边缘重构对锯齿型石墨烯纳米带电子输运的影响

实验研究表明石墨烯纳米带中广泛地存在边缘结构重构且稳定的边缘缺陷结构.本文采用第一性原理的计算方法研究了锯齿型石墨烯纳米带中边缘结构重构形成的两种不同缺陷结构对材料电子输运性能的影响.研究发现两种缺陷边缘结构对稳定纳米尺度位型结构和电子能带结构具有显著影响,它使得费米能级发生移动并引起了共振背散射.两种边缘缺陷重构均抑制了费米能级附近电子输运特性并导致不同区域的电子完全共振背散射,电导的抑制不仅与边缘缺陷结构的大小有关,它更取决于边缘缺陷重构位型引起的缺陷态的具体分布和电子能带的移动.     

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.     

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.     

具有连续反量子点的石墨烯纳米带中纯自旋流的实现

热自旋电子学结合了热电子学和自旋电子学二者的优势,在构建高速、低能耗器件技术上具有广泛的应用前景.本文基于密度泛函理论和非平衡格林函数相结合的方法,研究了在铁磁态石墨烯纳米带中沿带宽方向引入连续反量子点(六元环缺陷)以获得纯自旋流的模型.计算发现,在纳米带的单边引入反量子点会破坏纳米带结构的完整性,导致器件的透射谱在费米能级附近呈现"X"形交叉.在温度场下,不同自旋的电子朝相反方向流动,形成了自旋流和电荷流,并且通过微调器件的化学势可以获得电荷流为0,自旋流不为0的纯自旋流.结果表明,对于具有W条链宽的锯齿型石墨烯纳米带,当沿纳米带带宽方向连续引入反量子点数满足(W/2-1)时,即可获得最大的纯自旋流,这一研究结果为设计基于石墨烯纳米带的纯自旋流器件提供了有力的理论依据.     

石墨烯/BiOI纳米复合物电子结构和光学性质的第一性原理模拟计算

光催化材料在解决能源短缺和环境污染等问题方面具有广泛的应用前景,本文通过构建BiOI纳米薄膜并将其与石墨烯复合起来,得到具有较高的比表面积和良好的光催化活性的纳米复合物光催化材料.采用基于密度泛函理论的第一性原理方法分别计算了单层和双层BiOI纳米片及其与石墨烯复合结构的电子结构和光学性质,并考虑了BiOI中的Bi,O,I三种空位缺陷对电子结构和光学特性的影响.计算结果表明,由于BiOI和石墨烯之间的相互作用,在石墨烯和BiOI界面处自发发生电荷转移,形成电子-空穴对,且石墨烯衬底可有效提高BiOI对可见光的光吸收,提高其光催化活性.对空位缺陷的计算表明,Bi空位缺陷可促进石墨烯和BiOI之间的电荷转移,形成更多的层间电子-空穴对;相反,O和I空位缺陷则抑制层间电荷转移,减少电子-空穴对的生成.     

三角形石墨烯纳米片电子和磁学性质的尺寸效应

基于密度泛函理论, 本文研究了氢钝化锯齿形边缘三角形石墨烯纳米片的电子结构和磁学性质, 这种石墨烯纳米结构的基态表现出强烈的磁性边缘态和量子尺寸效应。 我们应用多种交换关联泛函, 对体系的自旋密度和电子结构进行了第一性原理计算和理论分析, 结果表明三角形石墨烯纳米片的总磁矩和自旋随尺寸线性变化,平均磁矩随着尺寸变大而增加, 并逐渐趋于一个定值。 与此同时, 体系的能隙随着尺寸增加而减小, 其中自旋不变能隙的调控对光学响应和光子激发有着重要意义。 计算得到的单电子能谱表明, 费米能级的简并度与体系尺寸成正比。 应用多种交换关联泛函的计算结果表明, 三角形石墨烯纳米片具有可调控的自旋和能隙, 为其在纳米级光电器件和磁性半导体的应用方面提供了理论依据.     

含氮SW缺陷对单壁碳纳米管电子结构和光学性质的影响

应用第一性原理密度泛函理论研究了单壁碳纳米管中Stone-Wales(SW)缺陷和氮掺杂情况下的电子结构和光学性质.研究发现,含氮SW缺陷单壁碳纳米管体系的总能降低,结合更稳定,且在费米能级附近出现一条半满的杂质带,并且随着氮掺杂位置的不同,掺杂能态出现显著差异.碳管的吸收和反射明显减弱且吸收峰和反射峰在低能区发生红移现象,在能量小于11eV附近均出现杂质特征峰.本文对计算结果进行了分析研究,可望为含氮SW缺陷碳管在光电材料中的应用提供理论依据. 关键词： 单壁碳纳米管 Stone-Wales缺陷 氮掺杂 光学性质     

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     

Rashba自旋-轨道耦合调制的单层半导体纳米结构中电子的自旋极化效应

利用现代材料生长技术纳米厚的半导体可以沿着良好的方向有序生长,形成层状半导体纳米结构.在这种半导体纳米结构中由于结构反演对称性破缺出现较强的自旋-轨道耦合,能有效消除半导体中电子的自旋简并,导致电子自旋极化效应,在自旋电子学领域中具有重要的应用.本文从理论上研究了单层半导体纳米结构中由Rashba型自旋-轨道耦合引起的电子自旋极化效应.由于Rashba型自旋-轨道耦合,相当强的电子自旋极化效应出现在该单层半导体纳米结构中.自旋极化率与电子的能量和平面内波矢有关,尤其是其可通过外加电场或半导体层厚度进行调控.因此,该单层半导体纳米结构可作为半导体自旋电子器件应用中的可控电子自旋过滤器.     

石墨烯纳米片磁有序和自旋逻辑器件第一原理研究

尺寸效应和拓扑阻挫能够在有限石墨烯纳米片段中形成磁有序,本文对能够产生大自旋或电子自旋反铁磁耦合的石墨烯有限片段进行合理分类,提出几种能够作为基本逻辑门的特殊结构并对其进行第一原理电子结构计算,为设计高密度超快自旋器件提供了有效方案和理论依据.计算结果证明：基于有限石墨烯片段的逻辑门结构能够在室温下进行错误率较低的可纠错运算.     

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.     

Effect of N doping and Stone-Wales defects on the electronic properties of graphene nanoribbons

The effects of nitrogen substitutional doping in the Stone-Wales (SW) defect on the electronic transport properties of zigzag-edged graphene nanoribbon (ZGNR) are studied by using density functional theory combined with nonequilibrium Green’s function. The transformation energies of all doped nanostructures are evaluated in terms of total energies and, furthermore, it is found that the impurity placed on the center of the ribbon is the most energetically favorable site. Nitrogen substitution gives rise to a complete electron backscattering region in doped configurations, and the location of which is dependent on the doping sites. The electronic and transport properties of doped ZGNRs are discussed. Our results suggest that modification of the electronic properties of ZGNR with topological defects by substitutional doping might not be significant for some doping sites.     

Anisotropic mechanical properties and Stone-Wales defects in graphene monolayer: A theoretical study

We investigate the mechanical properties of graphene monolayer via the density functional theoretical (DFT) method. We find that the strain energies are anisotropic for the graphene under large strain. We attribute the anisotropic feature to the anisotropic sp² hybridization in the hexagonal lattice. We further identify that the formation energies of Stone-Wales (SW) defects in the graphene monolayer are determined by the defect concentration and also the direction of applied tensile strain, correlating with the anisotropic feature.     

电场调控双层石墨烯纳米带的电子结构和光学性质

利用基于密度泛函理论的第一性原理方法,研究了外加电场作用下双层AA堆垛的Armchair边缘石墨烯纳米带(BAGNRs)的电子结构和光学性质. BAGNRs具有半导体特性,其带隙随带宽(宽度为4～12个碳原子)的增加而振荡性减小.当施加电场后,BAGNRs的带隙随着电场强度的增加而逐渐减小,带隙越大对电场值的变化越敏感.当电场值为0.5 V/?时,所有BAGNRs的带隙都为零. BAGNRs具有各向异性的光学性质,其介电函数在垂直极化方向为半导体特性,而在平行极化方向为金属特性.在外加电场的作用下,BAGNRs的介电函数、吸收系数、折射系数、反射系数、电子能量损失系数和光电导率,其峰值向低能量区域移动,即产生红移现象.电场增强了能带间的跃迁几率.纳米带宽度对这些光学性质参数具有不同程度的影响.研究结果解释了电场调控BAGNRs光学性质的规律和微观机理.     

Hallmark of perfect graphene

Using first-principles calculations we show that the adsorption of atomic hydrogen on graphene opens a substantial gap in the electronic density of states in which lies a spin-polarized gap state. This spin is quenched by the presence of a rotated C-C bond (a Stone-Wales defect) adjacent to or distant from the H atom. We explain these findings and discuss the implications for nanotubes and magnetic nanographene. Furthermore, we demonstrate that the combined effect of high curvature and a Stone-Wales defect makes H2 chemisorption close to being thermodynamically favorable.     

The effects of electron–phonon interaction on anisotropic RKKY interaction in graphene nanoribbon

We study the Ruderman–Kittle–Kasuya–Yosida (RKKY) interaction in doped armchair graphene nanoribbon. The effects of both external magnetic field and electron-Holstein phonon on RKKY interaction have been addressed. RKKY interaction as a function of distance between localized moments has been analyzed. It has been shown that a magnetic field along the z-axis mediates an anisotropic interaction which corresponds to a XXZ model interaction between two magnetic moments. In order to calculate the exchange interaction along arbitrary direction between two magnetic moments, we should obtain both transverse and longitudinal static spin susceptibilities of armchair graphene nanoribbon in the presence of electron-phonon coupling and magnetic field. The spin susceptibility components are calculated using the spin dependent Green’s function approach for Holstein model Hamiltonian. The effects of spin polarization on the dependence of exchange interaction on distance between moments are investigated via calculating correlation function of spin density operators. Our results show the influences of magnetic field on the spatial behavior of in-plane and longitudinal RKKY interactions are different in the presence of magnetic field.     

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.     

The importance of defects for carbon nanoribbon based electronics

The utilization of graphene nanoribbons for next generation nanoelectronics is commonly expected to depend on the controlled synthesis that yields a low density of defects. Edge roughness and vacancies have been shown to have a large impact on the performance of graphene nanoribbon transistors. In contrast, we show how certain defects can be used to enhance the electronic and magnetic properties of graphene nanoribbons. We explore the properties of hybrid graphene nanoribbons with armchair and zigzag features joined by an array of pentagon–heptagon structural defects. The graphene nanoribbons display an increased density of states at the Fermi level, and half metallicity in absence of external fields. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spin-dependent transport properties of debrominated tetrabromopolyaromatic with Cu or Co doping embedded between zigzag graphene nanoribbon electrodes

Using density functional theory combined with non-equilibrium Green's function method, we investigate the spin-dependent transport properties of debrominated tetrabromopolyaromatic (D-TBPA) molecules embedded between zigzag graphene nanoribbon electrodes, and the effects of copper and cobalt side doping have also been considered. Our results show that the copper doping can insert new energy levels around the Fermi Level and keep spin degeneration of band structure, the cobalt doping can also induce spin splitting. The results on spin transport properties of D-TBPAs embedded into zigzag graphene nanoribbon electrodes show that these systems exist spin filtering and negative differential resistance behaviors. Corresponding physical mechanism on the spin-dependent transport property has been revealed according to the frontier molecular orbital characteristics.