Bilayer Zigzag Graphene Nanoribbons Research Articles

Effects of transverse electric fields on Landau subbands in bilayer zigzag graphene nanoribbons

The magneto-electronic properties of quasi-one-dimensional zigzag graphene nanoribbons are investigated by using the Peierls tight-binding model. Quasi-Landau levels (QLLs), dispersionless Landau subbands within a certain region of -space, are resulted from the competition between magnetic and quantum confinement effects. In bilayer system, the interlayer interactions lead to two groups of QLLs, one occurring at the Fermi level and the other one occurring at higher energies. Transverse electric fields are able to distort energy spectrum, tilt two groups of QLLs, and cause semiconductor-metal transition. From the perspective of wave functions, the distribution of electrons is explored, and the evolution of Landau states under the influence of electric fields is clearly discussed. More interestingly, the band mixing phenomena exhibited in the energy spectrum are related to the state mixing, which can be seen by the wave functions. The density of states, which could be verified through surface inspections and optical experiments, such as scanning tunneling spectroscopy and absorption spectroscopy, is provided at last.

First-principles study of carrier-induced ferromagnetism in bilayer and multilayer zigzag graphene nanoribbons

We studied magnetism in bilayer and multilayer zigzag graphene nanoribbons (ZGNRs) through first-principles density functional theory calculations. We found that the magnetic ground state of bilayer ZGNRs is the C-type antiferromagnetic (AFM) state, which is the AFM order between intraplane-edge carbon atoms and ferromagnetic (FM) order between interplane edge carbon atoms. In the cases of infinitely stacked multilayer ZGNRs, i.e., zigzag graphite nanoribbons, the C-type AFM state is also the most stable. By carrier doping, we found that the magnetic ground state changed from the C-AFM state to the FM state and, thus, realized two-dimensional FM surface (edge) states of graphite with a metallic conductivity.

Noncollinear magnetism and half-metallicity in biased bilayer zigzag graphene nanoribbons

In this paper we study the edge polarizations and their consequences for a biased Bernal stacked bilayer graphene nanoribbon with zigzag termination. The magnetic states are classified according to the interlayer and intralayer couplings between the edge polarizations, and the magnetic phase diagram of doping versus bias voltage is given. A coplanar magnetic phase is found and the variation of its magnetic structure with bias voltage is investigated. For all the magnetic states, we also discuss the possibility of half-metallicity, and for a ribbon with perfect zigzag edges, we predict seven kinds of half-metallic states, which are characterized by their distinct magnetic structures and quantized electrical conductances along the ribbon.

Electric-field-induced destruction of quasi-Landau levels in bilayer graphenenanoribbons

The magneto-electronic properties of bilayer zigzag graphene nanoribbons are investigated by the Peierls tight-binding method. In the presence of magnetic fields, Landau quantization leads to the formation of Landau subbands. For the bilayer nanoribbons, these subbands are partially dispersionless in k-space and are called quasi-Landau levels (QLLs). Perpendicular electric fields, serving as the top gate, push the QLLs to higher state energy and split the flat subbands. From the evidence of band structure and density of states, the QLLs remain dispersionless and the corresponding peaks are still the main structure of density of states, which means that the material properties related to the QLLs are unchanged. However, the wave functions present a totally different evidence that the Landau wave functions are severely mixed, and the corresponding material properties would be strongly affected or destroyed. The wave functions provide an effective way to comprehend the characteristics of the flat subbands and Landau subbands. The energy spectra, density of states, and wave functions are discussed in detail.

Effects of the edge states on conductance and thermopower for the bilayer graphene nanoribbons

By using the nonequilibrium Green’s function combined with the tight-binding Hamiltonian, we have studied the effects of the edge states on the conductance and the thermopower for zigzag bilayer graphene nanoribbons (ZBGNs). It is shows that the band structure, conductance, and thermopower can be modulated by the boundary potentials and the bias voltages to the layers of the ZBGNs. When the boundary potentials are adjusted to the nearest neighbor hopping energy, two gapless edge modes with opposite velocities appear in the vicinity of the two Dirac points, and the quantized conductance has a transition from 2(n+1)G0 to 2(n+1/2)G0 with G0=2e2/h being the conductance unit and n an integer. Particularly, under the strong bias voltage, compare to the gapless perfect ZBGN, the thermopower can be enhanced more than twice for the gapless edge modes with opposite velocities. Combining the reduced thermal conductivity in few-layer graphene, our results show that the modulated ZBGNs are more reliable in thermoelectric application.

Transport properties of AA-stacking bilayer graphene nanoribbons

The transport properties of AA-stacking bilayer graphene nanoribbons (GNs) have been explored by using the nonequilibrium Green's function method and the Landauer–Büttiker formalism. It is found that in the case of zero bias, the interlayer coupling has pronounced effects on the conductance of bilayer GNs. The zigzag bilayer GNs remain metallic, but metallic armchair bilayer GNs will be semiconductor as the strength of interlayer coupling exceeds critical value. The first Van Hove singularities move close to the Dirac point for both armchair and zigzag bilayer GNs with the strength of interlayer coupling increasing. Some prominent conductance peaks around the Fermi energy are observed in zigzag bilayer GNs, when the top layer and bottom layer have different widths. In the presence of bias voltage, the I–V curves show that for armchair bilayer GNs, the interlayer interactions suppress current, while the interlayer interactions have almost no effect on the current for zigzag bilayer GNs. The ripples in bilayer GNs suppress electronic transport, especially for zigzag bilayer GNs.

Structural and Magnetic Instability of Bilayer and Trilayer Zigzag Graphene Nanoribbons

Structural and Magnetic Instability of Bilayer and Trilayer Zigzag Graphene Nanoribbons

Electronic Properties of Bilayer Zigzag Graphene Nanoribbons: First Principles Study

Based on the density functional theory, we calculate the dependence of the band structures of bilayer zigzag-edged graphene nanoribbons (BZGNRs) upon ribbon width, interlayer distance and stacking styles. Unlike monolayer zigzag GNR, whose energy gap is always zero under different ribbon widths, the gap of BZGNR varies greatly with the ribbon width or the interlayer distance. The greatest gaps for AA-stacking and AB-stacking BZGNRs are about 0.22eV and 0.12eV, respectively, which implies that gap-tuning of AA-BZGNRs is more effective than that of AB-BZGNRs. These results present a way to tune the band structures of BZGNRs and also provide theoretical guidance for the fabrication of GNR-based piezoelectric devices.

Spin-polarized energy-gap opening in asymmetric bilayer graphene nanoribbons

Electronic and magnetic properties of bilayer zigzag graphene nanoribbon (bZGNR) are studied using pseudopotential density functional method. The edge atoms in the top and bottom layers of bZGNR make a weak hybridization, which leads to electronic structures different from monolayer ZGNR. For asymmetric bZGNR, where the top and bottom layers have different widths, one edge is pinched by the interlayer bonding and the other sustains antiferromagnetic ordering. A small amount of charge transfer occurs from narrower to wider layer, producing spin-polarized electron and hole pockets. External electric field produces asymmetric energy-gap opening for each spin component, inducing half-metallicity in bZGNR.

Semiconducting to Half-Metallic to Metallic Transition on Spin-Resolved Zigzag Bilayer Graphene Nanoribbons

Our first-principles calculations show that the electronic properties of spin-resolved zigzag bilayer graphene nanoribbons (ZBGNRs) of antiferromagnetic edges are highly sensitive to the interlayer distance between nanoribbons. The energy gap of the antiferromagnetic ZBGNR decreases and the semiconducting ZBGNR finally becomes a metal with decreasing interlayer spacing. Coupling with a transverse electric field, the ZBGNR exhibits a semiconducting to half-metallic to metallic transition via interlayer distance compression. The charge redistribution of spin-up and spin-down and the change of the density of states on the edge atoms, which are driven by the coupled effect of nanomechanical compression and the electric field, contribute to the exceptional modification on the band structure of the ZBGNR.

Quasi-Landau levels in bilayer zigzag graphene nanoribbons

The magnetoelectronic properties of one-dimensional AB-stacked bilayer zigzag graphene nanoribbons are investigated by the Peierls tight-binding model. The competition between magnetic and quantum confinement effects induces quasi-Landau levels (QLLs). The interlayer interactions lead to two groups of QLLs, asymmetry of conduction and valence QLLs about the Fermi level, and splitting of the degenerate partial flat bands. The density of states exhibiting many prominent peaks, corresponding to QLLs, could be verified by experiments.

Magnetoconductance of graphene nanoribbons

The electronic and transport properties of monolayer and AB-stacked bilayer zigzag graphene nanoribbons subject to the influences of a magnetic field are investigated theoretically. We demonstrate that the magnetic confinement and the size effect affect the electronic properties competitively. In the limit of a strong magnetic field, the magnetic length is much smaller than the ribbon width, and the bulk electrons are confined solely by the magnetic potential. Their properties are independent of the width, and the Landau levels appear. On the other hand, the size effect dominates in the case of narrow ribbons. In addition, the dispersion relations rely sensitively on the interlayer interactions. Such interactions will modify the subband curvature, create additional band-edge states, change the subband spacing or the energy gap, and separate the partial flat bands. The band structures are symmetric or asymmetric about the Fermi energy for monolayer or bilayer nanoribbons, respectively. The chemical-potential-dependent electrical and thermal conductance exhibits a stepwise increase behaviour. The competition between the magnetic confinement and the size effect will also be reflected in the transport properties. The features of the conductance are found to be strongly dependent on the field strength, number of layers, interlayer interactions, and temperature.