Towards chirality-pure carbon nanotubes

Abstract

Current as-grown single-walled carbon nanotubes vary in diameter and chirality, which results in variations in their electronic and optical properties. Two approaches have been intensively studied to obtain chirality-pure nanotube structures and thus uniform properties for advanced applications. The first approach involves the post-synthesis separation according to the nanotubes' chiral vectors (n, m), and the second one involves direct synthes of carbon nanotubes with the same (n, m). This paper reviews the efforts along these two directions, with emphasis on the most recent progress of post-synthesis separation and the perspectives of controllable synthesis.

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Yani Zhang

Dr Yani Zhang was born in Shaanxi, China. She received her BSc degree in 2003 and her PhD degree in 2009 in Materials Science & Engineering, Northwestern Polytechnical University, China. She researched carbon fiber reinforced ceramic composites for ∼5 years in the National Key Lab of Thermostructure Composite Materials during her PhD. Currently, she is a research fellow in Nanyang Technological University (NTU), Singapore. Her research interests include carbon nanotube (CNT) materials, advanced nanotube fibers, composites, and ceramics, for functional and structural applications.

Lianxi Zheng

Prof. Lianxi Zheng received his BE degree in Electronic Engineering from Southeast University in China, and his Ph.D. on Physics from The University of Hong Kong. He has worked in Los Alamos National Laboratory as a director's postdoctoral fellow, and in CNT Technologies Inc. (USA) as a research scientist. In January 2008, he joined Nanyang Technological University, Singapore. His research interests include novel enabling technologies on nano-material synthesis, smart nanotube based sensors, nanoelectronics, and nanotube fibers & fiber composites.

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1. Introduction

A single-walled carbon nanotube (SWCNT) is a seamless cylinder structure that could be considered as a rolled-up graphene sheet. Owing to this unique structure, SWCNTs have excellent properties including nanoscale dimension, ballistic electron transport, high current capacity, and superior mechanical and chemical properties, making them the potential building blocks for next generation electronics.^1–4 SWCNT transistors, including single-electron transistors employing metallic nanotubes⁵ and field-effect transistors employing semiconducting nanotubes,⁶ have already been demonstrated. However, the electronic and optical properties of an SWCNT strongly depend on its diameter and chiral vector (the direction in which the graphene is rolled up). Approximately 67% of SWCNTs are semiconducting and 33% of them are metallic among all possible (n, m)s,^7–9 and the bandgap changes inversely with the diameter for semiconducting SWCNTs. As a result, an as-grown nanotube product from any synthesis methods, including arc discharge,¹⁰ laser ablation,¹¹ and chemical vapor deposition (CVD),¹² is a mixture of nanostructures with a broad distribution in diameter and chirality. Obtaining identical SWCNT structures, i.e. the same diameter and the same chirality, is then the major challenge for large-scale applications of SWCNTs, and has gained considerable research attention.^13,14

There exist two strategies for obtaining chirality-pure SWCNT structures: controllable synthesis and post-synthesis separation. In controllable synthesis, the diameter and chirality of a SWCNT are controlled through optimizing catalyst preparation and growth parameters,^15–17 based on an assumption that some particular SWCNTs are thermodynamically favorable under those conditions. And, more recently, seeded growth¹⁸ was proposed and demonstrated a promising future. In post-synthesis separation strategy, SWCNTs are chemically functionalized or physically encapsulated by surfactants, and then sorted out according to their structures.^19–22 By tuning the surfactant–SWCNT interaction, interested SWCNT structures could be separated from a starting mixture. This method is the most promising approach in the current stage, evidenced by the significant success in DNA–SWCNT separation.²³ Among many post-synthesis approaches, selective destruction is a strategy used to destroy un-wanted SWCNTs and keep the remainder for utilization. Particular methods include selective oxidation,^24,25 gas-phase etching,^26,27 and electrical breakdown.^28,29 This strategy is mainly used to separate nanotubes by conducting type, i.e. semiconducting from metallic, but shows limited selection on diameter and chirality and will not be discussed in this paper.

Therefore, this review paper will mainly focus on controllable synthesis and post-synthesis separations that have demonstrated a potential capability for chirality selection and diameter control, with emphasis on the most recent progress of post-synthesis separation and a relatively detailed review on the controllable synthesis.

2. Post-synthesis separation

For post-synthesis separation, SWCNTs need to be firstly dispersed in liquids, otherwise they tend to bond to each other through van der Waals forces to form bundles or large aggregates. By ultrasonically agitating an aqueous dispersion of raw SWCNTs in sodium dodecyl sulfate and then centrifuging, O'Connell³⁰ has successfully separated individual SWCNTs from bundles by taking advantage of the different sedimentation rates between them. Inspired by this work, many researchers have explored selective chemistry with reactivity that varies as a function of an SWCNT's electronic type, diameter, and chiral vector, in order to amplify the differences between various SWCNTs and then sort them accordingly. Furthermore, extensive study has been carried out on chemical functionalization of nanotubes, and the details can be found in several good review papers.^31–33 In general, chemical functionalization can be classified into two categories: covalent functionalization and non-covalent encapsulation. Covalent functionalization often induces substantial changes to the structure and properties of SWCNTs, while non-covalent functionalization has minimized perturbation of the SWCNTs. Both of them have been used for dispersing SWCNTs.

Following selective functionalization, SWCNTs could then be sorted by using a variety of techniques including electrophoresis,^34–38 ultracentrifugation,^20,39–43 and chromatography.^{19,23,44–47} Electrophoresis sorts SWCNTs according to their relative mobility in response to an electrical field. This method is favorable for length sorting of SWCNTs, because short SWCNTs have small molecular weight and then move fast under the electrical field. If an AC electrical field is applied, the method can be extended to sort SWCNTs according to their conducting type, owing to the difference in the dielectric constants of metallic and semiconducting SWCNTs. Ultracentrifugation, especially density gradient ultracentrifugation, has been used to sort SWCNTs according to their buoyant densities. By introducing a density gradient, the SWCNTs will be separated into different layers in the centrifuge tube according to their buoyant densities, which are in turn determined by their diameters. Chromatography is a technique to separate interested analyte from other molecular mixture based on the differential partitioning between the “mobile phase” and the “stationary phase”. A slight difference in partition coefficient will result in differential retention of analyte on the stationary phase. This method was first introduced for the separation of DNA-encapsulated SWCNTs by Zheng,¹⁹ and showed great promise in a recent study.²³

Numerous separation methods have been proposed by combining various selective surfactants with different separation techniques. For any combination, the principle of separation technique is relatively clear, but the mechanism of selective chemistry is very complicated. Therefore, in this paper we focus our discussion on the discrimination mechanism, and accordingly we can group related research work into two categories, chemical-affinity approach and surfactant-pattern approach, based on their research emphases on structure discrimination.

2.1 Affinity approach

In this approach, the structure discrimination originates from the structure-dependent chemical affinity between the surfactant molecules and SWCNT surfaces. Most of the sorting methods belong to this category, and the diameter-dependent binding has been observed for many common surfactants. One successful example is of using structure-discriminating surfactants to engineer subtle difference in SWCNTs' buoyant densities. By using a bile salt, sodium cholate (SC), to encapsulate SWCNTs, followed by density gradient ultracentrifugation, multiple regions of separated SWCNTs have been observed in the centrifuge tube.²⁰ As shown in Fig. 1a, different colors represent different layers of SWCNTs with different structures, with the most buoyant region corresponding to the semiconducting nanotubes (with different bandgaps) and lower sediment in the gradient corresponding to the bundles or aggregates. The successful separation of semiconducting SWCNTs by diameter and bandgap is supported by the optical absorption for different transitions in the 900–1340 nm range (first-order semiconducting transitions), as shown in Fig. 1b. The spectra in Fig. 1b also illustrate that SWCNTs of increasingly larger diameters are enhanced at increasingly larger densities. This observation contradicts the mass-volume ratio of intrinsic SWCNTs, indicating the diameter-dependent affinity of surfactants to SWCNTs, with more surfactants attaching to the larger diameter SWCNTs.

Fig. 1 Sorting SWCNTs by diameter. (a) SC encapsulated CoMoCAT-grown SWCNTs after centrifugating. Colored bands indicate the isolated SWCNTs sorted by diameter and bandgap. Bundles, aggregates and insoluble materials deposit at lower part of centrifuge tube. (b) Adsorption spectra indicate that SWCNTs of increasing diameter are more concentrated at larger densities. (Adapted with permission from ref. 20, © Nature.)

More interestingly, the density–structure relationship could be tuned by introducing a co-surfactant or changing the pH value of solution. As shown in Fig. 2, by increasing the pH to 8.5, the (7, 5) SWCNTs shift to lower buoyant density. Alternatively, by adding sodium dodecyl sulfate (SDS) to compete with the SC for non-covalent binding, (7, 5) and (9, 5)/(8, 7) SWCNTs shift to larger buoyant densities. Combining this method with multiple cycles of ultracentrifugation, it is technically reasonable to separate high purity SWCNTs according to their chiralities. Using a similar approach but different surfactant and solvent, (7, 5), (7, 6), (10, 5) and (9, 7) SWCNTs with respective enrichments of up to 90% have been separated from raw materials.²²

Fig. 2 Tuning the structure–density relationship. (6, 5), (7, 5) and (9, 5)/(8, 7) SWCNTs from CoMoCAT growth are represented by red, green and blue color, respectively. (a) SC with pH 7.4. (b) SC with Ph 8.5. (c) SDS/SC with pH 7.4. (Adapted with permission from ref. 20, © Nature.)

2.2 Surfactant-pattern approach

Besides chemical affinity, the structure patterns of surfactant molecules or surfactant assemblies were also used for chirality selection. It was found that geometrically constrained polyaromatic amphiphiles were selective to the chirality angle of SWCNTs, with a pentacenic-based amphiphile recognizing armchair SWCNTs and a quaterylene-based amphiphile discriminating zig-zag SWCNTs.⁴⁸ The organization of several other aromatic surfactants are also capable of the enrichment of a certain few (n, m) SWCNTs. In these studies, surfactants could form a chiral pattern around SWCNTs through hydrogen bonding between adjacent moieties. The interaction between the surfactants and SWCNTs was found to be strongly dependant on the chirality of SWCNTs, resulting in the discrimination of particular chiral SWCNTs.^21,23

The most promising approach in this category should be DNA–SWCNT chromatography. DNA sequences can form ordered structures on SWCNTs, allowing diameter and conducting type-sorting of SWCNTs through ion exchange chromatography (IEX).¹⁹ Experimental^19,23,49,50 and theoretical^51,52 studies indicate that DNA–SWCNT interaction and the resulting hybrid structure are dependent on both DNA sequence and SWCNT structure. When the sequence length is comparable to the van der Waals circumference of a typical SWCNT (1 nm in diameter), the IEX separation becomes extremely sensitive to the length of the DNA strand.²³ Authors have identified more than 20 DNA sequences, each of which allows purification of a particular (n, m) SWCNTs. Using these DNA sequences, 12 semiconducting SWCNTs have been successfully sorted from the starting mixture materials, evidenced by their absorption spectra shown in Fig. 3. In this study, the authors suggested that particular DNA strands could form a stable and well-ordered two-dimensional (2D) sheet through hydrogen bonding (Fig. 4a), and the 2D sheet could be further rolled up onto a particular SWCNT to form a stable barrel (Fig. 4b). This chiral and ordered DNA–SWCNT structure would minimize its van der Waals and hydrophobic interaction with IEX resin, allowing it to be separated and purified. Since each of the recognition sequences can form an ordered DNA barrel structure only on one particular (n, m) SWCNT, and a large number of sequences are available in DNA library, this method is expected to be used to separate any (n, m) SWCNTs in the future.

Fig. 3 Optical absorption spectra and atomic structures of 12 purified semiconducting SWCNTs and the starting HiPco mixture. (Adapted with permission from ref. 23, © Nature.)

Fig. 4 DNA–SWCNT structure. (a) 2D DNA sheet structure formed through hydrogen bonds. b) DNA barrel on a SWCNT formed by rolling up a 2D DNA sheet. (Adapted with permission from ref. 23, © Nature.)

3. Controllable synthesis

In parallel to the post-synthesis separation, tremendous efforts have been paid to controllable synthesis, aiming at obtaining chirality-pure SWCNT structures from one single process step. Although it is still under studying, we are gradually gaining controllability over the location, length, direction, diameter, defect and chirality of the SWCNT growth.^53–59 Here we review two important areas: diameter and chirality control through condition optimization and seeded nanotube growth.

3.1. Diameter and chirality control through condition optimization

Extensive efforts towards the controllable growth of nanotubes with specific diameter, chirality, and conducting type have been made, by modifying the catalyst type/materials,^60–62 support/substrate materials, synthesis temperature, gas flow rate, and carbon source feedstock/concentration.^63,58 These efforts could be summarized into three groups: (i) controlling catalyst type and particle size;⁶⁴ *(ii) controlling the growth conditions, such as temperature¹⁵ and pressure;^65–67 and (iii) using other top-down methods.

3.1.1. Controlling catalyst. Since nanotube growth is initiated by catalyst and the diameters of grown nanotubes are governed by the size of the catalyst particles, the progress of controlling the diameter and chirality of SWCNTs is then, to a large extent, constrained by the nature and size of the catalyst particles. Most of the work aiming at narrowing down the range of diameter distribution of nanotubes has been carried out through optimizing catalyst treatment or preparation. Chiang⁶⁸ reported that the diameters of SWCNTs, double-walled nanotubes (DWCNTs), and multi-walled nanotubes (MWCNTs) could be selectively controlled simply by tuning the catalyst particle size. Narrowly dispersed metal nanoparticles were prepared in atmospheric-pressure microplasma which decomposes organometallic precursors by electron impact dissociation. Suspended nanotubes synthesized from these nanoparticles were found to contain a high-purity (∼75%) of SWCNTs when the catalyst diameter was reduced to 2.2 nm. As shown in Fig. 5a, in the Raman spectrum of nanotubes grown from Ni particles with 2.2 nm mean diameter, three RBM peaks appear at 192, 214, and 254 cm⁻¹, which corresponds to a diameter range of 1.0–1.3 nm. In Fig. 5b, the Raman spectrum for nanotubes grown with 2.2 nm mean-diameter Ni particles is found to exhibit RBM intensities and a G-to-D ratio comparable to the purified HiPCO sample and significantly better than the HiPCO as-grown sample.⁶⁸ Ultrathin quasicrystal (QC) films of the catalyst were also studied for the synthesis of nanotubes by alcohol catalytic CVD. For example, Al–Cu–Fe ultrathin films were tested as catalysts, and found that they are good for the preferential synthesis of armchair-type SWCNTs, indicating that QC nanocluster catalysts played an important role in alcohol catalytic CVD.⁶⁹ Although controlling the particle size^70,71 and particle density⁷² has demonstrated success in nanotube diameter control, it encounters the problem of chirality control because various SWCNTs with similar diameter have different chiralities. Bimetallic catalysts may offer a solution for this problem. Recent studies on CoMo,^60,61 FeRu,⁶² and FeCo⁷³ have demonstrated the potential of chiral-specific growth, and narrow SWCNT diameter distribution has been achieved in the small-diameter range (<1.2 nm). Using Co-MCM-41 catalysts, Chen reported a narrow distribution of SWCNT diameter at optimized conditions (pre-reduction at 500–600 °C and growth at 750–800 °C).^74,75 By using different carbon precursors including CO, C₂H₅OH, CH₃OH, and C₂H₂ on Co–Mo catalysts, this group further synthesized the (n, m) selective SWCNTs. Carbon precursor pressure was found to be another key factor for the chirality control, and narrowly (n, m) distributed SWCNTs could only be obtained under high-pressure CO or vacuumed C₂H₅OH and CH₃OH.⁵⁸ Chiang also studied the link between catalyst composition and chirality distribution of the SWCNTs. It was demonstrated that the chirality distribution of as-grown SWCNTs could be altered by varying the composition of the Ni_xFe_1−x nanocatalysts.⁶⁸ In those studies, the role of the second metal is not very clear because the catalyst size and composition have not been independently controlled.^62,76 Nevertheless, rational design and precise fabrication of dimensionally and compositionally tunable nanocatalysts opens a potential route towards chiral-selective growth of SWCNTs.¹⁷

Fig. 5 (a) Micro Raman spectra of nanotubes synthesized in a flow furnace with different Ni particle mean diameters. (b) Comparison of micro Raman spectrum of nanotubes synthesized using 2.2 nm mean diameter Ni particles with as-grown and purified commercial HiPCO product samples. (Adapted with permission from ref. 68, © American Chemical Society.)

3.1.2. Controlling growth conditions. The diameter control could also be realized by controlling growth conditions such as temperature, growth time, carbon source concentration, and the gas flow rate. For example, low temperature (500–700 °C) synthesis of high-quality SWCNTs via microwave plasma assisted CVD was demonstrated by pyrolysis of methane over the Fe–Mo bimetallic catalyst nanoparticles supported on porous MgO powders. It was found that the lower the growth temperature, the smaller and more homogenous the tube diameter. As shown in Fig. 6, the nanotube diameters are 1.60 ± 0.55 nm at 800 °C, 1.00 ± 0.23 nm at 700 °C, 0.91 ± 0.15 nm at 600 °C, and 0.88 ± 0.12 nm at 500 °C, showing the feasibility of controlling the SWCNTs' diameters through tuning the growth temperature.⁷⁷ Nanotubes with controlled diameter distribution were also selectively grown by thermal decomposition of a botanical hydrocarbon, camphor, on a high-silica zeolite support impregnated with Fe–Co catalyst.⁷⁸ Liu studied the effect of carbon-feeding rate on the diameter distribution of SWCNTs, and found that SWCNT diameters were closely related to the carbon-feeding rate by selective activation of nanoparticles.⁵⁶ Recently, the catalytic decomposition of ethanol was also used to obtain high quality SWCNTs that were synthesized over Fe–Co/MgO catalyst by CVD, in which the quality and diameter distribution of SWCNTs were controlled by adjusting the ethanol concentration. After the efficient purification combining hydrochloric acid treatment, reflux of nitric acid and air oxidation, 98% purity SWCNTs were obtained.⁷⁹ More recently, a novel approach using ethylene as the carbon source for diameter control of SWCNTs was also reported.⁸⁰ It was found that increasing the flow rate of the ethylene would decrease the nanotube diameter, demonstrating the control of diameter distribution through changing the gas flow rate of ethylene.

Fig. 6 Plot of the average diameters with standard deviation for SWCNT samples grown under different temperatures (adapted with permission from ref. 77, © Elsevier).

3.1.3. Controlling with top-down methods. Except for the above efforts, several top-down methods, such as patterning growth, template synthesis, and patterning & rolling graphene sheets, have also been proposed to control the diameter and chirality. Kong⁸¹ previously described the strategy for making high-quality individual SWCNTs on silicon wafers patterned with micrometre-scale islands of catalytic material. The synthesized individual SWCNTs have diameters of 1–3 nm and lengths of up to tens of micrometres by CVD of methane on patterned substrates. Later, this selective area growth was used to further control the nanotubes' diameter. As shown in Fig. 7, by the selectively growing nanotube on the exposed edge of a thin film structure, Chopra⁸² demonstrated that the diameters of nanotubes have been controlled by the line width of reactive SiO₂ surface in a ferrocene/xylene CVD process. The resulting nanotube structures can be used for nanowiring or lithographic processes. More recently, an interesting method was presented to successfully synthesize the SWCNTs with a diameter range of 0.8–1.5 nm by confining Rh/Pd nanoparticles within a regular hexagonal arrayed one-dimensional channel of mesoporous silica, FSM-16,⁸³ as shown in Fig. 8. It was found that the obtained SWCNTs have a much narrower diameter distribution compared with the catalyst nanoparticles, indicating that the diameter of synthesized SWCNTs was confined by the channel size. A new concept of rolling patterned graphene nanoribbons/sheets was proposed to realize chirality control of SWCNTs. In this method, a top-down process of patterning, which is deterministic in nature with the tube diameter and chirality predefined by patterning, was proposed. Molecular dynamic simulation showed theoretical feasibility of adsorbate-assisted rolling of patterned graphene nanoribbons on graphite.⁸⁴ A more recent report also showed the experimental possibility of the electrostatic field on facilitating the rolling of graphene sheets and allowing the rolling any size of graphene,⁸⁵ however, the synthetic feasibility of this new approach to control chirality is still underway and needs further demonstration.

Fig. 7 Synthetic steps for the growth of carbon nanotubes from exposed face of Si/SiO₂/Si multilayer. (a) Cross-section of patterned Si/SiO₂/Si multilayer. (b) Post structure after photo resist liftoff and HF etching of surface oxide. (c) nanotube growth on exposed face of SiO₂ thin film. (Adapted with permission from ref. 82, © American Chemical Society.)

Fig. 8 Schematic representation of the SWCNT growth from 1D channels (adapted with permission from ref. 83, © Elsevier).

3.2. Seeded growth

Because the diameter variations that span many chiral indices are inevitable owing to the thermal vibrations in catalyst particles at elevated growth temperatures, growing specific (n, m) SWCNTs through condition optimization was speculated to be impossible.⁸⁶ Seeded growth, or nanotube cloning, was then proposed.¹⁸ In nanotube cloning, as shown in Fig. 9, a SWCNT segment with desired (n, m) would be prepared with open ends, upon which catalyst is docked. This segment, serving as a seed, is then placed into a CVD chamber for regrowth. It is expected that a new SWCNT will grow epitaxially from the seeded SWCNT under proper growth conditions, and thus preserve the structure (diameter and chirality) of the SWCNT seed.

Fig. 9 Sketch of nanotube cloning.

This idea was first proposed and demonstrated by Smalley's group.¹⁸ They started from an array of open-ended SWCNTs, and initialized a second CVD growth after “docking” catalyst atoms on nanotube's open tips. The elongation of a nanotube array was observed, and the regrown SWCNTs were found to have a diameter distribution similar to that of the original SWCNTs. Following this concept, they further demonstrated the nanotube regrowth from individual SWCNTs^87,88 to check whether or not such a regrowth process could be achieved at individual SWCNT level. As shown in Fig. 10, the amplification of a SWCNT can be clearly seen at the same location, indicated by the different nanotube lengths before and after regrowth. The height profiles of Atomic Force Microscopy (AFM) indicate that the diameter of amplified SWCNT is almost identical to the diameter of original seed SWCNT, demonstrating the success of regrowth. Future work is needed to clarify whether or not the amplified individual SWCNTs have preserved their original chiralities.⁸⁹

Fig. 10 AFM images of a seed SWCNT and its amplified SWCNT. (a) The starting 200 nm SWCNT indicated by the white arrow (the red arrow is the locator inscription). (b) A height profile of the seed-SWCNT showing a diameter of 0.73 nm. (c) Amplified SWCNT after regrowth. (d) Height profile of regrown SWCNT showing a nearly identical height of 0.72 nm over several points measured along its entire length. (e) The entire length of the amplified nanotube is 6.7 μm (between the yellow arrows). (Adapted with permission from ref. 87, © American Chemical Society.)

Catalyst-free regrowth was also used for SWCNT cloning.⁹⁰ By using open-end SWCNTs as seeds, duplicated SWCNTs have been directly grown from parent segments via an open-end growth mechanism without involving any metal catalyst, as shown in Fig. 11. Among 600 SWCNT seeds, about 56 (9%) of them show observable regrowth. By changing the substrate from SiO₂/Si to quartz, the regrowth yield has been improved from 9% to 40%. AFM height profiles show that newly grown SWCNTs have heights nearly identical to the original SWCNT seeds. More importantly, Raman spectra show the same RBM shift before and after regrowth, as shown in Fig. 11c, providing the first evidence of successful cloning at the individual SWCNT level.

Fig. 11 (a) SEM image of open-ended SWCNT seeds. (b) SEM image of the duplicate SWCNT grown from a SWCNT seed. The white arrow denotes the start of the regrowth. (c) Typical Raman spectroscopy across the SWCNT of the bottom in panel b, which shows the same RBM shift at 252.7 cm⁻¹ (the G-band was not shown here, and the other peak at 303 cm⁻¹ marked by * is from the SiO₂/Si substrate) and there are no other RBM peaks at low frequency. (Adapted with permission from ref. 90, © American Chemical Society.)

Although nanotube cloning has been demonstrated conceptually, the ultimate protocol would involve taking a single (n, m) SWCNT, cutting it into many short segments, using each of them as the seed for cloning, and repeating the process to obtain considerable amount chirality-pure SWCNT structure for applications.⁸⁷ Moving towards this goal, several particular issues need to be taken into consideration: (i) the synthesis process itself should be extremely stable so that no defects form during the growth; (ii) the growth mode should be tip growth for catalyst-docked cloning, otherwise the catalyst particles will be inserted between the seed SWCNTs and the regrown SWCNTs and; (iii) the regrowth should be an elongation process, i.e. non-nucleation growth. Recent progress on ultralong SWCNT synthesis may offer solutions for the three above-mentioned issues.

It is well known that the electronic properties of a SWCNT depend not only on its diameter and chirality, but also strongly on the defects.^57,91–95 It has been reported that a metal–semiconductor junction behaves like a rectifying diode with nonlinear transport characteristics that are strongly asymmetric with respect to bias polarity; the conductance in a metal–metal junction appears to be strongly suppressed and it displays a power-law dependence on temperature and applied voltage.⁵⁷ Unfortunately most SWCNTs possess a considerable amount of defects along their length.⁹⁶ Cloning alone does not guarantee identical electrical properties in final products if defects present. Therefore, developing a method of growing defect-free SWCNTs with a considerable length is an inevitable requirement for any cloning approach. Using ethanol chemical vapor deposition, our group has successfully synthesized individual SWCNTs that are up to 40 mm long.⁵³ The growth rate is as high as 11 μm per second. We believe that oxygen atoms present in ethanol contribute to such a growth by keeping the catalyst active for longer time by removing harmful amorphous carbon.^97,98 Raman spectra confirmed that almost all individual long SWCNTs maintain their diameter and chirality along their entire length, i.e. these SWCNTs are nearly defect-free. This synthesis method is especially suitable for nanotube cloning, with the high cloning efficiency offered by the ultralong length and fast growth rate of the initial seeds, and the possible identical structures guaranteed by defect-free growth.

The long SWCNTs appear to grow by a tip-growth mechanism,⁵³ that is, the catalytic Fe particles moved with the growing SWCNT tips, evidenced by the SWCNTs' morphologies shown in Fig. 12. A base growth with a stationary catalyst particle would require the whole SWCNT to overcome the van der Waals forces with the substrate and slide on the substrate, which is clearly impossible for obtaining ultralong and straight SWCNT on an Si surface. As shown in Fig. 12, short SWCNTs or dying SWCNTs show wavy and tangled morphologies, but long SWCNTs are relatively straight along most of their length, strongly suggesting a tip growth mode in this synthesis method for ultralong SWCNTs.

Fig. 12 Nanotube morphologies indicate the tip growth mode (scale bar is 10 μm). (Adapted with permission from ref. 53, © Nature.)

The key issue in nanotube cloning is how to keep the regrowth as an elongation process rather than nucleation of new nanotube structures. To address this issue, we have studied the nucleation kinetics of ultralong SWCNTs.⁹⁹ By counting the number density of SWCNTs and studying its temperature dependence, the nucleation process of SWCNTs could be studied separately from other growth kinetics. From the Arrhenius form relation, we found that the nucleation energy of such long SWCNTs is about 2.8 eV, which is obviously larger than the diffusion energy (∼1.5 eV) of cabon atoms in metal particles.^100,101 This big difference suggests that elongating an existing nanotube (through carbon diffusion) is energetically much more favorable than nucleating a new SWCNT structure. Ethanol CVD could then be a very promising technique for the seeded growth of SWCNTs to obtain controlled structures.

4. Conclusion

Comparing these two strategies of obtaining chirality-pure nanotube structures, post-synthesis separation seems to be a promising approach at the current stage. Both centrifugation and DNA chromatography have demonstrated the capability of chiral selection of CNTs. However, most of the present reports are focused on the separation of semiconducting nanotubes, with few reports on separation of metallic nanotubes. In addition, SWCNTs with a desirable chiral vector (n, m) only constitute a tiny fraction of any as-synthesized CNT aggregates, which limits nanotubes' large-scale applications even if the post-synthesis separation process is fully successful. At the same time, seeded growth (or nanotube cloning) of SWCNTs is still in the preliminary stage. Both seeded growth methods are faced currently with great challenges. For catalytic seeded growth, the biggest challenge is how to guarantee the regrowth occurs exclusively on CNT tips rather than from substrate or CNT sidewalls where it is also possible to attach the catalyst. The interaction of the catalyst with CNT seed and substrate during heating is also an issue. These issues are expected to be solved by a novel catalyst docking method. In open-end seeded growth, elongation is not a deterministic process, because the open-ended state is not energetically stable. The lower growth rate also gives a higher chance of defect formation. However, encouraged by the recent progress on the successful regrowth and defect-free synthesis of ultralong SWCNTs, we can be confident that, sooner or later, it will be possible for these methods to synthesize large quantities of SWCNTs with any pre-selected specific diameter and chirality. Nevertheless, the final solution should be the combination of synthesis control and post-synthesis separation, with controllable synthesis offering some degree of chirality-pure CNTs and post-synthesis separation providing further purification.

Acknowledgements

The authors highly acknowledge the financial support of the Singapore MOE tier 1 RG 26/08 research fund and NTU SUG fund.

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