Ab initio study of the electronic and transport properties of waved graphene nanoribbons

We apply the nonequilibrium Green's function method based on density functional theory to investigate the electronic and transport properties of waved zigzag and armchair graphene nanoribbons. Our calculations show that out-of-plane mechanical deformations have a strong influence on the band structures and transport characteristics of graphene nanoribbons. The computed I-V curves demonstrate that the electrical conductance of graphene nanoribbons is significantly affected by deformations. The relationship between the conductance and the compression ratio is found to be sensitive to the type of the nanoribbon. The results of our study indicate the possibility of mechanical control of the electronic and transport properties of graphene nanoribbons.     

L型石墨纳米结的热输运

利用非平衡格林函数方法研究了由半无限长扶手椅型和锯齿型边界石墨纳米带连接而成的L型石墨纳米结的热输运性质.结果表明,L型石墨纳米结的热导依赖于L型石墨纳米结的夹角和石墨纳米带的宽度.在L型石墨纳米结的夹角从30°增加到90°再增加到150°过程中,其热导显著增大.夹角为90°的L型石墨纳米结的热导随着扶手椅型纳米带宽度增加时,在低温区热导随着宽度的增大而降低,在高温区热导随宽度的增大而升高.对于夹角为150°的L型石墨纳米结,其热导无论是在低温区还是在高温区都随着锯齿型纳米带宽度的增加而降低.利用声子透射谱对这些热输运现象进行了合理的解释.研究结果阐明了不同L型石墨纳米结中的热输运机理,为设计基于石墨纳米结的热输运器件提供了重要的物理模型和理论依据. 关键词： 石墨纳米结 热输运 热导     

Electronic properties of MoS2 nanoribbons with disorder effects

A theoretical study of electronic properties on MoS₂ nanoribbon is made on focusing the calculation of zero bias transport in the presence of disorders. Disorders including intrinsic and extrinsic vacancies and also weak uniform scatter defects are considered. The calculations are based on the tight-binding Green's function formalism by including an iterative procedure. The Slater–Koster transformations are used to determine the parameters. This model reduces the numerical calculation time. The unsaturated atoms at the edge of armchair (zigzag) ribbon induce some mid-gap states with nearly high (low) localization, which act as scattering centers. The antiresonances of created quasi-localized states due to vacancy cause the conductance of the armchair nanoribbon to decrease. Finally, the zigzag ribbon provides the highest sensitivity as well as selectivity between the smaller energy range, in the presence of the single weak scatter with potential value of 2 eV at the edge of the ribbon.     

Electronic transport through superlattice-graphene nanoribbons

In this work, we study quantum transport properties of superlattice-graphene nanoribbons (SGNRs) attached to two semi-infinite metallic armchair graphene nanoribbon (AGNR) leads. The calculations are based on the tight-binding model and Green’s function method, in which localization length, density of states (DOS) and conductance of the system are calculated, numerically. By controlling the layered boron concentration, this kind of system can separate the extended states from the localized states. Our results may have important applications for building blocks in the nano-electronic devices based on GNRs.     

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.     

Fano resonance in electronic transport induced by the magnetic confinement in a graphene nanoribbon

Based on the Floquet scattering theory, a model of graphene-based electronic device is presented, in which electrical transport is controlled by adjusting Dirac fermions energy near resonance conditions. The presence of an oscillating field leads to the Fano resonance in transport through a magnetic structure in an armchair graphene nanoribbon (AGNR). The Fano resonance originates from bound states of the magnetic confinement, according to subband indices in the AGNR. The ballistic conductance is markedly affected by the Fano resonance due to the quasi-one-dimensional nature of AGNRs. The results may help realizing graphene electronics with the resonant characteristics in the conductance.     

Large negative differential resistance behavior in arsenene nanoribbons induced by vacant defects

We have studied the electronic structures of arsenene nanoribbons with different edge passivations by employing first-principle calculations. Furthermore, the effects of the defect in different positions on the transport properties of arsenene nanoribbons are also investigated. We find that the band structures of arsenene nanoribbons are sensitive to the edge passivation. The current-voltage characteristics of unpassivated and O-passivated zigzag arsenene nanoribbons exhibit a negative differential resistance behavior, while such a peculiar phenomenon has not emerged in the unpassivated and O-passivated armchair arsenene nanoribbons. The vacant defects on both top and bottom edges in unpassivated armchair arsenene nanoribbon can make its current-voltage characteristic also present a negative differential resistance behavior. After expanding the areas of the top and bottom defects in unpassivated armchair arsenene nanoribbon, the peak-to-valley ratio of the negative differential resistance behavior can be enlarged obviously, which opens another way for the application of arsenene-based devices with a high switching ratio.     

Effect of single magnetic atom on spin-polarized transport of armchair graphene nanoribbons

Using a perturbative method, the influence of a single magnetic impurity on the spin-polarized current flowing through a metallic armchair graphene nanoribbon is investigated theoretically. It is shown that the nonlinear correction depends strongly on the relative spin orientations in the two spin-polarized reservoirs. Also, the effects of magnetic impurity position and width of nanoribbon on the nonlinear conductance are discussed.     

Spin-dependent transport properties of an armchair boron-phosphide nanoribbon embedded between two graphene nanoribbon electrodes

Using non-equilibrium Green׳s function and ab initio calculations we investigate structural, electronic, and transport properties of a junction consisting of armchair hexagonal boron phosphide nanoribbon (ABPNR) contacted by two semi-infinite electrodes composed of armchair graphene nanoribbons (AGNRs). We consider three different configurations including the pristine AGNR–BP–GNR and substitutions for Iron atoms, namely on phosphorus and boron atoms at one edge of the BP nanoribbon. The spin current polarization in all these cases is extracted for each structure and bias. Such hybrid system is found to exhibit not only significant spin-filter efficiency (SFE) but also tunable negative differential resistance (NDR).     

Fano Factor in Strained Graphene Nanoribbon Nanodevices

We investigate the Fano factor in a strained armchair and zigzag graphene nanoribbon nanodevice under the effect of ac fheld in a wide range of frequencies at different temperatures(10 K-70 K). This nanodevice is modeled as follows: a graphene nanoribbon is connected to two metallic leads. These two metallic leads operate as a source and a drain. The conducting substance is the gate electrode in this three-terminal nanodevice. Another metallic gate is used to govern the electrostatics and the switching of the graphene nanoribbon channel. The substances at the graphene nanoribbon/metal contact are controlled by the back gate. The photon-assisted tunneling probability is deduced by solving the Dirac eigenvalue differential equation in which the Fano factor is expressed in terms of this tunneling probability. The results show that for the investigated nanodevice, the Fano factor decreases as the frequency of the induced ac fheld increases, while it increases as the temperature increases.In general, the Fano factors for both strained armchair and zigzag graphene nanoribbons are different. This is due to the effect of the uniaxial strain. It is shown that the band structure parameters of graphene nanoribbons at the energy gap, the C-C bond length, the hopping integral, the Fermi energy and the width are modulated by uniaxial strain. This research gives us a promise of the present nanodevice being used for digital nanoelectronics and sensors.     

A graphene quantum dot realized by an armchair graphene nanoribbon with line defect

The electron transport in a semiconducting armchair graphene nanoribbon with line defect is theoretically investigated, by coupling it to two normal metallic leads. It is found that the line defect induces a new localized quantum state near the Dirac point, and that the coupling between this state and the leads provides a channel for the resonant tunneling. This means that such a finite‐size nanoribbon can be viewed as a quantum dot. When two line defects are present simultaneously, a coupled quantum dot forms, leading to the splitting of the conductance peaks. With these results, we propose such a structure to be a promising candidate of an electron transistor. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of Stone-Wales Defect Symmetry on the Electronic Structure and Transport Properties of Narrow Armchair Graphene Nanoribbon

We report a first principles calculation to investigate the electron transport properties of defected armchair graphene nanoribbon (AGNR) influenced by Stone-Wales (SW) defect. The SW defect is found to be able to effectively influence the electronic structure of the defected AGNRs, and their electron transport behaviors can exhibit prominent differences depending on the symmetry of the nanostructured morphology. Moreover, our simulations have revealed that the introducing of the SW defect could be favorable for the electron transport of the defective AGNR. Our investigation has confirmed the possibility of tuning the electron transport of graphene nanoribbon by introducing a topological defect, which could be helpful to extending the field of applications for graphene nanoribbon-based nanodevices.     

Numerical study of localization length in disordered graphene nanoribbons

In this work, we study quantum transport properties of a defective graphene nanoribbon (DGNR) attached to two semi-infinite metallic armchair graphene nanoribbon (AGNR) leads. A line of defects is considered in the GNR device with different configurations, which affects on the energy spectrum of the system. The calculations are based on the tight-binding model and Green’s function method, in which localization length of the system is investigated, numerically. By controlling disorder concentration, the extended states can be separated from the localized states in the system. Our results may have important applications for building blocks in the nano-electronic devices based on GNRs.     

Strain Effects on Properties of Phosphorene and Phosphorene Nanoribbons: a DFT and Tight Binding Study

We perform comprehensive density functional theory calculations of strain effect on electronic structure of black phosphorus(BP) and on BP nanoribbons. Both uniaxial and biaxial strain are applied, and the dramatic change of BP's band structure is observed. Under 0-8% uniaxial strain, the band gap can be modulated in the range of 0.55-1.06 eV, and a direct-indirect band gap transition causes strain over 4% in the y direction. Under 0-8% biaxial strain, the band gap can be modulated in the range of 0.35-1.09 eV, and the band gap maintains directly.Applying strain to BP nanoribbon, the band gap value reduces or enlarges markedly either zigzag nanoribbon or armchair nanoribbon. Analyzing the orbital composition and using a tight-binding model we ascribe this band gap behavior to the competition between effects of different bond lengths on band gap. These results would enhance our understanding on strain effects on properties of BP and phosphorene nanoribbon.     

Theoretical calculation of excitonic binding energies and optical absorption spectra for Armchair graphene nanoribbons

In this paper the excitons of armchair graphene nanoribbons with layers of different width and thickness have been investigated. In this investigation, the band structure and energy gap of armchair graphene nanoribbons have been calculated using a tight-binding model including edge deformation effects (all edge atoms have been passivated with hydrogen atoms). Also, by calculating the conductance in armchair graphene nanoribbons (A-GNRs) optical absorption of armchair graphene nanoribbon in the single-electron approximation has been obtained. Finally, the binding energy of excitons in armchair graphene nanoribbons has been calculated using the Wannier model, Hartree-Fock approximation and the Bethe-Salpeter equation.     

Strain effect on graphene nanoribbon carrier statistic in the presence of non-parabolic band structure

The effect of tensile uniaxial strain on the non-parabolic electronic band structure of armchair graphene nanoribbon(AGNR) is investigated.In addition,the density of states and the carrier statistic based on the tight-binding Hamiltonian are modeled analytically.It is found that the property of AGNR in the non-parabolic band region is varied by the strain.The tunable energy band gap in AGNR upon strain at the minimum energy is described for each of n-AGNR families in the non-parabolic approximation.The behavior of AGNR in the presence of strain is attributed to the breakable AGNR electronic band structure,which varies the physical properties from its normality.The linear relation between the energy gap and the electrical properties is featured to further explain the characteristic of the deformed AGNR upon strain.     

Combined effect of Aharonov-Bohm effect and uniaxial strain on quantum conductance oscillations of carbon nanotube resonators

Using the π orbital tight-binding model and the multi-channel Laudauer-Büttiker formula, the combined effect of Aharonov-Bohm effect (induced by an axial magnetic field) and uniaxial strain on quantum conductance oscillations of the electronic Fabry-Perot resonators composed of armchair and metallic zigzag single-walled carbon nanotubes (SWNTs) has been studied. It is found that, for the case of the armchair SWNT, conductance oscillations near the band gap are dominated by Aharonov-Bohm effect, while the conductance oscillations in other regions are dominated by the uniaxial strains. The combined effect of Aharonov-Bohm effect and uniaxial strains on quantum conductance oscillations is not obvious. But, for the case of the metallic zigzag SWNTs, obvious single-channel transport and one or two conductance oscillations existing in two different gate voltage ranges were found by the combined effect of uniaxial strain and axial magnetic field.     

Band structure of deformed armchair nanoribbon with bond alternation

Electronic energy band structure of deformed armchair graphene nanoribbons with bond alternation is studied by the tight-binding approximation. In the presence of bond alternation, all armchair graphene nanoribbons become semiconducting with small band gap opened at center of the Brillouin zone. Under tensional strain, armchair graphene nanoribbons can become metallic at the critical values of deformation and we can control the band gap of nanoribbon by its strain.     

Electronic transport properties of junctions between carbon nanotubes and graphene nanoribbons

Using the tight-binding model and Green’s function method, we studied the electronic transport of four kinds of nanotube-graphene junctions. The results show the transport properties depend on both types of the carbon nanotube and graphene nanoribbon, metal or semiconducting. Moreover, the defect at the nanotube-graphene interface did not affect the conductance of the whole system at the Fermi level. In the double junction of nanotube/nanoribbon/nanotube, quasibound states are found, which cause antiresonance and result in conductance dips.     

周期性纳米洞内边缘氧饱和石墨烯纳米带的电子特性

利用基于密度泛函理论的第一性原理方法,研究了内边缘氧饱和的周期性凿洞石墨烯纳米带（G NR）的电子特性. 研究结果表明：对于凿洞锯齿形石墨烯纳米带（ZGNRs）,在非磁性态时不仅始终为金属,且金属性明显增强;反铁磁态（AFM）时为半导体的ZGNR,凿洞后可能成为金属;但铁磁态（FM）为金属的ZGNR,凿洞后一般变为半导体或半金属. 而对于凿洞的扶手椅形石墨烯（AGNRs）,其带隙会明显增加. 深入分析发现：这是由于氧原子对石墨烯纳米带边的电子特性有重要的影响,以及颈次级纳米带（NSNR）及边缘次级纳米带（ESNR）的不同宽度及边缘形状（锯齿或扶手椅形）能呈现出不同的量子限域效应. 这些研究对于发展纳米电子器件有重要的意义. 关键词： 石墨烯纳米带 纳米洞 内边缘氧饱和 电子特性