The quantum Hall effect in Graphene nano-ribbons (GNR) is investigated with the non-equilibrium Green s function (NEGF) based quantum transport model in the ballistic regime. The nearest neighbor tight-binding model based on pz orbital constructs the device Hamiltonian. GNRs of different edge geometries (Zigzag and Armchair) are considered. The magnetic field is included in both the channels and contact through Peierls substitution. Efficient algorithms for calculating the surface Green function are used to save computation time while simulating realistically large dimensions comparable to those used in experiments. Hall resistance calculations exactly reproduce the quantum Hall plateaus observed in the experiments. Use of large dimensions in the simulation is crucial in order to capture the quantum Hall effect in magnetic fields within experimentally relevant 10-20T.
We investigate the diffusive electron-transport properties of charge-doped graphene ribbons and nanoribbons with imperfect edges. We consider different regimes of edge scattering, ranging from wide graphene ribbons with (partially) diffusive edge scattering to ribbons with large width variations and nanoribbons with atomistic edge roughness. For the latter, we introduce an approach based on pseudopotentials, allowing for an atomistic treatment of the band structure and the scattering potential, on the self-consistent solution of the Boltzmann transport equation within the relaxation-time appro…
Interlayer Conductance in the Armchair Nanotube -- Zigzag Graphene Ribbon Parallel Contact: Theoretical Proposal of Detection of Wavefunction Growing from the Edge to the Center in the Graphene Ribbon
We study the thermoelectric performance of $90^0$-bent graphene nanoribbons containing nanopores for optimized design of multiple functional circuits including thermoelectric generators. We show that the thermal conductance of the $90^0$-bent ribbons is lower from few times to an order of magnitude compared to that of pristine armchair and zigzag straight ribbons. Consequently, the thermoelectric performance of the bent ribbons is better than its straight ribbon counterparts, in particular at high temperatures above 500 K. More importantly, the introduction of nanopores is demonstrated to stro…
Atomistic quantum transport simulation of realistically large devices is computationally very demanding. The widely used mode space (MS) approach can significantly reduce the numerical cost but good MS basis is usually very hard to obtain for atomistic full-band models. In this work, a robust and parallel algorithm is developed to optimize the MS basis for atomistic nanowires. This enables tight binding non-equilibrium Green's function (NEGF) simulation of nanowire MOSFET with realistic cross section of $\rm 10nm\times10nm$ using a small computer cluster. This approach is applied to compare th…
Third nearest neighbor parameterized tight biding model for graphene nano-ribbons
由Reich等人提出的第三近邻紧束缚模型[Phys. Rev. B 66，035412]很好地再现了2D石墨烯片的从头算能带结构。对于带状结构，现有的紧束缚参数组可以成功地解释所有扶手椅型带状结构的半导体行为。然而，他们仍然无法准确地描述带的斜率，而这个特征与电子的群速度和有效质量直接相关。在这项工作中，两个密度泛函理论和紧束缚计算进行，并介绍了一套新的紧束缚参数的第三近邻包括重叠条款。用这个模型得到的结果提供极好的协议与密度泛函理论的预测在大多数情况下的带状结构，即使在高能量区。此外，这套可以诱导电子空穴不对称性，表现在密度泛函理论。相关的结果也证明了扶手椅带的各种宽度，以及锯齿形结构，从而开辟了多尺度模拟的路线。
We investigate electronic transport in lithographically patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutrality point. Individual graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths and different crystallographic orientations. The temperature dependent conductance measurements show larger energy gaps opening for narrower ribbons. The sizes of these energy gaps are investigated by measuring the conductance in the non-linear response regime at low temperatures. We find that…
Thermoelectric measurements for graphene ribbons are currently performed on samples that include atomic disorder via defects and irregular edges. In this work, we investigate the thermopower or Seebeck coefficient of graphene ribbons within the linear response theory and the Landauer formalism, and we consider the diffusive regime taken as a limit of the ribbon aspect ratio. We find that the thermopower and the electronic conductivity depend not only on the aspect ratio, but also on chemical potential and temperature, which are set here as key parameters. The obtained numerical results with te…
The description of optical properties of subnanometer junctions is particularly challenging. Purely classical approaches fail, because the quantum nature of electrons needs to be considered. Here we report on a novel classical fully atomistic approach, ωFQ, based on the Drude model for conduction in metals, classical electrostatics and quantum tunneling. We show that ωFQ is able to reproduce the plasmonic behavior of complex metal subnanometer junctions with quantitative fidelity to full ab initio calculations. Besides the practical potentialities of our approach for large scale nanoplasmonic …
In spite of years of intense research, graphene continues to produce surprising results. Recently, it was experimentally observed that under certain conditions graphene can self-drive its tearing and peeling from substrates. This process can generate long, micrometer sized, folded nanoribbons without the action of any external forces. Also, during this cracking-like propagation process, the width of the graphene folded ribbon continuously decreases and the process only stops when the width reaches about few hundreds nanometers in size. It is believed that interplay between the strain energy of…
Quantum-mechanical calculations of electron magneto-transport in ideal graphene nanoribbons are presented. In noninteracting theory, it is predicted that an ideal ribbon that is attached to wide leads should reveal Fabry-Perot conductance oscillations in magnetic field. In the theory with Coulomb interaction taken into account, the oscillation pattern should rather be determined by the Aharonov-Bohm interference effect. Both of these theories predict the formation of quasi-bound states, albeit of different structures, inside the ribbon because of strong electron scattering on the interfaces be…
Using the tight-binding approach we study the electronic transport in a $\mathrm{MoS}_2$ zigzag ribbon with a spatially varying potential profile. Considering a ribbon with a smooth potential step in the Fermi energy regime where the transport is dominated by the edge modes, we find that the conductance exhibits sharp resonances due to the resonant transport through a n-p-n junction effectively created in the structure. We show that in a gated wire the current carried on the wire edges can be blocked despite the metallic band structure of the ribbon. For the Fermi energies corresponding to $\m…
The size-dependent electrical resistivity of single-layer graphene ribbons has been studied experimentally for ribbon widths from 16 nm to 320 nm. The experimental findings are that the resistivity follows a more dramatic trend than that seen for metallic nanowires of similar dimensions, due to a combination of surface scattering from the edges, band-gap related effects and shifts in the Fermi level due to edge effects. We show that the Charge Neutrality point switches polarity below a ribbon width of around 50 nm, and that at this point, the thermal coefficient of resistance is a maximum. The…
We study the optical response of nanoribbons made from the $α-T_3$ lattice under a weak magnetic field in the terahertz to far-infrared regime. It is found that the magnetic field can open a gap in the band structure and induce a new absorption peak with much reduced frequency in metallic armchair ribbons and a class of zigzag ribbons with particular boundaries. This tunable magneto-optical modulation effect is attributed to the interband transitions between the flat band and the propagating bands. By contrast, this magnetic modulation of gap opening and optical conductance is much weaker in m…
Magnetic Moment and Band Structure Analysis of Fe, Co, Ni-modified Graphene-nano- ribbon
采用第一性原理密度泛函理论计算了Fe、Co、Ni修饰的石墨烯纳米带（GNR）的磁性和能带特性。典型的晶胞分别为[C32 H2 Fe 1]、[C32 H2 Co 1]和[C32 H2 Ni 1]。Fe修饰的GNR的最稳定自旋态为Sz=4/2，而Co情况下的Sz=3/2，Ni情况下的Sz=2/2。Fe、Co和Ni的原子磁矩分别为3.63、2.49和1.26 μB，由于与石墨烯网络的磁耦合，这三个原子的磁矩比原子Hund定则的值有所降低。有可能通过与碳π共轭自旋系统的相互作用形成铁磁Fe，Co和Ni自旋阵列。通过展开一个Co改性的[C96 H6 Co 3]的单胞，比较了类铁磁和类亚铁磁自旋态.得出铁磁态是较稳定的磁态的结论。Co修饰的情况下的能带计算显示半金属结构，具有相对大的带隙（0.55eV）的上自旋，而小的间隙（0.05eV）的下自旋。这表明了自旋电子学应用的能力，如自旋过滤器。
The understanding and modeling of the heat transport across nanometer and sub-nanometer gaps where the distinction between thermal radiation and conduction become blurred remains an open question. In this work, we present a three-dimensional atomistic simulation framework by combining the molecular dynamics (MD) and phonon non-equilibrium Green's function (NEGF) methods. The relaxed atomic configuration and interaction force constants of metallic nanogaps are generated from MD as inputs into harmonic phonon NEGF. Phonon tunneling across gold-gold and copper-copper nanogaps is quantified, and i…
