Atomic correlation between adjacent graphene layers in double-wall carbon nanotubes

Atomic correlation between adjacent graphene layers was elucidated for double-wall carbon nanotubes (DWNTs) through a chiral index assignment of two nested nanotubes by high-resolution transmission electron microscopy. Our analysis provides a rather constant diameter difference close to 0.75 nm but no chiral angle correlation between the constituent nanotubes in the concentric DWNTs. The local atomic correlation as a commensurate graphene stacking was repeatedly found in eccentric DWNTs and circumscribed nanotubes, which should lead to elastic deformation and bundling of nanotubes.     

Band engineering of double-wall Mo-based hybrid nanotubes

Hybrid transition-metal dichalcogenides(TMDs) with different chalcogens on each side(X-TM-Y) have attracted attention because of their unique properties. Nanotubes based on hybrid TMD materials have advantages in flexibility over conventional TMD nanotubes. Here we predict the wide band gap tunability of hybrid TMD double-wall nanotubes(DWNTs) from metal to semiconductor. Using density-function theory(DFT) with HSE06 hybrid functional, we find that the electronic property of X-Mo-Y DWNTs(X = O and S, inside a tube; Y = S and Se, outside a tube) depends both on electronegativity difference and diameter difference. If there is no difference in electron negativity between inner atoms(X) of outer tube and outer atoms(Y) of inner tube, the band gap of DWNTs is the same as that of the inner one. If there is a significant electronegativity difference, the electronic property of the DWNTs ranges from metallic to semiconducting, depending on the diameter differences. Our results provide alternative ways for the band gap engineering of TMD nanotubes.     

Conductivity of double-walled carbon nanotubes at pressures of up to 30 GPa

Results from experimental studies of the electrical resistance of double-walled carbon nanotubes (DWNTs) in conducting diamond high-Pressure chambers at pressures of up to 30 GPa and room temperature are presented. The effect pressure has on the structure of the outer and inner tubes is analyzed. Ranges of pressure are found in which the electrical resistance of DWNTs changes considerably.     

n-type and p-type double-walled carbon nanotube field-effect transistors based on charge-transfer modulation

Electrical transport properties of double-walled carbon nanotubes (DWNTs) are modulated by encapsulating alkali-metal Cs atom or C₆₀ molecules via a plasma ion-irradiation method. The pristine DWNTs are found to exhibit ambipolar semiconducting behavior due to their small bandgap. In contrast, Cs and C₆₀ encapsulated DWNTs exhibit high performance unipolar n-type and p-type semiconducting behavior since they can operate as an electron donor and acceptor, respectively. Moreover, by controlling the filling level, p–n junction is found to be formed in DWNTs by Cs encapsulation. Our results indicate that DWNTs have great potential as building blocks for various electronic nano devices. PACS 73.63.Fg; 73.63.-b; 61.48.+c     

Synthesis and Raman scattering study of double-walled carbon nanotube peapods

Double-walled carbon nanotubes (DWNTs) encapsulating C₆₀ fullerenes were successfully synthesized by gas phase diffusion method. The obtained peapods were examined using high-resolution transmission electron microscopy (HRTEM). The HRTEM images indicate that the ordered packing phases of fullerene molecules inside are sensitively related to the inner-tube radius of DWNTs. Also, Raman measurements were carried out for the first time to characterize DWNTs peapods. There are obvious differences between the Raman spectrum of DWNTs peapods and that of SWNTs peapods. The intensities of resonances from C₆₀ in the former are much stronger than those in the latter. In addition, changes of tangential mode (TM) and radial breathing mode (RBM) of DWNTs after C₆₀ doping were observed. The possible reasons are discussed in the text.     

On the existence and stability of double-walled armchair silicon carbide nanotubes

A systematic study of type 1 armchair double-walled SiC nanotubes (DWNTs) (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 using the hybrid functional B3LYP (Becke’s three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all-electron 3-21G* basis set. The study indicates that the stabilities of the double-walled SiC nanotubes are of the same order as those of single-walled SiC nanotubes suggesting the possibilities of experimental synthesis of both single-walled and double-walled SiC nanotubes. The binding energy per atom or the cohesive energy of the double-walled nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. The formation energy of the DWNTs is found to be maximum when the interlayer separation is about 3.5 Å. The DWNTs (n,n)@(n+4,n+4) are found to have large formation energies. In particular, (5,5)@(9,9) DWNT is the most stable tube in our study with a binding energy per atom of 5.07 eV, the largest formation energy of 12.39 eV, an interlayer separation of 3.58 Å and a “band gap” of 1.97 eV. All double-walled SiC nanotubes are found to be semiconductors, with the band gaps decreasing from single-walled nanotubes to double-walled nanotubes.     

Dynamic column buckling of multi-walled carbon nanotubes under axial impact load

This paper studies the dynamic column buckling of multi-walled carbon nanotubes (MWNTs) under axial impact load. The analysis is based on the continuum mechanics model and a simplified model for the van der Waals forces between adjacent layers. By introducing initial imperfections for MWNTs and applying the method of preferred mode, a buckling condition is derived for the buckling load and associated buckling mode. In particular, explicit expressions are obtained for double-walled carbon nanotubes (DWNTs). Finally, numerical calculations are worked out for a DWNT and a five-layer MWNT with different length-to-radius ratios.     

Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene

We have measured a strictly linear pi plasmon dispersion along the axis of individualized single-wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single-wall carbon nanotubes. Comparative ab initio studies on graphene-based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects cause a mixing of electronic transitions, including the "Dirac cone," resulting in the observed linear dispersion.     

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.     

Formation and structure of circular-disc assemblies of double-walled carbon nanotubes from a catalytic CVD reaction

A large-size circular-disc (CD) assembly of carbon nanotubes (CNTs) was formed from injection catalytic chemical vapor deposition (CVD) reactions. The CNT-CD assembly, which has an outer diameter of 830 nm, consists of closely packed rings of double-walled carbon nanotubes (DWNTs), in twenty-eight circles, and stacked in five layers. A structural interpretation suggests that the CNT-CD assembly is coiled from a “tape-like” bundle of DWNTs, whose coiling to form the CNT-CD resembles the rolling up of tape to form a disc. The large CD assembly was formed within milliseconds in the transient gas flow reactions, and the coiling of nanotubes or “nucleation” of the circular assembly was associated with shears generated by gas disturbances in the vapor phase.     

Effects of vacancy percentage on the energy gap of zigzag single-wall carbon nanotubes

The influence of vacancy percentage on the energy gap of zigzag single-wall carbon nanotube is investigated by the Green's function method in coherent potential approximation. Our probes for various kinds of zigzag single-wall carbon nanotubes show that by increasing vacancy percentage the energy gap is also increased, so for metallic single-wall carbon nanotubes, a metallic to semi-metallic transition is occurred. However, any transition does not appear for semiconductor carbon nanotubes. So by controlling on concentration of vacancies, one can make a semiconductor SWCNT with a predetermined energy gap which is useful in nanoelectronic devices.     

Optical transitions in single-wall boron nitride nanotubes

Optical transitions in single-wall boron nitride nanotubes are investigated by means of optical absorption spectroscopy. Three absorption lines are observed. Two of them (at 4.45 and 5.5 eV) result from the quantification involved by the rolling up of the hexagonal boron nitride (h-BN) sheet. The nature of these lines is discussed, and two interpretations are proposed. A comparison with single-wall carbon nanotubes leads one to interpret these lines as transitions between pairs of van Hove singularities in the one-dimensional density of states of boron nitride single-wall nanotubes. But the confinement energy due to the rolling up of the h-BN sheet cannot explain a gap width of the boron nitride nanotubes below the h-BN gap. The low energy line is then attributed to the existence of a Frenkel exciton with a binding energy in the 1 eV range.     

Observation of ordered phases of fullerenes in carbon nanotubes

We have assembled molecular arrays of C60 inside double-walled carbon nanotubes (DWNTs) with internal diameters of 11-26 A and directly observed the existence of different crystalline phases of C60 previously predicted theoretically. The structure of the encapsulated C60 crystal is defined by the internal diameter of the DWNT, as the molecules adjust their packing arrangement in order to maximize van der Waals interactions. We have also shown that fullerenes in C(60)@DWNT interact with the outer layer of DWNTs, as demonstrated by the efficient filling of DWNTs with internal diameters of less than 12 A.     

Determination of optical isomers for left-handed or right-handed chiral double-wall carbon nanotubes

The handedness relationship between adjacent layers in nested double-wall carbon nanotubes (DWNTs) has been investigated for the first time. Our high-resolution electron microscopy analysis on a series of specimen tilts can successfully tell the handedness of each constituent nanotube in a DWNT, and therefore the chiral indices (n, m) including their optical isomers [(n, m) or (m, n)] of inner and outer nanotubes can be uniquely determined. It is shown that right-handed and left-handed nanotubes are equally distributed for both the inner and outer nanotubes in the examined specimens and a preferable handedness relationship between the adjacent layers in DWNT may exist.     

有限长双壁碳纳米管的电子输运性质

基于Landauer公式，研究了有限长的非公度和公度双壁碳纳米管的电子输运性顾，结果表明 ，双壁管的几何结构对其电子输运性质有显著的影响：非公度的双壁碳管的电导随能量的不 同，既可以是弹道型的，也可以是非弹道型的；由armchair管组成的公度的双壁碳管的电导 随能量变化呈现快速的电导振荡，并且此快速振荡叠加在背景慢振荡上，而zigzag管组成的 公度双壁管的电导随能量变化只有快速振荡、没有规则的慢振荡背景. 关键词： 碳纳米管 电子输运性质     

Cluster modeling of three types of double-walled armchair silicon carbide nanotubes

A systematic study of three types of armchair double-walled SiC 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 using the hybrid functional B3LYP (Becke’s three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all electron 3-21G* basis set. The study indicates that the stabilities of the double-walled SiC nanotubes are of the same order as those of single-walled SiC nanotubes suggesting the possibility of experimental synthesis of both single-walled and double-walled SiC nanotubes. The binding energy per atom or the cohesive energy of the double-walled nanotubes depends not only on the number of atoms but also on the coupling of the constituent singlewalled nanotubes and their types. A study of the binding energies, Mulliken charges, density of states and HOMO-LUMO gaps has been performed for all nanotubes from (n,n)@(n + 3,n + 3) to (n,n)@(n + 6,n + 6) (n = 3 ? 6). Type 2 DWNTs do not preserve the coaxial geometry when the difference in the chirality of the outer and inner tubes is 5 or less. For type 3, this occurs when the chirality difference is 4 or less. The gaps of types 2 and 3 DWNTs are lower than those of the corresponding single-walled nanotubes and are significantly less than those of type 1 DWNTs.     

单壁碳纳米管力学行为的数字散斑相关法实验研究

通过直接单向拉伸超长单壁碳纳米管束长绳,首次借助高精度数字散斑相关法,并结合显维放大技术,测量了单壁碳纳米管的弹性模量和拉伸强度。试验中观察了单壁碳纳米管束长绳的断裂过程。单壁碳纳米管束长绳通过改进的化学气相沉积技术生成。试验得到单壁碳纳米管的平均杨氏模量为129.0±70.3GPa,平均拉伸强度为1.95±0.56GPa,低于计算值和先前其它文献的试验值。     

Chemically active substitutional nitrogen impurity in carbon nanotubes

We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less than 1 at. %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.     

On the limiting physical adsorption of hydrogen in carbon materials

The specific features of hydrogen adsorption (and adsorption of other gases) at supercritical temperatures (specifically, the absence of capillary condensation and polymolecular adsorption and the appearance of a maximum in the adsorption isotherm in the pressure range 1–10 MPa) are discussed. Hydrogen adsorption decreases by an order of magnitude as the temperature increases from the critical temperature to the room value. The experimental adsorption isotherms in the supercritical range found in the literature are used to deduce a criterion of limiting hydrogen adsorption at various temperatures. Carbon adsorbents of different types (individual single-wall nanotubes, bundles of such nanotubes, multiwall nanotubes, and carbon fibers) are considered. A model of single graphite plane shows that the limiting hydrogen adsorption is 5 wt % at 77 K and 1 wt % at 293 K. These values can only be approached by adsorption in a material made of individual single-wall nanotubes. Methods to increase the adsorption are proposed.