Calculations of electric capacitance in carbon and BN nanotubes, and zigzag nanographite (BN, BCN) ribbons

Electronic states in nanographite ribbons with zigzag edges are studied using the extended Hubbard model with nearest neighbor Coulomb interactions. The electronic states with the opposite electric charges separated along both edges are analogous as nanocondensers. Therefore, electric capacitance, defined using a relation of polarizability, is calculated to examine nano-functionalities. We find that the behavior of the capacitance is widely different depending on whether the system is in the magnetic or charge polarized phases. In the magnetic phase, the capacitance is dominated by the presence of the edge states while the ribbon width is small. As the ribbon becomes wider, the capacitance remains with large magnitudes as the system develops into metallic zigzag nanotubes. It is proportional to the inverse of the width, when the system corresponds to the semiconducting nanotubes and the system is in the charge polarized phase also. The latter behavior could be understood by the presence of an energy gap for charge excitations. In the BN (BCN) nanotubes and ribbons, the electronic structure is always like that of semiconductors. The calculated capacitance is inversely proportional to the distance between the positive and negative electrodes.     

Rh在单壁碳纳米管上吸附的密度泛函理论研究

本文利用密度泛函理论研究了Rh原子在（6,6）单壁碳纳米管内外的吸附行为. 通过对Rh在单壁碳纳米管上不同吸附位的吸附构型与吸附能的研究发现: Rh吸附在管内、外的洞位最稳定, 且管外吸附比在管内强. 这是由于单壁碳纳米管的卷曲效应使得管外电荷密度比管内大造成的. 态密度分析表明, 吸附在管内外的Rh原子的5s电子均转移到了4d轨道上; Rh原子4d轨道上的电子转移到了（6, 6）碳管上, 使Rh带正电, 碳管带负电. 结合能带分析表明, Rh原子吸附在管内磁性较弱, 而吸附在管外较强. 关键词： 密度泛函理论 单壁碳纳米管 Rh原子 吸附     

Theoretical study of Si impurities in BN nanotubes

Density functional theory is employed to investigate the electronic and structural properties of substitutional Si impurities in a (10,0) BN nanotube. For the Si case, the band structure shows a level centered on the Si atom crossing the Fermi energy and no net spin is found. The Si introduces three localized exchange splitted Si levels in the gap. The formation energies show that the Si is likely to be present at N-rich conditions.Received: 9 December 2003, Published online: 28 May 2004PACS: 61.46. + w Nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals - 85.35.Kt Nanotube devices     

Crystal orbital study on the zigzag single-walled carbon nanotubes with multi-dichlorocarbene addition

Structure and electronic properties of zigzag single-walled carbon nanotubes with multi-dichlorocarbene addition are investigated using self-consistent field crystal orbital method. It is found that the addition can cause large deformation of the tubes and significantly modify the band structures. The addition can even cause semiconductor-metal phase transition. Furthermore, the mobility and conductivity are also calculated using the deformation potential approach for the addition systems.     

Interaction between methanol and single-walled carbon nanotubes: Density functional theory study

Density functional calculations have been performed to investigate the dependence of methanol interaction with the side walls of single-walled carbon nanotubes (SWCNTs) on the nanotube's type, curvature and chirality. The author's results show that methanol prefers to be physically adsorbed on semiconducting CNTs in comparison with the metallic one. It was found that the binding energy of methanol is increased for adsorption on larger-diameter nanotubes. Furthermore, we find that when a methanol molecule was adsorbed on higher chiral angle nanotubes the binding energy was increased. The study of the electronic structures and Mulliken analysis indicate that the methanol and CNT are interacting rather weakly, consistent with recent experimental observation.     

Theoretical study of defects of BN nanotubes: A molecular-mechanics study

We investigate the structure and the formation energy of the pentagon–heptagon pairs (5775) defects of BN nanotubes using a molecular-mechanics method. For (n, 0) BN nanotubes, the homoelemental bonds of the 5775 defect may be located nearly either parallel or perpendicularly in the direction of tube axis. The former is energetically favored and strongly affects the reduction of the strain with decreasing radius. The formation energy of the 5775 defect, which increases with increasing radius, is lower than that of the tetragon–octagon pairs (4884). The 5775 defect of BN nanotubes is structurally and energetically stable, as compared to the 4884 defect.     

Magnetic and electronic properties of zigzag BN nanoribbons with nonmetallic atom terminations: A first-principles study

The boron nitride (BN) nanosheet is an isostructural analog of graphene and can be viewed as the structure that C atoms in graphene are replaced with alternating B and N. The easily modulated band-gap of BN nanosheet by simply passivating its edge(s) makes it is promising for many potential applications in nanodevices and nanoelectronics. We further systematically theoretically study the magnetic and electronic properties of passivated-ZBNNR by nonmetallic atom(s), here. According to our calculations, all considered structures show magnetic feature, and the ZBNNRs can be metal or half-metal or semiconductor depending on the termination details. The great application-potential of the passivated-ZBNNRs is further confirmed based on our results.     

Boron nitride nanotubes with quadrangular cross sections: Density functional studies

Density functional theory (DFT) calculations have been performed to investigate the availabilities and properties of boron nitride nanotubes (BNNTs) with quadrangular cross sections. To achieve the purposes, the original structure of a representative BNNT was individually decorated by the carbon and silicon atoms to make the C-BNNT and Si-BNNT models. The sp³ hybridizations were set for the C and Si atoms to make possible the formation of the quadrangular cross sections for the BNNTs. The optimized results indicated that the investigated models could be stabilized; however, they showed different properties. The atomic scale properties based on computations of quadrupole coupling constants (C_Q) also approved different properties for the C-BNNT and Si-BNNT models. Moreover, the C_Q parameters indicated that the properties of C-BNNT could be considered similar to the original BNNT; however, more discrepancies were observed for the Si-BNNT.     

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.     

First-principles study on electron transport through BN-dimer embedded zigzag carbon nanotubes

First-principles calculations are performed for electron transmission through a metallic zigzag carbon nanotube with substitutional BN dimers parallel to the nanotube axis. The transmission coefficient is calculated in the energy range (around the charge neutrality point) in which there exist two degenerate subbands for each spin. Wave functions in the circumferential direction of one of the degenerate subbands can be chosen so as to have nodes at the position of a carbon dimer parallel to the nanotube axis. It is shown that the transmission probability of an incident wave with such wave-function nodes depends crucially on positions of BN dimers relative to the nodes. By placing each of dimers at one of the nodes, the transmission probability is substantially enhanced and is well described by the Born approximation in spite of spatially extended scattering potential due to ionized B and N. This suggests that the arrangement in the circumferential direction of various impurities influences transport through metallic zigzag carbon nanotubes.     

Density functional study of the reaction of O2 with a single site on the zigzag edge of graphene

The reaction between molecular oxygen and an isolated zigzag graphene edge has been studied using density functional theory at the B3LYP/6-31G(d) level of theory. The initial reaction forms a peroxide, ΔH = −135 kJ mol⁻¹. If the graphene edge is pre-oxidised, the dangling peroxy atom can (E_a = 91 kJ mol⁻¹) migrate across contiguous ketone groups until finding another vacant site and stabilizing as a ketone. However, if no further vacant sites are available, the peroxy oxygen has a number of other possibilities open to it, including desorption of an oxygen atom (E_a = 140 kJ mol⁻¹), migration via the basal plane to form a lactone (E_a = 147 kJ mol⁻¹), and direct interaction with an adjacent oxide to form the lactone or a carbonate (E_a = 146 kJ mol⁻¹). The combination of thermal energy and the heat released in the initial formation of the peroxy adduct is likely to be sufficient to overcome these secondary barriers at modest temperatures.Transfer of the dangling peroxy O to the basal plane produces an epoxide that is mobile on the basal surface (E_a = 40–80 kJ mol⁻¹) but that is transferred back to the edge upon coming into proximity of either a vacant edge site or ketone. The instability of the edge epoxide structure implies that it cannot play a significant role in carbon gasification through promoting the reactivity of ketones, contrary to earlier suggestions.The desorption of an oxygen atom creates a very active species capable of reacting with basal or edge sites as well as with oxygen complexes. The reaction of ketone + O has been reported to yield a five-membered ring + CO₂, leading to an overall stoichiometry which is consistent with the observed oxyreactivity of carbon surface oxides identified in isotopic labelling studies in which one O atom is gasified and the other forms a new surface oxide.     

Nonlinear conductivity of single-walled zigzag carbon nanotubes

The conductivity of the single-walled zigzag carbon nanotube system was studied in an alternating electric field with the intensity vector along the axis of nanotubes. The electronic carbon nanotube system was macroscopically considered in terms of the Boltzmann kinetic equation in the constant relaxation time approximation while omitting the interaction with the phonon subsystem. The nonlinear responses to the applied harmonic field were calculated and analyzed.     

Electronic properties of multi-defected zigzag carbon nanotubes

Electronic properties of multi-defected zigzag single-walled carbon nanotubes are investigated by use of the tight-binding Green's function method. The Stone-Wales defects and the vacancies are considered. We find that the conductance sensitively depends on the realistic defect configurations for the metallic zigzag carbon nanotubes. Interestingly, the electronic transport properties of the nanotubes with three vacancies can be considered as the sum effect of two double-vacancies, while those with Stone-Wal...     

Multiple-walled BN nanotubes obtained with a mechanical alloying technique

An experimental method to obtain multiple-walled nanotubes of BN using low energy is presented. The method is based on the use of mechanical alloying techniques with elemental boron powders and nitrogen gas mixed in an autoclave at room temperature. The chemical and structural characteristics of the multiple-walled nanotubes were obtained using different techniques, such as X-ray diffraction, transmission electron microscopy, EELS microanalysis, high-resolution electron microscopy images and theoretical simulations based on the multisliced approach of the electron diffraction theory. This investigation clearly illustrates the production of multiple-wall BN nanotubes at room temperature. These results open up a new kind of synthesis method with low expense and important perspectives for use in large-quantity production. PACS 61.16.Bg; 79.60.Jv; 61.46.1w; 61.50.Ah     

Dimer-covering resonating-valence-bond treatment of single-walled zigzag carbon nanotubes

Single-walled zigzag carbon nanotubes with h hexagons around the carbon nanotube, h ranging from 3 to 19, have been investigated from a resonating-valence-bond point of view. The energies calculated for the undoped h = 3n–1 zigzag carbon nanotubes, n integer, suggest that the two lowest-lying phases are degenerate. Therefore, de-confined low-energy topological spin defects would occur. Then, these carbon nanotubes should be conductors, in analogy to polyacethylene. In clear contrast, no such degeneracy is obtained for either, h = 3n+1 or h = 3n, so bound pairs of topological spin defects are expected to occur in these cases. Our findings provide further insights into electron correlation and exchange effects in carbon nanotubes.     

Electromagnetic solitons in bundles of zigzag carbon nanotubes

The possibility of forming solitons in zigzag carbon nanotubes is investigated using the coupled equations for the classical function of the electron distribution and the Maxwell equations for an electromagnetic field. It is demonstrated that the solitons are generated as a result of correlated changes in the classical distribution function and the electric field induced by nonequilibrium electrons of a carbon nanotube. The effective equation describing the dynamics of the electromagnetic field is derived. The existence of solitons is confirmed by the results of numerical calculations. The characteristics of solitons are investigated as a function of the diameter of zigzag carbon nanotubes.     

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.     

BN管状团簇到单壁纳米管的几何结构和电子性质

利用密度泛函理论(DFT),对氮化硼(BN)管状团簇的几何结构、稳定性和电子性质进行了研究.选取合适的BN结构单元作为结构生长基元,采用逐层生长的方式计算得到有限长度、不同截面尺寸的稳定管状团簇.结构中B-N交替排列,结构组成中的四元环和六元环数目均符合一般表达式.计算结果表明,通过适当组装管状团簇以及碳原子掺杂,可以制备出带隙可调的单壁氮化硼纳米管.     

F掺杂的BN纳米管及电导特性

利用一种新颖的催化生长方法,在生长BN纳米管的过程中直接引入F原子,获得了均匀F掺杂的BN纳米管.高分辨透射电子显微镜研究表明,构成BN纳米管的六元环由于F掺杂而被严重扭曲,纳米管壁由一些高度卷曲的连续片层构成.电学性质测量表明,相对于无掺杂的BN纳米管而言,F掺杂BN纳米管的电导显著增加.