Electronic transport for armchair graphene nanoribbons with a potential barrier

We theoretically investigate the electronic transport properties through a rectangular potential barrier embedded in armchair-edge graphene nanoribbons (AGNRs) of various widths. Using the Landauer formula and Dirac equation with the continuity conditions for all segments of wave functions at the interfaces between regions inside and outside the barrier, we calculate analytically the conductance and Fano factor for the both metallic and semiconducting AGNRs, respectively. It is shown that, by some numerical examples, at Dirac point the both types of AGNRs own a minimum conductance associated with the maximum Fano factor. The results are discussed and compared with the previous relevant works.     

Magnetoconductance of graphene nanoribbons

The electronic and transport properties of monolayer and AB-stacked bilayer zigzag graphene nanoribbons subject to the influences of a magnetic field are investigated theoretically. We demonstrate that the magnetic confinement and the size effect affect the electronic properties competitively. In the limit of a strong magnetic field, the magnetic length is much smaller than the ribbon width, and the bulk electrons are confined solely by the magnetic potential. Their properties are independent of the width, and the Landau levels appear. On the other hand, the size effect dominates in the case of narrow ribbons. In addition, the dispersion relations rely sensitively on the interlayer interactions. Such interactions will modify the subband curvature, create additional band-edge states, change the subband spacing or the energy gap, and separate the partial flat bands. The band structures are symmetric or asymmetric about the Fermi energy for monolayer or bilayer nanoribbons, respectively. The chemical-potential-dependent electrical and thermal conductance exhibits a stepwise increase behaviour. The competition between the magnetic confinement and the size effect will also be reflected in the transport properties. The features of the conductance are found to be strongly dependent on the field strength, number of layers, interlayer interactions, and temperature.     

Conductance of bilayer graphene nanoribbons with different widths

The electronic and transport properties of bilayer graphene nanoribbons with different width are investigated theoretically by using the tight-binding model. The energy dispersion relations are found to exhibit significant dependence on the interlayer interactions and the geometry of the bilayer graphene nanoribbons. The energy gaps are oscillatory with the upper ribbon displacement. For all four types of bilayer graphene nanoribbons, the bandgaps touch the zero value and exhibit semiconductor–metal transitions. Variations in the electronic structures with the upper ribbon displacement will be reflected in the electrical and thermal conductance. The chemical-potential-dependent electrical and thermal conductances exhibit a stepwise increase and spike behavior. These conductances can be tuned by varying the upper ribbon displacement. The peak and trench structures of the conductance will be stretched out as the temperature rises. In addition, quantum conductance behavior in bilayer graphene nanoribbons can be observed experimentally at temperature below 10 K.     

堆叠石墨片对锯齿型石墨纳米带电子输运的影响

采用格林函数方法研究了堆叠石墨片对锯齿型石墨纳米带电子输运性质的影响,计算了两种不同堆叠方式下锯齿型石墨纳米带的电导.研究发现,由于堆叠石墨片与石墨纳米带的耦合作用,锯齿型石墨纳米带的电导谱出现了电导谷.在远离费米能处,两种堆叠方式下的电导谷位置相近甚至重合;而在费米能附近,两种堆叠方式下的电导谷存在差异.此外,讨论了堆叠石墨片的几何尺寸对锯齿型石墨纳米带电子输运的影响.结果显示,随石墨片几何尺寸的增大,锯齿型石墨纳米带在两种堆叠方式下远离费米能处的电导谷逐渐向费米能方向移动,同时其费米能附近的电导谷在两种堆叠方式下的差异随石墨片尺寸的增大变得更为明显.研究结果表明,堆叠石墨片能够有效地调制锯齿型石墨纳米带的电子输运性质.     

扶手椅型石墨纳米带的双空位缺陷效应研究

采用基于密度泛函理论的第一性原理电子结构和输运性质计算,研究了扶手椅型石墨纳米带（具有锯齿边缘）的双空位缺陷效应.研究发现：双空位缺陷的存在并没有改变石墨纳米带的金属特性,但改变了费米面附近的能带结构.同时,双空位缺陷的取向对石墨纳米带的输运性质有很重要的影响.对于奇数宽度的纳米带,斜向双空位缺陷使得石墨带导电性能减弱,而垂直双空位能基本保留原有的线性伏安特性,导电性能降低较少;对于偶数宽度的纳米带,斜向双空位缺陷会使石墨带导电性能明显增强,而垂直双空位缺陷则具有完整石墨带的输运性质. 关键词： 石墨纳米带 585双空位缺陷 电子结构 输运性质     

铂掺杂扶手椅型石墨烯纳米带的电学特性研究

本文采用基于密度泛函理论(DFT)的第一性原理计算了铂原子填充扶手椅型石墨烯纳米带(AGNR)中双空位结构的电学性能.计算结果表明: 通过控制铂原子的掺杂位置, 可以实现纳米带循环经历小带隙半导体—金属—大带隙半导体的相变过程; 纳米带边缘位置是铂原子掺杂的最稳定位置, 边缘掺杂纳米带的带隙值随宽度的变化与本征AGNR一样可用三簇曲线表示, 但在较大宽度时简并成两条曲线, 一定程度上抑制了带隙值的振荡; 并且铂原子边缘掺杂导致宽度系数N_a = 3p和3p + 1(p是一个整数)的几个较窄纳米带的带隙中出现杂质能级, 有效地降低了其过大的带隙值. 此外, 铂掺杂AGNR的能带结构对掺杂浓度不是很敏感, 从而降低了对实验精度的挑战. 本文的计算有利于推动石墨烯纳米带在纳米电子学方面的应用.     

BN链掺杂的石墨烯纳米带的电学及磁学特性

基于密度泛函理论第一性原理系统研究了BN链掺杂石墨烯纳米带(GNRs)的电学及磁学特性, 对锯齿型石墨烯纳米带(ZGNRs)分非磁态(NM)、反铁磁态(AFM)及铁磁性(FM)三种情况分别进行考虑. 重点研究了单个BN链掺杂的位置效应. 计算发现: BN链掺杂扶手椅型石墨烯纳米带(AGNRs) 能使带隙增加, 不同位置的掺杂, 能使其成为带隙丰富的半导体. BN链掺杂非磁态ZGNR的不同位置, 其金属性均降低, 并能出现准金属的情况; BN链掺杂反铁磁态ZGNR, 能使其从半导体变为金属或半金属(half-metal), 这取决于掺杂的位置; BN链掺杂铁磁态ZGNR, 其金属性保持不变, 与掺杂位置无关. 这些结果表明: BN链掺杂能有效调控石墨烯纳米带的电子结构, 并形成丰富的电学及磁学特性, 这对于发展各种类型的石墨烯基纳米电子器件有重要意义. 关键词： 石墨烯纳米带 BN链掺杂 输运性质 自旋极化     

非近邻跳跃对扶手椅型石墨烯纳米带电子结构的影响

根据π电子的紧束缚模型,将电子的次近邻和第三近邻跳跃能考虑在内,得到扶手椅型石墨烯纳米带（AGRNs）能带结构的解析解.讨论了由次近邻和第三近邻电子跳跃引起的能带和能隙变化,发现次近邻和第三近邻跳跃分别对带隙产生增大和减小的影响. 比较了边界弛豫与非近邻跳跃之间的互相竞争关系. 当纳米带的宽度n为奇数时,二维石墨面的紧束缚模型中所固有的van Hove奇异性表现为AGRNs中的无色散带. 当AGRNs宽度增加时,能谱趋向于二维石墨烯时的能谱结构. 关键词： 扶手椅型石墨烯纳米带 非近邻跳跃 边界弛豫 电子结构     

锯齿型石墨纳米带叠层复合结的电子输运

基于紧束缚格林函数方法,研究了两半无限长锯齿型石墨纳米带叠层复合结的电子输运性质.结果表明,层间次近邻相互作用、叠层区长度及门电压对复合结的电子透射谱有重要调制作用.层间次近邻相互作用导致复合结的透射谱关于费米能呈现非对称性,与实验结果很好相符.低于费米能第一子能区内周期性出现透射系数为0和1的台阶,呈现全反射与透射现象.随散射结长度增加,透射系数在1内周期性振荡,呈现明显的量子干涉效应.在门电压调控下,低于费米能的透射系数出现了从1到0的转变,类似于开关效应.相关结果对基于石墨烯器件的设计与应用有指导意义.     

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     

含带隙石墨烯纳米带的自旋筛选输运

利用Landauer-Büttiker公式和非平衡格林函数方法,研究了在电荷和自旋偏压共同作用下的扶手椅型石墨烯纳米带的自旋相关的电子输运性质. 当系统存在两种偏压时,不用自旋的电子具有不同的偏压窗口. 同时,含带隙石墨烯纳米带具有与自旋无关的导电电压阈值. 通过设置适当的两种偏压值,系统可以产生易于调节的单一自旋的电流.     

石墨烯纳米带卷曲效应对其电子特性的影响

石墨烯纳米带 (GNRs) 是一种重要的纳米材料, 碳纳米管可看作是GNRs卷曲而成的无缝圆筒. 利用基于密度泛函理论的第一性原理方法, 系统研究了GNRs卷曲变形到不同几何构型时, 其电子特性, 包括能带结构 (特别是带隙) 、态密度、透射谱的变化规律. 结果表明: 无论是锯齿型GNRs (ZGNRs) 或扶手椅型GNRs (AGNRs), 在其卷曲成管之前, 其电子特性对卷曲形变均不敏感, 这意味着GNRs的电子结构及输运特性有较强地抵抗卷曲变形的能力. 当GNRs 卷曲成管后, ZGNRs和AGNRs表现出完全不同的性质, ZGNRs几乎保持金属性不变或变为准金属; 但AGNRs的电子特性有较大的变化, 出现不同带隙半导体、准金属之间的转变, 这也许密切关系到碳纳米管管口周长方向上的周期性边界条件及量子禁锢的改变. 这些研究对于了解GNRs电子特性的卷曲效应、以及GNRs与碳纳米管电子特性的关系 (结构与特性的关系) 有重要意义. 关键词： 石墨烯纳米带 卷曲效应 电子特性 密度泛函理论     

Equipartition of current in metallic armchair nanoribbon of graphene-based device

We numerically investigate the mesoscopic electronic transport properties of Bernal-stacked bilayer/trilayer graphene connected with four monolayer graphene terminals. In armchair-terminated metallic bilayer graphene, we show that the current from one incoming terminal can be equally partitioned into other three outgoing terminals near the charge-neutrality point, and the conductance periodically fluctuates, which is independent of the ribbon width but influenced by the interlayer hopping energy. This finding can be clearly understood by using the wave function matching method, in which a quantitative relationship between the periodicity, Fermi energy, and interlayer hopping energy can be reached. Interestingly, for the trilayer case, when the Fermi energy is located around the charge-neutrality point, the fractional quantized conductance 1/(4e²h) can be achieved when system exceeds a critical length.     

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.     

B与N掺杂对单层石墨纳米带自旋极化输运的影响

通过第一性原理计算研究了具有锯齿状边沿并且具有反铁磁构型的单层石墨纳米带的自旋极化输运.研究发现,在中心散射区同一位置掺入单个B和N原子,尽管对整个体系磁矩的影响完全相同,但对两个自旋分量电流的影响却完全相反.掺B时,自旋向上的电流显著大于自旋向下的电流;而掺N时,自旋向下的电流显著大于自旋向上的电流.这是由于不管掺B还是掺N都将打破自旋简并,使得导带和价带中自旋向上的能级比自旋向下的能级更高.掺B引入空穴,使完全占据的价带变为部分占据,从而自旋向上的能级正好处于费米能级,使得电子透射能力更强、电流更大,而自旋向下的能级则离费米能级较远使电子透射的能力较弱.掺N则引入电子,使得原来全空的导带变为部分占据,从而费米能级穿过导带中自旋向下的能级,使得自旋向下的电子比自旋向上的电子透射能力更强. 关键词： 自旋极化输运 单层石墨纳米带 第一性原理 非平衡格林函数     

Effect of electric field and magnetic field on spin transport in bilayer graphene armchair nanoribbons: A Monte Carlo simulation study

In this article we study the effect of external magnetic field and electric field on spin transport in bilayer armchair graphene nanoribbons (GNR) by employing semiclassical Monte Carlo approach. We include D'yakonov-Perel' (DP) relaxation due to structural inversion asymmetry (Rashba spin-orbit coupling) and Elliott-Yafet (EY) relaxation to model spin dephasing. In the model we neglect the effect of local magnetic moments due to adatoms and vacancies. We have considered injection polarization along z-direction perpendicular to the plane of graphene and the magnitude of ensemble averaged spin variation is studied along the x-direction which is the transport direction. To the best of our knowledge there has been no theoretical investigation of the effects of external magnetic field on spin transport in graphene nanoribbons. This theoretical investigation is important in order to identify the factors responsible for experimentally observed spin relaxation length in graphene 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     

石墨烯纳米带电子结构的紧束缚法研究

在推导出的一般复式格子的π电子紧束缚能量色散关系的基础上,通过假定石墨烯纳米带的电子横向限制势为无穷大硬壁势,导出石墨烯纳米带的能量色散关系及石墨烯纳米带或为金属或为半导体的条件.结果表明：石墨烯纳米带的电子结构与其几何构型（对称性及宽度）密切相关,所以通过控制几何构型,可将其调制成金属或不同带隙的半导体.这意味着石墨烯纳米带对于发展新型纳米器件具有重要意义. 关键词： 石墨烯纳米带 复式格子 紧束缚模型 电子结构     

Spin gapless armchair graphene nanoribbons under magnetic field and uniaxial strain

Using Green＇s function method, we investigate the spin transport properties of armchair graphene nanoribbons （AG- NRs） under magnetic field and uniaxial strain. Our results show that it is very difficult to transform narrow AGNRs directly from semiconductor to spin gapless semiconductors （SGS） by applying magnetic fields. However, as a uniaxial strain is exerted on the nanoribbons, the AGNRs can transform to SGS by a small magnetic field. The combination mode be- tween magnetic field and uniaxial strain displays a nonmonotonic arch-pattern relationship. In addition, we find that the combination mode is associated with the widths of nanoribbons, which exhibits group behaviors.     

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.