Influence of isoelectronic impurities on the electronic structure of BN nanotubes

Via the example of a (5, 5) boron-nitrogen armchair nanotube, the influence of isoelectronic substitutional impurities on the electronic structure of BN nanotubes has been investigated with the use of linear augmented cylindrical waves. The treatment is based on the local density approximation and the muffin-tin approximation for the electron potential. In this method, the electronic spectrum of a system is governed by the free motion of electrons in the interatomic space between cylindrical barriers and the electron scattering on atomic centers. It has been found that the substitution of one atom of N by P leads to the splitting of all twofold degenerate bands by 0.2 eV on average, a decrease in the energy gap from 3.5 to 2.8 eV, the separation of the s(P) band from the high-energy region of the s(B, N) band, as well as to the formation of the impurity π(P) and π*(P) bands, which form the valence-band top and conduction-band bottom in the doped system. The influence of an As atom on the electronic structure of (5, 5) BN nanotubes is qualitatively similar to the case of phosphorus, but the energy gap is smaller by 0.5 eV. The optical gap in the nanotubes is closed due to the effect of the Sb atom impurity. A substitution of one B atom by an Al atom results in the strong perturbation of the band structure and the energy gap in this case is only 1.6 eV in contrast to the weak indium-induced perturbation of the band structure of the BN nanotube. In the latter case, the energy gap is 2.9 eV. The above effects can be detected by the optical and photoelectron spectroscopy methods, as well as by measuring the electrical properties of the nanotubes. They can be used to create electronic devices based on boron-nitrogen nanotubes.     

Ab initio thermodynamic characteristics of the formation of oxygen vacancies,and boron,carbon, and nitrogen impurity centers in anatase

Using the ab initio projector augmented wave (PAW) method, calculations are performed for the electronic energy-band structure of titanium dioxide having the structure of anatase doped with boron, nitrogen, and carbon. Thermodynamic characteristics are determined for the formation of impurity centers, such as the preference energy for the interstitial position, the energy of impurity oxidation, and the energy of oxygen vacancy formation. It is shown that under the conditions of thermodynamic equilibrium the interstitial position of boron atoms is stable, whereas carbon atoms, depending on the oxygen pressure, can occupy both interstitial positions and substitutional positions of oxygen atoms, and nitrogen atoms replace oxygen atoms. It is shown that the presence of oxygen vacancies promotes the thermodynamic stability of carbon and nitrogen atoms. The obtained densities of electronic states correspond to ESR spectroscopy data, which indicates the presence of spin-polarized electrons in the states of the oxygen vacancy.     

碳纳米管负载矾的第一性原理研究

本文基于密度泛函第一性原理研究了原始和带有缺陷的(Stone-Wales缺陷和单空位缺陷)碳纳米管负载金属V的稳定构型.对于V吸附在原始碳纳米管(CNT)上时,V在内表面的吸附比外表面的吸附有更强的相互作用力,且六元环内表面结构最稳定.当V与Stone-Wales缺陷碳纳米管相互作用时,V原子易吸附在管外七元环C-C键的外表面和内表面处,这说明缺陷位置的有效结合使之局域化加强.而以单空位缺陷碳纳米管为载体时V最易吸附在外缺陷处,相当于碳纳米管的一个C被金属V原子取代,形成了3个V-C_(sur)键,这进一步表明SV管外吸附比管内吸附更容易.我们从上述三种构型载体中发现,金属V吸附在缺陷碳纳米管时的稳定性要优于原始碳纳米管,且SV缺陷对金属V的固定效果最好.     

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.     

Nonchiral BN Haeckelite nanotubes

A new class of boron-nitrogen (BN) nanotubes composed of tetragons, pentagons, hexagons, heptagons, and octagons is considered. By analogy with carbon nanotubes of the same topological structure, these nanotubes were called Haeckelites. The geometry, energetics, and electronic properties were studied in detail for two regular mutual arrangements of the polygons. It was found that Haeckelite nanotubes are dielectrics with the energy gap E_g = 3.24–4.09 eV. As the nanotube diameter increases, the energy gap E_g decreases, approaching the value for the corresponding planar Haeckelite layer. The ground-state energy of the Haeckelite BN nanotubes is 0.3 eV/atom higher than that of well-known hexagonal BN nanotubes.     

第一性原理计算绝缘体-金属转变临界掺杂浓度:Co重掺杂Si体系

基于密度泛函理论的第一性原理方法,本文旨在探索确定绝缘体-金属转变临界浓度的理论计算方法.以Co重掺杂Si为研究对象,构建并计算了10个Co不同掺杂浓度模型的晶体结构、杂质形成能及其电子性质.发现在Co掺杂Si体系的带隙中形成了杂质能级,杂质能级的位置和宽度随着Co浓度的增加呈线性变化.当Co掺杂浓度较高时杂质形成能逐渐稳定,且杂质能级穿过费米能级使体系表现出金属性.综合杂质形成能的变化趋势,以及杂质能级极小值与费米能级间的距离条件,可预测出发生绝缘体-金属转变的Co掺杂浓度为2.601Wingdings 2MC@10~(20) cm~(-3),与实验结果相一致.上述两条依据应用于S重掺杂Si体系和Se重掺杂Si体系同样成立.     

单壁碳纳米管杨氏模量的掺杂效应

用分子动力学方法研究了N,O,Si,P,S等5种杂质对扶手椅型(5，5)和锯齿型(9，0)单壁碳纳米管杨氏模量的影响.结果表明：直径为0.678和0.704 nm的扶手椅型(5，5)和锯齿型(9，0)碳纳米管在无掺杂时其杨氏模量分别为948和804 GPa.在掺杂浓度10％以下，碳纳米管的拉伸杨氏模量均随掺杂浓度增加近似呈线性下降规律，下降率以Si掺杂最大，N掺杂最小.对与C同周期的元素掺杂，随原子序数增加碳纳米管的杨氏模量下降率增大；与C不同周期的元素掺杂，碳纳米管的杨氏模量随掺杂浓度增加下降率更大，但 关键词： 碳纳米管 杂质 杨氏模量 分子动力学方法     

Tunable adsorption on carbon nanotubes

We investigated the adsorption of a single atom, hydrogen and aluminum, on single-wall carbon nanotubes from first principles. The adsorption is exothermic, and the associated binding energy varies inversely as the radius of the zigzag tube. We found that the adsorption of a single atom and related properties can be modified continuously and reversibly by the external radial deformation. The binding energy on the high curvature site of the deformed tube increases with increasing radial deformation. The effects of curvature and radial deformation depend on the chirality of the tube.     

He原子掺杂铝材料的第一性原理研究

采用基于第一性原理的平面波超软赝势方法,结合广义梯度近似(GGA),计算了铝及铝晶胞间隙位置掺入He原子后体系的几何结构、电子结构、总体能量和电荷布居值.计算结果表明:随着氦在金属铝中逐渐形成,铝晶胞体系会发生晶格畸变,但总的趋势是He在铝体系的八面体位置的晶格畸变小于其在四面体位置的晶格畸变.He在铝晶胞八面体和四面体间隙的杂质形成能分别为1.3367 eV和2.4411 eV.由此可知,He在铝晶胞中最稳定位置是八面体间隙位置.同时,文中还从原子尺度层面分析了He原子在铝晶胞中的占位及其键合性质,讨论 关键词： 铝材料 第一性原理 形成能     

Electronic structure of nitrogen-containing carbon nanotubes irradiated with argon ions: XPS and XANES studies

Using the methods of X-ray photoelectron (XPS) and X-ray absorption near edge structure (XANES) spectroscopies with synchrotron radiation, data on changes in the electronic structure and chemical composition of nitrogen-containing multiwalled carbon nanotubes (N-MWCNTs) upon their exposure to the radiation of argon ions with an energy of 5 keV are obtained. It is found that the exposure leads to an increase in the degree of defectiveness of the N-MWCNTs structure and to the carbon oxidation with formation of various oxygen-containing groups (C–OH, C=O/COOH, C–O–C/O–C–O, and CO₃). The presence of carbon–oxygen bonds on the surface of carbon nanotubes is associated with the formation of radiation defects. It is shown that an increase in the fraction of nitrogen atoms present in the substituting configuration in the N-MWCNTs wall structure due to the irradiation does not give rise to an increase in the density of the occupied states near the Fermi level against the background of an increase in the degree of structure defectiveness, carbon oxidation, and a decrease in the total nitrogen concentration. The obtained results show that the irradiation of N-MWCNTs with argon ions allows one to successfully functionalize their surface.

     

Ab initio study of surface electronic structure of phosphorus donor impurity on Ge(111)-(2×1) surface

We present the results of numerical modeling of the electronic properties of the Ge(111)-(2 × 1) surface in the vicinity of a P donor impurity atom near the surface. We have shown that, in spite of well-established bulk donor impurity energy level position at the very bottom of the conduction band, the surface donor impurity might produce an energy level below the Fermi energy, depending on impurity atom local environment. It has been demonstrated that the impurity located in subsurface atomic layers is visible in scanning tunneling microscopy experiment. The quasi-one-dimensional character of the impurity image observed in scanning tunneling microscopy experiments is confirmed by our computer simulations.     

γ射线辐照下多壁碳纳米管结构转变过程研究

采用60Coγ射线辐照纯净的多壁碳纳米管,用高分辨透射电镜和拉曼光谱,研究了多壁碳纳米管由石墨结构向无定形结构转变的演化过程.发现在γ射线辐照下,碳纳米管的外部石墨层逐渐失去最初的有序结构而向无定形结构转变.而且,随着γ射线辐照剂量的增加,无定形结构不断推进,而石墨层结构则不断减小,直至使整个碳纳米管变为一个中空的无定形纳米线结构.用原子位移理论和溅射机理对这种转变过程进行了分析.γ射线轰击碳纳米管击出碳原子,碳原子停留在晶格的间隙位置上产生间隙原子,在它原来的平衡位置则留下一个空位.当轰击粒子动能足够大时导致碰撞级联效应,无序结构增加.多数空位和间隙原子可能相互复合而彼此退火,但仍有少数原子作为间隙原子而造成晶格进一步缺陷.辐射也可以引起碳原子的溅射,溅射出来的碳原子沉积在碳纳米管的外壁上形成一层无定形碳结构.     

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.     

Energy dissipation mechanisms in carbon nanotube oscillators

Energy transfer from the translational degrees of freedom to phonon modes is studied for isolated systems of two coaxial carbon nanotubes, which may serve as a nearly frictionless nano-oscillator. It is found that for oscillators with short nanotubes (less than 30 A) a rocking motion, occurring when the inner tube is pulled about 1/3 out of the outer tube, is responsible for significant phonon energy acquisitions. For oscillators with long nanotubes translational energies are mainly dissipated via a wavy deformation in the outer tube undergoing radial vibrations. Frictional forces between 10(-17) and 10(-14) N per atom are found for various dissipative mechanisms.     

B and N-doped double walled carbon nanotube: a theoretical study

The structural and electronic properties of boron and nitrogen atom substitutional doping in (8,0)@(13,0) (semiconductor@semiconductor) and (6,0)@(13,0) (metallic@semiconductor) double walled carbon nanotubes, were obtained by using the first-principle calculations based on the density functional theory. In this framework, the electronic density plays a central role and it was obtained from a self-consistent field form. When boron or nitrogen substitutes a carbon atom the structure remains practically the same with negligible deformation observed around defects in all configurations considered. The electronic band structure results indicate that the boron doped systems behave as a p-type impurity, however, the nitrogen doped systems behave as an n-type impurity. In all the systems investigated here, we found that, in the cases of semiconductor@semiconductor tubes, they were the easiest to incorporate a B atom in the outer-wall and an N atom in the inner-wall of the nanotube.     

Mechanical Properties of Single-Walled （5,5） Carbon Nanotubes with Vacancy Defects

First-principles simulation is used to investigate the structural and mechanical properties of vacancy defective single-walled （5,5） carbon nanotubes. The relations of the defect concentration, distribution and characteristic of defects to Young＇s modulus of nanotubes are quantitatively studied. It is found that each dangling-bond structure （per supercell） decreases Young＇s modulus of nanotube by 6.1% for symmetrical distribution cases. However the concentrative vacancy structure with saturated atoms has less influence on carbon nanotubes. It is suggested that the mechanical properties of carbon nanotubes depend strongly upon the structure and relative position of vacancies in a certain defect concentration.     

碳纳米管封装Si20团簇的热稳定性研究

用分子动力学方法模拟一种特殊结构Si20 (表面异构的硅十二面体结构)填充到不同管径的扶手椅型碳纳米管中组成复合结构的热稳定性。通过能量分析和定量统计缺陷多边形数量等方法来研究这种结构在碳纳米管中的稳定性和结构演变情况。研究发现Si20的热稳定性和碳纳米管的直径关系密切;其在CNT(15, 15)中的热稳定性最好,当管径逐渐增加时,其热稳定性呈下降趋势;直至管径增加到CNT(21, 21),碳纳米管对Si20的空间限制作用变得很小,以至于不足以维持Si20的稳定。此外,Si原子因热振动替换碳纳米管中C原子而形成十二边形缺陷,这对碳纳米管的热稳定性有着明显的降低作用。     

Calculation of the energy of binding of titanium and scandium complexes to the surface of carbon nanotubes

Complexes of zigzag-type carbon nanotubes (CNTs) with transition metal atoms, scandium and titanium, were studied. It was demonstrated that the energy of binding of both atoms with a carbon surface decreases whereas the rate of diffusion along the surface increases with increasing nanotube diameter. The rate constant of migration of scandium atoms over a CNT surface are several orders of magnitude higher than that for titanium atoms, because the CNT surface—Sc atom binding energy is substantially lower.     

Existence and stability of co-axial and meshed double-walled armchair silicon nanotubes

A systematic study of armchair double-walled Si nanotubes (DWNT) (n, n)@(m, m) (3≤n≤6; 7≤m≤12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized with an all electron 3–21 G^⁎ basis set and the B3LYP functional. Analysis of the electronic structure properties of these tubes has also been performed with a larger basis set. The study indicates that the stabilities of the Si nanotubes are of the same order as those of single-walled Si nanotubes. It should be possible to experimentally synthesis both single-walled and double-walled Si nanotubes. The binding energy per atom or the cohesive energy of the nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. Nanotubes with small interlayer separations, called meshed tubes, do not hold the coaxial cylindrical structure after optimization. The SiNTs (n, n)@(n+3, n+3) are found to have large formation energies and binding energies per atom. For example, (3, 3)@(6, 6), (4, 4)@(7, 7), (5, 5)@(8, 8), and (6, 6)@(9, 9) all have large binding energies per atom, around 3.7 eV/atom. All Si nanotubes are found to be semiconductors. However, the band gap, in general, is observed to decrease from single walled nanotubes to double walled nanotubes.     

Potential energy distributions within and on single-walled and double-walled carbon nanotubes

The potential fields arising from dispersion forces induced by single- and double-walled carbon nanotubes of various chiral configurations and sizes on an argon test atom have been calculated, employing a discrete summation method that takes into account the translational symmetry of the carbon lattice of the different armchair and zigzag chiral conformations. The total interaction potential was considered to be pair-wise additive and described by the Lennard–Jones 6–12 potential expression. This has yielded radial field distributions as a function of axial distance within, outside, and between the walls these carbon nanotubes. It was found that these distributions depended not only upon position but also upon orientation of the axes with respect to the graphene sheets. The potential barriers at the open end of the carbon nanotubes have also been studied. The effect of surface curvature was evaluated by comparing a single graphite sheet to the corresponding segments of nanotubes containing the same number of carbon atoms. Using the calculated data, the Henry constants and diffusion coefficients have been computed together with their temperature dependencies and compared to other planar and microporous carbon systems.