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.     

Voltage-driven electronic transport and shot noise in armchair graphene nanoribbons

We study theoretically shot noise and minimal conductivity of electrons by evanescent states penetrating through clean graphene nanoribbons (GNRs). With increasing of the barrier voltage, we find that the minimum conductivity will increase to 4e²/πh and the maximum Fano factor will increase to 1/3. More interestingly, quantum oscillations can be tuned by the gate voltage and separated by tuning the barrier voltage     

Electronic transport properties on V-shaped-notched zigzag graphene nanoribbons junctions

Using nonequilibrium Green?s functions in combination with the density functional theory, the spin-dependent electronic transport properties on V-shaped notched zigzag-edged graphene nanoribbons junctions have been calculated. The results show that the electronic transport properties are strongly depending on the type of notch and the symmetry of ribbon. The spin-filter phenomenon and negative differential resistance behaviors can be observed. A physical analysis of these results is given.     

N掺杂对zigzag型石墨烯纳米带的能带结构和输运性质的影响

用第一性原理研究了N掺杂zigzag型石墨烯纳米带(z-GNRs)的能带结构、透射谱和电流电压特性,研究结果表明N掺杂将使得z-GNRs的能带结构中出现能隙,材料从金属转变为半导体;随着杂质浓度的增大,相同偏压下电流明显减小,同时体系费米面附近的透射率逐渐减小;z-GNRs的长度、宽度以及N原子的替代掺杂位置均会对输运性质产生影响,在宽度较小的情况下,掺杂浓度和掺杂位置两种因素共同影响体系的输运性质. 关键词： 石墨烯纳米带 N掺杂 能带结构 输运性质     

Fully spin-polarized transport induced by B doping in graphene nanoribbons

B-doping induced spin polarization in zigzag-edged graphene nanoribbons is studied by density functional calculations by two kinds of doping: (1) doping only one B atom in the central scattering region; (2) periodically doping in the whole system. It is found that even a single B dopant may cause large spin polarization in the current, which can be understood by the breaking of spin-degeneracy due to the impurity atoms and the Fermi level shift resulting from the hole-donating of the B atoms. More interestingly, 100% spin polarized current under finite bias is obtained through periodical doping although the transmission function around the Fermi level is not 100% spin polarized. This can be interpreted by a rigid shift model of the special band structures of the left and right leads in this case. It demonstrates that only transmission function at equilibrium conditions is not sufficient in the study of electron transport, but current should be considered in certain situations.     

Ballistic thermoelectric properties in double-bend graphene nanoribbons

Ballistic thermoelectric properties in double-bend graphene nanoribbons (GNRs) are investigated by using the nonequilibrium Green's function. We find that due to the elastic scattering caused by the interface mismatching, the thermal conductance contributed by phonons is greatly reduced, while ballistic transport behaviors for electrons are dramatically demolished, and even some gaps can be opened at antiresonance energies. Near these antiresonance gaps, the maximum value of ZT   (Z_Tmax$Z{T}_{\mathit{max}}$) can be observed, much larger than that for straight GNRs. Moreover, this Z_Tmax$Z{T}_{\mathit{max}}$ can be effectively tuned by modulating the length or width of double-bend GNRs.     

Thermoelectric properties of gated graphene ribbons in the ballistic regime

We investigate the thermoelectric properties of gated graphene ribbons in the ballistic transport limit using linear response theory and the Landauer formalism. The dependence of the electronic conductance, thermopower as well as electronic thermal conductance on both Fermi level and temperature are clarified and the validity of Wiedemann-Franz law is examined. The electronic part of thermoelectric figure of merit _ZTel${\mathit{ZT}}_{\mathit{el}}$ which gives an upper bound for the thermoelectric efficiency of the gated ribbons, is also calculated. It is shown that _ZTel${\mathit{ZT}}_{\mathit{el}}$ of wide and short gated ribbons is directly related to geometric aspect ratio of the graphene ribbon and for very short ribbons can exceed unity at room temperature. Our results could be useful in the design of efficient graphene-based thermoelectric devices.     

Electronic transport on graphene armchair-edge nanoribbons with Fermi velocity and potential barriers

The transport properties of Dirac fermions through armchair-edge graphene nanoribbons (AGNRs) with a single and double rectangular Fermi velocity $v_F$ and electrostatic potential U barriers is investigated. We employ a transfer matrix method (TMM) to compute the transmission coefficient of the full set of propagating mode which is used to obtain the conductance and Fano factor spectra for both metallic and semiconducting nanoribbons. We show that a reduced Fermi velocity within the barrier region can partially suppress the backscattering resulting from the electrostatic potential. In a double barrier structure, the emergence of high-order transmitting modes is shown to substantially reduce the Fano factor in the spectral region around U. These results indicate that the simultaneous tuning of $v_F$ and U in barrier regions can be explored to control the electronic transport in graphene-based nanoelectronics structures.     

Detection of valley currents in graphene nanoribbons

There are two valleys in the band structure of graphene zigzag ribbons, which can be used to construct valleytronic devices. We studied the use of a T junction formed by an armchair ribbon and a zigzag ribbon to detect the valley-dependent currents in a zigzag graphene ribbon. A current flowing in a zigzag ribbon is divided by the T junction into the zigzag and armchair leads and this separation process is valley dependent. By measuring the currents in the two outgoing leads, the valley-dependent currents in the incoming lead can be determined. The method does not require superconducting or magnetic elements as in other approaches and thus will be useful in the development of valleytronic devices.     

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.     

Atomistic simulations of divacancy defects in armchair graphene nanoribbons: Stability,electronic structure,and electron transport properties

Using the first principles calculations associated with nonequilibrium Green?s function, we have studied the electronic structures and quantum transport properties of defective armchair graphene nanoribbon (AGNR) in the presence of divacancy defects. The triple pentagon–triple heptagon (555–777) defect in the defective AGNR is energetically more favorable than the pentagon–octagon–pentagon (5–8–5) defect. Our calculated results reveal that both 5–8–5-like defect and 555–777-like defect in AGNR could improve the electron transport. It is anticipated that defective AGNRs can exhibit large range variations in transport behaviors, which are strongly dependent on the distributions of the divacancy defect.     

石墨烯纳米带电极区N掺杂电子输运性质研究

运用密度泛函理论和非平衡格林函数结合的方法,研究电极区N掺杂对扶手椅型石墨烯纳米带电子输运特性的影响.结果表明,与本征扶手椅型石墨烯纳米带电流-电压曲线相比,宽度为7的石墨烯纳米带电流-电压曲线表现出明显的不对称性,其中心N掺杂表现强烈的整流特性,整流系数达到102数量级,且将N原子从电极区中心位置移动到边缘,整流特性减弱.研究结果表明宽度为7的扶手椅型石墨烯纳米带出现强整流现象的原因主要是负向偏压下能量窗内没有透射峰引起的,该研究结果对将来石墨烯整流器件的设计具有重要的意义.     

Rippling and the external electric field on conductance in corrugated graphene nanoribbons molecular device

Molecular devices constructed using corrugated graphene nanoribbons (GNRs) are proposed in the paper. Recursive Green's function calculations show that the intrinsic ripples in graphene and the external electric field energy play important roles on the electron transport properties. Negative differential resistance is observed in zigzag corrugated GNRs. With the wavelength of the ripples decreasing, both the zigzag and armchair corrugated GNRs exhibit ON/OFF characteristics. On applying external electric field, current decreases dramatically in zigzag corrugated GNRs. These findings show that corrugated GNRs can be used to design functional nanoscale devices.     

Enhanced gas sensor based on nitrogen-vacancy graphene nanoribbons

We study the electron transport of nitrogen-vacancy zigzag graphene nanoribbons (ZGNRs) absorbing gas molecules. It is found that the nitrogen-vacancy ZGNRs are more sensitive to the gas molecules than the pristine ZGNRs. The gas molecules absorbed on the three-nitrogen vacancies lead to sharp resonant peaks on conductance, while those absorbed on the four-nitrogen vacancies lead to anti-resonant dips. Each kind of gas molecule can be detected by its own unique (different energy) resonant peaks (or dips). This indicates that the nitrogen vacancy can enhance the sensitivity to gas molecules, i.e., nitrogen-vacancy ZGNRs can serve as better gas sensors.     

Atomic and electronic structures of divacancy in graphene nanoribbons

First principles calculations have been performed to investigate the electronic structures and transport properties of defective graphene nanoribbons (GNRs) in the presence of pentagon-octagon-pentagon (5-8-5) defects. Electronic band structure results reveal that 5-8-5 defects in the defective zigzag graphene nanoribbon (ZGNR) is unfavorable for electronic transport. However, such defects in the defective armchair graphene nanoribbon (AGNR) give rise to smaller band gap than that in the pristine AGNR, and eventually results in semiconductor to metal-like transition. The distinct roles of 5-8-5 defects in two kinds of edged-GNR are attributed to the different coupling between π^? and π subbands influenced by the defects. Our findings indicate the possibility of a new route to improve the electronic transport properties of graphene nanoribbons via tailoring the atomic structures by ion irradiation.     

Tunneling of Dirac fermions in graphene through a velocity barrier with modulated by magnetic fields

We study magnetic field modulated transport properties of Dirac fermions in graphene, where Dirac fermions penetrate through a velocity barrier. We find strong wave vector filtering and resonant effect. The angular-dependent region of resonant tunneling is suppressed by tuning velocity barriers. We can also found that the confined states in this velocity barrier can be changed by the magnetic field. Various novel devices, such as wavevector filter and magnetic switches, may be constructed based on our observed phenomena.     

On ballistic transport in carbon nanotubes

We investigate theoretically the ballistic regime exhibited by conduction electrons in multiwalled carbon nanotubes in relation to the conductance quantization in these tubes. Starting from the fact that electron drift mobility is quantized in multiwall tubes, essential aspects related to both ballistic and diffusive regimes are discussed.     

Thermal transport in S-shaped graphene nano-junctions

By using nonequilibrium Green's function method, we study the phonon transport properties in S-shaped graphene nano-junctions (GNJs). Interesting transmission phenomenon is found. The transmission spectrum of low frequency phonon shows forbidden frequency band, by changing the width W₂ of the S-shaped GNJs. These low frequency forbidden bands are sensitive to their periodic geometric shape of GNJs. These thermal transport phenomena can be explained by analyzing the phonon transmission coefficient. This paper illustrates the thermal transport mechanisms in the different S-shaped GNJs, and the results could provide significant physical models and theoretical validity in designing the thermal devices based on the GNJs.     

The complex band structure for armchair graphene nanoribbons

Using a tight binding transfer matrix method,we calculate the complex band structure of armchair graphene nanoribbons.The real part of the complex band structure calculated by the transfer matrix method fits well with the bulk band structure calculated by a Hermitian matrix.The complex band structure gives extra information on carrier’s decay behaviour.The imaginary loop connects the conduction and valence band,and can profoundly affect the characteristics of nanoscale electronic device made with graphene nanoribbons.In this work,the complex band structure calculation includes not only the first nearest neighbour interaction,but also the effects of edge bond relaxation and the third nearest neighbour interaction.The band gap is classified into three classes.Due to the edge bond relaxation and the third nearest neighbour interaction term,it opens a band gap for N=3M 1.The band gap is almost unchanged for N=3M + 1,but decreased for N=3M.The maximum imaginary wave vector length provides additional information about the electrical characteristics of graphene nanoribbons,and is also classified into three classes.