Nucleation and growth of single-walled nanotubes: the role of metallic catalysts

Magnetic impurities in single-walled carbon nanotubes and graphene: a review

A review on magnetic impurities in single-walled carbon nanotubes and graphene: purification and detection of impurities and impurity-induced magnetism.

DFT study of Fe-Ni core-shell nanoparticles: stability, catalytic activity, and interaction with carbon atom for single-walled carbon nanotube growth

Metal catalysts play an important role in the nucleation and growth of single-walled carbon nanotubes (SWCNTs). It is essential for probing the nucleation and growth mechanism of SWCNTs to fundamentally understand the properties of the metal catalysts and their interaction with carbon species. In this study, we systematically studied the stability of 13- and 55-atom Fe and Fe-Ni core-shell particles as well as these particles interaction with the carbon atoms using the density functional theory calculations. Icosahedral 13- and 55-atom Fe-Ni core-shell bimetallic particles have higher stability than the corresponding monometallic Fe and Ni particles. Opposite charge transfer (or distribution) in these particles leads to the Fe surface-shell displays a positive charge, while the Ni surface-shell exhibits a negative charge. The opposite charge transfer would induce different chemical activities. Compared with the monometallic Fe and Ni particles, the core-shell bimetallic particles have weaker interaction with C atoms. More importantly, C atoms only prefer staying on the surface of the bimetallic particles. In contrast, C atoms prefer locating into the subsurface of the monometallic particles, which is more likely to form stable metal carbides. The difference of the mono- and bimetallic particles on this issue may result in different nucleation and growth mechanism of SWCNTs. Our findings provide useful insights for the design of bimetallic catalysts and a better understanding nucleation and growth mechanism of SWCNTs.

Insights into carbon nanotube and graphene formation mechanisms from molecular simulations: a review

The discovery of carbon nanotubes (CNTs) and graphene over the last two decades has heralded a new era in physics, chemistry and nanotechnology. During this time, intense efforts have been made towards understanding the atomic-scale mechanisms by which these remarkable nanostructures grow. Molecular simulations have made significant contributions in this regard; indeed, they are responsible for many of the key discoveries and advancements towards this goal. Here we review molecular simulations of CNT and graphene growth, and in doing so we highlight the many invaluable insights gained from molecular simulations into these complex nanoscale self-assembly processes. This review highlights an often-overlooked aspect of CNT and graphene formation-that the two processes, although seldom discussed in the same terms, are in fact remarkably similar. Both can be viewed as a 0D → 1D → 2D transformation, which converts carbon atoms (0D) to polyyne chains (1D) to a complete sp(2)-carbon network (2D). The difference in the final structure (CNT or graphene) is determined only by the curvature of the catalyst and the strength of the carbon-metal interaction. We conclude our review by summarizing the present shortcomings of CNT/graphene growth simulations, and future challenges to this important area.

Evidence of correlation between catalyst particles and the single-wall carbon nanotube diameter: a first step towards chirality control

Controlling the structure of single-wall carbon nanotubes during their synthesis by chemical vapor deposition remains a challenging issue. Here, using a specific synthesis protocol and ex situ transmission electron microscopy, we perform a statistical analysis of the structure of the tubes and of the catalyst particles from which they grow. We discriminate two nucleation modes, corresponding to different nanotube-particle junctions, that occur independently of the particle size. With the support of tight binding calculations, we show that a direct control of the nanotube diameter by the particle can only be achieved under growth conditions close to thermodynamic equilibrium.

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.

Mechanisms of single-walled carbon nanotube nucleation, growth, and healing determined using QM/MD methods

Since their discovery in the early 1990s, single-walled carbon nanotubes (SWNTs) have spawned previously unimaginable commercial and industrial technologies. Their versatility stems from their unique electronic, physical/chemical, and mechanical properties, which set them apart from traditional materials. Many researchers have investigated SWNT growth mechanisms in the years since their discovery. The most prevalent of these is the vapor-liquid-solid (VLS) mechanism, which is based on experimental observations. Within the VLS mechanism, researchers assume that the formation of a SWNT starts with co-condensation of carbon and metal atoms from vapor to form liquid metal carbide. Once the liquid reaches supersaturation, the solid phase nanotubes begin to grow. The growth process is partitioned into three distinct stages: nucleation of a carbon "cap-precursor," "cap-to-tube" transformation, and continued SWNT growth. In recent years, molecular dynamics (MD) simulations have come to the fore with respect to SWNT growth. MD simulations lead to spatial and temporal resolutions of these processes that are superior to those possible using current experimental techniques, and so provide valuable information regarding the growth process that researchers cannot obtain experimentally. In this Account, we review our own recent efforts to simulate SWNT nucleation, growth, and healing phenomena on transition-metal catalysts using quantum mechanical molecular dynamics (QM/MD) methods. In particular, we have validated each stage of the SWNT condensation mechanism using a self-consistent-charge density-functional tight-binding (SCC-DFTB) methodology. With respect to the nucleation of a SWNT cap-precursor (stage 1), we have shown that the presence of a transition-metal carbide particle is not a necessary prerequisite for SWNT nucleation, contrary to conventional experimental presumptions. The formation and coalescence of polyyne chains on the metal surface occur first, followed by the formation of the SWNT cap-precursor, "ring condensation", and the creation of an sp(2)-hybridized carbon structure. In our simulations, the nucleation process takes approximately 400 ps. This first step occurs over a much longer time scale than the second stage of SWNT condensation (approximately 50 ps). We therefore observe SWNT nucleation to be akin to the rate-limiting step of the SWNT formation process. In addition to the QM/MD simulation of various stages of SWNT nucleation, growth, and healing processes, we have determined the effects of temperature, catalyst composition, and catalyst size on the kinetics and mechanism of SWNT growth. With respect to temperature dependence, we observe a "sweet-spot" with respect to the efficiency of SWNT growth. In addition, Ni-catalyzed SWNT growth is observed to be 70-100% faster compared to Fe-catalyzed SWNT growth, depending on the catalyst particle size. We also observe a noticeable increase in SWNT growth rates using smaller catalyst particles. Finally, we review our recent QM/MD investigation of SWNT healing. In particular, we recount mechanisms by which adatom defects, monovacancy defects, and a "5-7 defect" are removed from a nascent SWNT. The effectiveness of these healing mechanisms depends on the rate at which carbon moieties are incorporated into the growing SWNT. Explicitly, we observe that healing is promoted using a slower carbon supply rate. From this rudimentary control of SWNT healing, we propose a route towards chirality-controlled SWNT growth.

Carbon Nanotubes Filled with Ferromagnetic Materials

Carbon nanotubes (CNT) filled with ferromagnetic metals like iron, cobalt or nickel are new and very interesting nanostructured materials with a number of unique properties. In this paper we give an overview about different chemical vapor deposition (CVD) methods for their synthesis and discuss the influence of selected growth parameters. In addition we evaluate possible growth mechanisms involved in their formation. Moreover we show their identified structural and magnetic properties. On the basis of these properties we present different application possibilities. Some selected examples reveal the high potential of these materials in the field of medicine and nanotechnology.

Interactions between metals and carbon nanotubes: at the interface between old and new materials

The article reviews the interaction between crystalline metals and carbon nanotubes in nanocomposite systems. Starting with an introduction to the chemical interaction between metal atoms and graphitic layers, an overview of the fields of nanotechnology is given where metal-carbon interaction comes into play. The interface between metals and carbon nanotubes is of interest in junctions between nanotubes and their periphery, for example in metallic contacts for electronic devices or in metal supports for carbon nanotube components. Furthermore, metals determine the catalytic growth of carbon nanotubes. The behaviour of individual metal atoms in or on carbon nanotubes is treated as well as the interaction between crystalline metals and nanotube surfaces. Emphasis is put on the common mechanisms of metal-carbon interaction that play a role in such different fields as the electrical transport through a metal-nanotube contact or the catalytic growth of nanotubes from metal particles.

Growth of chiral single-walled carbon nanotube caps in the presence of a cobalt cluster

Density functional theory is used to simulate nanotube growth by addition of a pair of carbon atoms to a composite chiral nanotube cap/cobalt cluster system, with caps corresponding to near-armchair (6, 5), (7, 5) and near-zigzag (9, 1) nanotubes. Two different carbon addition processes are evaluated: in the first, the new carbon atoms are located in the vicinity of the armchair site of the cap rim, and thus this process provides insight into the root-growth mechanism; in the second the carbon atoms are initially located under the cobalt cluster, and thus this process helps one to evaluate the dissolution of carbon inside the metal cluster. The geometric evolution and energetics of the system are used to explain features of the mechanism of nanotube growth. The root-growth reaction is shown to occur by displacement of a cobalt atom initially interacting with the armchair site while the added carbon atoms bond to each other forming a new hexagonal ring, whereas the carbon dissolution process shows formation of dimers inside the cluster only for the (6, 5) system. The energetics for both steps reveals that the dissolution stage is probably controlling the overall nanotube growth rate.

Rapid growth of a single-walled carbon nanotube on an iron cluster: density-functional tight-binding molecular dynamics simulations

Continued growth of a single-walled carbon nanotube (SWNT) on an Fe cluster at 1500 K is demonstrated using quantum chemical molecular dynamics simulations based on the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. In order to deal with charge transfer between carbon and metal particles and the multitude of electronic states, a finite electronic temperature approach is applied. We present trajectories of 45 ps length, where a continuous supply of carbon atoms is directed toward the C-Fe boundary between a 7.2 A long armchair (5,5) SWNT fragment and an attached Fe(38) cluster. The incident carbon atoms react readily at the C-Fe interface to form C- and C(2)-extensions on the tube rim that attach to the Fe cluster. These bridging sp-hybridized carbon fragments are vibrationally excited and highly mobile and, therefore, become engaged in frequent bond formation and breaking processes between their constituent C and the Fe atoms. The sp-hybridized carbon bridge dynamics and their reactions with the Fe-attached nanotube end bring about formations of new five-, six-, and seven-membered carbon rings extending the tube sidewall, resulting in overall continued growth of the nanotube on the Fe cluster up to nearly twice its length. Due to the random nature of new polygon formation, sidewall growth is observed as an irregular process without clear SWNT chirality preference. Compared to fullerene formation, heptagon formation is considerably promoted.

Effect of Rare-Earth Component of the RE/Ni Catalyst on the Formation and Nanostructure of Single-Walled Carbon Nanotubes

A systematic experimental study has been carried out on the efficiency of bimetallic catalysts based on Ni and the rare-earth elements Y, La, Ce, Nd, Gd, Tb, Dy, Ho, Er, and Lu (group A) and Eu, Sm, Yb, and Tm (group B) in the synthesis of single-walled carbon nanotubes (SWNTs). The two groups give quite different results when analyzed by a combination of SEM/TEM and Raman and UV-NIR spectroscopies. The elements in group A have an obvious catalytic effect and increase the yield of SWNTs dramatically, whereas those in group B are not efficient catalysts. The diameter distribution of the synthesized SWNTs was also affected by the rare-earth element used. For group A metals, there is a tendency that the fraction of small-diameter tubes decreases with decreasing ionic radius of the rare-earth element used. EDX and X-ray analyses indicate that group A metals deposit on the cathode deposits and form rare-earth carbides, whereas no group B metals are found in cathode deposits, except for a small amount of Tm present in the form of thulium carbide. Further analysis indicates that there is a very strong correlation between the ability to form rare-earth carbides and the catalytic efficiency for the formation of SWNTs.

Graphitic Encapsulation of Catalyst Particles in Carbon Nanotube Production

A new model is proposed for the encapsulation of catalyst metal particles by graphite layers that are obtained, for example, in low-temperature chemical vapor deposition production of carbon nanotubes (CNTs). In this model graphite layers are primarily formed from the dissolved carbon atoms in the metal-carbide particle when the particle cools. This mechanism is in good agreement with molecular dynamics simulations (which show that precipitated carbon atoms preferentially form graphite sheets instead of CNTs at low temperatures) and experimental results (e.g., encapsulated metal particles are found in low-temperature zones and CNTs in high-temperature regions of production apparatus, very small catalyst particles are generally not encapsulated, and the ratio of the number of graphitic layers to the diameter of the catalyst particle is typically 0.25 nm(-1)).

Structural and Reactivity Properties of Finite Length Cap-Ended Single-Wall Carbon Nanotubes

The reaction of C2 with growing single-wall carbon nanotubes of different chiralities is investigated using density functional theory. It is found that the energy of the frontier orbitals for (5,5) and (6,6) armchair carbon nanotubes exhibits periodic behavior with an increasing number of carbon atoms in the nanotube. Such periodic behavior induces oscillations in the reaction energy released by adsorption of C2 to the nanotube open edge. In contrast, the energy of the frontier orbitals of the (6,5) chiral tube remains constant as the number of C atoms increases, and the same stability is observed in the adsorption energy. It is suggested that this may be one of the reasons for the low percent of armchair single-wall carbon nanotubes found in the experimental synthesis.

Understanding catalysed growth of single-wall carbon nanotubes

Classical molecular dynamics simulations using a reactive force field, which allows simulation of bond-breaking and bond-forming, are carried out to investigate the several stages of a catalysed synthesis process of single-wall carbon nanotubes. The simulations assume instantaneous catalysis of a precursor gas on the surface of metallic nanoclusters, illustrating how carbon atoms dissolve in the metal cluster and then precipitate on its surface, evolving into various carbon structures, finally forming a cap which eventually grows to a single-wall nanotube. The results are discussed in the context of experimental synthesis results.

Stability of Supershort Single-Walled Carbon Nanotubes

How short can single-walled carbon nanotubes (SWNTs) be? How stable are such supershort SWNTs (ss-SWNTs)? This work is the first to address these questions. On the basis of binding energy (E(B)), standard heats of formation , and strain energy (E(S)), we found that SWNTs with only one benzene ring in the axial direction, which we refer to as supershort SWNTs (ss-SWNTs), can be thermodynamically stable. On the basis of the data of E(B), , and E(S), the relative stabilities of ss-SWNTs, fullerenes, polycyclic aromatic hydrocarbons, and butadiyne are discussed. This study has laid a theoretical foundation for the possible synthesis of ss-SWNTs.

Comparison of analytical techniques for purity evaluation of single-walled carbon nanotubes

We compare popular analytical techniques, including scanning and transmission electron microscopy (SEM and TEM), thermogravimetric analysis (TGA), and Raman and near-infrared (NIR) spectroscopy, for the evaluation of the purity of bulk quantities of single-walled carbon nanotubes (SWNTs). Despite their importance as imaging techniques, SEM and TEM are not capable of quantitatively evaluating the purity of typical inhomogeneous bulk SWNT samples because the image frame visualizes less than 1 pg of SWNT material; furthermore, there is no published algorithm to convert such images into numerical data. The TGA technique is capable of measuring the amount of metal catalyst in an SWNT sample, but does not provide an unambiguous separation between the content of SWNTs and carbonaceous impurities. We discuss the utilization of solution-phase near-infrared spectroscopy and solution-phase Raman spectroscopy to quantitatively compare arbitrary samples of bulk SWNT materials of different purities. The primary goal of this paper is to provide the chemical community with a realistic evaluation of current analytical tools for the purity evaluation of a bulk sample of SWNTs. The secondary goal is to draw attention to the growing crisis in the SWNT industry as a result of the lack of quality control and the misleading advertising by suppliers of this material.