Selective growth of individual multiwalled carbon nanotubes

Growth of individual, vertically aligned multiwalled carbon nanotubes (VACNT) on patterned Si wafers using dc plasma-enhanced CVD is described. The selective growth of individual VACNT within larger holes etched in Si is demonstrated for the first time.     

Effective electron drift mobility in multiwalled carbon nanotubes

We introduce the concept of effective electron drift mobility for multiwalled carbon nanotubes. This effective quantity is calculated under a quantum-box approach to conceive quantum transport in a given multiwalled carbon nanotube. In addition, effective motion time is determined.     

Analytical determination of the Fermi velocity in multiwalled carbon nanotubes

The quantized Fermi velocity in a multiwalled carbon nanotube is determined analytically by considering the tube as a one-dimensional quantum electrical conductor such that the above velocity scales with the number of layers of the tube. In fact, an expression for the magnitude of the quantized Fermi velocity is derived in terms of the free-electron mass and tube length. Comparison with previous theoretical results are made. In addition, from the expression for the Fermi velocity, the existence of an associated quantized electric field is discussed.     

Structural and vibrational properties of deformed carbon nanotubes

The fantastic variation of the physical properties of carbon nanotubes (CNTs) and their bundles under mechanical strain and hydrostatic pressure makes them promising materials for fabricating nanoscale electromechanical coupling devices or transducers. In this paper, we review the recent progress in this field, with much emphasis on our first-principles numerical studies on the structural and vibrational properties of the deformed CNTs under uniaxial and torsional strains, and hydrostatic pressure. The nonresonant Raman spectra of the deformed CNTs are also introduced, which are calculated by the first-principles calculations and the empirical bond polarizability model.      

Detection of invisible phonon modes in individual defect-free carbon nanotubes by gradient-field Raman scattering

We provide an effective method to investigate the field gradient effect in nanoconfined plasmon-matter interaction.Aligned ultralong SWNTs without defects were grown on marked substrates, followed by assembling gold nanoparticle clusters around individual nanotubes. The Raman scattering behavior of a nanotube placed in an atomic scale nanogap between adjacent nanoparticles was studied. In addition to the expected plasmon-induced Raman enhancement up to 103,the defect-free D-mode of an individual SWNT induced by gradient field is found for the first time. When the light is confined at atomic scale, gradient field Raman scattering becomes significant and dipole-forbidden phonon modes can be activated by quadrupole Raman tensor variation, indicating breakdown of the Raman selection rules.     

Fractional conductance quantization in multiwalled carbon nanotubes with disorder

A close relationship is established for describing fractional conductance quantization in multiwalled carbon nanotubes with disorder in the framework of the particle-in-a-box model. Our results agree well with experimental data in the context of nanotubes with defects.     

单壁碳纳米管的高压拉曼研究

采用金刚石对顶砧装置对直径分布在1.3 nm左右的单壁碳纳米管进行了高压拉曼光谱研究.实验结果表明随压力的增加碳管的截面形状发生了由圆到椭圆再到扁平的变化,这和我们之前的研究结果一致.从31 GPa卸压至常压后碳管的结构得到了较好的保持,这个压力值明显高于传统的Sp2键结构的碳材料所能稳定存在的压力范围(20 GPa以...     

Structural and vibrational properties of deformedcarbon nanotubes

The fantastic variation of the physical properties of carbon nanotubes (CNTs) and their bundles under mechanical strain and hydrostatic pressure makes them promising materials for fabricating nanoscale electromechanical coupling devices or transducers. In this paper, we review the recent progress in this field, with much emphasis on our first-principles numerical studies on the structural and vibrational properties of the deformed CNTs under uniaxial and torsional strains, and hydrostatic pressure. The nonresonant Raman spectra of the deformed CNTs are also introduced, which are calculated by the first-principles calculations and the empirical bond polarizability model.     

单壁手性碳纳米管Γ点E1和E2振动模式

在卷曲法计算得到的单壁碳纳米管12个非简并的本征频率与手性和管径关系的基础上，继续讨论了简并的本征频率中E₁和E₂的振动模式，以及其频率与手性和管径的关系. 关键词： 单壁碳纳米管 手性管 振动模式.     

Excitation energy dependence of Raman bands in multiwalled carbon nanotubes

Vertically aligned multiwalled carbon nanotubes (MWCNTs) were grown on 1‐ and 3‐nm cobalt (Co) films, at various growth times by microwave plasma enhanced chemical vapor deposition technique and their microstructural properties were analyzed with the help of Raman spectra that were obtained from different sources of laser excitation energies (E_L: 2.41, 1.96 and 1.58 eV). The variation of D and G band positions in MWCNTs grown on 1‐ and 3‐nm Co films follows a similar behavior, and an anomalous behavior was observed in the E_L dependence of the D′‐band wavenumber. In the second‐order spectra, the G′ band varied strongly according to structure with the laser excitation energy (E_L). The I_D/I_G ratio decreased with the increase of E_L for all MWCNTs; however, for a fixed E_L, the I_D/I_G dispersion is higher at lower E_L. The crystallite sizes were estimated using I_DI_G and E_L. We have shown that, for all MWCNTs, I_D/I_G ratio is inversely proportional to . Copyright © 2010 John Wiley & Sons, Ltd.     

Growth of carbon nanotubes using nanocrystalline carbon catalyst

The basic growth of carbon nanotubes (CNTs) involves dissociation of hydrocarbon molecules over a metal layer as a catalyst. Generally, the metals used for the catalyst include nickel, cobalt, gold, iron, platinum, and palladium. However, the metal catalyst used with CNTs could have a harmful influence on the electrical properties of electronic devices. Therefore, we propose the use of nanocrystalline carbon (nc-C) as the catalyst for the growth of CNTs. We used a nc-C catalyst layer deposited by the closed-field unbalanced magnetron (CFUBM) sputtering method, and CNTs were grown by the hot filament plasma-enhanced chemical vapor deposition (HF-PECVD) method with ammonia (NH₃) as a pretreatment and acetylene gas (C₂H₂) as a carbon source. The CNTs were grown on the nc-C layers pretreated with a variation of the pretreatment time. The characteristics of the pretreated nc-C layers and the grown CNTs were investigated by field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) measurements. Also, the structural variation of the pretreated nc-C layers was investigated by Raman measurement. We used the nc-C catalyst without metal, and we confirmed that our CNTs were composed with only carbon elements through an EDS measurement. Also, the pretreatment time was attributed to the growth of CNTs.     

Tunneling-current-induced light emission from individual carbon nanotubes

Tunneling electrons from a scanning tunneling microscope were used to excite light emission from individual multiwalled carbon nanotubes adsorbed on a highly ordered pyrolytic graphite surface. In the integral photon-intensity map, spatially uniform emission in the visible region was observed along the identical multiwalled carbon nanotubes. The emission spectra for the individual nanotubes showed unique profiles which differed with each nanotube, and were classified into two types. Our results indicate that the light emission is due to not the localized electronic states at the tube ends or defects but radiative transitions of electrons between the one-dimensional van Hove singularities, indicating that the two types of spectra are attributed to metallic and semiconducting nanotubes.     

Charge transport properties of composites of multiwalled carbon nanotube with metal catalyst and polymer: application to electromagnetic interference shielding

We report charge transport properties such as d.c. conductivity (σ_DC) and its temperature dependence for composites of poly(methyl methacrylate) (PMMA) and multiwalled carbon nanotubes (MWCNTs). The MWCNTs were synthesized through chemical vapor deposition with Fe or Co as catalyst. The MWCNTs were homogeneously dispersed in PMMA matrix through sonication to prepare MWCNT–PMMA composite films. We controlled mass concentration of MWCNTs in the composites, and the thickness of MWCNT–PMMA composite films was 20–400 μm. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy were used to study structure and homogeneity of the composites. The σ_DC at room temperature of MWCNT–PMMA composites increased as mass concentration of MWCNTs increased, which followed percolation theory. Electromagnetic interference (EMI) shielding efficiency (SE) of MWCNT–PMMA composites was measured in the frequency range of 50 MHz–3.5 GHz. We observed the increase of EMI SE of MWCNT–PMMA composites with increasing the concentration of MWCNTs.     

硅纳米颗粒在碳纳米管表面生长的分子动力学模拟

采用分子动力学模拟方法研究了硅纳米颗粒在碳纳米管上的生长，并分析了这种复合材料的基本结构.研究表明，由于硅原子和碳纳米管之间的相互作用以及碳纳米管的巨大的表面曲率，硅原子在碳纳米管表面不是形成覆盖碳纳米管的二维薄膜，而是生成具有三维结构的硅纳米颗粒.小纳米颗粒的结构和无基底条件下生成的颗粒结构基本一致.对于大纳米颗粒，不同于无基底条件下形成的球状纳米晶体硅结构，硅纳米颗粒沿管轴方向伸长，其结构为类似于硅晶体的无定形网络结构. 关键词： 纳米颗粒 碳纳米管 硅 分子动力学模拟     

非共振条件下单壁碳纳米管拉曼散射强度的计算

利用非共振情况下的键极化模型理论，对单壁碳纳米管的拉曼光谱强度进行了研究.考察了碳纳米管结构、入射光和散射光的偏振方向以及管轴的取向对散射光强度的影响.计算结果表明，光的偏振方向对拉曼散射强度影响较大，而手性对拉曼光谱的影响较小.针对碳管样品的实际情况，给出了无规取向碳管的拉曼散射光谱. 关键词： 碳纳米管 拉曼散射 声子 键极化     

Raman study on electrochemical lithium insertion into multiwalled carbon nanotubes

We have carried out in situ Raman studies during the electrochemical insertion of lithium ions (Li⁺) into pristine and thermally treated multiwalled carbon nanotubes (MWNTs). We found an improved structural integrity as well as the removal of defects in the thermally treated tubes. The different Li⁺ insertion behaviors above 0.5 V in as‐grown and thermally treated tubes could be explained by the presence of defects on the outer surface of the tubes. No change of Raman spectra from 2.8 to 0.8 V is characterized by the coverage of Li⁺ on the outer surface of tubes, whereas the upshift of G band and the absence of a separated G band below 0.75 V indicate the formation of diluted graphite intercalation below the stage‐2 phase (LiC₁₂). Copyright © 2008 John Wiley & Sons, Ltd.     

分子多振动模式的振动退相时间同时测量方法

利用作者自制的超连续谱激发时间分辨相干反斯托克斯拉曼散射(coherent anti-stokes Ranman scat-tering,CARS)谱仪同时测得反映分子组成的各种振动谱以及这些振动模式各自的退相时间.实验对苯甲腈样品分子振动谱中5个典型分子振动模式的振动弛豫过程进行了测量,一次获得了这种分子各振动模式对...     

Changes in the vibrational modes of carbon nanotubes induced by electron‐beam irradiation: resonance Raman spectroscopy

Influence of electron‐beam (e‐beam) irradiation on multi‐walled (MW) and single‐walled (SW) carbon nanotube films grown by microwave chemical vapor deposition technique is investigated. These films were subjected to an e‐beam energy of 50 keV from a scanning electron microscope for 2.5, 5.5, 8.0, and 15 h, and to 100 and 200 keV from a transmission electron microscope for a few minutes to ∼2 h continuously. Such conditions resemble an increased temperature and pressure regime enabling a degree of structural fluidity. To assess structural modifications, they were analyzed prior to and after irradiation using resonance Raman spectroscopy (RRS) in addition to in situ monitoring by electron microscopy. The experiments showed that with extended exposures, both types of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and formation of an intramolecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. RRS revealed that irradiation generated defects in the lattice as quantified through (1) variation of the intensity of radial breathing mode (RBM), (2) intensity ratio of D to G band (I_D/I_G), and (3) positions of the D and G bands and their harmonics (D* and G*) and combination bands (D + G). The increase in the defect‐induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while the SW ones transform from a semiconducting to a quasi‐metallic character. Softening of the q = 0 selection rule is suggested as a possible reason to explain these results. Furthermore, these studies provide a contrasting comparison between MW and SW nanotubes. Copyright © 2006 John Wiley & Sons, Ltd.     

Scattering by an array of parallel metallic carbon nanotubes

The scattering of electromagnetic wave by an array of parallel metallic single-walled carbon nanotubes is investigated based on the boundary-value method. Electronic excitations over each nanotube surface are modeled as an infinitesimally thin cylindrical layer of the free-electron gas. The scattering cross section of both transverse magnetic （TM） and transverse electric （TE） uniform plane waves by the system at normal incidences is obtained.     

Efficient and facile one pot carboxylation of multiwalled carbon nanotubes by using oxidation with ozone under mild conditions

In this study, oxidation of carbon nanotubes with ozone in the presence of hydrogen peroxide was investigated. The reaction was performed under clean and mild conditions and oxidized products with high concentration of oxygenated groups were yielded. The reaction products were characterized with attenuated total reflectance (ATR), Raman spectroscopy, scanning electron microscopy (SEM), X-ray diffractometry (XRD), back titration, X-ray photoelectron spectroscopy (XPS) and the dispersion behavior of the oxidized multiwalled carbon nanotubes (MWCNTs) was also studied. The results confirmed the presence of high concentrations of oxidative groups on the carbon nanotubes (CNTs) treated by the method of the present work.