公度双壁碳纳米管层间耦合对其场发射特性影响的研究

采用紧束缚能带理论，利用所提出的考虑卷曲效应的紧束缚能量哈密顿量，建立了公度双壁碳纳米管(DWNT)的能带结构模型；基于碳纳米管(CNT)发射电流与其能带结构的相关性，定量分析了公度DWNT的层间耦合作用对其场发射电流的影响.结果表明：在层间耦合作用下，DWNT的带结构中部分简并能级发生劈裂，同时使禁带宽度发生改变.前一个因素增加了电子发射的通道，后一个因素改变价带中参与发射的电子数量，导致在一定外电场下，DWNT与其外层的SWNT相比，场发射电流有一定程度的增加，且半导体性管发射电流增幅比金属性管大，在 关键词： 公度双壁碳纳米管 能带结构 层间耦合作用 卷曲效应     

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     

石墨烯纳米带卷曲效应对其电子特性的影响

石墨烯纳米带 (GNRs) 是一种重要的纳米材料, 碳纳米管可看作是GNRs卷曲而成的无缝圆筒. 利用基于密度泛函理论的第一性原理方法, 系统研究了GNRs卷曲变形到不同几何构型时, 其电子特性, 包括能带结构 (特别是带隙) 、态密度、透射谱的变化规律. 结果表明: 无论是锯齿型GNRs (ZGNRs) 或扶手椅型GNRs (AGNRs), 在其卷曲成管之前, 其电子特性对卷曲形变均不敏感, 这意味着GNRs的电子结构及输运特性有较强地抵抗卷曲变形的能力. 当GNRs 卷曲成管后, ZGNRs和AGNRs表现出完全不同的性质, ZGNRs几乎保持金属性不变或变为准金属; 但AGNRs的电子特性有较大的变化, 出现不同带隙半导体、准金属之间的转变, 这也许密切关系到碳纳米管管口周长方向上的周期性边界条件及量子禁锢的改变. 这些研究对于了解GNRs电子特性的卷曲效应、以及GNRs与碳纳米管电子特性的关系 (结构与特性的关系) 有重要意义. 关键词： 石墨烯纳米带 卷曲效应 电子特性 密度泛函理论     

手性环状碳纳米管的电子结构及磁化特性

基于石墨平面π电子的紧束缚模型,导出了手性(chiral)环状碳纳米管(TCNTs)在磁场中的电子结构,从而对TCNTs量子特性和磁化规律进行了较深入研究.第Ⅰ,Ⅱ类TCNTs在磁场中存在金属—半导体的连续转变.计算分析表明:在T=0K时,第Ⅰ,Ⅱ类TCNTs的磁化强度随磁通量Φ呈线性周期性变化(以磁通量子Φ₀=h/e为周期),并且磁化强度对手性角θ、管口半径r,以及温度T极为敏感,但与环半径R无关.手性TCNTs的磁化强度比椅形或锯齿形TCNTs的磁化强度强很多;而第Ⅲ类TCNTs 关键词： 手性环状碳纳米管 电子结构 磁化特性     

单壁碳纳米扶手椅、锯齿管声子色散关系的计算

引用石墨经验力常数计算碳纳米管声子色散关系时，必须处理由二维平面卷曲形成三维实体纳米管所引入的问题. 报道对一系列扶手椅和锯齿单壁碳纳米管计及卷曲效应的声子色散关系的计算结果. 基于实际的数值计算结果，以及对单壁碳纳米扶手椅、锯齿管结构的对称性分析，讨论了Brillouin区中心Γ点晶格振动模的分类. 关键词： 碳纳米管 声子色散关系     

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     

单壁碳纳米管微分电导在高压和强磁场下的实验研究

单壁碳纳米管在高压下会发生结构相变，导致金属型的碳纳米管变成半导体.相变后碳纳米管中电子的库仑关联的表现形式发生变化，从Luttinger liquid行为转变成环境量子涨落行为.同时，相变后电子波函数的相位关联导致弱局域化行为的出现.为了研究库仑关联和相位关联之间是否有相互影响，使用金刚石对顶砧和液压自锁高压包在0—10 GPa准静压范围内测量了单层碳纳米管样品在低温和不同磁场下的微分电导随偏压的依赖关系.实验结果表明，相位关联和库仑关联是两种独立的效应，各自影响着电子的输运行为. 关键词： 单层碳纳米管 高压 微分电导     

Ar^＋离子辐照对碳纳米管电子结构的影响

应用ＸＰＳ和ＡＥＳ研究了碳纳米管结构稳定性，结果表明：低能电子束辐照不会改变碳纳米管的电子结构；相反，低能Ａｒ＾＋离子束即使辐照强度低剂０．２ｍＡ／ｃｍ＾２，也会引起碳纳米管电子结构的明显变化。     

4A碳纳米管及相关结构的嵌锂特性(英文)

碳纳米管独特的一维结构和强烈的卷曲效应为外来原子提供了理想的嵌入通道。本文全面总结了近年来我们对直径仅为4A的三种单壁碳纳米管嵌锂特性的密度泛函研究工作。我们具体讨论了体系嵌锂后的结构、能量、电子、电化学等特性。由于这些超小直径的碳纳米管最初合成于沸石晶体的纳米管道,我们也讨论了碳纳米管?沸石晶体复合体系的嵌锂特性。另外,我们还研究了由(5,0)和(14,0)碳纳米管组成的双壁碳纳米管体系的嵌锂特性。我们的理论计算表明,超小直径碳纳米管及相关结构作为锂离子电池负极材料具有很好的应用前景。     

石墨炔管能带和力学性质的第一性原理研究

基于第一性原理计算,这篇文章研究了单壁锯齿型和扶手型石墨炔管的几何结构、电子结构以及杨氏模量.计算表明:石墨炔管是一类具有一定能隙的直接带隙半导体管,其带隙在0.4-1.3eV的能量范围,且随管径的增大而变小.而石墨炔管的杨氏模量在0.44-0.50Tpa区间变化.对于锯齿型石墨炔管,其杨氏模量随着半径的增大而变小而锯齿型石墨炔管的杨氏模量随其半径的增大而增大.     

Thermal conductivity for single-walled carbon nanotubes from Einstein relation in molecular dynamics

Equilibrium molecular dynamics based Einstein relation with an appropriate definition for integrated heat current (i.e., with modified energy moment) are combined to quantify the thermal conductivity of individual single-walled carbon nanotubes, armchair, zigzag and chiral tubes. The thermal conductivity has been investigated as a function of three parameters, tube radius, length and chirality at and near room temperature with Brenner potential model. Thermal conductivity is found to have unusually high value and varies with radius, length and chirality of tubes. Also the thermal conductivity at temperature range from 50 to 100 K is found to have a maximum value. For 12.1 nm tube length, the thermal conductivity has converging trend which its value dependents on the tube radius and chirality. Tubes with large radius have lower values of thermal conductivity. Furthermore, the results show that armchair tubes have large values of the thermal conductivity comparing with zigzag and chiral tubes. It seems possible to uncover carbon nanotubes thermal properties based on measurements having heat dependence by adding another methods for calculations.     

多壁碳纳米管储氢的物理吸附特性

采用巨正则蒙特卡罗方法 ,模拟常温、1 0MPa下氢在扶手椅型多壁壁碳纳米管中的物理吸附过程 .氢分子之间、氢分子与碳原子之间的相互作用采用Lennard Jones势能模型 .研究了双壁碳纳米管外 (内 )径固定而内 (外 )径改变时的物理吸附储氢情况 ,发现氢分子主要储存在双壁碳纳米管的管壁附近 ,当双壁碳纳米管的内外管壁间距由 0 .34nm增大到 0 .6 1或 0 .88nm时可有效增加物理吸附储氢量 ,并给出了相应的理论解释 .在此基础上 ,计算了管壁间距为 0 .34、0 .6 1和 0 .88nm时的三壁碳纳米管的物理吸附储氢量 ,并与相同条件下单壁和双壁碳纳米管的物理吸附储氢量作了比较 ,发现多壁碳纳米管的物理吸附储氢量随碳管层数的增加而减小 .     

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.     

Electron interactions and scaling relations for optical excitations in carbon nanotubes

Recent fluorescence spectroscopy experiments on single wall carbon nanotubes reveal substantial deviations of observed absorption and emission energies from predictions of noninteracting models of the electronic structure. Nonetheless, the data for nearly armchair nanotubes obey a nonlinear scaling relation as a function of the tube radius R. We show that these effects can be understood in a theory of large radius tubes, derived from the theory of two dimensional graphene where the Coulomb interaction leads to a logarithmic correction to the electronic self-energy and marginal Fermi liquid behavior. Interactions on length scales larger than the tube circumference lead to strong self-energy and excitonic effects that compete and nearly cancel so that the observed optical transitions are dominated by the graphene self-energy effects.     

Adsorption on carbon nanotubes: quantum spin tubes, magnetization plateaus, and conformal symmetry

We formulate the problem of adsorption onto the surface of a carbon nanotube as a lattice gas on a triangular lattice wrapped around a cylinder. This model is equivalent to an XXZ Heisenberg quantum spin tube. We find density plateau structures for armchair, zigzag, and chiral nanotubes. The zigzags are special and have extensive zero temperature entropy plateaus in the classical limit. Quantum effects lift the degeneracy, leaving gapless excitations described by a c = 1 conformal field theory with compactification radius quantized by the tube circumference.     

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.     

Stability and electronic structure of InN nanotubes from first-principles study

The stability and electronic structure of hypothetical InN nanotubes were studied by first-principles density functional theory. It was found that the strain energies of InN nanotubes are smaller than those of carbon nanotubes of the same radius. Single-wall zigzag InN nanotubes were found to be semiconductors with a direct band gap while the armchair counterparts have an indirect band gap. The band gaps of nanotubes decrease with increasing diameter, similar to the case of carbon nanotubes.     

Phonon and thermal properties of achiral single wall carbon nanotubes

A detailed theoretical study of the phonon and thermal properties of achiral single wall carbon nanotubes has been carried out using force constant model considering up to third nearest-neighbor interactions. We have calculated the phonon dispersions, density of states, radial breathing modes (RBM) and the specific heats for various zigzag and armchair nanotubes, with radii ranging from 2.8 Å to 11.0 Å. A comparative study of phonon spectrum with measured Raman data reveals that the number of Raman active modes for a tube does not depend on the number of atoms present in the unit cell but on its chirality. Calculated phonon modes at the zone center more or less accurately predicted the Raman active modes. The radial breathing mode is of particular interest as for a specific radius of a nanotube it is found to be independent of its chirality. We have also calculated the variation of RBM and G-band modes for tubes of different radii. RBM shows an inverse dependence on the radius of the tube. Finally, the values of specific heat are calculated for various nanotubes at room temperature and it was found that the specific heat shows an exponential dependence on the diameter of the tube.     

Consideration of mechanical properties of single-walled carbon nanotubes under various loading conditions

In this article, mechanical properties of single-walled carbon nanotubes (SWCNTs) with various radiuses under tensile, compressive and lateral loads are considered. Stress–strain curve, elastic modulus, tensile, compressive and rotational stiffness, buckling behaviour, and critical axial compressive load and pressure of eight different zigzag and armchair SWCNTs are investigated to figure out the effect of radius and chirality on mechanical properties of nanotubes. Using molecular dynamic simulation (MDS) method, it can be explained that SWCNTs have higher Young’s modulus and tensile stiffness than compressive elastic modulus and compressive stiffness. Critical axial force of zigzag SWCNT is independent from the radius, but that of armchair type rises by increasing of radius, also these two types show different buckling modes.