门电压控制的硅烯量子线中电子输运性质

硅烯是由单层硅原子形成的二维蜂窝状晶格结构,具有石墨烯类似的电学性质,由于硅烯中存在比较强的自旋轨道耦合而备受关注.本文利用非平衡格林函数方法研究了门电压控制的硅烯量子线中电子输运性质和能带结构.研究发现,只有在较强的门电压下,而且硅烯量子线具有较好的锯齿形或扶手椅形边界而不存在额外硅原子时,硅烯量子线中才存在无能隙的自旋极化边缘态.另外,计算结果表明这种门电压控制的硅烯量子线中边缘态在每个能谷处自旋是极化的.这些计算结果将为实验上利用电场制作硅烯纳米结构提供理论支持.     

有限尺寸石墨烯的电子态

根据π电子的紧束缚模型, 通过有限系统的Bloch定理方法, 解析计算了有限尺寸石墨烯的电子态和能带. 研究发现, 其电子态有且只有两类, 分别是驻波态和边缘态.驻波态时, 波函数形式是两个方向都是正弦函数; 边缘态时, 波函数形式是Armchair边界的方向是双曲正弦函数, Zigzag边界的方向是正弦函数. 其能带由总碳原子数N个离散的本征值组成, 推导了定量计算边缘态的本征值个数的表达式, 并通过态密度来分析边缘态的存在和与无限大情况的一致性. 所有的分析中数值结果与解析理论都完全一致, 当两个受限方向都变成无限长时, 可以得到与无限大石墨烯相同的结果. 关键词： 紧束缚模型 石墨烯 边缘态 态密度     

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     

Carbon nanoribbons and nanotubes based on δ-graphyne: A first-principles study

As a stable allotropy of two-dimensional (2D) carbon materials, δ-graphyne has been predicted to be superior to graphene in many aspects. Using first-principles calculations, we investigated the electronic properties of carbon nanoribbons (CNRs) and nanotubes (CNTs) formed by δ-graphyne. It is found that the electronic band structures of CNRs depend on the edge structure and the ribbon width. The CNRs with zigzag edges (Z-CNRs) have spin-polarized edge states with ferromagnetic (FM) ordering along each edge and anti-ferromagnetic (AFM) ordering between two edges. The CNRs with armchair edges (A-CNRs), however, are semiconductors with the band gap oscillating with the ribbon width. For the CNTs built by rolling up δ-graphyne with different chirality, the electronic properties are closely related to the chirality of the CNTs. Armchair (n, n) CNTs are metallic while zigzag (n, 0) CNTs are semiconducting or metallic. These interesting properties are quite crucial for applications in δ-graphyne-based nanoscale devices.     

Structural, electronic, and magnetic properties of pristine and oxygen-adsorbed graphene nanoribbons

The structural, electronic and magnetic properties of pristine and oxygen-adsorbed (3,0) zigzag and (6,1) armchair graphene nanoribbons have been investigated theoretically, by employing the ab initio pseudopotential method within the density functional scheme. The zigzag nanoribbon is more stable with antiferromagnetically coupled edges, and is semiconducting. The armchair nanoribbon does not show any preference for magnetic ordering and is semiconducting. The oxygen molecule in its triplet state is adsorbed most stably at the edge of the zigzag nanoribbon. The Stoner metallic behaviour of the ferromagnetic nanoribbons and the Slater insulating (ground state) behaviour of the antiferromagnetic nanoribbons remain intact upon oxygen adsorption. However, the local magnetic moment of the edge carbon atom of the ferromagnetic zigzag ribbon is drastically reduced, due to the formation of a spin-paired C-O bond.     

Band gap engineering in silicene: A theoretical study of density functional tight-binding theory

In this work, we performed first principles calculations based on self-consistent charge density functional tight-binding to investigate different mechanisms of band gap tuning of silicene. We optimized structures of silicene sheet, functionalized silicene with H, CH₃ and F groups and nanoribbons with the edge of zigzag and armchair. Then we calculated electronic properties of silicene, functionalized silicene under uniaxial elastic strain, silicene nanoribbons and silicene under external electrical fields. It is found that the bond length and buckling value for relaxed silicene is agreeable with experimental and other theoretical values. Our results show that the band gap opens by functionalization of silicene. Also, we found that the direct band gap at K point for silicene changed to the direct band gap at the gamma point. Also, the functionalized silicene band gap decrease with increasing of the strain. For all sizes of the zigzag silicene nanoribbons, the band gap is near zero, while an oscillating decay occurs for the band gap of the armchair nanoribbons with increasing the nanoribbons width. At finally, it can be seen that the external electric field can open the band gap of silicene. We found that by increasing the electric field magnitude the band gap increases.     

单空位缺陷对石墨纳米带电子结构和输运性质的影响

基于第一性原理电子结构和输运性质计算,研究了单空位缺陷对单层石墨纳米带(包括zigzag型和armchair型带)电子性质的影响.研究发现,单空位缺陷使石墨纳米带在费米面上出现一平直的缺陷态能带;单空位缺陷的引入使zigzag型半导体性的石墨纳米带变为金属性,这在能带工程中有重要的应用价值;奇数宽度的armchair型石墨纳米带表现出金属特性,有着很好的导电性能,同时,偶数宽度的armchair型石墨带虽有金属性的能带结构,但却有类似半导体的伏安特性;单空位缺陷使得奇数宽度的armchair石墨纳米带导电 关键词： 石墨纳米带 单空位缺陷 电子结构 输运性质     

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.     

Energy gap modulation of graphene nanoribbons by F termination

Under the generalized gradient approximation (GGA), the electronic properties are studied for the F-terminated graphene nanoribbons (GNRs) with either zigzag edge (ZGNRs) or armchair edge (AGNRs) by using the first-principles projector augmented wave potential within the density function theory (DFT) framework. The results show that an edge state appears at the Fermi level E_F in the broader F-terminated ZGNRs, but does not appear in all the F-terminated AGNRs due to their dimerized C-C bonds at edge. The density of states (DOS) and projected DOS (PDOS) analyses show that the F-terminated ZGNRs are metallic and have a sharp peak at the Fermi level when the width is large enough. In contrast, the AGNRs are always semiconductors independent of their width. The charge density contours analyses shows that the C-F bond is an ionic bond due to a much stronger electronegativity of the F atom than that of the C atom. However, all kinds of the C-C bonds display a typical nonpolar covalent bonding feature.     

Edge states of epitaxially grown graphene on 4H-SiC(0001) studied by scanning tunneling microscopy

The edge properties of single layer graphene epitaxially grown on partially graphitized 4H-SiC(0001) surface have been investigated with scanning tunneling microscopy (STM). We directly observed the atomic-structure dependency of the super structures in the vicinity of armchair and zigzag edges due to the different kinds of symmetry-breaking at those two edges.     

First-principles study on structural and electronic properties of AlNSix heterosheet

Under generalized gradient approximation (GGA), the structural and electronic properties of AlN and Si sheets, hydrogen terminated AlN and Si nanoribbons with hexagonal morphology and 2, 4, 6 zigzag chains across the ribbon width and the hexagonally bonded heterosheets AlNSi_x (x=2, 4, and 6) consisting of hexagonal networks of AlN (h-AlN) strips and silicene sheets with zigzag shaped borders have been investigated using the first-principles projector-augmented wave (PAW) formalism within the density function theory (DFT) framework. The AlN sheet is an indirect semiconductor with a band gap of 2.56 eV, while the Si sheet has a metallic character since the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) meet at one k point from Γ to Z. In the semiconductor 6-ZAlNNR, for example, the states of LUCB and HOVB at zone boundary Z are edge states whose charges are localized at edge Al and N atoms, respectively. In metallic 6-ZSiNR, a flat edge state is formed at the Fermi level E_F near the zone boundary Z because its charges are localized at edge Si atoms. The hybridizations between the edge states of h-AlN strips and silicene sheets result in the appearance of border states in the zigzag borders of heterosheets AlNSi_x whose charges are localized at two atoms of the borders with either bonding or antibonding π character.     

Characteristics of Li diffusion on silicene and zigzag nanoribbon

We perform a density functional study on the adsorption and diffusion of Li atoms on silicene sheet and zigzag nanoribbons. Our results show that the diffusion energy barrier of Li adatoms on silicene sheet is 0.25 eV, which is much lower than on graphene and Si bulk. The diffusion barriers along the axis of zigzag silicene nanoribbon range from 0.1 to0.25 eV due to an edge effect, while the diffusion energy barrier is about 0.5 eV for a Li adatom to enter into a silicene nanoribbon. Our calculations indicate that using silicene nanoribbons as anodes is favorable for a Li-ion battery.     

Spin-polarized transport in multiterminal silicene nanodevices

The spin-polarized transport properties of multiterminal silicene nanodevices are studied using the tight binding model and Landauer–Buttier approach. We propose a four-terminal 2-shaped junction device and two types of three-terminal T-shaped junction devices, which are made of the crossing of a zigzag and an armchair silicene nanoribbon. If the electrons are injected into the metallic lead, the near-perfect spin polarization with 100% around the Fermi energy can be achieved easily at the other semiconducting leads. Thus the multiterminal silicene nanodevices can act as controllable spin filters.     

Shot noise fluctuations in disordered graphene nanoribbons near the Dirac point

Random fluctuations of the shot-noise power in disordered graphene nanoribbons are studied. In particular, we calculate the distribution of the shot noise of nanoribbons with zigzag and armchair edge terminations. We show that the shot noise statistics is different for each type of these two graphene structures, which is a consequence of the presence of different electron localizations: while in zigzag nanoribbons electronic edge states are Anderson localized, in armchair nanoribbons edge states are absent, but electrons are anomalously localized. Our analytical results are verified by tight binding numerical simulations with random hopping elements, i.e., off diagonal disorder, which preserves the symmetry of the graphene sublattices.     

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.     

Reprint of : Shot noise fluctuations in disordered graphene nanoribbons near the Dirac point

Random fluctuations of the shot-noise power in disordered graphene nanoribbons are studied. In particular, we calculate the distribution of the shot noise of nanoribbons with zigzag and armchair edge terminations. We show that the shot noise statistics is different for each type of these two graphene structures, which is a consequence of the presence of different electron localizations: while in zigzag nanoribbons electronic edge states are Anderson localized, in armchair nanoribbons edge states are absent, but electrons are anomalously localized. Our analytical results are verified by tight binding numerical simulations with random hopping elements, i.e., off diagonal disorder, which preserves the symmetry of the graphene sublattices.     

Transport properties of AA-stacking bilayer graphene nanoribbons

The transport properties of AA-stacking bilayer graphene nanoribbons (GNs) have been explored by using the nonequilibrium Green's function method and the Landauer–Büttiker formalism. It is found that in the case of zero bias, the interlayer coupling has pronounced effects on the conductance of bilayer GNs. The zigzag bilayer GNs remain metallic, but metallic armchair bilayer GNs will be semiconductor as the strength of interlayer coupling exceeds critical value. The first Van Hove singularities move close to the Dirac point for both armchair and zigzag bilayer GNs with the strength of interlayer coupling increasing. Some prominent conductance peaks around the Fermi energy are observed in zigzag bilayer GNs, when the top layer and bottom layer have different widths. In the presence of bias voltage, the I–V curves show that for armchair bilayer GNs, the interlayer interactions suppress current, while the interlayer interactions have almost no effect on the current for zigzag bilayer GNs. The ripples in bilayer GNs suppress electronic transport, especially for zigzag bilayer GNs.     

单壁碳纳米管能带及其电子特性研究

从能带理论出发,采用电子紧束缚能量色散关系,推导锯齿,扶手椅和手性单壁碳纳米管(SWCNT)的电子能带结构表达式,指出单壁碳纳米管或为金属或为半导体的判据。结果表示:单壁碳纳米管的电子结构与其几何结构密切相关,如扶手椅型单壁碳纳米管是金属性的,而对其它类型的单壁碳纳米管是与碳纳米管的手性指数有关,只有手性指数n和m的差别等于3的倍数时,单壁碳纳米管是金属性的,否则会显出有带隙的半导体特性。这意味着单壁碳纳米管是由特殊的电子传输和光学性质,在纳米电子学领域具有巨大的潜在应用价值。     

石墨烯-六方氮化硼面内异质结构的扫描隧道显微学研究

石墨烯-六方氮化硼面内异质结构因可调控石墨烯的能带结构而受到广泛关注. 本文介绍了在超高真空体系内, 利用两步生长法在两类对石墨烯分别有强和弱电子掺杂的基底, 即Rh(111)和Ir(111)上制备石墨烯-六方氮化硼单原子层异质结构. 通过扫描隧道显微镜及扫描隧道谱对这两种材料的形貌和电子结构进行研究发现: 石墨烯和六方氮化硼倾向于拼接生长形成单层的异质结构, 而非形成各自分立的畴区; 在拼接边界处, 石墨烯和六方氮化硼原子结构连续无缺陷; 拼接边界多为锯齿形型, 该实验结果与密度泛函理论计算结果相符合; 拼接界面处的石墨烯和六方氮化硼分别具有各自本征的电子结构, 六方氮化硼对石墨烯未产生电子掺杂效应.     

Self-passivating edge reconstructions of graphene

Planar reconstruction patterns at the zigzag and armchair edges of graphene were investigated with density-functional theory. It was unexpectedly found that the zigzag edge is metastable and a planar reconstruction spontaneously takes place at room temperature. The reconstruction changes electronic structure and self-passivates the edge with respect to adsorption of atomic hydrogen from a molecular atmosphere.