Carbon Nanotubes

A carbon SWNT can be visualized as a hollow cylinder formed by rolling a planar sheet of hexagonal graphite (unit-cell parameters a = 0.246, c = 0.669 nm). It can be uniquely described by a vector C = nn + m 2, where ai and a2 are reference unit vectors as defined in Fig. 14.1.9. The SWNT is generated by rolling up the sheet such that the two end-points of the vector C are superimposed. The tube is denoted as (n, m) with n m, and its diameter D 

Generation of the chiral (8,4) carbon nanotube by rolling a graphite sheet along the vector C = naj + ma2, and definition of the chiral angle 9. The reference unit vectors aj and a2 are shown, and the broken lines indicate the directions for generating achiral zigzag and armchair nanotubes. 

The tubes with m = n are called armchair and those with m = 0 arc referred to as zigzag. All others are chiral with the chiral angle 6 defined as that between the vectors C and ai 0 can be calculated from the equation 

Lateral view of three kinds of carbon nanotubes with end caps (a) armchair (5,5) capped by one-half of (T.. (b) zigzag (9,0) capped by one-half of Cgo, and (c) an enantiomorphic pair of chiral SWNTs each capped by a hemisphere of icosahedral fullerene C140 

The values of 6 lies between 0° (for a zigzag tube) and 30° (for an armchair tube). Note that the mirror image of a chiral (n, m) nanotube is specified by (n + m, —m). The three types of SWNTs are illustrated in Fig. 14.1.10. 

Carbon nanotubes are rolled up tubes of carbon made up of a graphitic hexagonal structure. They can either be open or capped at the end by carbon pentagons which give all Fullerenes their closed curvature. 

1993 Structural rigidity of carbon nanotubes 1993 Synthesis of single-wall nanotubes 

1998 Chemical Vapor Deposition synthesis of aligned nanotube films 

1998 Synthesis of nanotube peapods in which a nanotube includes a rowof Ceo molecules 2000 Thermal conductivity of nanotubes 

2001 Integration of carbon nanotubes for logic circuits and intrinsic superconductivity of carbon nanotubes 

Carbon nanotubes can be grown on conducting Si, Au, Pt, and glass. The first three substrates are useful for making nanotubes into electrochemical electrodes. There are three ways nanotubes have been configured as an electrochemical electrode. First, nanotubes have been made into the equivalent of a carbon paste electrode by dispersion into mineral 

Potentiodynamic and potentiostatic methods can also be used to activate nanotubes. Similar to chemical oxidation, electrochemical pretreatment can effectively remove impurities and cause the creation of carbon-oxygen functional groups at the exposed edge plane and defect sites. The same authors reported on an electrochemical pretreatment that involved potentiostating the electrode at +1.7 V vs. Ag/AgCl in pH 7 phosphate buffer for 3 min followed by 3 min at —1.5 V (84). Both the chemical and electrochemical oxidations improved the electrode response (smaller voltammetric A p and larger values) for Fe(CN)g , serotonin, and caffeic acid. 

Carbon nanotubes can also act as nucleating agents for polymer crystallization. 

The discovery of carbon nanotubes (CNTs) is generally attributed to Sumio lijima who in 1991 discovered tubular forms of rolled-up graphene sheets (sheets of hexagonal sp -bonded C atoms) with bucky ball-like end caps. lijima s paper brought definitive news of such structures to the western world where it set off a flurry of research activity. 

SWNTs have very unusual properties that depend on their chirality (the sense and degree of twist in their structure). The chirality is measured by the chiral vector Ch defined 

The development of ways to make SWNTs and control their structure as well as ways to manipulate them and incorporate them into devices is at the forefront of research in nanotechnology. Recent developments in CVD growth of nanotubes have resulted in the ability to grow bundles of CNTs that can be harvested and spun into fibers to make a super thread. The individual CNTs in a super thread are bonded by van der Waals forces, but it is possible to alter their mechanical and electrical properties by heat treating and irradiation. The sides of CNTs have been functionalized to bind with epoxies and polymers to form composites and their tips have been functionalized to serve as chemical sensors, atomic force microscope (AFM) tips, ion and electron emitters, and for other novel applications. 

As shown in Table 4.11 carbon nanotubes have now been studied in various polymers as a means of improving their electrical properties. 

Other substances, which have been used to modify the electrical properties of polymers include clay [18], carbon block [19], graphite [20], bentonite [21], carbon fibre [22], silica [23, 24] and montmorillonite [25]. 

Since the discovery of the carbon nanotubes, there has been considerable work on other layered materials such as M0S2, WS2 and BN to explore the formation of nanotubes of these materials. Indeed several of them have been synthesized and characterized [21-23]. Similarly, nanowires of various inorganic materials have also been made [21]. In this chapter, we shall present the various important aspects of carbon nanotubes including their preparation, structure, mechanism of formation, chemical substitution, properties and applications. The methodologies developed for synthesizing nanowires and nanotubes of various inorganic materials as well as their salient features will also be discussed [21-24]. 

An inkjet ink that contains carbon nanotubes (CNTs), flake graphites, an organic carrier, a binder, a surfactant, a film enhancer and a solvent has been described (18). The CNTs to be used can be purified by the following steps (18)  

Heating the CNTs in air flow at about 350°C for about 2 /z to remove amorphous carbons, 

Soaking the treated CNTs in about 36% hydrochloric acid for about 24 h to remove metal catalysts, 

Rinsing the isolated CNTs with deionized water, and 

The CNTs can be chemically modified with fimctional groups such as -COOH, -CHO, -NH2, and -OH on the walls or at the end portions after the add treatment. These functional groups can cause the CNTs to become soluble and dispersible in the solvent. 

The imaging of single-walled carbon nanotubes (SWNTs) has become the most frequent application of TERS [23-25]. SWNTs have generated intense interest due to their potential applications in nanotechnology. Four types of Raman mode are usually observed in the TER spectra of SWNT the radial breathing modes (RBM), two graphitic bands (G, G ), and the disordered (D) band. The positions of these bands are vibrational signatures of the state of the SWNT, for example, its defect density, chirality, and so on. 

With these dehnitions, we can then visualize both the atomic structure and the electronic structure of C nanotubes. 

There are three types oftubular structures the first corresponds tom = Oor(n, 0), which are referred to as zig-zag tubes the second corresponds to m =nov(n, n), which are referred to as armchair tubes and the third corresponds to m 7 n or n, m), which are referred to as chiral tubes. Since there are several ways to define the same chiral tube with different sets of indices, we will adopt the convention that m n, which produces a unique identification for every tube. Examples of the three types of tubes and the corresponding vectors along the tube axis and perpendicular to it are shown in Fig. 13.9. The first two types of tubes are quite simple and correspond to regular cylindrical shapes with small basic repeat units. The third type is more elaborate because the hexagons on the surface of the cylinder form a helical structure. This is the reason why the basic repeat units are larger for these tubes. 

The fact that the tubes can be described in terms of the two new vectors that are parallel and perpendicular to the tube axis, and are both multiples of the primitive lattice vectors of graphene, also helps to determine the electronic structure of the tubes. To first approximation, this will be the same as the electronic structure of a graphite plane folded into the Brillouin Zone determined by the reciprocal lattice 

It is actually convenient to define two new vectors, in terms of which both zig-zag and armchair tubes can be easily described. These vectors and their corresponding reciprocal lattice vectors are 

In terms of these vectors, the zig-zag and armchair tubes are described as 

Carbon nano tubes were first discovered by Iijima in 1991 during the course of their work on fullerenes. Nanotubes belong to a promising group of nanomaterials. Although other nano tubes based on boron nitride and molybdenum have also been reported, currently CNTs are by far the most important group, and may be characterized in two classes single-walled (SW) CNTs and multi-walled (MW) CNTs. 

The field of carbon nanotube research was launched in 1991 by the initial experimental observation of carbon nanotubes by transmission electron microscopy (TEM) [151], and the subsequent report of conditions for the synthesis of large quantities of nanotubes [152,153]. Though early work was done on 

The earliest observations of carbon nanotubes with very small (nanometer) diameters [151, 158, 159] are shown in Fig. 14. Here we see results of high resolution transmission electron microscopy (TEM) measurements, providing evidence for m-long multi-layer carbon nanotubes, with cross-sections showing several concentric coaxial nanotubes and a hollow core. One nanotube has 

The diameter distribution of single-wall carbon nanotubes is of great interest for both theoretical and experimental reasons, since theoretical studies indicate that the physical properties of carbon nanotubes are strongly dependent on the nanotube diameter. Early results for the diameter distribution of Fe-catalyzed single-wall nanotubes (Fig. 15) show a diameter range between 0.7 nm and 1.6 nm, with the largest peak in the distribution at 1.05 nm, and with a smaller peak at 0.85 nm [154]. The smallest reported diameter for a single-wall carbon nanotube is 0.7 nm [154], the same as the diameter of the Ceo molecule (0.71 nm) [162]. 

Whereas multi-wall carbon nanotubes require no catalyst for their growth, either by the laser vaporization or carbon arc methods, catalyst species are necessary for the growth of the single-wall nanotubes [156], while two different catalyst species seem to be needed to efficiently synthesize arrays of single wall carbon nanotubes by either the laser vaporization or arc methods. The detailed mechanisms responsible for the growth of carbon nanotubes are not yet well understood. Variations in the most probable diameter and the width of the diameter distribution is sensitively controlled by the composition of the catalyst, the growth temperature and other growth conditions. 

The circumference of any carbon nanotube is expressed in terms of the chiral vector = nai ma2 which connects two crystallographically equivalent sites on a 2D graphene sheet [see Fig. 16(a)] [162]. The construction in 

Since nearly two decades ago that lijima [203] reported the observation of carbon nanotubes, numerous researchers studied physical and chemical properties of this new form of carbons. From unique electronic properties and a thermal conductivity higher than diamond to mechanical properties (stiffness, strength, resilience) higher any current material, carbon nanotubes offer tremendous opportunities for the development of new materials. 

Carbon nanotubes can be imagined as a sheet of graphite that has been rolled into a tube. The properties of nanotubes depend on atomic arrangement, the diameter and length of the tubes, and the morphology, or nano structure. Nanotubes exist as either single-walled (SWCNT) or multi-walled (MWCNT) structures. The MWCNTs are composed of concentric SWCNTs [202]. 

The primary synthesis methods for single and multi-walled carbon nanotubes include arc-discharge [203, 204], lase ablation [205], gas-phase catalytic growth from carbon monoxide [206], and chemical vapor deposition (CVD) from hydrocarbons [207-209], The scale-up limitation of arc discharge and laser ablation methods would make them cost prohibitive. One unique aspect of CVD technique is its ability to synthesize aligned arrays of carbon nanotubes with controlled diameter and length. The details on these methods go beyond the scope of this chapter. 

The exceptional mechanical and physical properties of carbon nanotubes along with low density make this new form of carbon an excellent candidate for composite reinforcement. The understanding thermo-mechanical properties of nanotube-based composite require knowledge of the elastic, fracture, and interface interaction of nanotube. 

According to wide range of properties of carbon fibers, they are suited to different applications. The properties of some commercially available fibers are given in Table 9.10. 

Vm Fig. 28.22 (a) Vectors on a graphene sheet that define achiral zigzag and armchair carbon nanotubes. The angle 0 is defined as being 0° for the zigzag structure. The bold lines define the shape of the open ends of the tube, (b) An example of an armchair carbon nanotube, (c) An example of a zigzag carbon nanotube. 

Delft University of Technology Science Photo Library 

Tethering ordered assemblies of carbon nanotubes to surfaces is an important step towards producing materials suitable for applications in microelectronic devices. Thiol derivatives can be anchored to gold surfaces, and derivatives of the type formed in reaction 28.28 are suitable for this purpose (structure 28.9). 

The following terms have been introduced in this chapter. Do you know what they mean  

Ladd (1994) Chemical Bonding in Solids and Fluids, Ellis Horwood, Chichester. 

Chemical modification of CNT by covalent bonding is one of the important methods for improving their surface characteristics. Because of the extended Jt-network of the sp -hybridised nanotubes, CNT have a tendency for covalent attachment, which 

Considering the low physical characteristics of biopolymers, fillers are recommended for the reinforcement of their electrical, mechanical and thermal properties. Following the discovery of CNT, much work has been done regarding their application as fillers in other polymers, for improving the properties of the matrix polymer. At first CNT were used as a filler in epoxy resin, by the alignment method. Later on, numerous studies have focused on CNT as excellent substitutes for conventional nanofillers in nanocomposites and recently, many polymers and biopolymers have been reinforced by CNT. As already mentioned, these nanocomposites have remarkable characteristics, compared to the bulk materials, due to their imique properties. 

Several parameters affect the mechanical properties of the composites, including proper dispersion and a large aspect ratio of the filler, interfacial stress transfer, a good alignment of reinforcement, and solvent selection. 

The uniformity and stability of nanotube dispersion in polymer matrices are most important parameters for the performance of composites. A good dispersion leads to efficient load transfer concentration centres in composites and to uniform stress distribution. Many scientists have reviewed the dispersion and functionalisation techniques of CNT for polymer-based nanocomposites, as well as their effects on the properties of CNT/polymer nanocomposites. They demonstrated that the control of these two factors led to uniform dispersion. Overall, the results showed that a proper dispersion enhanced a variety of mechanical properties of nanocomposites. 

Researchers examined the effect of solvent selection on the mechanical properties of CNT/polymer composites fabricated from double-walled nanotubes and polyvinyl alcohol in different solvents. It was shown that solvent selection can have a dramatic effect on the mechanical properties of CNT/polymer composites. Also, a critical 

One particularly interesting, and likewise promising, market with unprecedented growth rates appears to exist for carbon nanotubes (CNTs), the global demand for which in electronic, automotive and aerospace and defense applications is summarized in Table 6.7. 

CNTs have demonstrated major potential in both micro- and nanoelectronic components, in lifestyle products and sports equipment, as electrode materials for batteries, as supercapacitors, as fuel cells and novel actuator devices as well as electronic displays and, in particular, as novel drug delivery systems (McWilliams, 2006 Brand ct al., 2007). Unfortunately, however, the large-scale biomedical application of CNTs is still pending as serious doubts must first be resolved in relation to their potential biotoxicity. 

According to a study conducted by Oliver (2007), the market volume of CNTs in the US jumped from US 51 million in 2006 to US 79 million in 2007. An AAGR of 74%, which was even more impressive than that predicted by Freedonia Custom Research (2006) (see Table 6.7), should result in a market volume of US 807 million in 2011. The application fields of CNTs include energy storage (batteries), electronics, and composite materials, with the latter role being expected to account for 80% of global CNT production. 

FIGURE 3.58 Water adsorption-desorption isotherms (289 K) for SWCNT and AC measured with HNMR AC. (Taken from Chem. Phys. Lett., 421, Mao, S., Kleinhammes, A., and Wu, Y., NMR study of water adsorption in single-walled carbon nanotubes, 513-517, 2006. Copyright 2006, with permission from Elsevier.) 

Nuclear Magnetic Resonance Studies of Interfacial Phenomena 

FIGURE 3.59 NMR spectra of open (solid lines) and closed (dashed lines) CNT at different tempera- 

An increase in the water content from 70 to 280 mg/g the free surface energy value increases by a factor of 3.2 (Table 3.10, Ys). Consequently, the area of contacts between water and MWCNT surface 

FIGURE 3.61 (a) Temperature dependences of the amounts of unfrozen water, bound to MWCNTs 

Since it was initially reported [21], several methods have been presented in order to attach DNA onto CNTs, including adsorption. First, transmission electron microscopy showed that the DNA molecules tended to cover the surface of the nanotubes evenly, suggesting a strong interaction with the carbon surface [24]. 

The regular system of hydrogen bonds in DNA is destroyed in DNA/NaOH solution and the DNA molecule is partly transformed from a double spiral to a chaotic ball [118]. This transformation may promote the interaction of DNA molecules with CNTs. The ssDNA adsorption on CNTs was greater than for dsDNA molecules [117,118], suggesting that the adsorption of DNA on CNT is presumably via hydrophobic interactions between the nanotubes and the hydrophobic bases on DNA. 

Since 1976, studies on the thermal treatment of hydrocarbons (CVD) allowed researchers to isolate and observe long fibers and filaments (hollow fibers), carbon of several micrometers in length, but with very small diameter. However, this discovery made jointly by French and Japanese researchers [26] had a limited impact because the crystalline structure of these objects was inaccessible. In the early 90s, another Japanese team from NEC Labs proposed the helical structure of carbon nanocylinders (Fig. 5. If), which was quickly confirmed by several teams around the world [1]. In 1993, tubes with almost no thickness (formed from a single sheet of graphene) were synthesized, isolated and analyzed [27]. These were named single-wall carbon nanotubes (SWNT) because of their nanometer diameter. 

Currently, numerous other structures related to CNTs have been cataloged, including  

Ideal CNTs can be conceptualized as a graphene sheet rolled to form a cylinder with a few nanometers of diameter, micrometers and even inches [32] of length, while the ends are closed by half-fullerenes on each side (Fig. 5.2). 

Depending on the amount of rolled sheets, the CNTs can be divided into two categories those with only one wall, called single wall (SWNT—Single-Wall Nanotubes) or multi-wall (MWNT— Multi-WaU Carbon Nanotnbes). The latter is the result of several coaxial cylinders with a distance similar to the distance between the walls of graphene sheets that make np graphite (0.335 nm [34]).The number of sheets or walls of MWNTs can vary widely, from two (also eaUed DWNT, Double-Wall Carbon Nanotubes) to dozens of walls [35, 36]. MWNTs are usually isolated in synthesis products, while SWNTs are usually produced in the form of bundles of nanotubes [37]. 

The way in which these graphene sheets are rolled determines the atomic structure of the CNT, which is described in terms of chirality (helicity) of the tube, defined by the chiral vector Ch and chiral angle 6, Fig. 5.3a. The numbers (n, m) are integers and ai and a2 are the unit vectors of the hexagonal lattice of the graphene sheet. 

CNTs are metallic or semiconducting based on the exact way the CNTs are wrapped. Multi-waUed CNTs are metallic in most cases as there are so many layers and the probability is very high of having one wrapped layer in the group to being metallic. 

CNTs find applications in the areas such as micro electronics, field emission displays. X-ray sources and gas sensors. Single waUed and multi-waUed CNTs can be grown using high pressure arcs, laser ablation and chemical vapour deposition. 

Where V the voltage is applied across the electrodes and d is the gap spacing between the electrodes. If one of the electrodes is replaced by a sharp protrusion or a carbon nanotube, then tiis interpreted as the minimum distance between the electrodes. So the local field (within 1-2 nm of the surface atoms) is much higher than the applied field. So the field is very high close to the tip and causes field emission of electrons above a threshold value. The shape of the electric field that spawns from the carbon nanotube in vacuum is also found to be near Gaussian [21]. 

St e 3 gregated pores formed among isolated or bundled CNTs 

At first, we will expound on SWNTs. Purified SWNTs show a reversible saturation composition of Lii.yCe, simulation results show that possibility of Li intercalation in SWNT bundles can reach a high saturation density of LisCe, higher than the LiCe ideal value for graphite and reported MWNTs. During the Li intercalation process, the Li fills the SWNT bundle continuously and randomly, thus there are no staging phenomena, which are opposed to that of graphite.  

The encapsulations of organic molecules, like 9,10-dichloro-anthracene, jS-carotene, and coronene was found to be effective to increase the reversible storage capacity. Especially for the SWNT with coronene, the reversible capacity is 736 mAh/g, which is about 2.5 times greater than that of original empty tube. This is due to the steric hindrance of the organic molecules in the tube. The electrol3 e molecules and solvated Li ions cannot enter the tube. 

The desolvation of the solvated Li ions occur at the entrance of the tubes, while the organic molecules offer accommodation of Li ions. However, the cycling performance and coulombic efficiency still need to be improved for practical application.  

As to MWNTs, they were investigated as an anode material for reversible Li-ion intercalation as early as 1998. Lots of methods were adopted for preparation of MWNTs, and different MWNTs show various electrochemical performance. Compared to SWNTs, the irreversible capacity of MWNTs is quite large. A typical example 

Comparison of (a) aligned CNT electrode array and (b) entangled CNT matrix. 

CNT s are fabricated by a number of different methods including arc discharge, CVD, high pressure carbon monoxide (HiPco), and laser ablation. Laser ablation mainly produces SWNTs but is more expensive than the CVD and arc discharge techniques [51]. Alternatively, CVD methodology allows 

TEM image of bare MWNT used for high power supercapacitor. Low surface area is caused by tight interspacing between graphitic carbon walls. Source Zhou, R. et al. 2010. Nanotechnology, 21, 345701. With permission.) 

All CNT production strategies are expensive due to the high energy processes, extensive purification, scalability issues, and size control required for commercialization. The high cost, especially for SWNTs, is a limiting factor in their adoption into ESs. Another major limitation to the adoption of CNTs in ESs is the low surface area, less than 500 m. g i compared to areas of AC devices. Commercial HiPco production generates exclusively SWNTs that attain BET surface areas as high as 800 m. g-i [52]. This surface area is still far below those of AC materials. 

Bundled CNT s prevent electrolyte access to all but the outermost tubes, significantly reducing the active surface areas of the materials [52]. To optimize accessible area, steps must be taken to separate bundles by sonication and by additions of stabilizers or dispersants [53]. Similar to ACs, electrochemical oxidation with KOH can be performed to increase the surface areas and the capacitances of CNTs [52]. The oxidation increases area by uncapping the nanotubes and exposing more internal surface area. Too much oxidation or large concentrations of dispersants can negatively alter CNT performance, so care must be taken to optimize the properties. 

Due to their unique properties, CNTs have attracted considerable attention since their discovery in 1991 by lijima (50). Their high electrical conductivity, great mechanical strength, and low percolation threshold values make them attractive as additive materials for precipitates in lithium-ion batteries (51). 

Recently, high performance LiMno.8Feo.2PO4 was prepared with both etched and functionalized multi-walled CNTs and ketjenblacks (52). Ketjenblack is an electroconductive carbon black from AKZO Chemicals B.V. Corp. In detail, the preparation was done as follows (52)  

Preparation 2-5 The materials were synthesized using a chemical vapor deposition process that resulted in tubes with an average diameter of 10 nm, a length of 10 jim, and a purity of 95%. The functionalization with 

The multi-walled CNTs functionalized with carboxylic groups exhibited a better affinity toward cathode materials than pristine-based nanotubes. Also, the electrochemical performance of the LiMno.8Feo.2PO4 cathode materials was improved by using multi-walled CNTs shortened by vigorous mechanical mixing, in comparison to pristine long multi-walled CNTs samples. 

Moreover, the use of multi-walled CNTs together with Ketjen-blacks showed better electrochemical performance than when Ket-jenblacks or multi-walled CNTs were used separately (52). Ket-jenblacks are electroconductive carbon black powders with high surface areas, available from AkzoNobel. 

Until recently, MWCNTs had not reached the same performances as the SGCNTs. Conunercial MWCNTs have a typical capacity close to 250 mAh g. After puiificatimi, the capacity raises to a 400 mAh g . However, chemically drilled MWCNT (DMWCNT) has been prepared recently by solid state process [126]. 

In this work, cobalt oxide particles have been deposited on the surface of the nanotubes, and have been removed after an oxidation process, leading to 4 nm-sized holes. The reversible capacity of the DMWCNT cycled between 0.02 and 3.0 V exceeded 600 mAh g remaining constant over the 20 cycles were the material has been tested. An improvement has been achieved for an interface-controlled MWCNT structure, synthesized through a two-step process of catalyst deposition and chemical vapor deposition (CVD) and directly grown on a copper current collector [127], 

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Metal-carbide clusters are relevant to the fonnation of both endohedral fullerenes and carbon nanotubes [1351. There also exists a class of apparently stable metal-carbide cluster ions, = Ti, V, Cr, Zr and Hf), called... 

Flafner J FI, Bronikowski M J, Azamian B R, Nikolaev P, Rinzier A G, Colbert A T, Smith K A and Smalley R E 1998 Catalytic growth of single-wall carbon nanotubes from metal particles Chem. Phys. Lett. 296 195... 

Ebbesen T W 1997 Carbon Nanotubes—Preparation and Properties (Boca Raton, FL Chemical Rubber Company)... 

In this work, simple (single-use) biosensors with a layer double stranded (ds) calf thymus DNA attached to the surface of screen-printed carbon electrode assembly have been prepared. The sensor efficiency was significantly improved using nanostructured films like carbon nanotubes, hydroxyapatite and montmorillonite in the polyvinylalcohol matrix. 

There are many applications for diamonds and related materials, e.g., diamondlike carbon films, and there are potential applications for Fullerenes and carbon nanotubes that have not yet been realised. However, the great majority of engineering carbons, including most of those described in this book, have graphitic microstructures or disordered graphitic microstructures. Also, most engineering carbon materials are derived firom organic precursors by heat-treatment in inert atmospheres (carbonisation). A selection of technically-... 

Drcssclhaus, M.S., Dresselhaus, G. and Eklund, P.C., Science of Fullerenes and Carbon Nanotubes, 1996, Academic Press, San Diego. 

Carbon Nanotubes Preparation and Properties, ed. T.W. Ebbesen, 1997, CRC Press, Boca Raton. 

The structure-property relations of fullerenes, fullerene-derived solids, and carbon nanotubes are reviewed in the context of advanced technologies for carbon-hased materials. The synthesis, structure and electronic properties of fullerene solids are then considered, and modifications to their structure and properties through doping with various charge transfer agents are reviewed. Brief comments are included on potential applications of this unique family of new materials. 

Regarding a historical perspective on carbon nanotubes, very small diameter (less than 10 nm) carbon filaments were observed in the 1970 s through synthesis of vapor grown carbon fibers prepared by the decomposition of benzene at 1100°C in the presence of Fe catalyst particles of 10 nm diameter [11, 12]. However, no detailed systematic studies of such very thin filaments were reported in these early years, and it was not until lijima s observation of carbon nanotubes by high resolution transmission electron microscopy (HRTEM) that the carbon nanotube field was seriously launched. A direct stimulus to the systematic study of carbon filaments of very small diameters came from the discovery of fullerenes by Kroto, Smalley, and coworkers [1], The realization that the terminations of the carbon nanotubes were fullerene-like caps or hemispheres explained why the smallest diameter carbon nanotube observed would be the same as the diameter of the Ceo molecule, though theoretical predictions suggest that nanotubes arc more stable than fullerenes of the same radius [13]. The lijima observation heralded the entry of many scientists into the field of carbon nanotubes, stimulated especially by the un-... 

Fig. 14. High resolution TEM observations of three multi-wall carbon nanotubes with N concentric carbon nanotubes with various outer diameters do (a) N = 5, do = 6.7 nm, (b) N = 2, do = 5.5 nm, and (c) N = 7, do = 6.5 nm. The inner diameter of (c) is d = 2.3 nm. Each cylindrical shell is described by its own diameter and chiral angle [151].

Fig. 17. Schematic models for a single-wall carbon nanotubes with the nanotube axis normal to (a) the 6 = 30° direction (an armchair (n, n) nanotube), (b) the 0 = 0°...

Because of the speeial atomie arrangement of the earbon atoms in a carbon nanotube, substitutional impurities are inhibited by the small size of the carbon atoms. Furthermore, the serew axis disloeation, the most eommon defeet found in bulk graphite, is inhibited by the monolayer strueture of the Cfj() nanotube. For these reasons, we expeet relatively few substitutional or struetural impurities in single-wall earbon nanotubes. Multi-wall carbon nanotubes frequently show bamboo-like defects associated with the termination of inner shells, and pentagon-heptagon (5 - 7) defects are also found frequently [7]. 

Structurally, carbon nanotubes of small diameter are examples of a onedimensional periodic structure along the nanotube axis. In single wall carbon nanotubes, confinement of the stnreture in the radial direction is provided by the monolayer thickness of the nanotube in the radial direction. Circumferentially, the periodic boundary condition applies to the enlarged unit cell that is formed in real space. The application of this periodic boundary condition to the graphene electronic states leads to the prediction of a remarkable electronic structure for carbon nanotubes of small diameter. We first present... 

The ID electronic energy bands for carbon nanotubes [170, 171, 172, 173, 174] are related to bands calculated for the 2D graphene honeycomb sheet used to form the nanotube. These calculations show that about 1/3 of the nanotubes are metallic and 2/3 are semiconducting, depending on the nanotube diameter di and chiral angle 6. It can be shown that metallic conduction in a (n, m) carbon nanotube is achieved when... 

Fig. 19. The energy gap FJ, for a general chiral single-wall carbon nanotube as a function of 100 kidt, where dt is the nanotube diameter in A [179].

Closely related to the ID dispersion relations for the carbon nanotubes is the ID density of states shown in Fig. 20 for (a) a semiconducting (10,0) zigzag carbon nanotube, and (b) a metallic (9,0) zigzag carbon nanotube. The results show that the metallic nanotubes have a small, but non-vanishing 1D density of states, whereas for a 2D graphene sheet (dashed curve) the density of states... 

Experimental measurements to test the remarkable theoretical predictions of the electronic structure of carbon nanotubes are difficult to carry out because... 

Early transport measurements on individual multi-wall nanotubes [187] were carried out on nanotubes with too large an outer diameter to be sensitive to ID quantum effects. Furthermore, contributions from the inner constituent shells which may not make electrical contact with the current source complicate the interpretation of the transport results, and in some cases the measurements were not made at low enough temperatures to be sensitive to 1D effects. Early transport measurements on multiple ropes (arrays) of single-wall armchair carbon nanotubes [188], addressed general issues such as the temperature dependence of the resistivity of nanotube bundles, each containing many single-wall nanotubes with a distribution of diameters d/ and chiral angles 6. Their results confirmed the theoretical prediction that many of the individual nanotubes are metallic. 

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