file:///Untitled Topics in Applied Physics 80 ISSN: 0303-4216 (printed version) ISSN: 1437-0859 (electronic version) ISBN: 3-540-41086-4 Carbon Nanotubes Editors: M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris M. S. Dresselhaus, G. Dresselhaus, Ph. Avouris (2001) Topics Appl. Phys. 80: pp. V – VIII Foreword and Preface Mildred S. Dresselhaus and Phaedon Avouris (2001) TAP 80: pp. 1 – 9 Introduction to Carbon Materials Research Mildred S. Dresselhaus and Morinobu Endo (2001) TAP 80: pp. 11 – 28 Relation of Carbon Nanotubes to Other Carbon Materials Hongjie Dai (2001) TAP 80: pp. 29 – 53 Nanotube Growth and Characterization Jean-Christophe Charlier and Sumio Iijima (2001) TAP 80: pp. 55 – 81 Growth Mechanisms of Carbon Nanotubes Reshef Tenne and Alex K. Zettl 2001) TAP 80: pp. 81 – 112 Nanotubes from Inorganic Materials Steven G. Louie (2001) TAP 80: pp. 113 – 145 Electronic Properties, Junctions, and Defects of Carbon Nanotubes Zhen Yao, Cees Dekker, and Phaedon Avouris (2001) TAP 80: pp. 147 – 171 Electrical Transport Through Single-Wall Carbon Nanotubes Teri Wang Odom, Jason H. Hafner, and Charles M. Lieber (2001) TAP 80: pp. 173 – 211 Scanning Probe Microscopy Studies of Carbon Nanotubes Riichiro Saito and Hiromichi Kataura (2001) TAP 80: pp. 213 – 247 Optical Properties and Raman Spectroscopy of Carbon Nanotubes Jörg H. Fink and Philippe Lambin (2001) TAP 80: pp. 247 – 272 Electron Spectroscopy Studies of Carbon Nanotubes James Hone (2001) TAP 80: pp. 273 – 286 Phonons and Thermal Properties of Carbon Nanotubes Boris I. Yakobson and Phaedon Avouris (2001) TAP 80: pp. 287 – 327 Mechanical Properties of Carbon Nanotubes László Forró and Christian Schönenberger (2001) TAP 80: pp. 329 – 391 Physical Properties of Multi-wall Nanotubes Pulickel M. Ajayan and Otto Z. Zhou (2001) TAP 80: pp. 391 – 425 Applications of Carbon Nanotubes1 of 1 14/09/2003 19:04
Forewordby R. E. Smalley, Chemistry Nobel Lauveate 1996Since the discovery of the fullerenes in 1985 my research group and I havehad the privilege of watching from a central location the worldwide scientiﬁccommunity at work in one of its most creative, penetrating, self-correcting,and divergent periods of the last century. In his recent book, “The Transpar-ent Society”, David Brin discusses the virtues of an open, information richsociety in which individuals are free to knowledgeably criticize each other,to compete, to test themselves and their ideas in a free market place, andthereby help evolve a higher level of the social organism. He points out thatmodern science has long functioned in this mode, and argues that this opencriticism and appeal to experiment has been the keystone of its success. Thisnew volume, Carbon Nanotubes, is a wonderful example of this process atwork. Here you will ﬁnd a summary of the current state of knowledge in thisexplosively growing ﬁeld. You will see a level of creativity, breadth and depthof understanding that I feel conﬁdent is beyond the capability of any singlehuman brain to achieve in a lifetime of thought and experiment. But manyﬁne brains working independently in the open society of science have done itvery well indeed, and in a very short time. While the level of understanding contained in this volume is immense, itis clear to most of us working in this ﬁeld that we have only just begun. Thepotential is vast. Here we have what is almost certainly the strongest, stiﬀest,toughest molecule that can ever be produced, the best possible molecularconductor of both heat and electricity. In one sense the carbon (fullerene)nanotube is a new man-made polymer to follow on from nylon, polypropylene,and Kevlar. In another, it is a new “graphite” ﬁber, but now with the ultimatepossible strength. In yet another, it is a new species in organic chemistry, andpotentially in molecular biology as well, a carbon molecule with the almostalien property of electrical conductivity, and super-steel strength.
VI Foreword Can it be produced in megatons? Can it be spun into continuous ﬁbers? Can it grown in organized arrays or as a perfect single crystal? Can it be sorted by diameter and chirality? Can a single tube be cloned? Can it be grown enzymatically? Can it be assembled by the molecular machinery of living cells? Can it be used to make nanoelectronic devices, nanomechanical memories, nano machines, . . .? Can it be used to wire a brain?There is no way of telling at this point. Certainly for many researchers, thebest, most exciting days of discovery in this ﬁeld are still ahead. For therest of us, it will be entertaining just to sit back and watch the worldwideorganism of science at work. Hold on to your seats! Watch the future unfold.Houston, TexasJanuary 2001 Richard E. Smalley
PrefaceCarbon nanotubes are unique nanostructures with remarkable electronic andmechanical properties, some stemming from the close relation between car-bon nanotubes and graphite, and some from their one-dimensional aspects.Initially, carbon nanotubes aroused great interest in the research communitybecause of their exotic electronic structure. As other intriguing propertieshave been discovered, such as their remarkable electronic transport proper-ties, their unique Raman spectra, and their unusual mechanical properties,interest has grown in their potential use in nanometer-sized electronics andin a variety of other applications, as discussed in this volume. An ideal nanotube can be considered as a hexagonal network of carbonatoms that has been rolled up to make a seamless hollow cylinder. These hol-low cylinders can be tens of micrometers long, but with diameters as small as0.7 nm, and with each end of the long cylinder “capped with half a fullerenemolecule, i.e., 6 pentagons”. Single-wall nanotubes, having a cylindrical shellwith only one atom in thickness, can be considered as the fundamental struc-tural unit. Such structural units form the building blocks of both multi-wallnanotubes, containing multiple coaxial cylinders of ever-increasing diameterabout a common axis, and nanotube ropes, consisting of ordered arrays ofsingle-wall nanotubes arranged on a triangular lattice. The ﬁrst reported observation of carbon nanotubes was by Iijima in 1991for multi-wall nanotubes. It took, however, less than two years before single-wall carbon nanotubes were discovered experimentally by Iijima at the NECResearch Laboratory in Japan and by Bethune at the IBM Almaden Labora-tory in California. These experimental discoveries and the theoretical work,which predicted many remarkable properties for carbon nanotubes, launchedthis ﬁeld and propelled it forward. The ﬁeld has been advancing at a breath-taking pace ever since with many unexpected discoveries. These exciting de-velopments encouraged the editors to solicit articles for this book on thetopic of carbon nanotubes while the ﬁeld was in a highly active phase ofdevelopment. This book is organized to provide a snapshot of the present status of thisrapidly moving ﬁeld. After the introduction in Chap. 1, which provides somehistorical background and a brief summary of some basic subject matterand deﬁnitions, the connection between carbon nanotubes and other carbonmaterials is reviewed in Chap. 2. Recent developments in the synthesis and
VIII Prefacepuriﬁcation of single-wall and multi-wall carbon nanotubes are discussed inChap. 3. This is followed in Chap. 4 by a review of our present limited un-derstanding of the growth mechanism of single-wall and multi-wall carbonnanotubes. Chapter 5 demonstrates the generality of tubular nanostructuresby discussing general principles for tubule growth, and providing the readerwith numerous examples of inorganic nanotube formation. The unique elec-tronic structure and properties of perfect and defective carbon nanotubes arereviewed from a theoretical standpoint in Chap. 6. The electrical properties,transport, and magneto-transport measurements on single-wall nanotubesand ropes, as well as simple device structures based on carbon nanotubes arepresented in Chap. 7. Scanning tunneling microscopy is used to study thatnanotube electronic structure and spectra. The use of nanotubes as atomicforce microscope tips for ultra-high resolution and chemically sensitive imag-ing is also discussed in Chap. 8. The application of optical spectroscopy tonanotubes is presented in Chap. 9. In this chapter, the discussion of the opti-cal properties focuses on the electronic structure, the phonon structure, andthe coupling between electrons and phonons in observations of resonance Ra-man scattering and related phenomena. The contribution made by electronspectroscopies to the characterization of the electronic structure of the nano-tubes is discussed in Chap. 10, in comparison with similar studies devoted tographite and C60 . This is followed in Chap. 11 by a brief review of the phononand thermal properties, with emphasis given to studies of the speciﬁc heatand the thermal conductivity, which are both sensitive to the low-dimensionalaspects of carbon nanotubes. Chapter 12 discusses experiments and theoryon the mechanical properties of carbon nanotubes. Linear elastic parameters,non-linear instabilities, yield strength, fracture and supra-molecular interac-tions are all reviewed. Chapter 13 discusses transport measurements, magne-totransport properties, electron spin resonance, and a variety of other exoticproperties of multiwall nanotubes. The volume concludes in Chap. 14 witha brief review of the present early status of potential applications of carbonnanotubes. Because of the relative simplicity of carbon nanotubes, we expect them toplay an important role in the current rapid expansion of fundamental studieson nanostructures and their potential use in nanotechnology. This simplicityallows us to develop detailed theoretical models for predicting new phenom-ena connected with these tiny, one-dimensional systems, and then look forthese phenomena experimentally. Likewise, new experimental eﬀects, whichhave been discovered at an amazingly rapid rate, have provided stimulus forfurther theoretical developments, many of which are expected to be broadlyapplicable to nanostructures and nanotechnology research and development.Cambridge, Massachusetts Mildred S. DresselhausYorktown Heights, New York Gene DresselhausJanuary 2001 Phaedon Avouris
Relation of Carbon Nanotubes to OtherCarbon MaterialsMildred S. Dresselhaus1 and Morinobu Endo21 Department of Electrical Engineering and Computer Science and Department of Physics MIT, Cambridge, MA 02139, USA firstname.lastname@example.org Faculty of Engineering Department of Electrical and Electronic Engineering Shinshu University, Nagano-shi, 380 Japan email@example.comAbstract. A review of the close connection between the structure and propertiesof carbon nanotubes and those of graphite and its related materials is presentedin order to gain new insights into the exceptional properties of carbon nanotubes.The two dominant types of bonding (sp2 and sp3 ) that occur in carbon materi-als and carbon nanotubes are reviewed, along with the structure and properties ofcarbon materials closely related to carbon nanotubes, such as graphite, graphitewhiskers, and carbon ﬁbers. The analogy is made between the control of the prop-erties of graphite through the intercalation of donor and acceptor species with thecorresponding doping of carbon nanotubes.Carbon nanotubes are strongly related to other forms of carbon, especially tocrystalline 3D graphite, and to its constituent 2D layers (where an individualcarbon layer in the honeycomb graphite lattice is called a graphene layer). Inthis chapter, several forms of carbon materials are reviewed, with particularreference to their relevance to carbon nanotubes. Their similarities and dif-ferences relative to carbon nanotubes with regard to structure and propertiesare emphasized. The bonding between carbon atoms in the sp2 and sp3 conﬁgurationsis discussed in Sect. 1. Connections are made between the nanotube curva-ture and the introduction of some sp3 bonding to the sp2 planar bonding ofthe graphene sheet. The unusual properties of carbon nanotubes are derivedfrom the unusual properties of sp2 graphite by imposing additional quan-tum conﬁnement and topological constraints in the circumferential directionof the nanotube. The structure and properties of graphite are discussed inSect. 2, because of their close connection to the structure and properties ofcarbon nanotubes, which is followed by reviews of graphite whiskers and car-bon ﬁbers in Sect. 3 and Sect. 4, respectively. Particular emphasis is given tothe vapor grown carbon ﬁbers because of their especially close connection tocarbon nanotubes. The chapter concludes with brief reviews of liquid carbonM. S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.): Carbon Nanotubes,Topics Appl. Phys. 80, 11–28 (2001)c Springer-Verlag Berlin Heidelberg 2001
12 Mildred S. Dresselhaus and Morinobu Endoand graphite intercalation compounds in Sect. 5 and Sect. 6, respectively,relating donor and acceptor nanotubes to intercalated graphite.1 Bonding Between Carbon AtomsCarbon-based materials, clusters, and molecules are unique in many ways.One distinction relates to the many possible conﬁgurations of the electronicstates of a carbon atom, which is known as the hybridization of atomic or-bitals and relates to the bonding of a carbon atom to its nearest neighbors. Carbon is the sixth element of the periodic table and has the lowest atomicnumber of any element in column IV of the periodic table. Each carbon atomhas six electrons which occupy 1s2 , 2s2 , and 2p2 atomic orbitals. The 1s2orbital contains two strongly bound core electrons. Four more weakly boundelectrons occupy the 2s2 2p2 valence orbitals. In the crystalline phase, the va-lence electrons give rise to 2s, 2px , 2py , and 2pz orbitals which are importantin forming covalent bonds in carbon materials. Since the energy diﬀerencebetween the upper 2p energy levels and the lower 2s level in carbon is smallcompared with the binding energy of the chemical bonds, the electronic wavefunctions for these four electrons can readily mix with each other, therebychanging the occupation of the 2s and three 2p atomic orbitals so as to en-hance the binding energy of the C atom with its neighboring atoms. Thegeneral mixing of 2s and 2p atomic orbitals is called hybridization, whereasthe mixing of a single 2s electron with one, two, or three 2p electrons is calledspn hybridization with n = 1, 2, 3 [1,2]. Thus three possible hybridizations occur in carbon: sp, sp2 and sp3 , whileother group IV elements such as Si and Ge exhibit primarily sp3 hybridiza-tion. Carbon diﬀers from Si and Ge insofar as carbon does not have inneratomic orbitals, except for the spherical 1s orbitals, and the absence of nearbyinner orbitals facilitates hybridizations involving only valence s and p orbitalsfor carbon. The various bonding states are connected with certain structuralarrangements, so that sp bonding gives rise to chain structures, sp2 bondingto planar structures and sp3 bonding to tetrahedral structures. The carbon phase diagram (see Fig. 1) guided the historical synthesis ofdiamond in 1960 , and has continued to inspire interest in new forms ofcarbon, as they are discovered . Although we have learned much aboutcarbon since that time, much ignorance remains about the possible phases ofcarbon. While sp2 bonded graphite is the ground state phase of carbon underambient conditions, at higher temperatures and pressures, sp3 bonded cubicdiamond is stable. Other regions of the phase diagram show stability rangesfor hexagonal diamond, hexagonal carbynes [5,6,7], and liquid carbon . Itis believed that a variety of novel π-electron carbon bulk phases remain tobe discovered and explored. In addition to the bulk phases featured in the carbon phase diagram,much attention has recently focussed on small carbon clusters , since the
Relation of Carbon Nanotubes to Other Carbon Materials 13Fig. 1. A recent version of the phase diagram of carbon . Solid lines representequilibrium phase boundaries. A: commercial synthesis of diamond from graphiteby catalysis; B: rapid solid phase graphite to diamond synthesis; C: fast trans-formation of diamond to graphite; D: hexagonal graphite to hexagonal diamondsynthesis; E: shock compression graphite to hexagonal diamond synthesis; F: shockcompression graphite to cubic-type diamond synthesis; B, F, G: graphite or hexag-onal diamond to cubic diamond synthesis; H, I, J: compressed graphite acquiresdiamond-like properties, but reverts to graphite upon release of pressurediscovery of fullerenes in 1985 by Kroto et al.  and of carbon nanotubesin 1991 by Iijima . The physical reason why these nanostructures form isthat a graphene layer (deﬁned as a single 2D layer of 3D graphite) of ﬁnitesize has many edge atoms with dangling bonds,indexdangling bonds and thesedangling bonds correspond to high energy states. Therefore the total energy ofa small number of carbon atoms (30–100) is reduced by eliminating danglingbonds, even at the expense of increasing the strain energy, thereby promotingthe formation of closed cage clusters such as fullerenes and carbon nanotubes. The rolling of a single graphene layer, which is a hexagonal networkof carbon atoms, to form a carbon nanotube is reviewed in this volumein the introductory chapter , and in the chapters by Louie  andSaito/Kataura , where the two indices (n, m) that fully identify eachcarbon nanotube are speciﬁed [9,15]. Since nanotubes can be rolled froma graphene sheet in many ways [9,15], there are many possible orientationsof the hexagons on the nanotubes, even though the basic shape of the carbonnanotube wall is a cylinder. A carbon nanotube is a graphene sheet appropriately rolled into a cylinderof nanometer size diameter [13,14,15]. Therefore we can expect the planar sp2bonding that is characteristic of graphite to play a signiﬁcant role in carbonnanotubes. The curvature of the nanotubes admixes a small amount of sp3
14 Mildred S. Dresselhaus and Morinobu Endobonding so that the force constants (bonding) in the circumferential direc-tion are slightly weaker than along the nanotube axis. Since the single wallcarbon nanotube is only one atom thick and has a small number of atomsaround its circumference, only a few wave vectors are needed to describe theperiodicity of the nanotubes. These constraints lead to quantum conﬁnementof the wavefunctions in the radial and circumferential directions, with planewave motion occurring only along the nanotube axis corresponding to a largenumber or closely spaced allowed wave vectors. Thus, although carbon nano-tubes are closely related to a 2D graphene sheet, the tube curvature and thequantum conﬁnement in the circumferential direction lead to a host of prop-erties that are diﬀerent from those of a graphene sheet. Because of the closerelation between carbon nanotubes and graphite, we review brieﬂy the struc-ture and properties of graphite in this chapter. As explained in the chapterby Louie , (n, m) carbon nanotubes can be either metallic (n − m = 3q,q = 0, 1, 2, . . .) or semiconducting (n−m = 3q±1, q = 0, 1, 2, . . .), the individ-ual constituents of multi-wall nanotubes or single-wall nanotube bundles canbe metallic or semiconducting [13,15]. These remarkable electronic propertiesfollow from the electronic structure of 2D graphite under the constraints ofquantum conﬁnement in the circumferential direction . Actual carbon nanotube samples are usually found in one of two forms:(1) a Multi-Wall Carbon Nanotube (MWNT) consisting of a nested coaxialarray of single-wall nanotube constituents , separated from one anotherby approximately 0.34 nm, the interlayer distance of graphite (see Sect. 2),and (2) a single wall nanotube rope, which is a nanocrystal consisting of ∼10–100 Single-Wall Nanotubes (SWNTs), whose axes are aligned parallel to oneanother, and are arranged in a triangular lattice with a lattice constant thatis approximately equal to dt + ct , where dt is the nanotube diameter and ctis approximately equal to the interlayer lattice constant of graphite.2 GraphiteThe ideal crystal structure of graphite (see Fig. 2) consists of layers in whichthe carbon atoms are arranged in an open honeycomb network containingtwo atoms per unit cell in each layer, labeled A and B. The stacking of thegraphene layers is arranged, such that the A and A atoms on consecutivelayers are on top of one another, but the B atoms in one plane are over theunoccupied centers of the adjacent layers, and similarly for the B atoms onthe other plane . This gives rise to two distinct planes, which are labeledby A and B. These distinct planes are stacked in the ‘ABAB’ Bernal stackingarrangement shown in Fig. 2, with a very small in-plane nearest-neighbordistance aC−C of 1.421 ˚, an in-plane lattice constant a0 of 2.462 ˚, a c-axis A Alattice constant c0 of 6.708 ˚, and an interplanar distance of c0 /2 = 3.354 ˚. A A 4This crystal structure is consistent with the D6h (P 63 /mmc) space group andhas four carbon atoms per unit cell, as shown in Fig. 2.
Relation of Carbon Nanotubes to Other Carbon Materials 15(a) (b)Fig. 2. (a) The crystal structure of hexagonal single crystal graphite, in whichthe two distinct planes of carbon hexagons called A and B planes are stacked inan ABAB... sequence with P 63/mmc symmetry. The notation for the A and Bplanes is not to be confused with the two distinct atoms A and B on a singlegraphene plane (note a rhombohedral phase of graphite with ABCABC... stackingalso exists ). (b) An STM image showing the trigonal network of highly orientedpyrolytic graphite (HOPG) in which only one site of the carbon hexagonal networkappears, as for example, the B site, denoted by black balls in (a) Since the in-plane C-C bond is very strong and the nearest-neighbor spac-ing between carbon atoms in graphite is very small, the in-plane lattice con-stant is quite stable against external perturbations. The nearest neighborspacing between carbon nanotubes is essentially the same as the interplanar ˚spacing in graphite (∼3.4 A). One consequence of the small value of aC−C ingraphite is that impurity species are unlikely to enter the covalently bondedin-plane lattice sites substitutionally (except for boron), but rather occupysome interstitial position between the graphene layer planes which are bondedby a weak van der Waals force. These arguments also apply to carbon nano-tubes and explain why the substitutional doping of individual single wallcarbon nanotubes with species other than boron is diﬃcult. The weak inter-planar bonding of graphite allows entire planes of dopant atoms or moleculesto be intercalated between the carbon layers to form intercalation compounds.Also carbon nanotubes can adsorb dopant species on their external and in-ternal surfaces and in interstitial sites between adjacent nanotubes, as isdiscussed in Sect. 6. The graphene layers often do not stack perfectly and do not form theperfect graphite crystal structure with perfect Bernal ‘ABAB’ layer stack-ing. Instead, stacking faults are often formed (meaning departures from theABAB stacking order). These stacking faults give rise to a small increase inthe interlayer distance from the value 3.354 ˚ in 3D graphite until a value of Aabout 3.440 ˚ is reached, at which interplanar distance, the stacking of the Aindividual carbon layers become uncorrelated with essentially no site bond-
16 Mildred S. Dresselhaus and Morinobu Endoing between the carbon aatoms in the two layers. The resulting structure ofthese uncorrelated 2D graphene layers is called turbostratic graphite [1,18].Because of the diﬀerent diameters of adjacent cylinders of carbon atoms ina multiwall carbon nanotube [15,16], the structural arrangement of the ad-jacent carbon honeycomb cylinders is essentially uncorrelated with no sitecorrelation between carbon atoms on adjacent nanotubes. The stacking ar-rangement of the nanotubes is therefore similar in behavior to the graphenesheets in turbostratic graphite. Thus, perfect nanotube cylinders at a largespatial separation from one another should be able to slide past one anothereasily. Of signiﬁcance to the properties expected for carbon nanotubes is the factthat the electronic structure of turbostratic graphite, a zero gap semiconduc-tor, is qualitatively diﬀerent from that of ideal graphite, a semimetal witha small band overlap (0.04 eV). The electronic structure of a 2D graphenesheet  is discussed elsewhere in this volume , where it is shown thatthe valence and conduction bands of a graphene sheet are degenerate by sym-metry at the special point K at the 2D Brillouin zone corner where the Fermilevel in reciprocal space is located . Metallic carbon nanotubes have anallowed wavevector at the K-point and therefore are eﬀectively zero gap semi-conductors like a 2D graphene sheet. However, semiconducting nanotubes donot have an allowed wavevector at the K point (because of quantum conﬁne-ment conditions in the circumferential direction) [14,15], thus resulting in anelectronic band gap and semiconducting behavior, very diﬀerent from that ofa graphene sheet. Several sources of crystalline graphite are available, but diﬀer somewhatin their overall characteristics. Some discussion of this topic could be helpfulto readers since experimentalists frequently use these types of graphite sam-ples in making comparisons between the structure and properties of carbonnanotubes and sp2 graphite. Natural single-crystal graphite ﬂakes are usually small in size (typicallymuch less than 0.1 mm in thickness), and contain defects in the form of twin-ning planes and screw dislocations, and also contain chemical impurities suchas Fe and other transition metals, which make these graphite samples lessdesirable for certain scientiﬁc studies and applications. A synthetic single-crystal graphite, called “kish” graphite, is commonlyused in scientiﬁc investigations. Kish graphite crystals form on the surface ofhigh carbon content iron melts and are harvested as crystals from such hightemperature solutions . The kish graphite ﬂakes are often larger thanthe natural graphite ﬂakes, which makes kish graphite the material of choicewhen large single-crystal ﬂakes are needed for scientiﬁc studies. However,these ﬂakes may contain impurities. The most commonly used high-quality graphitic material today is HighlyOriented Pyrolytic Graphite (HOPG), which is prepared by the pyrolysis ofhydrocarbons at temperatures of above 2000◦C and the resulting pyrolytic
Relation of Carbon Nanotubes to Other Carbon Materials 17carbon is subsequently heat treated to higher temperatures to improve itscrystalline order [21,22]. When stress annealed above 3300◦C, the HOPG ex-hibits electronic, transport, thermal, and mechanical properties close to thoseof single-crystal graphite, showing a very high degree of c-axis alignment. Forthe high temperature, stress-annealed HOPG, the crystalline order extends toabout 1 µm within the basal plane and to about 0.1µm along the c-direction.This material is commonly used because of its good physical properties, highchemical purity and relatively large sample sizes. Thin-ﬁlm graphite materi-als, especially those based on Kapton and Novax (polyimide) precursors, arealso prepared by a pyrolysis/heat treatment method, and are often used.3 Graphite WhiskersA graphite whisker is a graphitic material formed by rolling a graphene sheetup into a scroll . Except for the early work by Bacon , there is lit-tle literature about graphite whiskers. Graphite whiskers are formed in a dcdischarge between carbon electrodes using 75–80 V and 70–76 A. In the arcapparatus, the diameter of the positive electrode is smaller than that of thenegative electrode, and the discharge is carried out in an inert gas using ahigh gas pressure (92 atmospheres). As a result of this discharge, cylindri-cal boules with a hard shell were formed on the negative electrode. Whenthese hard cylindrical boules were cracked open, scroll-like carbon whiskersup to ∼3 cm long and 1–5µm in diameter were found protruding from thefracture surfaces. The whiskers exhibited great crystalline perfection, highelectrical conductivity, and high elastic modulus along the ﬁber axis. Sincetheir discovery , graphite whiskers have provided the benchmark againstwhich the performance of carbon ﬁbers is measured. The growth of graphitewhiskers by the arc method has many similarities to the growth of carbonnanotubes , especially MWNTs which do not require the use of a catalyst,except that graphite whiskers were grown at a higher gas pressure than iscommonly used for nanotube growth. While MWNTs are generally found tobe concentric cylinders of much smaller outer diameter, some reports havebeen given of scroll-like structures with outer diameters less than 100 nm .4 Carbon FibersCarbon ﬁbers represent an important class of graphite-related materialswhich are closely connected to carbon nanotubes, with regard to structureand properties. Despite the many precursors that can be used to synthe-size carbon ﬁbers, each having diﬀerent cross-sectional morphologies (Fig. 3),the preferred orientation of the graphene planes is parallel to the ﬁber axisfor all carbon ﬁbers, thereby accounting for the high mechanical strengthof carbon ﬁbers . Referring to the various morphologies in Fig. 3, the as-prepared vapor-grown ﬁbers have an “onion skin” or “tree ring” morphology
18 Mildred S. Dresselhaus and Morinobu EndoFig. 3. Sketch illustrating the morphology of Vapor-Grown Carbon Fibers (VGCF):(a) as-deposited at 1100◦ C , (b) after heat treatment to 3000◦ C . The mor-phologies for commercial mesophase-pitch ﬁbers are shown in (c) for a “PAC-man”cross section with a radial arrangement of the straight graphene ribbons and a miss-ing wedge and (d) for a PAN-AM cross-sectional arrangement of graphene planes.In (e) a PAN ﬁber is shown, with a circumferential arrangement of ribbons in thesheath region and a random structure in the coreand after heat treatment to about 2500◦ C bear a close resemblance to carbonnanotubes (Fig. 3a). After further heat treatment to about 3000◦C, the outerregions of the vapor grown carbon ﬁbers form facets (Fig. 3b), and becomemore like graphite because of the strong interplanar correlations resultingfrom the facets [1,25]. At the hollow core of a vapor grown carbon ﬁber isa multiwall (and also a single wall) carbon nanotube (MWNT), as shown inFig. 4, where the MWNT is observed upon fracturing a vapor grown carbonﬁber . Of all carbon ﬁbers, the faceted vapor grown carbon ﬁbers (Fig. 3b)are closest to crystalline graphite in both crystal structure and properties. The commercially available mesophase pitch-based ﬁbers, are exploitedfor their extremely high bulk modulus and high thermal conductivity, whilethe commercial PAN (polyacrylonitrile) ﬁbers are widely used for their hightensile strength . The high modulus of the mesophase pitch ﬁbers is relatedto the high degree of c-axis orientation of adjacent graphene layers, while thehigh strength of the PAN ﬁbers is related to defects in the structure. Thesestructural defects inhibit the slippage of adjacent graphene planes relative toeach other, and inhibit the sword-in-sheath failure mode (Fig. 5) that dom-inates the rupture of a vapor grown carbon ﬁber . Typical diameters forindividual commercial carbon ﬁbers are ∼ 7µm, and they can be very long.These ﬁbers are woven into bundles called tows and are then wound up asa continuous yarn on a spool. The remarkable high strength and modulusof carbon ﬁbers (Fig. 6) are responsible for most of the commercial interest
Relation of Carbon Nanotubes to Other Carbon Materials 19 Fig. 4. (a) Carbon nanotube exposed on the breakage edge of a vapor grown car- (a) bon ﬁber as grown (a) and heat-treated at 3000◦ C (b). The sample is fractured by pulverization and the core diameter is ∼5 nm. (b) These photos suggest a struc- tural discontinuity between the nanotube core of the ﬁber and the outer carbon lay- ers deposited by chemical vapor deposition techniques. The photos show the strong mechanical properties of the nanotube core which maintain its form after breakage of (b) the periphery  Fig. 5. The sword-in-sheath fail- ure mode of heat treated vapor grown carbon ﬁbers. Such failure modes are also observed in multi- wall carbon nanotubes in these ﬁbers and these superior mechanical properties (modulus and ten-sile strength) should be compared to steel, for which typical strengths andbulk modulus values are 1.4 and 207 GPa, respectively . The excellent me-chanical properties of carbon nanotubes are closely related to the excellentmechanical properties of carbon ﬁbers, though notable diﬀerences in behaviorare also found, such as the ﬂexibility of single wall carbon nanotubes and thegood mechanical properties of multiwall nanotubes (with only a few walls)under compression, in contrast with carbon ﬁbers which fracture easily undercompressive stress along the ﬁber axis. Vapor-grown carbon ﬁbers can be prepared over a wide range of diame-ters (from ∼10 nm to more than 100µm) and these ﬁbers have central hollowcores. A distinction is made between vapor grown carbon ﬁbers and ﬁbers
20 Mildred S. Dresselhaus and Morinobu Endo Fig. 6. The breaking strength of various types of carbon ﬁbers plotted as a function of Young’s modulus. Lines of constant strain can be used to estimate the breaking strains [1,27,28]with diameters in the range 10–100 nm, which are called nanoﬁbers, and ex-hibit properties intermediate between those of typical vapor grown carbonﬁbers, on the one hand, and MWNTs, on the other . The preparation ofvapor grown carbon ﬁbers is based on the growth of a thin hollow tube ofabout 100 nm diameter (a nanoﬁber) by a catalytic process based on ultra-ﬁneparticles (∼10 nm diameter) which have been super-saturated with carbonfrom the pyrolysis of a hydrocarbon gas at ∼1050◦C [1,29]. The thickeningof the vapor-grown carbon ﬁber occurs through an epitaxial growth process,whereby the hydrocarbon gas is dehydrogenated at the ∼1050◦C growth tem-perature, and the carbon deposit is adsorbed on the surface of the growingﬁber. Subsequent heat treatment to ∼2500◦C anneals the disordered car-bon deposit and results in vapor grown carbon ﬁbers with a tree ring coaxialcylinder morphology . Further heat treatment to 2900◦C results in facetedﬁbers (Fig. 3b) which exhibit structural and electronic properties very closeto those of single crystal graphite [1,29]. If the growth process is stoppedbefore the thickening step starts, MWNTs are obtained . These vapor grown carbon ﬁbers show (h, k, l) X-ray diﬀraction lines in-dicative of the 3D graphite structure and a semimetallic band overlap andcarrier density similar to 3D graphite. The infrared and Raman spectra areessentially the same as that of 3D graphite. Vapor-grown carbon ﬁbers andnanoﬁbers with micrometer and several tens of nanometer diameters, respec-tively, provide intermediate materials between conventional mesophase pitch-derived carbon ﬁbers (see Fig. 3) and single wall carbon nanotubes. Since theirsmallest diameters (∼10 nm) are too large to observe signiﬁcant quantum con-ﬁnement eﬀects, it would be diﬃcult to observe band gaps in their electronicstructure, or the radial breathing mode in their phonon spectra. Yet subtlediﬀerences are expected between nanoﬁber transport properties, electronic
Relation of Carbon Nanotubes to Other Carbon Materials 21structure and Raman spectra relative to the corresponding phenomena ineither MWNTs of graphitic vapor grown carbon ﬁbers. At present, little isknown in detail about the structure and properties of nanoﬁbers, except thattheir properties are intermediate between those of vapor grown carbon ﬁbersand MWNTs.4.1 History of Carbon Fibers in Relation to Carbon NanotubesWe provide here a brief review of the history of carbon ﬁbers, the macroscopicanalog of carbon nanotubes, since carbon nanotubes have become the focusof recent developments in carbon ﬁbers. The early history of carbon ﬁbers was stimulated by needs for materialswith special properties, both in the 19th century and more recently afterWorld War II. The ﬁrst carbon ﬁber was prepared by Thomas A. Edisonto provide a ﬁlament for an early model of an electric light bulb. Speciallyselected Japanese Kyoto bamboo ﬁlaments were used to wind a spiral coilthat was then pyrolyzed to produce a coiled carbon resistor, which could beheated ohmically to provide a satisfactory ﬁlament for use in an early modelof an incandescent light bulb . Following this initial pioneering work byEdison, further research on carbon ﬁlaments proceeded more slowly, sincecarbon ﬁlaments were soon replaced by a more sturdy tungsten ﬁlament inthe electric light bulb. Nevertheless research on carbon ﬁbers and ﬁlamentsproceeded steadily over a long time frame, through the work of Sch¨tzenberger uand Sch¨tzenberger (1890) , Pelabon , and others. Their eﬀorts were umostly directed toward the study of vapor grown carbon ﬁlaments, showingﬁlament growth from the thermal decomposition of hydrocarbons. The second applications-driven stimulus to carbon ﬁber research camein the 1950’s from the needs of the space and aircraft industry for strong,stiﬀ light-weight ﬁbers that could be used for building lightweight compositematerials with superior mechanical properties. This stimulation led to greatadvances in the preparation of continuous carbon ﬁbers based on polymer pre-cursors, including rayon, polyacrylonitrile (PAN) and later mesophase pitch.The late 1950’s and 1960’s was a period of intense activity at the Union Car-bide Corporation, the Aerospace Corporation and many other laboratoriesworldwide. This stimulation also led to the growth of a carbon whisker (see Sect. 3), which has become a benchmark for the discussion of the mechan-ical and elastic properties of carbon ﬁbers. The growth of carbon whiskerswas also inspired by the successful growth of single crystal whisker ﬁlamentsat that time for many metals such as iron, non-metals such as Si, and oxidessuch as Al2 O3 , and by theoretical studies , showing superior mechanicalproperties for whisker structures . Parallel eﬀorts to develop new bulk syn-thetic carbon materials with properties approaching single crystal graphiteled to the development of highly oriented pyrolytic graphite (HOPG) in 1962by Ubbelohde and co-workers [36,37], and HOPG has since been used as oneof the benchmarks for the characterization of carbon ﬁbers.
22 Mildred S. Dresselhaus and Morinobu Endo While intense eﬀort continued toward perfecting synthetic ﬁlamentarycarbon materials, and great progress was indeed made in the early 1960’s, itwas soon realized that long term eﬀort would be needed to reduce ﬁber de-fects and to enhance structures resistive to crack propagation. New researchdirections were introduced because of the diﬃculty in improving the structureand microstructure of polymer-based carbon ﬁbers for high strength and highmodulus applications, and in developing graphitizable carbons for ultra-highmodulus ﬁbers. Because of the desire to synthesize more crystalline ﬁlamen-tous carbons under more controlled conditions, synthesis of carbon ﬁbers bya catalytic Chemical Vapor Deposition (CVD) process proceeded, laying thescientiﬁc basis for the mechanism and thermodynamics for the vapor phasegrowth of carbon ﬁbers in the 1960’s and early 1970’s . In parallel to thesescientiﬁc studies, other research studies focused on control of the processfor the synthesis of vapor grown carbon ﬁber [38,39,40,41], leading to themore recent commercialization of vapor grown carbon ﬁbers in the 1990’s forvarious applications. Concurrently, polymer-based carbon ﬁber research hascontinued worldwide, mostly in industry, with emphasis on greater control ofprocessing steps to achieve carbon ﬁbers with ever-increasing modulus andstrength, and on ﬁbers with special characteristics, such as very high thermalconductivity, while decreasing costs of the commercial products. As research on vapor grown carbon ﬁbers on the micrometer scale pro-ceeded, the growth of very small diameter ﬁlaments less than 10 nm (Fig. 7),was occasionally observed and reported [42,43], but no detailed systematicstudies of such thin ﬁlaments were carried out. An example of a very thin va-por grown nanoﬁber along with a multiwall nanotube is shown in the brightﬁeld TEM image of Fig. 7 [42,43,44,45]. Reports of thin ﬁlaments below 10 nm inspired Kubo  to ask whetherthere was a minimum dimension for such ﬁlaments. Early work [42,43] onvapor grown carbon ﬁbers, obtained by thickening ﬁlaments such as the ﬁberdenoted by VGCF (Vapor Grown Carbon Fiber) in Fig. 7, showed very sharplattice fringe images for the inner-most cylinders. Whereas the outermostlayers of the ﬁber have properties associated with vapor grown carbon ﬁbers, Fig. 7. High-resolution TEM micrograph showing carbon a vapor grown carbon nanoﬁber (VGCF) with an diameter less than 10 nm and a nanotube [42,43,44,45]
Relation of Carbon Nanotubes to Other Carbon Materials 23there may be a continuum of behavior of the tree rings as a function of diam-eter, with the innermost tree rings perhaps behaving like carbon nanotubes. Direct stimulus to study carbon ﬁlaments of very small diameters moresystematically  came from the discovery of fullerenes by Kroto and Smal-ley . In December 1990 at a carbon-carbon composites workshop, paperswere given on the status of fullerene research by Smalley , the discoveryof a new synthesis method for the eﬃcient production of fullerenes by Huﬀ-man , and a review of carbon ﬁber research by Dresselhaus . Discus-sions at the workshop stimulated Smalley to speculate about the existenceof carbon nanotubes of dimensions comparable to C60 . These conjectureswere later followed up in August 1991 by discussions at a fullerene workshopin Philadelphia  on the symmetry proposed for a hypothetical single-wallcarbon nanotubes capped at either end by fullerene hemispheres, with sugges-tions on how zone folding could be used to examine the electron and phonondispersion relations of such structures. However, the real breakthrough oncarbon nanotube research came with Iijima’s report of experimental observa-tion of carbon nanotubes using transmission electron microscopy . It wasthis work which bridged the gap between experimental observation and thetheoretical framework of carbon nanotubes in relation to fullerenes and astheoretical examples of 1D systems. Since the pioneering work of Iijima ,the study of carbon nanotubes has progressed rapidly.5 Liquid CarbonLiquid carbon refers to the liquid phase of carbon resulting from the meltingof pure carbon in a solid phase (graphite, diamond, carbon ﬁbers or a varietyof other carbons). The phase diagram for carbon shows that liquid carbon isstable at atmospheric pressure only at very high temperatures (the meltingpoint of graphite Tm ∼ 4450 K) . Since carbon has the highest meltingpoint of any elemental solid, to avoid contamination of the melt, the cruciblein which the carbon is melted must itself be made of carbon, and suﬃcientheat must be focused on the sample volume to produce the necessary temper-ature rise to achieve melting [8,51]. Liquid carbon has been produced in thelaboratory by the laser melting of graphite, exploiting the poor interplanarthermal conductivity of the graphite , and by resistive heating , thetechnique used to establish the metallic nature of liquid carbon. Although diamond and graphite may have diﬀerent melting temperatures,it is believed that the same liquid carbon is obtained upon melting either solidphase. It is likely that the melting of carbon nanotubes also forms liquidcarbon. Since the vaporization temperature for carbon (∼4700 K) is onlyslightly higher than the melting point (∼4450 K), the vapor pressure overliquid carbon is high. The high vapor pressure and the large carbon–carbonbonding energy make it energetically favorable for carbon clusters rather thanindependent atoms to be emitted from a molten carbon surface . Energetic
24 Mildred S. Dresselhaus and Morinobu Endoconsiderations suggest that some of the species emitted from a molten carbonsurface have masses comparable to those of fullerenes . The emission offullerenes from liquid carbon is consistent with the graphite laser ablationstudies of Kroto, Smalley, and co-workers . Resistivity measurements on a variety of vapor grown carbon ﬁbers pro-vided important information about liquid carbon. In these experiments ﬁberswere heated resistively by applying a single 28µs current pulse with currentsup to 20 A . The temperature of the ﬁber as a function of time was deter-mined from the energy supplied in the pulse and the measured heat capacityfor bulk graphite  over the temperature range up to the melting point,assuming that all the power dissipated in the current pulse was convertedinto thermal energy in the ﬁber. The results in Fig. 8 for well-graphitizedﬁbers (heat treatment temperatures THT = 2300◦C, 2800◦C) show an ap-proximately linear temperature dependence for the resistivity ρ(T ) up to themelting temperature, where ρ(T ) drops by nearly one order of magnitude,consistent with metallic conduction in liquid carbon (Fig. 8). This ﬁgure fur-ther shows that both pregraphitic vapor grown carbon ﬁbers (THT = 1700◦C,2100◦C), which are turbostratic and have small structural coherence lengths,and well graphitized ﬁbers, all show the same behavior in the liquid phase,although their measured ρ(T ) functional forms in the solid phase are verydiﬀerent. Measurements of the resistance of a carbon nanotube through themelting transition have not yet been carried out.Fig. 8. The electrical resistivity vs temperature for vapor grown carbon ﬁbers withvarious heat treatment temperatures (THT = 1700, 2100, 2300, 2800◦ C). The sharpdecrease in ρ(T ) above ∼4000 K is identiﬁed with the melting of the carbon ﬁbers.The measured electrical resistivity for liquid carbon is shown 
Relation of Carbon Nanotubes to Other Carbon Materials 256 Graphite Intercalation CompoundsBecause of the weak van der Waals interlayer forces associated with the sp2bonding in graphite, graphite intercalation compounds (GICs) may be formedby the insertion of layers of guest species between the layers of the graphitehost material [56,57], as shown schematically in Fig. 9. The guest speciesmay be either atomic or molecular. In the so-called donor GICs, electrons aretransferred from the donor intercalate species (such as a layer of the alkalimetal potassium) into the graphite layers, thereby raising the Fermi level EFin the graphitic electronic states, and increasing the mobile electron concen-tration by two or three orders of magnitude, while leaving the intercalatelayer positively charged with low mobility carriers. Conversely, for acceptorGICs, electrons are transferred to the intercalate species (which is usuallymolecular) from the graphite layers, thereby lowering the Fermi level EF inthe graphitic electronic states and creating an equal number of positivelycharged hole states in the graphitic π-band. Thus, electrical conduction inGICs (whether they are donors or acceptors) occurs predominantly in thegraphene layers and as a result of the charge transfer between the intercalateand host layers. The electrical conductivity between adjacent graphene layersis very poor. Among the GICs, Li-based GICs are widely commercialized in Fig. 9. Schematic model for a graphite intercala- tion compound showing the stacking of graphite layers (networks of hexagons on a sheet) and of intercalate (e.g., potassium) layers (networks of large hollow balls). For this stage 1 compound, each carbon layer is separated by an intercalate layer 
26 Mildred S. Dresselhaus and Morinobu EndoLi-ion secondary batteries for cell phones, personal computers, and electricvehicle batteries . Carbon nanotubes also provide a host material for intercalation of donors(e.g., alkali metals) or acceptors (e.g., halogens such as bromine and iodine).In the case of nanotubes, the guest species is believed to decorate the exte-rior of the single wall nanotubes, and also to enter the hollow cores of thenanotubes. A similar charge transfer process is observed for alkali metalsand halogens in SWNTs, based on measuring the downshifts and upshifts inthe characteristic Raman G-band mode frequencies, similar to the standardcharacterization techniques previously developed for GICs [9,56]. Also, thedistances of the dopants to the nearest-neighbor carbon atoms are similarfor doped carbon nanotubes and for GICs [9,56]. Practical applications ofintercalated carbon nanotubes are also expected in analogy with Li-GICs,especially for super high capacity batteries.References 1. M. S. Dresselhaus, G. Dresselhaus, K. Sugihara, I. L. Spain, H. A. Goldberg, Graphite Fibers and Filaments, Springer Ser. Mater. Sci., Vol. 5 (Springer, Berlin, Heidelberg 1988) 12, 16, 17, 18, 19, 20, 22 2. B. T. Kelly, Physics of Graphite (Applied Science, London 1981) 12 3. F. P. Bundy, W. A. Bassett, M. S. Weathers, R. J. Hemley, H. K. Mao, A. F. Goncharov, Carbon 34, 141–153 (1996) 12, 13 4. F. P. Bundy. Solid State Physics under Pressure: Recent Advance with Anvil Devices, ed. by S. Minomura, (Reidel, Dordrecht 1985) p. 1 12, 23 5. A. G. Whittaker, E. J. Watts, R. S. Lewis, E. Anders, Science 209, 1512 (1980) 12 6. A. G. Whittaker, P. L. Kintner, Carbon 23, 255 (1985) 12 7. V. I. Kasatochkin, V. V. Korshak, Y. P. Kudryavtsev, A. M. Sladkov, I. E. Sterenberg, Carbon 11, 70 (1973) 12 8. M. S. Dresselhaus, J. Steinbeck, Tanso 132, 44–56 (1988). (Journal of the Japanese Carbon Society) 12, 23 9. M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon Nanotubes (Academic, New York 1996) 12, 13, 2610. H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, R. E. Smalley, Nature (London) 318, 162–163 (1985) 13, 23, 2411. S. Iijima, Nature (London) 354, 56 (1991) 13, 2312. M. S. Dresselhaus, P. Avouris, chapter 1 in this volume 1313. S. G. Louie, chapter 6 in this volume 13, 1414. R. Saito, H. Kataura, chapter 9 in this volume 13, 1615. R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical Properties of Carbon Nanotubes (Imperial College Press, London 1998) 13, 14, 1616. L. Forr´, C. Sch¨nenberger, chapter 13 in this volume 14, 16 o o17. R. W. G. Wyckoﬀ, Crystal Structures, (Interscience) New York 1964, Vol. 1 14, 1518. J. Maire, J. M´ring, Proceedings of the First Conference of the Society of e Chemical and Industrial Conference on Carbon and Graphite (London, 1958) p. 204 16
Relation of Carbon Nanotubes to Other Carbon Materials 2719. P. R. Wallace, Phys. Rev. 71, 622 (1947) 1620. S. B. Austerman, Chemistry and Physics of Carbon, Vol. 7, P. L. Walker, Jr. (Ed.) (Dekker, New York 1968) p. 137 1621. A. W. Moore, Chemistry and Physics of Carbon, Vol. 11, P. L. Walker, Jr., P. A. Thrower (Eds.), (Dekker, New York 1973) p.69 1722. A. W. Moore, Chemistry and Physics of Carbon, Vol. 17, P. L. Walker, Jr., P. A. Thrower (Eds.), (Dekker, New York 1981) p. 233 1723. R. Bacon, J. Appl. Phys. 31, 283–290 (1960) 17, 2124. J. C. Charlier , Carbon Nanotubes, M. S. Dresselhaus, G. Dresselhaus, P. Avouris (Eds.), (Springer, Berlin, 2000. Springer Series in Solid-State Sci- ences) 17, 2025. T. C. Chieu, G. Timp, M. S. Dresselhaus, M. Endo, A. W. Moore, Phys. Rev. B 27, 3686 (1983) 1826. M. Endo and M. S. Dresselhaus, Science Spectra (2000) (in press) 1927. M. Endo, A. Katoh, T. Sugiura, M. Shiraishi, Extended Abstracts of the 18th Biennial Conference on Carbon, (Worcester Polytechnic Institute, 1987) p. 151 2028. M. Endo, T. Momose, H. Touhara, N. Watanabe, J. Power Sources 20, 99 (1987) 2029. M. Endo, CHEMTECH 18 568 (1988) (Sept.) 2030. M. Endo, K. Takeuchi, K. Kobori, K. Takahashi, H. Kroto, A. Sarkar, Carbon 33, 873 (1995) 2031. T. A. Edison, US Patent 470,925 (1892) (issued March 15, 1892) 2132. P. Sch¨tzenberger, L. Sch¨tzenberger, Compt. Rendue 111, 774 (1890) 21 u u33. C. H. Pelabon, Compt. Rendue 137, 706 (1905) 2134. C. Herring, J. K. Galt, Phys. Rev. 85, 1060 (1952) 2135. A. P. Levitt, Whisker Technology (Wiley-Interscience, New York 1970) 2136. A. W. Moore, A. R. Ubbelohde, D. A. Young, Brit. J. Appl. Phys. 13, 393 (1962) 2137. L. C. F. Blackman, A. R. Ubbelohde, Proc. Roy. Soc. (London) A266, 20 (1962) 2138. T. Koyama, Carbon 10, 757 (1972) 2239. M. Endo, T. Koyama, Y. Hishiyama, Jap. J. Appl. Phys. 15, 2073–2076 (1976) 2240. G. G. Tibbetts, Appl. Phys. Lett. 42, 666 (1983) 2241. G. G. Tibbetts, J. Cryst. Growth 66, 632 (1984) 2242. M. Endo, Mecanisme de croissance en phase vapeur de ﬁbres de carbone (The growth mechanism of vapor-grown carbon ﬁbers), PhD thesis, University of Orleans, Orleans, France, (1975) (in French) 2243. M. Endo, PhD thesis, Nagoya University, Japan, (1978) (in Japanese) 2244. A. Oberlin, M. Endo, T. Koyama, Carbon 14, 133 (1976) 2245. A. Oberlin, M. Endo, T. Koyama, J. Cryst. Growth 32, 335–349 (1976) 2246. R. Kubo, (private communication to M. Endo at the Kaya Conference) (1977) 2247. R. E. Smalley, DoD Workshop in Washington, DC (Dec. 1990) 2348. D. R. Huﬀman, DoD Workshop in Washington, DC (Dec. 1990) 2349. M. S. Dresselhaus, DoD Workshop in Washington, DC (Dec. 1990) 2350. M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, University of Pennsylvania Workshop (August 1991) 23
28 Mildred S. Dresselhaus and Morinobu Endo51. J. Steinbeck, G. Dresselhaus, M. S. Dresselhaus, Int. J. Thermophys. 11, 789 (1990) 23, 2452. J. Heremans, C. H. Olk, G. L. Eesley, J. Steinbeck, G. Dresselhaus, Phys. Rev. Lett. 60, 452 (1988) 23, 2453. J. S. Speck, J. Steinbeck, G. Braunstein, M. S. Dresselhaus, T. Venkatesan, in Beam-Solid Interactions and Phase Transformations, MRS Symp. Proc., Vol. 51, 263, H. Kurz, G. L. Olson, J. M. Poate (Eds.) (Materials Research Society Press, Pittsburgh PA, 1986) 2354. F. P. Bundy, H. M. Strong, R. H. Wentdorf, Jr. Chemistry and Physics of Carbon, Vol. 10, P. L. Walker, Jr., P. A. Thrower (Eds.), (Dekker, New York 1973) p. 213 2455. W. R¨dorﬀ, E. Shultze, Z. Anorg. allg. Chem. 277, 156 (1954) 25 u56. M. S. Dresselhaus, G. Dresselhaus, Adv. Phys. 30, 139–326 (1981) 25, 2657. H. Zabel, S. A. Solin (Eds.) Graphite Intercalation Compounds I: Structure and Dynamics, (Springer, Berlin, Heidelberg 1990) 2558. M. Endo, C. Kim, T. Karaki, Y. Nishimura, M. J. Matthews, S. D. M. Brown, M. S. Dresselhaus, Carbon 37, 561–568 (1999) 26
Nanotube Growth and CharacterizationHongjie DaiDepartment of Chemistry, Stanford UniversityStanford, CA 94305-5080, USAhdai@chem.stanford.eduAbstract. This chapter presents a review of various growth methods for carbonnanotubes. Recent advances in nanotube growth by chemical vapor deposition(CVD) approaches are summarized. CVD methods are promising for producinghigh quality nanotube materials at large scales. Moreover, controlled CVD growthstrategies on catalytically patterned substrates can yield ordered nanotube archi-tectures and integrated devices that are useful for fundamental characterizationsand potential applications of nanotube molecular wires.In 1991, Iijima of the NEC Laboratory in Japan reported the ﬁrst observa-tion of multi-walled carbon nanotubes (MWNT) in carbon-soot made by arc-discharge . About two years later, he made the observation of Single-WalledNanoTubes (SWNTs) . The past decade witnessed signiﬁcant research ef-forts in eﬃcient and high-yield nanotube growth methods. The success innanotube growth has led to the wide availability of nanotube materials, andis a main catalyst behind the recent progress in basis physics studies andapplications of nanotubes. The electrical and mechanical properties of carbon nanotubes have cap-tured the attention of researchers worldwide. Understanding these propertiesand exploring their potential applications have been a main driving force forthis area. Besides the unique and useful structural properties, a nanotube hashigh Young’s modulus and tensile strength. A SWNT can behave as a well-deﬁned metallic, semiconducting or semi-metallic wire depending on two keystructural parameters, chirality and diameter . Nanotubes are ideal sys-tems for studying the physics in one-dimensional solids. Theoretical and ex-perimental work have focused on the relationship between nanotube atomicstructures and electronic structures, electron-electron and electron-phononinteraction eﬀects . Extensive eﬀort has been taken to investigate the me-chanical properties of nanotubes including their Young’s modulus, tensilestrength, failure processes and mechanisms. Also, an intriguing fundamen-tal question has been how mechanical deformation in a nanotube aﬀects itselectrical properties. In recent years, progress in addressing these basic prob-lems has generated signiﬁcant excitement in the area of nanoscale science andtechnology. Nanotubes can be utilized individually or as an ensemble to build func-tional device prototypes, as has been demonstrated by many research groups.Ensembles of nanotubes have been used for ﬁeld emission based ﬂat-panelM. S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.): Carbon Nanotubes,Topics Appl. Phys. 80, 29–53 (2001)c Springer-Verlag Berlin Heidelberg 2001
30 Hongjie Daidisplay, composite materials with improved mechanical properties and elec-tromechanical actuators. Bulk quantities of nanotubes have also been sug-gested to be useful as high-capacity hydrogen storage media. Individual nano-tubes have been used for ﬁeld emission sources, tips for scanning probe mi-croscopy and nano-tweezers. Nanotubes also have signiﬁcant potential as thecentral elements of nano-electronic devices including ﬁeld eﬀect transistors,single-electron transistors and rectifying diodes. The full potential of nanotubes for applications will be realized until thegrowth of nanotubes can be optimized and well controlled. Real-world ap-plications of nanotubes require either large quantities of bulk materials ordevice integration in scaled-up fashions. For applications such as compositesand hydrogen storage, it is desired to obtain high quality nanotubes at thekilogram or ton level using growth methods that are simple, eﬃcient andinexpensive. For devices such as nanotube based electronics, scale-up willunavoidably rely on self-assembly or controlled growth strategies on surfacescombined with microfabrication techniques. Signiﬁcant work has been car-ried out in recent years to tackle these issues. Nevertheless, many challengesremain in the nanotube growth area. First, an eﬃcient growth approach tostructurally perfect nanotubes at large scales is still lacking. Second, growingdefect-free nanotubes continuously to macroscopic lengths has been diﬃcult.Third, control over nanotube growth on surfaces should be gained in order toobtain large-scale ordered nanowire structures. Finally, there is a seeminglyformidable task of controlling the chirality of SWNTs by any existing growthmethod. This chapter summarizes the progress made in recent years in carbonnanotube growth by various methods including arc-discharge, laser ablationand chemical vapor deposition. The growth of nanotube materials by Chem-ical Vapor Deposition (CVD) in bulk and on substrates will be focused on.We will show that CVD growth methods are highly promising for scale-upof defect-free nanotube materials, and enable controlled nanotube growth onsurfaces. Catalytic patterning combined with CVD growth represents a novelapproach to ordered nanowire structures that can be addressed and utilized.1 Nanotube Growth MethodsArc-discharge and laser ablation methods for the growth of nanotubes havebeen actively pursued in the past ten years. Both methods involve the conden-sation of carbon atoms generated from evaporation of solid carbon sources.The temperatures involved in these methods are close to the melting temper-ature of graphite, 3000–4000◦C.1.1 Arc-Discharge and Laser AblationIn arc-discharge, carbon atoms are evaporated by plasma of helium gas ig-nited by high currents passed through opposing carbon anode and cathode
Nanotube Growth and Characterization 31(Fig. 1a). Arc-discharge has been developed into an excellent method forproducing both high quality multi-walled nanotubes and single-walled nano-tubes. MWNTs can be obtained by controlling the growth conditions such asthe pressure of inert gas in the discharge chamber and the arcing current. In1992, a breakthrough in MWNT growth by arc-discharge was ﬁrst made byEbbesen and Ajayan who achieved growth and puriﬁcation of high qualityMWNTs at the gram level . The synthesized MWNTs have lengths on theorder of ten microns and diameters in the range of 5-30 nm. The nanotubes aretypically bound together by strong van der Waals interactions and form tightbundles. MWNTs produced by arc-discharge are very straight, indicative oftheir high crystallinity. For as grown materials, there are few defects suchas pentagons or heptagons existing on the sidewalls of the nanotubes. Theby-product of the arc-discharge growth process are multi-layered graphiticparticles in polyhedron shapes. Puriﬁcation of MWNTs can be achieved byheating the as grown material in an oxygen environment to oxidize away thegraphitic particles . The polyhedron graphitic particles exhibit higher ox-idation rate than MWNTs; nevertheless, the oxidation puriﬁcation processalso removes an appreciable amount of nanotubes. For the growth of single-walled tubes, a metal catalyst is needed in thearc-discharge system. The ﬁrst success in producing substantial amounts ofSWNTs by arc-discharge was achieved by Bethune and coworkers in 1993 .They used a carbon anode containing a small percentage of cobalt catalystin the discharge experiment, and found abundant SWNTs generated in thesoot material. The growth of high quality SWNTs at the 1–10 g scale wasachieved by Smalley and coworkers using a laser ablation (laser oven) method(Fig. 1b) . The method utilized intense laser pulses to ablate a carbontarget containing 0.5 atomic percent of nickel and cobalt. The target wasplaced in a tube-furnace heated to 1200◦C. During laser ablation, a ﬂowof inert gas was passed through the growth chamber to carry the grownnanotubes downstream to be collected on a cold ﬁnger. The produced SWNTsare mostly in the form of ropes consisting of tens of individual nanotubesclose-packed into hexagonal crystals via van der Waals interactions (Fig. 2).The optimization of SWNT growth by arc-discharge was achieved by Journetand coworkers using a carbon anode containing 1.0 atomic percentage ofyttrium and 4.2 at. % of nickel as catalyst . In SWNT growth by arc-discharge and laser ablation, typical by-productsinclude fullerenes, graphitic polyhedrons with enclosed metal particles, andamorphous carbon in the form of particles or overcoating on the sidewalls ofnanotubes. A puriﬁcation process for SWNT materials has been developedby Smalley and coworkers  and is now widely used by many researchers.The method involves reﬂuxing the as-grown SWNTs in a nitric acid solutionfor an extended period of time, oxidizing away amorphous carbon species andremoving some of the metal catalyst species.
32 Hongjie Dai Fig. 1 a–c. Schematic experi- mental setups for nanotube growth methods The success in producing high quality SWNT materials by laser-ablationand arc-discharge has led to wide availability of samples useful for studyingfundamental physics in low dimensional materials and exploring their appli-cations.1.2 Chemical Vapor DepositionChemical vapor deposition (CVD) methods have been successful in makingcarbon ﬁber, ﬁlament and nanotube materials since more than 10–20 yearsago [10,11,12,13,14,15,16,17,18,19].1.2.1 General Approach and MechanismA schematic experimental setup for CVD growth is depicted in Fig. 1c. Thegrowth process involves heating a catalyst material to high temperaturesin a tube furnace and ﬂowing a hydrocarbon gas through the tube reactorfor a period of time. Materials grown over the catalyst are collected upon
Nanotube Growth and Characterization 33 Fig. 2 a,b. Single-walled nanotubes grown by laser ablation (courtesy of R. Smalley)cooling the system to room temperature. The key parameters in nanotubeCVD growth are the hydrocarbons, catalysts and growth temperature. Theactive catalytic species are typically transition-metal nanoparticles formedon a support material such as alumina. The general nanotube growth mech-anism (Fig. 3) in a CVD process involves the dissociation of hydrocarbonmolecules catalyzed by the transition metal, and dissolution and saturationof carbon atoms in the metal nanoparticle. The precipitation of carbon fromthe saturated metal particle leads to the formation of tubular carbon solids insp2 structure. Tubule formation is favored over other forms of carbon such asgraphitic sheets with open edges. This is because a tube contains no danglingbonds and therefore is in a low energy form. For MWNT growth, most of theCVD methods employ ethylene or acetylene as the carbon feedstock and thegrowth temperature is typically in the range of 550–750◦C. Iron, nickel orcobalt nanoparticles are often used as catalyst. The rationale for choosingthese metals as catalyst for CVD growth of nanotubes lies in the phase di-agrams for the metals and carbon. At high temperatures, carbon has ﬁnitesolubility in these metals, which leads to the formation of metal-carbon solu-tions and therefore the aforementioned growth mechanism. Noticeably, iron,
34 Hongjie Dai Fig. 3. Two general growth modes of nanotube in chemi- cal vapor deposition. Left di- agram: base growth mode. Right diagram: tip growth modecobalt and nickel are also the favored catalytic metals used in laser ablationand arc-discharge. This simple fact may hint that the laser, discharge andCVD growth methods may share a common nanotube growth mechanism,although very diﬀerent approaches are used to provide carbon feedstock. A major pitfall for CVD grown MWNTs has been the high defect densitiesin their structures. The defective nature of CVD grown MWNTs remainsto be thoroughly understood, but is most likely be due to the relativelylow growth temperature, which does not provide suﬃcient thermal energyto anneal nanotubes into perfectly crystalline structures. Growing perfectMWNTs by CVD remains a challenge to this day.1.2.2 Single-Walled Nanotube Growth and OptimizationFor a long time, arc-discharge and laser-ablation have been the principalmethods for obtaining nearly perfect single-walled nanotube materials. Thereare several issues concerning these approaches. First, both methods rely onevaporating carbon atoms from solid carbon sources at ≥ 3000◦C, whichis not eﬃcient and limits the scale-up of SWNTs. Secondly, the nanotubessynthesized by the evaporation methods are in tangled forms that are diﬃ-cult to purify, manipulate and assemble for building addressable nanotubestructures. Recently, growth of single-walled carbon nanotubes with structural perfec-tion was enabled by CVD methods. For an example, we found that by usingmethane as carbon feedstock, reaction temperatures in the range of 850–1000◦C, suitable catalyst materials and ﬂow conditions one can grow highquality SWNT materials by a simple CVD process [20,21,22,23]. High tem-
Nanotube Growth and Characterization 35perature is necessary to form SWNTs that have small diameters and thus highstrain energies, and allow for nearly-defect free crystalline nanotube struc-tures. Among all hydrocarbon molecules, methane is the most stable at hightemperatures against self-decomposition. Therefore, catalytic decompositionof methane by the transition-metal catalyst particles can be the dominantprocess in SWNT growth. The choice of carbon feedstock is thus one of thekey elements to the growth of high quality SWNTs containing no defectsand amorphous carbon over-coating. Another CVD approach to SWNTs wasreported by Smalley and coworkers who used ethylene as carbon feedstockand growth temperature around 800◦ C . In this case, low partial-pressureethylene was employed in order to reduce amorphous carbon formation dueto self-pyrolysis/dissociation of ethylene at the high growth temperature. Gaining an understanding of the chemistry involved in the catalyst andnanotube growth process is critical to enable materials scale-up by CVD .The choice of many of the parameters in CVD requires to be rationalized inorder to optimize the materials growth. Within the methane CVD approachfor SWNT growth, we have found that the chemical and textural proper-ties of the catalyst materials dictate the yield and quality of SWNTs. Thisunderstanding has allowed optimization of the catalyst material and thusthe synthesis of bulk quantities of high yield and quality SWNTs . Wehave developed a catalyst consisting of Fe/Mo bimetallic species supportedon a sol-gel derived alumina-silica multicomponent material. The catalyst ex-hibits a surface are of approximately 200 m2 /g and mesopore volume of 0.8mL/g. Shown in Fig. 4 are Transmission Electron Microscopy (TEM) andScanning Electron Microscopy (SEM) images of SWNTs synthesized withbulk amounts of this catalyst under a typical methane CVD growth condi-tions for 15 min (methane ﬂow rate = 1000 mL/min through a 1 inch quartztube reactor heated to 900◦ C). The data illustrates remarkable abundanceof individual and bundled SWNTs. Evident from the TEM image is that thenanotubes are free of amorphous carbon coating throughout their lengths.The diameters of the SWNTs are dispersed in the range of 0.7–3 nm with apeak at 1.7 nm. Weight gain studies found that the yield of nanotubes is upto 45 wt.% (1 gram of catalyst yields 0.45 gram of SWNT). Catalyst optimization is based on the ﬁnding that a good catalyst ma-terial for SWNT synthesis should exhibit strong metal-support interactions,possess a high surface area and large pore volume. Moreover, these texturalcharacteristics should remain intact at high temperatures without being sin-tered . Also, it is found that alumina materials are generally far superiorcatalyst supports than silica. The strong metal-support interactions allowhigh metal dispersion and thus a high density of catalytic sites. The inter-actions prevent metal-species from aggregating and forming unwanted largeparticles that could yield to graphitic particles or defective multi-walled tubestructures. High surface area and large pore volume of the catalyst supportfacilitate high-yield SWNT growth, owing to high densities of catalytic sites
36 Hongjie Dai Fig. 4. Bulk SWNT ma- terials grown by chem- ical vapor deposition of methane. (a) A low mag- niﬁcation TEM image. (b) A high magniﬁca- tion TEM image. (c) An SEM image of the as-grown materialmade possible by the former and rapid diﬀusion and eﬃcient supply of carbonfeedstock to the catalytic sites by the latter. Mass-spectral study of the eﬄuent of the methane CVD system has beencarried out in order to investigate the molecular species involved in the nano-tube growth process . Under the typical high temperature CVD growthcondition, mass-spectral data (Fig. 1.2.2) reveals that the eﬄuent consistsof mostly methane, with small concentrations of H2 , C2 and C3 hydrocar-bon species also detected. However, measurements made with the methanesource at room temperature also reveals similar concentrations of H2 and C2 –C3 species as in the eﬄuent of the 900◦ C CVD system. This suggests thatthe H2 and C2 –C3 species detected in the CVD eﬄuent are due to impuri-ties in the methane source being used. Methane in fact undergoes negligibleself-pyrolysis under typical SWNT growth conditions. Otherwise, one would
Nanotube Growth and Characterization 37 Fig. 5. Mass spectrum recorded with the ef- ﬂuent of the methane CVD system at 900◦ Cobserve appreciable amounts of H2 and higher hydrocarbons due to methanedecomposition and reactions between the decomposed species. This result isconsistent with the observation that the SWNTs produced by methane CVDunder suitable conditions are free of amorphous carbon over-coating. The methane CVD approach is promising for enabling scale-up of defect-free nanotube materials to the kilogram or even ton level. A challenge istwheter it is possible to enable 1 g of catalyst producing 10, 100 g or evenmore SWNTs. To address this question, one needs to rationally design andcreate new types of catalyst materials with exceptional catalytic activities,large number of obtain active catalytic sites for nanotube nucleation with agiven amount of catalyst, and learn how to grow nanotubes continuously intomacroscopic lengths. A signiﬁcant progress was made recently by Liu and coworkers in obtain-ing a highly active catalyst for methane CVD growth of SWNTs . Liuused sol-gel synthesis and supercritical drying to produce a Fe/Mo catalystsupported on alumina aerogel. The catalyst exhibits an ultra-high surfacearea (∼ 540 m2 /g) and large mesopore volume (∼ 1.4 mL/g), as a result ofsupercritical drying in preparing the catalyst. Under supercritical conditions,capillary forces that tend to collapse pore structures are absent as liquid andgas phases are indistinguishable under high pressure. Using the aerogel cat-alyst, Liu and coworkers were able to obtain ∼ 200% yield (1 g of catalystyielding 2 g of SWNTs) of high quality nanotubes by methane CVD. Ev-idently, this is a substantial improvement over previous results, and is anexcellent demonstration that understanding and optimization of the catalystcan lead to scale-up of perfect SWNT materials by CVD. The growth of bulk amounts of SWNT materials by methane CVD hasbeen pursued by several groups. Rao and coworkers used a catalyst basedon mixed oxide spinels to growth SWNTs . The authors found that goodquality and yield of nanotubes were obtainable with FeCo alloy nanoparticles.Colomer and coworkers recently reported the growth of bulk quantities ofSWNTs by CVD of methane using a cobalt catalyst supported on magnesiumoxide . They also found that the produced SWNTs can be separated fromthe support material by acidic treatment to yield a product with about 70–80% of SWNTs.
38 Hongjie Dai1.2.3 Growth Mode of Single-Walled Nanotubes in CVDThe states of nanotube ends often contain rich information about nanotubegrowth mechanisms . High resolution TEM imaging of the SWNTs syn-thesized by the methane CVD method frequently observed closed tube endsthat are free of encapsulated metal particles as shown in Fig. 6. The oppo-site ends were typically found embedded in the catalyst support particleswhen imaged along the lengths of the nanotubes. These observations sug-gest that SWNTs grow in the methane CVD process predominantly via thebase-growth process as depicted in ﬁgure 3 [10,14,16,22,29]. The ﬁrst stepof the CVD reaction involves the absorption and decomposition of methanemolecules on the surface of transition-metal catalytic nanoparticles on thesupport surface. Subsequently, carbon atoms dissolve and diﬀuse into thenanoparticle interior to form a metal-carbon solid state solution. Nanotubegrowth occurs when supersaturation leads to carbon precipitation into a crys-talline tubular form. The size of the metal catalyst nanoparticle generally dic-tates the diameter of the synthesized nanotube. In the base-growth mode, thenanotube lengthens with a closed-end, while the catalyst particle remains onthe support surface. Carbon feedstock is thus supplied from the ‘base’ wherethe nanotube interfaces with the anchored metal catalyst. Base-growth oper-ates when strong metal-support interactions exist so that the metal speciesremain pinned on the support surface. In contrast, in the tip-growth mecha-nism, the nanotube lengthening involves the catalyst particle lifted oﬀ fromthe support and carried along at the tube end. The carried-along particle isresponsible for supplying carbon feedstock needed for the tube growth. Thismode operates when the metal-support interaction is weak . In the methane CVD method, we have found that enhancing metal-support interactions leads to signiﬁcant improvement in the performance ofthe catalyst material in producing high yield SWNTs . This is due tothe increased catalytic sites that favor base-mode nanotube growth. On theFig. 6 a,b. TEM images of the ends of SWNTs grown by CVD
Nanotube Growth and Characterization 39other hand, catalysts with weak metal-support interactions lead to aggrega-tion of metal species and reduced nanotube yield and purity. Large metalparticles due to the aggregation often lead to the growth of multi-layeredgraphitic particles or defective multi-walled tube structures. Metal-supportinteractions are highly dependent on the type of support materials and thetype of metal precursor being used in preparing the catalyst .1.3 Gas Phase Catalytic GrowthIt has also been demonstrated that catalytic growth of SWNTs can be grownby reacting hydrocarbons or carbon monoxide with catalyst particles gener-ated in-situ. Cheng and coworkers reported a method that employs benzeneas the carbon feedstock, hydrogen as the carrier gas, and ferrocene as thecatalyst precursor for SWNT growth . In this method, ferrocene is va-porized and carried into a reaction tube by benzene and hydrogen gases.The reaction tube is heated at 1100–1200◦C. The vaporized ferrocene de-composes in the reactor, which leads to the formation of iron particles thatcan catalyze the growth of SWNTs. With this approach however, amorphouscarbon generation could be a problem, as benzene pyrolysis is expected tobe signiﬁcant at 1200◦C. Smalley and coworkers has developed a gas phasecatalytic process to grow bulk quantities of SWNTs . The carbon feed-stock is carbon monoxide (CO) and the growth temperature is in the rangeof 800–1200◦C. Catalytic particles for SWNT growth are generated in-situby thermal decomposition of iron pentacarbonyl in a reactor heated to thehigh growth temperatures. Carbon monoxide provides the carbon feedstockfor the growth of nanotubes oﬀ the iron catalyst particles. CO is a very stablemolecule and does not produce unwanted amorphous carbonaceous materialat high temperatures. However, this also indicates that CO is not an eﬃcientcarbon source for nanotube growth. To enhance the CO carbon feedstock,Smalley and coworkers have used high pressures of CO (up to 10 atm) tosigniﬁcantly speed up the disproportionation of CO molecules into carbon,and thus enhance the growth of SWNTs. The SWNTs produced this wayare as small as 0.7 nm in diameter, the same as that of a C60 molecule. Theauthors have also found that the yield of SWNTs can be increased by intro-ducing a small concentration of methane into their CO high pressure reactorat 1000–1100◦C growth temperatures. Methane provides a more eﬃcient car-bon source than CO and does not undergo appreciable pyrolysis under theseconditions. The high pressure CO catalytic growth approach is promising forbulk production of single-walled carbon nanotubes.2 Controlled Nanotube Growthby Chemical Vapor DepositionRecent interest in CVD nanotube growth is also due to the idea that alignedand ordered nanotube structures can be grown on surfaces with control thatis not possible with arc-discharge or laser ablation techniques [23,32].
40 Hongjie Dai2.1 Aligned Multi-Walled Nanotube StructuresMethods that have been developed to obtain aligned multi-walled nanotubesinclude CVD growth of nanotubes in the pores of mesoporous silica, an ap-proach developed by Xie’s group at the Chinese Academy of Science [33,34].The catalyst used in this case is iron oxide particles created in the poresof silica, the carbon feedstock is 9% acetylene in nitrogen at an overall 180torr pressure, and the growth temperature is 600◦ C. Remarkably, nanotubeswith lengths up to millimeters are made (Fig. 7a) . Ren has grown rel-atively large-diameter MWNTs forming oriented ‘forests’ (Fig. 7b) on glasssubstrates using a plasma assisted CVD method with nickel as the catalystand acetylene as the carbon feedstock around 660◦C . Our group has been devising growth strategies for ordered multi-walledand single-walled nanotube architectures by CVD on catalytically patternedsubstrates [23,32]. We have found that multi-walled nanotubes can self-assem-ble into aligned structures as they grow, and the driving force for self-align-ment is the Van der Waals interactions between nanotubes . The growthapproach involves catalyst patterning and rational design of the substrateto enhance catalyst-substrate interactions and control the catalyst particle Fig. 7. Aligned multi-walled nano- tubes grown by CVD methods. (a) An ultra-long aligned nanotube bundle (courtesy of S. Xie). (b) An oriented MWNT forest grown on glass substrate (courtesy of Z. Ren)
Nanotube Growth and Characterization 41size. Porous silicon is found to be an ideal substrate for this approach andcan be obtained by electrochemical etching of n-type silicon wafers in hy-droﬂuoric acid/methanol solutions. The resulting substrate consisted of athin nanoporous layer (pore size ∼ 3 nm) on top of a macroporous layer (withsubmicron pores). Squared catalyst patterns on the porous silicon substrateare obtained by evaporating a 5 nm thick iron ﬁlm through a shadow maskcontaining square openings. CVD growth with the substrate is carried outat 700◦C under an ethylene ﬂow of 1000 mL/min for 15 to 60 min. Figure 8shows SEM images of regularly positioned arrays of nanotube towers grownfrom patterned iron squares on a porous silicon substrates. The nanotubetowers exhibit very sharp edges and corners with no nanotubes branchingaway from the blocks. The high resolution SEM image (Fig. 8c) reveals thatthe MWNTs (Fig. 8c inset) within each block are well aligned along the direc-tion perpendicular to the substrate surface. The length of the nanotubes andthus the height of the towers can be controlled in the range of 10–240µm byvarying the CVD reaction time. The width of the towers is controlled by thesize of the openings in the shallow mask. The smallest self-oriented nanotubetowers synthesized by this method are 2µm × 2µm. The mechanism of nanotube self-orientation involves the nanotube base-growth mode . Since the nanoporous layer on the porous silicon substrateserves as an excellent catalyst support, the iron catalyst nanoparticles formedon the nanoporous layer interact strongly with the substrate and remainpinned on the surface. During CVD growth, the outermost walls of nano-tubes interact with their neighbors via van der Waals forces to form a rigidbundle, which allows the nanotubes to grow perpendicular to the substrate(Fig. 8d). The porous silicon substrates exhibit important advantages overplain silicon substrates in the synthesis of self-aligned nanotubes. Growth onsubstrates containing both porous silicon and plain silicon portions ﬁnds thatnanotubes grow at a higher rate (in terms of length/min) on porous siliconthan on plain silicon. This suggests that ethylene molecules can permeatethrough the macroporous silicon layer (Fig. 8d) and thus eﬃciently feed thegrowth of nanotubes within the towers. The nanotubes grown on poroussilicon substrates have diameters in a relatively narrow range since cata-lyst nanoparticles with a narrow size distribution are formed on the poroussupporting surface, and the metal-support interactions prevent the catalyticmetal particles from sintering at elevated temperatures during CVD.2.2 Directed Growth of Single-Walled NanotubesOrdered, single-walled nanotube structures can be directly grown by methaneCVD on catalytically patterned substrates. A method has been devised togrow suspended SWNT networks with directionality on substrates contain-ing lithographically patterned silicon pillars [25,37]. Contact printing is usedto transfer catalyst materials onto the tops of pillars selectively. CVD of
42 Hongjie DaiFig. 8. Self-oriented MWNT arrays grown by CVD on a catalytically patternedporous silicon substrate. (a) SEM image of tower structures consisted of alignednanotubes. (b) SEM image of the side view of the towers. (c) A high magniﬁcationSEM image showing aligned nanotubes in a tower. Inset: TEM image showing twoMWNTs bundling together. (d) Schematic diagram of the growth processmethane using the substrates leads to suspended SWNTs forming nearly or-dered networks with the nanotube orientations directed by the pattern of thepillars (Fig. 9). The growth approach starts with developing a liquid-phase catalyst pre-cursor material that has the advantage over solid-state supported catalysts inallowing the formation of uniform catalyst layers and for large-scale catalyticpatterning on surfaces [25,37]. The precursor material consists of a triblockcopolymer, aluminum, iron and molybdenum chlorides in mixed ethanol andbutanol solvents. The aluminum chloride provides an oxide framework whenoxidized by hydrolysis and calcination in air. The triblock copolymer directsthe structure of the oxide framework and leads to a porous catalyst structureupon calcination. The iron chloride can lead to catalytic particles needed forthe growth of SWNTs in methane CVD. The catalyst precursor material isﬁrst spun into a thin ﬁlm on a poly-dimethyl siloxane (PDMS) stamp, fol-lowed by contact-printing to transfer the catalyst precursor selectively onto
Nanotube Growth and Characterization 43Fig. 9. Directed growth of suspended SWNT. (a) A nanotube power-line structure.(b) A square of nanotubes. (c) An extensive network of suspended SWNTsthe tops of pillars pre-fabricated on a silicon substrate. The stamped sub-strate is calcined and then used in CVD growth. Remarkably, the SWNTs grown from the pillar tops tend to direct frompillar to pillar. Well-directed SWNT bridges are obtained in an area of thesubstrate containing isolated rows of pillars as shown in Fig. 9a, where sus-pended tubes forming a power-line like structure can be seen. In an areacontaining towers in a square conﬁguration, a square of suspended nanotubebridges is obtained (Fig. 9b). Suspended SWNTs networks extending a largearea of the substrate are also formed (Fig. 9c). Clearly, the directions of theSWNTs are determined by the pattern of the elevated structures on the sub-strate. Very few nanotubes are seen to extend from pillars and rest on thebottom surface of the substrate. The self-directed growth can be understood by considering the SWNTgrowth process on the designed substrates . Nanotubes are nucleated onlyon the tower-tops since the catalytic stamping method does not place any cat-alyst materials on the substrate below. As the SWNTs lengthen, the methaneﬂow keeps the nanotubes ﬂoating and waving in the ‘wind’ since the ﬂow ve-locity near the bottom surface is substantially lower than that at the level ofthe tower-tops. This prevents the SWNTs from being caught by the bottomsurface. The nearby towers on the other hand provide ﬁxation points for thegrowing tubes. If the waving SWNTs contact adjacent towers, the tube-towervan der Waals interactions will catch the nanotubes and hold them aloft.
44 Hongjie Dai Growing defect-free nanotubes continuously to macroscopic lengths hasbeen a challenging task. In general, continuous nanotube growth requiresthe catalytic sites for SWNTs remaining active indeﬁnitely. Also, the carbonfeedstock must be able to reach and feed into the catalytic sites in a continu-ous fashion. For CVD growth approaches, this means that catalyst materialswith high surface areas and open pore structures can facilitate the growth oflong nanotubes, as discussed earlier. We have found that within the methane CVD approach, small concen-trations (∼0.03%) of benzene generated in-situ in the CVD system can leadto an appreciable increase in the yield of long nanotubes. Individual SWNTswith length up to 0.15 mm (150µm) can be obtained . An SEM image ofan approximately 100µm long tube is shown in Fig. 10. The SWNTs appearcontinuous and strung between many pillars. In-situ generation of benzeneis accomplished by catalytic conversion of methane in the CVD system us-ing bulk amounts of alumina supported Fe/Mo catalyst . This leads toenhanced SWNT growth, and a likely explanation is that at high tempera-tures, benzene molecules are highly reactive compared to methane, thereforeenhancing the eﬃciency of carbon-feedstock in nanotube growth. However,we ﬁnd that when high concentrations of benzene are introduced into theCVD growth system, exceedingly low yields of SWNTs result with virtuallyno suspended tubes grown on the sample. This is because high concentra-tions of benzene undergo extensive pyrolysis under the high temperatureCVD condition, which causes severe catalyst poisoning as amorphous carbonis deposited on the catalytic sites therefore preventing SWNT growth.Fig. 10. SEM image of a CVD grown approximately 100 m long SWNT strungbetween silicon pillars2.3 Growth of Isolated Nanotubes on Speciﬁc Surface SitesA CVD growth strategy has been developed collaboratively by our groupand Quate to grow individual single-walled nanotubes at speciﬁc sites on ﬂatsilicon oxide substrates . The approach involves methane CVD on sub-strates containing catalyst islands patterned by electron beam lithography.‘Nanotube chips’ with isolated SWNTs grown from the islands are obtained.Atomic Force Microscopy (AFM) images of SWNTs on such nanotube-chipsare shown in Fig. 10, where the synthesized nanotubes extending from thecatalyst islands are clearly observed. This growth approach readily leads