Effect of a gap opening on the conductance of graphene superlattices (original) (raw)

Transport Properties in Gapped Bilayer Graphene

2021

We investigate transport properties through a rectangular potential barrier in AB-stacked bilayer graphene (AB-BLG) gapped by dielectric layers. Using the Dirac-like Hamiltonian with a transfer matrix approach we obtain transmission and reflection probabilities as well as the associated conductance. For two-band model and at normal incidence, we find extra resonances appearing in transmission compared to biased AB-BLG, which are Fabry-Pérot resonance type. Now by taking into account the inter-layer bias, we show that both of transmission and anti-Klein tunneling are diminished. Regarding four band model, we find that the gap suppresses transmission in an energy range by showing some behaviors look like ”Mexican hats”. We examine the total conductance and show that it is affected by the gap compared to AA-stacked bilayer graphene. In addition, we find that the suppression in conductance is more important than that for biased AB-BLG.

Band structure, density of states, and transmission in graphene bilayer superlattices

2010

The energy spectrum and density of states of graphene bilayer superlattices (SLs) are evaluated. We take into account doping and/or gating of the layers as well as tunnel coupling between them. In addition, we evaluate the transmission through such SLs and through single or double barriers. The transmission exhibits a strong dependence on the direction of the incident wave vector.

Symmetry-induced band-gap opening in graphene superlattices

Physical Review B, 2010

We study nxn honeycomb superlattices of defects in graphene. The considered defects are missing p z orbitals and can be realized by either introducing C atom vacancies or chemically binding simple atomic species at the given sites. Using symmetry arguments we show how it is possible to open a gap when n = 3m + 1, 3m + 2 (m integer), and estimate its value to have an approximate square-root dependence on the defect concentration x = 1/n 2 . Tight-binding calculations confirm these findings and show that the induced-gaps can be quite large, e.g. ∼ 100 meV for x ∼ 10 −3 . Gradient-corrected density functional theory calculations on a number of superlattices made by H atoms adsorbed on graphene are in good agreement with tight-binding results, thereby suggesting that the proposed structures may be used in practice to open a gap in graphene.

Electrical Conductance of Graphene with Point Defects

Acta Physico-Chimica Sinica

Graphene is one of the most promising materials in nanotechnology and has attracted worldwide attention and research interest owing to its high electrical conductivity, good thermal stability, and excellent mechanical strength. Perfect graphene samples exhibit outstanding electrical and mechanical properties. However, point defects are commonly observed during fabrication which deteriorate the performance of graphene based-devices. The transport properties of graphene with point defects essentially depend on the imperfection of the hexagonal carbon atom network and the scattering of carriers by localized states. Furthermore, an in-depth understanding of the effect of specific point defects on the electronic and transport properties of graphene is crucial for specific applications. In this work, we employed density functional theory calculations and the non-equilibrium Green's function method to systematically elucidate the effects of various point defects on the electrical transport properties of graphene, including Stone-Waals and inverse Stone-Waals defects; and single and double vacancies. The electrical conductance highly depends on the type and concentration of point defects in graphene. Low concentrations of Stone-Waals, inverse Stone-Waals, and single-vacancy defects do not noticeably degrade electron transport. In comparison, DV585 induces a moderate reduction of 25%-34%, and DV55577 and DV5555-6-7777 induce significant suppression of 51%-62% in graphene. As the defect concentration increases, the electrical conductance reduces by a factor of 2-3 compared to the case of graphene monolayers with a low concentration of point defects. These distinct electrical transport behaviors are attributed to the variation of the graphene band structure; the point defects induce localized states near the Fermi level and result in energy splitting at the Dirac point due to the breaking of the intrinsic symmetry of the graphene honeycomb lattice. Double vacancies with larger defect concentrations exhibit more flat bands near the Fermi energy and more localized states in the defective region, resulting in the presence of resonant peaks close to the Fermi energy in the local density of states. This may cause resonant scattering of the carriers and a corresponding reduction of the conductance of graphene. Moreover, the partial charge densities for double vacancies and point defects with larger concentrations exhibit enhanced localization in the defective region that hinder the charge carriers. The electrical conductance shows an exponential decay as the defect concentration and energy splitting increase. These theoretical results provide important insights into the electrical transport properties of realistic graphene monolayers and will assist in the fabrication of high-performance graphene-based devices.

Robust optical conductivity in gapped graphene

We study the optical conductivity in the low-energy regime of gapped mono-and bilayer graphene. A scaling relation is found, in which the four parameters frequency, gap, Fermi energy and temperature appear only as combination of three independent parameters. The ratio of the optical conductivity of bilayer and mononlayer graphene is exactly 2.

Angle-dependent bandgap engineering in gated graphene superlattices

AIP Advances, 2016

Graphene Superlattices (GSs) have attracted a lot of attention due to its peculiar properties as well as its possible technological implications. Among these characteristics we can mention: the extra Dirac points in the dispersion relation and the highly anisotropic propagation of the charge carriers. However, despite the intense research that is carried out in GSs, so far there is no report about the angular dependence of the Transmission Gap (TG) in GSs. Here, we report the dependence of TG as a function of the angle of the incident Dirac electrons in a rather simple Electrostatic GS (EGS). Our results show that the angular dependence of the TG is intricate, since for moderated angles the dependence is parabolic, while for large angles an exponential dependence is registered. We also find that the TG can be modulated from meV to eV, by changing the structural parameters of the GS. These characteristics open the possibility for an angle-dependent bandgap engineering in graphene.

The conduction gap in double gate bilayer graphene structures

Journal of Physics: Condensed Matter, 2010

Using the nonequilibrium Green function method, the electrical behavior of a double gate bilayer graphene structure is investigated. Due to energy bandgap opening when potential energies in the layers are different, a clear gap of electrical current is observed. The sensitivity of this phenomenon to device parameters (gate length, temperature) has been considered systematically. It appears that the threshold voltage can be controlled by tuning the gate voltages and/or the Fermi energy. Our obtained results may be useful and provide new suggestions for further experimental investigations.

Transport properties of antidot superlattices of graphene nanoribbons

Physical Review B, 2009

In this work we show a theoretical study of the electronic and transport properties of superlattices formed by a periodic structure of vacancies ͑antidots͒ on graphene nanoribbons. The systems are described by a singleband tight-binding Hamiltonian and also by ab initio total energy density-functional theory calculations. The quantum conductance is determined within the Green's function formalism, calculated by real-space renormalization techniques. A series of well defined gap structures on the conductance as a function of the Fermi energy is observed. This strongly depends on the period of the vacancies on the nanoribbon and on the internal geometrical structure of the supercell. Controlling these parameters could be possible to modulate the electronic response of the systems.

Band structure and gaps of triangular graphene superlattices

2010

General properties of long wavelength triangular graphene superlattice are studied. It is shown that Dirac points with and without gaps can arise at a number of high symmetry points of the Brillouin Zone. The existence of gaps can lead to insulating behavior at commensurate fillings. Strain and magnetic superlattices are also discussed.