Effects of heteroatom (boron or nitrogen) substitutional doping on the electronic properties of graphene nanoribbons

The electronic properties of graphene nanoribbons (GNRs) with heteroatom (boron or nitrogen) substitutional doping at different sites are investigated by performing first-principles calculations based on density functional theory. The calculated results show that boron substitutional doping changes the conducting characteristics of GNRs to half-metallic. In contrast, nitrogen substitutional doping results in retention of the half-metallic characteristics of GNRs. It is predicted that the theoretical results may be valuable to the design of GNR-based spintronics devices.     

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     

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     

Towards graphene nanoribbon-based electronics

The successful fabrication of single layer graphene has greatly stimulated the progress of the research on graphene. In this article, focusing on the basic electronic and transport properties of graphene nanoribbons (GNRs), we review the recent progress of experimental fabrication of GNRs, the theoretical and experimental investigations of physical properties, and device applications of GNRs. We also briefly discuss the research efforts on the spin polarization of GNRs in relation to the edge states.      

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     

界面铁掺杂锯齿形石墨烯纳米带的自旋输运性能

基于密度泛函理论第一原理系统研究了界面铁掺杂锯齿(zigzag)形石墨烯纳米带的自旋输运性能, 首先考虑了宽度为4的锯齿(zigzag)形石墨烯纳米带, 构件了4个纳米器件模型, 对应于中心散射区的长度分别为N=4, 6, 8和10个石墨烯单胞的长度, 铁掺杂在中心区和电极的界面. 发现在铁磁(FM)态, 四个器件的β自旋的电流远大于α自旋的电流, 产生了自旋过滤现象; 而界面铁掺杂的反铁磁态模型, 两种电流自旋都很小, 无法产生自旋过滤现象; 进一步考虑电极的反自旋构型, 器件电流显示出明显的自旋过滤效应. 探讨了带宽分别为5和6的纳米器件的自旋输运性能, 中心散射区的长度为N=6个石墨烯单胞的长度, FM 态下器件两种自旋方向的电流值也存在较大的差异, β自旋的电流远大于α自旋电流. 这些结果表明: 界面铁掺杂能有效调控锯齿形石墨烯纳米带的自旋电子, 对于设计和发展高极化自旋过滤器件有重要意义.     

双空位缺陷石墨纳米带的电子结构和输运性质研究

基于第一原理电子结构和输运性质计算,研究了585双空位拓扑缺陷对锯齿（zigzag）型石墨纳米带（具有椅型（armchair）边）电子结构和输运性质的影响.研究发现,585双空位缺陷的存在使得锯齿型石墨纳米带的能隙增大,并在能隙中出现了一条局域于缺陷处的缺陷态能带,双空位缺陷的取向也影响其能带结构.另外,585双空位缺陷对能隙较小的锯齿型石墨纳米带输运性质的影响较大,而对能隙较大的锯齿型石墨纳米带影响很小,缺陷取向并不显著影响纳米带的输运性质. 关键词： 石墨纳米带 585空位缺陷 电子结构 输运性质     

掺杂菱形BN片的石墨烯纳米带的电子特性

采用基于密度泛函理论的第一性原理方法,系统研究掺杂菱形BN片的石墨烯纳米带的电子特性.掺杂使扶手椅型石墨烯纳米带（AGNRs）的带隙增大,不同位置掺杂AGNRs的带隙大小略有差异.在无磁性态,无论是否掺杂,锯齿型石墨烯纳米带（ZGNRs）都为金属.在铁磁态,掺杂使ZGNRs由金属转变为半导体.而处于反铁磁态时,无论是否掺杂,ZGNRs都为半导体,掺杂使其带隙发生改变.掺杂的AGNRs和ZGNRs的结构稳定,掺杂ZGNRs的基态为反铁磁态.掺杂菱形BN片可以有效调控GNRs的电子特性.     

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.     

Spatially separated spin carriers in three-ports graphene nanoribbons

Exporting the different spin signals to different ports is of practical importance to graphene-based spintronic devices. In this work, we have designed a three-ports graphene nanoribbon (GNR) device by inserting square-shaped carbon tetragon (CT) into GNR symmetrically, and calculated the magnetic moment distribution and transmission spectrum by using first-principles calculation and quantum transport simulation. Our results show that CT can bring non-equivalent path for two spin transport channels resulting in one spin is easier to transport than the other in each output port. Overall whole model, the spin states would be separated in real space but degenerated in energy. After correcting the device with asymmetric edge hydrogenation, we can achieve spatially separated spin carriers in real space and stable spin transporting. Our results suggest this model can serve as the most basic logic device for applying in future spintronics.     

Electronic properties of boron and nitrogen doped graphene nanoribbons and its application for graphene electronics

On the basis of density functional theory calculations, we have systematically investigated the electronic properties of armchair-edge graphene nanoribbons (GNRs) doped with boron (B) and nitrogen (N) atoms. B (N) atoms could effectively introduce holes (electrons) to GNRs and the system exhibits p- (n-) type semiconducting behavior after B (N) doping. According to the electronic structure calculations, Z-shape GNR-based field effect transistors (FETs) is constructed by selective doping with B or N atoms. Using first-principles quantum transport calculations, we demonstrate that the B-doped p-type GNR-FETs can exhibit high levels of performance, with high ON/OFF ratios and low subthreshold swing. Furthermore, the performance parameters of GNR-FETs could be controlled by the p-type semiconducting channel length.     

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.     

Electronic transport of graphene nanoribbons within recursive Green’s function

In this work, we introduce a recursive Green’s function method for investigating electronic transport in a graphene nanoribbons (GNRs) quantum wire with armchair (AGNR) and zigzag (ZGNR) edges which attached to two semi-infinite square lattice leads. This model reduces numerical calculations time and enables us to use Green’s function method to investigate transport in a supperlattice device. Therefore, we consider AGNR and ZGNR devices attached to metallic semi-infinite square lattice leads, taking into account the effects of longitudinal and wide of the wire. Our calculations are based on the tight-binding model, which the recursive Green’s function method is used to solve inhomogeneous differential equations. We concentrate on the electrical conductance and current for various length and wide size of the wire. Our numerical results show that the transport properties are strongly affected by the quantum interference effect and the lead interface geometry to the device. By controlling the type of contact and wire geometry, this kind of system can explain the antiresonance states at the Fermi energy. Our results can serve as a base for developments in designing nano-electronic devices.     

Quasiparticle energies and band gaps in graphene nanoribbons

We present calculations of the quasiparticle energies and band gaps of graphene nanoribbons (GNRs) carried out using a first-principles many-electron Green's function approach within the GW approximation. Because of the quasi-one-dimensional nature of a GNR, electron-electron interaction effects due to the enhanced screened Coulomb interaction and confinement geometry greatly influence the quasiparticle band gap. Compared with previous tight-binding and density functional theory studies, our calculated quasiparticle band gaps show significant self-energy corrections for both armchair and zigzag GNRs, in the range of 0.5-3.0 eV for ribbons of width 2.4-0.4 nm. The quasiparticle band gaps found here suggest that use of GNRs for electronic device components in ambient conditions may be viable.     

4H-SiC MOSFET的温度特性研究

对4HSiCMOSFET的器件结构和温度特性进行了研究,总结了器件的结构参数对特性的影响,比较了不同温度下的输出特性以及饱和漏电流、阈值电压、跨导、导通电阻与温度的变化关系,模拟结果表明4HSiCMOSFET具有优异的温度特性,在800K下可以正常工作 关键词： 4H-SiC MOSFET     

A-Z-A型石墨烯场效应晶体管吸附 效应的第一性原理研究

利用第一性原理的计算方法, 研究了A-Z-A型GNR-FET的电子结构和输运性质及其分子吸附效应. 得到了以下结论: 纯净的A-Z-A型GNR-FET具有典型的双极型晶体管特性, 吸附分子的存在会使纳米带能隙变小. 对于吸附H, H₂, H₂O, N₂, NO, NO₂, O₂, CO₂和SO₂分子的情况, A-Z-A型GNR-FET仍然保持着场效应晶体管的基本特征, 但吸附不同类型的分子会使GNR-FET的输运特性发生不同程度的改变; 对于吸附OH分子的情况, 输运特性发生了本质的改变, 完全不具有场效应晶体管的特性. 这些研究结果将有助于石墨烯气体探测器的工程实现, 并对应用于不同环境中GNR-FET的设计具有重要指导意义.     

Energy gaps in graphene nanoribbons

Based on a first-principles approach, we present scaling rules for the band gaps of graphene nanoribbons (GNRs) as a function of their widths. The GNRs considered have either armchair or zigzag shaped edges on both sides with hydrogen passivation. Both varieties of ribbons are shown to have band gaps. This differs from the results of simple tight-binding calculations or solutions of the Dirac's equation based on them. Our ab initio calculations show that the origin of energy gaps for GNRs with armchair shaped edges arises from both quantum confinement and the crucial effect of the edges. For GNRs with zigzag shaped edges, gaps appear because of a staggered sublattice potential on the hexagonal lattice due to edge magnetization. The rich gap structure for ribbons with armchair shaped edges is further obtained analytically including edge effects. These results reproduce our ab initio calculation results very well.     

Electronic and magnetic properties of copper-family-element atom adsorbed graphene nanoribbons with zigzag edges

We have investigated the electronic and magnetic properties of copper-family-element (CFE) atom adsorbed graphene nanoribbons (GNRs) with zigzag edges using first-principles calculations based on density functional theory. We found that CFE atoms energetically prefer to be adsorbed at the edges of nanoribbons. Charges are transferred between the CFE atom and carbon atoms at the edge, which reduce the local magnetic moment of carbon atoms in the vicinity of adsorption site and change the electronic structure of GNRs. As a result, Cu adsorbed zigzag GNR is a semiconductor with energy band gap of 0.88 eV in beta-spin and energy gap of 0.22 eV in alpha-spin, while Ag adsorbed zigzag GNR and Au adsorbed zigzag GNR are both half-metallic with the energy gaps of 0.68 eV and 0.63 eV in beta-spin, respectively. These results show that CFE atom adsorbed zigzag GNRs can be applied in nanoelectronics and spintronics.     

Spin-polarized transport properties of a pyridinium-based molecular spintronics device

By applying a first-principles approach based on non-equilibrium Green's functions combined with density functional theory, the transport properties of a pyridinium-based “radical-π-radical” molecular spintronics device are investigated. The obvious negative differential resistance (NDR) and spin current polarization (SCP) effect, and abnormal magnetoresistance (MR) are obtained. Orbital reconstruction is responsible for novel transport properties such as that the MR increases with bias and then decreases and that the NDR being present for both parallel and antiparallel magnetization configurations, which may have future applications in the field of molecular spintronics.     

Effect of Chemical Doping on the Electronic Transport Properties of Tailoring Graphene Nanoribbons

The electronic transport properties of a molecular junction based on doping tailoring armchair-type graphene nanoribbons(AGNRs)with different widths are investigated by applying the non-equilibrium Green's function formalism combined with first-principles density functional theory.The calculated results show that the width and doping play significant roles in the electronic transport properties of the molecular junction.A higher current can be obtained for the molecular junctions with the tailoring AGNRs with W=11.Furthermore,the current of boron-doped tailoring AGNRs with widths W=7 is nearly four times larger than that of the undoped one,which can be potentially useful for the design of high performance electronic devices.