau levels. The behavior of the top and bottom layers may be reversed by rotating the triaxial strain by 60°. (© 2015 WILEY ‐ VCH Verlag GmbH &Co. KGaA, Weinheim) Part of Focus Issue on ”Carbononics” (Eds.: Pawel Hawrylak, Francois Peeters, Klaus Ensslin) The influENce of mechanical deformations on the electronic properties of bilayer graphENe can be described by an effective pseudo ‐ magnetic field. The authors show that in ‐ plane triaxial strain, applied to both layers equally, can break the layer symmetry leading to differENt pseudo ‐ Landau level spectra in the top and bottom layers of graphENe. [ABSTRACT FROM AUTHOR]

Silicene is an exciting two-dimensional material that shares many of graphene's electronic properties, but differs in its structural buckling. This buckling allows opening a bandgap in silicene through the application of a perpendicular electric field. Here we show that this buckling also enables highly effective modulation of silicene's conductance by means of an in-plane electric field applied through silicene side gates, which can be realized concurrently within the same silicene monolayer. We illustrate this by using silicene to implement Self-Switching Diodes (SSDs), which are two-dimensional field effect nanorectifiers realized within a single silicene monolayer. Our quantum simulation results show that the atomically-thin silicene SSDs, with sub-10 nm dimensions, achieve a current rectification ratio that exceeds 200, without the need for doping, representing a 30 fold enhancement over graphene SSDs. We attribute this enhancement to a bandgap opening due to the in-plane electric field, as a consequence of silicene's buckling. Our results suggest that silicene is a promising material for the realization of planar field effect devices. [ABSTRACT FROM AUTHOR]

The dielectric functions of atomically thin graphene, silicene, and arsenene have been investigated as a function of temperature. With zero energy gap, more carriers in graphene and silicene are thermally excited as temperature increases and intraband transition strengthens, resulting in the strengthened absorption peak. Yet with large energy gap, interband transition dominates optical absorption of arsenene but it reduces as lattice vibration enhances, inducing the redshift and decreased absorption peak. To validate the theoretical method, the calculated optical constants of isolated graphene are compared with ellipsometry results and demonstrate good agreement. [ABSTRACT FROM AUTHOR]

Electron valley, a degree of freedom that is analogous to spin, can lead to novel topological phases in bilayer graphene. A tunable bandgap can be induced in bilayer graphene by an external electric field, and such gapped bilayer graphene is predicted to be a topological insulating phase protected by no-valley mixing symmetry, featuring quantum valley Hall effects and chiral edge states. Observation of such chiral edge states, however, is challenging because inter-valley scattering is induced by atomic-scale defects at real bilayer graphene edges. Recent theoretical work has shown that domain walls between AB- and BA-stacked bilayer graphene can support protected chiral edge states of quantum valley Hall insulators. Here we report an experimental observation of ballistic (that is, with no scattering of electrons) conducting channels at bilayer graphene domain walls. We employ near-field infrared nanometre-scale microscopy (nanoscopy) to image in situ bilayer graphene layer-stacking domain walls on device substrates, and we fabricate dual-gated field effect transistors based on the domain walls. Unlike single-domain bilayer graphene, which shows gapped insulating behaviour under a vertical electrical field, bilayer graphene domain walls feature one-dimensional valley-polarized conducting channels with a ballistic length of about 400 nanometres at 4 kelvin. Such topologically protected one-dimensional chiral states at bilayer graphene domain walls open up opportunities for exploring unique topological phases and valley physics in graphene. [ABSTRACT FROM AUTHOR]

We have systematically investigated the effect of local strain on electronic properties of graphene by first-principles calculations. Two major types of local strain, oriented along the zigzag and the armchair directions, have been studied. We find that local strain with a proper range and strength along the zigzag direction results in opening of significant band gaps in graphene, on the order of 10-1 eV; whereas, local strain along the armchair direction cannot open a significant band gap in graphene. Our results show that appropriate local strain can effectively open and tune the band gap in graphene; therefore, the electronic and transport properties of graphene can also be modified. [ABSTRACT FROM AUTHOR]

We report a scanning tunneling spectroscopy study systematically performed on graphene nanoribbons (GNRs) with various widths and layer numbers. The GNRs are formed on highly oriented pyrolytic graphite (HOPG) by crystallographic etching, as reported by Datta and co-workers [Nano Lett. 8, 1912 (2008)]. Regardless of the width and layer numbers, GNRs having zigzag edges exhibit a peak at the Fermi energy in their local density of states (LDOS) when measured near the edges, whereas no peak appears away from the edges. On the other hand, a depression of the LDOS emerges at the Fermi energy in the case of a GNR having armchair edges with no relation to the measured position in an identical GNR. The energy gap of the LDOS depression monotonically decreases with increasing GNR width, whereas there is no apparent dependence on the layer numbers. By comparison with the band structure calculated by a nearest-neighbor tight-binding method, it is suggested that the overlap of wave functions between the topmost layer and the underlayers is negligible, resulting in an LDOS similar to that on an isolated monolayer GNR even on an HOPG substrate. From the quantitative scaling of energy gaps (Egap) of LDOS depression with respect to GNR widths (W), the relation between the two is obtained as Egap = 1.9[eV nm]/W. [ABSTRACT FROM AUTHOR]

A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as γ0 = 3.1 eV, γ1 = 0.39 eV, and γ4 = 0.22 eV. [ABSTRACT FROM AUTHOR]

Transition metal dichalcogenides (TMDs) have emerged as exciting 2D materials beyond graphene due to their promising applications in the field of electronics and optoelectronics. Hence, the ability to produce controllable and uniformly thick TMD sheets over a large area is of utmost important for large-scale applications. Here, a facile method of synthesizing large-area, layer-controlled WS2, and MoS2 sheets by sulfurization of their corresponding thin metal films is reported. A metal film, which is deposited by magnetron sputtering method, can be adjusted to produce, with great control, the desired sheet thickness down to a monolayer. Various characterization techniques, such as Raman, photoluminescence, and transmission electron microscopy, were used to evaluate the grown films. The results confirmed some of the exotic properties of TMDs such as the thickness dependent band-gap transition (indirect to direct band gap) and Raman shift. Devices made directly on the as-grown film showed modest mobility, ranging from 0.005 to 0.01cm² V-1 s-1 . Our synthesis method is simple and could also be used to synthesize other TMDs. [ABSTRACT FROM AUTHOR]

Despite the noble electronic properties of graphene, its industrial application has been hindered mainly by the absence of a stable means of producing a band gap at the Dirac point (DP). We report a new route to open a band gap (Eg) at DP in a controlled way by depositing positively charged Na+ ions on single layer graphene formed on 6H-SiC(0001) surface. The doping of low energy Na+ ions is found to deplete the π* band of graphene above the DP, and simultaneously shift the DP downward away from Fermi energy indicating the opening of Eg. The band gap increases with increasing Na+ coverage with a maximum Eg >= 0.70 eV. Our core-level data, C 1s, Na 2p, and Si 2p, consistently suggest that Na+ ions do not intercalate through graphene, but produce a significant charge asymmetry among the carbon atoms of graphene to cause the opening of a band gap. We thus provide a reliable way of producing and tuning the band gap of graphene by using Na+ ions, which may play a vital role in utilizing graphene in future nano-electronic devices. [ABSTRACT FROM AUTHOR]

Using density functional theory calculations with van der Waals corrections, we investigated how the interlayer orientation affects the structure and electronic properties of MoS2-graphene bilayer heterostructures. Changing the orientation of graphene with respect to MoS2 strongly influences the type and the value of the electronic bandgap in MoS2, while not significantly altering the binding energy between the layers or the interlayer spacing. We show that the physical origin of this tunable bandgap arises from variations in the S-S interplanar distance (MoS2 thickness) with the interlayer orientation, variations which are caused by electron transfer away from the Mo-S bonds. [ABSTRACT FROM AUTHOR]

We present first-principles calculations on the bandgap of graphene on a layer of hexagonal boron nitride in three different stacking configurations. Relative stability of the configurations is identified and bandgap tunability is demonstrated through the application of an external, perpendicularly applied electric field. We carefully examine the bandgap's sensitivity to both magnitude of the applied field as well as separation between the graphene and hexagonal boron nitride layers. Features of the band structure are examined and configuration-dependent relationships between the field and bandgap are revealed and elucidated through the atomprojected density of states. These findings suggest the potential for opening and modulating a bandgap in graphene as high as several hundred meV. [ABSTRACT FROM AUTHOR]

Our first-principles full-potential density functional theory calculations show that a ZnS monolayer (ML-ZnS), which is predicted to adopt a graphene-like planar honeycomb structure with a direct band gap, undergoes strain-induced modifications in its structure and band gap when subjected to in-plane homogeneous biaxial strain (δ). ML-ZnS gets buckled for compressive strain greater than 0.92% ; the buckling parameter Δ(= 0.00 Å for planar ML-ZnS) linearly increases with increasing compressive strain (Δ = 0.435 Å at δ = −5.25%). A tensile strain of 2.91% turns the direct ML-ZnS band gap into indirect. Within our considered strain values of |δ| < 6%, the band gap shows linearly decreasing (non-linearly increasing as well as decreasing) variation with tensile (compressive) strain. These predictions (based on our calculations with two atoms per unit cell) may be exploited in future for potential applications in strain sensors and other nano-devices such as nano-electromechanical systems and nano-optomechanical systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Capacitance measurements provide a powerful means of probing the density of states. The technique has proved particularly successful in studying 2D electron systems, revealing a number of interesting many-body effects. Here, we use large-area high-quality graphene capacitors to study behavior of the density of states in this material in zero and high magnetic fields. Clear renormalization of the linear spectrum due to electron-electron interactions is observed in zero field. Quantizing fields lead to splitting of the spinand valley-degenerate Landau levels into quartets separated by interaction-enhanced energy gaps. These many-body states exhibit negative compressibility but the compressibility returns to positive in ultrahigh B. The reentrant behavior is attributed to a competition between field-enhanced interactions and nascent fractional states. [ABSTRACT FROM AUTHOR]

We propose and analyze an approach to secure a high on/off current ratio in a graphene field effect transistor (FET) by introducing a physical gap along the channel rather than by attempting to open the energy bandgap of graphene. The device simulation results of the newly proposed device structure reveal highly suppressed off-state current of ∼10-9 A/μm, an on/off current ratio of more than seven orders of magnitude, and a subthreshold slope of 2.23 mV/decade more than a 20-fold reduction relative to the theoretical limitation of conventional metal-oxide-semiconductor FETs. [ABSTRACT FROM AUTHOR]

The article discusses the advanced material graphene. The author notes that publications and patents relating to graphene have increased worldwide between 200-2010 with some applications, including conducting inks and flexible coatings, being close to, or already on, the market. Topics include the fundamental advances of graphene in electronics as a replacement for silicon and the challenges of replacing silicon, such as the absence of an electronic band gap in graphene.

We examined the graphene and carbon nanotubes in 5 groups according to their structural and electronic properties by using ab initio density functional theory: zigzag (metallic and semiconducting), chiral (metallic and semiconducting), and armchair (metallic).We studied the structural and electronic properties of the 3D supercell graphene and isolated SWCNTs. So, we reported comprehensively the graphene and SWCNTs that consist of zigzag (6, 0) and (7, 0), chiral (6, 2) and (6, 3), and armchair (7, 7). We obtained the energy band graphics, band gaps, charge density, and density of state for these structures. We compared the band structure and density of state of graphene and SWCNTs and examined the effect of rolling for nanotubes. Finally, we investigated the charge density that consists of the 2D contour lines and 3D surface in the XY plane. [ABSTRACT FROM AUTHOR]

Helicity-dependent photocurrent in monolayer graphene has been the subject of intense debate, and was recently ascribed to photon drag and circular photogalvanic effects. Unlike inversion symmetric monolayer graphene with no band gap, the most stable case of two-layer graphene, AB-stacked bilayer graphene, has broken inversion symmetry and can have a band gap upon electrical gating. Here we report the experimental determination of the photocurrent response of mono- and bilayer graphene as a function of light polarization, as well as carrier density and polarity. The mono- and bilayer graphene data show qualitative features in common with the photocurrent contribution that is expected to arise from the photon drag effect. On the other hand, the photocurrent due to the circular photogalvanic effect in bilayer (monolayer) graphene has asymmetric (symmetric) dependence on carrier density, which is attributed to particle-hole asymmetry. [ABSTRACT FROM AUTHOR]

The article discusses research efforts in graphene-based electronics and the graphene field-effect which is inefficient in digital application. It mentions graphene being sliced into nanoribbons to overcome bandgap is suitable for most logic applicatons. Topics include heterojunctions, electronic doping and electron-beam lithography.

Structural, electronic, and magnetic behaviors of 5d transition metal (TM) atom substituted divacancy (DV) graphene are investigated using first-principles calculations. Different 5d TM atoms (Hf, Ta, W, Re, Os, Ir, and Pt) are embedded in graphene, these impurity atoms replace 2 carbon atoms in the graphene sheet. It is revealed that the charge transfer occurs from 5d TM atoms to the graphene layer. Hf, Ta, and W substituted graphene structures exhibit a finite band gap at high symmetric K-point in their spin up and spin down channels with 0.783 , 1.65 , and 1.78 magnetic moments, respectively. Ir and Pt substituted graphene structures display indirect band gap semiconductor behavior. Interestingly, Os substituted graphene shows direct band gap semiconductor behavior having a band gap of approximately 0.4 eV in their spin up channel with 1.5 magnetic moment. Through density of states (DOS) analysis, we can predict that d orbitals of 5d TM atoms could be responsible for introducing ferromagnetism in the graphene layer. We believe that our obtained results provide a new route for potential applications of dilute magnetic semiconductors and half-metals in spintronic devices by employing 5d transition metal atom-doped graphene complexes. [ABSTRACT FROM AUTHOR]

Graphene decorated with 5d transitional metal atoms is predicted to exhibit many intriguing properties; for example iridium adatoms are proposed to induce a substantial topological gap in graphene. We extensively investigated the conductivity of single-layer graphene decorated with iridium deposited in ultra-high vacuum at low temperature (7 K) as a function of Ir concentration, carrier density, temperature, and annealing conditions. Our results are consistent with the formation of Ir clusters of ~100 atoms at low temperature, with each cluster donating a single electronic charge to graphene. Annealing graphene increases the cluster size, reducing the doping and increasing the mobility. We do not observe any sign of an energy gap induced by spin-orbit coupling, possibly due to the clustering of Ir. [ABSTRACT FROM AUTHOR]

The article focuses on a study on the use of graphene in digital electronics applications by Guanxiong Liu and colleagues by overcoming absence of intrinsic energy bandgap and using nonlinear output characteristics of transistors.