Band Gap In Bilayer Graphene Research Articles

Effect of intrinsic & Rashba spin-orbit interactions (SOI) in case of bilayer graphene

In this study, we investigated how various SOI affect the properties of bilayer graphene’s energy band structure. We have considered intralayer interactions in this work. We focused on the system without biasing. We found that the intrinsic spin-orbit interaction (ISOI) can produce an energy band gap in bilayer graphene. On the other hand, in the case of an intralayer Rashba spin-orbit interaction (RSOI) different energy bands exist. The results shown depicts how the behaviour of the energy bands remains the same when we are increasing the number of atoms (N) in a unit cell of bilayer graphene.

Band Gap Opening in Bilayer Graphene-CrCl3/CrBr3/CrI3 van der Waals Interfaces.

We report experimental investigations of transport through bilayer graphene (BLG)/chromium trihalide (CrX3; X = Cl, Br, I) van der Waals interfaces. In all cases, a large charge transfer from BLG to CrX3 takes place (reaching densities in excess of 1013 cm-2), and generates an electric field perpendicular to the interface that opens a band gap in BLG. We determine the gap from the activation energy of the conductivity and find excellent agreement with the latest theory accounting for the contribution of the σ bands to the BLG dielectric susceptibility. We further show that for BLG/CrCl3 and BLG/CrBr3 the band gap can be extracted from the gate voltage dependence of the low-temperature conductivity, and use this finding to refine the gap dependence on the magnetic field. Our results allow a quantitative comparison of the electronic properties of BLG with theoretical predictions and indicate that electrons occupying the CrX3 conduction band are correlated.

Quantum capacitive coupling between large-angle twisted graphene layers

Large-angle twisted bilayer graphene (tBLG) is known to be electronically decoupled due to the spatial separation of the Dirac cones corresponding to individual graphene layers in the reciprocal space. The close spacing between the layers causes strong capacitive coupling, opening possibilities for applications in atomically thin devices. Here, we present a self-consistent quantum capacitance model for the electrostatics of decoupled graphene layers, and further generalize it to deal with decoupled tBLG at finite magnetic field and large-angle twisted double bilayer graphene at zero magnetic field. We probe the capacitive coupling through the conductance, showing good agreement between simulations and experiments for all the systems considered. We also propose a new experiment utilizing the decoupling effect to induce a huge and tunable bandgap in bilayer graphene by applying a moderately low bias. Our model can be extended to systems composed of decoupled graphene multilayers as well as non-graphene systems, opening a new realm of quantum-capacitively coupled materials.

Sizable Band Gap in Epitaxial Bilayer Graphene Induced by Silicene Intercalation.

Opening a band gap in bilayer graphene (BLG) is of significance for potential applications in graphene-based electronic and photonic devices. Here, we report the generation of a sizable band gap in BLG by intercalating silicene between BLG and Ru substrate. We first grow high-quality Bernal-stacked BLG on Ru(0001) and then intercalate silicene to the interface between the BLG and Ru, which is confirmed by low-energy electron diffraction and scanning tunneling microscopy. Raman spectroscopy shows that the G and 2D peaks of the intercalated BLG are restored to the freestanding-BLG features. Angle-resolved photoelectron spectroscopy measurements show that a band gap of about 0.2 eV opens in the BLG. Density functional theory calculations indicate that the large-gap opening results from a cooperative contribution of the doping and rippling/strain in the BLG. This work provides insightful understanding on the mechanism of band gap opening in BLG and enhances the potential of graphene-based device development.

Thermoelectric properties of gapped bilayer graphene

Unlike in conventional semiconductors, both the chemical potential and the band gap in bilayer graphene (BLG) can be tuned via application of external electric field. Among numerous device implications, this property also designates BLG as a candidate for high-performance thermoelectric material. In this theoretical study we have calculated the Seebeck coefficients for abrupt interface separating weakly- and heavily-doped areas in BLG, and for a more realistic rectangular sample of mesoscopic size, contacted by two electrodes. For a given band gap () and temperature (T) the maximal Seebeck coefficient is close to the Goldsmid–Sharp value , the deviations can be approximated by the asymptotic expression , with the electron charge −e, the Boltzmann constant , and . Surprisingly, the effects of trigonal warping term in the BLG low-energy Hamiltonian are clearly visible at few-Kelvin temperatures, for all accessible values of meV. We also show that thermoelectric figure of merit is noticeably enhanced (ZT > 3) when a rigid substrate suppresses out-of-plane vibrations, reducing the contribution from ZA phonons to the thermal conductivity.

Electrical noise inside the band gap of bilayer graphene

Bilayer graphene (BLG), endowed with its electrostatically tunable band gap, plays a special role in next generation 2D materials based electronics. However, important aspects of its device characteristics are not fully understood, including in particular, the mechanism of electrical noise in its gapped state, which is essential to its operation. Here, we present the first systematic study of 1/f noise in conductance G in a dual-gated BLG as we tune the Fermi energy inside its large band gap. The normalized conductance noise initially increases dramatically by over an order of magnitude, which our analysis indicates, as arising due to percolative charge transport. When the Fermi energy reaches the charge neutrality point, we observe a remarkably constant irrespective of the magnitude of band gap, even though G decreases exponentially with band gap. We discuss that this is likely to be consequence of fluctuations in activated charge carriers far from the Fermi energy, enabled by charge traps of the substrate. Our results provide a coherent description of low frequency electrical noise in gapped BLG, paving the way for its implementation in the next-generation devices as well as representing benchmark for other gapped 2D materials.

Band Gap Calculations of Bilayer Graphene and Bilayer Armchair Graphene Nanoribbon

Graphene has a high potential for use in electronic devices because it has a unique characteristic, electrons in massless grasses and has a speed close to the speed of light. Graphene conductivity at temperature space is 40 000 cm2·V-1·s-1 in the air. In addition, thermal conductivity can reach 2 800 W·m-1·K-1. The mechanical properties of graphene are very strong and flexible, and optical properties it has a white light transmittance of 95%. Therefore, graphene is suitable for transparent screen and the flexible electrode. The energy band gap of monolayer graphene can be changed by reducing the size, as for the band gap of bilayer graphene was affected by symmetry between two layers. So, it is interesting to research the band gap characteristics of bilayer armchair graphene nanoribbon (AGNR). In this paper, we have calculated the band gap of bilayer graphene and bilayer AGNR in various parameters such as band width and symmetry between two layers. The method used is tight binding method by only taking into account its nearest neighbors only. The results obtained from this simulation show that the band gap will be more open if asymmetry of the structure bilayer graphene is increasing and the band gap of AGNR will be more open if ribbon width decreased, is not affected by asymmetry.

Difference in gating and doping effects on the band gap in bilayer graphene

A band gap is opened in bilayer graphene (BLG) by applying an electric field perpendicular to the layer, which offers versatility and controllability in graphene-based electronics. The presence of the band gap has been confirmed using double-gated BLG devices in which positive and negative gate voltages are applied to each side of BLG. An alternative method to induce the electric field is electron and hole doping of each side of BLG using electron-transfer adsorbates. However, the generation of the band gap by carrier doping is still under investigation. Here, we determined whether the electron/hole doping can produce the electric field required to open the band gap by measuring the temperature dependence of conductivity for BLG placed between electron-donor self-assembled monolayers (SAMs) and electron-acceptor molecules. We found that some devices exhibited a band gap and others did not. The potentially irregular and variable structure of SAMs may affect the configuration of the electric field, yielding variable electronic properties. This study demonstrates the essential differences between gating and doping.

First principles study of tunable band gap in bi layer Graphene (BLG)

Graphene being two-dimensional crystalline form of carbon hold great promise for nanoscale electronics and photonics. Graphene's promise can't be realized unless a band gap is introduced. As with monolayer Graphene, bilayer Graphene also has a zero band gap and thus has attracted considerable attention for its study. The structure and electrical properties of bi layer Graphene (BLG) sheets adsorbed with metals are studied using first-principles calculations. There is a tunable band gap in BLG by applying a vertical electric field, but the operation of BLG requires two gates to create a band gap and tune channel's conductance individually. The band gap is generally proportional to the charge transfer density. Using first principle calculations, we propose an alternative scheme to open a band gap in ABC-stacked BLG via single-side adsorption. We find that the band gap of ABC-stacked BLG can even be opened if the bond symmetry is broken. A clear transmission gap which is comparable with the band gap is observed in simulated single-gated SnCl4-adsorbed ABC-stacked bilayer Graphene. The adsorption of BLG on SnCl4 is a weak physisorption, but a band gap can be opened. Calculations of electron density show a weak coupling between Graphene substrate and SnCl4 metal and a shift in the Fermi level is also observed. This fundamental study helps in BLG device study due to inevitable Graphene/metal contact which renders metal-adsorbed BLG a promising channel in single-gated devices.

Band gap opening of bilayer graphene by graphene oxide support doping

In contrast to the metallic monolayer graphene by graphene oxides (GOs) doping, the sizable band gap of bilayer graphene is opened by GOs.

Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings.

The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states toward the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7–20 meV. Orbital splittings of 4–10 meV and a valley splitting of about 3 meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques.

Electric gating induced bandgaps and enhanced Seebeck effect in zigzag bilayer graphene ribbons

We theoretically investigate the effect of a transverse electric field generated by side gates and a vertical electric field generated by top/back gates on energy bands and transport properties of zigzag bilayer graphene ribbons (Bernal stacking). Using atomistic tight binding calculations and Green’s function formalism we demonstrate that a bandgap is opened when either field is applied and even enlarged under simultaneous influence of the two fields. Interestingly, although vertical electric fields are widely used to control the bandgap in bilayer graphene, here we show that transverse fields exhibit a more positive effect in terms of modulating a larger range of bandgap and retaining good electrical conductance. The Seebeck effect is also demonstrated to be enhanced strongly—by about 13 times for a zigzag bilayer graphene ribbon with 16 chain lines. These results may motivate new designs of devices made of bilayer graphene ribbons using electric gates.

A promising way to open an energy gap in bilayer graphene.

There has been huge research interest in the energy gap problem of monolayer and bilayer graphene due to their great potential in practical applications. Herein, based on first-principles calculations, we report a promising way to open a large band gap in bilayer graphene (BLG) by sandwiching it between two substrates, although this is not usually expected to occur due to the weak interlayer interactions dominated by van der Waals forces. Taking surface-functionalized boron-nitrides as substrates, we predict from first-principles calculations that BLG can have energy gaps ranging from 0.35 eV to 0.55 eV, depending on the substrates and stacking order. Compared to other methods of band-gap manipulation in BLG, the structural integrity of BLG is well-preserved in our study, and the predicted energy gap is suitable for electric devices. Since the proposed method is easily realized in experiments, our results will hopefully accelerate the application of graphene in semiconductor devices and promote the development of graphene technology.

Tunable Band Gap in Bilayer Graphene by Trimesic Acid Molecular Doping

We are presenting the results of first-principles study of the electronic band structure properties of molecular doping of bilayer graphene with one-sided adsorption of a single and self-assembled trimesic acid (TMA) monolayer which exhibits a p-type dopant semiconductor. Our study demonstrates that the effect of charge transfer between the adsorbate and substrate is the origin of perturbation in inversion symmetry of bilayer graphene and a result of band gap opening. Meanwhile, doping of bilayer graphene with an organic molecule produces an internal electric (built-in) field between the bottom and top layer due to charge asymmetry. The self-assembly of trimesic acid (TMA, benzene-1,3,5-tricarboxylic acid) on bilayer graphene with the two most network arrangements improves the stability and band gap energy of the adlayer by formation of reliable hydrogen bonding. The various patterns applied to adsorption on bilayer graphene allow us to tune the induced band gap where the existence of a linear trend of energy gap with number of carriers demonstrates consistency between experimental and theoretical studies. Accordingly these data lead to further implications for nanoelectronics and nanophotonics devices.

Tunable band gaps in graphene/GaN van der Waals heterostructures

Van der Waals (vdW) heterostructures consisting of graphene and other two-dimensional materials provide good opportunities for achieving desired electronic and optoelectronic properties. Here, we focus on vdW heterostructures composed of graphene and gallium nitride (GaN). Using density functional theory, we perform a systematic study on the structural and electronic properties of heterostructures consisting of graphene and GaN. Small band gaps are opened up at or near the Γ point of the Brillouin zone for all of the heterostructures. We also investigate the effect of the stacking sequence and electric fields on their electronic properties. Our results show that the tunability of the band gap is sensitive to the stacking sequence in bilayer-graphene-based heterostructures. In particular, in the case of graphene/graphene/GaN, a band gap of up to 334 meV is obtained under a perpendicular electric field. The band gap of bilayer graphene between GaN sheets (GaN/graphene/graphene/GaN) shows similar tunability, and increases to 217 meV with the perpendicular electric field reaching 0.8 V Å − 1.

(Invited) Quantum Capacitance Measurement of Bilayer Graphene

The ability to electrostatically tune the band gap in bilayer graphene attracts great attention. Although the theory predicts the band gap of ~250meV, the off-current is considerably reduced only at the temperature lower than 1 mK and generally high at room temperature. This is explained by the variable range hopping in gap states. However, there should be intrinsically no interface states in graphene because there is no dangling bonds on the basal plane. The origin for the gap states is still open question. Moreover, in the actual device operation, the high on-current as well as the low off-current are crucial. There are four sub-bands in bilayer graphene; two in the conduction bands and two in the valence bands. However, the contribution of the sub-bands to the electrical transport for on-current is quite limited due to the unreliable quality of the high-ktop gate insulator on bilayer graphene.Recently, we demonstrate the ultra-high displacement of ~8 V/nm (n=~4×1013 cm-2) in bilayer graphene using the solid state Y2O3 top gate [1], which has been reached only by the ion gating so far. The ultra-high displacement provides the access to both the carrier response issue in the largely-opened band gap and the inter-band scattering issue at the high carrier density. In this study, we focus on the quantum capacitance (C Q) measurements [2] for bilayer graphene because the density of states (DOS) can be extracted through C Q=e 2 DOSand the scattering issues can be excluded. The frequency dispersion in C-V curve reveals that the carriers in bilayer graphene electrically communicate with trap sites within the band gap. The local breakdown of A-B stacking, which results in the local conduction sites, might be the origin for the gap states. Moreover, the systematic comparison of I-V and C-V curves reveals that the filling of carriers in the high energy sub-bands results in the reduction of the conductivity due to the inter-band scattering.[1] K. Nagashio, K. Kanayama, T. Nishimura, A. Toriumi, IEDM Tech. 2013.[2] K. Nagashio, T. Nishimura, A. Toriumi, APL, 2013, 102, 173507.

Bound state energy of a Coulomb impurity in gapped bilayer graphene

Application of a perpendicular electric field induces a band gap in bilayer graphene, and it also creates a ``Mexican hat'' structure in the dispersion relation. This structure has unusual implications for the hydrogen-like bound state of an electron to a Coulomb impurity. We calculate the ground state energy of this hydrogen-like state as a function of the applied interlayer voltage and the effective fine structure constant. Unlike in the conventional hydrogen atom, the resulting wave function has many nodes even in the ground state. Further, the electron state undergoes ``atomic collapse'' into the Dirac continuum both at small and large voltage.

Opening a band gap without breaking lattice symmetry: a new route toward robust graphene-based nanoelectronics.

Developing graphene-based nanoelectronics hinges on opening a band gap in the electronic structure of graphene, which is commonly achieved by breaking the inversion symmetry of the graphene lattice via an electric field (gate bias) or asymmetric doping of graphene layers. Here we introduce a new design strategy that places a bilayer graphene sheet sandwiched between two cladding layers of materials that possess strong spin-orbit coupling (e.g., Bi2Te3). Our ab initio and tight-binding calculations show that a proximity enhanced spin-orbit coupling effect opens a large (44 meV) band gap in bilayer graphene without breaking its lattice symmetry, and the band gap can be effectively tuned by an interlayer stacking pattern and significantly enhanced by interlayer compression. The feasibility of this quantum-well structure is demonstrated by recent experimental realization of high-quality heterojunctions between graphene and Bi2Te3, and this design also conforms to existing fabrication techniques in the semiconductor industry. The proposed quantum-well structure is expected to be especially robust since it does not require an external power supply to open and maintain a band gap, and the cladding layers provide protection against environmental degradation of the graphene layer in its device applications.

(Invited) Toward Ambient-Stable Molecular Gated Graphene-FET: A Donor/Acceptor Hybrid Architecture to Achieve Bandgap in Bilayer Graphene

In this work we report a technique to improve the Ion/Ioff ratio in bilayer graphene FET by asymetrical doping of layers. Doping is achived by n-doping the bottom layer by depositing bilayer graphene flakes on NH2-SAM modified SiO2 substrate and hole doping the top layer via coating the device with a film of F4TCNQ-containing polystyrene. Asymmetric surface doping of bilayer graphene can induce an electric field between both layers which results in opening of an electronic bandgap due to symmetry breaking. DFT modelling shows an effective electric field of ~1.5 V/nm between the layers and field effect measurements show an increase of Ion/Ioff ratio in bilayer FET up to 135 due to opening of the bandgap also, a Schottky barrier of ~60 meV at the interface of the semiconducting bilayer graphene and the electric contact is observed.

(Invited) Toward Ambient-Stable Molecular Gated Graphene-FET: A Donor/Acceptor Hybrid Architecture to Achieve Bandgap in Bilayer Graphene

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