Experimental study of the Raman strain rosette based on the carbon nanotube strain sensor

This work presents a new technique named Raman strain rosette for the micro‐strain measurement of both Raman active and Raman inactive materials. The technique is based on the theoretical model of the carbon nanotube (CNT) strain sensor that applies the resonance and polarization Raman properties of CNTs and calculates the synthetic contributions of uniformly dispersed CNTs to the entire Raman spectrum. In our work, the proposed technique is applied in different experiments on the Raman inactive materials, such as step‐by‐step uniaxial tensile and Raman mapping around a circular hole. The experimental results reached by the Raman strain rosette are consistent with the actual values as a whole. This study verifies that the Raman strain rosette is applicable to quantitative measurement of all the in‐plane components of the strain tensor (including both normal and shear strains) by three polarized Raman detections for each sampling spot on a microscale. The technique is further applicable to achieving the strain fields through Raman mapping. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

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.      

The effect of conjugation length distribution on the properties of modified PPV

The aim of this work was to demonstrate the optical properties of a copolymer involving alternating poly(para-phenylene–vinylene) and ether groups derived from the conjugation length distribution. For this, the time-dependent density functional theory (TD-DFT) was used. Thus, different conjugation lengths were considered, where the correlation of the corresponding theoretical spectra, including Raman, optical absorption and photoluminescence, allows us to conclude that the conjugation length distribution is caused by a random distribution of the ether blocks. The weight of each conjugation length representing its contribution is shown by the use of the Raman spectra. Furthermore, the theoretical spectra arising from the response of different conjugation lengths are obtained and compared with the experimental ones.     

Effects of microwave plasma treatment on the field emission properties of printed carbon nanotubes/Ag nano-particles films

The effects of Ar microwave plasma treatment on field emission properties of the printed carbon nanotubes (CNTs) cathode films using Ag nano-particles as binder were investigated. The field emission J-E characteristics were measured at varied plasma treatment time. Significant improvement in emission current density, emission stability and uniformity were achieved for the Ar treated CNTs films, even though the plasma treatment increased the turn on electric field slightly. High-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy revealed the microstructural changes of CNTs after the plasma treatment. The improved field emission properties of CNTs film can be attributed to the generation of a high density of structural defects after treatment, which increased greatly the possible emission active sites. Besides, the formation of the sharpened and open-ended CNTs tips is all helpful for improving the field emission properties of the treated CNTs.     

Synthesis and field emission properties of carbon nanotubes grown in ethanol flame based on a photoresist-assisted catalyst annealing process

Carbon nanotubes (CNTs) have been grown directly on a Si substrate without a diffusion barrier in ethanol diffusion flame using Ni as the catalyst after a photoresist-assisted catalyst annealing process. The growth mechanism of as-synthesized CNTs is confirmed by scanning electron microscopy, high resolution transmission-electron microscopy and energy-dispersive spectroscopy. The photoresist is the key for the formation of active catalyst particles during annealing process, which then result in the growth of CNTs. The catalyst annealing temperature has been found to affect the morphologies and field electron emission properties of CNTs significantly. The field emission properties of as-grown CNTs are investigated with a diode structure and the obtained CNTs exhibit enhanced characteristics. This technique will be applicable to a low-cost fabrication process of electron-emitter arrays.     

Field emission properties of electrophoretic deposition carbon nanotubes film

The field emission properties of electrophoretic deposition(EPD) carbon nanotubes (CNTs) film have been improved by depositing CNTs onto the titanium (Ti)-coated Si substrate, followed by vacuum annealing at 900 °C for 2 h, and the enhanced emission mechanism has been studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and Raman spectroscopy. Field emission measurements showed that the threshold electric field was decreased and the emission current stability was improved compared to that of EPD CNTs film on bare Si substrate. XRD and Raman spectroscopy investigations revealed that vacuum annealing treatment not only decreased the structural defects of CNTs but made a titanium carbide interfacial layer formed between CNTs and substrate. The field emission enhancement could be attributed to the improved graphitization of CNTs and the improved contact properties between CNTs and substrate including electrical conductivity and adhesive strength due to the formed conductive titanium carbide.     

Structural dependence of nonlinear elastic properties for carbon nanotubes using a continuum analysis

Based on molecular mechanics coupled with the atomistic-based continuum theory, a structural mechanics approach is presented to examine the nonlinear elastic properties of carbon nanotubes (CNTs) subjected to large axial deformations. According to molecular mechanics, the interaction force between atoms is modeled using the Morse potential. The nanoscale continuum theory is established to directly incorporate the Morse potential function into the constitutive model of CNTs. In this paper, we simulate and examine the influence of CNT structures on the stress–strain response. The linear elastic property of CNTs is independent of the helicity of the hexagonal carbon lattice along the tubes, while their nonlinear elastic behavior shows a larger chirality dependence. The present theoretical approach supplies a set of very simple formulas and is able to serve as a good approximation of the mechanical properties of CNTs. PACS 62.20.-x; 62.20.Dc; 62.25.+g     

单轴、双轴应变Si拉曼谱应力模型

本文依据拉曼光谱原理, 基于Secular方程及拉曼选择定则分别获得了单轴、 双轴(001), (101), (111)应变Si材料应变张量与拉曼谱线移动的定量关系, 并在此基础上, 基于广义胡克定律最终建立了单轴、双轴(001), (101), (111)应变Si材料拉曼谱峰与应力的理论关系模型. 该模型建立过程详细、系统, 所得结果全面、量化, 可为应变Si材料应力的测试分析提供重要理论参考.     

Mapping the intracellular distribution of carbon nanotubes after targeted delivery to carcinoma cells using confocal Raman imaging as a label-free technique

The uptake of carbon nanotubes (CNTs) by mammalian cells and their distribution within cells is being widely studied in recent years due to their increasing use for biomedical purposes. The two main imaging techniques used are confocal fluorescence microscopy and transmission electron microscopy (TEM). The former, however, requires labeling of the CNTs with fluorescent dyes, while the latter is a work-intensive technique that is unsuitable for in situ bio-imaging. Raman spectroscopy, on the other hand, presents a direct, straightforward and label-free alternative. Confocal Raman microscopy can be used to image the CNTs inside cells, exploiting the strong Raman signal connected to different vibrational modes of the nanotubes. In addition, cellular components, such as the endoplasmic reticulum and the nucleus, can be mapped. We first validate our method by showing that only when using the CNTs' G band for intracellular mapping accurate results can be obtained, as mapping of the radial breathing mode (RBM) only shows a small fraction of CNTs. We then take a closer look at the exact localization of the nanotubes inside cells after folate receptor-mediated endocytosis and show that, after 8-10 h incubation, the majority of CNTs are localized around the nucleus. In summary, Raman imaging has enormous potential for imaging CNTs inside cells, which is yet to be fully realized.     

高能球磨对碳纳米管形貌及场致发射显示特性的影响

用化学气相沉积法制备了碳纳米管,进行了不同时间的球磨处理。用扫描电镜、拉曼光谱对其形貌和结构进行了表征。对不同球磨条件下的碳纳米管制备成阴极,进行了场致发射特性的测试。结果表明,高能球磨会对碳纳米管的形貌、结构及场致发射性能有明显的影响。球磨时间为0．5～1h时,可以使碳纳半管变短而均匀,且场致发射电性能与未处理时相近,即有低的阈值电场和高的发射电流密度,从而使发射时在阳极上产生的荧光点密度大大增加,发光均匀。但研磨时间过长会改变碳纳半管结构,使其非晶化或石墨化,导致其场致发射性能和显示效果变差。     

Detailed profiling of CNTs arrays along the growth window in a floating catalyst reactor

We report a detailed longitudinal and depth profiles of multi-wall carbon nanotubes (CNTs) arrays synthesized using xylene and ferrocene in a floating catalyst reactor. Point to point analyses of the CNTs grown in a “growth window” with CNTs arrays longer than 0.5 mm were performed using optical microscopy, Raman spectroscopy, FESEM, high-resolution TGA/DTA, and TEM techniques. The heights of the CNTs arrays show a maximum at a mid point of the growth window, while a reverse trend of minimum is observed for iron-to-CNTs atomic ratios. The ratio of amorphous carbon to CNTs sharply increases along the growth window and from the bottom to top of CNTs arrays. The CNTs diameter also increases along the growth window, due to deposition of the amorphous carbon, which can be almost removed by temperature programmed oxidation up to around 500 °C. A base growth mechanism, the variations of catalyst content, residence time and temperature profile along the growth window, the adsorption and decomposition of polycyclic aromatic hydrocarbons to amorphous carbon, and a limited diffusion of hydrocarbon species through the arrays covered by excessive amorphous carbon may explain the results.     

Predicting the nonlinear tensile behavior of carbon nanotubes using finite element simulation

Carbon nanotubes (CNTs) possess extremely high mechanical properties and could be the ultimate reinforcing materials for the development of nanocomposites. In this work, a Finite Element (FE) model based on the molecular mechanics theory was developed to evaluate tensile properties of single-walled carbon nanotubes (SWCNTs). The deformation and fracture of carbon nanotubes under tensile strain conditions were studied by common FE software, Ansys. In this model, individual carbon nanotube was simulated as a frame-like structure, and the primary bonds between two nearest-neighboring atoms were treated as beam elements. The beam element properties were determined via the concept of energy equivalence between molecular dynamics and structural mechanics. So far, several researches have studied the elastic behavior of CNTs, and its nonlinearity is not well understood. The novelty of the model lies on the use of nonlinear beam elements to evaluate SWNTs tensile failure. The obtained calculated mechanical properties show good agreement with existing numerical and experimental results.     

One-pot synthesis of one-dimensional array Pt nanoparticles on carbon nanotubes via a facile microwave polyol method

High-density attachment and one-dimensional array Pt nanoparticles (NPs) on carbon nanotubes (CNTs) to generate Pt/CNTs heterostructures are obtained via one-pot microwave polyol method. The morphology, composition of as-obtained Pt/CNTs heterostructures is characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD). The Raman spectrum and Fourier transform infrared (FTIR) spectrum show the introduction of defects or functional groups on CNTs surface, which are crucial factors to assist the nucleation and growth of Pt NPs along the skeleton of CNTs.     

Raman spectroscopy of strained GeSi alloys deposited on Ge substrates

The Raman scattering method has been successfully used to investigate the properties of GeSi alloys deposited on Ge substrates in this paper. The effect of Si composition and strain in the GeSi alloy on the Raman shifts of Ge-Ge, Ge-Si and Si-Si phonon modes is studied. The relationship between them have been derived by the assumption that the Raman shifts is nearly linear with Si composition and strain in the GeSi alloys. The experimental data show reasonable agreement with the fits.     

Band gap of carbon nanotubes under combined uniaxial–torsional strain

Within tight-binding model, the band gaps of armchair and zigzag carbon nanotubes (CNTs) under both uniaxial tensile and torsional strains have been studied. It is found that the changes in band gaps of CNTs depend strongly on the strain type. The torsional strain can induce a band gap for armchair CNTs, but it has little effect on band gap of the zigzag CNTs. While the tensile strain has great effect on band gap of zigzag CNTs, but it has no effect on that of the armchair CNTs. More importantly, when both the tensile and torsional strains are simultaneously applied to the CNTs, the band gap changes of armchair CNTs are not equal to a simple sum over those induced separately by uniaxial tensile and torsional strains. There exists a cooperative effect between two kinds of strains on band gap changes of armchair CNTs. But for zigzag CNTs, the cooperative effect was not found. Analytical expressions for the band gaps of armchair and zigzag CNTs under combined uniaxial–torsional strains have been derived, which agree well with the numerical results.     

LPCVD法制备碳纳米管薄膜及其场发射性能的研究

采用低压化学气相沉积法(LPCVD)在镍片上直接制备碳纳米管(CNTs)薄膜,系统地研究了生长温度(500~800℃)对碳纳米管薄膜形貌、结构及场发射性能的影响,并对此方法的生长机理进行了分析。当温度从500℃升高到650℃时,碳纳米管的生长速率随着温度升高而增大,而温度继续上升,速率则明显减小。利用扫描电镜(SEM)和拉曼光谱仪表征和检测了碳纳米管薄膜的形貌和结构。碳纳米管的管径、长度、一致性和晶化程度随温度都有明显的变化。同时还对碳纳米管薄膜的场发射特性进行了测试,对其场发射机理进行了深入地探讨,表明温度对碳纳米管的性能有很大影响,并存在最优化的温度条件。实验结果表明碳纳米管薄膜的形貌、结构及其场发射性能可通过生长温度进行一定范围的控制。     

Carbon nanotube bundles and thin layers probed by micro-Raman spectroscopy

Raman spectra of single-wall carbon nanotubes (CNTs) either in the form of micrometer sized bundles or thin layers prepared by dilution and sonication of powders have been compared. We have been able to collect the Raman spectrum of nanotube bundles that are not in touch with the substrate, and therefore not affected by interactions with the substrate surface. This spectrum resulted to be similar to that of the precursor nanotube powders, whereas relevant changes in the Raman spectrum are detected when the diluted powders form very thin layers on either metallic or insulating surfaces, as probed by confocal microraman imaging on well defined areas of the CNTs layers. In the case of thin layers, the intensity of the Raman D band, detected between 1 320 and 1 340 cm^-1 and ascribed to disorder effects, is strongly enhanced. This enhancement occurs independently on the kind of substrate. Received 2 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: sangalet@dmf.bs.unicatt.it     

Hydrogen plasma enhanced alignment on CNT-STM tips grown by liquid catalyst-assisted microwave plasma-enhanced chemical vapor deposition

Carbon nanotubes are grown directly on a scanning tunneling microscopy tip by liquid catalyst-assisted microwave-enhanced chemical vapor deposition, and effects of hydrogen plasma treatment on the tip have been investigated in detail by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Raman spectroscopy. The unaligned CNTs on the as-grown tip apex have been realigned and reshaped by subsequent hydrogen plasma treatment. The diameter of CNTs is enlarged mainly due to amorphous layers being re-sputtered over their outer shells.     

Preparation of highly conductive CNTs/polyaniline composites through plasma pretreating and in-situ polymerization

In this study, carbon nanotubes (CNTs) were pretreated by plasma, and further in-situ polymerized with aniline to achieve uniform CNTs/polyaniline (PANI) composites with high conductivity. The highest conductivity (2.946 S/cm) of CNTs/PANI composites under optimum plasma treating parameters is much higher than those without CNTs pretreatment or premodified by acid oxidation method. The scanning electronic microscope (SEM) pictures indicate that smooth surfaces of pristine CNTs (diameter: 20-40 nm) changes into rough structures and the size increases to around 90 nm, which is further proved by the X-ray diffraction (XRD), the Fourier transform infrared spectroscopy (FTIR), and the Raman measurements. Thermogravimetric analysis (TGA) results reveal that the thermal stability of CNTs/PANI composite is better than that of pure PANI.