Phonon trigonal warping effect in graphite and carbon nanotubes

The one-dimensional structure of carbon nanotubes leads to quantum confinement of the wave vectors for the electronic states, thus making the double resonance Raman process selective, not only of the magnitude, but also of the direction of the phonon wave vectors. This additional selectivity allows us to reconstruct the phonon dispersion relations of 2D graphite, by probing individual single wall carbon nanotubes of different chiralities by resonance Raman spectroscopy, and using different laser excitation energies. In particular, we are able to measure the anisotropy, or the trigonal warping effect, in the phonon dispersion relations around the hexagonal corner of the Brillouin zone of graphite.     

Electric field aligned growth of single-walled carbon nanotubes

Electric field aligned, single-walled carbon nanotubes are grown between electrodes using thermal chemical vapour deposition (CVD) of methane. The growth occurs on a thin film layered catalyst of aluminium, iron and molybdenum patterned on top of electrodes. The nanotubes bridge 10 μm sized electrode gaps and have a typical diameter of less than 2 nm as measured by Raman spectroscopy and atomic force microscopy. We present electrical transport measurements on a directly grown nanotube which shows p-type semiconducting behaviour.     

Covalent functionalization of multi-walled carbon nanotubes by lipase

Lipase from Candida rugosa was covalently anchored onto acid-treated multi-walled carbon nanotubes (MWNTs) through a self-catalytic mechanism. A variety of characterization techniques including FTIR, Raman spectroscopy, and XPS were employed to demonstrate the formation of the ester linkage between lipase and MWNTs. The MWNTs-lipase biocomposites showed significantly increased solubility in some common-used organic solvents, such as THF, DMF and chloroform. This study may offer a novel and facile route for covalent modification of carbon nanotubes, and expand the potential utilization of both lipases and MWNTs in the fields of biocatalyst and biosensor.     

Synthesis and characterization of polycaprolactone-grafted carbon nanotubes via click reaction

Polycaprolactone (PCL) was covalently grafted on the surface of carbon nanotubes by a simple click reaction of propargyl-terminated PCL (propargyl-PCL) and carbon nanotubes (CNTs) containing azide groups (MWNT-N₃). Propargyl-PCL was synthesized by the ring-opening polymerization of ε-caprolactone using propargyl alcohol and stannous octoate. MWNT-N₃ was prepared from MWNT having 2-bromoisobutyryl groups (MWNT-Br) with sodium azide by azidation. The melting temperature of propargyl-PCL was shifted to the high temperature in PCL-grafted MWNT. The thermal stability of PCL-grafted MWNT was enhanced as compared to that of propargyl-PCL. PCL was coated on the surface of MWNT with a high density of PCL chains, which showed good solubility of PCL-grafted MWNT in organic solvents. PCL-grafted MWNT was characterized with FT-IR, ¹H NMR, Raman, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscopy.     

The HSAB principle as a means to interpret the reactivity of carbon nanotubes

Polycyclic curved aromatic fragments (C₂₄H₁₂) have been employed as models of the single-walled carbon nanotubes (n,0), where n varies from 4 up to 30. Those structures were chosen on the basis of the analysis of the strain energy values calculated for the models possessing various sizes. The flat coronene structure has been chosen as a molecular fragment topologically resembling the honeycomb lattice in order to investigate the relation between the curvature and reactivity of the sidewalls of SWNTs. In the current study we took into account the interaction of CO and NH₂ (treated as probe molecules) with the exterior surface of nanotubes. Obtained results illustrate that both total as well as local hardness and/or molecular electrostatic potential (MEP) can be a good measure for the reactivity if the influence of geometrical changes is considered. The systematic theoretical studies also show that the calculated interaction energies of sorbed CO on those models are related to the both types of hardness. On the other hand, in the case of amidogen sorbed on the nanotube surface the correlation between the binding energy and MEP is visible. Those differences can be explained by various kinds of the adsorption mechanism, i.e. physical or chemical adsorption, respectively.     

Enhanced interface interaction between modified carbon nanotubes and magnesium matrix

Carbon nanotube (CNT)/metal interface interaction is critical to the mechanical properties of CNT-reinforced metal matrix composites (MMCs). In this paper, in order to realize the chemical modification of the interface interaction between CNTs and Mg matrix, different types of defects (monovacancy, carbon and oxygen adatoms, as well as p-type boron and n-type nitrogen substitution) are introduced in CNTs to investigate the effect of the defects on the interface interaction (E_ib) between CNT and Mg (0 0 0 1) surface. Moreover, two models (adsorption model and interface model) are compared and validated to investigate the interface interaction. It is revealed that the CNT with the carbon adatom has the highest E_ib with the Mg (0 0 0 1), and the effect of boron doping on E_ib is superior to the intermediate oxygen which has already been proved experimentally in the enhancement of the interface interaction in MMCs. In terms of the electronic structure analysis, we reveal the micro-mechanism of the increase of E_ib under the action of different types of defects, and propose that the presence of holes (boron dopant) and the unsaturated electrons in CNTs can generate the chemical interaction between CNT and Mg matrix effectively. Our results are of great scientific importance to the realization of robust interfacial bonding between CNTs and Mg matrix via the reinforcement modification, so as to enhance the mechanical properties of CNTs reinforced Mg matrix composites.     

Preparation and physical/electrochemical characterization of carbon nanotube-chitosan modified pencil graphite electrode

In this work, preparation and characterization of single-walled carbon nanotube-chitosan (SWNT-chitosan) modified disposable pencil graphite electrode (PGE) was carried out. Firstly, commercial single-walled carbon nanotube was purified and characterized using thermal gravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDX) for this purpose. Purified SWNT was mixed with chitosan polymer for preparing their composite. Then, PGE was modified with this composite. The characterization of the modified electrode was carried out using atomic force microscopy (AFM). The electrochemical behaviour of the obtained electrode was investigated and compared with the electrochemical behaviour of chitosan modified and unmodified PGEs using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and alternative current (AC) impedance spectroscopy. In order to obtain more sensitive electrochemical signals, the effect of SWNT concentration was studied. This modified electrode also showed electrocatalytic effect for hydrogen evolution.     

Optical absorption of graphite and single-wall carbon nanotubes

A review of the electronic dipole transitions in graphite and single-wall carbon nanotubes is presented. Because of its singular electronic structure, the optical absorption matrix element as a function of wave vector has a node in the two-dimensional Brillouin zone of graphite, which depends linearly on the optical polarization direction. In the case of the single-wall carbon nanotubes, the dipole selection rule and the van Hove singularity in the joint density of states will give a characteristic behavior, which is observed by luminescence and resonance Raman spectroscopy. PACS 78.30.Na; 78.20.Bh; 78.66.Tr; 63.22.+m; 36.20.Kd; 36.20.Ng     

Electron diffraction on graphite nanocrystals in the walls of carbon nanotubes

The structure and defects in the walls and cavities of carbon nanotubes were examined by electron diffraction, transmission electron microscopy, energy dispersive X-ray analysis and Raman spectroscopy. The predominating defects in the walls of the nanotubes are graphite nanocrystals having a preferential orientation in the direction of the nanotube axis, and another significant type of defects are particles of the catalyst in the nanotube cavities. In the cavities, also the presence of molybdenum was proved having its origin in the catalyst.     

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.     

半导体性单壁碳纳米管与其石墨衬底表面相互作用的第一性原理计算

利用基于密度泛函理论的计算方法,研究了处于石墨（HOPG）表面的半导体性单壁碳纳米管,发现碳纳米管下面的石墨受碳管的作用向下凹陷,而纳米管本身虽然保持其形状不变,但它的电子态受石墨衬底影响,造成导带下移,禁带宽度明显减小. 关键词： 单壁碳纳米管 密度泛函理论 局域密度近似方法     

Interaction of C60 with carbon nanotubes and graphite

The interaction of C60 with single-wall carbon nanotubes (SWNTs) and graphite is studied experimentally by thermal desorption spectroscopy and theoretically by molecular-mechanics and molecular-dynamics calculations. The van der Waals parameters and force field for C60-graphene and C60-SWNT interactions are derived from the low-coverage C60 binding energy to the graphite surface. We use these to compare the efficiency of different mechanisms by which C60 can be encapsulated into SWNTs.     

Chemisorption of atomic hydrogen in graphite and carbon nanotubes

An orthogonal tight-binding model of the carbon-hydrogen interaction was modified to deal with the different hybridization states of atomic hydrogen on carbon surfaces, without explicitly including charge self-consistency. The resulting model has great flexibility and computational efficiency, generally with a good quantitative accuracy. The non-self-consistent C-H model was tested by calculating structural properties of small hydrocarbons and simple polymers, and against ab initio results of H binding to both perfect and defective graphite. The model was employed to study the chemisorption properties and dynamics of atomic hydrogen on perfect and defective surfaces of graphite and carbon nanotubes.     

Enhancing the thermoelectric performance through the mutual interaction between conjugated polyelectrolytes and single-walled carbon nanotubes

We present a method of constructing composites composed of conjugated polyelectrolytes(CPEs)and singlewalled carbon nanotubes(SWCNTs)to obtain a high-performing flexible thermoelectric generator.In this approach,three kinds of polymers,namely,poly[(1,4-(2,5-didodecyloxybenzene)-alt-2,5-thiophene](P1),poly[(1,4-(2,5-bis-sodium butoxysulfonate-phenylene)-alt-2,5-thiophene](P2),and poly[(1,4-(2,5-bis-acid butoxysulfonic-phenylene)-alt-2,5-thiophene](P3)are designed,synthesized and complexed with SWCNTs as thermoelectric composites.The electrical conductivities of the CPEs/SWCNTs(P2/SWCNTs,and P3/SWCNTs)nanocomposites are much higher than those of non-CPEs/SWCNTs(P1/SWCNTs)nanocomposites.Among them,the electrical conductivity of P2/SWCNTs with a ratio of 1:4 reaches 3686 S·cm^-1,which is 12.4 times that of P1/SWCNTs at the same SWCNT mass ratio.Moreover,CPEs/SWCNTs composites(P2/SWCNTs)display remarkably improved thermoelectric properties with the highest power factor(PF)of 163μW·m^-1·K^-1.In addition,a thermoelectric generator is fabricated with P2/SWCNTs composite films,and the output power and power density of this generator reach 1.37μW and 1.4 W·m;(cross-section)at△T=70 K.This result is over three times that of the thermoelectric generator composed of non-CPEs/SWCNTs composite films(P1/SWCNTs,0.37μW).The remarkably improved electrical conductivities and thermoelectric properties of the CPEs/SWCNTs composites(P2/SWCNTs)are attributed to the enhanced interaction.This method for constructing CPEs/SWCNTs composites can be applied to produce thermoelectric materials and devices.     

Electron-phonon interaction and transport in semiconducting carbon nanotubes

We calculate the electron-phonon scattering and binding in semiconducting carbon nanotubes, within a tight-binding model. The mobility is derived using a multiband Boltzmann treatment. At high fields, the dominant scattering is interband scattering by LO phonons corresponding to the corners K of the graphene Brillouin zone. The drift velocity saturates at approximately half the graphene Fermi velocity. The calculated mobility as a function of temperature, electric field, and nanotube chirality are well reproduced by a simple interpolation formula. Polaronic binding give a band-gap renormalization of approximately 70 meV, an order of magnitude larger than expected. Coherence lengths can be quite long but are strongly energy dependent.     

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     

单壁碳纳米管在石墨基底上运动的分子动力学模拟

采用分子动力学模拟方法研究单壁碳纳米管在石墨基底上的运动.首先碳纳米管在基底弛豫至平衡状态，然后对其施加一固定外力，撤去外力后，碳纳米管在基底上逐渐减速至停止.为了研究管径、手性角对运动方式的影响，本文选择了C(10,10)，C(10,9)，C(10,8)，C(10,5)，C(10,0)，C(8,8)六种单壁碳纳米管进行模拟.结果表明，碳纳米管在石墨基底上的运动方式由手性角决定，与管径无关.手性角等于30°时，碳纳米管与石墨基底之间为公度结构，碳纳米管的运动出现周期性的滑动和翻滚现象；手性角大于28.3°小于30°时，碳纳米管一边向前滑动一边滚动；手性角小于26.3°时，碳纳米管在基底上滑动.碳纳米管的手性角决定了它与石墨基底接触界面的微观构型，从而决定了碳纳米管的运动方式. 关键词： 分子动力学模拟 碳纳米管 动能 结构公度性     

On-site repulsion as the source of pairing in carbon nanotubes and intercalated graphite

We show that different non-conventional superconductors have one fundamental feature in common: pair eigenstates of the Hamiltonian are repulsion-free, the W = 0 pairs. In extended Hubbard models, pairing can occur for reasonable parameter values. For (N, N) nanotubes the binding energy of the pair depends strongly on the filling and decreases towards a reduced but nonzero value for the graphite sheet N → ∞. Received 13 July 2002 Published online 29 November 2002     

Defect electron states in carbon nanotubes and graphite from the NEXAFS spectroscopy data

Experimental results of studying the electronic structure of single-walled and multi-walled carbon nanotubes as well as graphite by X-ray absorption spectroscopy (or NEXAFS spectroscopy) are presented. The C1s absorption spectra are measured with high energy resolution using the equipment of the Russian-German beamline of the BESSY electron storage ring. Features found in absorption spectra of carbon nanotubes and graphite for the first time are interpreted in the case of carbon nanotubes as the contribution of electron states appearing due to the imperfection of their structure both under the nonequilibrium synthesis conditions and during the subsequent producing manipulations.