Growth of carbon nanotubes from ring carbon clusters

The possibility of growing single-wall carbon nanotubes from ring carbon clusters that appear at a certain stage of cooling carbon vapor is discussed. Such a technique could allow one to grow single-wall nanotubes without introducing a macroscopic amount of a catalyst and to retain nanotubes open during their growth. An analysis performed using semiempirical quantum-chemical methods shows that, when catalyst atoms interact with the edge of an already formed nanotube surface, the bonds of these atoms with carbon tend to occupy positions normal to the generatrix of the nanotube. This situation is natural for transition-metal atoms, since they favor the destruction of pentagonal cycles at the edge of the surface. The destruction mechanism consists in the fact that pentagons incorporate carbon atoms from the outside and become hexagons. The dependence of this tendency on the type of catalyst atom is considered.     

Growth and morphology of carbon nanostructures on nickel oxide nanoparticles in catalytic chemical vapor deposition

The present study explores the conditions favorable for the growth of cylindrical carbon nanostructures such as multi-walled carbon nanotube (MWCNT) and carbon nanofiber by catalytic chemical vapor deposition (CCVD) method using nickel oxide-based catalyst nanoparticles of different average sizes as well as different levels of doping by copper oxide. The role of doping and the average size have been related to the observed melting behavior of nanoparticles of nickel oxide by thermal and diffraction analysis, and the importance of melting has been highlighted in the context of growth of cylindrical nanostructures. In the reducing environment prevailing in the CCVD chamber due to decomposition of flowing acetylene gas at elevated temperature, there is extensive reduction of oxide nanoparticles. Lack of melting and faster flow of carbon-bearing gases favor the formation of a carbon deposit cover over the catalyst nanoparticles giving rise to the formation of nanobeads. Melting allows rapid diffusion of carbon from the surface to inside catalyst particles, and reduced flow of gas lowers the rate of carbon deposit, both creating conditions favorable for the formation of cylindrical nanostructures, which grows around the catalyst particles. Smaller particle size and lower doping favor growth of MWCNT, while growth of fiber is commonly observed on larger particles having relatively higher level of doping.     

碳纳米管表面金纳米颗粒的形成与结构转变

利用分子动力学模拟研究了室温下金纳米颗粒在碳纳米管表面的结构和作用能.研究结果表明,金纳米颗粒随着尺寸的增大会发生不同于孤立状态下的结构转变.当原子数小于130时,颗粒属于无序结构;当原子数大于140时,呈现面心立方晶体结构.小金纳米颗粒和碳纳米管结合紧密,相互作用能正比于面对碳纳米管的颗粒表面面积. 关键词： 金纳米颗粒 碳纳米管 分子动力学模拟     

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.     

Model predictions and experimental characterization of Co-Pt alloy clusters

Model and real cobalt-platinum alloy clusters are compared in terms of structure, composition and segregation. Canonical and semi grand canonical Metropolis Monte Carlo simulations are performed to model these clusters, using embedded atom (EAM) and modified embedded atom (MEAM) potentials. All of them correctly predict the bulk L1₂ Co₃Pt and CoPt₃ structures as well as the L1₀ CoPt phase. However, the lattice parameters, phase stability and the L1₀-fcc order-disorder transition temperature are at variance. Segregation predictions with EAM and MEAM potentials are contradictory. Experimentally, mixed clusters with various compositions were deposited by Low Energy Cluster Beam on amorphous carbon at room temperature. Their size distribution, crystalline structure and composition were examined by Transmission Electron Microscopy (TEM). Clusters with the same size distributions were modelled. Both experiment and modelling show their crystallographic parameters to continuously correspond to the fcc CoPt chemically disordered phase. Diffraction measurements indicate surface segregation of the specie in excess, in agreement with EAM predictions for the Co-rich phase. The consequences on magnetic properties are discussed.     

MEAM势与Tersoff势比较研究——碳化硅熔化与凝固行为

运用分子动力学方法对比模拟研究了碳化硅的体熔化、表面熔化和晶体生长过程.分别采用MEAM 势和Tersoff势两种势函数描述碳化硅.结果表明:体熔化时,两种势函数描述的SiC的原子平均能量、 Lindemann指数和结构有序参数与温度的变化关系相似,但MEAM势对应的体熔点(4250 K)比Tersoff势(4750 K) 的要高.表面熔化时,两种势函数描述的SiC在相同的过热度下熔化速度相近;而在相同的温度条件下,MEAM 作用的SiC表面熔化速度更快.这是由于MEAM势SiC的热力学熔点(3338 K)低于Tersoff势SiC的热力学熔点 (3430 K)的缘故.两种势函数作用的SiC在晶体生长方面差异很大.MEAM势SiC的晶体生长速度与过冷度有关, 过冷度约为400 K时晶体生长速度最快.但Tersoff势SiC晶体却在过冷度为0—1000 K的范围内均不能生长. 综合考虑,MEAM势比Tersoff势能更好地描述碳化硅的熔化和凝固行为.     

Functionalization of ultradispersed diamond for DNA detection

Carbon nanotubes have been synthesized by catalytic chemical vapour deposition of acetylene diluted with argon using three different catalysts, namely, nickel formate, cobalt formate and ferrocene. The synthesis was carried out at 700°C in a quartz reactor for 30 minutes. Thermal analysis was carried out in order to determine the yield of the nanotube. It was found that the deposit contains 86% nanotube, with nickel-based catalyst, which was the maximum. The yield of nanotube was 71 times that of the nickel loading. The TEM images reveal helical type of nanotubes with iron catalyst while cobalt and nickel catalysts yielded straight nanotubes. This technique can be explored for the bulk production of carbon nanotube in an economic way.     

Characterization of carbon nanotubes exposed to Na or bombarded with Na at room temperature

We report an electron spectroscopy study on low energy Na⁺ ion implantation and Na atom intercalation in single walled and multi walled carbon nanotube mat samples. Our results show that these two different methods yield quite different dopant spatial distribution since implanted sodium atoms remain on the surface while intercalated alkali metal particles readily diffuse in the bulk.     

Particle size effects on Wulff constructions

The equilibrium structure of small particles is analysed by minimising the total surface energy of atomistic clusters. Large deviations from the bulk Wulff construction are identified for fairly large (~10 nm) particles due to sphere packing corrections. These act as additional edge terms which can be significantly larger than the true edge terms. For a simplified fcc model, it is shown that the fraction of (100) surface drops markedly as the particle size drops because of these packing effects. This can lead to very large particle size effects for a face sensitive catalytic reaction. It is also pointed out that these packing corrections link very small (< 100 atom) cluster to very large particles.     

Vibrational properties of vacancy in Au using modified embedded atom method potentials

Much improved result for lattice dynamics of Au is obtained with the use of modified embedded atom method (MEAM) potentials compared to the earlier embedded atom method (EAM) potentials. The MEAM potentials along with experimental phonons are utilised to calculate local spectra of neighbours of vacancy using Green's function method. The local spectrum of first neighbour of vacancy shows general loss of modes at lower frequencies with a resonance like sharp peak near the top end of the spectrum. The spectrum of second neighbour shows small changes from the host spectra except a pronounced dip in the middle. In accordance with the obtained features of local spectra of neighbours the calculated mean-square displacements are lower for both first and second neighbours as compared to that of host atoms. The calculated formation entropy is in reasonable agreement with other calculations and available experimental values.     

Quasi-one-dimensional fullerene-nanotube composites: Structure, formation energetics, and electronic properties

Carbon nanotubes coated with close-packed C₆₀ (or C₇₀) fullerenes, which are “attached” to the nanotubes by van der Waals forces, are considered and classified as a new class of nanocomposites. Semiempirical and molecular-dynamics calculations reveal the most energetically stable systems and show that a topological (Stone-Wales) defect on a nanotube can promote a more favorable “attachment” of fullerene to the nanotube. It has been shown that the molecular interaction of the fullerene coating with the nanotube leads to a significant change in its electronic spectrum, namely, to the formation of minibands including a large number of branches associated with the lift of the degeneracy of levels of C₆₀ and to the consolidation of branches of the carbon nanotube into the Brillouin zone smaller than that in the carbon nanotube. This fact should strongly change the interaction of light with such a nanocomposite as compared to carbon nanotubes and fullerenes, which provides prospect of its application in photovoltaics.     

Selective growth of carbon nantoubes on SiO2/Si substrate

Three-step raising temperature process was employed to fabricate carbon nanotubes by pyrolysis of ferrocene/melamine mixtures on silica and single crystalline silicon wafers respectively. Then the morphologies, structures and compositions of obtained carbon nanotubes are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscope (EDX) and electron energy-loss spectroscopy (EELS). TEM and SEM observation shows that on silica substrate, high-oriented carbon nanotube can grow compactly to form continuous film on both frontal and cross-section surfaces, but on silicon substrate, only can form on cross-section surface. These carbon nanotubes have much irregular cup-like structure, and with outer diameter varying from 25 nm to 35 nm. At the top end of carbon nanotube there is a catalyst particle. EDX analysis reveals that the particle are iron cluster, and EELS spectrum indicates that the nanotube is composed of pure carbon. Finally, the effect of substrate surface roughness on the growth behavior of carbon nanotubes has been discussed.     

Electron emission from arrays of carbon nanotubes/fibres

The overall aim of this work is to produce arrays of field emitting microguns, based on carbon nanotubes, which can be utilised in the manufacture of large area field emitting displays, parallel e-beam lithography systems and electron sources for high frequency amplifiers. This paper will describe the work carried out to produce patterned arrays of aligned multiwall carbon nanotubes (MWCNTs) using a dc plasma technique and a Ni catalyst. We will discuss how the density of the carbon nanotube/fibres can be varied by reducing the deposition yield through nickel interaction with a diffusion layer or by direct lithographic patterning of the Ni catalyst to precisely define the position of each nanotube/fibre. Details of the field emission behaviour of the different arrays of MWCNTS will also be presented.     

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     

Growth of carbon nanotubes from C<Subscript>60</Subscript>

Carbon nanotubes can be obtained from a multitude of molecular precursors in chemical vapor deposition (CVD) processes. Here we demonstrate that the use of C₆₀ as the carbon feedstock gas in an iron-catalyzed thermal CVD experiment leads to the formation of films of multi-walled carbon nanotubes. The critical role of the diameter of the catalyst particles in determining the efficiency of nanotube growth is clearly demonstrated. Electron microscopy and Raman spectroscopy were employed for the characterisation of the nanotube material. The structural properties of the individual nanotubes show distinctive differences to acetylene-grown multi-walled nanotubes. PACS 81.07.De; 81.10.Bk     

Nucleation and growth of SWNT: TEM studies of the role of the catalyst

This paper reviews transmission electron microscopy studies, combining high resolution imaging and electron energy loss spectroscopy, of the nucleation and growth of carbon single wall nanotubes with a particular emphasis on the nanotubes obtained from the evaporation-based elaboration techniques. Inspection of samples obtained from different synthesis routes shows that in all cases nanotubes are found to emerge from catalyst particles and that they have grown perpendicular or parallel to the surface according to whether they have been synthesized via evaporation-based methods or CCVD methods. Whereas the latter case corresponds to the well-known situation of carbon filaments growth, the former case strongly suggests another formation and growth process, which is described and its different steps discussed in detail. In this model, formation of the nanotubes proceeds via solvation of carbon into liquid metal droplets, followed by precipitation, at the surface of the particles, of excess carbon in the form of nanotubes through a nucleation and root growth process. It is argued that the nucleation of the nanotubes, which compete with the formation of graphene sheets wrapping the surface of the particle, necessarily results from a surface instability induced by the conditions of segregation. The nature and the origin of this instability was studied in the case of the class of catalyst Ni–R.E. (R.E.=Y, La, Ce, …) in order to identify the influence of the nature of the catalyst. The respective roles played by Ni and R.E. have been identified. It is shown that carbon and rear-earth co-segregate and self-assemble at the surface of the particle in order to form a surface layer destabilizing the formation of graphene sheets and providing nucleation sites for nanotubes growing perpendicular to the surface. To cite this article: A. Loiseau et al., C. R. Physique 4 (2003).     

The van der Waals energy of an atom near a carbon nanotube

Using a unified macroscopic QED formalism, an integral equation for the van der Waals energy of a two-level atomic system near a carbon nanotube is derived. The equation is valid for both strong and weak atom-vacuum field coupling. By solving it numerically, the inapplicability of weak coupling-based van der Waals interaction models in the close vicinity of the nanotube surface is demonstrated. It is also shown that encapsulation of doped atoms into the nanotube is energetically more favorable than their outside adsorption by the nanotube surface.     

C2H4/Ni(100)近边X射线吸收谱的多重散射理论研究

本文用多重散射原子集团方法计算并分析了C₂H₄/Ni(100)系统的碳K边近边X射线吸收精细结构(NEXAFS)谱,确定了乙烯吸附在Ni(100)表面上的几何结构。结果表明,乙烯是吸附在垂直桥位上的,其中碳原子与表面最近的镍原子距离是1.70?,而乙烯分子平面倾斜于表面50°,同时发现,氢和镍之间的相互作用对结构的形成有很大的作用,这些结果得到了不同途径的验证。 关键词：     

Encapsulation of methane molecules into carbon nanotubes

Methane gas (CH₄) is a chemical compound comprising a carbon atom surrounded by four hydrogen atoms, and carbon nanotubes have been proposed as possible molecular containers for the storage of such gases. In this paper, we investigate the interaction energy between a CH₄ molecule and a carbon nanotube using two different models for the CH₄ molecule, the first discrete and the second continuous. In the first model, we consider the total interaction as the sum of the individual interactions between each atom of the molecule and the nanotube. We first determine the interaction energy by assuming that the carbon atom and one of the hydrogen atoms lie on the axis of the tube with the other three hydrogen atoms offset from the axis. Symmetry is assumed with regard to the arrangement of the three hydrogen atoms surrounding the carbon atom on the axis. We then rotate the atomic position into 100 discrete orientations and determine the average interaction energy from all orientations. In the second model, we approximate the CH₄ molecule by assuming that the four hydrogen atoms are smeared over a spherical surface of a certain radius with the carbon atom located at the center of the sphere. The total interaction energy between the CH₄ molecule and the carbon nanotube for this model is calculated as the sum of the individual interaction energies between both the carbon atom and the spherical surface and the carbon nanotube. These models are analyzed to determine the dimensions of the particular nanotubes which will readily suck-up CH₄ molecules. Our results determine the minimum and maximum interaction energies required for CH₄ encapsulation in different tube sizes, and establish the second model of the CH₄ molecule as a simple and elegant model which might be exploited for other problems.     

Thermodynamics approach of the formation of Ni catalyst particles for carbon nanotubes growth

Size-dependent thermodynamic parameters, such as Gibbs free energy, enthalpy and entropy, for the transition of a Ni nanofilm to catalyst particles for subsequent carbon nanotube growth have been explored. In this investigation, we consider the derived equations of the size-dependent melting temperature of nanosolids based on our previous works. Using this thermodynamic approach, it is found that the diameter of Ni particles is 3 times greater than the thickness of the original film. From the critical and stable sizes of transformed Ni nanoparticles, a minimum film thickness for transformation of film to nanoparticles was obtained. Our predictions are in good agreement with experimental results.