Raman spectra of double-wall carbon nanotubes under extreme uniaxial stress

We investigated the pressure dependence of the Raman frequencies and intensities of the D and G bands of double-wall carbon nanotubes under strong uniaxial conditions. Using moissanite anvils, we observed for the first time the evolution of the D band under extreme stress/pressure conditions. We find that the difference between D and G frequencies remains constant over the whole stress range. In addition, we observe that double-wall carbon nanotubes behave elastically up to the maximum uniaxial stress reached in our experiments, which is estimated to be about 12 GPa.     

Rayleigh scattering studies on inter-layer interactions in structure-defined individual double-wall carbon nanotubes

Although the inter-layer coupling in layered materials has attracted considerable interest due to its importance in determining physical properties of two-dimensional systems, studies on the inter-layer coupling in one-dimensional systems have so far been limited. Double-wall carbon nanotubes (DWCNTs) are one of the most fundamental and ideal model systems to study the inter-layer coupling in one-dimensional systems. In this work, Rayleigh scattering spectroscopy and transmission electron microscopy are used to characterize the electronic transition between inner-and outer-nanotubes of the exactly same individual DWCNT. We find that the inter-layer coupling is strong, leading to downshifts in most of the optical transition energies (up to ～0.2 eV) compared to isolated CNTs. We also find that the presence of metallic tubes lead to stronger shifts. The inter-layer screening of Coulomb interactions is one of the key factors in explaining the observed results.      

Controllable preparation and properties of single-/double-walled carbon nanotubes

In this paper, we discussed recent studies done in our laboratories with a floating catalyst chemical vapor deposition (CVD) method. We can grow single- or double-walled carbon nanotubes (SWNTs/DWNTs) with different kinds of catalysts. Single-walled carbon nanotubes without amorphous carbon coating were prepared by thermally decomposing acetylene (C₂H₂) at the temperature range of 750–1200 °C with ferrocene as catalyst. While with sulfur promoted ferrocene catalyst, double-walled carbon nanotubes were mass-produced by pyrolizing C₂H₂ at the temperature range of 900–1100 °C. Furthermore, tunable growth of DWNTs with different diameter was achieved in our experiment. It is found that DWNTs produced at lower carbon partial pressure have much smaller inner tubes, even DWNTs with the smallest inner diameter of 0.4 nm was found in here. As convenient and effective tool, radial breathing mode (RBM) of Raman scattering technique can be used to distinguish SWNTs from DWNTs. In further studies of Raman scattering with DWNTs, the possible match of the inner tubes and the outer tubes according to the RBM bands was assigned, and different chirality types were discussed according to the diameter and chirality dependence of resonant Raman vibration. We also investigated the temperature-dependent frequency shift of resonant Raman spectra of DWNTs in the range of 78–650 K. We found that different RBM peaks, which are relative to different tube diameters, have different temperature coefficient of frequency shift, and the larger diameter carbon nanotubes have more RBM frequency downshift with increasing temperature. It is ascribed to the RBM frequency variation to the temperature dependence of the stretching force constant of C–C bond. Besides, Polarized Raman spectra were preformed on well-aligned SWNTs structure fabricated through post-growth method and found that the angular dependence of Raman intensity is consistent well with the predictions of the resonance Raman theory.     

Structure determinations of double-wall carbon nanotubes grown by catalytic chemical vapor deposition

We report an application of nanoarea electron diffraction for structure determination of double-wall carbon nanotubes (DWNT) grown by catalytic chemical vapor deposition. The structures of 30 tubes were determined from experimental diffraction patterns. Among these tubes, the inner and outer wall structure of 18 tubes was precisely determined by comparison with kinematic electron diffraction simulations. For the structure of the DWNTs, our experiment revealed a mixture of semiconducting-metallic (S-M), S-S and M-M tubes. The spacing between the two walls ranges from 0.335 nm to 0.384 nm. Most DWNTs are incommensurate and chiral.     

Redox protein noncovalent functionalization of double-wall carbon nanotubes: electrochemical binder-less glucose biosensor

Double wall carbon nanotubes are noncovalently functionalized with redox protein and such assembly is used for construction of electrochemical binder-less glucose biosensor. Redox protein glucose oxidase performs as biorecognition element and double wall carbon nanotubes act both as immobilization platform for redox enzyme and as signal transducer. The double carbon nanotubes are characterized by cyclic voltammetry and specific surface area measurements; the redox protein noncovalently functionalized double wall carbon nanotubes are characterized in detail by X-ray photoelectron spectroscopy, cyclic voltammetry, amperometry, and transmission electron microscopy.     

In situ Raman spectroelectrochemistry as a tool for the differentiation of inner tubes of double-wall carbon nanotubes and thin single-wall carbon nanotubes

In situ Raman spectroelectrochemistry has been used to distinguish between thin single-wall carbon nanotubes (SWCNT) and the inner tubes of double-wall carbon nanotubes (DWCNT). The spectroelectrochemical method is based on the different change of the electronic structure of the inner tube in DWCNT and that of SWCNT during electrochemical charging, which is reflected in the Raman spectra. During electrochemical charging the inner tubes of DWCNT exhibit a delayed attenuation of the intensities of their Raman modes as referred to the behavior of SWCNT of similar diameter. The changes are pronounced for the radial breathing mode (RBM), and thus, these modes are diagnostic for the distinction of inner tubes of DWCNT from the thin SWCNT. The different sensitivities of inner and outer tubes to the applied electrochemical charging is a simple analytical tool for differentiation of SWCNT and DWCNT in a mixture. The significance of the proposed method is demonstrated on a commercial DWCNT sample.     

电子束辐照下单壁碳纳米管的结构不稳定性

利用透射电镜(TEM)原位观察了一端固定一端自由和两端固定的单壁碳纳米管(SWNT(s))在电子束辐照下的结构不稳定性.研究发现,一端固定一端自由的SWNT优先轴向和径向收缩后颈缩,最后形成一个个碳笼紧密相连的收缩结构("碳笼-碳笼"结构);两端固定的SWNT仅径向收缩后颈缩,最后形成许多碳笼相连的类似结构.此外,后者在电子束辐照下断开后又会重新粘合起来,表现出很强的表面塑性流变或湿润效应.这些电子束辐照诱导SWNTs非热激活结构不稳定性现象可以用我们最近提出的表面纳米曲率效应和能量束超快诱导软模和点阵失稳进行全新、全面、正确的解释.     

由不同碳源合成及制备纳米碳管的进展

纳米碳管可通过石墨、煤炭及炭黑等高含碳固体物，甲烷、乙烯或苯之类的有机化合物，一氧化碳和碳化硅等含碳无机化合物的转化来制备，甚至还可由低分子芳烃化合物来逐级合成。根据各种不同碳源，按不同转化过程及制备方法进行了总结与归纳，期望能对纳米碳管的合成及制备有一总体理解；通过简要、系统地介绍各种转化过程及制备方法，并对其中一些有发展前途的方法特别是电弧法、激光烧蚀法及化学气相沉积法的最新改进和发展进行了考察，分析了这些方法的优缺点；最后概述了目前纳米碳管规模制备方法的进展情况。     

Functionalization of single- and multi-walled carbon nanotubes with cationic amphiphiles for plasmid DNA complexation and transfection

The possibility of delivering DNA efficiently to cells represents a crucial issue for the treatment of both genetic and acquired diseases. However, even although the efficiency of non-viral transfection systems has improved in the last decade, none have yet proven to be sufficiently effective in vivo. We report herein our results on the functionalization of single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) by two cationic amphiphiles (lipid RPR120535 and pyrenyl polyamine), their use for the complexation of plasmid DNA, and their efficiency in transfecting cells in vitro. The experiments have shown that the efficiency of transfection is higher when using SWNT instead of MWNT, and that transfection efficiency is similar or slightly higher when using nanoplexes (SWNT/lipid RPR120535/DNA) instead of lipoplexes (lipid RPR120535/DNA) and several orders of magnitude higher than that of naked DNA. This study therefore shows both that the transfection is better when using SWNTs and that it is dependent on the nature of the amphiphilic molecules adsorbed on the nanotubes.      

In situ Raman study on single- and double-walled carbon nanotubes as a function of lithium insertion

We investigated the electrochemical lithium ion (Li(+)) insertion/desertion behavior on highly pure and bundled single- and double-walled carbon nanotubes (SWNTs and DWNTs) using an in situ Raman technique. In general, two storage sites could host Li(+) in SWNT and DWNT bundles when varying an external potential: a) the outer surface sites, and b) the interstitial spaces within the bundles. The most sensitive changes in the tangential mode (TM) of the Raman spectra upon doping with Li(+) can be divided into two regions. The first region was found from 2.8 to 1.0 V (the coverage of Li(+) on the outer surface of a bundled nanotube) and was characterized by the loss of resonant conditions via partial charge transfer, where the G(+) line of the SWNT and the TM of the outer tube of DWNTs experienced a highly depressed intensity, but remained almost constant in frequency. The appearance of a Breit-Wigner-Fano (BWF) profile provided strong evidence of metallic inner tubes within DWNTs. The second region was observed when the applied potentials ranged from 0.9 to 0 V and was characterized by Li(+) diffusion into the interstitial sites of the bundled nanotube material. This phenomenon invoked a large downshift of the G(-) band in SWNTs, and a small downshift of the TM of the inner tube of DWNTs caused by expansion of the C--C bonds due to the charge transferred to the nanotubes, and the disappearance of the BWF profile through the screening effect of the interstitial Li(+) layers.     

Symmetry analysis of relative motions in a double-wall carbon nanotube

The binding energy of a double-wall carbon nanotube (DWNT) is theoretically studied as a function of the relative longitudinal shift and relative rotation of the component single-wall carbon nanotubes (SWNTs). It is shown that the binding energy is an oscillating function of the relative shift and rotation, with the oscillation period depending on the relations between symmetry elements of the SWNTs. The results of numerical calculations of the binding energy of DWNTs, performed in the approximation of weak van der Waals interlayer interaction, are presented.     

Analysis of different techniques for modelling excitons in carbon nanotubes

It is now widely accepted that exciton effects play an important role in the optical processes of carbon nanotubes. However, previous theoretical work has tended to concentrate on an understanding of exciton binding energies, with little attention paid to calculations of exciton sizes. In this work we employ two approaches: a k·p model and a variational technique, and we examine their ability to make simultaneous predictions of exciton binding energy and size, while comparing our results to previous work taken from the literature.     

不同基底上碳纳米管的制备及生长机理

以二茂铁为催化前驱物,C2H2为碳源,N2为载气,采用浮动催化法,在蓝宝石、单晶硅、石英、玻璃以及碳纤维基底上制备了不同形貌和结构的碳纳米管.利用扫描电子显微镜和Raman光谱对碳纳米管的形貌和微结构进行了表征.结果表明在蓝宝石和石英基底上所生长的碳纳米管薄膜具有较好的定向性和较高的石墨化程度;单晶硅基底上碳纳米管薄膜呈"底疏顶密"分布;而玻璃和碳纤维基底上所生长的碳纳米管不具有定向性.最后,对不同基底对碳纳米管生长的影响进行了分析和讨论.     

Field emission properties of carbon nanotubes with different morphologies

The field emission behavior of base-model well-aligned carbon nanotubes (Base-CNTs), curled carbon nanotubes (Curled-CNTs), and tip-model well-aligned CNTs (Tip-CNTs) was examined. The nanotubes were fabricated by means of direct current plasma-enhanced chemical vapor deposition using different ammonia (NH₃) pre-treatment plasma currents. The turn-on electric field values required to obtain a 10-nA current for Base-CNTs, Curled-CNTs, and Tip-CNTs were determined at 3.8, 4.3, and 4.9 V/μm, respectively. The field enhancement factor γ of Base-CNTs, calculated from a Fowler–Nordheim plot, was higher than that for the Curled-CNTs and Tip-CNTs. In the presence of a strong electric field, argon ion irradiation permanently straightened the as-grown Curled-CNTs films. The straightening process enhanced the emission properties of the as-grown Curled-CNTs films by decreasing the turn-on field and increasing the total emission current. Thus, morphology parameters of the MWNTs significantly affect the emission properties of CNTs.     

碳纳米管"种子"上生长碳纳米管和氮掺杂碳纳米管

以碳纳米管(CNTs)作"种子",液体石蜡和三聚氰胺分别作碳源和碳/氮源,利用爆炸辅助化学气相沉积法在CNTs"种子"上合成出新的CNTs和氮掺杂碳纳米管(CNx).透射电镜(TEM)和电子能量损失谱(EELS)测试表明:新合成的CNTs具有空心管状结构,而CNx呈竹节状且氮的原子分数高达17.3%.CNTs"种子"的作用归因于其端部的纳米级弯曲和开放性边缘具有吸附并外延Cn/CN物种的功能.     

The reinforcement role of different amino-functionalized multi-walled carbon nanotubes in epoxy nanocomposites

Functionalization with different amino groups of multi-walled carbon nanotubes was achieved and nanotube-reinforced epoxy nanocomposites were prepared by mixing amino-functionalized multi-walled carbon nanotubes with epoxy resin. Differential scanning calorimetry, thermogravimetric analysis and bending tests were used to investigate the thermal and mechanical properties of the nanocomposites. The results showed that different kinds of amino-functionalized multi-walled carbon nanotubes would have different effects on the thermal and mechanical properties of the nanocomposites. The reinforcement mechanism of amino-functionalized multi-walled carbon nanotubes in epoxy resin was discussed.     

Boron nitride and carbon double-wall hetero-nanotubes: first-principles calculation of electronic properties

First-principles calculations based on density functional theory with the generalized gradient approximation were carried out to investigate the electronic properties of boron nitride and carbon double-wall hetero-nanotubes of different chirality and size. The results show that the electronic structures of the double-wall hetero-nanotubes near the Fermi level are dominated by the p?electrons of carbon atoms, regardless of whether the carbon nanotube is inside or outside the boron nitride nanotube. Double-wall hetero-nanotubes consisting of semiconducting carbon and boron nitride nanotubes are semiconductors. An opening of a band gap is observed for armchair carbon and boron nitride double-wall hetero-nanotubes with small intertube spacing due to the intertube interaction and the changes of symmetry.     

A facile method to fabricate silica-coated carbon nanotubes and silica nanotubes from carbon nanotubes templates

Silica-coated multiwalled carbon nanotubes (MWCNTs) have been prepared by the sol–gel polymerization of tetraethoxysilane (TEOS) in the presence of the acid-oxidized MWCNTs at room temperature, followed by oxidizing the MWCNTs templates at high temperature in air to produce hollow silica nanotubes. The thickness and architectures of silica shell were well controlled by rationally adjusting the concentration of TEOS, and by adding cationic surfactant as a structure-directing agent. These results also give a clear answer to prove the fact that the structures of spherical silica particles can be fully “copied” to the coating shell and the wall of silica nanotubes when prepared by the same method as the synthesis of silica particles in the presence of templates.     

Vibration analysis of single-walled carbon nanotubes using different gradient elasticity theories

The present work aims at investigating the vibrational characteristics of single-walled carbon nanotubes (SWCNTs) based on the gradient elasticity theories. The small-size effect, which plays an essential role in the dynamical behavior of nanotubes, is captured by applying different gradient elasticity theories including stress, strain and combined strain/inertia ones. The theoretical formulations are established based upon both the Euler–Bernoulli and the Timoshenko beam theories. To validate the accuracy of the present analysis, molecular dynamics (MDs) simulations are also conducted for an armchair SWCNTs with different aspect ratios. Comparisons are made between the aforementioned different gradient theories as well as different beam assumptions in predicting the free vibration response. It is shown that implementation of the strain gradient elasticity by incorporating inertia gradients yields more reliable results especially for shorter length SWCNTs on account of two small scale factors corresponding to the inertia and strain gradients. Also, the difference between two beam models is more prominent for low aspect ratios and the Timoshenko beam model demonstrates a closer agreement with MD results.     

Vibration analysis of single-walled carbon nanotubes using different gradient elasticity theories

The present work aims at investigating the vibrational characteristics of single-walled carbon nanotubes (SWCNTs) based on the gradient elasticity theories. The small-size effect, which plays an essential role in the dynamical behavior of nanotubes, is captured by applying different gradient elasticity theories including stress, strain and combined strain/inertia ones. The theoretical formulations are established based upon both the Euler–Bernoulli and the Timoshenko beam theories. To validate the accuracy of the present analysis, molecular dynamics (MDs) simulations are also conducted for an armchair SWCNTs with different aspect ratios. Comparisons are made between the aforementioned different gradient theories as well as different beam assumptions in predicting the free vibration response. It is shown that implementation of the strain gradient elasticity by incorporating inertia gradients yields more reliable results especially for shorter length SWCNTs on account of two small scale factors corresponding to the inertia and strain gradients. Also, the difference between two beam models is more prominent for low aspect ratios and the Timoshenko beam model demonstrates a closer agreement with MD results.