[Show abstract][Hide abstract] ABSTRACT: In this work, the optical responses of graphene superlattices, i.e. graphene subjected to a periodic scalar potential, are theoretically reported. The optical properties were studied by investigating the optical conductivity, which was calculated using the Kubo formalism. It was found that the optical conductivity becomes dependent on the photon polarization and is suppressed in the photon energy range of (0, Ub), where Ub is the potential barrier height. In the higher photon energy range, i.e. Ω > Ub, the optical conductivity is, however, almost identical to that of pristine graphene. Such behaviors of the optical conductivity are explained microscopically through the analysis of the elements of optical matrices and effectively through a simple model, which is based on the Pauli blocking mechanism.
[Show abstract][Hide abstract] ABSTRACT: We show that periodic scalar potentials can induce the localization of some electronic states in graphene. Particularly, localized states are found at energies outside the potential variation range and embedded in the continuum spectrum of delocalized ones. The picture of the connection of wave functions with typical symmetries defined in relevant-edge nanoribbons is employed to explain the formation of the electronic structure and to characterize/classify eigen-states in graphene superlattices.
[Show abstract][Hide abstract] ABSTRACT: A numerical scheme based on the tight-binding description for pz-electrons in graphene was developed to study the formation and behaviors of plasmons in this two-dimensional electron system. The random phase approximation has been used to calculate the dielectric function for arbitrary temperature and doping level. We show that at zero-doping, only one kind of plasmons of long wavelength is observed at sufficiently high temperature. At finite doping, such plasmons exist even at zero temperature, but strongly damped, due to the interplay between the intra- and inter-band transition processes. Particularly, we show a significant dependence of the plasmon spectrum on the wave-vector direction in the regime of high doping, which is the reflection of the anisotropy of the energy surfaces far from the Dirac point.
Physica E Low-dimensional Systems and Nanostructures 04/2014; 58:101–105. DOI:10.1016/j.physe.2013.11.020 · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study several graphene devices able to generate non-linear effects in the current─voltage characteristics and in particular negative differential resistance (NDR) effects. This theoretical investigation is based on numerical charge transport simulation in the Green's function approach applied to a tight-binding Hamiltonian for particles in graphene. Depending on the device, the physical mechanism involved in the NDR effect may be different: (i) the mismatch of modes between left and right sides of a P+/P zigzag ribbon junction, (ii) the modulation of interband tunnelling in P/N junctions (tunnel diodes and tunnel field-effect transistors) or (iii) the modulation of chiral tunnelling in ‘conventional’ graphene transistors. We emphasize the advantages of exploiting different approaches of bandgap engineering in the form of graphene nanoribbons (GNRs) or nanomesh lattices (GNM), the latter resulting from a periodic array of nanoholes in graphene sheets. In particular, such nanostructuring allows us to design position-dependent bandgaps in devices, which is shown to make possible the optimization of device operation and, here, to get very high peak-to-valley ratio of the NDR. In the case of GNR nanostructuring, it is shown that appropriate bandgap engineering can even make the current─voltage characteristics of tunnel diodes weakly sensitive to the atomic edge disorder. Finally, GNM lattices are shown to be a very promising way to open large bandgaps in wide sheets of graphene and to introduce bandgaps locally with a view to optimizing the device operation and performance.
Journal of Physics D Applied Physics 02/2014; 47(9). DOI:10.1088/0022-3727/47/9/094007 · 2.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A physical model is proposed to clarify the electron transport through graphene-metal interfaces. It is based on an effective description of the coupling between the graphene π-bands and the metal sd- and d-bands. Applying this model to vertically symmetrical metal-graphene-metal structures, we show that the current-voltage characteristics can be either linear or non-linear with a negative or positive differential resistance depending on the dominance of the d-like or s-like electrons in the metal as well as the graphene-metal coupling.
[Show abstract][Hide abstract] ABSTRACT: An effective approach of quantum transport of Dirac carriers in mono- and bi-layer graphene structures and devices is presented. Initially based on the Green's function formalism to treat the Dirac Hamiltonian of massless particles in two-dimensional mono-layer graphene, the model has been extended to to small bandgap materials and to bi-layer graphene with massive carriers. It is applied to investigate some transport problems as the minimum conductivity, the tunneling properties the spin-polarized transport through single-barrier structures, and the operation of graphene field-effect transistors.
Computational Electronics (IWCE), 2010 14th International Workshop on; 11/2010
[Show abstract][Hide abstract] ABSTRACT: An atomistic Green's function approach to simulating electron and phonon transport in graphene nanoribbons (GNRs) is presented. Phonons are described by an empirical Force-Constant model including interactions up to the fifth nearest neighbour while the nearest neighbour tight-binding Hamiltonian is used for electrons. The model was applied to investigate the edge dependence of GNR thermoelectric properties. The factor of merit ZT appears to be strongly enhanced in mixed-edge ribbons.
Computational Electronics (IWCE), 2010 14th International Workshop on; 11/2010
[Show abstract][Hide abstract] ABSTRACT: Graphene, a carbon material discovered in 2004 by a group of scientists at the University of Manchester, UK, has been attracting significant attention in both fundamental and applied studies. Due to the rapid increase in the number of articles on this material since its discovery, a range of readers, particularly those just beginning to learn about this material, are turning to various different sources. The purpose of this article is to create a bridge between the key aspects of this material in experimental and theoretical investigations, as well as in fundamental and applied studies, aiming to provide a basic understanding of this material for those who are new to it. The presentation in this article is thus not particularly academic. The content focuses on four themes, including fabrication methods, basic properties, potential for application and some typical research directions for this magic carbon material.
Advances in Natural Sciences: Nanoscience and Nanotechnology 10/2010; 1(3):033001. DOI:10.1088/2043-6254/1/3/033001
[Show abstract][Hide abstract] ABSTRACT: In this work, the effect of coupling between metallic electrodes and graphene is discussed. We demonstrate that the transport properties of graphene at the charge neutrality point are very sensitive to this coupling. By introducing a model based on two real parameters, namely the real and the imaginary parts of the self-energy which describes the metal-graphene coupling, the obtained results of charge conductivity versus the Fermi energy reproduce well the essential features of experimental data such as the asymmetry between electrons and holes. Additionally, the possible role of scattering processes in the enhancement of the density of states, and thus of the minimum of conductivity, at the charge neutrality point is also discussed. This work is believed to be helpful for further studies of graphene-based devices wherein metallic electrodes may have a major impact on electrical characteristics.
[Show abstract][Hide abstract] ABSTRACT: High crystalline quality ZnS nanowires were fabricated using the thermal evaporation method. They were then oxidized in air at different temperatures to form a one-dimensional protuberant ZnO/ZnS structure. It was argued that the oxidation at low enough temperature can significantly improve the quality of the ZnS nanowires by passivating dangling bonds on the nanowire surface as the absorption of oxygen atoms. This study provides a simple approach for synthesizing optically active ZnO/ZnS heterostructures.
[Show abstract][Hide abstract] ABSTRACT: We investigate the transport properties of p<sup>+</sup>/p junctions based on zigzag-edge graphene strips by means of numerical quantum simulation. The p<sup>+</sup> and p domains are created by field effect using appropriate gate electrodes. A negative differential resistance behavior is predicted regardless of the evenness/oddness of the zigzag line number of the ribbon with peak-to-valley current ratio reaching the value of 10 at room temperature. Besides the role of the parity selective rule, the phenomenon is explained as resulting from the suppression of the coherent transition due to the mismatch of modes in the left and right sides of the junction. The influence of various factors governing the peak-to-valley current ratio is analyzed. In particular, it is found that the negative differential resistance may be severely affected by the roughness of ribbon edges.
[Show abstract][Hide abstract] ABSTRACT: We analyze the spin-dependent transport in single ferromagnetic gate structures based on armchair graphene nanoribbon (GNR) using the non-equilibrium Green's function method in a tight binding model. It is shown that the spin polarized current oscillates as a function of the gate-induced barrier height. For perfect GNRs, the larger the energy band gap, the stronger the oscillation of the spin polarization. However, though the edge roughness of the ribbons tends to enlarge the band gap, it also strongly reduces the conductance which finally degrades the spin polarized current.
Journal of Physics Conference Series 11/2009; 19332. DOI:10.1088/1742-6596/193/1/012100
[Show abstract][Hide abstract] ABSTRACT: Using the nonequilibrium Green’s functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbons controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effects of width-dependent band gap and of the nature of contacts. The oscillation of spin polarization in graphene nanoribbons with a large band gap is strong in comparison with that in infinite graphene sheets. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference in electronic structure between normal conductor and graphene generates confined states which have a strong influence on the transport properties of the device. This study suggests that the device should be carefully designed to obtain a high controllability of spin-polarized current.
[Show abstract][Hide abstract] ABSTRACT: Using the nonequilibrium Green’s function theory, transport properties of nanoscale graphene structures deposited on a SiO <sub>2</sub>/ Si substrate have been investigated taking into account the influence of both lattice defects and charged impurities. The calculation argues the metallic lead-graphene coupling responsible for the asymmetric transport of electrons and holes, and shows that the conductivity is generally suppressed by these scattering processes. However, at the charge neutrality point, the screening seems to weaken such a suppression, leading to the minimum conductivity value of 4e<sup>2</sup>/πh even for the impurity density higher than 10<sup>12</sup> cm <sup>-2</sup> , while it is strongly diminished to zero for the vacancy density of 10<sup>11</sup> cm <sup>-2</sup> . Obtained results for the conductivity and the charge mobility are also discussed to highlight available experimental data.
[Show abstract][Hide abstract] ABSTRACT: The Wigner Monte Carlo approach is shown to provide an efficient way to study quantum transport in the presence of scattering and to connect semi-classical to quantum transport. The study of resonant tunneling diodes highlights the physics of the impact of scattering on resonant tunneling, and on electron decoherence and localization. The simulation of nano-MOSFET evidences a mixed regime, where both quantum transport and scattering play a significant role.
Simulation of Semiconductor Processes and Devices, 2008. SISPAD 2008. International Conference on; 10/2008
[Show abstract][Hide abstract] ABSTRACT: Motivated by recent studies on the use of graphene for new concepts of electronic/spintronic devices, the authors develop an efficient calculation method based on the nonequilibrium Green’s function to solve the quantum relativisticlike Dirac’s equation that governs the low-energy excited states in graphene. The approach is then applied to investigate the electronic transport and the spin polarization in a single-graphene barrier structure. The obtained results are presented and analyzed in detail aiming to highlight typical properties of the considered graphene structure as well as the efficiency of the developed approach that both may be helpful for further development in electronic devices and in spintronics.
[Show abstract][Hide abstract] ABSTRACT: Using the nonequilibrium Green’s function formalism, the authors investigate the effect of the electron-phonon interaction on the current and shot noise in one dimensional resonant tunneling structures. Besides the well-known current behavior, they particularly show that the shot noise may be enhanced over the Poissonian value due to the phonon-assisted tunneling effect. The observed super-Poissonian noise is then interpreted as a result of the competition between the coherent and sequential current components.