Encyclopedia of Nanoscience and Nanotechnology 9781588830579, 1588830578

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Encyclopedia of Nanoscience and Nanotechnology Volume 1 Number 1 2004 Aligned Carbon Nanotubes

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Xianbao Wang; Yunqi Liu; Daoben Zhu Alkanethiol Self-Assembled Monolayers

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J. Justin Gooding Analytical Characterization of Single Wall Carbon Nanotubes

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Sivaram Arepalli; Pasha Nikolaev; Olga Gorelik Analytical Ultracentrifugation of Nanoparticles

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Helmut Cölfen Applications of Electrodeposited Nanostructures

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G. Palumbo; J. L. McCrea; U. Erb Applications of Focused Ion Beam in Nanotechnology

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V. J. Gadgil; F. Morrissey Atomic Manipulation by Scanning Tunneling Microscopy

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K.-F. Braun; G. Meyer; F. Moresco; S.-W. Hla; K. Morgenstern; S. Fölsch; J. Repp; K.-H. Rieder Atomic Structure of Defects in Semiconductors

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I. Yonenaga Atomic Structure of Diamond Surfaces

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J. M. Perez; R. E. Stallcup II Atomic Wires

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I. N. Yakovkin Augmented Waves for Nanomaterials

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Pavel N. D'yachkov Bimetallic Ferrofluids

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Michael Hilgendorff Biocompatible Core-Shell Nanoparticles for Biomedicine

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Xiaoxiao He; Xia Lin; Kemin Wang; Liang Chen; Ping Wu; Yin Yuan; Weihong Tan Bioconjugated Silica Nanoparticles for Bioanalysis

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Xiaojun Zhao; Lisa R. Hilliard; Kemin Wang; Weihong Tan Biodoped Sol-Gel Polymer Nanocomposites

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Iqbal Gill

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Biogenic Nanoparticles

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Joseph M. Slocik; Marc R. Knecht; David W. Wright Biological Molecules in Nanodevices

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Stephen C. Lee; Mark A. Ruegsegger; Mauro Ferrari Bionanodevices

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Thomas Schalkhammer Bionanomotors

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Dmitri Grigoriev; Dieter Moll; Jeremy Hall; Peixuan Guo Bismuth Nanostructured Materials

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Joshua T. Moore; Charles M. Lukehart Boron-Carbon Nitride Nanohybrids

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Yoke Khin Yap Boron Nitride Nanotubes

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Dmitri Golberg; Yoshio Bando C60-Based Materials

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J. G. Hou; A. D. Zhao; Tian Huang; Shan Lu Calixarenes

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Chebrolu P. Rao; Mishtu Dey Cantilever-Based Sensors

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D. Then; C. Ziegler Carbon Nanomaterials

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Maheshwar Sharon Carbon Nanostructures for Cold Electron Sources

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P. Gröning; L. Nilsson; P. Ruffieux; R. Clergereaux; O. Gröning Carbon Nanotube Growth by Chemical Vapor Deposition

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M. Meyyappan Carbon Nanotube Sensors

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Jun Li; Hou Tee Ng Carbon Nanotubes: Synthesis by Arc Discharge Technique

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Yoshinori Ando Carbon Nanotube-Based Field Emitters

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Seong Chu Lim; Hee Jin Jeong; Kay Hyeok An; Dong Jae Bae; Young Hee Lee; Young Min Shin; Young Chul Choi Carbon Nanotube-Based Supercapacitors

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Young Hee Lee; Kay Hyeok An; Ji Young Lee; Seong Chu Lim

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Carbon Nanotubes in Composite Materials

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Ch. Laurent; A. Peigney Catalysis by Gold Nanoparticles

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Masatake Haruta Catalytic Synthesis of Carbon Nanotubes and Nanofibers

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Kenneth B. K. Teo; Charanjeet Singh; Manish Chhowalla; William I. Milne Ceramic Nanoparticle Synthesis

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Xiangdong Feng; Michael Z. Hu Ceramic Nanopowders

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Yong S. Cho; Howard D. Glicksman; Vasantha R. W. Amarakoon Charge Carrier Dynamics in Nanoparticles

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Christian D. Grant; Thaddeus J. Norman Jr.; Jin Z. Zhang Chemical Derivatization of Carbon Nanotube Tips

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Vladimir A. Basiuk; Elena V. Basiuk (Golovataya-Dzhymbeeva) Chemical Synthesis of Nanoparticles

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H. Bönnemann; K. S. Nagabhushana Chemically Prepared Magnetic Nanoparticles

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M. A. Willard; L. K. Kurihara; E. E. Carpenter; S. Calvin; V. G. Harris Chemistry of Carbon Nanotubes

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V. N. Khabashesku; J. L. Margrave Chiral Macrocycles

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Suk Joong Lee; Wenbin Lin Copyright © 2004 American Scientific Publishers

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Encyclopedia of Nanoscience and Nanotechnology

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Aligned Carbon Nanotubes Xianbao Wang, Yunqi Liu, Daoben Zhu Chinese Academy of Sciences, Beijing, People’s Republic of China

CONTENTS 1. Introduction 2. Carbon Nanotubes and Their Preparation Methods 3. Alignments and Patterns of Multiwalled Carbon Nanotubes 4. Alignments of Single-walled Carbon Nanotubes 5. Properties and Applications of Aligned Carbon Nanotubes 6. Summary Glossary References

1. INTRODUCTION Significant progress has been made in the science of carbon nanotubes (CNTs) since the publication of Iijima’s milestone paper in 1991 [1]. If there is one thing which has characterized fullerene and nanotube sciences, it is serendipity [2]. The discovery of buckminsterfullerene itself was a wonderful accident, and nanotubes were an unanticipated by-product of the bulk synthesis of C60 . CNTs have captured the imagination of physicists, chemists, and materials scientists alike. Their intriguing electronic, magnetic, optical, and mechanical properties, coupled with their unusual molecular shape and size, have made CNTs very promising as functional components for building molecular-based electronics, nanomachines, and nanoscale biomedical devices as well as composites as structural materials. Physicists have been attracted to their extraordinary electronic properties, chemists to their potential as “nanotest-tubes,” and materials scientists to their amazing stiffness, strength, and resilience. Nanotechnologists, mostly from the three aforementioned areas, have discussed and explored possible nanotube-based nanodevices such as nanogears, nanobearings, single electron transistors, quantum computers, and so on. Many review articles [3–9], books [2, 10–14], and special issues of journals [15–17] have been devoted to this topic to collect the ISBN: 1-58883-057-8/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

new developments and concepts in this rapidly developing interdisciplinary field. Following the discovery of CNTs, single-walled carbon nanotubes (SWNTs) reported simultaneously by Iijima and Ichihashi [18] and Bethune et al. [19] in 1993, which are ideal models of one-dimensional materials with unique structural and electronic properties, have generated great interest for use in a broad range of potential nanodevices. Perhaps the largest volume of research into nanotubes has been devoted to the electronic properties, of which the field emission study was extensive, profound, and highly near to a practicable product. CNTs are known to be very good electron emitters. This is why since their discovery there has been a lot of speculation and experiments about the use of nanotubes in the construction of flat panel devices. The first study on CNT field emission (FE) properties was carried out by Rinzler et al. [20] from Rice University. They found that field emission of electrons from individually mounted CNTs has been dramatically enhanced when the nanotube tips are opened by laser evaporation or oxidative etching. A short time-hereafter, the field emission from a film of postsynthesized reduced CNT alignments was made by de Heer and co-workers [21]. However, a prerequisite for a major breakthrough in the area would be the perfect alignments of CNTs on a suitable surface. This chapter is mainly focused on the synthesis and applications of aligned carbon nanotubes, including SWNTs and multiwalled carbon nanotubes (MWNTs). The organization of this chapter is as follows: Section 2 first considers three classical methods for synthesizing nanotubes, including arc discharge, laser ablation, and chemical vapor deposition (CVD), especially comparing the advantages and disadvantages of each synthesis method, and then presents the new preparation techniques developed in the last two years. In Section 3, the fabrications of various alignments and patterns are discussed in detail for MWNTs, and the emphasis in this section is on the controllable fabrication of nanotube alignments. Alignments of SWNTs are described in Section 4. Section 5 gives a discussion of properties and application of aligned CNTs, and the FE properties are of great concern. Finally, future research consideration and challenges are presented in Section 6.

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 1: Pages (1–15)

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2. CARBON NANOTUBES AND THEIR PREPARATION METHODS A carbon nanotube consists of one or more seamless cylindrical shells of graphitic sheets. The nanotube is typically closed at each end, according to Euler’s theorem [22], by the introduction of pentagons in the hexagonal network. A theoretical model of a CNT with two closed ends was simulated by Endo and Kroto [23]. However, an experimental observation of the tube with both ends closed has seldom been reported for two main reasons. One is that most CNTs, regardless of the synthesis methods (such as arc discharge, laser ablation, and catalyst decomposition of hydrocarbon), are attached by either open ends, or one closed carbon cap, or ends capped with metal particles. The other is that the large length/diameter ratio (>1000) prevents one from simultaneously observing two ends of the tubes in the same field of vision under a transmission electron microscope (TEM), though tubes with two caps exist in the product. In an earlier report [24], it was also noticed that nanotubes produced using the arc discharge technique can be fairly short, and these structures with two capped ends can be observed under high resolution transmission electron microscopy. However, the structure is so small and asymmetric, with a diameter of at most 40 nm, that it should be precisely defined as an elongated nanoparticle of graphitic carbon rather than a well-defined CNT with capped ends. We reported a perfect carbon nanotube with two graphitic caps observed in the products by pyrolysis of metal phthalocyanine [25]. Such a large CNT with capped ends is of great interest for the formation mechanism of nanotubes. CNTs are most attractive because of their fascinating features. What makes nanotubes so special is the combination of dimension, structure, and topology that translates into a whole range of superior properties. The basic constitution of the nanotube lattice is the C–C covalent bond (as in graphite planes) which is one of the strongest in nature. The perfect alignment of the lattice along the tube axis and the closed topology endow nanotubes with in-plane properties of graphite such as high conductivity, excellent strength and stiffness, chemical specificity, and inertness together with some unusual properties such as the electronic structure, which is dependent on lattice helicity and elasticity. In addition, the nanodimensions provide a large surface area that could be useful in mechanical and chemical applications. The surface area of MWNTs has been determined by BET techniques and is ∼10–20 m2 /g, which is higher than that of graphite but small compared to activated porous carbons. This value for SWNT is expected to be an order of magnitude higher. Similarly, due to the relatively large hollow channels in the center of nanotubes, their density should be very low compared to graphite. Rough estimates suggest that SWNT density could be as small as 0.36 g/cm3 , and MWNT density could range between 1 and 2 g/cm3 depending on the constitution of the samples [7].

2.1. Classical Synthesis Methods Nanotubes are not always perfect seamless shells of graphite. Their quality depends on the method used to generate them and the exact conditions of the particular method. Making

Aligned Carbon Nanotubes

nanotubes is simple, but making good quality samples with high yields and highly graphitized shells is not trivial. There are several methods to produce CNTs, but three classical methods including arc discharge, laser ablation, and a catalytic technique remain the most practical for scientific purposes and realistic applications. The arc method [26, 27] remains by far the best technique for the synthesis of highquality nanotubes simply because the process has a very high temperature of 4000 K. However, this method suffers from a number of disadvantages [14]. First, it is labor intensive and requires some skill to achieve a satisfactory level of reproducibility. Second, the yield is rather low, since most of the evaporated carbon is deposited on the walls of the vessel rather than on the cathode, and the materials that are formed in the deposit contain substantial amounts of nanoparticle and other graphitic debris. Third, it is a batch rather than a continuous process, and it does not easily lend itself to scale-up. The amount of nanotubes that can be produced is limited. Progress in this direction has been hampered somewhat by a lack of understanding of the growth mechanism of tubes in the arc. The laser evaporation technique developed by Smalley’s group [28–30] appears to produce the highest yield and best quality materials, but the high powered lasers required for this method will obviously not be available in every laboratory. The synthesis of MWNTs in this way has been carried out by a pure graphite target [29]. In 1995, they reported the development of the laser synthesis technique which enabled them to prepare SWNTs with a target of a metal–graphite composite instead of pure graphite [28]. Subsequent refinements to this methd led to the production of SWNTs with unusually uniform diameter [30]. CVD of hydrocarbon over metal catalyst has been another classical method to produce carbon materials. Various forms of carbon fibers, filaments, and MWNTs have been synthesized by this technique in the past [31–34]. In general, metal catalytic particles are exposed to a medium containing hydrocarbon gaseous species, and the formation of nanotubes is catalyzed [35]. During growth, good uniformity in size of the tube is achieved by controlling the size of the seeded catalyst particles, and the process can be easily scaled up to produce large amounts of materials. In some cases, when the catalysts are prefabricated into patterned arrays, well-aligned nanotube assemblies are produced, which will be discussed in detail (Section 3). This seems a promising direction for further research. However, the quality of tubes produced in this way has been rather poor compared with the arc and laser methods. As shown in Figure 1, a nanotube prepared by a catalytic pyrolysis of iron phthalocyanines is a structure with lots of topological defects in a wall [36]. There is a further serious weakness of all these techniques for preparing nanotubes; they produce a wide range of the size and structures. This may not be a problem for some applications, but it could be a drawback in areas where specific tube structures with uniform properties are needed, such as in nanoelectronics. Progress in this direction has been hampered somewhat by a lack of understanding of the growth mechanism of tubes. Although a number of theories [37–40] have been put forward, none of which has gained universal acceptance, most questions remain unanswered and the uncertainty surrounding nanotube growth

Aligned Carbon Nanotubes

3 advantages favoring the scaling-up of this new method. However, this method suffers from harsh synthesis conditions (a high temperature of 800  C, a high pressure of 100 MPa, and a long time of 48 h), and electron microscopy observations revealed that very short multiwalled tubes (hundreds of nanometers) exist together with the aggregates of needlelike and polygonal nanoparticles. Compared with this method, a solvothermal route to MWNTs has been developed under a lower temperature (350  C) and shorter time (8 h) [46]. This catalyst-assembly benzene-thermal route was carried out to produce CNTs using reduction of hexachlorobenzene by metallic potassium in the presence of Co/Ni catalyst. The synthesis temperature was low, but the quantity of the nanotubes was poor.

2.2.2. Supercritical CO2 Technique

Figure 1. A carbon nanotube with many topological defects synthesized by chemical vapor deposition. Reprinted with permission from [37], M. Endo and H. W. Kroto, J. Phys. Chem. 96, 6941 (1992). © 1992, Elsevier Science.

mechanisms has impeded progress in the development of more controlled synthesis techniques.

2.2. Preparation Methods of Nanotubes In the last two years, a few new synthesis methods were developed to produce CNTs to solve one or more problems during the conventional preparation process. These ways take respective advantages over the conventional methods to a certain extent and were of important potential for basical research as a new route to produce CNTs. However, the samples of the nanotubes produced by all those new methods existed in lower quality and less perfect structures than the classical ones. A number of experimental conditions should be optimized before these synthesis methods are developed to produce nanotubes with high quantity and desirable structures to meet scientific research and practical applications.

Carbon dioxide is nonflammable, essentially nontoxic, and environmentally benign [47]. Motiei et al. [48] reported the structures of graphitic concentric shells grown by supercritical CO2 . It is found that MWNTs and nested fullerenes could be prepared from dry ice in the presence of Mg by heating the precursors in a closed vessel at the autogenic pressure of the mixture. This method avoids the complexities of using a flowing gas at controlled pressures and high temperature and requires no technically complex equipment.

2.2.3. Solid-State Metathesis Reaction The solid-state metathesis reaction has been developed over the past few years into a simple and effective route to materials that are difficult to synthesize by conventional methods [49–51]. These highly exothermic and self-propagating reactions initiated by a heated filament often use molecular precursors to produce crystalline products. An exchange reaction between carbon halides and lithium acetylide catalyzed by cobalt dichloride enables the rapid synthesis of CNTs by the following equation [52]: C2 Cl6 + 3 Li2 C2 → 8 C (nanotubes) + 6 LiCl. Without a catalyst, only graphitic and amorphous carbon forms. These reactions, once optimized, will likely require cheaper precursors, more preparation, and less expensive equipment than existing methods. However, along with multi- and single walled nanotubes, graphite-encapsulated cobalt nanoparticles, free carbons, and cobalt metals were found. These shortages should be overcome before the extensive application of this method.

2.2.1. Hydrothermal or Solvothermal Synthesis

2.2.4. Flame Synthesis

The oxidative effect of hot water on amorphous carbon is currently used for purification of CNTs by hydrothermal treatment (immersion in water at moderate and high temperatures and pressures) [41, 42]. It was found that the hydrothermal processing method could be developed to synthesize nanostructural carbon [43, 44]. Recently, MWNTs have been synthesized in the absence of metal catalyst by hydrothermal treatment of amorphous carbon in pure water at 800  C and 100 MPa [45]. The hydrothermal nanotubes are free of amorphous carbon after treatment. The homogeneity of hydrothermal processes and availability of amorphous carbon materials, without the need for catalyst, are

Combustion is widely used in industrial processes for largescale materials synthesis, such as for carbon black and metal oxides [53, 54]. These flame processes are well known for their many desirable features including continuous processing and energy efficiency. Recent investigations have sought to take advantages of these advantages to synthesize SWNTs in aerosol form [55, 56]. It was found that SWNTs could be produced by a binary or ternary gas mixture of CO/C2 H2 /H2 with a metal catalyst at 700  C in a two-stage flame. This flame system afforded the advantage that the catalyst formation step could be separated from the nanotube growth steps, which allowed investigations of catalyst particle size

Aligned Carbon Nanotubes

4 dependencies upon nanotube growth unlike methods that employ in-situ generation and concurrent growth. Apart from the methods presented previously, there are many other methods such as a gas phase synthesis [57], a reduction catalysis route at a low temperature of 200  C [58], and a solar approach [59], although these methods to produce nanotubes suffer from different drawbacks.

3. ALIGNMENTS AND PATTERNS OF MULTIWALLED CARBON NANOTUBES CNTs are the most promising materials anticipated to impact future nanoscience and nanotechnology. Their unique structural and electronic properties have generated great interest for use in a broad range of potential nanodevices [60, 61]. Most of these applications will require a fabrication method capable of producing CNT alignments or patterns with uniform structures and periodic arrangements to meet device requirements. Therefore, the ability to controllably obtain ordered or pattering CNT architectures is important to both fundamental characterizations and potential applications [62]. Controlled synthesis involving CVD has been studied as an effective strategy to order or pattern CNTs on a variety of surfaces [63–67]. This has been stimulated by its simplicity, its ability to yield structures that are two- or three-dimensional (2D or 3D) periodic over large area, and its potential to be much less expensive than other synthesis methods such as an arc discharge and a laser ablation. In these early attempts, most of the works to date have focused on fabricating 2D CNT alignments or patterns over large areas because of their paramount importance for obtaining scale-up functional devices. However, in contrast to these efforts, little research on the preparation of 3D CNT alignments or patterns has been reported because of many technical difficulties, although the 3D structures show more unique and potential applications such as photonic devices [68, 69], data storage [70, 71], and ultrahydrophobic materials [72, 73]. In this section, various fabricating methods are discussed for alignments and patterns of MWNTs, including 2D or 3D structures.

3.1. Preparation of 2D Nanotube Alignments on Different Substrates Alignments or patterns of CNTs are particularly important for fabricating functional devices such as field emitters and nanoelectronics. Earlier attempt to manipulate nanotubes for these application have been made by postgrowth methods such as cutting a polymer resin–nanotube composite [74] or drawing a nanotube–ethanol suspension through a ceramic filter [75]. In the past few years, a great quantity of research on the fabrication of 2D CNT alignments has been reported [63–67, 76–78]. Although all these 2D CNT alignments were synthesized by CVD, different technical routes were adopted by different research groups. The main differences of these synthesis methods may be summarized as the following factors: a substrate and a precursor. The substrates that are used to support the CNTs alignments may be put into two categories: porous templates (mesoporous

silica [63], nanochannel alumina [76], etc.) and plain plates (quartz glass [77], single crystal silicon plate [78], glass [65], etc.). As far as the precursor is concerned, there are two main materials: hydrocarbons [66, 76] and metal organic compounds [77]. When hydrocarbons are used as precursors, metal catalysts (such as Fe, Co, Ni) should be added to grow CNTs. However, as for metal organic compounds no additional metal catalyst is necessary since these precursors contain both the metal catalyst and carbon source required for CNT growth. The following discussion is mainly focused on the substrates where the nanotube alignments grow.

3.1.1. Planar Substrate Quartz glass is a good planar substrate for growing wellaligned nanotubes [64, 77, 79]. Terrones et al. [64] described a method for generating aligned nanotubes by pyrolysis of 2-amino-4, 6-dichloro-triazine over thin films of a cobalt catalyst patterned on a silica substrate by laser etching. We have grown well-aligned nanotubes with large diameters on quartz glass substrate by pyrolysis of a metal phthalocyanine under Ar/H2 flow at a temperature of 950  C [77]. We investigated the influence of the growth time and precursor level on the structures of MWNTs. It was found that both the outer diameter and length of aligned nanotubes increased with increasing growth time and precursor level. The outer diameters of well-aligned nanotubes range from 25 to 250 nm, and the length ranges from 1 to 500 m. However, the microstructures of aligned naotubes are independent of the growth temperature and the Ar/H2 flow rate in our case. Figure 2a is a scanning electron microscopy (SEM) image showing a large area of well-aligned CNTs with uniform diameter and length. Figure 2b is a typical TEM image of the bamboo-shaped nanotubes [80] produced by pyrolysis of 0.5 g iron phthalocyanine in 12 min. The graphite sheath sliding out from the Fe particle surface accounts for the formation of the compartments of bamboolike CNTs. The driving force of the sliding was caused by the stress accumulated

100nm -100µm

(a)

(b)

Figure 2. A SEM micrograph showing large area well-aligned CNTs perpendicular to the surface of the substrate. (b) A typical TEM image of the CNTs produced by pyrolysis of 0.5 g FePc in 12 min. Reprinted with permission from [77], X. B. Wang et al., Chem. Phys. Lett. 340, 419 (2001). © 2001, Elsevier Science.

Aligned Carbon Nanotubes

in the graphite sheath due to the segregation of carbon atoms from the inside of the sheath [80]. Apart from quartz glass, we can produce aligned nanotubes on various substrates made of other materials such as single crystalline silicon, iron plate, nickel plate, cobalt plate, and so on. Glass is the most common material for panel display. The ability to prepare aligned nanotubes over a glass substrate makes them more suitable for electron emission applications. Recently, Ren et al. [65, 81] reported the growth of large scale well-aligned CNT arrays on nickel-coated glass at temperatures below 666  C by plasma-enhanced hot filament CVD. Acetylene gas was used as the carbon source and ammonia gas was used as a catalyst and dilution gas. It was found that NH3 plays a crucial catalytic role together with the nickel layer to promote the growth of the nanotubes, and nickel thickness plays a very important role in determining the diameters. Nickel wafers were found to be another good substrate for producing nanotube alignments. Chen et al. have grown aligned graphitic nanofibers on single crystalline Ni (100) [82] and polycrystalline Ni substrate [83] via plasma assisted hot filament CVD using a gas mixture of nitrogen and methane. Small Ni particles on the substrate surface generated by the plasma acted as a catalyst for the growth of nanofibers.

3.1.2. Porous Template Mesoporous silica containing iron nanoparticles was used as a substrate when well-aligned CNTs were first synthesized by Xie et al. [63]. The mesoporous silica was prepared by a sol–gel process from tetraethoxysilane hydrolysis in iron nitrate aqueous solution [84]. This method to produce large areas of highly ordered, isolate long nanotubes is based on CVD. The tubes are up to about 50 m long and well graphitized. The growth direction of the nanotubes can be controlled by the pores from which the nanotubes grow. Porous silicon, a light-emitting material [85, 86], is an ideal substrate for growing organized nanotubes. Fan et al. [60] have prepared porous silicon patterned with Fe film by electron beam evaporation through shadow masks and then on this substrate carried out the synthesis of self-oriented regular arrays of CNTs by catalyst decomposition of ethylene at 700  C. Porous silicon substrate exhibits an important advantage over plain silicon for synthesizing nanotubes. The investigation showed that the nanotubes grew at a higher rate on porous silicon than on silicon. The well-ordered nanotubes can be used as electron FE arrays. Scaling-up of the synthesis process should be compatible with the existing semiconductor processes and allows the development of nanotubes devices integrated into silicon technology. Highly ordered arrays of CNTs can be grown by pyrolysis of acetylene on cobalt within a hexagonal close-packed nanochannel alumina template at 650  C [76]. The method is based on template growth. An illustration of a typical fabrication process flow is shown in Figure 3a. The process begins with the anodization of high purity (99.999%) alumina on a desired substrate. The next step is to deposit electrochemically a small amount of cobalt catalyst into the bottom of the template channels. The ordered arrays of nanotubes (Fig. 3b)

5 (a)

Aluminium

NCA template Al anodization

Cobalt catalyst

Carbon nanotubes

Co deposition

C2H2 pyrolysis

(b)

200 nm

Figure 3. (a) Schematic of fabrication process. (b) SEM image of the resulting ordered arrays of CNTs synthesized using the nanochannel alumina template. Reprinted with permission from [76], J. Li et al., Appl. Phys. Lett. 75, 367 (1999). © 1999, American Institute of Physics.

are grown in a flow of a mixture of 10% acetylene in nitrogen. There are several features in this fabrication technique [87, 88]. First, each pore of the template is filled with one nanotube, which defines the tube diameter, and the tube diameter distribution throughout the arrays is narrow. Second, the controlled variation of the nanotube size, density, and array spacing depends on easily adjustable parameters such as the anodizing voltage, electrolyte composition, and temperature. Tube lengths of up to 100 m can be obtained by varying the length of the pores in the alumina template, which can be achieved by varying the time of anodization. Finally, the method allows inexpensive production of large arrays of ordered nanotubes.

3.2. Controllable Fabrication of CNT Alignments 3.2.1. Selective Positioning Growth for Patterns Selective positioning growth of CNT patterns is arousing a wide range of research interest [89–91]. In the past few years, several methods have been developed to site-selectively grow nanotubes. Photolithographic Approach Wei et al. [89, 90] have used a CVD method with gas-phase catalyst delivery to direct the assembly of CNTs in a variety of predetermined orientations onto silicon/silica substrates, of which patterning was generated by photolithography followed by a combination of wet and/or dry etching. Figure 4 shows some striking examples of organized nanotube patterns grown on preselected substrate sites by the photolithographic approach. There is no nanotube grown on silicon, but the aligned nanotubes grow readily on silica in a direction that is normal to the substrate surface [90]. The preference of nanotubes to grow

Aligned Carbon Nanotubes

6

d = 10 µm

d = 2 µm

d = 5 µm

Si

Figure 4. SEM images of organized nanotube patterns grown on preselected substrate sites by a photolithographic approach. Scale bars, 100 m. Reprinted with permission from [89], B. Q. Wei et al., Nature 416, 495 (2002). © 2002, Nature Publishing Group.

selectively on and normal to silica surfaces allows the sites of nucleation and the direction of growth to be controlled. Yang et al. [67] have also reported the fabrication of patterns of perpendicularly aligned nanotubes with resolution down to m scale by pyrolysis of iron phthalocyanines onto a quartz substrate prepatterned with a photoresist film. Electron Beam Lithographic Approach With a electron beam lithographic technique, patterned growth of freestanding nanotubes on nickel dots on silicon can be achieved by plasma-enhanced hot filament CVD [92]. The thin film nickel pattern as a catalyst for growing nanotubes was fabricated on a silicon wafer by a standard microlithographic method and metal evaporators. Well-separated, single MWNTs grew on each dot of an array of ∼100 nm nickel dots under an acetylene/ammonia mixture at below 660  C. The diameter and height depend on the nickel dot size and growth time, respectively. Using this method, devices requiring freestanding vertical CNTs such as scanning probe microscopy, FE flat panel displays, etc. can be fabricated. Soft Lithographic Approach Most microfabrication methods mentioned start with photolithography to form a pattern in a photoresist on the substrate. Although these techniques are very widely used, they are incompatible for solution such as gels, some polymers, some organic and organometallic species, and biological molecules. To pattern these materials successfully, the patterned photoresist must be impermeable to the reagents used and the deposited materials should not be compromised by solvents used for the liftoff. Methods other than photolithography

often involve a shadow mask formed from a rigid metal. The air gap between the mask and substrate makes the use of rigid shadow masks to pattern materials from solution impossible [60]. One micropatterning technique that circumvents some of these drawbacks is soft lithography [93–95], which uses a patterned elastomer fabricated from poly(dimethylsiloxane) as the mask, stamp, or mold. Because the elastomer can conform to and seal reversibly against the contours of a surface, it can be used as a mask or a stamp. Soft lithography has become a very promising technique for micro/nanostructuring a wide range of materials. Various strategies, including microcontact printing (CP) [96–98] and micromolding [99], have been developed for nanoscale patterning. Kind et al. [96] elucidated some important aspects of using CP to pattern silicon substrate with catalysts followed by the growth of CNTs on the activated regions. In brief, a patterned and inked elastomeric stamp is used to print a catalyst as a pattern onto a substrate (Fig. 5). The growth of MWNTs follows from the catalytic decomposition of acetylene on the printed pattern of the catalyst. Changing the concentration of the catalyst in the ink solution allows one to tune the density of the nanotubes from single, randomly oriented nanotubes to densely packed arrays of nanotubes oriented normal to the substrate. This approach relied on rigid solid-state substrates such as silicon and alumina to achieve aligned nanotubes, which SiO2

Si

Printing

Inked stamp

Catalyst

Catalyst

CVD

Carbon nanotubes

Figure 5. Procedure for the patterned growth of carbon nanotubes by microcontact printing a Fe(III)-based catalyst precursor onto silicon wafers. The stamp is inked with an ethanolic solution of Fe(III) and then printed onto the substrate. The growth of carbon nanotubes proceeds by the catalytic decomposition of acetylene. Reprinted with permission [96], H. Kind et al., Langmuir 16, 6877 (2000). © 2000, American Chemical Society.

Aligned Carbon Nanotubes

may limit its wider scope of application. Hence, it would be interesting and challenging to investigate the possibility of performing in-situ selective growth on a novel substrate such as polymers and its direct integration with elastomeric polymers to fabricate practical devices. Ng et al. [98] have recently reported the combination of soft-lithographymediated growth and surface wettability manipulation of an elastomeric polymer to achieve in-situ highly site-selective growth of multiwalled nanotubes arrays on elastomeric substrates patterned with catalyst precursors. The realization of this approach may provide new methodologies for flexible FE display, implantable sensors, etc. Biasing Growth Approach The investigations by Avigal and Kalish [100] showed that patterns of MWNTs could be obtained by positively biasing the substrate during growth. Growth was performed in a flowing mixture of 7% CH4 in Ar onto Co covered Si held at 800  C with and without the presence of an electric field. It was found that the tube alignment occurs only when a positive bias is applied to the substrate whereas no aligned growth occurs under negative bias and no tube growth is observed with no field. Therefore, selective area biasing may permit selected area growth of vertically aligned CNTs, a process that may find many applications. Fabrication of micropatterned nanotubes remains both scientific and technically challenging. Many methods have been presented in preceding parts of this section to produce nanotube patterns. However, these synthetic methods suffer from complex pre- or postsynthesis manipulation. In 2000, we have developed a simple method [101] for large-scale synthesis of CNTs (up to several square centimeters) aligned in a direction normal to the substrate surface (typically, quartz glass plates). Unexpectedly, we found honeycomblike aligned CNTs by pyrolysis of nickel–cobalt phthalocyanine (designated as Ni–CoPc), which was synthesized by roasting the mixtures of metal salts, phthalanlione, urea, and ammonium molybdate. A typical experimental procedure is as follows: a clean quartz glass plate (4 × 2 × 01 cm) was placed in a flow reactor consisting of a quartz glass tube and a furnace fitted with an independent temperature controller: A flow of Ar–H2 (1:1, v/v, 20 cm3 min−1 ) was then introduced into the quartz tube during heating. After the central region of the furnace reached 950  C, a quartz boat with 0.5 g Ni–CoPc was placed in the region where the temperature was about 500–600  C. After 5 min heating, CNTs grew in a direction normal to the substrate surface. Figure 6 is a set of SEM micrographs showing the honeycomblike shape of CNTs. As can be seen in Figure 6a, the honeycomblike CNTs are close together, and the diameters of the honeycombs range from 15 to 80 m. Figure 6b is a higher magnification image of an area in Figure 6a and clearly shows the size and distribution uniformity of honeycombs. Figure 6c is a magnification image of a typical honeycomb shown in Figure 6a (square frame). The external diameter of the honeycomb is 80 m, which contains a hollow inner cavity with diameter of 30 m. The CNTs, forming the honeycombs, grow out from the inner cavity perpendicular uniformly to the substrate surface and then extend all around from the opening of the inner cavity, and finally twin with those of other honeycombs along its outside line.

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Figure 6. (a) SEM micrograph of honeycomblike aligned CNTs. (b) A higher magnification image of honeycomblike aligned CNTs. (c) Magnification image of a typical honeycomb shown in Figure 2a. Reprinted with permission from [101], X. B. Wang et al., Appl. Phys. A 71, 347 (2000). © 2000, Springer-Verlag.

It is interesting that the same patterns of honeycomblike CNTs, two years later, were synthesized by prolysis of ferrocene and xylene on thermally oxidized silicon wafers [102].

3.2.2. Controllable Growth of Structures and Density of Aligned Nanotubes Controllable Growth of Diameters and Lengths Since nanotubes were first discovered in 1991, several advances in synthesis have led to the production of tubes in larger quanlities [63, 103] and higher purities [104–106]. Different diameter, length, and chairality of nanotubes give rise to diverse physical and mechanical properties [10, 107, 108]. However, controlling diameter, length, and charity has never been easy. Recently, several approaches for controlling diameters of SWNTs have been made. As far as the arc discharge method is concerned, the diameter of SWNTs can be changed by selecting metal catalysts [109] and ranging the pressure of helium gas [110]. For the laser ablation method, the diameters of SWNTs can be controlled by varying the furnace temperature [111] and laser pulse power [112]. However, the research on controlled growth of MWNTs, especially aligned CNTs, falls behind in contrast with that of SWNTs. Choi et al. [113] have synthesized aligned CNTs on Ni-deposited Si substrate using microwave plasma-enhanced CVD. They found that the diameter, growth rate (length), and density of CNTs could be controlled systematically by the grain size of Ni thin films. With decreasing the gain size of Ni thin films, the diameter of the nanotubes decreased, whereas the growth rate and density increased. The gain size of Ni films varied with the radio frequency (rf) power density during the rf magnetron sputtering process. As mentioned in Section 3.1.1, we can control the diameters and lengths of aligned CNTs by varying the growth time and the precursor level. Research results show that

Aligned Carbon Nanotubes

8 the length of the aligned nanotubes increases with the increase of the growth time, and the mean diameters does sharply, too. The diameter distribution width (a difference of between the largest and smallest diameter of a sample) shifts systematically to larger region with increasing growth time, which indicates that the outer diameters of nanotubes become inhomogenous in longer growth time. The template approach, shown in Section 3.1.2, can control effectively the diameter and length of aligned nanotubes by changing the nanochannel size and the thickness of template substrate, respectively. Recently, Jeong et al. [114] reported nanotube alignments with a narrow length distribution by etching the aluminum oxide template away. Sonication of aligned nanotubes on the template in an acetone solution cut the overgrown tubes effectively, resulting in short MWNTs. Site Density Controlling of Aligned CNTs Although the diameter and the length of aligned nanotubes can be easily controlled by changing the catalyst particle size, the growth time, etc., control of the site density is still challenging. For well-aligned nanotubes, tuning of site density is very important for certain applications, such as FE, nanoelectronic arrays, etc., because of the shielding effect of the dense arrays. Previous methods used to induce the site density include electron beam lithography [92], photolithography [115], microcontract printing [96], shadow mask [60], etc. However, all these methods either require expensive equipment and intensive labor or cannot control the site density in large area. Recently, research efforts to control the site density have been successful. Pulse-current electrochemical deposition has been used to prepare Ni nanoparticles that are used as the catalysts for the growth of aligned CNTs [116]. The nucleation site density of the Ni nanoparticles was controlled by changing the magnitude and duration of the pulse current. The site density of the aligned nanotubes varied from 105 to 108 cm−2 (Fig. 7).

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Figure 7. SEM images of aligned CNTs with site densities of (a) 75 × 105 (b) 2 × 106 , (c) 6 × 106 , (d) 2 × 107 , and (e) 3 × 108 cm−2 , and (f) a single standing CNT. Reprinted with permission from [116], Y. Tu et al., Appl. Phys. Lett. 80, 4018 (2002). © 2002, American Institute of Physics.

substrate surface. A few pillar-shaped structures of CNTs grow out from the 2D alignments in a well-distributed mode, which characterizes the 3D CNT alignments. The CNT posts with a diameter of about 3.4 m are 7.8 m higher - 50µm

- 50µm

3.3. 3D Alignments and Patterns of CNTs In the preceding part of this section, we gave a wide coverage on the fabrication of alignments and patterns of 2D nanotubes with selective positioning and controlled growth. However, in sharp contrast to the research enthusiasm for 2D aligned CNTs, the research on 3D alignment and patterns of nanotubes is sparse. We have developed a simple method for the large-scale synthesis of 3D aligned CNT patterns on quartz glass substrate [62, 117]. A typical experimental procedure is as follows: a flow reactor, consisting of a quartz tube and a furnace fitted with an independent temperature controller, was heated to 950  C under a flow of Ar/H2 (1:1, v/v, 20 cm3 min−1 . Almost immediately after the transfer of a quartz glass plate (4 × 2 × 01 cm) from acetone solution to the central region (950  C) of the furnace, a quartz boat with 0.5 g of FePc was placed in the region where the temperature was 550  C. After 5 min CNTs grew in a direction normal to the substrate surface. Figure 8a shows an SEM image of 3D regular arrays of nanotubes aligned along the direction perpendicular to the

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Figure 8. (a) SEM images of 3D regular arrays of nanotubes aligned along the direction perpendecular to the substrate surface. SEM images of 3D nanotube patterns: (b) ringlike castles and (c) a 490-m-long crucian carp without a tail and fins. Reprinted with permission from [117], X. B. Wang et al., Chem. Commun. 8, 751 (2001). © 2001, The Royal Society of Chemistry.

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9

than the 2D nanotube alignments, whose height is mainly 6 m from the quartz substrate. In addition to the pillarshaped 3D nanotube alignments, most interesting patterns made of nanotubes arrays, such as ringlike castles (Fig. 8b) and a 490-m-long crucian carp without a tail and fins (Fig. 8c), were observed under similar experimental conditions. Although the growth mechanism for these patterns is incomplete at present, we think that the substrate should be responsible for their formation. At the original stage, acetone soaked on the quartz glass substrate was rapidly carbonized at high temperature before volatilization to form the patterns. The resulting carbon, together with iron atom by pyrolysis of FePc, generates metal carbide, which is a more activated catalyst to grow CNTs than metal [80]. Thus, the CNTs in the pattern region formed by metal carbide grow more rapidly than those by metal. This accounts for the formation of alignments and patterns of 3D CNTs. Apart from this, both the strong van der Waals interactions between the tubes and the high surface density of the growing nanotubes serve as additional advantages for the constituent nanotube to be “uncoiled” and allow the aligned nanotubes to develop on the quartz substrate. In spite of the interesting patterns of 3D nanotubes, the results were unexpected. On the basis of the previous experiments, we have recently realized the controllable fabrication of 3D nanotubes patterns with features of high resolution by a vacuum deposition technique through shadow masks [118]. Using SEM, we observed 3D aligned CNTs, consisting of two streaked ribbons with widths of 6.5 and 8.4 m, respectively (Fig. 9), by pyrolysis of FePc for 5 min. The nanotubes in the ribbon area are about 1.4 m higher than those of 2D alignments (beyond the ribbon region). The growth rates of the nanotubes in and beyond the ribbon region were about 25 and 20.3 nm/s, respectively. It is only the difference of growth rates that results in the formation of 3D nanotube alignment. These 3D micropatterns of well-aligned

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Figure 9. SEM images of 3D nanotube alignments consisting of two ribbonlike structures (6.5 and 8.5 m). The nanotubes in the ribbon region are 1.4 m higher than those elsewhere. Reprinted with permission from [118], X. B. Wang et al., Adv. Mater. 14, 1557 (2002). © 2002, Wiley–VCH.

nanotubes were also prepared on photolithographically prepatterned substrates by the same method [119]. This opened the way for fabricating 3D nanotube micropatterns and thus developing novel nanoelectronic devices.

4. ALIGNMENTS OF SINGLE-WALLED CARBON NANOTUBES Alignments of SWNTs have been envisioned to enhance performance of various technologically important devices such as sensors [120], field emitters, and organic light-emitting diodes [121]. Immobilizing the random SWNTs into a controlled orientation would be an extremely important step for these real device application of the nanotubes. However, the creation of SWNT alignments has fallen far behind MWNT arrays because of the technical difficulty in handling or aligning individual SWNTs to ideal locations.

4.1. Chemical Alignments Chemical alignment is an effective method to order SWNTs perpendicular to the substrate by shortening SWNTs in an oxidizing environment, followed by chemical modification of the carboxyl end groups. Short and long SWNTs have exhibited considerable affinity for amine-functionalized substrates, although they tend to orient parallel to the substrate [122, 123]. Thiol functionalization of SWNTs resulted in better alignment on gold substrate; nevertheless, this system was plagued by low surface coverage and long adsorption time [124]. Recently, Wu et al. developed an alternative method for the assembly of oxidatively shortened SWNTs on silver surfaces [125]. This technique is based on the spontaneous adsorption (self-assembly) of the COOH groups at the open ends of CNTs onto silver surfaces. Their studies revealed that most of the SWNTs (ca. 80%) assembled on silver surfaces have a bundle size of 65 ± 05 nm, possibly suggesting the selective adsorption of SWNTs on silver. More recently, dense arrays of monolayer and multilayer assemblies of shortened SWNTs have been demonstrated using a metal assisted organization process from nonaqueous media [126]. The self-assembly, which was performed on a substrate such as glass, silicon (110) wafers with native oxide and quartz crystal microbalance resonator, consisted of sequential dipping in an aqueous solution of FeCl3 followed by immersion in DMF dispersed shortened SWNTs and separated by intermedia washing in DMF. A monolayer of densely packed, needlelike domains was obtained after 30 min immersion in nonaqueous dispersions of shortened SWNTs. This geometry is believed to be the result of a high concentration of carboxy groups on the severed edges of shortened SWNTs, hydroxy functionalization of Fe3+ -decorated surfaces, and strong hydrophobic interactions between adjacent shortened SWNTs.

4.2. Alignments by an Electrical or Magnetic Field Applying an electrical or magnetic field may be a simple and alternative method to align CNTs. Bubke et al. [127] reported that MWNTs dispersed in ethanol can be aligned by an electrical field. Due to the orientation of these elongated particles, the nanotube alignments exhibited optical

10 anisotropy. Thick films of aligned SWNTs and ropes have been produced by filtration/deposition from suspension in strong magnetic field [128, 129]. Recently