Growth Mechanism Of Carbon Nanotubes Research Articles

Different growth mechanisms of vertical carbon nanotubes by rf- or dc-plasma enhanced chemical vapor deposition at low temperature

Vertically aligned carbon nanotubes (CNTs) were synthesized using FeNi or Fe sputtered catalyst layers on glass substrates by radio frequency or direct current plasma enhanced chemical vapor deposition (rf- or dc-PECVD). This article compared the growth mechanisms of CNTs synthesized by rf- and dc-PECVD, based on gas flow rate, plasma power, and catalysts. Tip growth CNTs were produced at 180 °C, 10 SCCM (SCCM denotes cubic centimeter per minute at STP) CH4, and 30 W by rf-PECVD using 8 or 4 nm FeNi or 4 nm Fe island films sputtered onto glass substrates. CNTs could not grow using dc-PECVD under the same deposition conditions; tip growth of CNTs occurred at 180 °C, 15 SCCM CH4, and 50 W using dc-PECVD with sputtered FeNi island catalysts on increasing the plasma power and CH4 flow rate. This article explained why rf-PECVD provided more efficient decomposition of gas molecules than dc-PECVD by plasma theory. The major difference between rf- and dc-PECVD was the higher concentration of reactive radicals in the former. However, in dc-PECVD, the CNT growth was well aligned vertically. FeNi thin film catalysts exhibited higher activity and better wetting ability than the Fe island thin film catalysts.

Catalyzed Growth of Carbon Nanotube with Definable Chirality by Hybrid Molecular Dynamics−Force Biased Monte Carlo Simulations

Metal-catalyzed growth mechanisms of carbon nanotubes (CNTs) were studied by hybrid molecular dynamics-Monte Carlo simulations using a recently developed ReaxFF reactive force field. Using this novel approach, including relaxation effects, a CNT with definable chirality is obtained, and a step-by-step atomistic description of the nucleation process is presented. Both root and tip growth mechanisms are observed. The importance of the relaxation of the network is highlighted by the observed healing of defects.

Chemical Vapor Deposition of Carbon Nanotubes: A Review on Growth Mechanism and Mass Production

This review article deals with the growth mechanism and mass production of carbon nanotubes (CNTs) by chemical vapor deposition (CVD). Different aspects of CNT synthesis and growth mechanism are reviewed in the light of latest progresses and understandings in the field. Materials aspects such as the roles of hydrocarbon, catalyst and catalyst support are discussed. Many new catalysts and new carbon sources are described. Growth-control aspects such as the effects of temperature, vapor pressure and catalyst concentration on CNT diameter distribution and single- or multi-wall formation are explained. Latest reports of metal-catalyst-free CNT growth are considered. The mass-production aspect is discussed from the perspective of a sustainable CNT technology. Existing problems and challenges of the process are addressed with future directions.

An Updated Review of Synthesis Parameters and Growth Mechanisms for Carbon Nanotubes in Fluidized Beds

Research published since 2006 on the synthesis of carbon nanotubes (CNTs) using chemical vapor deposition (CVD) in a fluidized bed reactor is reviewed. A complete account of experimental procedures, including upstream treatments (catalyst preparation, calcination, and reduction), synthesis conditions, and downstream processes (purification) is presented in an attempt to determine the effect of these variables on carbon nanotube morphology, diameter, yield, and quality. The formation and growth mechanisms of carbon nanotubes by CVD is reviewed in detail in an attempt to account for discrepancies in the properties of CNTs produced from experiments at superficially similar conditions. This reveals that the underlying variables that appear to control growth are not directly manipulated in the CVD process; rather they are determined by complex interactions between variables. Thus, the current “change-one-factor-at-a-time” experimental paradigm, which assumes orthogonal variables, is the most likely source of t...

Atomic carbon adsorption on Ni nanoclusters: a DFT study

Atomic carbon is a key intermediate interacting with transition metal clusters during the growth of carbon nanotube (CNT). Present density functional calculations studied the initial carbon adsorption on four Ni nanoclusters (N13, N15, N38, and N55). Our results show that carbon atoms preferentially adsorb on high-coordination sites, and carbon adsorption energies are larger on smaller Ni clusters. Ni cluster reconstruction plays an important role in creating more stable subsurface adsorption sites. The migration of adsorbed carbon atom on the surface threefold hollow site into the underlying interstitial subsurface positions is thermodynamically and kinetically feasible. The results indicate that the investigation of CNT growth mechanism should include both surface and subsurface carbon atoms, coupled with surface reconstruction of Ni nanoclusters.

Properties, synthesis, and growth mechanisms of carbon nanotubes with special focus on thermal chemical vapor deposition

Carbon nanotubes (CNTs) have been extensively investigated in the last decade because their superior properties could benefit many applications. However, CNTs have not yet made a major leap into industry, especially for electronic devices, because of fabrication challenges. This review provides an overview of state-of-the-art of CNT synthesis techniques and illustrates their major technical difficulties. It also charts possible in situ analyses and new reactor designs that might enable commercialization. After a brief description of the CNT properties and of the various techniques used to synthesize substrate-free CNTs, the bulk of this review analyzes chemical vapor deposition (CVD). This technique receives special attention since it allows CNTs to be grown in predefined locations, provides a certain degree of control of the types of CNTs grown, and may have the highest chance to succeed commercially. Understanding the primary growth mechanisms at play during CVD is critical for controlling the properties of the CNTs grown and remains the major hurdle to overcome. Various factors that influence CNT growth receive a special focus: choice of catalyst and substrate materials, source gases, and process parameters. This review illustrates important considerations for in situ characterization and new reactor designs that may enable researchers to better understand the physical growth mechanisms and to optimize the synthesis of CNTs, thus contributing to make carbon nanotubes a manufacturing reality.

Importance of Cr 2O 3 layer for growth of carbon nanotubes on superalloys

We present a detailed investigation of the formation of stable catalyst particles on Inconel substrates, an important step in the growth mechanism of carbon nanotubes on these substrates. The chemical nature of the interaction of catalyst and substrate was investigated using XPS, high-resolution cross-sectional TEM, and EDS studies. The results indicate that Cr 2O 3, an electrically conductive oxide, plays an important role in stabilizing nanoclusters of Fe catalyst under typical growth conditions. The nature of growth on a number of other superalloys is also presented and is examined with respect to their chemical composition.

Six-Membered-Ring-Based Radical Mechanism for Catalytic Growth of Carbon Nanotubes with Benzene Precursor

The growth mechanism of carbon nanotubes (CNTs) catalyzed by nickel catalyst with benzene precursor has been studied by using density functional theory calculations. The typical initial process of the graphene growth is considered as the formation of biphenyl on Ni(111) surface with two benzene molecules and the minimum energy pathway for this formation has been investigated in detail. It is found that the carbon−hydrogen bond could be selectively dissociated while the carbon−carbon connection still remained. Thus the dehydrogenated hexagonal rings are generated, which could form the graphene sheet on the catalyst surface through radical incorporation for sequential growth of CNTs. The calculations rationalize the six-membered-ring-based growth mechanism previously proposed according to in situ experimental results. The train of thought of this radical mechanism also should be enlightening for the synthesis of doped CNTs with some suitable heterocyclic precursors, which is of great importance for function...

Thermal Analysis Study of the Growth Kinetics of Carbon Nanotubes and Epitaxial Graphene Layers on Them

With the aim of improving the yield and purity of carbon nanotubes (CNTs), we have studied the chemical vapor deposition (CVD) growth of CNTs with Fe−Mo powder catalyst and acetylene precursor by using the technique of in situ simultaneous thermal analysis (STA). We could distinguish this way the rapid catalytic growth stage of CNTs and the slow epitaxial growth stage of graphitic impurities on the nanotubes, which were discovered to be graphene layers. Both reactions were first order, and the activation energies were determined to be 177 and 189 kJ/mol, respectively. In the rapid-growth stage, the growth of CNTs could be further tuned from kinetically controlled regime to gas-diffusion-controlled regime, where the CNTs exhibit a longer growth time and a higher yield, using appropriately shaped crucibles. These results are helpful for both elucidating the growth mechanism of CNTs and improving the quality of mass-produced 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.

A multiscale approach for modeling the early stage growth of single and multiwall carbon nanotubes produced by a metal-catalyzed synthesis process

A parametrized mesoscale model for the early stage growth of isolated single or multiwall carbon nanotubes (CNTs) has been developed in order to investigate the effects of metal catalyst particle size and composition on CNT growth mechanism during synthesis via a substrate-supported, catalytic chemical vapor deposition process. The model is based on a coarse-grained graphene sheet, represented by a two-dimensional simply connected triangular mesh, with parameters for the surface curvature, bond stretching, carbon-carbon interaction, and carbon-catalyst interaction determined by classical molecular dynamics simulations using a bond-order potential derived from ab initio calculations. The mesoscale simulations show that the initial type of CNT growth is strongly influenced by the surface interaction energy between the graphene sheet and metal catalyst particle, rate of carbon deposition, and particle size. As expected, single wall tubes are produced from small catalyst particles at low deposition rates, but increasing the strength of carbon-catalyst interaction energy or carbon deposition rate results in double or even multiwall CNT structures, formed by folding or involution of the graphene sheet. For the range of model parameters investigated, all single wall CNTs with a diameter greater than 6.6 nm exhibited a kink-collapse transition once a certain critical tube length was reached.

Effect of Mo in Co-Mo/MgO catalysts on the synthesis yield and structure of carbon nanotubes

Carbon nanotubes (CNTs) were synthesized at 650–770°C using Co/MgO and Co–Mo/MgO as catalysts and ethylene as carbon source. The dependence of the carbon yield on Mo amount in catalyst, synthesis temperature and time were investigated, a nd the structures of CNTs were characterized using FESEM, TEM, and Raman spectroscopy. It was found that CNTs with “bamboo-like” structure were synthesized using Co–Mo/MgO catalysts, in contrast to typical multi-walled carbon nanotubes with hollow inner tube obtained from Co/MgO catalyst. Addition of 5–10% Mo in catalysts resulted in the highest carbon yield, e.g. 5500% for 60 min synthesis, which is about 20 times higher than that of Co/MgO. The addition of Mo also extended the lifespan of catalyst for the growth of CNTs. TEM observation showed that the growth of CNTs was in a root-growth mode. The structure of CNTs obtained from Co–Mo/MgO catalysts was explained based on the growth mechanism of CNTs.

Morphology study of carbon nanospecies grown on carbon fibers by thermal CVD technique

Multiwalled carbon nanotubes were uniformly grown directly on Ni catalyzed carbon fibers by catalytic decomposition of acetylene at 700 °C using thermal chemical vapour deposition process. Different process parameters such as growth temperature, time and gas flow rates were investigated in relation to morphology and yield of the products. The overall yield of the nanospecies demonstrated an initial nonlinear increase and eventually stabilized with increasing deposition time while it was found reaching to maxima at 700 °C and drops rapidly at temperatures on either side. The morphology of nanostructures was examined extensively using scanning and transmission electron microscopy which revealed formation of various carbon nanostructures such as nanofilaments, nanofibers, nanoladders, nanospirals, nanojunctions, nanocones, nanoonions etc. accompanying nanotubes. High resolution transmission electron microscopy images clearly showed single crystal structure of the catalyst particle and tip-growth mechanism of carbon nanotubes growth. Non-tip growth mechanism was observed in especial case of ferrocene derived floating Fe catalyst.

Mechanisms for Catalytic CVD Growth of Multiwalled Carbon Nanotubes

Carbon nanotubes possess unique properties that make them potentially ideal materials for various technological applications. However, a basic growth mechanism explaining the way metallic atoms interact with carbon to nucleate, grow and heal CNTs still needs to be understood. In this review paper we describe the mechanisms of catalytic chemical vapor deposition growth of multi-walled carbon nanotubes and carbon nanofibers, the role of various parameters that govern their growth kinetics and the knowledge added to singlewalled nanotube growth. We also examine future strategies needed to reveal complete knowledge of the growth mechanisms of carbon nanotubes.

“In situ” observations of carbon nanotube generation in CVD cell coupled to spectrometers

AbstractPreliminary results of Raman spectroscopy are presented on “in situ” growth of carbon nanotubes in a CVD‐cell which is optionally coupled to a mass spectrometer (MS). The system allows to analyze the differences in the content of the types of carbon nanotubes in the sample during the processes after 2 min, 5 min, 10 min, 15 min, 20 min and 30 min. The evolved gases in the throughout synthesis were analyzed in the MS mode. Two kinds of experiments were performed: in argon atmosphere at atmospheric pressure and under vacuum at low pressure, both with CH4 as a carbon feedstock. The temperature of the experiments was set up to 600 °C. The detailed study of the process can lead to a better understanding of the carbon nanotube growth mechanism. The analysis of these phenomena is supported by high resolution transmission electron microscopy (HR‐TEM). In addition, thermogravimetric analyses of the finally obtained products in the two experiments are presented. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of Negative Electric Field Biasing on Growth of Vertically Aligned Carbon Nanotubes Using Chemical Vapor Deposition

Vertically aligned carbon nanotubes (CNTs) were grown on a Si substrate by thermal chemical vapor deposition (CVD) and direct current plasma-enhanced CVD with a negative electric field. By contrast, under a positive electric field, CNTs grew randomly. The morphologies of the CNTs grown on Si and quartz substrates were compared to clarify the mechanism of aligned CNT growth. CNTs grown on quartz showed a random structure regardless of the polarity and amplitude of applied electric field. It is noted that the Fe catalysts on the tips of aligned CNTs were apparently elongated compared with those of randomly grown CNTs. The present observations suggest that aligned CNT growth might be due to the electrostatic force acting on the electrons drawn toward the tips of CNTs by negative electric field.

Fabrication and Growth Mechanism of Carbon Nanotubes by Chemical Vapor Deposition

Fabrication and Growth Mechanism of Carbon Nanotubes by Chemical Vapor Deposition

Nanocatalysts with Tunable Properties Derived from Polystyrene-b-poly(vinyl pyridine) Block Copolymer Templates for Achieving Controllable Carbon Nanotube Synthesis

Chemical vapor deposition, one of major carbon nanotube synthesis techniques, has demonstrated great promise for substrate-based carbon nanotube device applications. To understand carbon nanotube growth mechanisms for achieving controllable synthesis, thereby producing carbon nanotubes with consistent and predefined behavior for materializing their highly touted properties, it is imperative to develop a methodology to create nanocatalyst systems controllably with great tunability. In this paper, a comparative study of polystyrene-b-poly(4-vinyl pyridine) versus polystyrene-b-poly(2-vinyl pyridine) as templates for creating nanocatalysts is presented. Creating uniform and periodically ordered nanocatalyst arrays with tunable size and spacing is demonstrated. A scaling relationship between nanocatalyst spacing and polystyrene block length for polystyrene-b-poly(2-vinyl pyridine) is elicited. The potential of synthesizing nanocatalysts with adjustable composition is discussed. The ability to tailor nanocatalyst size and spacing by varying block lengths has been demonstrated to promote controllable synthesis of carbon nanotubes with adjustable diameter and density. Finally, an example of using such highly controlled nanocatalysts derived from the block copolymer template approach, to study the effect of substrate on carbon nanotubes synthesis, is described.

Synthesis of carbon nanbotubes by plasma-enhanced CVD process: gas phase study of synthesis conditions

To support experimental investigations, a model based on Chemkin™ software was used to simulate gas phase and surface chemistry during plasma-enhanced catalytic CVD of carbon nanotubes. According to these calculations, gas phase composition, etching process and growth rates are calculated. The role of several carbon species, hydrocarbon molecules and ions in the growth mechanism of carbon nanotubes is presented in this study. Study of different conditions of gas phase activation sources and pressure is performed.

Carbon nanotube growth mechanism switches from tip- to base-growth with decreasing catalyst particle size

The growth of carbon nanotubes by a plasma assisted catalytic chemical vapor deposition was investigated using cobalt, nickel and iron catalyst particles of different sizes. For the three catalysts examined, it was shown that the growth mode switches from “tip-growth” for large particles (>>5 nm) to “base-growth” for smaller ones (<5 nm). While single-walled nanotubes and those with few walls (typically <7 walls) grow from their base, larger multi-walled nanotubes are fed with carbon via their tips which support the catalyst particle. A growth scenario involving two different pathways for carbon diffusion is proposed in order to explain the change in growth mode.