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.     

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.     

石墨烯纳米带量子点中的量子混沌现象

在20 mK的极低温下测量了石墨烯纳米带量子点的电子输运性质,观测到清晰的库仑阻塞菱形块和对应量子点激发态的电导峰.对库仑阻塞近邻电导峰间距和峰值进行了统计分析,发现其统计分布分别满足无规矩阵理论描述的Wigner-Dyson分布和Porter-Thomas分布,说明石墨烯纳米带量子点在低温下出现了量子混沌现象.还讨论了这种长方形量子点中量子混沌的可能成因. 关键词： 石墨烯纳米带 量子点 库仑阻塞 量子混沌     

Yb2.75C60价带光电子能谱

用角积分紫外光电子能谱技术测量了Yb2 75C6 0 薄膜的价带电子态密度分布 .相纯Yb2 75C6 0 样品通过C1s芯态x射线电子谱峰的位移表征 .结果表明Yb2 75C6 0 是半导体 ,在费米能级处几乎没有电子态分布 .Yb 6s电子态和C6 0 LU MO能带的杂化效应不可忽略 ,有部分Yb 6s电子分布在Yb C6 0 杂化能带上 .     

边缘修饰的石墨烯纳米带电子特性的第一性原理研究

本文采用第一性原理计算方法,研究了zigzag型石墨烯纳米带在边缘采用不同基团（包括氢原子、羟基、酮基、氢和羟基共同饱和）进行修饰后电子特性的改变,计算了能带结构、态密度和电荷差分密度。结果分析表明,不同基团修饰的影响本质上可归结于不同的边缘杂化方式。边缘sp2杂化方式对GNRs体系内层原子的电子状态影响很小,没有改变zigzag-GNRs的金属性;而边缘sp3杂化的体系在能带结构中打开了一个带隙,此带隙随纳米带宽度的增加而逐渐减小。其中GNRs-H2 体系和GNRs-H2O体系发生了由金属性向半导体性的转变,而GNRs-O体系费米能级升高并且进入了导带,依然呈现金属性。利用这种边缘修饰非常易于调控GNRs的电子能带结构。     

边缘修饰的石墨烯纳米带电子特性的第一性原理研究

本文采用第一性原理计算方法,研究了zigzag型石墨烯纳米带在边缘采用不同基团（包括氢原子、羟基、酮基、氢和羟基共同饱和）进行修饰后电子特性的改变,计算了能带结构、态密度和电荷差分密度。结果分析表明,不同基团修饰的影响本质上可归结于不同的边缘杂化方式。边缘sp2杂化方式对GNRs体系内层原子的电子状态影响很小,没有改变zigzag-GNRs的金属性;而边缘sp3杂化的体系在能带结构中打开了一个带隙,此带隙随纳米带宽度的增加而逐渐减小。其中GNRs-H2 体系和GNRs-H2O体系发生了由金属性向半导体性的转变,而GNRs-O体系费米能级升高并且进入了导带,依然呈现金属性。利用这种边缘修饰非常易于调控GNRs的电子能带结构。     

石墨烯纳米网电导特性的能带机理：第一原理计算

通过第一原理电子结构计算来研究有序多孔纳米网的电导特性变化的能带机理.能带结构分析结果表明:石墨烯纳米网超晶格(3m,3n)(m和n为整数)的电子本征态在布里渊区中心点发生四重简并;碳空位孔洞规则排列形成的石墨烯纳米网具有由简并态分裂形成的宽度可调带隙,无论石墨烯的两个子晶格是否对等.在具有磁性网孔阵列的石墨烯纳米网中,反铁磁耦合使对称子晶格的反演对称性增加了一项量子限制条件,导致能带结构在K点的二重简并态分裂成带隙.通过控制网孔密度能够有效调节石墨烯纳米网的带隙宽度,为实现新一代石墨烯纳米电子器件提供了理论依据.     

Rb吸附石墨烯纳米带电子性质和光学性质的研究

本文基于密度泛函理论的第一性原理方法了计算了Rb、O和H吸附石墨烯纳米带的差分电荷密度、能带结构、分波态密度和介电函数,调制了石墨烯纳米带的电子性质和光学性质,给出了不同杂质影响材料光学特性的规律.结果表明本征石墨烯纳米带为n型直接带隙半导体且带隙值为0.639 eV;Rb原子吸附石墨烯纳米带之后变为n型简并直接带隙半导体,带隙值为0.494eV;Rb和O吸附石墨烯纳米带变为p型简并直接带隙半导体,带隙值增加为0.996eV;增加H吸附石墨烯纳米带后,半导体类型变为n型直接带隙半导体,且带隙变为0.299eV,带隙值相对减小,更有利于半导体发光器件制备.吸附Rb、O和H原子后,石墨烯纳米带中电荷密度发生转移,导致C、Rb、O和H之间成键作用显著.吸附Rb之后,在费米能级附近由C-2p、Rb-5s贡献;增加O原子吸附之后,O-2p在费米能级附近贡献非常活跃,杂化效应使费米能级分裂出一条能带;再增加H原子吸附之后,Rb-4p贡献发生蓝移,O-2p在费米能级附近贡献非常强,费米能级分裂出两条能带.Rb、O和H的吸附后,明显调制了石墨烯纳米带的光学性质.     

外加横向电场作用下石墨烯纳米带电子结构的密度泛函紧束缚计算

采用基于密度泛函理论的紧束缚方法计算研究了外加横向电场对边缘未加氢/加氢钝化的扶手椅型石墨烯纳米带的电子结构及电子布居数的影响.计算结果表明,石墨烯纳米带的能隙变化受其宽带影响.当施加沿其宽度方向的横向外加电场时,纳米带的能带结构及态密度都会产生较大的变化.对于具有半导体性的边缘未加氢纳米带,随着所施加电场强度的增加,会发生半导体-金属的转变.同时,电场也会对能级分布产生显著影响.外加电场导致纳米带内原子上电子布居数分布失去对称性,电场强度越大,其布居数不对称性越明显.边缘加氢钝化可以显著改变纳米带内原子上的布居数分布.     

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.     

Effect of substitutional impurities on the electronic transport properties of graphene

Density-functional theory in combination with the nonequilibrium Green's function formalism is used to study the effect of substitutional doping on the electronic transport properties of hydrogen passivated zig-zag graphene nanoribbon devices. B, N and Si atoms are used to substitute carbon atoms located at the center or at the edge of the sample. We found that Si-doping results in better electronic transport as compared to the other substitutions. The transmission spectrum also depends on the location of the substitutional dopants: for single atom doping the largest transmission is obtained for edge substitutions, whereas substitutions in the middle of the sample give larger transmission for double carbon substitutions. The obtained results are explained in terms of electron localization in the system due to the presence of impurities.     

边界对石墨烯量子点非线性光学性质的影响

石墨烯作为一种新型非线性光学材料,在光子学领域具有重要的应用前景,引起研究人员的极大兴趣.本文运用量子化学计算方法研究了边界引入碳碳双键(C=C)和掺杂环硼氮烷(B₃N₃)环对石墨烯量子点非线性光学性质和紫外-可见吸收光谱的影响.研究发现,扶手椅边界上引入C=C双键后,六角形石墨烯量子点分子结构对称性降低,电荷分布对称性发生破缺,导致分子二阶非线性光学活性增强.石墨烯量子点在从扶手椅型边界向锯齿型边界过渡的过程中,随着边界C=C双键数目的增加,六角形石墨烯量子点和B3N3掺杂六角形石墨烯量子点的极化率和第二超极化率分别呈线性增加.此外,边界对石墨烯量子点的吸收光谱也有重要影响.无论是石墨烯量子点还是B₃N₃掺杂石墨烯量子点,扶手椅型边界上引入C=C双键导致最高占据分子轨道能级升高,最低未占分子轨道能级的降低,前线分子轨道能级差减小,因而最大吸收波长发生了红移.中心掺杂B₃N₃环后会增大石墨烯量子点的分子前线轨道能级差,导致B3N3掺杂后的石墨烯量子点紫外-可见吸收光谱发生蓝移.本文研究为边界修饰调控石墨烯量子点非线性光学响应提供了一定的理论指导.     

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.     

Transport properties in a line defect superlattice of graphene

It was recently reported that a kind of graphene line defect can be fabricated in a controllable experimental way. In the present work we theoretically investigate the band structure and the electronic transport properties of a graphene superlattice formed by embedding periodically line defects in the graphene lattice. Based on the calculated results, we suggest that such a superlattice can be used as a quantum wire array which can carry much larger current than a single graphene nanoribbon. A remarkable advantage of this superlattice over other quantum wires is that the electronic transport in it is insensitive to scattering effects except that the scattering potential range is smaller than the graphene lattice constant. Moreover, we find that the anisotropy of the Dirac cone presented in this superlattice has a nontrivial influence on the universal minimal conductivity and the sub-Poissonian shot noise of graphene.     

Modeling of lightly-doped drain and source contact with boron and nitrogen in graphene nanoribbon

Graphene, a monolayer carbon atoms arranged in hexagonal honeycomb lattice possesses impressive electronic properties. It is utilized as channel, source and drain contact in graphene nanoribbon field-effect transistor (GNRFET). Zigzag graphene nanoribbon (ZGNR) is used as semi-metallic drain and source terminal to a pristine armchair graphene nanoribbon (AGNR) that acts as a semiconducting channel. In addition, a single dopant, either nitrogen or boron is added to create lightly-doped drain and source contact. The electronic properties of graphene nanoribbon (GNR) with lightly-doped drain and source contacts are obtained from tight-binding approach. With self-energy matrices, the lightly-doped contacts Hamiltonian matrices are combined with the pristine channel Hamiltonian matrix. The density of states (DOS) are simulated based on the non-equilibrium Green's Function (NEGF) formalism. Our findings are then compared with published research work. Furthermore, it is demonstrated that the DOS of the overall GNR structure still retain a small band gap and possess semiconducting properties when the channel is connected to semi-metallic contact at the drain and source terminal.     

Graphene for Thermoelectric Applications: Prospects and Challenges

Thermoelectric power generators require high-efficiency thermoelectric materials to transform waste heat into usable electrical energy. An efficient thermoelectric material should have high Seebeck coefficient and excellent electrical conductivity as well as low thermal conductivity. Graphene, the first truly 2D nanomaterial, exhibits unique properties which suit it for use in thermoelectric power generators, but its application in thermoelectrics is limited by the high thermal conductivity and low Seebeck coefficient resulting from its gapless spectrum. However, with the possibility of modification of graphene's band structure to enhance Seebeck coefficient and the reduction of its thermal conductivity, it is an exciting prospect for application in thermoelectric power generation. This article examines the electronic, optical, thermal, and thermoelectric properties of graphene systems. The factors that contribute to these material properties in graphene systems like charge carriers scattering mechanisms are discussed. A salient aspect of this article is a synergistic perspective on the reduction of thermal conductivity and improvement of Seebeck coefficient of graphene for a higher thermoelectric energy conversion efficiency. In this regard, the effect of graphene nanostructuring and doping, forming of structural defects, as well as graphene integration into a polymer matrix on its thermal conductivity and Seebeck coefficient is elucidated.     

Electronic and transport properties of boron-doped graphene nanoribbons

We report a spin polarized density functional theory study of the electronic and transport properties of graphene nanoribbons doped with boron atoms. We considered hydrogen terminated graphene (nano)ribbons with width up to 3.2 nm. The substitutional boron atoms at the nanoribbon edges (sites of lower energy) suppress the metallic bands near the Fermi level, giving rise to a semiconducting system. These substitutional boron atoms act as scattering centers for the electronic transport along the nanoribbons. We find that the electronic scattering process is spin-anisotropic; namely, the spin-down (up) transmittance channels are weakly (strongly) reduced by the presence of boron atoms. Such anisotropic character can be controlled by the width of the nanoribbon; thus, the spin-up and spin-down transmittance can be tuned along the boron-doped nanoribbons.     

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.     

氢化与非氢化石墨烯纳米条带的密度泛函研究

基于密度泛函理论的第一性原理计算方法,研究了宽度N=8的边缘氢化和非氢化条带的结构和电子性质. 研究表明,扶手形无氢化石墨纳米条带的边缘碳原子是以三重键相互结合,它在边缘的成键强度比氢化时要高,具有更强的化学活性,可作为纳米化学传感器的基础材料. 能带结构计算表明,无论是扶手形条带还是锯齿形条带,它们都是具有带隙的半导体,且无氢化条带的带隙要比氢化的条带带隙宽度大,氢化对于条带的电子性质具有显著修饰作用. 通过锯齿形石墨纳米条带顺磁性、铁磁性和反铁磁性的计算,发现反铁磁的状态最稳定,并且边缘磁性最强,这有利于条带在自旋电子器件中的应用. 关键词： 石墨纳米条带 成键机理 电子结构 自旋分布