Gas-phase synthesis of nitrogen-containing carbon nanotubes and their electronic properties

Nitrogen-containing carbon nanotubes are synthesized using a gas-phase reaction. The synthesis of nitrogen-doped carbon nanotubes from 100 to 500 Å in diameter is accomplished through pyrolysis of acetonitrile (CH₃CN) at a temperature of 800°C. Cobalt and nickel metallic particles formed upon thermal decomposition of a mixture of maleate salts are used as catalysts. The materials synthesized are investigated by scanning and transmission electron microscopy. Analysis of the x-ray photoelectron spectra demonstrates that the content of nitrogen atoms in three nonequivalent charge states is approximately equal to 3%. A comparison of the CK _α x-ray fluorescence spectrum of the carbon nanotubes synthesized through electric-arc evaporation of graphite and the x-ray fluorescence spectrum of the nitrogen-containing carbon nanotubes prepared by catalytic decomposition of acetonitrile indicates that, in the latter case, the spectrum contains a certain contribution from the sp ³ states of carbon atoms. The temperature dependences of the electrical conductivity for different types of multi-walled carbon nanotubes are compared. The difference observed in the temperature dependences of the electrical conductivity is associated with the presence of additional scattering centers in nitrogen-containing carbon nanotubes.     

Ni原子链填充碳纳米管的能量、电子结构和磁性的第一性原理计算

在广义梯度近似(GGA)下,利用密度泛函理论(DFT)框架下的第一性原理投影缀加波(PAW)赝势方法,研究了单根Ni原子链填充扶手椅型(n,n)(5≤n≤9)单壁碳纳米管的能量、电子结构和磁性.结果表明(5,5)碳纳米管直径过小排斥Ni原子链的插入,(6,6)碳纳米管是容纳Ni原子链的最小碳纳米管,特别是Ni原子链位于其中心轴线上时的形成能最低.以Ni@(6,6)和Ni@(7,7)系统为例,计算并分析了其自旋极化能带结构,电子总态密度,分波态密度和磁性,发现Ni原子的3d态电子 关键词： Ni原子链 碳纳米管 电子结构 磁性能     

Effects of oxygen-containing defect complex on the electronic structures and transport properties of single-walled carbon nanotubes

The electronic structures and transport properties of (10,0)$(10,0)$ single-walled carbon nanotube ((10,0)$(10,0)$ (SWNT)) with oxygen-containing defect complex are investigated using density functional theory in combination with nonequilibrium Green?s function method. The complex delocalizes the local states of (10,0)$(10,0)$ SWNT induced by mono- and di-vacancy but strengthens the localization of the states induced by the Stone–Wales defect. As a result, the complex partially restores the transport properties of (10,0)$(10,0)$ SWNT with vacancies, but reduces the transmission of (10,0)$(10,0)$ SWNT with Stone–Wales defect. However, the oxygen-containing defect complex only slightly influences the transmission gap and threshold voltage of the system.     

Imaging the elastic properties of coiled carbon nanotubes with atomic force microscopy

Coiled carbon nanotubes were produced catalytically by thermal decomposition of hydrocarbon gas. After deposition on a silicon substrate, the three-dimensional structure of the helix-shaped multiwalled nanotubes can be visualized with atomic force microscopy. Helical structures of both chiralities are present in the nanotube deposits. For larger coil diameters ( >170 nm), force modulation microscopy allows one to probe the local elasticity along the length of the coil. Our results agree with the classical theory of elasticity. Similar to the case of straight nanotubes, the Young modulus of coiled multiwalled nanotubes remains comparable to the very high Young modulus of hexagonal graphene sheets.     

锯齿型碳纳米管的结构衍生及电子特性

利用密度泛函理论研究锯齿型单、双壁碳纳米管从核到管状团簇直至纳米管的逐层结构衍生.研究结果表明五边形结构在管状团簇生长中发挥关键作用.此外,基于管状团簇的研究,运用周期性边界条件得到锯齿型单、双壁碳纳米管,并通过计算能带和态密度研究其电子特性.对单壁(n,0)和双壁(n,0)@(2n,0)碳纳米管,当n=3q(q为整数)时,具有金属或窄带隙半导体特性;n?=3q时,具有较宽带隙半导体特性,且带隙随管径的增加而减小.然而,小管径碳纳米管受曲率效应的明显影响,n?=3q的(4,0),(4,0)@(8,0)和(5,0)@(10,0)均呈现金属性;n=3q的(6,0)@(12,0)则表现出明显的半导体特性.     

卷曲效应对单壁碳纳米管电子结构的影响

利用计入卷曲效应的单壁碳纳米管(SWCNT)的能量色散关系,计算最低导带的电子速度及有效质量,并与不计入卷曲效应的结果进行了比较.计算结果表明:卷曲效应对电子速度及有效质量的影响与SWCNT的类型密切相关,金属锯齿型SWCNT对卷曲效应最为敏感,其次是扶手椅型SWCNT,最不敏感的是半导体锯齿型SWCNT.由此可以推断,卷曲效应对金属锯齿型SWCNT电子结构及低偏压输运特性影响最大,其次是扶手椅型SWCNT,影响最不明显的是半导体锯齿型SWCNT.这些结果与实验测量及密度泛函理论计算结果完全一致. 关键词： 单壁碳纳米管 卷曲效应 电子速度 电子有效质量     

并苯纳米环[6]CA及其衍生物的电子结构和光物理性质的密度泛函理论研究

采用密度泛函理论PBE0方法在6-31G(d, p) 基组水平上对比研究并六苯纳米环[6]CA及BN取代纳米环[6]CA-BN的几何结构及电子性质. 同时探讨锂离子掺杂对不同体系的芳香性、前线分子轨道、电子吸收光谱及传输性质的影响. 通过电离势、亲合势及重组能的计算, 预测纳米环体系得失电子的能力及传输性能. 结果表明:[6]CA的能隙很小, BN取代后, 能隙明显增大; 锂离子掺杂到两种纳米环中, 在不明显改变前线分子轨道分布的前提下, 几乎同步降低了最高占据轨道、 最低未占据轨道能级, 锂离子掺杂使载流子传输性能得到很大改善; 电子吸收光谱拟合发现, BN取代使吸收光谱很大程度蓝移, 吸收强度明显减小; 而锂离子掺杂对光谱的强度及吸收范围没有明显影响. 关键词： 碳纳米环 硼氮纳米环 锂离子掺杂 密度泛函理论     

Electronic structures and transport properties of sulfurized carbon nanotubes

The electronic and transport properties of side-walled sulfurized (8, 0) zigzag carbon nanotube were investigated by using density functional theory coupled with a non-equilibrium Green function approach. It is found that the adsorption of the sulfur chains largely reduces the bandgap of the semiconducting (8, 0) carbon nanotube, even changing it into a metallic one. More importantly, the transmission eigenstates around the Fermi level are contributed by not only the sulfur chains but also the complex system made of the sulfur chains and the single-walled carbon nanotube. Our results provide a method to improve the conductivity and utilization rate of the surface in the electrodes of supercapacitor which are made of the carbon nanotubes.     

Stability and electronic properties of carbon nanotubes doped with transition metal impurities

We apply first-principles method to investigate the effect of the diameter on the stability and electronic properties of zigzag carbon nanotubes doped with iron, nickel and manganese impurity atoms. In this contribution we follow the evolution of the electronic and structural properties as a function of the nanotube diameter. As a general result, we found that the binding energy decreases with the increasing nanotube radius. Additionally, depending on the interaction of transition metal impurity with the tubular carbon structure, it is observed that the total magnetization varies with the tube diameter due to hybridization and confinement effects. It is also shown that such magnetization varies with the curvature radius, increasing for manganese impurity atoms and decreasing for iron and nickel.     

Low-energy electronic properties of pairs of identical carbon nanotubes

The tight-binding model is employed to study the low-energy electronic properties of aligned pairs of identical single-wall carbon nanotubes with the intertube interactions. The rotational symmetry about the tube axes is totally broken, and the intertube interactions hybridize the atomic states on each tube to create new sub-bands. Sub-band spacing, sub-band curvature, band-edge states, and energy gaps are sensitive to stacking types and are also dependent on the radius and the chirality of the tubes. The systems could be metal, semimetal, or semiconductor depending on their stacking types. In particular, an armchair pair keeps the band structures linear like a single tube if the pair has a glide symmetry with respect to the plane between its constituent tubes. Breaking this symmetry makes the pair semimetallic or semiconducting. However, there are no such properties for chiral and zigzag pairs. The variations in electronic structures of these pairs are more complicated and more sensitive to the tube radii. Instead of being like a rope or a large bundle, the stacking-type dependent behavior is more similar to commensurate double-wall carbon nanotubes.     

Effects of phosphorus-doping upon the electronic structures of single wall carbon nanotubes

The phosphorus-doped single wall carbon nanotube (PSWCNT) is studied by using First-Principle methods based on Density Function Theory (DFT). The formation energy, total energy, band structure, geometry structure and density of states are calculated. It is found that the formation energy of the P-doped single carbon nanotubes increases with diameters; the total energy of carbon nanotubes with the same diameter decreases as the doping rate increases. The effects of impurity position on the impurity level are discussed. It illustrates that the position of the impurity level may depend on the C-P-C bond angle. According to the above results, it is feasible to substitute a carbon atom with a phosphorus atom in SWCNT. It is also found that P-doped carbon nanotubes are N type semiconductor. Supported by the Natural Science Foundation of Fujian Province of China (Grant No. A0220001)     

Structural and electronic properties of carbon nanotubes under hydrostatic pressures

We studied the structural and electronic properties of carbon nanotubes under hydrostatic pressures based on molecular dynamics simulations and first principles band structure calculations. It is found that carbon nanotubes experience a hard-to-soft transition as external pressure increases. The bulk modulus of soft phase is two orders of magnitude smaller than that of hard phase. The band structure calculations show that band gap of (10, 0) nanotube increases with the increase of pressure at low pressures. Above a critical pressure (5.70GPa), band gap of (10, 0) nanotube drops rapidly and becomes zero at 6.62GPa. Moreover, the calculated charge density shows that a large pressure can induce an {sp}²-to-{sp}³ bonding transition, which is confirmed by recent experiments on deformed carbon nanotubes.     

The geometrical and electronic structures of open-end fully functionalized single-walled carbon nanotubes

We have investigated the geometrical and electronic structures of open-end single-walled carbon nanotubes (SWNTs) having chemically modified tips, using semi-empirical AM1 and density functional theory methods. The hydroxyl (–OH), carboxyl (–COOH) and amide (–CONH₂) functional groups were used to saturate the open-ends of nanotubes. The effects of functional groups were studied by comparison with the pristine tubes, of which the tubular lengths vary from two to ten unit-cells (40 Å). The results show that the C–C bond lengths of all model tubes are only slightly different, and the behavior of converging bond lengths in COOH– and CONH₂-SWNTs is very similar to the pristine tube. Tip functionalization alters the frontier orbitals of the pristine tube, but these effects seem to rapidly decrease as the tubule becomes longer. In general, it can be concluded that the geometrical and electronic structures of pristine tubes after tube-end “full” functionalization will be preserved, hence supporting that more real-world “partially” functionalized SWNTs can be used in the same way as the pristine version in most application areas.     

Nanoscale atomic waveguides with suspended carbon nanotubes

We propose an experimentally viable setup for the realization of one-dimensional ultracold atom gases in a nanoscale magnetic waveguide formed by single doubly-clamped suspended carbon nanotubes. We show that all common decoherence and atom loss mechanisms are small, guaranteeing a stable operation of the trap. Since the extremely large current densities in carbon nanotubes are spatially homogeneous, our proposed architecture allows for creation of a very regular trapping potential for the atom cloud. Adding a second nanowire allows creation of a double-well potential with a moderate tunneling barrier which is desired for tunneling and interference experiments with the advantage of tunneling distances being in the nanometer regime. PACS 03.75.Gg; 03.75.Dg; 73.63.Fg     

N-doping and coalescence of carbon nanotubes: synthesis and electronic properties

Self-assembly pyrolytic routes to large arrays (<2.5 cm²) of aligned CN_x nanotubes (15–80 nm OD and <100 μm in length) are presented. The method involves the thermolysis of ferrocene/melamine mixtures (5:95) at 900–1000 °C in the presence of Ar. Electron energy loss spectroscopy (EELS) reveals that the N content varies from 2–10%, and can be bonded to C in two different fashions (double-bonded and triple-bonded nitrogen). The electronic densities of states (DOS) of these CN_x nanotubes, using scanning tunneling spectroscopy (STS), are presented. The doped nanotubes exhibit strong features in the conduction band close to the Fermi level (0.18 eV). Using tight-binding and ab initio calculations, we confirm that pyridine-like (double-bonded) N is responsible for introducing donor states close to the Fermi Level. These electron-rich structures are the first example of n-type nanotubes. Finally, it will be shown that moderate electron irradiation at 700–800 °C is capable of coalescing single-walled nanotubes (SWNTs). The process has also been studied using tight-binding molecular dynamics (TBMD). Vacancies induce the coalescence via a zipper-like mechanism, which has also been observed experimentally. These vacancies trigger the organization of atoms on the tube lattices within adjacent tubes. These results pave the way to the fabrication of nanotube heterojunctions, robust composites, contacts, nanocircuits and strong 3D composites using N-doped tubes as well as SWNTs. Received: 10 October 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Structure and electronic properties of the double-wall nanotubes constructed from SiO2 nanotubes encapsulated inside zigzag carbon nanotubes

This paper presents ab initio self-consistent field crystal orbital calculations on the structures, stabilities, elastic and electronic properties of the double-wall nanotubes made of SiO(2) nanotubes encapsulated inside zigzag carbon nanotubes based on density functional theory. It is found that formation of the combined systems is energetically favorable when the nearest distance between the two constituents is in the area of the van der Waals effect. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. Based on the deformation potential theory and effective mass approximation, the mobilities of charge carriers are calculated to be in the range of 10(2)-10(4) cm(2) V(-1) s(-1), the same order of magnitude as those of the corresponding zigzag carbon nanotubes. The Young's moduli are also calculated for the combined systems.     

扶手椅型氮化镓纳米管电子结构与传输特性的研究

本文采用密度泛函理论和非平衡格林函数对扶手椅型氮化镓纳米管（n,n）（2≤ n ≤10）的电子结构和输运性质进行了研究。结果表明,所有的扶手椅型氮化镓纳米管都是间接带隙半导体,带隙随着纳米管直径的增加而增加,并且得到了两极体系下氮化镓纳米管的电流-电压曲线。氮化镓纳米管的半导体特性随着纳米管直径的增加越来越明显,电子态密度和电子透射光谱都具有脉冲型尖峰并且最大峰值随着n的增加而增加。这说明电子态密度和电子透射光谱峰在能量范围内,有较好的对应关系。     

Prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on surfaces of single-walled carbon nanotubes along their axes can lead to a modification of cylindrical carbon cores of these single-walled carbon nanotubes to carbon cores that have a nearly prismatic shape (prismatic modification). In faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. It has been established that the main characteristics of the single-walled carbon nanotubes thus modified are rather close to the corresponding characteristics of the related isostructural polymer conjugated systems (such as cis-polyenes, polyphenylenes, poly(periacenes), or polyphenantrenes). Fragments of model nanotubes of the (n, n) and (n, 0) types that contain up to 360 carbon atoms and their derivatives doped with fluorine atoms have been calculated using the semiempirical parametric method 3.     

Geometry and electronic properties of single vacancies in achiral carbon nanotubes

An ideal single vacancy can be formed by removing one carbon atom from a hexagonal network. The vacancy is one of the most important defect structures in carbon nanotubes (CNTs). Vacancies can affect the mechanical, chemical, and electronic properties of CNTs. We have systematically investigated single vacancies and their related point defects for achiral, single-walled carbon nanotubes (SWNTs) using first-principles calculations. The structures around single vacancies undergo reconstruction without constraint, forming ground-stateor metastable-state structures. The 5-1DB and 3DB point defects can be formed in armchair CNTS, while the 5-1DB-P and 5-1DB-T point defects can be formed in zigzag CNTs. The related point defects can transform into each other under certain conditions. The formation energies of armchair CNTs change smoothly with the tube radius, while in the case of the 3DB defect, as the radius get larger, the formation energies tend towards a constant value.     

Anti-prismatic modifications of single-walled carbon nanotubes and their electronic properties: Regular adsorption of fluorine atoms on graphene surfaces of nanotubes

The regular adsorption of fluorine atoms on the surfaces of single-walled carbon nanotubes along their cylindrical axes leads to a modification of cylindrical carbon skeletons of these single-walled carbon nanotubes into carbon skeletons that have a nearly “anti-prismatic” shape (anti-prismatic modifications). In the faces of these modified single-walled carbon nanotubes, there can arise quasi-one-dimensional isolated carbon conjugated subsystems (tracks) with different structures. Model fragments of nanotubes of the (n, 0) type that contain up to 360 carbon atoms and their derivatives with regularly adsorbed fluorine atoms on the graphene surface have been calculated using the semiempirical PM3 method. It has been found that the main properties of the single-walled carbon nanotubes modified by the above method are determined by the character of the conjugation of the electrons in isolated carbon tracks, which is close to the character of the conjugation of the electrons in the initial single-walled carbon nanotubes.