Bilayer Graphene Nanostructures Research Articles

Resistive Switching in Bigraphene/Diamane Nanostructures Formed on a La3Ga5SiO14 Substrate Using Electron Beam Irradiation.

Memristors, resistive switching memory devices, play a crucial role in the energy-efficient implementation of artificial intelligence. This study investigates resistive switching behavior in a lateral 2D composite structure composed of bilayer graphene and 2D diamond (diamane) nanostructures formed using electron beam irradiation. The resulting bigraphene/diamane structure exhibits nonlinear charge carrier transport behavior and a significant increase in resistance. It is shown that the resistive switching of the nanostructure is well controlled using bias voltage. The impact of an electrical field on the bonding of diamane-stabilizing functional groups is investigated. By subjecting the lateral bigraphene/diamane/bigraphene nanostructure to a sufficiently strong electric field, the migration of hydrogen ions and/or oxygen-related groups located on one or both sides of the nanostructure can occur. This process leads to the disruption of sp3 carbon bonds, restoring the high conductivity of bigraphene.

Fabrication of three-dimensional sandwich-like silver nanowire network coated bilayer graphene nanostructures for flexible display.

High-quality transparent conductive film is very important for flexible display, especially with super high transmittance, good conductivity, and excellent stability. In this work, the synthesis and characterization of a three-dimensional sandwich-like structure, including a silver nanowire network and PEDOT:PSS coated by two layers of graphene, a transparent conductive layer named GAPG, were demonstrated. The transparent conductive layer made full use of the plasma effect of silver nanowires and the properties of graphene, including high light transmittance, metalloid conductivity, and excellent mechanical recovery. The PEDOT:PSS enabled silver nanowires and double graphene to form a solid whole, which greatly improved the stability of the membrane. Its sheet resistance was 48.3Ω/◻, and its electron mobility was 14,663cm2/(V⋅s), which were superior to single-layer graphene and other transparent conductive layers. Compared with other flexible conductive materials, GAPG had more advantages in mechanical properties, a wider viewing angle, excellent bending recovery, and more stable performance. When the film was bent, its light transmittance was only ±0.3Ω, and the change rate was less than ±0.2%. The scattering of silver nanowires and PEDOT:PSS particles made the problem of display angle fully solved. So GAPG is a potential flexible conductive material, which can be used in electronic paper and other flexible devices to greatly improve their performance.

Elastic Properties of Bilayer Graphene Nanostructures with Closed Holes

The elastic moduli of bilayer graphene nanomeshes, i.e., nanomeshes of bilayer graphene, where layers at the edges of “closed” holes are coupled to each other by a continuous network of sp2-hybridized atoms, have been calculated by ab initio methods. Structures with different configurations of holes in layers with AA, AB, and 30° stackings have been studied. It has been shown that the ultimate tensile strength of the nanomeshes under consideration is higher than that of graphene nanostructures and is comparable with the ultimate tensile strength of bilayer graphene and single-layer carbon nanotubes. A possible application of such strong nanomeshes as nanocontainers for hydrogen storage and other compressed gases has been also discussed.

Self-assembled triangular graphene nanostructures: Evidence of dual electronic response

Structural and electronic properties of bilayer graphene films and nanostructures obtained through the graphitization of SiC(0001) were investigated in this work using scanning tunneling microscopy and spectroscopy. We report on the observation of triangular nanostructures which result from extended stacking faults in the SiC substrate and their effects on graphene layers that are formed on top of them. Spectroscopic measurements revealed distinct electronic responses as a function of the local hydrogen intercalation. Spectroscopic signatures ranging from single- to double-layer graphene, as well as intermediate states were observed as a consequence of the (in)complete hydrogen intercalation process. High resolution topographic scanning tunneling microscopy images at resonant bias voltages inside triangular nanostructures reveal that the bottom layer of the bilayer graphene film is still bonded to the substrate. Therefore, the triangular nanostructures present edges and facets with the coexistence of carbon atoms in sp3 and sp2 hybridizations. Using atomistic calculations we have modeled the local density of states of these objects reproducing their electronic response. The generation of regions with distinct electronic responses is potentially interesting for high-density data storage with hidden bit capabilities.

Gate-tunable valley currents, nonlocal resistances, and valley accumulation in bilayer graphene nanostructures

Using the B\"{u}ttiker-Landauer formulation of transport theory in the linear response regime, the valley currents and non-local resistances of bilayer graphene nanostructures with broken inversion symmetry are calculated. It is shown that broken inversion symmetry in bilayer graphene nanostructures leads to striking enhancement of the non-local 4-terminal resistance and to valley currents several times stronger than the conventional electric current when the Fermi energy is in the spectral gap close to the energy of Dirac point. The scaling relation between local and non-local resistances is investigated as the gate voltage varies at zero Fermi energy and a power-law is found to be satisfied. The valley velocity field and valley accumulation in four-terminal bilayer graphene nanostructures are evaluated in the presence of inversion symmetry breaking. The valley velocity and non-local resistance are found to scale differently with the applied gate voltage. The unit cell-averaged valley accumulation is found to exhibit a dipolar spatial distribution consistent with the accumulation arising from the valley currents. We define and calculate a {\em valley capacitance} that characterizes the valley accumulation response to voltages applied to the nanostructure's contacts.

Evidence of electronic cloaking from chiral electron transport in bilayer graphene nanostructures

The coupling of charge carrier motion and pseudospin via chirality for massless Dirac fermions in monolayer graphene has generated dramatic consequences, such as the unusual quantum Hall effect and Klein tunneling. In bilayer graphene, charge carriers are massive Dirac fermions with a finite density of states at zero energy. Because of their non-relativistic nature, massive Dirac fermions can provide an even better test bed with which to clarify the importance of chirality in transport measurement than massless Dirac fermions in monolayer graphene. Here, we report an electronic cloaking effect as a manifestation of chirality by probing phase coherent transport in chemical-vapor-deposited bilayer graphene. Conductance oscillations with different periodicities were observed on extremely narrow bilayer graphene heterojunctions through electrostatic gating. Using a Fourier analysis technique, we identified the origin of each individual interference pattern. Importantly, the electron waves on the two sides of the potential barrier can be coupled through the evanescent waves inside the barrier, making the confined states underneath the barrier invisible to electrons. These findings provide direct evidence for the electronic cloaking effect and hold promise for the realization of pseudospintronics based on bilayer graphene.

Gate tunable layer selectivity of transport in bilayer graphene nanostructures

Recently it was found that bilayer graphene may exhibit regions with and without van der Waals coupling between the two layers. We show that such structures can exhibit a strong layer selectivity when current flows through the coupled region and that this selectivity can be tuned by means of electrostatic gating. Analysing how this effect depends on the type of bilayer stacking, the potential on the gates and the smoothness of the boundary between the coupled and decoupled regions, we show that nearly perfect layer selectivity is achievable in these systems. This effect can be further used to realise a tunable layer switch.

Synthesis of quasi-free-standing bilayer graphene nanoribbons on SiC surfaces.

Scaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications.

Superconducting properties of lithium-decorated bilayer graphene

The present study provides a comprehensive theoretical analysis of the superconducting phase in selected lithium-decorated bilayer graphene nanostructures. The numerical calculations, conducted within the Eliashberg formalism, give quantitative estimations of the most important thermodynamic properties such as the critical temperature, specific heat, critical field and others. It is shown that discussed lithium-graphene systems present enhancement of their thermodynamic properties comparing to the monolayer case, e.g., the critical temperature can be raised to . Furthermore, estimated characteristic thermodynamic ratios exceed predictions of the Bardeen-Cooper-Schrieffer theory suggesting that the considered lithium-graphene systems can be properly analyzed only within the strong-coupling regime.

Electronic Spectral Function of Monolayer and Bilayer Graphene Nanostructures

Electronic Spectral Function of Monolayer and Bilayer Graphene Nanostructures

Electron flow in split-gated bilayer graphene

We present transport measurements on a bilayer graphene sheet with a homogeneous back gate and a split top gate. The electronic transport data indicate the capability of directing electron flow through bilayer graphene nanostructures purely defined by electrostatic gating. Comparing the transconductance data recorded for different top gate geometries—continuous barrier and split gate—the observed transport features for the split gate can be attributed to the interference effects inside the narrow opening.

Local topological phase transitions in periodic condensed matter systems

Topological properties of a periodic condensed matter system are global features of its Brillouin zone (BZ). In contrast, the validity of effective low energy theories is usually limited to the vicinity of a high symmetry point in the BZ. We derive a general criterion under which the control parameter of a topological phase transition localizes the topological defect in an arbitrarily small neighbourhood of a single point in k-space upon approaching its critical value. Such a local phase transition is associated with a Dirac-like gap closing point, whereas a flat band transition is not localized in k-space. This mechanism and its limitations are illustrated with the help of experimentally relevant examples such as HgTe/CdTe quantum wells and bilayer graphene nanostructures.

Signature of quantum interference and the Fano resonances in the transmission spectrum of bilayer graphene nanostructure

The well-known asymmetric Fano resonances that results from the quantum interference between the discrete and the continuum states are noted for the first time in the ballistic transmission spectrum of the bilayer graphene tunneling structures. This unconventional tunneling transmission, in stark contrast to the monolayer graphene and to the conventional heterostructures, arises due to the quadratic dispersion of the chiral charge carriers. If the Klein tunneling (the phenomenon for normal incidence) is an unusual characteristic of the massless chiral particles, then the Fano tunneling (the phenomenon for low glancing incidence) would be the specialty for the massive chiral particles. The characteristic features of the Fano line shape are found to be highly sensitive to the direction of incidence of the charge carriers, the applied homogeneous electric field, and to the barrier height. The sharp anti-resonance at the center of the tunneling band arising due to the destructive interference between the electron and the holelike states could probably be responsible for the high negative differential conductance (NDC) in the bilayer graphene. The tunneling conductance in the double barrier structure exhibits a resonant peak with a sharp NDC region for the Fermi energy less than or equal to half of the barrier height. The present findings might have great implications in the preparation of NDC-based devices using bilayer graphene nanostructures.

Tunable Fano resonances in the ballistic transmission and tunneling lifetime in a biased bilayer graphene nanostructure

Dynamics of the Dirac fermions, in particular the transmission coefficient and the resonant tunneling lifetime are studied across a bilayer graphene electrostatic double barrier structure modulated by an in plane homogeneous electric field. Asymmetric Fano type resonances are noted for the first time in the transmission spectrum of the bilayer graphene nanostructures and are found to be highly sensitive to the direction of incidence of the charge carriers and the applied homogeneous electric field. The effect of the external field on the extended and the evanescent modes is also analysed. Resonant tunneling lifetime is found to be highly anisotropic in nature. The chiral carriers are either accelerated or decelerated by the electric field depending on the energy of the quasi-bound states, the angle of incidence and the magnitude of the applied field. Effects of the external field on the localization of the chiral carriers are also discussed.