Carbon nanotubes – Production and industrial applications

Materials & Design Materials and Design 28 (2007) 1477–1489 www.elsevier.com/locate/matdes

Carbon nanotubes – Production and industrial applications Melissa Paradise 1, Tarun Goswami

*

Department of Mechanical Engineering, The T.J. Small College of Engineering, Ohio Northern University, Ada, OH 45810, United States Received 8 August 2005; accepted 10 March 2006 Available online 5 May 2006

Abstract Carbon nanotubes are discussed in this paper from the time of their discovery to present day applications. Specifically the production methods, properties and industrial applications of carbon nanotubes are reviewed. Production methods include classical approaches such as the arc method, chemical vapor deposition, laser ablation, and electric arc discharge along with new methods which are being tested such as through solar energy, plasma and microgravity environments. The electrical and mechanical properties and actual structure of carbon nanotubes are discussed in detail. Both current applications of carbon nanotubes along with potential uses are also elucidated in this review. The data has been compiled from open literature to comment on trends in behavior of the carbon nanotubes. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Single wall nanotubes; Multi-wall nanotubes; Nanometer; Chemical vapor deposition; Arc discharge; Carbon

1. Introduction Carbon is known to be the most versatile element that exists on the earth. It has many different properties which can be used in different ways depending on how the carbon atoms are arranged. For more than 6000 years carbon has been used for the reduction of metal oxides. Carbon in the form of graphite was discovered in 1779, and 10 years later in the form of a diamond. It was then determined that both of these forms belong to a family of chemical elements. It was not until about 200 years later that the next advancements in carbon took place. In 1985 Kroto, Smalley and Curl2 discovered fullerenes [1]. A few years later the carbon nanotube was discovered. Carbon nanotubes (CNT) were first discovered in 1991, by Sumio Iijima,3 in fullerene soot [2,3]. It was a product of the carbon-arc discharge method, which is similar to *

Corresponding author. Tel.: +1 419 772 2385; fax: +1 419 772 2404. E-mail address: [email protected] (T. Goswami). 1 Junior student in the Department of Mechanical Engineering Ohio Northern University, 45810, United States. 2 Recipients of 1996 Nobel Prize in Chemistry for the discovery of fullerenes. 3 Recipient of 2002 Benjamin Franklin medal in Physics for his work on carbon nanotubes. 0261-3069/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2006.03.008

the method used for fullerenes preparation. In this form, carbon is arranged in tubular formations on a nanoscopic level. To observe such materials a high resolution transmission electron microscopy was used [3,4]. Carbon nanotubes are a completely new type of carbon fibre which comprises coaxial cylinders of graphite sheets, which range from 2 to 50 sheets [5]. The first observations Sumio made [3] were of multi-walled nanotubes, and it was not until two years later when single wall nanotubes were observed. Ijima along with Ichihasi [6] used carbon electrodes with a small amount of iron and filled the chamber around the carbon arc with methane and argon gas which yielded the single wall carbon nanotube. Single wall nanotubes are basically a single fullerene molecule that has been stretched out so their length is a million times its diameter [7]. Around this same time Donald Bethune and colleagues also observed the single wall carbon nanotube [4]. In 1996 Smalley synthesized bundles of single wall carbon nanotubes for the first time [5]. The name carbon nanotube is derived from their size which is only a few nanometers wide. By definition carbon nanotubes are cylindrical carbon molecules with properties that make them potentially useful in extremely small scale electronic and mechanical applications. These tubes consist of rolled up hexagons, 10,000 times thinner than a human hair. Ideal nanotubes can be described as a seamless

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cylinder of rolled up hexagonal networks of carbon atoms, which is capped with half a fullerene molecule at the end [2]. Their strength is one to two orders of magnitude and weight six times lighter than steels. Possible applications range from semiconductors, electronic memory, drive products, and medical delivery systems to uses in plastics such as automobile body panels, paint, tires and as flame retardants in polyethylene and polypropylene [9]. Carbon nanotubes have been the focus of considerable study because of their unusual strength along with excellent mechanical, electrical, thermal and magnetic properties [1– 100]. Nanotechnology has been recently supported with Nanotechnology Research and Development Act allowing $3.7 billion over the next four years to be administered by the National Nanotechnology Initiative with plans to create a National Nanotechnology Program (NNP) [10] in the United States. 2. Production of carbon nanotubes Various methods since arc growth have been explored to produce carbon nanotubes. Essentially nanotube structures are all formed in the same way but the process which causes the formation differs, Fig. 1. The first method for the production of multi-wall carbon nanotubes was through arc growth [14] Fig. 1(a), but most attractive method commercially used is condensation–vaporization densation (CVD) method. Under this method there are different ways to induce the carbon vaporization such as the electric arc discharge, continuous or pulsed laser ablation, or solar energy [11] Fig. 1(b). Chemical methods have also been found to synthesize carbon materials such as the catalytic decomposition of hydrocarbons, the production by electrolysis (Fig. 1(c)), heat treatment of a polymer, the low temperature solid pyrolysis, or the in situ catalysis [15]. Recently a catalytic chemical vapor deposition (CCVD) has also been experimented which may prove to be better than the regular CVD method [12]. Some other methods which also have been found to work in the production of carbon nanotubes is the plasma torch method [13] the underwater alternating current (AC) electric arc method [14] and production in a microgravity environment [8].

Fig. 1. Schematic representation of various processes used to produce CNTs: (a) Electric-arc method used at the University of Montpelier (France). (b) Schematic representation of oven laser-vaporization apparatus used at Rice University (Houston, Texas, USA). (c) Electrolysis experimental system (Brighton, UK). (d) Arc discharge and CNT formation and transport in the sheath. (e) Arc-discharge technique. (f) Laser ablation process. (g) Solar furnace from Odeillo (France). (h) Solar experimental chamber used in Odeillo (France).

3. CVD process In the CVD process growth involves heating a catalyst material to high temperatures (500–1000 °C) in a tube furnace using a hydrocarbon gas through the tube reactor over a period of time [16]. The basic mechanism in this process is the dissociation of hydrocarbon molecules catalyzed by the transition metal and saturation of carbon atoms in the metal nanoparticle [16]. Precipitation of carbon from the metal particle leads to the formation of tubular carbon solids in a sp2 structure [16]. The characteristics of the carbon nanotubes produced by CVD method depend on the working conditions such as the temperature and the pressure of operation, the volume and

concentration of methane, the size and the pretreatment of metallic catalyst, and the time of reaction. Many times a catalyst is added to speed up the process, to lower high production costs, and improve the quality of the final product [17]. The type of carbon nanotube produced depends on the metal catalyst used during the gas phase delivery [18]. In the CVD process single wall nanotubes are found to be produced at higher temperatures with a well-dispersed and supported metal catalyst while multi wall nanotubes are formed at lower temperatures and even with the absence of a metal catalyst [19], Fig. 2. To eliminate impurities formed during the process such as graphite compounds, amorphous carbon, fullerenes, coal and metal nanoparticles a purification

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Fig. 1 (continued)

is needed. This is achieved by oxidative treatments in the gaseous phase, liquid phase, acid treatment, micro filtration, thermal treatment and ultrasound methods. After the process is complete the samples need to be characterized further. Techniques such as Raman scattering (RS), thermal gravimetric analysis (TGA), scanning electronic microscopy (SEM) and atomic force microscopy (AFM) have been used for such characterization [15]. 4. Arc method The arc method [2], in which carbon nanotubes were discovered, is carried out in low pressure He or other neutral atmosphere (Fig. 1(a)). Seales reaction chambers and vac-

uum equipment are needed to provide the atmosphere. The products are known to be well graphitized but there are some problems with this method. The growth needs to be interrupted to remove the product from the chamber [2]. The most widely used process in producing carbon nanotubes is the electric arc discharge method, Fig. 1(d– e). This same process is also used in producing fullerenes. In this method an electric arc discharge is generated between two graphite electrodes under inert atmosphere of helium or argon. A very high temperature is obtained which allows the sublimation of the carbon. Two kinds of synthesis can be performed in the arc: evaporation of pure graphite or co-evaporation of graphite and metal [11]. For the carbon nanotubes to be obtained, purification

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Fig. 2. TEM micrograph of a multi-walled carbon nanotube.

by gasification with oxygen or carbon dioxide is needed [20]. The first successful production of multi wall nanotubes at the gram level was developed in 1992 by Ebbesen and Ajayan [21]. For single wall nanotubes to be obtained a metal catalyst is needed and this first success of achieving substantial amounts came in 1993 by Bethune coworkers [22]. Process parameters involve small gaps between electrodes (>1 mm), high current (100 A), plasma between the electrode at about 4000 K, voltage range (30–35 V) under specified electrode dimensions. 5. Laser ablation method The laser ablation method is the second technique for producing carbon nanotubes which is very useful and powerful (Fig. 1(f)). This process is known to produce carbon nanotubes with the highest quality and high purity of single walls [23]. Laser ablation was the first technique used to generate fullerenes in clusters. In this process, a piece of graphite is vaporized by laser irradiation under an inert atmosphere. This results in soot containing nanotubes which are cooled at the walls of a quartz tube. Two kinds of products are possible: multi walled carbon nanotubes or single walled carbon nanotubes [11]. For this process a purification step by gasification is also needed to eliminate carbonaceous material. The effect of the gasification depends on the type of reactant used [24]. The first growth of high quality single wall nanotubes was achieved by Smalley and coworkers [25]. 6. Other methods Another method which is still being explored is through solar energy (Fig. 1(g–h)). It was used only for fullerene production until 1996. In this method nanotubes are now produced using highly concentrated sunlight from a solar

furnace. The sunlight is focused on a graphite sample and vaporizes the carbon. Soot containing the nanotubes is then condensed in a dark zone of a reactor, which is collected in a filter and water cooled [11]. Carbon nanotubes can also be produced under chemical methods. The catalytic decomposition of hydrocarbons is performed in a flow furnace at high temperatures. It results in four structural forms: amorphous carbon layers on the surface of the catalyst, filaments of amorphous carbon, graphite layers covering metal particles, and multi wall carbon nanotubes. Electrolysis produces carbon nanotubes by passing an electric current in a molten ionic salt between graphite electrodes [11]. Other methods which have been recently developed such as the plasma torch method, was designed on the basis that carbon nanotubes would naturally grow in any environment in which both appropriate metal atoms and carbon atoms are present. The underwater AC electric arc method actually combines the underwater growth with the use of an AC controlled power supply. Using environments such as microgravity can also help lead to better nanotubes and production by eliminating the effects of uncontrolled buoyancy [7]. Some of the methods are more effective than others but a problem that all methods face is the ability for the carbon nanotubes to self align. Many applications of carbon nanotubes require controlled growth of aligned carbon nanotubes with surface modification. Controlled synthesis of well aligned nanotubes in predetermined patterns is particularly important in terms of fundamental studies and applications [26] (Fig. 3). Depending on which substrate is being used in the CVD process two-dimensional (2D) or threedimensional (3D) micropatterns can be produced [26]. Self-alignment is a key technology in silicon device manufacturing and could benefit nanomechanical fabrication processes because patterned layers can be produced without additional lithography steps and could provide more accurate alignment than lithography. One successful method has been performed through the synthesis of carbon nanotubes in an enhanced CVD process on Si wafers and patterned Si wafers with parallel line arrays and holes and using Fe and CoSix as a catalyst [27]. This process successfully produced carbon nanotubes and carbon nanorods which were aligned and parallel to the substrate which favors applications towards microelectronic devices [27]. 7. Structure Carbon nanotubes are built from sp2 carbon units and consist of honeycomb lattices and are a seamless structure. They are tubular having a diameter of a few nanometers but lengths of many microns. MWNTs are closed graphite tubules rolled like a graphite sheet, Fig. 2. Diameters usually range between 2 and 25 nm, and the distance between sheets is about 0.34 nm [28], Fig. 3. single-walled carbon nanotubes’ (SWNT) are made of a single seamlessly rolled graphite sheet with a typical diameter of about 1.4 nm which is similar to a buckyball (C60) [16] (Fig. 4). They

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Fig. 3. Micrographs showing carbon nanotubes (a) macrograph of carbon nanotubes, (b) scanning electron micrographs of CNTs at 10,000 and 20,000 magnification, (c) aligned carbon nanotubes.

have a tendency to form in bundles which are parallel in contact and consist of tens to hundreds of nanotubes [29]. Depending on how the grapheme walls of the nanotube are rolled together they can result in an armchair, zigzag or chiral shapes (Fig. 5). These groups are distinguished by their unit cells which are determined by the chiral vector given by the equation: C h ¼ n^ a1 þ m^ a2 where ^ a1 and ^a2 are unit vectors in the two-dimensional hexagonal lattice, and n and m are integers. Another important parameter is the chiral angle, which is the angle between Ch and ^a1 (Fig. 6). When n = m and the chiral angle is 30 degrees it is known as an armchair type. When m or n are zero and

the chiral angle is equal to zero the nanotube is known as zigzag. Chiral nanotubes are therefore when the chiral angles are between 0° and 30°. The diameter is found by 2 2 1=2 € the equation d t ¼ ðO3=pÞa , where ac–c cc ðm þ mn þ n Þ is the distance between neighboring carbon atoms in the flat sheet. The phase difference is known to be 2P, where, for example, 10 hexagons are around the circumference of a zigzag type, the 11th would collide with the first when it comes around the circumference once [30]. The chiral angles along with diameter determine the properties of the nanotube. Studies of optical properties of nanotubes show that in most cases they act as semi

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Fig. 6. Schematic diagram showing how a hexagonal sheet of graphite is ‘rolled’ to form a carbon nanotube [33].

Fig. 4. Structures of (a) diamond, graphite, and fullerene (from R.E. Smalley), (b) a single-wall helical carbon nanotube [3].

est occupied orbital, has finite density neighboring carbon atoms in the flat sheet. The phase difference is known to be 2P, levels for a metallic tube and zero for a semiconductor. The density state occurs at sharp peaks as the energy level is increased [4]. 8. Properties

Fig. 5. Illustrations of the atomic structure of (a) an armchair and (b) a ziz-zag nanotube [33].

conductors but in a few rare cases they act as metallic. This metallic behavior only happens when n  m = 3L and L = 0, resulting in the (HOMO–LUMO) fundamental gap being 0.0 eV. The electronic properties are a result of the electrons being normal to the nanotube axis. While acting as a semi conductor the fundamental gap was found to be 0.5 eV, which was a function of the diameter which causes them to exist as ropes in their native state [30]. The energy gap is found by Egap = 2y0acc/d, where y0 is the C–C tight bonding overlap energy (2.7 ± 0.1 eV), acc is the nearest neighbor C–C distance (0.142 nm), and d is the diameter. Studies also showed that a small gap would exist because of P/r bonding orbital and P*/r* anti-bonding orbital at the Fermi level. The Fermi energy is the high-

Carbon nanotubes are unique nanostructures which are known to have remarkable electronic and mechanical properties. These characteristics have sparked great interest in their possible uses for nano-electronic and nanomechanical devices. Properties of carbon nanotubes can also be expanded to thermal and optical properties as well. Carbon nanotubes are predicted to have high stiffness and axial strength as a result of the carbon–carbon sp2 bonding [31]. Studies exploring the elastic response, inelastic behavior and buckling yield strength and fracture need to be conducted to find practical uses of the nanotubes. The mechanical properties of a solid must ultimately depend on the strength of its interatomic bonds. With knowledge of known properties of crystal graphite the mechanical properties of carbon nanotubes can be predicted with some confidence [32]. Experimental and theoretical results have shown an elastic modulus of greater than 1 TPa (that of a diamond is 1.2 TPa) and have reported strengths 10–100 times higher than the strongest steel at a fraction of the weight [33]. It has been predicted that carbon nanotubes have the highest Young’s modulus of all different types of composite tubes such as BN, BC3, BC2N, C3N4, CN, etc. [34] (Table 1). The definition of Young’s modulus involves the second derivative of the energy with respect to the applied stress/strain. In general, the strength of the chemical bonds determines the actual value of Young’s modulus and smaller diameters result in a smaller Young’s modulus. However, in tests conducted on carbon nanotubes show that little dependence exists on the diameter of the tube with Young’s modulus, which does help to hypothesize that carbon nanotubes do possess the highest Young’s modulus which is expected around

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Table 1 Mechanical properties of carbon nanotubes [5] Material

Young’s modulus (GPa)

Tensile strength (GPa)

Density (g/cm3)

Single wall nanotube Multi wall nanotube Steel Epoxy Wood

1054 1200 208 3.5 16

150 150 0.4 0.005 0.008

2.6 7.8 1.25 0.6

1 TPa [35]. Experiments conducted have resulted in tensile strengths in the range from 11 to 63 GPa, with dependence on the outer shell diameter, which is not far from the theoretical yield strength of 100 GPa. Due to high in-plane tensile strength of graphite, both single and multi wall carbon nanotubes, are expected to have large bending constants since they mostly depend on Young’s modulus. The nanotube has been found to be very flexible. It can be elongated, twisted, flattened, or bent into circles before fracturing. Simulations conducted by Bernholc and colleagues indicate it can regain their original shape. Their ‘kink-like’ ridges allow the structure to relax elastically while under compression, unlike carbon fibers which fracture easily [4]. The unique elastic and inelastic properties have brought about more studies on the durability of carbon nanotubes. For single wall nanotubes simulations of deformations showed that each shape change corresponded directly to an abrupt release in energy and a singularity in the stress/strain curve. The nanotubes were found to have an extremely large breaking strain which decreased with temperature. However, it was concluded single wall nanotubes were subject to buckling under high pressure, which is responsible for the pressure induced abnormalities of vibration modes and electrical resistivity (Fig. 7). The elastic modulus, Poisson’s ratio and bulk modulus were all found to be directly affected by the tubes radius. A max bulk modulus was found to be 38 GPa with samples having a radius of 0.6 nm. For multi-wall nanotubes the properties were a little more complicated to calculate. An empirical lattice dynamics model was used, which showed that multi-wall nanotubes were insensitive to parameters such as the chirality, tube radius, and the number of layers. Thermal properties including specific heat and thermal conductivity of carbon nanotubes are determined primarily by the phonons [31]. A phonon is a quantum acoustic energy similar to the photon. Phonons are a result of lattice vibrations observed in the Raman spectra [4]. Especially at low temperatures the phonon contribution to these quantities dominates and is due to the acoustic phonons. The measurements of thermoelectric power of nanotube systems give direct information for the type of carriers and conductivity mechanisms. Theoretical and experimental results show superior electrical properties of carbon nanotubes. They can produce electric current carrying capacity 1000 times higher than copper wires [36]. For 1D systems cylindrical surface, transla-

Fig. 7. TEM micrograph and computer simulation of nanotube buckling [33].

tional symmetry with a screw axis could affect the electronic structures and related properties. The electronic capabilities possessed by carbon nanotubes are seen to arise predominately from interlayer interactions, rather than from interlayer interactions between multilayers within a single carbon nanotube or between different nanotubes [37]. These optical properties have proved to be especially unique with capabilities of acting as either a metallic or semiconductor, which depends on tubule diameter and chiral angle. Studies have shown that metallic conduction can be achieved without introduction of doping effects. For semiconducting nanotubes the band gaps have been found to be proportional to a fraction of the diameter and without relation to the tubule chirality [37]. The I-tight-binding model within the zone folding scheme shows, one third of carbon nanotubes are found to be metallic while two thirds are semiconducting, depending on their indices [31]. Calculations based on the use of r and P bands, due to curvature induced mixing of these bands, are used to predict that some metallic nanotubes are very-small-band-gap semiconducting nanotubes [38] (Fig. 8). The symmetry of the structures basically relates all the calculations in both single and multi-wall carbon nanotubes. Electronic properties of bundles of single wall nanotubes can be derived, assuming the intertube interactions are not strong enough to change the band structure. Broken symmetry caused by interactions between tubes in a bundle create a pseudogap of about 0.2 and 0.1 eV [39]. This pseudogap, which is created can modify electronic properties such as semimetallike temperature dependence of the electrical conductivity and a finite gap in the infrared absorption spectrum is predicted [31].

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Fig. 8. Band-gap values vs. nanotube diameters define nanotubes as metallic or semiconducting [5].

9. Applications Carbon nanotubes have attracted a great deal of attention world wide with their unique properties which are leading to many promising applications. Potential practical applications have been reported such as chemical sensors [40], field emission materials [41], catalyst support [42], electronic devices [43], high sensitivity nanobalance for nanoscopic particles [43], nanotweezers [44], reinforcements in high performance composites, and as nanoprobes in meteorology and biomedical and chemical investigations, anode for lithium ion in batteries [45], nanoelectronic devices [46], supercapacitors [47] and hydrogen storage [48]. New applications are likely in the diamond industry since experiments have shown the conversion of carbon nanotubes to diamond under high pressure and high temperatures with the presence of a certain catalyst [49]. These are just a few possibilities that are currently being explored. As research continues, new applications will also develop. 10. Composites Given the mechanical properties that have been reported on carbon nanotubes, an entire new class of composite materials may be possible with the use of carbon nanotubes. The first commercially recognized use for multi wall nanotubes was electrically conducting components in polymer composites [50]. The matrices used in carbon nanotubes incorporated into composites can improve the electrical properties which can act as a polymer, metal, or metal oxide [14]. Carbon nanotube metal or metal oxide composites have been made to improve electrical conductivity. For applications in polymer nanocomposites the elastic and fracture properties of carbon nanotubes must be understood along with interactions at the nanotube matrix interface. The performance of carbon nanotubes in a polymer or ceramic matrix is well above traditional fillers such as carbon black or ultra

fine metal powders [51]. The major difference from conventional fiber-reinforced composites in that the scale is narrowed down to nanometers instead of micrometers [33]. Large similarities between mechanical properties of a polymer film and a SWNT matrix exist in that both have high viscoelasticity that can be evaluated using a nanoindentor [52]. It would be difficult to replace all carbon fibers in their uses since there has been so much work done with them. It is better for carbon nanotube research to look to a new market rather replace the old. The great novelty with carbon nanotubes is that they can achieve high stiffness along with high strength [34]. Also studies have shown that carbon nanotubes do perform as reinforcing elements with polymer [53], ceramic [54] and metallic matrices [55], but without alignment their performance in terms of strength and stiffness fall short of traditional carbon fibers. For industrial applications as composites large quantities of nanotubes will be needed. It has been found that the best method for high quantity and low cost production of nanotubes is provided through the CVD method. Cost factors also lead more to the use of multi wall nanotubes rather than single wall nanotubes [50]. Incorporating nanotubes into plastics can lead to a dramatically increased modulus of elasticity and strength in structural materials. The main problem still lies in producing the nanotubes so they are uniformly dispersed, achieving nanotube-matrix adhesion providing stress transfer and intra bundle sliding in single wall nanotubes [50]. Promising results have been observed by Biercuk and others to overcome these problems by increasing Vickers hardness with single wall nanotubes and increasing the modulus of elasticity and breaking stress in polystyrene using multiwall nanotubes [56]. Nanotube reinforced composites have already been successfully created. Experiments on a fully integrated nanotube composite using single wall nanotubes demonstrated dramatic enhancement of mechanical properties. To produce these composites a reaction of terminal diamines with alkycarboxl groups attached to single wall nanotubes in the course of dicarboxxlic acid acyl peroxide treatment was needed. The ultimate strength and shear modulus increased from 30% to 70% with only the addition of 1–4 wt% of single wall nanotubes. The strain to failure also increased showing an increase in toughness [57] (Fig. 9). Rubber compounds reinforced by nanotubes are potential applications in tire industry. By replacing the carbon black with carbon nanotubes improved skid resistance and reduced abrasion of the tire have been found in experimental results [58]. Carbon nanotubes may provide a safer, faster, and eventually cheaper transportation [59] in the future. Although expectations of carbon nanotubes are very high for their use in composites there has been some speculation against the results they produce when mixed with some polymers and plastics. Carbon nanotubes themselves are superior conductors by themselves but they may not exhibit the same level of conductivity when integrated into other materials [60]. Experiments have shown the conductivity to increase thermal conductivity by two or threefold

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Fig. 9. Presence of 5 wt% multi wall carbon nanotubes results in a steeper slope in the stress–strain curve [66].

when it should have been close to 50 fold [60]. The problem is that carbon nanotubes vibrate at much higher frequencies than the atoms in surrounding material which causes the resistance to be so high the thermal conductivity is limited [60]. Inducing stronger bonds between the nanotube and other material might help in solving the problem [60]. The use of carbon nanotubes to improve materials will be investigated in the future as production increases and applicability in industrial settings become possible. 11. Sensors and probes Carbon nanotubes have proved to have some advantages for sensing applications. Their small size with larger surface; high sensitivity, fast response and good reversibility at room temperature enable them as a gas molecule sensor [61]; enhanced electron transfer when used as electrodes in electrochemical reactions [62]; and easy protein immobilization with retention of activity as potential biosensors [62] are among some of the desirable applications. Studies have shown that surface modification performed on aligned carbon nanotubes even furthers the sensitivity of nanotube sensors [25]. The main advantage of these sensors are the nanscopic size of the nanotube sensing element and the corresponding nanoscopic size of the material required for a response [50]. The mechanical robustness of the nanotubes and the low buckling force increase the probe life along and minimizes damage during repeated hard crashes into substrates [49]. The cylindrical shape and small tube diameter also allow for imaging in narrow deep crevices and improve resolution in comparison to conventional nanoprobes, especially for high sample feature heights [63]. Electronic properties suggest carbon nanotubes will be able to mediate electron transfer reactions with electro active species in a solution when used as electrode material [64]. This leads to the idea that carbon nanotube based electrodes can be used in miniature chemical sensing [65]. Electrode materials with carbon nanotubes resulted in better behavior than traditional carbon electrodes including good conducting ability and high chemical stability [29]. The electrical

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resistivity of single wall nanotubes have been found to change sensitively on exposure to gaseous ambients containing NO2 , NH3 , and O2. By monitoring this change the presence of gases could be detected. Results showed are at least an order of magnitude faster than those currently available and that they could be operated at room temperature or at higher temperatures for sensing applications [66]. This sensing application is now being researched for its use on automotive tires. A tiny sensor would be able to monitor and report tire pressure to the driver while being able to withstand extreme temperature and vibrations [58]. Since multi wall nanotubes are conducting they can be used as scanning probes on microscope tips in instruments such as a scanning tunneling microscope (STM), atomic force microscope (AFM) and electrostatic force microscopes (Fig. 10). With their ultra high sensitivity, high resolution electron microscopes which have sub-nanoscale accuracy have the ability to obtain information on the atomic arrangement, element identification and electronic structure of nanocarbon materials [67]. Nanotubes tips can also be used for high resolution imaging or as active tools for surface manipulation. On an AFM tip they can be controlled like tweezers to pick up and release nanoscale structures [68]. Nanoscopic tweezers have been made that are driven by the electrostatic interaction between two nanotubes on a probe tip [69]. Studies have shown the reversible bending of nanotubes can be used to alter their conduction. Optimal designs such as the zigzag and armchair nanotubes were observed to have a difference in mechanical response at large bending and the current passing through metallic structures decreasing at larger bending angles as the semiconductor increases [70]. The correspondence between mechanical response and electronic transport has been proven potential applications of nanotubes in such applications as nano-electro-mechanical sensors and even switches [71].

Fig. 10. Use of a MWNT as an AFM tip At the center of the vapor grown carbon fiber (VGCF) is a MWNT which forms the tip. The VGCF provides a convenient and robust technique for mounting the MWNT probe for use in a scanning probe instrument [66].

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Aligned multi-wall carbon nanotubes are now being used for the development of an amperometric biosensor [70]. Electrodes modified with carbon nanotubes are used for the immobilization of enzymes and other redox proteins on the ends of aligned nanotube arrays [72], on the walls of carbon nanotubes [73] and inside nanotubes [74]. It has been shown that small proteins can be entrapped into the inner channel of opened carbon nanotubes by simple absorption [75]. Azamaian et al. [73] demonstrated the principal where glucose oxidase was absorbed along the length of carbon nanotubes and randomly distributed on a glassy carbon electrode. The key in this design is the establishment of electron transfer between enzyme active site and electrochemical conducer [71]. Small surface area leads to constraints on enzyme loading [76]. Carbon nanotubes posses the high surface area needed along with the structure dependant metallic character to promote electron transfer reactions at low potentials [77]. Based on results, chemical etching was proven to be most efficient when opening carbon nanotubes and allowing the entrance of the enzyme at the inner shell [71]. Basic electronic properties of semiconducting carbon nanotubes change when placed in a magnetic field [78]. The ‘‘band gap’’ shrank which is unique among known materials [78]. Nanotubes band gaps are comparable with silicon and gallium arsenide which are currently the mainstays of the computer industry because their narrow band gaps correspond with how much electricity it takes to flip a transistor from ‘on to off’ [78]. With the possibility of carbon nanotubes band gap disappearing all together in the presence of stronger magnetic fields, they could take over the roles of silicon and gallium arsenide potentially revolutionizing the computer industry [78]. 12. Field emission devices Field emission is a quantum effect when compared to thermionic emission. For technological applications, elec-

Table 2 Threshold electrical field values for different materials for a 10 mA/cm2 current density [66] Material

Threshold electrical field (V/m)

Mo tips Si tips p-type semiconducting diamond Undoped, defective CVD diamond Amorphous diamond Cs-coated diamond Graphite powder (<1 mm size) Nanostructured diamonda Carbon nanotubesb

50–100 50–100 130 30–120 20–40 20–30 17 3–5 (unstable >30 mA/cm2) 1–3 (stable at 1 A/cm2)

a b

Heat-treated in H plasma. Random SWNT film.

tron emissive materials should have low threshold emission fields and should be stable at high current density [66] (Table 2). Carbon nanotubes posses the right combination of properties: nanometer size diameter, structural integrity, high electrical conductivity, and chemical stability that make good electron emitters [79]. The first field emission from carbon nanotubes was performed in 1995 by Rinzler from single isolated multi wall nanotubes [80] and by multi wall nanotube film by de Heer [81]. Research on electronic devices has since focused primarily on the use of single and multi wall carbon nanotubes as field emission electron sources [82] for flat panel displays [83], lamps [84], gas discharge tubes providing surge protection [85], and X-ray [86] and microwave generators [87]. A potential applied between a nanotube coated surface and an anode creates high electric fields which is a result of a small radius of the nanofiber tip and the length of the nanofiber [50]. The local fields cause electrons to tunnel from the nanotube tip to the tunnel. This process of nanotube tip electron emission differs from that of bulk metals because it arises from discrete energy states instead of continuous electronic bands and its behavior depends on the nanotube tip structure, single wall nanotubes [88] or multi wall nanotubes [84] (Fig. 11).

Fig. 11. Left: Schematic of a prototype field emission display using carbon nanotubes. Right: A prototype 4.5_ field emission display fabricated by Samsung using carbon nanotubes (image provided by Dr. W. Choi of Samsung Advanced Institute of Technologies).

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13. Flat panel displays

Table 3 Hydrogen storage of carbon nanotubes to other carbon materials [66]

Flat panel displays are one of the more lucrative applications of carbon nanotubes but are also the most technically complex. Nanotubes are at an advantage over liquid crystal displays since they have low power consumption, high brightness, a wide viewing angle, a fast response rate and a wide operating system [50]. In the actual process electric fields direct the field-emitted electrons toward the anode where phosphorus produces light for the flat panel display [50]. Prototype matrix-addressable diode flat panel displays have been constructed at Northwestern University [66]. One demonstration consists of nanotube-epoxy stripes on the cathode glass plate and phosphor coated indium-tinoxide (ITO) stripes on the anode plate [89]. Pixels are then formed at the intersection of the cathode and anode stripes. Pulses of ±150 V are switched among anode and cathode stripes to produce an image [66].

Material

Max. wt%

H2 T (K)

P (MPa)

SWNTs (low purity) SWNTs (high purity) GNFs (tubular) GNFs (herringbone) GNS (platelet) Graphite GNFs Li-GNFs Li-graphites K-GNFs K-graphite SWNTs (high purity) SWNTs (50% pure)

5–10 4 11.26 67.55 53.68 4.52 0.4 20 14 14 5.0 8.25 4.2

133 300 298 298 298 298 298–773 473–673 473–674 <313 <313 80 300

0.040 0.040 11.35 11.35 11.35 11.35 0.101 0.101 0.101 0.101 0.101 7.18 10.1

14. Nanotube-based lamps Nanotube-based lamps are similar to displays comprising of a nanotube-coated surface opposing a phosphorcoated substrate, but they are less technically challenging and require less investment [50]. With lifetimes expected in excess of 8000 h they can look to replace environmentally problematic mercury-based fluorescent lamps used in stadium style displays [84]. Nanotube-based gas discharge tubes might also find commercial use in protecting telecommunications networks from power surges [85]. Another application arises if a metal target is used to replace the phosphorescent screen at the anode. This causes the accelerating voltage to increase producing X-rays instead of light [50]. The compact geometry of the nanotube based X-ray lead to potential uses for X-ray endoscopes and medical exploration [50]. 15. Energy storage Graphite, carbonaceous materials and carbon fiber electrodes have been used for decades in fuel cells, batteries and several other electrochemical applications [90]. Carbon nanotubes are now being considered for energy storage and production because of their small dimensions, a smooth surface topology, and perfect surface specificity since only the graphite planes are exposed in their structure [66]. The efficiency of the fuel cells is determined by the rate of electron transfer at carbon electrodes, which has been shown by several experiments to be fastest on carbon nanotubes [91]. The area of hydrogen storage is one of the most active studies involving energy storage yet also the most controversial. Extremely high and reversible hydrogen storage has been reported in materials containing single wall nanotubes [92] along with graphite nanofibers fibers [93] which has attracted interest both in industry along with the academic world (Table 3). The problem remains, however, in a lack of understanding of the basic mechanisms of hydro-

gen storage in these materials. The main ways to store hydrogen is by metal hybrids, cryo-absorption, and by the gas phase in metal hybrids [66]. Due to carbon nanotubes cylindrical shape and geometry, and nanometer – scale diameters, it has been predicted that they will be able to store liquid as gas in the inner cores through capillary effect improving metal hybrid batteries [94]. 16. Electrochemical devices Carbon nanotubes have been studied for their potential uses as electrodes for devices that use electrochemical double layer charge injection because of their high electrochemically accessible surface area of porous nanotube arrays combined with high electric conductivity [50]. Examples of such applications include ‘‘Supercapacitors’’ which have capacitances much larger than ordinary dielectric based capacitor and electrochemical actuators which may potentially be used in robots [50]. The capacitance for an electrochemical device depends on the separation between the charge on the electrode and countercharge in the electrolyte. Since this distance is about a nanometer for nanotubes in electrodes compared to a micrometer in ordinary dielectric capacitors, extremely large capacitances result from the high nanotube surface are accessible to the electrolyte [50]. The use of nanotubes as electrodes in lithium batteries is a possibility because of the high reversible component of storage capacity at high discharge rates [50]. The reversible capacity reported with single wall nanotubes is 1000 mA h/g compared to 372 mA h/g for graphite [95] and 708 mA h/g for ball milled graphite [79]. 17. Nanometer-sized electronic devices Recent advances have led to the idea that nanotubes will be useful for downsizing circuit dimensions. Presently, current-induced electromigration causes conventional metal wires interconnects to fail when the diameter becomes too small [50]. The covalently bonded structure of carbon nanotubes militates against similar breakdown of nanotube wires and because of ballistic transport the intrinsic resistance of

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the nanotube should essentially vanish [50]. Experimental results have shown that metallic single wall nanotubes can carry up to 109 A/cm2 compared to current densities for normal metals being only 105 A/cm2 [96]. The research of field effect transistors (NT-FETs) aims to replace source drain channel structure with a nanotube. Transistors assembled with carbon nanotubes may or may not work however depending on whether the chosen nanotube is semiconducting or metallic, which the operator has no control over [50]. It might be possible to peel back layers from multi-wall nanotubes to achieve desired properties but advances in microlithography are still needed to perfect this reduction method. Recent developments have focused the media attention to nanotube nanoelectronic applications [50]. Crossed single wall nanotubes have been used in producing three and four-terminal electronic devices [97] along with nonvolatile memory that functions like a electromechanical relay [98]. Nanotube transistors [99] have also been reported using integrated nanotubes which may lead to large scale integration. Patterned growth of carbon nanotubes on silicon wafers [100] may prove to be the step needed to integrate nanotubes into electronics. 18. Conclusions Carbon nanotubes may have only recently caught the attention of the world but many advances have been made since their discovery about a decade ago. They are unique nanostructures that display the desirable properties of any other known material. The techniques of production have also come a long way but still have some room to be more efficient and cost effective. They have amazing electronic and mechanical properties which lead to incredible forms of strength, and conductivity. Due to these qualities the field of applications is almost endless. From reinforcements in composites, sensors and probes, energy storage, electrochemical devices and nanometer sized electronics carbon nanotubes could revolutionize the world. Acknowledgements One of the authors (T.G.) acknowledge Mr. Tom Hughes of Applied Science Inc. for providing insights on this subject and data. Mr. David Bennett assisted with the illustrations. This research was a part of summer research experience for undergraduates funded by Ohio Northern University. References [1] Scharff P. New carbon materials for research and technology. Carbon 1998;36(5–6):481–6. [2] Biro LP, Horvath ZE, Szlamas L, Kertesz K, Weber F, Juhasz G, et al. Continuous carbon nanotube production in underwater AC electric arc. Chem Phys Lett 2003:399–402. [3] Burstein E. A major milestone in nanoscale material science: the 2002 Benjamin Franklin Medal in Physics presented to Sumio Iijima, 2003;340(3–4):221–42.

[4] Dresselhaus MS, Dresselhaus G, Eklund PC. Science of fullerenes and carbon nanotubes. New York: Academic Press; 1996. [5] Yamabe T. Recent development of carbon nanotubes. Synthetic Met 1995:1511–8. [6] Iijima S, Ichihashi T. Single shell carbon nanotubes of one diameter. Nature 1993;636:603. [7] Alford JM, Mason GR, Feikema DA. Formation of carbon nanotubes in a microgravity environment, Sixth international microgravity combustion workshop, NASA Glenn Research Center, Cleveland, OH, CP-2001-210826, 2001. p. 293–6, May 22–24. [8] Massachusetts Technology Collaborative and The Nano Science and Technology Institute, Nanotechnology and its impact on industry. Industry Summaries, 1997. [9] Dunn T, Morse BB, Pendleton PC. Nanotechnology: The big picture, The newsletter of the MIT enterprise forum of Cambridge, 2004;22(8). [10] Laplaze D, Alvarez L, Badie JM, Flamant G. Carbon nanotubes: dynamics of synthesis processes. Carbon 2002:1621–34. [11] Journet C, Bernier P. Production of carbon nanotubes. Appl. Phys. A 1998:1–9. [12] Endo M. Mass production, selective formation, and applications of carbon nanotubes; Shinsu University; 1995. [13] Chen C, Perry W, Xu H, Jiang Y, Phillips J. Plasma torch production of macroscopic carbon nanotube structures. Carbon 2003:2555–60. [14] Liu Y, Gao L. A study of electrical properties of nanotube–NiFe2O4 composites: effect of surface treatment of carbon nanotubes. Carbon 2005:47–52. [15] Izaskun B, Ingio F, Ainara G, Belsue M, Roberto M. Synthesis purification and characterization of carbon nanotubes. Poster 2004:13–7. [16] Dai H. Carbon nanotubes: opportunities and challenges. Surface Sci 2002;500(1–3):218–41. [17] Resasco DE, Alvarez WE, Pompeo F, Balzano L, Herrera JE, Kitiyanan B, et al. J Nanopart Res 2002;4:131. [18] Johnson R. CVD process tames carbon nanotube growth. EE Times, 2002. [19] Qingwen L, Jin Y, Zhogfan L. Dependence of the formation of carbon nanotubes on the chemical structures of hydrocarbons. Adv Nanomater Nanodev 2002:59–71. [20] Morishita K, Takarada T. Scanning electron microscope observation of the purification behavior of carbon nanotubes. J Mater Sci 1999;34(6):1169–74. [21] Ebbesen TW, Ajayan PM. Large-scale synthesis of carbon nanotubes. Nature 1992;358:220–2. [22] Bethune DS, Kiang CH, DeVries M, Gorman G, Savoy R, Vazquez J, et al. Cobalt-catalysed growth of carbon nanotubes with singleatomic-layer walls. Nature 1993;363:605–7. [23] Chiang M, Liu K, Lai T, Tsai C, Cheng H, Lin I. Electron field emission properties of pulsed laser deposited carbon films containing carbon nanotubes. J Vac Sci Technol B 2001;19(3):1034–9. [24] Srivastava D, Brenner DW, Schall JD, Ausman KD, Yu M, Ruoff RS. J. Phys. Chem. B [in press]. [25] Soundarrajan P, Patil A, Liming D. Surface modification of aligned carbon nanotube arrays for electrochemical sensing applications. American Vacuum Society; 2002. p. 1198–201. [26] Wang B, Liu X, Wu D, Wang H, Jiang J, Wang X, et al. Controllable preparation of patterns of aligned carbon nanotubes on metals and metal-coated silicon substrates. J Mater Chem 2003:1124–6. [27] Chang H, Lin C, Kuo C. Iron and cobalt silicide catalysts-assisted carbon nanostructures on the patterned Si substrates. Thin Solid Films 2002;420–421:219–24. [28] Ajayan PM. Chem Rev 1999;99:1787. [29] Zhao Q, Gan Z, Zhuang Q. Electrochemical sensors based on carbon nanotubes. Electroanalysis 2002(23):1609–13. [30] Saito Y, Uemura S. Field emission from carbon nanotubes and its application to electron sources. Carbon 1999;38(2):169–82.

M. Paradise, T. Goswami / Materials and Design 28 (2007) 1477–1489 [31] Popov V. Carbon nanotubes: properties and application. Mater Sci Eng R. Rep 2004;43(3):61–102. [32] Ruoff R, Lorents D. Mechanical and thermal properties of carbon nanotubes. Carbon 1995;33(7):925–30. [33] Thoteson E, Ren Z, Chou T. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61(13):1899–912. [34] Delmotte JP, Rubio A. Mechanical properties of carbon nanotubes: a fiber digest for beginners. Carbon 2002;40(10):1729–34. [35] Whittaker, Brink J, Husseni M, Linford G, Davis M. Self-aligned mechanical attachment of carbon nanotubes to silicon dioxide structures by selective silicon dioxide chemical-vapor deposition. Appl Sci Lett 2003;83(25). [36] Collins PG, Avouris P. Nanotubes for electronics. Sci Am 2000;283:62–9. [37] Dresselhaus M, Dresselhaus G, Saito R. Physics of carbon nanotubes. Carbon 1995;33(7):883–91. Pergamon. [38] Hamada N, Sawada S, Oshiyama A. New one-dimensional conductors: graphitic microtubules. Phys Rev Lett 1992;68:1579. [39] Kwon YK, Toma’nek D. Electronic and structural properties of multiwall carbon nanotubes. Phys Rev B 1998;58:R16001. [40] Ong JK, Franklin NR, Zhou C, Chapline MG, Peng S, Cho K, et al. Science 2000;287:622. [41] Tans SJ, Verschueren RM, Dekker C. Nature 1998:393. [42] Planeix JM, Coustel N, Coq B, Brotons V, Kumbhar PS, Dutartre R, et al. J Am Chem Soc 1994;116:7935. [43] Saito S. Science 1997;278:77. [44] Kim P, Lieber CM. Science 1999;286:2148. [45] Che G, Lakshmi BB, Fisher ER, Martin CR. Nature 1996;384:147. [46] Collins PG, Zettle A, Bando H, Thess A, Smally RE. Science 1997;278:100. [47] Che G, Lakshmi BB, Fisher ER, Martin CR. Nature 1998;393:346. [48] Journet C, Maser WK, Bernier P, Loiseau A, Lamy de la M, Chapelle S, et al. Nature 1997;308:756. [49] Wang WK, Cao LM. Transformation of carbon nanotubes to diamond at high pressure and high temperature. Russ Phys J 2001;44(2):178–82. [50] Baugham R, Zakhidov A, Heer W. Carbon nanotubes – the route toward applications. Science 2002;297:787–92. [51] Rutkofsky M, Banash M, Rajagopal R, Chen J. Zyvex Application 2005. [52] Salvetat JP, Loubet JL [unpublished results]. [53] Schadler LS, Giannaris SC, Ajayan PM. Load transfer in carbon nanotube epoxy composites. Appl Phys Lett 1998;73(26):3842–4. [54] Peigney A, Laurent C, Rousset A. Synthesis and characterization of alumina matrix nanocomposites containing carbon nanotubes. Key Eng Mat 1997;132–136:743–6. [55] Kuzumaki T, Miyazawa K, Ichinose H, Ito K. Processing of carbon nanotube reinforced aluminum composite. J Mater Res 1998;13(9):2445–9. [56] Biercuk MJ et al. Appl Phys Lett 2002;80:2767. [57] Zhu J, Peng H, Rodriguez F, Margrave J, Khabashesku V, Imam A, et al. Reinforcing epoxy polymer composites through covalent integration of functionalized nanotubes. Adv Functional Mater 2004:643–8. [58] Smith J. Slicing it extra thin, Tireview, 2005. Available from: http:// www.tireview.com. [59] Bernard A. Applied sciences aims its nanotubes at the near-term market for polymers, SMALLTIMES, 2002. Available from: http:// www.smalltimes.com. [60] Rensselaer Polytechnic Institute, Thermal superconductivity in carbon nanotubes not so ‘Super’ when added to certain materials, Nano Tsunami, 2003. Available from: http://www.sciencedaily.com/ releases/2003/11/031112072719.htm.

[61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71]

[72] [73] [74]

[75] [76] [77] [78]

[79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99]

[100]

1489

Tang ZK et al. Science 2001;292:2462. Tennent HG. US Patent 4,663,230 (5 May 1987). Hafner JH, Cheung CL, Leiber CM. Nature 1999;398:761. Britto PJ, Santhanam KSV, Ajayan PM. Bioelectrochem Bioenerg 1996;41:121. Kong NR, Franklin C, Zhou MC, Chapline S, Peng K, Cho HD. Science 2000;287(622):406–15. Ajayan P, Zhou O. Applications of carbon nanotubes, carbon nanotubes. University of North Carolina; 2001. p. 391–425. Yumura M. Carbon nanotube industrial applications, Research Center for Advanced Carbon Materials; 2004, No. 10. Kim P, Lieber CM. Science 1999;286(2148):414. Kim P, Lieber CM. Science 1999;286(2148). Lee C, Liu X, Zhou C. Carbon nanotube field-effect inverters. Appl Phys Lett 2001;79(20). Farajian A, Yakobson B, Mizeseki H, Kawazoe Y. Electronic transport through bent carbon nanotubes: nanoelectromechanical sensors and switches. Phys Rev 2003;67:1–6. Gao M, Dai LM, Wallace GG. Electroanalysis 2003;15:1089. Azamian BR, Davis JJ, Coleman KS, Bagshaw CB, Green MLH. J Am Chem Soc 2002;124:12664. Gooding J. Nanostructuring electrodes with carbon nanotubes: a review on electrochemistry and applications for sensing. Electrochimica 2004;50(15):3049–60. Iijima S, Barbec C, Maiti A, Bernholc J. J Chem Phys 1996;104:2089. Lammert PE, Zhang P, Crespi VH. Phys Rev Lett 2000;84:2453. Tekleab D, Carroll DL, Samsonidze GG, Yakobson BI. Phys Rev B 2001;64:035419. Rice University, Magnetic forces may turn some nanotubes into metals, Nano Tsunami, 2004. Available from: http://www.sciencedaily.com/releases/2004/05/040521073051.htm. Bonnard JM, Salvetat JP, Stockli T, de Herr WA, Forro L, Chatelain A. Appl Phys Lett 1998;73(918):396. Rinzler AG, Hafner JH, Nikolaev P, Lou L, Kim SG, Tomanek D, et al. Science 1995;269:1550–3. De Heer WA, Chatelain A, Ugarte D. Science 1995;270:1179–80. de Heer WA, Chaˆtelain A, Ugarte D. Science 1995;270. Lee NS et al. Diam Relat Mater 2001;10:265. Saito Y, Uemura S. Carbon 2000;38:169. Rosen R et al. Appl Phys Lett 2000;76:1668. Sugie H et al. Appl Phys Lett 2001;78:2578. Zhou O. personal communication. Rinzler AG et al. Science 1995;269:1550. Wang QH, Setlur AA, Lauerhaas JM, Dai JY, Seelig EW, Chang RH. Appl Phys Lett 1998;72(2912):399–400. McCreery RL. Bard AJ, editor. Electroanalytical Chemistry, vol. 17. New York: Marcel Dekker; 1991. p. 401. Nugent J, Santhanam KSV, Rubio A, Ajayan PM. J Phys Chem 401 [in press]. Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Nature 1997;386(377):404–5. Chambers A, Park C, Baker RTK, Rodriguez NM. J Phys Chem B 1998;102(4253):404. Pederson, Broughton M. J Phys Rev Lett 1992;69(2689):405. Gao B et al. Chem Phys Lett 1999;307:153. Iijima S. Nature 1991;354(56):391. Fuhrer MS et al. Science 2000;288:494. Rueckes T et al. Science 2000;289:94. Bachtold A, Hadley P, Nakanishi T, Dekker C. Science 2001;294:1317. published online 4 October 2001 (10.1126/ science.1065824). Franklin NR, Li Y, Chen RJ, Javey A, Dai H. Appl Phys Lett 2001;79:4571.