Growth Mechanism Of Single-walled Carbon Nanotubes Research Articles

Atomic-Scale Evidence of Catalyst Evolution for the Structure-Controlled Growth of Single-Walled Carbon Nanotubes.

ConspectusKnowing how nanomaterials nucleate and dynamically evolve at the nanoscale is crucial to understanding and in turn controlling the structure and properties of a wide variety of materials, among which single-walled carbon nanotubes (SWCNTs) with chirality-dependent properties is a typical example. Catalyst takes a central role in guiding the SWCNT growth. An in-depth understanding of the growth mechanism of SWCNTs requires knowledge of the catalyst dynamic behavior during the chemical vapor deposition process, where real-time atomic-scale observations are needed. The high spatial, temporal, and energy resolution makes the state-of-the-art aberration-corrected environmental transmission electron microscope (ETEM) a superior tool for tracking the catalyst evolution and the SWCNT growth.Several key factors and processes, including the catalyst stability, carbon diffusion pathway, nucleation site, and growth modes of nanotubes, greatly influence the structure of SWCNTs. This Account summarizes our recent progress in the ETEM investigation of the dynamic catalyst behavior and nucleation of SWCNTs. We first compare the different growth modes of SWCNTs on two types of catalyst-stable solid intermetallic Co7W6 and unstable monometallic catalysts. Then we address the origin of different growth modes and chirality selectivity by revealing the atomic-scale stability and evolution of catalysts under carbon feed conditions and the observation of the in situ growth of SWCNTs on catalysts. We also discuss the catalyst-support interaction and the possible influence on SWCNT growth. In the end, we summarize the present achievements and future challenges.We carefully compare the difference in the ordinary Co catalyst and Co7W6 catalyst which has shown great chirality selectivity in SWCNT growth. Direct imaging by ETEM demonstrated that solid catalysts initiated the growth of SWCNTs with diameters smaller (dNT) than those of the catalyst particles (dNP) (dNT < dNP), whereas molten catalyst nanoparticles produced SWCNTs with similar diameters (dNT ≈ dNP). ETEM combined with in situ synchrotron X-ray absorption spectroscopy demonstrated that the Co7W6 catalyst maintained a solid stable structure under carbon feed conditions at 700-1000 °C, demonstrating the feasibility in acting as a structure template to grow SWCNTs. By contrast, the state and composition of the Co catalyst were changing during SWCNT growth. The near-surface lattice spacings of Co7W6 remained unchanged under carbon feed condition with carbon diffusion on the surface, whereas the solid Co catalyst underwent dynamic expansion and contraction due to carbon penetration into and precipitation out of Co nanoparticles. These two different pathways of carbon diffusion on or in catalysts indicate the distinctly different growth mechanisms of SWCNTs: the epitaxial growth of SWCNTs with specified chirality on the facets of Co7W6 nanocrystals and the nonselective growth of SWCNTs by the Co catalyst with Co/CoC3 as the active species. Besides the SWCNT-catalyst interface, the catalyst-support interface is also of importance in SWCNT growth. The atomic-scale information on catalyst dynamics provides a deep mechanistic understanding of SWCNT growth and will boost the development of the structure-controlled synthesis of SWCNTs and other nanomaterials.

(Invited) Structure Characterization of Intermetallic Compound Catalysts and Single-Walled Carbon Nanotubes

Single-walled carbon nanotubes (SWNTs) have shown great potentials in various fields attributing to their unique structure-dependent properties, therefore, the structure-controlled preparation of SWNTs is a crucial issue for their advanced applications (e.g. carbon-based nanoelectronics) and has been a great challenge for about two decades. We developed a strategy to produce SWNTs with specific chirality by using tungsten-based intermetallic compound nanocrystals as catalysts. The characterization of catalyst structure is very important to reveal the growth mechanism of SWNTs. We used transmission electron microscopy (TEM) including high-resolution TEM, scanning TEM, as well as image simulation to probe the W6Co7 catalyst structure before and after the SWNT growth. In-situ TEM performed at 1100 oC was used to prove that the W6Co7 catalysts are crystalized and keep stable structure. High-resolution TEM measurements assisted with DFT simulations on the interface of SWNT/catalyst show that the chirality-specific growth is due to a good structural match between the nanotube and the metal atom arrangement of planes in the catalyst. The chirality characterization is also very important. We found that the electronic Raman scattering (ERS) can offer an accurate chirality assignment for metallic tubes. We also developed a bilayer plot for determining the chiralities of SWNTs under complex environment.

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.

Selective synthesis of large diameter, highly conductive and high density single-walled carbon nanotubes by a thiophene-assisted chemical vapor deposition method on transparent substrates

Selective synthesis of single-walled carbon nanotubes (SWNTs) with controlled properties is an important research topic for SWNT studies. Here we report a thiophene-assisted chemical vapor deposition (CVD) method to directly grow highly conductive SWNT thin films on substrates, including transparent ones. By adding low concentration thiophene into the carbon feedstock (ethanol), the as-prepared carbon nanotubes demonstrate an obvious up-shift in the diameter distribution while the single-walled structure is still retained. In the proposed mechanism, the change in the diameter is sourced from the increase in the carbon yield induced by the sulfur-containing compound. Such SWNTs are found to possess high conductivity with 95% SWNTs demonstrating on/off ratios lower than 100 in transistors. More importantly, it is further demonstrated that this method can be used to directly synthesize dense SWNT networks on transparent substrates which can be utilized as transparent conductive films (TCFs) with very high transparency. Such TCFs can be applied to fabricate a light modulating window as a proof-of-concept. The present work provides important insights into the growth mechanism of SWNTs and great potential for the preparation of TCFs with high scalability, easy operation and low cost.

Synthesis of uniform single-wall carbon nanotubes using Mn12 clusters as the catalyst precursor

Structure-selective synthesis of single-walled carbon nanotubes (SWCNTs) is of critical importance while an elaborative choice of catalysts is one of the feasible strategies toward such a purpose. Consequently, an investigation of how to synthesize uniform single-walled carbon nanotubes (SWCNTs) using Mn12 clusters as the catalyst precursor by means of mist flow chemical vapor deposition (CVD) has been implemented. As a result, according to Raman mapping and calculated diameter distribution, the diameter of as-grown SWCNTs is homogeneously around 1.51±0.31nm even though Mn is rarely used as the catalyst for the synthesis of SWCNTs individually. The promising results may enlighten us with new experimental information for understanding the growth mechanism of SWCNTs, which is beneficial for their controllable synthesis in the future.

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.

Growth of Single-Walled Carbon Nanotubes from Well-Defined POSS Nanoclusters Structure

High-quality single-walled carbon nanotubes (SWNTs) with narrow diameter distribution can be generated from well-defined Si 8 O 12 nanoclusters structure which form from thermal decomposition of chemically modified polyhedral oligomeric silsesquioxane (POSS). The nanosized Si x O y particles were proved to be responsible for the SWNT growth and believed to be the reason for the narrow diameter distribution of the as-grown SWNTs. This could be extended to other POSS. The SWNTs grown from the nanosized Si x O y particles were found to be semiconducting enriched SWNTs (s-SWNTs). A facile patterning technology, direct photolithography, was developed for generating SWNT pattern, which is compatible to industrial-level fabrication of SWNTs pattern for device applications. The metal-free growth together with preferential growth of s-SWNTs and patterning in large scale from the structure-defined silicon oxide nanoclusters not only represent a big step toward the control growth of SWNTs and fabrication of devices for applications particularly in nanoelectronics and biomedicine but also provide a system for further studying and understanding the growth mechanism of SWNTs from nanosized materials and the relationship between the structure of SWNT and nonmetal catalysts.

Chemical vapor deposition growth of single-walled carbon nanotubes with controlled structures for nanodevice applications.

Single-walled carbon nanotubes (SWNTs), a promising substitute to engineer prospective nanoelectronics, have attracted much attention because of their superb structures and physical properties. The unique properties of SWNTs rely sensitively on their specific chiral structures, including the diameters, chiral angles, and handedness. Furthermore, high-performance and integrated circuits essentially require SWNT samples with well-aligned arrays, of single conductive type and of pure chirality. Although much effort has been devoted to chemical vapor deposition (CVD) growth of SWNTs, their structure control, growth mechanism, and structural characterizations are still the primary obstacles for the fabrication and application of SWNT-based nanodevices. In this Account, we focus on our established CVD growth methodology to fulfill the requirements of nanodevice applications. A rational strategy was successfully exploited to construct complex architectures, selectively enrich semiconducting (s) or metallic (m) SWNTs, and control chirality. First, well-aligned and highly dense SWNT arrays are beneficial for nanodevice integration. For the directed growth mode, anisotropic interactions between the SWNTs and the crystallographic structure of substrate are crucial for their growth orientation. Just as crystals possess various symmetries, SWNTs with controlled geometries have the corresponding turning angles. Their complex architectures come from the synergetic effect of lattice and gas flow directed modes. Especially, the aligned orientations of SWNTs on graphite are chirality-selective, and their chiral angles, handedness, and (n,m) index have been conveniently and accurately determined. Second, UV irradiation and sodium dodecyl sulfate (SDS) washing-off methods have been explored to selectively remove m-SWNTs, leaving only s-SWNT arrays on the surface. Moreover, the UV-assisted technique takes the advantages of low cost and high efficiency and it directly produces a high ratio of s-SWNT arrays. We also designed a smart scotch tape to sort out the s-SWNTs and m-SWNTs from the as-grown mixture with 3-aminopropyl-triethoxysilane and triethoxyphenylsilane as glues, respectively. This is analogous to the mechanical exfoliation of a graphene sheet. Third, the obtained SWNT intramolecular junctions obtained by temperature-mediated CVD indicate that temperature can seriously affect the SWNT's chirality during its growth. Importantly, the cloning method can validate the chirality-controlled growth of SWNTs, and the cloning efficiency is significantly improved on a quartz surface. Well-aligned SWNT arrays with a high density and controlled structures are highly desirable for carbon nanoelectronics. We hope that the advanced methodology used here will promote their controlled preparation and provide insights into the growth mechanism of SWNTs.

The growth mechanism of single-walled carbon nanotubes with a controlled diameter

Understanding the growth mechanism of carbon nanotube is important to control the structure and properties of carbon nanotubes. We examined several aspects of the single wall carbon nanotubes (SWCNTs) prepared by chemical vapor deposition (CVD) processes using ethanol with the additions of ferrocene and thiophene, focusing on the potential mechanisms leading to the differences in the diameters, structures, and conductivities of the SWCNTs formed. The diameter of SWCNTs could be controlled from ∼1.0 to ∼5.8nm by adjusting the concentration of thiophene used. The larger diameter SWCNTs has a higher thermal stability than smaller-diameter SWCNTs. With increasing diameter, the electrical properties of the SWCNT change from mainly semi-conducting to metallic-like. The growth mechanism of SWCNTs was subsequently studied to understand the mechanism of thiophene controlled SWCNTs formation. This study could lead to improved control over the diameter and electronic properties of SWCNTs.

Single-walled carbon nanotube synthesis on SiO2/Si substrates at very low pressures by the alcohol gas source method using a Pt catalyst

A platinum catalyst was used for single-walled carbon nanotube (SWCNT) growth on SiO2/Si substrates using an alcohol gas source method, a type of cold-wall chemical vapor deposition. Compared to Co, a conventional transition metal catalyst, the optimal ethanol pressure was considerably reduced in the growth at 700°C, and SWCNTs could be grown even at an ambient ethanol pressure of 1×10−5Pa. Raman spectroscopy measurements showed that the G/Si ratios of SWCNTs grown at 700°C with the Pt catalyst under an ethanol pressure between 1×10−4 and 1×10−1Pa was larger than that grown with Co catalyst under optimal conditions (700°C, 1×10−1Pa), indicating that the Pt catalyst is suitable for SWCNT growth under a low ethanol pressure. In addition, the diameter distributions of SWCNTs grown with the Pt catalyst were narrower than those grown with the Co catalyst. Taking into account the results by transmission electron microscopy observation, the diameter reduction was caused by the smaller migration distance of Pt on the substrate. Based on these results, we discuss the growth mechanism of SWCNTs from the Pt catalyst.

Evaluation of metallic and semiconducting single-walled carbon nanotube characteristics

The nature of the mixed electronic type metallic (M-) and semiconducting (S-) single-walled carbon nanotubes (SWNTs) synthesized by current methods has posed a key challenge for the development of high performance SWNT-based electronic devices. The precise measurements of M- to S-SWNT ratio in as-grown or separated samples are of paramount importance for the controlled synthesis, separation and the realization of various applications. The objective of this review is to provide comprehensive overview of the progress achieved so far for measuring the M/S ratio both on individual and collective levels of SWNT states. We begin with a brief introduction of SWNT structures/properties and discussion of the problems and difficulties associated with precise measurement of the M/S ratio, and then introduce the principles for obtaining distinguished signals from M-and S-SWNTs. These techniques are classified into different groups based either on the single/ensemble detection of SWNT samples or on the principles of techniques themselves. We then present the M/S ratio evaluation results of these methods, with emphasis on scanning probe microscopy (SPM)-based detection techniques. Finally, the prospects of precise and large-scale measurement of M/S ratio in achieving controlled synthesis and understanding growth mechanism of SWNTs are discussed.

How Catalysts Affect the Growth of Single‐Walled Carbon Nanotubes on Substrates

Direct growth of single-walled carbon nanotubes (SWNTs) on flat substrates by chemical vapor deposition (CVD) is very important for the application of SWNTs in nanodevices. In the growth process, catalysts play an important role in controlling the structure of SWNTs. Over the years, we have systematically studied the size-controlled synthesis of Fe-based nanoparticles and the CVD growth of SWNTs, especially the horizontally aligned SWNTs, catalyzed by these produced nanoparticles. Some new catalysts were also developed. Among them, Cu is shown to be a superior catalyst for growing SWNT arrays on both silicon and quartz substrates and Pb is a unique catalyst from which one can obtain SWNTs without any metallic contaminant. SWNTs prepared with both Cu and Pb are very suitable for building high-performance nanodevices. These studies are also very helpful for further understanding the growth mechanism of SWNTs.

Surface growth of single-walled carbon nanotubes from ruthenium nanoparticles

In this paper, we report that ruthenium is an active and efficient catalyst for growth of single-walled carbon nanotubes (SWNTs) by a chemical vapor deposition (CVD) process for the first time. High density random and horizontally superlong well-oriented SWNTs on substrate can be fabricated via CH 4 or EtOH as carbon source under suitable conditions. Scanning and transition electron microscopy investigations, Raman spectroscopy and atomic force microscopy measurements show the tubular structure, the high crystallinity, and the properties of the grown nanotubes. The results show that the SWNTs from ruthenium have better structural uniformity with less defects and provides an alternative catalyst for SWNTs growth. The successful growth of SWNTs by Ru catalyst provides new experimental information for understanding the growth mechanism of SWNTs, which may be helpful for their controllable synthesis.

Metal-Catalyst-Free Growth of Single-Walled Carbon Nanotubes

We present a metal-catalyst-free CVD process for the high-efficiency growth of single-walled carbon nanotubes (SWNTs) on surface. By applying a 30-nm-thick SiO(2) sputtering deposited Si or Si/SiO(2) wafer as substrate and CH(4) as a carbon source, dense and uniform SWNT networks with high quality can be obtained without the presence of any metal species. Moreover, a simple patterned growth approach, using a scratched Si/SiO(2) wafer as substrate, is also presented for the growth of SWNTs with good position controllability. Our finding of the growth of SWNTs via a metal-catalyst-free process will provide valuable information for understanding the growth mechanism of SWNTs in-depth, which accordingly will facilitate the controllable synthesis and applications of carbon nanotubes.

Manganese-Catalyzed Surface Growth of Single-Walled Carbon Nanotubes with High Efficiency

We demonstrate that manganese (Mn) can catalyze the growth of single-walled carbon nanotubes (SWNTs) with high efficiency via a chemical vapor deposition process. Dense and uniform SWNT films with high quality were obtained by using a Mn catalyst, as characterized by scanning electron microscopy, Raman spectroscopy, atomic force microscopy, and transmission electron microscopy. Moreover, we found that the surface property of the substrate plays a critical role in the growth efficiency of SWNTs. By deposition of a thin oxide layer (SiO2 or Al2O3) on the top of a SiO2/Si substrate, the growth efficiency of SWNTs was dramatically improved. The successful growth of SWNTs by Mn catalyst provides new experimental information for understanding the growth mechanism of SWNTs, which may be helpful for their controllable synthesis.

In-situ Observation of CVD Growth of Carbon Nanotubes with a Scanning Electron Microscope

We have developed a scanning electron microscope (SEM) equipped with a chemical vapor deposition system to investigate the growth processes and bundling processes of single-walled carbon nanotubes (SWNTs). The SWNT growth and in-situ SEM observation were performed on planar and micro-pillar-patterned substrates. Even an isolated SWNT could be observed using the enhanced secondary electron emission due to electron-beam-induced-current on insulator, or when it was suspended between pillar structures. On the planer substrate, SWNT grows upward from the surface in the initial growth stage, but falls to the surface in the later stage. On the patterned substrate, it forms a bridge between pillars in the initial growth stage, and then bundles with other SWNTs in the later stage. Understanding the growth mechanism of SWNTs is essential for the control of their diameter and chirality of them, and useful for the assembly of new SWNT architectures.

Effects of synthesis time for synthesizing single-walled carbon nanotubes over Mo–Fe–MgO catalyst and suggested growth mechanism

The influence of synthesis time on the synthesis of single-walled carbon nanotubes (SWCNTs) over Mo–Fe–MgO catalyst by decomposing CH 4 in Ar was studied by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Very short and isolated SWCNTs with poor quality and high defect level were obtained and the carbon yield rate was low at 2 min. The SWCNTs were longer and the graphitization degree was higher as the synthesis time was increased. At 30 min, long bundles of SWCNTs with lower defect level were obtained, and most of the growth had taken place within 30 min. After this time, nearly no SWCNTs were generated without the presence of the active catalytic sites, but amorphous graphite was still deposited due to the continuous decomposition of CH 4. Carbon yield rate increased strongly with time up to 10 min. However, with longer growth time, the increase of carbon yield rate slowed, and the change of carbon yield rate was small after 30 min. The growth mechanism of SWCNTs over Mo–Fe–MgO catalyst was proposed according to the experimental results. The solid state nano-particles with high melting point on the surface of molten Fe–C droplets may be the active catalytic sites and play an important role in the formation of the SWCNT nucleus. The precipitation of carbon from the saturated Fe–C droplets leads to the formation of tubular carbon solids with an sp 2 structure.

Spectroscopic study during single-wall carbon nanotubes production by Ar, H 2, and H 2–Ar DC arc discharge

A new method to produce a macroscopic oriented web (30 cm in length) of single-wall carbon nanotubes (SWNTs) has been developed in our laboratory. In order to understand the growth mechanism of SWNTs, the optical emission spectra during SWNTs production in pure Ar, H 2 gas, and H 2–Ar mixture gas were investigated. In pure Ar gas, Fe spectra are strongly appeared, in which SWNTs could not be formed, but in pure H 2 gas, Fe spectra almost disappeared in which small amount of SWNTs were formed. In the case of H 2–Ar gas, Fe and C 2 species were commonly identified, in which SWNTs were highly produced. H 2–Ar gas provides the optimum condition for high production and high quality of SWNTs. Spectroscopic study during carbon nanotubes production by DC arc discharge provides the useful method to under the growth mechanism of nanotubes.

カーボンナノチューブの大量合成と触媒効果

Carbon nanotubes have potential applications in a variety of fields, such as composites, field-emission displays, electrodes, gas adsorbent, and catalyst supports. For putting these applications into industry, it is very important to establish the mass production of nanotubes. Here, we introduce our recent progress on the large-scale synthesis of carbon nanotubes, in particular, single-wall carbon nanotubes (SWNTs), based on the gas-phase flow reaction. By spraying the Co-Mo nanoparticles dissolved in toluene to the vertical furnace, we succeeded in selectively obtaining SWNTs and multi-wall carbon nanotubes (MWNTs). It was found that the structure and quality of nanotubes depend on the concentration of the thiophene additives, the hydrogen gas flow rate, and the injection rate of the colloidal solution. The growth mechanism of SWNTs and MWNTs are discussed in terms of catalytic effects. Other large-scale synthetic methods for SWNTs, such as high pressure CO disproportionation (HiPco) and the supported catalysts-based fluidized bed reactions are also introduced.

On the iron-catalysed growth of single-walled carbon nanotubes and encapsulated metal particles in the gas phase

The production of single-walled carbon nanotubes (SWNT) by catalytic disproportionation of carbon monoxide on iron particles in the gas phase was studied. The addition of hydrogen to the reactants was found to increase the SWNT yield relative to the competing formation of encapsulated iron particles. Adding acetylene, however, did not result in a significant change of the SWNT growth but gave rise to the formation of unwanted self-pyrolysis products. Iron particles are seen to decorate the SWNT and are shown to be single crystalline with several layers of encapsulation by graphitic carbon. All these observations are discussed with respect to the catalytic growth mechanism of SWNT and encapsulated metal particles from CO and hydrocarbon molecules.