Properties Of Bilayer Graphene Research Articles

Enhanced electron-phonon coupling in doubly aligned hexagonal boron nitride bilayer graphene heterostructure

The relative twist angle in heterostructures of two-dimensional (2D) materials with similar lattice constants result in a dramatic alteration of the electronic properties. Here, we investigate the electrical and magnetotransport properties in bilayer graphene (BLG) encapsulated between two hexagonal boron nitride (hBN) crystals, where the top and bottom hBN are rotationally aligned with bilayer graphene with a twist angle $\theta_t\sim 0^{\circ} \text{and}~ \theta_b < 1^{\circ}$, respectively. This results in the formation of two moir\'e superlattices, with the appearance of satellite resistivity peaks at carrier densities $n_{s1}$ and $n_{s2}$, in both hole and electron doped regions, together with the resistivity peak at zero carrier density. Furthermore, we measure the temperature(T) dependence of the resistivity ($\rho$). The resistivity shows a linear increment with temperature within the range 10K to 50K for the density regime $n_{s1} <n<n_{s2}$ with a large slope d$\rho$/dT $\sim$ 8.5~$\Omega$/K. The large slope of d$\rho$/dT is attributed to the enhanced electron-phonon coupling arising due to the suppression of Fermi velocity in the reconstructed minibands, which was theoretically predicted, recently in doubly aligned graphene with top and bottom hBN. Our result establishes the uniqueness of doubly aligned moire system to tune the strength of electron-phonon coupling and to modify the electronic properties of multilayered heterostructures.

Electronic property of bilayer graphene on pristine and rhenium-doped MoS2

Abstract First-principles calculation has been performed to investigate the electronic structure of bilayer graphene (BLG) on MoS2 layer with and without rhenium(Re) atom substitution. The results show that the band gap of MoS2/BLG is 30 meV and is enlarged to 98 meV with the substitution of molybdenum (Mo) atom by Re atom in MoS2. By analyzing the change density difference of the two systems, the existence of MoS2 causes the inequivalent charge distribution between the two graphene layers, which induces the band gap. Dopant Re atom enlarges the inequivalent electron distribution, resulting larger band gap.

Transport properties of bilayer graphene due to charged impurity scattering: Temperature-dependent screening and substrate effects

We calculate the zero-temperature conductivity of bilayer graphene (BLG) impacted by Coulomb impurity scattering using four different screening models: unscreened, Thomas–Fermi (TF), overscreened and random phase approximation (RPA). We also calculate the conductivity and thermal conductance of BLG using TF, zero- and finite-temperature RPA screening functions. We find large differences between the results of the models and show that TF and finite-temperature RPA give similar results for diffusion thermopower S[Formula: see text]. Using the finite-temperature RPA, we calculate temperature and density dependence of S[Formula: see text] in BLG on SiO2, HfO2 substrates and suspended BLG for different values of interlayer distance c and distance between the first layer and the substrate d.

Gas sensitive properties of bilayer graphene for NO 2

Gas sensitive properties of bilayer graphene for NO ₂

Spin Transport Investigations in Bilayer Graphene

We study the spin-based electron transport properties in bilayer graphene with half-metallic ferromagnetic (HMF) CrO2 electrodes. Out-of-plane bilayer graphene has been examined in this paper. A high magneto-resistance ratio (MR) of ∼99% is obtained in the bilayer structure for a potential range of 0 to 0.8 V, after which MR drops because of the increase in spin-up current in the anti-parallel configuration at 1.2 V. High spin filtration of ∼100% is obtained at all bias points. Transmission spectrum studies show that a large number of electronic states are present at the Fermi level in bilayer graphene, which would result in high current.

Mapping of Bernal and non-Bernal stacking domains in bilayer graphene using infrared nanoscopy.

Bilayer graphene (BLG) shows great potential as a new material for opto-electronic devices because its bandgap can be controlled by varying the stacking orders, as well as by applying an external electric field. An imaging technique that can visualize and characterize various stacking domains in BLG may greatly help in fully utilizing such properties of BLG. Here we demonstrate that infrared (IR) scattering-type scanning near-field optical microscopy (sSNOM) can visualize Bernal and non-Bernal stacking domains of BLG, based on the stacking-specific inter- and intra-band optical conductivities. The method enables nanometric mapping of stacking domains in BLG on dielectric substrates, augmenting current limitations of Raman spectroscopy and electron microscopy techniques for the structural characterization of BLG.

Influence of Polymer Residues on the Double Resonance Process of Bilayer Graphene

PMMA is commonly used for graphene transfer and device processing. However, it leaves a thin layer of polymer residues after standard acetone cleaning and causes electrical and thermal performance degradation of graphene devices. In this paper, we present a research of the impact of polymer residues on surface morphologies and electronic properties of bilayer graphene using Raman spectroscopy in combination with atomic force microscopy (AFM). The electronic structure of bilayer graphene is well captured in its Raman spectrum, of which the 2D band reveals four double resonance Raman scattering processes. The Raman analyses show universal blueshifts of the G band and the four 2D sub-bands P11, P12, P21, P22, as well as reduced intensity ratios of the sub-bands to the G band I(Pij)/I(G) after surface contamination by polymer residues, implying an electronic structure modulation in bilayer graphene. The effects are mainly attributed to p-type doping and extrinsic scattering induced by residual impurities and defects.

Hybrid matrix method for stable numerical analysis of the propagation of Dirac electrons in gapless bilayer graphene superlattices

Gapless bilayer graphene (GBG), like monolayer graphene, is a material system with unique properties, such as anti-Klein tunneling and intrinsic Fano resonances. These properties rely on the gapless parabolic dispersion relation and the chiral nature of bilayer graphene electrons. In addition, propagating and evanescent electron states coexist inherently in this material, giving rise to these exotic properties. In this sense, bilayer graphene is unique, since in most material systems in which Fano resonance phenomena are manifested an external source that provides extended states is required. However, from a numerical standpoint, the presence of evanescent-divergent states in the eigenfunctions linear superposition representing the Dirac spinors, leads to a numerical degradation (the so called Ωd problem) in the practical applications of the standard Coefficient Transfer Matrix (K) method used to study charge transport properties in Bilayer Graphene based multi-barrier systems. We present here a straightforward procedure based in the hybrid compliance-stiffness matrix method (H) that can overcome this numerical degradation. Our results show that in contrast to standard matrix method, the proposed H method is suitable to study the transmission and transport properties of electrons in GBG superlattice since it remains numerically stable regardless the size of the superlattice and the range of values taken by the input parameters: the energy and angle of the incident electrons, the barrier height and the thickness and number of barriers. We show that the matrix determinant can be used as a test of the numerical accuracy in real calculations.

Carrier Transport Properties of Bilayer Graphene Obtained via Hall Measurements.

The electron transport characteristics of bilayer graphene were investigated in terms of changes in the temperature from 2 to 300 K. The purpose of this study was a confirming the reported values5 of the carrier density and the mobility for using a ballistic quantum transport experiment. The mechanical exfoliation method was adopted to get the best quality of bilayer graphene and the number of layers of graphene was identified by the Raman spectra. Our bilayer graphene was a slightly n-doped state with the neutrality point at Vg = -7.5 V. Hall measurements provided an accurate means to evaluate the carrier density and the mobility. In magnetic field of -9 to 9 T, clear Shubnikov-de Haas oscillations were measured in bilayer graphene, indicating the presence of a clear electron quantum transport mechanism. The mobility and the carrier density of the bilayer graphene at 2 K were measured to be 9,400 cm2/V·s and 3 x 10(11) cm(-2), respectively.

Tunneling Plasmonics in Bilayer Graphene.

We report experimental signatures of plasmonic effects due to electron tunneling between adjacent graphene layers. At subnanometer separation, such layers can form either a strongly coupled bilayer graphene with a Bernal stacking or a weakly coupled double-layer graphene with a random stacking order. Effects due to interlayer tunneling dominate in the former case but are negligible in the latter. We found through infrared nanoimaging that bilayer graphene supports plasmons with a higher degree of confinement compared to single- and double-layer graphene, a direct consequence of interlayer tunneling. Moreover, we were able to shut off plasmons in bilayer graphene through gating within a wide voltage range. Theoretical modeling indicates that such a plasmon-off region is directly linked to a gapped insulating state of bilayer graphene, yet another implication of interlayer tunneling. Our work uncovers essential plasmonic properties in bilayer graphene and suggests a possibility to achieve novel plasmonic functionalities in graphene few-layers.

Linear magnetoresistance in mosaic-like bilayer graphene

Contrary to common belief, bilayer graphene is not defect-free: the abundance of partial dislocations leads to a mosaic-like network structure. As a result, as now shown, the magnetoresistance of bilayer graphene depends linearly, rather than quadratically, on the external magnetic field. The magnetoresistance of conductors usually has a quadratic dependence on magnetic field1, however, examples exist of non-saturating linear behaviour in diverse materials2,3,4,5,6. Assigning a specific microscopic mechanism to this unusual phenomenon is obscured by the co-occurrence and interplay of doping, mobility fluctuations and a polycrystalline structure7,8. Bilayer graphene has virtually no doping fluctuations, yet provides a built-in mosaic tiling due to the dense network of partial dislocations9,10. We present magnetotransport measurements of epitaxial bilayer graphene that exhibits a strong and reproducible linear magnetoresistance that persists to B = 62 T at and above room temperature, decorated by quantum interference effects at low temperatures. Partial dislocations thus have a profound impact on the transport properties in bilayer graphene, a system that is frequently assumed to be dislocation-free. It further provides a clear and tractable model system for studying the unusual properties of mosaic conductors.

Shift-enriched optical properties in bilayer graphene

The relative shift between graphene layers (d) significantly changes the electronic and optical properties of bilayer graphenes (BLGs). It induces band anticrossings, low-energy saddle points, the shift of subbands, and the change of state degeneracies of middle-energy saddle points. The number, frequencies, and spacings of peaks in single-particle excitation spectra are all affected by the relative shift. Moreover d induces anisotropy in the optical excitation, except for AA- and AB-stacked BLGs. The anisotropy and the d-dependent excitation spectra could be used to identify the interlayer shift.

First-principles study of bilayer graphene on BN/Co(111): van der Waals density functional approach

We have investigated the magnetic property of bilayer graphene adsorbed on BN/Co(111) by using the van der Waals density functional method. We found that the BN layer adjacent to the Co(111) substrate showed a buckling geometry while the BN layer next to the graphene layer became flat. According to previous results, a one-monolayer (ML) thickness of graphene on BN/Co(111) displayed a spin polarized state, but we observed that the half metallic state disappeared in bilayer graphene on BN/Co(111) while Co(111) was still half metallic as found in graphene(1ML)/BN/Co(111). We found that the surface graphene layer had a gap whereas the first interface graphene on BN/Co(111) showed an ordinary metallic state. Thus, we realized that the electronic structure of bilayer graphene grown on BN/Co(111) had been significantly altered compared with that of graphene(1ML)/BN/Co(111) system.

Perpendicular Electric Field Effect on Electronic Properties of Bilayer Graphene

Perpendicular Electric Field Effect on Electronic Properties of Bilayer Graphene

Electronic properties of BN-doped bilayer graphene and graphyne in the presence of electric field

In the present paper, we have used density functional theory to study electronic properties of bilayer graphene and graphyne doped with B and N impurities in the presence of electric field. It has been demonstrated that a band gap is opened in the band structures of the bilayer graphene and graphyne by B and N doping. We have also investigated influence of electric field on the electronic properties of BN-doped bilayer graphene and graphyne. It is found that the band gaps induced by B and N impurities are increased by applying electric field. Our results reveal that doping with B and N, and applying electric field are an effective method to open and control a band gap which is useful to design carbon-based next-generation electronic devices.