Diffusion- and reaction-limited growth of carbon nanotube forests

Molecular dynamics simulation of carbon nanotube growth under a tensile strain

We performed molecular dynamics simulations of carbon nanotube (CNT) to elucidate the growth process in the floating catalyst chemical vapor deposition method (FCCVD). FCCVD has two features: a nanometer-sized cementite (Fe3 C) particle whose melting point is depressed because of the larger surface-to-volume ratio and tensile strain between the growing CNT and the catalyst. The simulations, including these effects, demonstrated that the number of 6-membered rings of the (6,4) chiral CNT constantly increased at a speed of 1 mm / s at 1273 K , whereas those of the armchair and zigzag CNTs were stopped in the simulations and only reached half of the numbers for chiral CNT. Both the temperature and CNT chirality significantly affected CNT growth under tensile strain.

Synthesis of MWCNTs by chemical vapor deposition of methane using FeMo/MgO catalyst: role of hydrogen and kinetic study

This study aims to investigate the role of hydrogen on CNTs synthesis and kinetics of CNTs formation. The CNTs were synthesized by catalytic chemical vapor deposition of methane over FeMo/MgO catalyst. The experimental results revealed that hydrogen plays an important role in the structural changes of catalyst during the pre-reduction process. The catalyst structure fully transformed into metallic FeMo phases, resulting in an increased yield of 5 folds higher than those of the non-reduced catalyst. However, the slightly larger diameter and lower crystallinity ratio of CNTs was obtained. The hydrogen co-feeding during the synthesis can slightly increase the CNTs yield. After achieving the optimum amount of hydrogen addition, further increase in hydrogen would inhibit the methane decomposition, resulting in lower product yield. The hydrogenation of carbon to methane was proceeded in hydrogen co-feed process. However, the hydrogenation was non-selective to allotropes of carbon. Therefore, the addition of hydrogen would not benefit neither maintaining the catalyst stability nor improving the crystallinity of the CNT products. The kinetic model of CNTs formation, derived from the two types of active site of dissociative adsorption of methane, corresponded well to the experimental results. The rate of CNTs formation greatly increases with the partial pressure of methane but decreases when saturation is exceeded. The activation energy was found to be 13.22 kJ mol-1, showing the rate controlling step to be in the process of mass transfer.

Growth and Performance of High-Quality SWCNT Forests on Inconel Foils as Lithium-Ion Battery Anodes

Large-scale production of vertically aligned single-walled carbon nanotubes (VA-SWCNTs) on metal foils promises to enable technological advancements in many fields, from functional composites to energy storage to thermal interfaces. In this work, we demonstrate growth of high-quality (G/D > 6, average diameters ∼ 2-3 nm, densities > 10¹² cm^-2) VA-SWCNTs on Inconel metal for use as a lithium-ion battery (LIB) anode. Scale-up of SWCNT growth on Inconel 625 to 100 cm² exhibits nearly invariant CNT structural properties, even when synthesis is performed near atmospheric pressure, and this robustness is attributed to a growth kinetic regime dominated by the carbon precursor diffusion in the bulk gas mixture. SWCNT forests produced on large-area metal substrates at close to atmospheric pressure possess a combination of structural features that are among the best demonstrated so far in the literature for growth on metal foils. Leveraging these achievements for energy applications, we demonstrate a VA-SWCNT LIB anode with capacity >1200 mAh/g at 1.0C and stable cycling beyond 300 cycles. This robust synthesis of high-quality VA-SWCNTs on metal foils presents a promising route toward mass production of high-performance CNT devices for a broad range of applications.

A highly stable, nanotube-enhanced, CMOS-MEMS thermal emitter for mid-IR gas sensing

The gas sensor market is growing fast, driven by many socioeconomic and industrial factors. Mid-infrared (MIR) gas sensors offer excellent performance for an increasing number of sensing applications in healthcare, smart homes, and the automotive sector. Having access to low-cost, miniaturized, energy efficient light sources is of critical importance for the monolithic integration of MIR sensors. Here, we present an on-chip broadband thermal MIR source fabricated by combining a complementary metal oxide semiconductor (CMOS) micro-hotplate with a dielectric-encapsulated carbon nanotube (CNT) blackbody layer. The micro-hotplate was used during fabrication as a micro-reactor to facilitate high temperature (>700 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{\circ }$$\end{document}∘C) growth of the CNT layer and also for post-growth thermal annealing. We demonstrate, for the first time, stable extended operation in air of devices with a dielectric-encapsulated CNT layer at heater temperatures above 600 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{\circ }$$\end{document}∘C. The demonstrated devices exhibit almost unitary emissivity across the entire MIR spectrum, offering an ideal solution for low-cost, highly-integrated MIR spectroscopy for the Internet of Things.

Dynamic State and Active Structure of Ni-Co Catalyst in Carbon Nanofiber Growth Revealed by in Situ Transmission Electron Microscopy

Alloy catalysts often show superior effectiveness in the growth of carbon nanotubes/nanofibers (CNTs/CNFs) as compared to monometallic catalysts. However, due to the lack of an understanding of the active state and active structure, the origin of the superior performance of alloy catalysts is unknown. In this work, we report an in situ transmission electron microscopy (TEM) study of the CNF growth enabled by one of the most active known alloy catalysts, i.e., Ni-Co, providing insights into the active state and the interaction between Ni and Co in the working catalyst. We reveal that the functioning catalyst is highly dynamic, undergoing constant reshaping and periodic elongation/contraction. Atomic-scale imaging combined with in situ electron energy-loss spectroscopy further identifies the active structure as a Ni-Co metallic alloy (face-centered cubic, FCC). Aided by the molecular dynamics simulation and density functional theory calculations, we rationalize the dynamic behavior of the catalyst and the growth mechanism of CNFs and provide insight into the origin of the superior performance of the Ni-Co alloy catalyst.

Metal Cluster Size-Dependent Activation Energies of Growth of Single-Chirality Single-Walled Carbon Nanotubes inside Metallocene-Filled Single-Walled Carbon Nanotubes

By combining in situ annealing and Raman spectroscopy measurements, the growth dynamics of nine individual-chirality inner tubes (8,8), (12,3), (13,1), (9,6), (10,4), (11,2), (11,1), (9,3) and (9,2) with diameters from ~0.8 to 1.1 nm are monitored using a time resolution of several minutes. The growth mechanism of inner tubes implies two successive stages of the growth on the carburized and purely metallic catalytic particles, respectively, which are formed as a result of the thermally induced decomposition of metallocenes inside the outer SWCNTs. The activation energies of the growth on carburized Ni and Co catalytic particles amount to 1.85–2.57 eV and 1.80–2.71 eV, respectively. They decrease monotonically as the tube diameter decreases, independent of the metal type. The activation energies of the growth on purely metallic Ni and Co particles equal 1.49–1.91 eV and 0.77–1.79 eV, respectively. They increase as the tube diameter decreases. The activation energies of the growth of large-diameter tubes (d_t = ~0.95–1.10 nm) on Ni catalyst are significantly larger than on Co catalyst, whereas the values of small-diameter tubes (d_t = ~0.80–0.95 nm) are similar. For both metals, no dependence of the activation energies on the chirality of inner tubes is observed.

Influencing factors and growth kinetics analysis of carbon nanotube growth on the surface of continuous fibers

Carbon nanotubes (CNTs) were continuously grown on the surface of the moving carbon fiber by chemical vapor deposition method using a custom-designed production line to prepare composite reinforcements on a large-scale. The systematic study of different parameters affecting the CNT growth revealed simple growth kinetics, which helps to control the surface morphology and structural quality of CNTs. Since hydrogen maintains the activity of the catalyst, it promotes the growth of CNTs in a continuous process. The increase of acetylene partial pressure promotes the accumulation of amorphous or graphite carbon on the catalyst surface, resulting in the decrease of CNT growth rate when acetylene concentration reaches 40%. The growth temperature significantly affects the CNT diameter and structural quality. As the temperature increases, the crystallinity of the tube wall increases obviously, and the CNT diameter increases due to the aggregate growth of the catalyst particles. According to the Arrhenius formula, the apparent activation energy is observed to be 0.67 eV, which proves that both bulk diffusion and surface diffusion exist when activated carbon passes through the catalyst to form CNTs.

Digital Isotope Coding to Trace the Growth Process of Individual Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) are attracting increasing attention as an ideal material for high-performance electronics through the preparation of arrays of purely semiconducting SWCNTs. Despite significant progress in the controlled synthesis of SWCNTs, their growth mechanism remains unclear due to difficulties in analyzing the time-resolved growth of individual SWCNTs under practical growth conditions. Here we present a method for tracing the diverse growth profiles of individual SWCNTs by embedding digitally coded isotope labels. Raman mapping showed that, after various incubation times, SWCNTs elongated monotonically until their abrupt termination. Ex situ analysis offered an opportunity to capture rare chirality changes along the SWCNTs, which resulted in sudden acceleration/deceleration of the growth rate. Dependence on growth parameters, such as temperature and carbon concentration, was also traced along individual SWCNTs, which could provide clues to chirality control. Systematic growth studies with a variety of catalysts and conditions, which combine the presented method with other characterization techniques, will lead to further understanding and control of chirality, length, and density of SWCNTs.

Chirality-dependent growth of single-wall carbon nanotubes as revealed inside nano-test tubes

Growth dynamics of single-wall carbon nanotubes (SWCNTs) have been studied with nickelocene as a precursor encapsulated in the interior of template SWCNTs. By means of multi-laser Raman spectroscopy, growth curves of nine different SWCNTs, (8,8), (12,3), (13,1), (9,6), (10,4), (11,2), (11,1), (9,3) and (9,2), have been determined upon in situ annealing at various temperatures. The data reveal that the nanotubes grow through fast and slow reaction pathways with high and low activation energies, respectively. While the activation energy of the slow growth is independent of the nanotube's chiral vector, that of the fast growth exhibits a monotonic increase as the tube diameter reduces from ∼1.1 down to 0.8 nm and no dependency on the chiral angle, which can be attributed to the size-dependent properties of catalyst clusters. The chirality dependent catalytic growth properties exploited in this study provide the basis for a large-scale synthesis of single-chiral vector SWCNTs.

Investigation of growth dynamics of carbon nanotubes

The synthesis of single-walled carbon nanotubes (SWCNTs) with defined properties is required for both fundamental investigations and practical applications. The revealing and thorough understanding of the growth mechanism of SWCNTs is the key to the synthesis of nanotubes with required properties. This paper reviews the current status of the research on the investigation of growth dynamics of carbon nanotubes. The review starts with the consideration of the peculiarities of the growth mechanism of carbon nanotubes. The physical and chemical states of the catalyst during the nanotube growth are discussed. The chirality selective growth of nanotubes is described. The main part of the review is dedicated to the analysis and systematization of the reported results on the investigation of growth dynamics of nanotubes. The studies on the revealing of the dependence of the growth rate of nanotubes on the synthesis parameters are reviewed. The correlation between the lifetime of catalyst and growth rate of nanotubes is discussed. The reports on the calculation of the activation energy of the nanotube growth are summarized. Finally, the growth properties of inner tubes inside SWCNTs are considered.

High quality and high performance adsorption of Congo red using as-grown MWCNTs synthesized over a Co-MOF as a catalyst precursor via the CVD method

A Co-MOF has been synthesized and characterized, which was firstly used as a combined catalyst precursor to synthesize MWCNTs with high performance in the adsorption of CR.

Real-Time Imaging of Self-Organization and Mechanical Competition in Carbon Nanotube Forest Growth

The properties of carbon nanotube (CNT) networks and analogous materials comprising filamentary nanostructures are governed by the intrinsic filament properties and their hierarchical organization and interconnection. As a result, direct knowledge of the collective dynamics of CNT synthesis and self-organization is essential to engineering improved CNT materials for applications such as membranes and thermal interfaces. Here, we use real-time environmental transmission electron microscopy (E-TEM) to observe nucleation and self-organization of CNTs into vertically aligned forests. Upon introduction of the carbon source, we observe a large scatter in the onset of nucleation of individual CNTs and the ensuing growth rates. Experiments performed at different temperatures and catalyst particle densities show the critical role of CNT density on the dynamics of self-organization; low-density CNT nucleation results in the CNTs becoming pinned to the substrate and forming random networks, whereas higher density CNT nucleation results in self-organization of the CNTs into bundles that are oriented perpendicular to the substrate. We also find that mechanical coupling between growing CNTs alters their growth trajectory and shape, causing significant deformations, buckling, and defects in the CNT walls. Therefore, it appears that CNT-CNT coupling not only is critical for self-organization but also directly influences CNT quality and likely the resulting properties of the forest. Our findings show that control of the time-distributed kinetics of CNT nucleation and bundle formation are critical to manufacturing well-organized CNT assemblies and that E-TEM can be a powerful tool to investigate the mesoscale dynamics of CNT networks.

Predictive Synthesis of Freeform Carbon Nanotube Microarchitectures by Strain-Engineered Chemical Vapor Deposition

High-throughput fabrication of microstructured surfaces with multi-directional, re-entrant, or otherwise curved features is becoming increasingly important for applications such as phase change heat transfer, adhesive gripping, and control of electromagnetic waves. Toward this goal, curved microstructures of aligned carbon nanotubes (CNTs) can be fabricated by engineered variation of the CNT growth rate within each microstructure, for example by patterning of the CNT growth catalyst partially upon a layer which retards the CNT growth rate. This study develops a finite-element simulation framework for predictive synthesis of complex CNT microarchitectures by this strain-engineered growth process. The simulation is informed by parametric measurements of the CNT growth kinetics, and the anisotropic mechanical properties of the CNTs, and predicts the shape of CNT microstructures with impressive fidelity. Moreover, the simulation calculates the internal stress distribution that results from extreme deformation of the CNT structures during growth, and shows that delamination of the interface between the differentially growing segments occurs at a critical shear stress. Guided by these insights, experiments are performed to study the time- and geometry-depended stress development, and it is demonstrated that corrugating the interface between the segments of each microstructure mitigates the interface failure. This study presents a methodology for 3D microstructure design based on "pixels" that prescribe directionality to the resulting microstructure, and show that this framework enables the predictive synthesis of more complex architectures including twisted and truss-like forms.

Understanding properties of engineered catalyst supports using contact angle measurements and X-ray reflectivity

There is significant interest in broadening the type of catalyst substrates that support the growth of high-quality carbon nanotube (CNT) carpets. In this study, ion beam bombardment has been utilized to modify catalyst substrates for CNT carpet growth. Using a combination of contact angle measurements (CAMs) and X-ray reflectivity (XRR) for the first time, new correlations between the physicochemical properties of pristine and engineered catalyst substrates and CNT growth behavior have been established. The engineered surfaces obtained after exposure to different degrees of ion beam damage have distinct physicochemical properties (porosity, layer thickness, and acid-base properties). The CAM data were analyzed using the van Oss-Chaudhury-Good model, enabling the determination of the acid-base properties of the substrate surfaces. For the XRR data, a Fourier analysis of the interference patterns enabled extraction of layer thickness, while the atomic density and interfacial roughness were extracted by analyzing the amplitude of the interference oscillations. The dramatic transformation of the substrate from "inactive" to "active" is attributed to a combined effect of substrate porosity or damage depth and Lewis basicity. The results reveal that the efficiency of catalyst substrates can be further improved by increasing the substrate basicity, if the minimum surface porosity is established. This study advances the use of a non-thermochemical approach for catalyst substrate engineering, as well as demonstrates the combined utility of CAM and XRR as a powerful, nondestructive, and reliable tool for rational catalyst design.

The Application of Gas Dwell Time Control for Rapid Single Wall Carbon Nanotube Forest Synthesis to Acetylene Feedstock

One aspect of carbon nanotube (CNT) synthesis that remains an obstacle to realize industrial mass production is the growth efficiency. Many approaches have been reported to improve the efficiency, either by lengthening the catalyst lifetime or by increasing the growth rate. We investigated the applicability of dwell time and carbon flux control to optimize yield, growth rate, and catalyst lifetime of water-assisted chemical vapor deposition of single-walled carbon nanotube (SWCNT) forests using acetylene as a carbon feedstock. Our results show that although acetylene is a precursor to CNT synthesis and possesses a high reactivity, the SWCNT forest growth efficiency is highly sensitive to dwell time and carbon flux similar to ethylene. Through a systematic study spanning a wide range of dwell time and carbon flux levels, the relationship of the height, growth rate, and catalyst lifetime is found. Further, for the optimum conditions for 10 min growth, SWCNT forests with ~2500 μm height, ~350 μm/min initial growth rates and extended lifetimes could be achieved by increasing the dwell time to ~5 s, demonstrating the generality of dwell time control to highly reactive gases.

The relationship between the growth rate and the lifetime in carbon nanotube synthesis

We report an inverse relationship between the carbon nanotube (CNT) growth rate and the catalyst lifetime by investigating the dependence of growth kinetics for ∼330 CNT forests on the carbon feedstock, carbon concentration, and growth temperature. We found that the increased growth temperature led to increased CNT growth rate and shortened catalyst lifetime for all carbon feedstocks, following an inverse relationship of a fairly constant maximum height. For the increased carbon concentration, the carbon feedstocks fell into two groups where ethylene/butane showed an increased/decreased growth rate and a decreased/increased lifetime indicating different rate-limiting growth processes. In addition, this inverse relationship held true for different types of CNTs synthesized by various chemical vapor deposition techniques and continuously spanned a 1000-times range in both the growth rate and catalyst lifetime, indicating the generality and fundamental nature of this behavior originating from the growth mechanism of CNTs itself. These results suggest that it would be fundamentally difficult to achieve a fast growth with a long lifetime.

State of the art of nanoforest structures and their applications

Forest-like nanostructures, their syntheses, properties, and applications are reviewed.

Discovery of wall-selective carbon nanotube growth conditions via automated experimentation

Applications of carbon nanotubes continue to advance, with substantial progress in nanotube electronics, conductive wires, and transparent conductors to name a few. However, wider application remains impeded by a lack of control over production of nanotubes with the desired purity, perfection, chirality, and number of walls. This is partly due to the fact that growth experiments are time-consuming, taking about 1 day per run, thus making it challenging to adequately explore the many parameters involved in growth. We endeavored to speed up the research process by automating CVD growth experimentation. The adaptive rapid experimentation and in situ spectroscopy CVD system described in this contribution conducts over 100 experiments in a single day, with automated control and in situ Raman characterization. Linear regression modeling was used to map regions of selectivity toward single-wall and multiwall carbon nanotube growth in the complex parameter space of the water-assisted CVD synthesis. This development of the automated rapid serial experimentation is a significant progress toward an autonomous closed-loop learning system: a Robot Scientist.

Synergetic chemical coupling controls the uniformity of carbon nanotube microstructure growth

Control of the uniformity of vertically aligned carbon nanotube structures (CNT "forests"), in terms of both geometry and nanoscale morphology (density, diameter, and alignment), is crucial for applications. Many studies report complex and sometimes unexplained spatial variations of the height of macroscopic CNT forests, as well as variations among micropillars grown from lithographically patterned catalyst arrays. We present a model for chemically coupled CNT growth, which describes the origins of synergetic growth effects among CNT micropillars in proximity. Via this model, we propose that growth of CNTs is locally enhanced by active species that are catalytically produced at the substrate-bound nanoparticles. The local concentration of these active species modulates the growth rate of CNTs, in a spatially dependent manner driven by diffusion and local generation/consumption at the catalyst sites. Through experiments and numerical simulations, we study how the uniformity of CNT micropillars can be influenced by their size and spacing within arrays and predict the widely observed abrupt transition between tangled and vertical CNT growth by assigning a threshold concentration of active species. This mathematical framework enables predictive modeling of spatially dependent CNT growth, as well as design of catalyst patterns to achieve engineered uniformity.

Kinetics of laser-assisted carbon nanotube growth

Laser-assisted chemical vapour deposition (CVD) growth is an attractive mask-less process for growing locally aligned carbon nanotubes (CNTs) in selected places on temperature sensitive substrates. The nature of the localized process results in fast carbon nanotube growth with high experimental throughput. Here, we report on the detailed investigation of growth kinetics related to physical and chemical process characteristics. Specifically, the growth kinetics is investigated by monitoring the dynamical changes in reflected laser beam intensity during growth. Benefiting from the fast growth and high experimental throughput, we investigate a wide range of experimental conditions and propose several growth regimes. Rate-limiting steps are determined using rate equations linked to the proposed growth regimes, which are further characterized by Raman spectroscopy and Scanning Electron Microscopy (SEM), therefore directly linking growth regimes to the structural quality of the CNTs. Activation energies for the different regimes are found to be in the range of 0.3-0.8 eV.

Metal-catalyst-free growth of carbon nanotubes/carbon nanofibers on carbon blacks using chemical vapor deposition

Carbon black can act as catalysts to grow carbon nanotubes or carbon nanofibers through a metal-catalyst-free thermal chemical vapor deposition.

Carbon nanotube growth from Langmuir-Blodgett deposited Fe3O4 nanocrystals

We investigate colloidal Fe(3)O(4) nanocrystals as a catalyst system for carbon nanotube (CNT) growth that allows for decoupling the CNT growth step from the catalyst shaping and activation step. The system consists of 6.4 nm Fe(3)O(4) nanocrystals synthesized using a solution-based thermal decomposition reaction and, subsequently, transferred as hexagonally ordered Langmuir-Blodgett (LB) monolayers on TiN substrates. We demonstrate for the first time aligned CNT growth from LB deposited nanocrystals on a metallic underlayer. The hexagonally ordered monolayers of catalyst particles show promising stability up to the CNT growth temperature. In situ TEM heating experiments were performed to find this onset of particle deformation and showed stability of the nanoparticles up to 600 °C. The particle coalescence at high temperatures was also evidenced by the increasing CNT diameter, from 9.5 nm at 580 °C to 16 nm at 630 °C. By choosing to work at temperatures below the onset particle coalescence temperature, equivalent CNT diameters were obtained under different catalyst activation and growth conditions. The high stability of the catalyst on the metallic underlayer enables us to study CNT growth kinetics independently of the catalyst shaping step. This work opens a route towards combining growth studies with an electrical evaluation of the CNT growth as the TiN can be used as the bottom contact.

CVD-grown horizontally aligned single-walled carbon nanotubes: synthesis routes and growth mechanisms

Single-walled carbon nanotubes (SWCNTs) have attractive electrical and physical properties, which make them very promising for use in various applications. For some applications however, in particular those involving electronics, SWCNTs need to be synthesized with a high degree of control with respect to yield, length, alignment, diameter, and chirality. With this in mind, a great deal of effort is being directed to the precision control of vertically and horizontally aligned nanotubes. In this review the focus is on the latter, horizontally aligned tubes grown by chemical vapor deposition (CVD). The reader is provided with an in-depth review of the established vapor deposition orientation techniques. Detailed discussions on the characterization routes, growth parameters, and growth mechanisms are also provided.

Incremental growth of short SWNT arrays by pulsed chemical vapor deposition

Very short arrays of continuous single-wall carbon nanotubes (SWNTs) are grown incrementally in steps as small as 25 nm using pulsed chemical vapor deposition (CVD). In-situ optical extinction measurements indicate that over 98% of the nanotubes reinitiate growth on successive gas pulses, and high-resolution transmission electron microscopy (HR-TEM) images show that the SWNTs do not exhibit segments, caps, or noticeable sidewall defects resulting from repeatedly stopping and restarting growth. Time-resolved laser reflectivity (3-ms temporal resolution) is used to record the nucleation and growth kinetics for each fast (0.2 s) gas pulse and to measure the height increase of the array in situ, providing a method to incrementally grow short nanotube arrays to precise heights. Derivatives of the optical reflectivity signal reveal distinct temporal signatures for both nucleation and growth kinetics, with their amplitude ratio on the first gas pulse serving as a good predictor for the evolution of the growth of the nanotube ensemble into a coordinated array. Incremental growth by pulsed CVD is interpreted in the context of autocatalytic kinetic models as a special processing window in which a sufficiently high flux of feedstock gas drives the nucleation and rapid growth phases of a catalyst nanoparticle ensemble to occur within the temporal period of the gas pulse, but without inducing growth termination.

Growth of ultrahigh density single-walled carbon nanotube forests by improved catalyst design

We have grown vertically aligned single-walled carbon nanotube forests with an area density of 1.5 × 10(13) cm(-2), the highest yet achieved, by reducing the average diameter of the nanotubes. We use a nanolaminate Fe-Al(2)O(3) catalyst design consisting of three layers of Al(2)O(3), Fe, and Al(2)O(3), in which the lower Al(2)O(3) layer is densified by an oxygen plasma treatment to increase its diffusion barrier properties, to allow a thinner catalyst layer to be used. This high nanotube density is desirable for using carbon nanotubes as interconnects in integrated circuits.

Growth kinetics of vertically aligned carbon nanotube arrays in clean oxygen-free conditions

Vertically aligned carbon nanotubes (CNTs) are an important technological system, as well as a fascinating system for studying basic principles of nanomaterials synthesis; yet despite continuing efforts for the past decade many important questions about this process remain largely unexplained. We present a series of parametric ethylene chemical vapor deposition growth studies in a "hot-wall" reactor using ultrapure process gases that reveal the fundamental kinetics of the CNT growth. Our data show that the growth rate is proportional to the concentration of the carbon feedstock and monotonically decreases with the concentration of hydrogen gas and that the most important parameter determining the rate of the CNT growth is the production rate of active carbon precursor in the gas phase reaction. The growth termination times obtained with the purified gas mixtures were strikingly insensitive to variations in both hydrogen and ethylene pressures ruling out the carbon encapsulation of the catalyst as the main process termination cause.

Flux-dependent growth kinetics and diameter selectivity in single-wall carbon nanotube arrays

The nucleation and growth kinetics of single-wall carbon nanotubes in aligned arrays have been measured using fast pulses of acetylene and in situ optical diagnostics in conjunction with low pressure chemical vapor deposition (CVD). Increasing the acetylene partial pressure is shown to decrease nucleation times by three orders of magnitude, permitting aligned nanotube arrays to nucleate and grow to micrometers lengths within single gas pulses at high (up to 7 μm/s) peak growth rates and short ∼0.5 s times. Low-frequency Raman scattering (>10 cm(-1)) and transmission electron microscopy measurements show that increasing the feedstock flux in both continuous- and pulsed-CVD shifts the product distribution to large single-wall carbon nanotube diameters >2.5 nm. Sufficiently high acetylene partial pressures in pulsed-CVD appear to temporarily terminate the growth of the fastest-growing, small-diameter nanotubes by overcoating the more catalytically active, smaller catalyst nanoparticles within the ensemble with non-nanotube carbon in agreement with a growth model. The results indicate that subsets of catalyst nanoparticle ensembles nucleate, grow, and terminate growth within different flux ranges according to their catalytic activity.

Gas dwell time control for rapid and long lifetime growth of single-walled carbon nanotube forests

The heat history (i.e., "dwell time") of the carbon source gas was demonstrated as a vital parameter for very rapid single-walled carbon nanotube (SWNT) forest growth with long lifetime. When the dwell time was raised to 7 s from the 4 s used for standard growth, the growth rate increased to 620 μm/min: a benchmark for SWNT forest growth on substrates. Importantly, the increase in growth rate was achieved without decreasing either the growth lifetime or the quality of the SWNTs. We interpret that the conversion rate of the carbon feedstock into CNTs was selectively increased (versus catalyst deactivation) by delivering a thermally decomposed carbon source with the optimum thermal history to the catalyst site.

Carbon diffusion from methane into walls of carbon nanotube through structurally and compositionally modified iron catalyst

To understand diffusion processes occurring inside Fe catalysts during multiwall carbon nanotube (MWCNT) growth, catalysts were studied using atomic-resolution scanning transmission electron microscopy combined with electron energy-loss spectroscopy. Nanotube walls emanate from structurally modified and chemically complex catalysts that consist of cementite and a 5 nm amorphous FeOx cap separated by a 2-3 nm thick carbon-rich region that also contains Fe and O (a-C:FexOy). Nonuniform distribution of carbon atoms throughout the catalyst base reveals that carbon molecules from the gas phase decompose near the catalyst multisection junction, where the MWCNT walls terminate. Formation of the a-C:FexOy region provides the essential carbon source for MWCNT growth. Two different carbon diffusion mechanisms are responsible for the growth of the inner and outer walls of each MWCNT.

Carbon nanotube mass production: principles and processes

Our society requires new materials for a sustainable future, and carbon nanotubes (CNTs) are among the most important advanced materials. This Review describes the state-of-the-art of CNT synthesis, with a focus on their mass-production in industry. At the nanoscale, the production of CNTs involves the self-assembly of carbon atoms into a one-dimensional tubular structure. We describe how this synthesis can be achieved on the macroscopic scale in processes akin to the continuous tonne-scale mass production of chemical products in the modern chemical industry. Our overview includes discussions on processing methods for high-purity CNTs, and the handling of heat and mass transfer problems. Manufacturing strategies for agglomerated and aligned single-/multiwalled CNTs are used as examples of the engineering science of CNT production, which includes an understanding of their growth mechanism, agglomeration mechanism, reactor design, and process intensification. We aim to provide guidelines for the production and commercialization of CNTs. Although CNTs can now be produced on the tonne scale, knowledge of the growth mechanism at the atomic scale, the relationship between CNT structure and application, and scale-up of the production of CNTs with specific chirality are still inadequate. A multidisciplinary approach is a prerequisite for the sustainable development of the CNT industry.

Recent progress on the growth mechanism of carbon nanotubes: a review

Tremendous progress has been achieved during the past 20 years on not only improving the yields of carbon nanotubes and move progressively towards their mass production, but also on gaining a profound fundamental understanding of the nucleation and the growth processes. Parameters that influence the yield but also the quality (e.g., microstructure, homogeneity within a batch) are better understood. The influence of the carbon precursor, the reaction conditions, the presence of a catalyst, the chemical and physical status of the latter, and other factors have been extensively studied. The purpose of the present Review is not to list all the experiments reported in the literature, but rather to identify trends and provide a comprehensive summary on the role of selected parameters. The role of the catalyst occupies a central place in this Review as a careful control of the metal particle size, particle dispersion on the support, the metastable phase formed under reaction conditions, its possible reconstruction, and faceting strongly influence the diameter of the carbon nanotubes, their structure (number of walls, graphene sheet orientation, chirality), their alignment, and the yield. The identified trends will be compared with recent observations on the growth of graphene. Recent results on metal-free catalysts will be analyzed from a different perspective.

Multiple alkynes react with ethylene to enhance carbon nanotube synthesis, suggesting a polymerization-like formation mechanism

Thermal treatments of feedstock gases (e.g., C(2)H(4)/H(2)) used during carbon nanotube (CNT) synthesis result in the formation of a complex mixture of volatile organic compounds and polycyclic aromatic hydrocarbons. Some of these are likely important CNT precursors, while others are superfluous and possibly degrade product quality, form amorphous carbon, and/or contribute to growth termination. To simulate the effect of thermal treatment without this chemical complexity, we delivered trace amounts of individual hydrocarbons, along with ethylene and hydrogen, to a cold-wall atmospheric pressure reactor containing a locally heated metal catalyst (Fe on Al(2)O(3)). Using these compound-specific experiments, we demonstrate that many alkynes (e.g., acetylene, methyl acetylene, and vinyl acetylene) accelerate multiwalled CNT formation with this catalyst system. Furthermore, ethylene is required for enhanced CNT growth, suggesting that the alkyne and ethylene may react in concert at the metal catalyst. This presents a distinct CNT formation mechanism where the chemical precursors may be intact during C-C bond formation, such as in polymerization reactions, challenging the widely accepted hypothesis that precursors completely dissociate into C (or C(2)) units before "precipitating" from the metal. Armed with these mechanistic insights, we were able to form high-purity CNTs rapidly with a 15-fold improvement in yield, a 50% reduction in energetic costs, and order of magnitude reduction in unwanted byproduct formation (e.g., toxic and smog-forming chemicals and greenhouse gases).

Pulsed growth of vertically aligned nanotube arrays with variable density

The density of vertically aligned carbon nanotube arrays is shown to vary significantly during normal growth by chemical vapor deposition and respond rapidly to changes in feedstock flux. Pulsing the feedstock gas to repeatedly stop and start nanotube growth is shown to induce density variations up to a factor of 1.6 within ca. 1-2 μm long layers, allowing the synthesis of new array architectures with distinct regions of controllable length and density variation. Z-Contrast scanning transmission electron microscopy of corresponding sections of the arrays is used to provide unambiguous measurements of these density variations. Time-resolved optical reflectivity measurements of the height and optical extinction coefficient of the growing arrays are shown to provide a real-time diagnostic of both array density and growth kinetics.