拓扑缺陷的不同分布对单壁碳纳米管电学性能的影响

在紧束缚近似基础上,利用扩展的Su-Schriffer-Heeger(SSH)模型,在实空间研究了在完整的"zigzag"碳纳米管中分别引入5/7,5/6/7,5/6/6/7拓扑缺陷所构成的(9,0)-(8,0),(9,0)-(7,0)和(9,0)-(6,0)三种异质结的电学性能.通过研究表明:这些拓扑缺陷不仅改变碳管的直径,而且支配费米能级附近的电学行为.并计算了(9,0)-(8,0),(9,0)-(7,0)和(9,0)-(6,0)系统的电子态密度,对这3种异质结的能带结构和电子态密度进行了比较.结果表明:五边形和七边形在碳管中分布的不同对碳管电学性能的影响明显不同.因此,可以研制出基于这些异质结的不同的电子器件基元.     

拓扑缺陷对单壁碳纳米管电子结构及其光学光谱的影响

应用密度泛函理论计算了半导体型单壁碳纳米管(7,0)和(8,0)以及其发生镜像对称和非镜像对称Stone-Wales形变、形成异质结(7,0)—(8,0)情况下的能带结构、吸收光谱、反射光谱,并对计算结果进行了比较。研究发现： 引入拓扑缺陷态后,碳纳米管的能带结构发生了明显的变化,费米能级在不同缺陷情况下移动方向不一致;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;在光子能量约为E=13 eV处各碳管的吸收谱和反射谱中均出现一特征峰,并且在引入缺陷以后该特征峰向高能区移动。文章对计算结果进行了分析和探讨,可望利用这种拓扑缺陷的引入而产生的光电特性来设计碳管光电器件。     

Vacancy cluster-limited electronic transport in metallic carbon nanotube

We investigate the electronic properties of metallic (7,7) carbon nanotubes (CNT) in the presence of a variety of tetra- and hexa-vacancy defects, by using the first principles density functional theory (DFT) combined with the non-equilibrium Green’s function technique. From the view point of energetic stability large vacancies tend to split into pentagon and heptagon (5-7) defects. However, this does not preclude the presence of “holes” in the carbon nanotube by the nanoelectronic lithography technique. We show that the states linked to large vacancies hybridize with the extended states of the nanotubes to modify their band structure. As a consequence, the hole-like defects in the CNT lead to more prominent electronic transport compared to the situation in the defective CNT consisting of pentagon-heptagon pair defects. Our study suggests the possibility to improve the electronic properties of a defective carbon nanotube via morphological modifications induced by irradiation techniques.     

Curvature-induced metallization of double-walled semiconducting zigzag carbon nanotubes

We report total-energy electronic-structure calculations that provide energetics and electronic structures of double-walled carbon nanotubes consisting of semiconducting (n,0) nanotubes. We find that optimum spacing between the walls of the nanotubes is slightly larger than the interlayer spacing of the graphite. We also find that the electronic structures of the double-walled nanotubes with the inner (7,0) nanotube are metallic with multicarrier characters in which electrons and holes exist on inner and outer nanotubes, respectively. Interwall spacing and curvature difference are found to be essential for the electron states around the Fermi level.     

Effect of carbon network defects on the electronic structure of semiconductor single-wall carbon nanotubes

For a single-wall (14, 0) carbon nanotube, the total density of electronic states of the ideal structure and of some possible defect structures is calculated in the framework of the band theory approach using Gaussian-type orbitals and the approximation of the generalized density gradient. It is shown that allowance for defects of the atomic structure of a nanotube makes it possible to adequately describe the existing experimental data on nanotube electronic structure. In the framework of the same approach, the total density of electronic states is calculated for an intermolecular contact of (5, 5) and (10, 0) single-wall carbon nanotubes formed due to the creation of a 5–7 defect. It is shown that the electronic states related to the contact region and the 5–7 defect lie in vicinity of the Fermi level.     

Coherent transport through intramolecular junction of single-wall carbon nanotubes

We have constructed four types single-wall carbon nanotube intramolecular junctions (IMJs) of (5,5)/(8,0), (5,5)/(10,0), (5,5)/(9,0)A, and (5,5)/(9,0)B along a common axis, and calculated their electronic and transport properties using a tight binding-based Green's function approach that is particular suitable for realistic calculation of electronic transport property in extended system. Our results show that quasi-localized states can appear in the metal/semiconductor heterojunctions ((5,5)/(8,0) and (5,5)/(10,0)junctions), which is desirable for the design of a quantum device; and the conductance of M-M IMJs is very sensitive to the connectivity of the matching tubes, certain configurations of connection completely stop the flow of electron, while others permit the transmission of the current through the interface. These results may have implications for the device assembly and manipulation process of all carbon nanotubes-based microelectronic elements. Received 14 January 2003 / Received in final form 25 February 2003 Published online 4 June 2003 RID="a" ID="a"e-mail: lfyzz@yahoo.com.cn     

Bonding and energy dissipation in a nanohook assembly

Combining total energy and molecular dynamics calculations, we explore the suitability of nanotube-based hooks for bonding. Our results indicate that a large force of 3.0 nN is required to disengage two hooks, which are formed by the insertion of pentagon-heptagon pairs in a (7,0) carbon nanotube. Nanohooks based on various nanotubes are resilient and keep their structural integrity during the opening process. Arrays of hooks, which are permanently anchored in solid surfaces, are a nanoscale counterpart of velcro fasteners, forming tough bonds with a capability of self-repair.     

Role of Stone–Wales defects on the functionalization of (8,0) single wall carbon nanotubes by the amine group: Ab initio study

Using first-principle calculations, we have investigated the chemical functionalization of (8,0) zigzag single wall carbon nanotubes (SWNTs) by the amine group on Stone–Wales (SW) defects. The binding of NH₂ with the defective (8,0) nanotube was explored and the preferential grafting sites have been identified. On the other hand, the modifications induced by SW defect and functional groups in the structural and electronic properties of (8,0) SWNT have also been investigated. The role of SW defects in the chemical reactivity of carbon nanotubes was well identified.     

Hexagonal silicon nanotube confined inside a carbon nanotube: A first-principles study

We studied the stability, geometrical structures and electronic energy band of hexagonal silicon nanotube (SiNT) confined inside carbon nanotubes based on first-principle calculations. The results show that the encapsulating process of SiNT is exothermic in (9,9) carbon nanotube while endothermic in (8,8) and (7,7) carbon nanotubes. When the SiNT is inserted into (9,9) carbon nanotube, the insertion energy is about 0.09 eV. Energy band of SiNT@(9,9) nanotube is not distorted greatly compared with the superposition of bands of isolated SiNT and (9,9) carbon nanotube. Especially, a parabolic band occurs near the Fermi level of energy band in SiNT@(7,7) nanotube. Such a band could be a nearly free electronic state originating from carbon nanotube. Moreover, we discuss the variation of total energy as the SiNT rotates around its axis inside carbon nanotubes.     

接枝羧基的有限长碳纳米管电子结构的第一性原理研究

基于局域密度泛函理论，采用第一性原理方法，建立了对(5,5)型和(9,0)型有限长碳纳米管接枝羧基官能团的原子模型，通过计算其电子分布和态密度的变化，讨论羧基官能团对碳纳米管电子结构和电子输运特性的影响. 计算表明，接枝羧基的碳纳米管,其电子结构明显改变，其费米能级上的电子态密度下降；最高占据轨道上的非定域程度减弱,致使电子输运性能呈下降趋势. 关键词： 碳纳米管 密度泛函理论 电子结构     

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     

Theory of B(2)O and BeB(2) nanotubes: new semiconductors and metals in one dimension

We describe two new boron-based nanotubes: B(2)O and BeB(2). Both are isoelectronic to graphite, have reasonable curvature energies, and have already been made in their bulk planar forms. The lowest energy allotrope of planar single-layer B(2)O is a semiconductor with a moderate band gap. The local density approximation band gap of the corresponding (3,0) B(2)O nanotube [similar in size to (9,0) carbon nanotube tube] is direct and around 1.6 eV, within a range inaccessible to previous C or BN nanotubes. Single-layer BeB(2) has a fascinating structure: the Be atoms rest above the boron hexagonal faces, nearly coplanar to the boron sheet. The unusual K-point pi-pi(*) Fermi-level degeneracy of graphite survives, while a new nearly pointlike Fermi surface appears at the M point. As a result, BeB(2) nanotubes are uniformly metallic.     

Direct observation of localized defect States in semiconductor nanotube junctions

We report scanning tunneling microscopy of semiconductor-semiconductor carbon nanotube junctions with different band gaps. Characteristic features of the wave functions at different energy levels, such as a localized defect state, are clearly exhibited in the atomically resolved scanning tunneling spectroscopy. The peaks of the Van Hove singularity on each side penetrate and decay into the opposite side across the junction over a distance of approximately 2 nm. These experimental features are accounted for, with the help of tight-binding calculation, by the presence of pentagon-heptagon pair defects at the junction.     

管长和管径对单壁碳纳米管电导的影响

基于紧束缚模型，发展转移矩阵方法研究了单壁碳纳米管的导电性质.研究表明，由于卷曲效应，锯齿型(3k,0)管(k为整数)出现窄的电导沟，其大小与能隙一致.在费米能附近，电子输运不仅与管径和管长紧密相关，而且电子在不同能量下可能出现弹道的、扩散的和经典的三种不同输运特征. 关键词： 碳纳米管 转移矩阵 电导     

First-principles calculations of carbon nanotubes adsorbed on Si(001)

We report first-principles calculations on the adsorption of a metallic (6,6) single-walled carbon nanotube (SWCN) on the Si(001) surface. We find stable geometries for the nanotube between two consecutive dimer rows where C-Si chemical bonds are formed. The binding energy in the most stable geometry is found to be 0.2 eV/A. Concerning the electronic properties, the most stable structure shows an increase in the density of states near the Fermi level due to the formation of C-Si bonds enhancing the metallic character of the nanotube by the contact with the surface. These properties may lead one to consider metallic SWCNs adsorbed on Si substrates for interconnections and contacts on future nanoscale devices. Finally, the nature of the nanotube-surface interaction for nanotubes of larger diameters is also discussed.     

Electronic Transport Properties of （7,0） Semiconducting Carbon Nanotube

Electronic transport properties of a finite （7,0） carbon nanotube （CNT） coupled to Au （111） surfaces are investigated with a fully nonequilibrium Green＇s functions method combined with the density functional theory. The results show that the coupling effect between the CNT and Au electrode plays an important role in the transport properties, which leads to the formation of a high plateau in the transmission spectrum around Fermi energy. In addition, the current-voltage characteristic of the （7,0） CNT coupled to Au electrodes is different from an isolated （7,0） CNT.     

First-principle study of interaction of molecular hydrogen with BC<Subscript>3</Subscript> composite single-walled nanotube

The physisorption of molecular hydrogen in BC₃ composite single-walled nanotube, investigated using density functional theory, was compared with single-walled carbon nanotube. Both external and internal adsorption sites of these two nanotubes have been studied with the hydrogen molecular axis oriented parallel to the nanotube wall. The calculated results show that: ([see full textsee full text]) the physisorption energies of a H₂ molecule are larger for BC₃(8,0) composite nanotube than for C(8,0) nanotube at all adsorption sites examined. ([see full textsee full text]) For these two nanotubes, the physisorption energies are larger for hydrogen bound inside the nanotubes than for adsorption outside the nanotubes. The different behavior between these two nanotubes is explained by the contour plots of electron density and charge-density difference of them. The present computations suggest that BC₃ nanotube may be a better candidate for hydrogen storage than carbon nanotube.     

Designing different nanomagnets via delocalization of cobalt magnetic moment inside narrow single walled carbon nanotubes

Density functional theory calculations were employed to study the effects of chirality and diameter of single walled carbon nanotubes (SWCNTs) on electronic, structural and magnetic properties of cobalt-doped (9,0), (5,5) and (5,0) nanotube systems. The (9,0) and (5,5) SWCNTs have similar diameters but different chiralities, whereas the (5,0) tube has a very small diameter. The Co-SWCNT systems are considered with four different possible arrangements, three of which are stable and only substitution of the Co with one of the carbon atoms on the surface of the SWCNTs is an exemption. Although the quasi-metallic band gap of the (9,0) SWCNT is eliminated by the cobalt doping process, metallic features of the (5,5) and (5,0) nanotubes remain unchanged. On the other hand, delocalization of the cobalt’s magnetization and inducement of a noticeable magnetization to the tubes provide a vast area of possible total magnetizations for the Co-SWCNT systems. The results are applicable to spintronics and useful for designing other nanomagnetic systems.     

锯齿型单壁碳纳米管能隙的第一性原理研究

本文采用基于广义梯度近似(GGA)的第一性原理方法,对锯齿型单壁碳纳米管(7,0)、(8,0)、(9,0)、(12,0)的能带结构和能隙进行理论计算.结果表明,各孤立管能隙值跟紧束缚(TB)近似所推出的结论有很大的出入,但跟实验观测结果比较接近.本文对引起这一差异的原因做了初步探讨.     

Electronic transport properties of junctions between carbon nanotubes and graphene nanoribbons

Using the tight-binding model and Green’s function method, we studied the electronic transport of four kinds of nanotube-graphene junctions. The results show the transport properties depend on both types of the carbon nanotube and graphene nanoribbon, metal or semiconducting. Moreover, the defect at the nanotube-graphene interface did not affect the conductance of the whole system at the Fermi level. In the double junction of nanotube/nanoribbon/nanotube, quasibound states are found, which cause antiresonance and result in conductance dips.