Electronic properties of Z-shaped graphene nanoribbon under uniaxial strain

Based on tight-binding approximation and a generalized Green's function method, the effect of uniaxial strain on the electron transport properties of Z-shaped graphene nanoribbon (GNR) composed of an armchair GNR sandwiched between two semi-infinite metallic armchair GNR electrodes is numerically investigated. Our results show that the increase of uniaxial strain enhances the band gap and leads to a metal-to-semiconductor transition for Z-shaped GNR. Furthermore, in the Landauer–Büttiker formalism, the current–voltage characteristics, the noise power resulting from the current fluctuations and Fano factor of strained Z-shaped GNR are explored. It is found the threshold voltage for the current and the noise power increased so that with reinforcement of the uniaxial strain parameter strength, the noise power goes from the Poisson limit to sub-Poisson region at higher bias voltages.     

Effects of vertical strain on zigzag graphene nanoribbon with a topological line defect

We report the elastic, electronic and magnetic properties of naked ZGNRs with topological line defects (LD-ZGNRs) lying symmetrically on the ribbon's middle under vertical strains at four types of line defect atoms by using a first-principles approach. By changing the position and size of the local deformation of the ribbon, the optimal position is obtained. Moreover, an apparent spin-splitting of the energy band is obtained when a local deformation is created by the vertically applied strain.     

Effect of disorder with long-range correlation on transport in graphene nanoribbon

Transport in disordered armchair graphene nanoribbons (AGR) with long-range correlation between quantum wire contacts is investigated by a transfer matrix combined with Landauer's formula. The metal-insulator transition is induced by disorder in neutral AGR. Therein, the conductance is one conductance quantum for the metallic phase and exponentially decays otherwise, when the length of AGR approaches infinity and far longer than its width. Similar to the case of long-range disorder, the conductance of neutral AGR first increases and then decreases while the conductance of doped AGR monotonically decreases, as the disorder strength increases. In the presence of strong disorder, the conductivity depends monotonically and non-monotonically on the aspect ratio for heavily doped and slightly doped AGR, respectively. For edge disordered graphene nanoribbon, the conductance increases with the disorder strength of long-range correlated disordered while no delocalization exists, since the edge disorder induces localization.     

Spin thermopower and thermoconductance in a ferromagnetic graphene nanoribbon

The spin thermoelectric properties of a zigzag edged ferromagnetic (FM) graphene nanoribbon are studied theoretically by using the non-equilibrium Green's function method combined with the Landauer-Büttiker formula. By applying a temperature gradient along the ribbon, under closed boundary conditions, there is a spin voltage ΔV(s) inside the terminal as the response to the temperature difference ΔT between two terminals. Meanwhile, the heat current ΔQ is accompanied from the 'hot' terminal to the 'cold' terminal. The spin thermopower S?=?ΔV(s)/ΔT and thermoconductance κ?=?ΔQ/ΔT are obtained. When there is no magnetic field, S versus E(R) curves show peaks and valleys as a result of band selective transmission and Klein tunneling with E(R) being the on-site energy of the right terminal. The results are in agreement with the semi-classical Mott relation. When |E(R)|??M, the quantized value of [Formula: see text] appears. In the quantum Hall regime, because Klein tunneling is suppressed, S peaks are eliminated and the quantized value of κ is much clearer. We also investigate how the thermoelectric properties are affected by temperature, FM exchange split energy and Anderson disorder. The results indicate that S and κ are sensitive to disorder. S is suppressed for even small disorder strengths. For small disorder strengths, κ is enhanced and for moderate disorder strengths, κ shows quantized values.     

Quantum capacitance of the armchair-edge graphene nanoribbon

The quantum capacitance, an important parameter in the design of nanoscale devices, is derived for armchair-edge single-layer graphene nanoribbon with semiconducting property. The quantum capacitance oscillations are found and these capacitance oscillations originate from the lateral quantum confinement in graphene nanoribbon. Detailed studies of the capacitance oscillations demonstrate that the local channel electrostatic potential at the capacitance peak, the height and the number of the capacitance peak strongly depend on the width, especially a few nanometres, of the armchair-edge graphene nanoribbon. It implies that the capacitance oscillations observed in the experiments can be utilized to measure the width of graphene nanoribbon. The results also show that the capacitance oscillations are not seen when the width is larger than 30 nm.     

Four-terminal impedance of a graphene nanoribbon based structure

The four-terminal impedance is studied in a typical graphene nanoribbon based structure. When two additional voltage probes are attached, the results show that at the Dirac point, both the real and imaginary parts of the impedance are negative. As the Fermi energy deviates from the Dirac point, the real part of impedance oscillates with its sign changing frequently, while the imaginary part becomes vanishingly small. The phase incoherent processes introduced by the voltage probes contribute to inelastic scattering and charge redistribution in the central device region. As a result, the measured conductance is substantially different from the two-terminal measurement of a perfect graphene nanoribbon, indicating the important role of voltage probes in realistic four-terminal measurement.     

Andreev reflection in a patterned graphene nanoribbon superconducting heterojunction

In the study, an improved superconducting heterojunction is made up of a zigzag graphene nanoribbon, which is patterned by a triangle and supports localized edge mode. Since all the localized edge modes stem from a pattern operation, the structure features of the pattern exert an enormous function on the coherent quantum transport. Especially, the patterned modes can enhance the Andreev reflection largely both in the ferromagnetic nanoribbon edge and the antiferromagnetic nanoribbon edge. The spin resolved zero bias conductances, in sharp contrast to its counterpart in the infinite width superconducting heterojunction, exhibit the different dependence on the patterned ferromagnetic interaction.     

Adiabatic quantum pump in a zigzag graphene nanoribbon junction

The adiabatic electron transport is theoretically studied in a zigzag graphene nanoribbon(ZGNR) junction with two time-dependent pumping electric fields. By modeling a ZGNR p–n junction and applying the Keldysh Green's function method, we find that a pumped charge current is flowing in the device at a zero external bias, which mainly comes from the photon-assisted tunneling process and the valley selection rule in an even-chain ZGNR junction. The pumped charge current and its ON and OFF states can be efficiently modulated by changing the system parameters such as the pumping frequency, the pumping phase difference, and the Fermi level. A ferromagnetic ZGNR device is also studied to generate a pure spin current and a fully polarized spin current due to the combined spin pump effect and the valley valve effect. Our finding might pave the way to manipulate the degree of freedom of electrons in a graphene-based electronic device.     

Self-assembly of graphene nanoribbon ring on metallic nanowires

Molecular dynamics simulations demonstrate that metallic nanowires (NWs) can activate and guide the self-assembly of graphene nanoribbon rings (GNR), allowing them to adopt a bilayered helical configuration on NWs. This unique technology attributes to the combined effects of the van der Waals force and the π–π stacking interaction. The size and chirality effects of GNR on the self-assembly of GNR–NW system are calculated. Diverse NWs, acting as an external force, can initiate the conformational change of the GNRs to form bilayered helical structures. The stability of the formed nanosystems is further analyzed for numerous possible applications.     

Valley selection rule in a Y-shaped zigzag graphene nanoribbon junction

The valley valve effect was predicted in a straight zigzag graphene nanoribbon （ZGR） p/n junction. In this work, we address a possible valley selection rule in a Y-shaped ZGR junction. By modeling the system as a three-terminal device and calculating the conductance spectrum, we found that the valley valve effect could be preserved in the system and the Y-shaped connection does not mix the valley index or the pseudoparities of quasiparticles. It is also shown that the Y-shaped ZGR device can be used to separate spins in real space according to the unchanged valley valve effect. Our finding might pave a way to manipulate and detect spins in a multi-terminal graphene-based spin device.     

氢化与非氢化石墨烯纳米条带的密度泛函研究

基于密度泛函理论的第一性原理计算方法,研究了宽度N=8的边缘氢化和非氢化条带的结构和电子性质. 研究表明,扶手形无氢化石墨纳米条带的边缘碳原子是以三重键相互结合,它在边缘的成键强度比氢化时要高,具有更强的化学活性,可作为纳米化学传感器的基础材料. 能带结构计算表明,无论是扶手形条带还是锯齿形条带,它们都是具有带隙的半导体,且无氢化条带的带隙要比氢化的条带带隙宽度大,氢化对于条带的电子性质具有显著修饰作用. 通过锯齿形石墨纳米条带顺磁性、铁磁性和反铁磁性的计算,发现反铁磁的状态最稳定,并且边缘磁性最强,这有利于条带在自旋电子器件中的应用. 关键词： 石墨纳米条带 成键机理 电子结构 自旋分布     

The modulation effects on Landau levels in graphene nanoribbon

Magnetic-modulation effects in one-dimensional graphene nanoribbon have been investigated by the Peierls tight-binding model. The energy dispersion exhibits unusual oscillatory behavior which is mainly dominated by the modulation strength, the period, and the ribbon width. The main features of energy band are directly reflected in density of states, such as the structure, the height, the position, and the number of prominent peaks.     

Effects of edge chemistry doping on graphene nanoribbon mobility

Doping of semiconductor is necessary for various device applications. Exploiting chemistry at its reactive edges was shown to be an effective way to dope an atomically thin graphene nanoribbon (GNR) for realizing new devices in recent experiments. The carrier mobility limited by edge doping is studied as a function of the GNR width, doping density, and carrier density by using ab initio density functional and parameterized tight binding simulations combined with the non-equilibrium Green's function formalism for quantum transport. The results indicate that for GNRs wider than about 4 nm, the mobility scales approximately linearly with the GNR width, inversely proportional to the edge doping concentration and decreases for an increasing carrier density. For narrower GNRs, dependence of the mobility on the GNR width and carrier density can be qualitatively different.     

Controllable tuning of the electronic transport in pre-designed graphene nanoribbon

We make use of ab initio density functional theory calculation to explore the electronic and transport properties of zigzag-edged graphene nanoribbon (ZGNR) with peculiar designed electronic transport channels by tailoring the atomic configuration of the nanostructure. Tailoring the atomic structure has significant influences on the electronic transport of the defective nanostructure, and eventually the metal-semiconducting transition are identified with the increasing number of missing atoms. Our results demonstrate that pre-designed graphene nanoribbon by selective tailoring with high precision is expected to be served as the basic component for nanoelectronic device.     

Structural defects influence on the conductance of strained zigzag graphene nanoribbon

In this paper, we investigate the influence of point structural defects on the transport properties of zigzag graphene nanoribbons (ZGNRs) under uniaxial strain field, using the numerical studies based on the ab-initio calculation, the standard tight-binding model and Green's functions. The calculation results show that the direction of applied strain and defect type significantly affect the conductance properties of ZGNRs. The conductance of the defective nanoribbons generally decreases and some dips corresponding to complete electron backscattering is appeared. This behavior is originated from the different coupling between the conducting electronic states influenced by the wave function modification around the Fermi energy which depends on the defect type. We show that the presence of defects leads to a significant increase in local current. Furthermore, we have investigated the strain-tunable spin transport of defective ZGNRs in the presence of the exchange magnetic field and Rashba spin-orbit coupling (RSOC).     

A biosensor based on graphene nanoribbon with nanopores: a first-principles devices-design

A biosensor device,built from graphene nanoribbons(GNRs) with nanopores,was designed and studied by firstprinciples quantum transport simulation.We have demonstrated the intrinsic transport properties of the device and the effect of different nucleobases on device properties when they are located in the nanopores of GNRs.It was found that the device’s current changes remarkably with the species of nucleobases,which originates from their different chemical compositions and coupling strengths with GNRs.In addition,our first-principles results clearly reveal that the distinguished ability of a device’s current depends on the position of the pore to some extent.These results may present a new way to read off the nucleobases sequence of a single-stranded DNA(ssDNA) molecule by such GNRs-based device with designed nanopores     

Effect of the deflection strain on the atomic and electronic structure of a graphene nanoparticle

The results of numerical simulation of the deflection strain of a graphene nanoparticle 36.9 Å long and 41.18 Å wide are presented. The nanoparticle is deflected by nanoindentation. A platinum pyramid with the face-centered cubic lattice is considered as an atomic-force microscope tip. It is found that the graphene nanoparticle withstands a force of 437.83 nN, and its tensile strength is 126 GPa. It is shown that the nanoparticle deflection improves its emission properties. The particle conductivity remains almost unchanged with increasing deflection. The maximum π-electron shell overlap and the significant redistribution of the electron charge density are characteristic of atoms of the graphene nanoparticle with the largest curvature.