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。     

电子在固体中的运动

固体(本文指结晶体)的主要特性是其原子或分子的规则排列.不同晶体各种物理性质上的千差万别(例如有金属、半导体和绝缘体之分),都深刻地与电子在晶体中的能量状态及运动规律相联系着. 在晶体中,一个电子的运动是与数目众多的格点上的原子(或分子)以及数目更多的其它电子的运动相关联.物理学家们分析了各种实验和数据,颇费心思地将这种多体问题简化为单电子问题,建立了可信赖的单电子模型──能带论. 一、自由电子气模型1.电子公有化-自由电子气 以金属钠为例来说明这个模型.金属钠为体心立方结构.每个原子在闭壳层外有一个价电子.这个电子…     

氮化硼纳米带功能化碳纳米管的热自旋输运性质

热自旋电子学器件结合了自旋电子学和热电子学各自的优点,对人类可持续发展具有重要作用.本文研究了锯齿形BN纳米带(ZBNRs)共价功能化碳纳米管(SWCNT)的电子结构,发现ZBNRs-B-(6,6)SWCNT为磁性半金属,nZBNRs-B-(6,6)SWCNT(n=2—8)为磁性金属;nZBNRs-N-(6,6)SWCNT(n=1—8)为双极化铁磁半导体;4ZBNRs-B-(4,4)SWCNT和4ZBNRs-N-(4,4)SWCNT为磁性半金属,4ZBNRs-B-(m,m)SWCNT(m=5—9)为磁性金属;4ZBNRs-N-(m,m)SWCNT(m=5—9)为双极化铁磁半导体.然后,基于锯齿形BN纳米带共价功能化碳纳米管设计了新型热自旋电子学器件,发现基于ZBNRs-N-(6,6)SWCNT的器件具有热自旋过滤效应;而8ZBNRs-N-(6,6)SWCNT和nZBNRs-B-(6,6)SWCNT(n=1,8)都存在自旋相关塞贝克效应.这些发现表明BN纳米带功能化碳纳米管在热自旋电子学器件方面具有潜在的应用.     

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     

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.     

碳纳米管吸附铜原子的密度泛函理论研究

采用密度泛函方法对铜原子在有限长（5,5）椅型单壁碳纳米管的吸附行为进行了研究.计算结果表明,铜原子吸附在管外壁要比吸附在管内壁能量上更为有利,在管外壁碳原子顶位吸附最佳,属于明显的化学吸附.且用前线轨道理论对其成键特性进行了分析,表明在顶位吸附时主要由铜原子的4s轨道电子与碳纳米管中耦合的σ-π键形成新的σ键.此外还对比计算了两种典型位置电子密度,发现顶位吸附的成键中有更大的电子云重叠.进一步表明在某些情况下铜碳原子可以成键. 关键词： 碳纳米管 铜原子 成键特性     

SmN晶体的电子结构和高压相变的第一性原理

利用基于密度泛函理论的第一性原理,研究了SmN晶体的电子结构和高压相变. SmN晶体的电子结构具有半金属特征,多数自旋电子显示金属导电性,少数自旋电子显示半导体导电性. 高压相变的结果显示,SmN晶体经历从NaCl型(B1)到CsCl型(B2)结构转变的压致结构相变,相变压力117 GPa. 弹性系数的结果显示,在环境压力下SmN晶体的弹性系数满足玻恩稳定条件,标志着B1相是力学稳定结构. 声子谱结果显示,在环境压力下B1相是热力学稳定结构,与弹性系数的计算结果一致.     

氧吸附对单壁碳纳米管的电子结构和光学性能的影响

用密度泛函理论计算了氧分子物理吸附在半导体型单壁碳纳米管的束缚能，能带结构和吸收 光谱.计算结果指出氧分子吸附在碳纳米管表面的优先位置，研究发现氧吸附对碳管的电子 输运特性和吸收光谱有着重要的影响，并对光致氧分子解吸附的现象进行了理论分析. 关键词： 单壁碳纳米管 氧物理吸附 能带结构 吸收光谱     

掺氮碳化硅纳米管电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算,对本征碳化硅纳米管和掺氮碳化硅纳米管的电子结构进行了计算.计算表明本征（8,0）碳化硅为直接带隙半导体,能带间隙为0.94 eV;掺氮浓度为1.56%和3.12％的碳化硅纳米管的能带间隙减小为0.83 eV和0.74 eV.从差分电荷密度可以看出,能带间隙的减小是氮硅键与碳硅键相比共价成键能力降低的结果. 关键词： 碳化硅纳米管 掺氮 第一性原理 电子结构     

C/SiC纳米管异质结电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理计算,对扶手椅型(4,4)和(6,6)及锯齿型(8,0)和(10,0)C/SiC纳米管异质结的电子结构进行了研究.结果表明两类异质结结构都表现为半导体特性.扶手椅型纳米管异质结形成了Ⅰ型异质结,电子和空穴都限制在碳纳米管部分.锯齿型纳米管异质结中价带顶主要分布在碳纳米管部分及C/SiC界面处,而导带底均匀分布在整个纳米管异质结上.这两种异质结结构在未来纳米器件中具有潜在的应用价值. 关键词： C/SiC纳米管异质结 第一性原理 电子结构     

Carbon systems of polymerized nanotubes: Crystal and electronic structures

Molecular dynamic calculations are carried out for the (P, T) phase diagram of a covalent compound of cross-linked carbon single-wall nanotubes (SWNT) and for the structures and electronic spectra of the novel crystals of polymerized carbon nanotubes. It is shown that the transformation of covalently bonded nanotubes in a close-packed conducting structure cardinally modifies their electronic properties. The P-SWNT crystal becomes semiconducting and, upon complete transformation of sp ²-hybridized carbon atoms into sp ³-hybridized ones, it becomes an insulator.     

Characteristic features of the π-electron states of carbon nanotubes

A simple tight-binding model of the π-electron states of carbon nanotubes is analyzed. The symmetry group of nanotubes and its relation to electronic structure are discussed. The applicability of the simple model is analyzed on the basis of numerical calculations of the electronic states of energy-optimized nanotubes, performed in a parametric tight-binding model that takes account of the carbon valence electrons. Numerical data on the gap widths of optimized nanotubes and data obtained from the zone-folding model employed in the literature are presented. Zh. éksp. Teor. Fiz. 111, 2074–2085 (June 1997)     

First-principles study of nanotubes within the tetragonal,hexagonal and dodecagonal cycle structures

A systematic study has been done on the structural and electronic properties of carbon, boron nitride and aluminum nitride nanotubes with structure consisting of periodically distributed tetragonal (T ≡A₂X₂), hexagonal (H ≡A₃X₃) and dodecagonal (D ≡A₆X₆) (AX=C₂, BN, AlN) cycles. The method has been performed using first-principles calculations based on density functional theory (DFT). The optimized lattice parameters, density of state (DOS) curves and band structure of THD-NTs are obtained for (3, 0) and (0, 2) types. Our calculation results indicate that carbon nanotubes of these types (THD-CNTs) behave as a metallic, but the boron nitride nanotubes (THD-BNNTs) (with a band gap of around 4 eV) as well as aluminum nitride nanotubes (THD-AlNNTs) (with a band gap of around 2.6 eV) behave as an semiconductor. The inequality in number of atoms in different directions is affected on structures and diameters of nanotubes and their walls curvature.     

First principles investigation of magnesium antimonite semiconductor compound in two different phases under hydrostatic pressure

We investigate the electronic and the structural properties of Mg₃Sb₂ in cubic and hexagonal phases using the full potential-linearized augmented plane wave (FP-LAPW) method within the framework of density functional theory. The effects of hydrostatic pressure on band gap, bandwidths of bands under Fermi energy labeled by B1 and B2 from the top, the energy gap between B1 and B2 (anti-symmetry gap) and also effective masses of electrons and holes are studied using optimized lattice parameters. We observe that the hydrostatic pressure decreases the band gap and the anti-symmetry gap while it increases the bandwidths of all bands below the Fermi energy. The effective masses of electrons and holes for the hexagonal phase depend on pressure in the Γ→Λ direction. In the cubic phase the effective mass of electrons is independent of pressure and the effective mass of holes depend on the pressure in the Γ→N direction.     

Chemisorption of single fluorine atoms on the surface of zigzag single-walled carbon nanotubes: A model calculation

We report a model calculation of the chemisorption energies ΔE_ads of single fluorine atoms on the outer surface of zigzag single-walled carbon nanotubes (Z-SWCNTs) (p,0) with p ranging from 11 to 21. A simplified model based on an effective-mass theory is adopted to describe the electronic structure of the nanotubes. Chemisorption is treated within the Anderson–Newns approach, which takes account of Coulomb interaction between adsorbate electrons. Considering adsorption of an adatom directly on top of a surface carbon atom, we find that in the case of a fluorine atom bonded to the sidewall of the nanotubes, the absolute values of ΔE_ads are in the range for Z-SWCNTs with typical diameters of , larger |ΔE_ads| values being associated with semiconducting tubes. For the latter ones, |ΔE_ads| decreases rather significantly as the radius R of the tubes increases, tending towards the “infinite” radius graphene case, whereas for metallic tubes |ΔE_ads| slightly increases with increasing R. The localized acceptor states induced by a fluorine atom in the band gap of the semiconducting tubes are found to be responsible for such difference in the behaviour of ΔE_ads for the two above-mentioned types of tubes. The results obtained shed light on the possible mechanism of the atomic fluorine adsorption-induced hole-doping of the semiconducting tubes, which might significantly affect the transport properties of these tubes.     

Comparison of influence of incorporated 3d-, 4d- and 4f-metal chlorides on electronic properties of single-walled carbon nanotubes

In the present work, the channels of single-walled carbon nanotubes were filled with melts of ZnCl₂, CdCl₂, and TbCl₃ by a capillary method with subsequent slow cooling. The detailed study of electronic structure of filled nanotubes was performed using Raman, optical absorption, and X-ray photoelectron spectroscopy. The obtained data are in mutual agreement and it proves that the filling of carbon nanotube channels with all these salts leads to the charge transfer from nanotube walls to the incorporated compounds, thus acceptor doping of nanotubes takes place. It was found out that encapsulated terbium chloride has the largest influence on the electronic properties of carbon nanotubes.     

Band structure and CK α emission of ultrathin nanotubes

An attempt is made to calculate the energy bands and spectra of the characteristic CK _α emission of small-diameter carbon nanotubes. The calculated spectra for the nanotubes are compared with similar spectra for graphite monolayers used as a test object and with known experimental results for nanotubes. It is concluded that the x-ray emission spectra can be used to identify thin carbon nanotubes. A classification of solid-phase carbon is proposed which takes into account the position of carbon nanotubes in the family of allotropic carbon forms. The type of hybridization of the electrons in the carbon atom is used as the criterion for classification.     

Raman evidence of the interaction between multiwalled carbon nanotubes and nanostructured TiO2

Nanocomposites of carbon nanotubes and titanium dioxide (TiO₂) have attracted much attention due to their photocatalytic properties. Although many examples in the literature have visualized these nanocomposites by electron microscopic images, spectroscopic characterization is still lacking with regard to the interaction between the carbon nanotube and TiO₂. In this work, we show evidence of the attachment of nanostructured TiO₂ to multiwalled carbon nanotubes (MWNTs) by Raman spectroscopy. The nanostructured TiO₂ was characterized by both full‐width at half‐maximum (FWHM) and the Raman shift of the TiO₂ band at ca 144 cm⁻¹, whereas the average diameter of the crystallite was estimated as approximately 7 nm. Comparison of the Raman spectra of the MWNTs and MWNTs/TiO₂ shows a clear inversion of the relative intensities of the G and D bands, suggesting a substantial chemical modification of the outermost tubes due to the attachment of nanostructured TiO₂. To complement the nanocomposite characterization, scanning electronic microscopy and X‐ray diffraction were performed. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of the electronic structure of single-walled carbon nanotubes filled with cobalt bromide

Single-walled carbon nanotubes have been filled with cobalt bromide. The microstructure, optical properties, and effect of the introduced CoBr₂ compound on the electronic structure of the nanotubes have been studied. It has been shown that the electron density in the resulting nanocomposites is transferred from the walls of the nanotubes to the nanocrystals of cobalt bromide, which is an electron acceptor.     

Studies of chemical diffusion in La1-X SrXCoO3-δ

Recent investigations of superconductivity in carbon nanotubes have shown that a single-wall zig-zag nanotube can become superconducting at around 15?K. Theoretical studies of superconductivity in nanotubes using the traditional phonon exchange model, however, give a superconducting transition temperature T _c less than 1?K. To explain the observed higher critical temperature we explore the possibility of the plasmon exchange mechanism for superconductivity in nanotubes. We first calculate the effective interaction between electrons in a nanotube mediated by plasmon exchange and show that this interaction can become attractive. Using this attractive interaction in the modified Eliashberg theory for strong coupling superconductors, we then calculate the critical temperature T _c in a single-wall nanotube. Our theoretical results can explain the observed T _c in a single-wall nanotube. In particular, we find that T _c is sensitively dependent on the dielectric constant of the medium, the effective mass of the electrons and the radius of the nanotube. We then consider superconductivity in a bundle of single-wall nanotubes and find that bundling of nanotubes does not change the critical temperature significantly. Going beyond carbon nanotubes we show that in a metallic hollow nanowire T _c has some sort of oscillatory behaviour as a function of the surface number density of electrons.