Individuals, grasses, and forests of single- and multi-walled carbon nanotubes grown by supported Co catalysts of different nominal thicknesses

The relationships among the nominal thickness of Co catalyst, the structure of the catalyst particles, and the structure of carbon nanotubes (CNTs) growing from the catalyst during chemical vapor deposition were investigated. Various morphologies of CNTs such as individuals, random networks parallel to the surface of the substrate (‘grasses’), and vertically aligned forests of single- and multi-walled carbon nanotubes were grown by only varying the nominal thickness of catalyst under the same reaction condition. These different morphologies at the same growth time were due to the different areal density rather than to the length of CNTs. With increasing nominal thickness of catalyst, the catalyst particles changed in diameter while their areal density remained relatively almost constant. The change in diameter possibly affected the number ratio of active catalyst particles to the whole particles, which in turn affected the areal density of CNTs and yielded the various morphologies. Longer growth time increased the CNT length, which caused further change in CNT morphologies from individuals to grasses and grasses to forests.     

Phase states and magnetic properties of iron nanoparticles in carbon nanotube channels

The structure, phase composition, and magnetic properties of carbon nanotubes filled with iron nanoparticles and obtained by thermolysis of a mixture of ferrocene and C₆₀ fullerene or ferrocene and orthoxylene at a temperature of 800°C are investigated. Electron microscopy, X-ray diffraction, and Mössbauer spectroscopy data lead to the conclusion that carbon nanotubes are multilayer systems partially filled with iron nanoparticles and/or nanorods. Metallic inclusions in nanotube channels form α-Fe, γ-Fe, and Fe₃C phases. The concentration of each phase in the samples is determined. It is shown that 10–20-nm iron clusters in nanotubes exhibit magnetic properties typical of bulk phases of iron. High elasticity of carbon nanotube walls facilitates stabilization of the high-temperature γ-Fe phase; the relative concentration of this phase in a sample can be increased by lowering the concentration of ferrocene in the initial reaction mixture.     

Surface growth of single-walled carbon nanotubes from ruthenium nanoparticles

In this paper, we report that ruthenium is an active and efficient catalyst for growth of single-walled carbon nanotubes (SWNTs) by a chemical vapor deposition (CVD) process for the first time. High density random and horizontally superlong well-oriented SWNTs on substrate can be fabricated via CH₄ or EtOH as carbon source under suitable conditions. Scanning and transition electron microscopy investigations, Raman spectroscopy and atomic force microscopy measurements show the tubular structure, the high crystallinity, and the properties of the grown nanotubes. The results show that the SWNTs from ruthenium have better structural uniformity with less defects and provides an alternative catalyst for SWNTs growth. The successful growth of SWNTs by Ru catalyst provides new experimental information for understanding the growth mechanism of SWNTs, which may be helpful for their controllable synthesis.     

Zinc oxide catalyzed growth of single-walled carbon nanotubes

We demonstrate that zinc oxide can catalyze the growth of single-walled carbon nanotubes (SWNTs) with high efficiency by a chemical vapor deposition process. The zinc oxide nanocatalysts, prepared using a diblock copolymer templating method and characterized by atomic force microscopy (AFM), were uniformly spaced over a large deposition area with an average diameter of 1.7 nm and narrow size distribution. Dense and uniform SWNTs films with high quality were obtained by using a zinc oxide catalyst, as characterized by scanning electron microscopy (SEM), Raman spectroscopy, AFM, and high-resolution transmission electron microscopy (HRTEM).     

A study of the effect of different catalysts for the efficient CVD growth of carbon nanotubes on silicon substrates

We report on the identification of efficient combinations of catalyst, carbon feedstock, and temperature for the ethanol chemical vapour deposition (CVD) growth of single-wall carbon nanotubes (SWCNTs) onto silicon substrates.Different catalyst preparations, based on organometallic salts (Co, Fe, Mo, Ni acetate, and bimetallic mixtures), have been spin coated onto thermally grown silicon dioxide on silicon chips to perform tests in a temperature range between 500 and 900 °C.The samples have been then characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. Assuming the growth of high-quality isolated nanotubes as target, the ratio in Raman spectra between the intensity of the G peak and of the D peak has been used as the main parameter to evaluate the performance of the catalytic process. A comparison made for both single metals and bimetallic mixtures points out best conditions to achieve efficient CVD growth of SWCNTs.     

采用旋转涂膜法制备基底生长的定向碳纳米管阵列

 采用化学气相沉积技术，利用旋转涂膜法制备催化剂基底材料，通过对涂膜过程中的角速度、旋转时间以及基底还原过程中温度的控制改变催化剂颗粒的分布状态，获得了粒径均匀分布的催化剂基底，该基底上催化剂颗粒集中分布在47～62 nm区间，再利用该基底生长出定向碳纳米管阵列。运用扫描电镜、透射电镜、拉曼光谱仪对样品进行了表征。结果表明旋转涂膜法制备的基底平整性好于普通的滴膜法，且较其它基底制备方法具有简单易控、可使催化剂均匀分散等特点。利用该基底制备的碳纳米管阵列定向性良好。     

Magnetic binary nanofillers

Magnetic binary nanofillers containing multiwall carbon nanotubes (MWCNT) and hercynite were synthesized by Chemical Vapor Deposition (CVD) on Fe/AlOOH prepared by the sol–gel method. The catalyst precursor was fired at 450 °C, ground and sifted through different meshes. Two powders were obtained with different particle sizes: sample A (50–75 μm) and sample B (smaller than 50 μm). These powders are composed of iron oxide particles widely dispersed in the non-crystalline matrix of aluminum oxide and they are not ferromagnetic. After reduction process the powders are composed of α-Fe nanoparticles inside hercynite matrix. These nanofillers are composed of hercynite containing α-Fe nanoparticles and MWCNT. The binary magnetic nanofillers were slightly ferromagnetic. The saturation magnetization of the nanofillers depended on the powder particle size. The nanofiller obtained from powder particles in the range 50–75 μm showed a saturation magnetization 36% higher than the one formed from powder particles smaller than 50 μm. The phenomenon is explained in terms of changes in the magnetic environment of the particles as consequence of the presence of MWCNT.     

The effect of Fe and Ni catalysts on the growth of multiwalled carbon nanotubes using chemical vapor deposition

The effect of Fe and Ni catalysts on the synthesis of carbon nanotubes (CNTs) using atmospheric pressure chemical vapor deposition (APCVD) was investigated. Field emission scanning electron microscopy (FESEM) analysis suggests that the samples grow through a tip growth mechanism. High-resolution transmission electron microscopy (HRTEM) measurements show multiwalled carbon nanotubes (MWCNTs) with bamboo structure for Ni catalyst while iron filled straight tubes were obtained with the Fe catalyst. The X-ray diffraction (XRD) pattern indicates that nanotubes are graphitic in nature and there is no trace of carbide phases in both the cases. Low frequency Raman analysis of the bamboo-like and filled CNTs confirms the presence of radial breathing modes (RBM). The degree of graphitization of CNTs synthesized from Fe catalyst is higher than that from Ni catalyst as demonstrated by the high frequency Raman analysis. Simple models for the growth of bamboo-like and tubular catalyst filled nanotubes are proposed.     

Effect of nickel,iron and cobalt on growth of aligned carbon nanotubes

The effect of pure nickel, iron and cobalt on growth of aligned carbon nanotubes was systematically studied by plasma-enhanced hot-filament chemical vapor deposition. It is found that the catalyst has a strong effect on the nanotube diameter, growth rate, wall thickness, morphology and microstructure. Ni yields the highest growth rate, largest diameter and thickest wall, whereas Co results in the lowest growth rate, smallest diameter and thinnest wall. The carbon nanotubes catalyzed by Ni have the best alignment and the smoothest and cleanest wall surface, whereas those from Co are covered with amorphous carbon and nanoparticles on the outer surface. The carbon nanotubes produced from Ni catalyst also exhibit a reasonably good graphitization. Therefore, Ni is considered as the most suitable catalyst for growth of aligned carbon nanotubes. Received: 30 November 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Effects of catalysts on the internal structures of carbon nanotubes and corresponding electron field-emission properties

Using a chemical vapor deposition (CVD) method, multi-walled carbon nanotubes with uniform diameters of approximately 10 nm were synthesized on silicon substrates by the decomposition of acetylene using Fe, Co and Ni as the catalysts. Catalyst effects on the internal structures of the carbon nanotubes were evident in the Fe, Co and Ni catalyzed nanotubes. Although these nanotubes demonstrated similar morphologies, due to the variety of internal structures, the nanotubes synthesized from different catalysts demonstrated various electron field-emission characteristics including turn-on field, threshold field and field enhancement factor. Compared with carbon nanotubes from Ni catalyst, nanotubes from Fe and Co with the same diameters have better field-emission properties. Graphite layers in nanotubes from Fe and Co are much straighter and more parallel to the tube axis with fewer defects. For instance, the turn-on field and threshold field for nanotubes from Ni are 5 V/m and 9 V/m, respectively. These electric fields are much higher than those for nanotubes from Fe, which are 0.35 V/m and 2.8 V/m, respectively. This could be due to the effect of catalysts on the work function of nanotubes, since the catalyst particle usually terminates the free end of the nanotube, and the influence of internal structure on electron transportation along the nanotube axis. Therefore, this study suggests that besides a small diameter, good graphitization (crystallization) is an important prerequisite for a good carbon nanotube emitter. PACS 79.70.+q; 68.37.Lp; 81.07.De     

Controllable growth of individual, uniform carbon nanotubes by thermal chemical vapor deposition

We report on the controllable growth of individual, uniform carbon nanotubes using thermal chemical vapor deposition (CVD). We performed a detailed study of the various factors influencing the growth of single nanotubes. In particular, we investigated the role played by catalyst layer thickness, catalyst dot size, deposition temperature, and gas source pressure on the growth process of straight, single nanotubes. Straight, individual nanotubes with uniform diameter can be obtained by decomposition of 0.1 mbar of acetylene at a temperature of 800 °C over a 5 nm thick nickel film that is patterned into square dots with dimensions below 500 nm. We compare the performance of thermal CVD and of plasma enhanced CVD for growing individual nanotubes.     

Effect of catalyst preparation on the yield of carbon nanotube growth

Multi-wall carbon nanotubes (MWCNTs) were synthesized by catalytic chemical vapor deposition (CVD) on catalytic iron nanoparticles dispersed in a silica matrix, prepared by sol gel method. In this contribution, variation of gelation condition on catalyst structure and its influence on the yield of carbon nanotubes growth was studied. The precursor utilized were tetraethyl-orthosilicate and iron nitrate. The sols were dried at two different temperatures in air (25 or 80 °C) and then treated at 450 °C for 10 h. The xerogels were introduced into the chamber and reduced in a hydrogen/nitrogen (10%v/v) atmosphere at 600 °C. MWCNTs were formed by deposition of carbon atoms from decomposition of acetylene at 700 °C. The system gelled at RT shows a yield of 100% respect to initial catalyst mass whereas the yield of that gelled at 80 °C was lower than 10%. Different crystalline phases are observed for both catalysts in each step of the process. Moreover, TPR analysis shows that iron oxide can be efficiently reduced to metallic iron only in the system gelled at room temperature. Carbon nanotubes display a diameter of about 25–40 nm and several micron lengths. The growth mechanism of MWCNTs is base growth mode for both catalysts.     

Selective growth, characterization, and field emission performance of single-walled and few-walled carbon nanotubes by plasma enhanced chemical vapor deposition

Single-walled carbon nanotubes (SWCNTs) and few-walled carbon nanotubes (FWCNTs) have been selectively synthesized by plasma enhanced chemical vapor deposition at a relative low temperature (550 °C) by tuning the thickness of iron catalyst. The parametric study and the optimization of the nanotube growth were undertaken by varying inductive power, temperature, catalyst thickness, and plasma to substrate distance. When an iron film of 3-5 nm represented the catalyst thickness for growing FWCNT arrays, SWCNTs were synthesized by decreasing the catalyst thickness to 1 nm. The nanotubes were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Electron field emission properties of the nanotubes indicate that the SWCNTs exhibit lower turn-on field compared to the FWCNTs, implying better field emission performance.     

Quality of horizontally aligned single-walled carbon nanotubes: Is methane as carbon source better than ethanol?

Chemical vapor deposition (CVD) growth of horizontally aligned single-walled carbon nanotubes (SWNTs) was studied using two representative carbon source sources: ethanol and methane. The resulting SWNTs were compared for similar reaction conditions which were based on the formation of Ni metal nanoparticles selective electrochemical deposition (SED) on the defect sites of SWNTs. The products were analyzed by Raman spectroscopy and SEM. The results demonstrate that methane was much better carbon source for growing high quality horizontal alignment of SWNTs than ethanol due to the etching effects of OH radicals on the SWNTs.     

用微米级LaNi5为催化剂生长的碳管的ESR谱研究

用微米级LaNi₅合金粉末为催化剂, 以乙炔为原料, 采用化学气相沉积(CVD)法合成了多壁碳纳米管. 在100~290 K温度下测量了41 μm≤d≤150 μm粒径催化剂制备的不同直径分布的碳纳米管的电子自旋共振（ESR）谱,研究了测量温度、微米级催化剂粒径及制备过程的氢气氛对生成的碳纳米管的ESR谱线型、g因子、线宽的影响. 发现碳纳米管的g因子随其直径的增大而增大,分别为2.040 0（催化剂粒径41 μm≤d≤50 μm, 碳纳米管的直径分布为10 nm到20 nm）和2.089 8（催化剂粒径100 μm≤d≤150 μm,碳纳米管的直径分布为70 nm到120 nm）. 发现小管径纳米管的ESR谱图有一个峰, 而大管径纳米管的ESR谱图有两个峰A和B, 且随测量温度的升高, 峰B强度增大.     

On the morphology of carbon nanotubes growing from catalyst particles: Formulation of the model

A model is constructed for the growth of nanotubes from metal catalyst particles supersaturated with carbon. An island of the graphene plane on the catalyst surface serves as a nucleus for the formation of nanotubes with different morphologies. The dependence of the type of nanotube nucleating from an island on the catalyst particle size and the minimum number of carbon-metal interaction parameters is determined. These parameters are calculated using the semiempirical quantum-chemical methods. The results of calculations in the framework of the proposed model are compared with the experimental data obtained for the simultaneous formation of nanotubes of several types.     

Mo-Fe催化下SWCNTs制备中温度窗口的研究

本文以Mo1-Fe10-MgO为催化剂, Ar为载气, CH4为碳源, 在不同的温度下制备SWCNTs。利用显微激光拉曼研究了不同的制备温度对生成SWCNTs的质量的影响, 得到制备SWCNTs的温度窗口, 给出最佳制备温度。     

Comparison of growth stress measurements with modelling in thin iron oxide films

High temperature oxidation of metals leads to residual stresses both in the metal and in the growing oxide. In this work, the evolution of this residual stresses is theoretically predicted in the growing oxide layers. The origin of these stresses is based on a microstructural model. Using experimental results providing from the oxidation kinetics, and an analysis proposed to describe the growth strain occurring in the thin layers, a set of equations is established allowing determining the stresses evolution with oxidation time. Then, the model is compared with experimental results obtained on both α-Fe and phosphated α-Fe, oxidised at different temperatures. Numerical data are extracted from experiments either with an asymptotic formulation or with an inverse method. These two methods give good agreement with experiments and allow extracting the model parameters.     

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.     

New aspects on pulsed laser deposition of aligned carbon nanotubes

We have grown carbon nanotubes (CNT) by pulsed laser deposition (PLD) at 1000 °C in Ar atmosphere. A Nd/YAG laser was used for irradiation of a graphite target containing Ni and Co rods. High-resolution scanning electron microscopy (HRSEM) and transmission electron microscopy (TEM) images showed that “closed” carbon nanotubes were grown between clusters of metallic particles, so that the individual nanotubes were arranged in parallel to each other forming a shape of “Rope-Bridge”. The nanotubes structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and their type was found to be of MWNT, containing about five SWNT. Total diameter was 5-20 nm and their length was about 1 μm. High homogeneous distribution carbon nanotubes were grown and different structures were observed such as well-aligned carbon nanotubes, bamboo-like and Y-junction carbon nanotubes.