CARBONNANOTUBES

Carbon is an extraordi-nary element, consider-

ing the diversity of materialsit forms. Ranging from spar-kling gems to sooty filth,these materials have beenstudied and used for centu-ries, and carbon science waslong thought to be a maturefield. So when a whole newclass of carbon materials—the fullerenes, such as C6o—appeared in the last decade,many scientists were sur-prised.12 The consequences have reached well beyond thefullerenes themselves to include major changes in ourconcepts and understanding of long-known carbon mate-rials. It is in this context that the story of carbon nano-tubes starts.3

HistoryWith the study of C60 and C70, it was soon realized thatan infinite variety of closed graphitic structures could beformed, each with unique properties. All that was neces-sary to create such a structure was to have 12 pentagonspresent to close the hexagonal network, as explained byEuler's theorem (see the box on page 28). Given that C70was already slightly elongated as compared to C60, tubularfullerenes were imagined.

In 1991, when a group at the Naval Research Labo-ratory first submitted a theoretical paper on the electronicstructure of such small tubes,4 the results were deemedtoo speculative for publication because the synthesis ofnanotubes seemed unlikely in the near future. The re-searchers predicted, among other things, that by simplyturning a sheet of graphite into a small tube, the structurewould have a carrier density similar to that of metals,unlike graphite. And they predicted that the tube wouldhave no Peierls distortion (bond alternation resulting inthe formation of a gap) at room temperature, unlike otherconjugated materials such as polyacetylene.

At the same time, in Japan, Sumio Iijima of NEChad been using transmission electron microscopy to ana-lyze a sample of carbon soot received from Yoshinori Andoof Meijo University. The sample had been retrieved froma carbon arc machine normally used to make C60. Iijimaobserved that the sample contained tubules. Although

THOMAS EBBESEN is a principal researcher at the NECFundamental Research Laboratory in Tsukuba, Japan. He iscurrently a visiting scientist at the NEC Research Institute inPrinceton, New Jersey.

Seamless cylindrical shells of graphiticcarbon have novel mechanical and

electronic properties that suggest newhigh-strength fibers, submicroscopic

test tubes and, perhaps, newsemiconductor materials

Thomas W. Ebbesen

cylindrical carbon tubes andfibers were known,5 thesenanotubes appeared per-fectly graphitized andcapped at each end with pen-tagons, just like the fullerenemolecules. Most important ofall, Iijima noticed that thecarbon atoms in each nano-tube's closed shells were ar-ranged with various degreesof helicity: The path of carb-on bonds formed a spiralaround the tube.6

The excitement of this discovery was amplified whenseveral theoretical studies revealed that the nanotubewould be either metallic or a semiconductor, dependingnot only on the diameter but also on the helicity.7 Froma materials point of view, carbon nanotubes were seen asthe ultimate fiber, with an exceptional strength-to-weightratio. So by the spring of 1992, the expectations fornanotubes were running very high. The problem was thatthe nanotubes were present only in minute quantities inthe carbonaceous deposits, so that it would be hard toextract them or do any studies of their properties.

At that time, our molecular science group in the NECFundamental Research Laboratory in Tsukuba, Japan,was busy studying fullerenes, which we generated withthe carbon arc method.2 One day while trying to makemodified C60 with boron, we found quite by chance thatunder different operating conditions the arc would producenanotubes in high yields.8 The deposits came in a tubularform as shown in figure 1, with an outer hard shell andan inner soft fibrous core containing the nanotubes andnanoparticles. With gram quantities now in hand, it waspossible to start evaluating nanotube properties. Lastyear there were more than 180 papers published on thesubject, and what follows is only a small sampling of thetotal activity in the field.

Structure and productionHow does one define a carbon nanotube? Ideally, a nano-tube consists of one or more seamless cylindrical shells ofgraphitic sheets. In other words, each shell is made ofsp2 (trivalentl carbon atoms that form a hexagonal net-work without any edges. A nanotube can be thought ofas a tubular microcrystal of graphite. The tube is typicallyclosed at each end, according to Euler's theorem (see box),by the introduction of pentagons in the hexagonal network.Figure 2 shows examples of multishell nanotubes. Eachlayer in the tip contains at least six pentagons. Theinterlayer spacing is about 0.34 nanometers and typicalof turbostratic graphite, in which the position of each layer

26 JUNE 1996 PHYSICS TODAY • 1996 American Institute of Physics. S-0031-9228-960M20-6

STAGES IN NANOTUBE PRODUCTION, a: Tubular depositabout 6 mm in diameter formed in a carbon arc. Its cross

section reveals an inner black fibrous core material containingmillimeter-sized fibers, one of which is shown in b. The fiber

in b is about 1 mm in diameter and is in turn composed of50-fim bundles, c: A 5-yum-wide portion of one such bundle,revealing nanotubes, smaller bundles and other carbonaceous

particles. See figure 3a for further magnification of thecontent. The sample was prepared in helium at 500 torr. (a isfrom ref. 7; b and c are from T. W. Ebbesen et a!., Cbem. Phys.

Lett. 209, 83, 1993.) FIGURE 1

relative to the next is not correlated. A given nanotubewill be composed of shells having different helicities. Infact, the different degrees of helicity in each shell arenecessary to obtain the best fit between the successiveshells in a tube and minimize the interlayer distance. Thenanotubes are typically longer than a micrometer, withdiameters ranging from 1 to 20 nm. As a result of thesedimensions and this aspect ratio, nanotubes are expectedto have some features of low-dimensional materials.

Nanotubes are not always perfect seamless shells ofgraphite. Their quality depends on the method used togenerate them and the exact conditions of the particularmethod. Making nanotubes is simple, but making good-quality samples with high yields and highly graphitizedshells—that is, a continuous seamless hexagonal net-work—is not trivial.

There are now several methods3 for making nano-tubes, but the carbon arc method8 remains the mostpractical for scientific purposes and yields the most highlygraphitized tubes simply because the process has a veryhigh temperature—of 4000 K. Only with the properlygraphitized material can one expect to find any correlationbetween theoretical predictions and experiments. Fur-thermore these samples can be purified, as I discuss below.

The deposit generated in the arc has a hierarchicalstructure, with nanotubes organized in small bundles,which are themselves organized (together with nanopar-ticles) into 50-/xm fibers packed to form larger fibers visibleto the eye, as shown in figure 1. The alignment of thenanotubes in the deposit along the axis of the arc current,the yield of nanotubes and their structural quality alldepend on the conditions of the arc. The most criticalparameters are the inert gas pressure, the growth rate,

the cooling rate, the stability of the arc plasma and manyvariables that are hard to quantify.38 Nanotube yields ofabout 60% of the core material are obtainable underoptimal conditions.8

The growth mechanism of the nanotubes is a complexand fascinating subject in itself. Why do nanotubes formin the first place—and in such high yields—at such hightemperatures when they are not thermodynamically themost stable structures? In other words, why tubes andnot balls? This and other important questions were raisedand discussed by Richard Smalley of Rice University in

JUNE 1996 PHYSICS TODAY 27

Euler and the Nanotubehe flat sp2 hexagonal sheets of graphite can be giventhree-dimensional shapes by sp2-sp3 rehybridization and

by the introduction of rings other than hexagons. The gra-phitic geometry induced by the presence of pentagons andheptagons clearly follows Euler's theorem, which relates thenumber of vertices, edges and faces of an object. From it amore practical expression can be derived for sp2 hexagonalnetworks:'

. . . 2n4 + n5 - n7 - 2n8. . . = = 12 (1-g)

Here, nx (x > 2) is the number of polygons having x sides(notice that it is not necessary to count the hexagons) and g is

the genus (g = 0 for a closed• sphere, g = 1 for a torus having a

hole, and so on). This equation isvery useful in that it tells what thepresence of rings (polygons) does

/ to the geometry of the hexagonalnetwork and how many are nec-essary to obtain a closed struc-ture—that is, a total disclination of4TT. For instance, for g = 0, rc5 =12. In other words, 12 pentagons

are necessary to close the hexagonal network in the absence ofheptagons, octagons and so on. This is the case of C60. Eachtip of the closed nanotubes corresponds to half a sphere, andthus the ends must contain at least 6 pentagons, as illustratedabove.

If 12 pentagons induce a total of 4rr disclination, then onepentagon induces a 4TT/12 (also refered to as a 60°) disclination.Heptagons produce a negative 60° disclination. (See the sketchat the top of the next column.)

The addition of one heptagon requires the presence of oneextra pentagon to close the structure. In other words, a

pentagon-heptagonpair produces nonet disclinationbut may changethe diameter andthe helicity of a

nanotube. If the pentagon and heptagon are attached by oneside as a linked pair, they may be very hard to detect in ananotube, but if many of them happen to be oriented in a similardirection, then thenanotube will be-come wider andwider as illustrated atthe right.

An example ofsuch a wideningnanotube is shownin figure 2b, fromwhich it can be esti-mated that a penta-gon-heptagon pairmust be presentevery 3 nm. Noticethat the nanotube isopen at the tip, form-ing a structure simi-lar to half a torus.According to Euler'stheorem, such a torus can be formed by having six pentagons onthe outer periphery and six heptagons on the inner periphery.

Referencea. H. Tferrones, A. L. Mackay, Carbon 30, 1251 (1992). B. I.

Dunlap, Phys. Rev. B 49, 5643 (1994).

trying to understand the nanotube growth mechanism.He suggested that the high electric field, due to the voltagedrop in the space-charge sheath near the cathode, mightbe responsible for keeping the nanotubes open and mightresult in elongated structures.9

More detailed analysis of the physics of the arc plasmaby Eugene Gamaly of Australian National Universityindicates that the electric field is not after all the majorplayer and that the growth of nanotubes is the result ofthe competition between two types of carbon species pre-sent near the cathode surface: the anisotropic unidirec-tional carbon ions accelerated across the gap, and thethermally evaporated carbon from the cathode with iso-tropic velocity distribution.10 In other words, the intro-duction of an axis of symmetry in the reaction zone dueto the unidirectional carbon species results in elongatedstructures. The suggested bimodal distribution of carbonspecies seems to agree well with the bimodal distributionof products—namely, nanotubes and nanoparticles (whichare polyhedral and have no particular axis of symmetry—as can be seen in figure 3a.3

Analysis shows that such an axis of symmetry alwaysexists in the reaction coordinates in all the methods usedto generate nanotubes. For instance, in the catalyticgrowth of single-shell nanotubes, the catalytic particlesprovide this asymmetry in three dimensions. In thistechnique, metal catalysts such as cobalt, iron and nickelare mixed into the carbon rods consumed in the arcplasma, resulting in significant quantities of single-shellnanotubes with diameters centered around 1.2 nm. This

method was considered very promising when it was firstreported by Don Bethune and his coworkers at IBM andIijima and Tetsuya Ichihashi at NEC, because these nano-tubes would allow a much better comparison with theorythan would the multishell nanotubes.11 However, sepa-rating these single-shell nanotubes from the residual met-al and soot has been a big obstacle, and developing apurification technique remains the biggest challenge.3

It took a long time to find a way of purifying themultishell carbon arc tubes. At NEC, after trying variousstandard purification methods without success, we discov-ered that the nanotubes could be purified by oxidation inan oven at about 1000 K, with the nanoparticles beingconsumed faster than the nanotubes.3 Oxidation worksbecause the nanotubes are consumed from the tip inward,and therefore their greater aspect ratio gives them asurvival advantage; figure 3b shows the dramatic differ-ences after oxidation. Hence, although the yield was quitesmall (about I've), macroscopic quantities of purified mul-tishell nanotubes were finally available and could bestudied for their properties.

DefectsAs with any other material, the issue of defects cannot beneglected when trying to measure and analyze nanotubeproperties. The required purity and perfection depend onthe type of property to be measured. And the quality ofthe nanotube sample depends sensitively on the method-ology. So it is not surprising that results have not alwaysbeen consistent from one research group to another. This

28 JUNE 1996 PHYSICS TODAY

NANOTUBE tip structures(a,b,c) and a filled nanotube(d). All are multishellnanotubes. The one in b isvase shaped with an open tiphaving a semitoroidal structure(see box for explanation). Thenanotube in c has a carbonpentagon (P) and heptagon(H), resulting in a thinner tubebefore closing at the very tip.The nanotube in d is filledwith bismuth oxide. The scaleof each of these transmissionelectron micrographs isindicated by the fringe spacings,which are about 0.34 nm.(b is from T. W. Ebbesen, T.Takada, Carbon, vol. 33, p.973, 1995; d is from P. M.Ajayan etal, Nature 367, 522,1993.) FIGURE 2

fact has led to suggestions that typical nanotubes are nottubes but are composed of small pieces of graphitic do-mains patched together like papier-mache or rolled up likescrolls. There is no doubt that such structures exist, butthey are not representative of all samples.

There is much evidence to support the view thatnanotubes are seamless cylinders. For instance, whennanotubes are oxidized in an oven at high temperature,they are consumed from the tip inward, layer by layer ashas been shown independently by Edman Tsang, PeterHarris and Malcolm Green at the University of Oxfordand our own group.12 If nanotubes had seams, one wouldexpect that the seams would oxidize first, everywherealong the tube.

That does not imply that the nanotubes are defectfree. They may contain a number of defects, which fallinto three categories: topological defects, rehybridizationdefects and incomplete bonding.

Topological defects are due to the presence of ringsother than hexagons in the structure (except at the ends,where six pentagons are necessary for closure). It appearsthat a common topological defect is the pentagon-hepta-gon pair in which the pentagon and heptagon are attachedto each other, forming what chemists know as the azulinestructure. This 5/7 pair produces no net disclination (seebox) but may slightly change the diameter and the helicity,or chirality, of the tube, depending on its orientationrelative to the tube axis. Such 5/7 pairs are hard to detectunless many happen to be oriented in the same overalldirection, in which case the nanotube diameter will gradu-ally widen. Such widening is observed from time to time(figure 2), and frequencies of one 5/7 pair every 3 nm canbe estimated.

Mingqi Liu and John Cowley of Arizona State Uni-versity have shown that nanotubes are not always cylin-drical but may be polygonal.13 The presence of the pen-tagons at the tip (see box) might induce nanotubes to growwith a polygonal cross section instead of relaxing to acylindrical shape. The edges have a stronger sp3 characterthan do the flat areas, the more so the greater the degree

of curvature, forming sp3 defect lines in the sp2 network.These rehybdrization defects will no doubt affect theproperties.

Finally, incomplete bonding due to dislocation edgesare also observed, but their frequencies do not suggestthat they are a major problem in normal arc-producedsamples.

One might wonder why the nanotubes grown in thearc at 4000 K still contain defects. It is most likely dueto the fact that kinetics, and not thermodynamics, domi-nate the growth process.3 As a result, there is not suffi-cient time for annealing the structure. As with othergraphitic materials, further high temperature treatment(at about 3100 K) reduces the amount of defects, as dis-cussed below.

Electronic propertiesThe electronic properties of carbon nanotubes are of greatinterest in light of the theoretical predictions, but theyhave also been the most challenging to measure due tothe small diameter of the tubes. Only very recently hasone been able to attach probes directly to single nanotubesto measure transport properties. Most studies have beenon bulk material, from which a great amount of informa-tion has been learned. Electron spin resonance (ESR)studies, for example, reveal that a fraction of the nano-tubes are indeed metallic (or narrow bandgap semicon-ductors). And there has been no indication of Peierlsdistortion (such as instability to bond alternation inpolyacetylene), in agreement with theoretical calcula-tions.4'7 Furthermore, ESR studies of carbon nanotubesalso illustrate well the issues of sample preparation anddefects, as results have varied significantly from one groupto another.

Seeing this problem, we decided that annealing thenanotubes at high temperature in argon would reduce theamount of defects. Indeed, the results are not only dif-ferent, but cleaner. A single conduction-electron signalhaving Pauli paramagnetic temperature dependence isobserved, and no localized spins appear at low temperature

JUNE 1996 PHYSICS TODAY 29

CORE FIBROUS MATERIAL of a carbonarc deposit (figure lc) before (a) andafter (b) purification by gas-phaseoxidation. Notice in a the presence ofpolyhedral particles, also known ascarbon nanoparticles. The transmissionelectron micrographs cover a regionabout 2.5-yu.m wide. FIGURE 3

after annealing. Perhaps the most dramatic change canbe seen in the g-value (roughly the ratio of the spin-orbitcoupling energy and the energy separating the coupledstates), which becomes almost temperature independent af-ter annealing, as shown in figure 4. The average Pauliparamagnetic susceptibility of the annealed purifiednanotubes is greater than 4.5 x 10~s electromagnetic unitsper gram, which is larger than that of graphite. Thismeasure is a minimum because if the sample containswide-bandgap semiconductor nanotubes that go undetectedin the ESR, the susceptibility will be even larger.

Recently, Luc Langer and his coworkers at the Catho-lic Universities of Louvain and Leuwen in Belgium werethe first to succeed in attaching probes directly to a singlemultishell nanotube to measure transport properties.They attached two leads to a nonannealed nanotube andmeasured the magnetoconductance down to 20 mK undervarious field strengths. The results (figure 5a) show someremarkable features that are consistent with quantum

transport in a weakly disordered andlow-dimensional electronic system.Both two-dimensional weak localiza-tion and the presence of a Landaulevel (quantized state of an electronin a magnetic field) at the crossing ofthe valence and conductance bandscontribute to the positive magnetocon-ductance. At very low temperatures,aperiodic fluctuations, known as uni-versal conductance fluctuations, ap-pear superimposed on the weak local-ization and Landau level effects(figure 5b). The fact that such anonannealed nanotube has features ofa disordered transport system is con-sistent with the ESR studies before andafter annealing, which indicate thepresence of defects in such samples.

The greatest challenge ahead is toplace four probes directly on a singlenanotube. That would allow accuratemeasurement of the transport proper-ties of both annealed and nonannealednanotubes and thereby enable compari-son with theoretical predictions about

the effect of the diameter and helicity on those properties.

Material propertiesAs with carbon fibers, perhaps the most important poten-tial application of carbon nanotubes is based on the useof their mechanical properties, in particular their highstrength-to-weight ratio. In this regard, seamless nano-tubes are expected to be the ultimate fiber. Again, dueto their very small size, there are not yet any hard experi-mental values for their tensile strength, Youngs modulusand so on. There are, however, many observations thatnanotubes are very strong and flexible. For example, nano-tubes will buckle and deform, but they recover withoutany damage, as shown on the cover. This property stemsfrom the ability of the sp2 carbon atoms to rehybridizewhen the bonds are deformed out of plane, the degree ofsp2-sp3 rehybridization being dependent on the curvature.At the same time, the in-plane rigidity and strength ofgraphite sheets must also be present in the nanotubes.

30 JUNE 1996 PHYSICS TODAY

TEMPERATURE DEPENDENCE of the g values of the conductionelectron spin resonance of purified nanotubes before (blue

circles) and after (red circles) high-temperature annealing atabout 3100 K. (From M. Kosaka, T. W. Ebbesen, H. Hiura,

K. Tanigaki, Chem. Pbys. Lett, 233, 47, 1995.) FIGURE 4

2.03

intH******1*'"*'"*'***'***1

100 200TEMPERATURE (kelvin)

300

Other features of nanotubes that offer intriguing pos-sibilities for material science and nanoscale experimentsare the inner hollow cavity and the outer surface. Forinstance, the inner hollow cavity can serve as a nanoscaletest tube or mold, while the outer shell could be decoratedto yield catalysts with unique properties due to the highcurvature.14

For such goals, it is extremely important to under-stand the wetting properties of nanotubes, because theygovern what liquid will spontaneously cover the outersurface, be drawn inside by capillarity, mix to form acomposite and so on. Capillarity is an indicator of wettingability, as can be seen from the Young-Laplace equation

AP = (2y/r)cos 0

which relates the pressure difference \P across the liq-uid/vapor interface in a capillary to the surface tensiony of the liquid, the radius of curvature r of the meniscusand the contact angle 0 between meniscus and surface.The contact angle is a direct measure of the strength ofliquid/solid interactions relative to the cohesive forces inthe liquid. The liquid will be drawn spontaneously intothe capillary when IP is positive—in other words, when0 is less than 90°. In such a case the contact angle 6 issaid to be wetting. It is hard to predict the contact angle,and so we decided to evaluate the wetting properties ofnanotubes by testing their capillarity action on varioussubstances according to their surface tension. The results,shown in the table, indicate that nanotubes are wet onlyby low-surface-tension liquids, with a limit of about 200mN/m. Hence, typical metals will not be drawn in bycapillarity. Only by applying outside pressure sufficientlyhigh to compensate for the negative AP will the liquidmetal go in where it can be trapped by lowering thetemperature below its melting point.14

Preceeding and later capillarity work on metal oxidesby P. M. Ajayan and his colleagues all fit within thislimit.15 Figure 2d gives an example of a nanotube filledwith bismuth oxide. Most recently, the researchers haveshown that V2O5 (y = 88 mN/m) will cover both the insideand outside of the nanotube, acting like a template forthe formation of nanoceramic fibers. We predicted thatthe low-surface-tension limit was sufficiently high to allowwetting by organic solvents and thus was favorable fordoing chemistry. This idea was confirmed independentlyby a simple and elegant experiment by the Oxford re-

searchers.16 They used nitric acid (y = 43 mN/m) to openthe nanotube tips by oxidation and fill the nanotube witha metallic compound dissolved in the acid. In other words,the nitric acid acts first as a tube opener and then as alow-surface-tension carrier to introduce material insidethe nanotube that otherwise would not have gone inspontaneously. This technique should be very useful forfilling nanotubes with a variety of materials.

Future directionsCarbon nanotubes are at the crossroads of traditionalcarbon fibers and fullerenes.3 Although carbon sciencecan help researchers to understand nanotubes, nanotubesthemselves provide new insights into the more traditionalcarbon materials. This quality in itself is an indirectapplication of nanotube research. Nanotubes derive uniqueproperties from their dimensions and topology. For in-stance, experiments indicate that nanotubes can be usedas atomic-scale field emitters17 and as pinning materialin high-Tc superconductors.18 Further studies of their

Nanotube wetting abilities""

Substance

HNO,

S

Cs

Rb

V2O,

Se

Pb oxides

Bi oxides

Te

Pb

Hg

Ga

Surface tension(millinewtons per meter)

43

61

67

77

80

97

(PbO - 132)

(Bi2O3 -200)

190

470

490

710

Wetting

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

No

No

No

'"Wetting properties of nanotubes in terms of surface tension of selectedsubstances in their liquid state. The Pb and Bi oxides had unknownstoichiometry in the experiments.

JUNE 1996 PHYSICS TODAY 31

G'owO2

OQ

OU

I I0.01 0.1 1 10

TEMPERATURE (kelvin)100

wuz<uQOUowo<

0 2 4 6 8 10

MAGNETIC FIELD (tesla)

mechanical properties are extremely important, becausethat category is where the direct application perhaps hasthe most potential; nanotube-reinforced materials are onepossibility. Low-cost methods for generating large quan-tities of perfectly graphitized nanotubes will be necessaryfor such bulk applications. In this regard, further analysisof their growth mechanism is warranted. Nanotubesmade from elements other than carbon also open excitingavenues for future research.19

Although there has been considerable activity in thisfield, much work remains to be done, in particular directlyon individual nanotubes. This work is perhaps the mostchallenging, but it opens the door to broader studiesrelating bulk properties to graphitic materials with specificgeometrical designs. Only when researchers can do thissystematically will they be able to fully tap the wealththat is hidden in carbon.

References1. H. W. Kroto, J. R. Heath, S. C. O'Brien, R. F. Curl, R. E. Smal-

ley, Nature 318, 162 (1985).2. W. Kratschmer, L. D. Lamb, K. Fostiropoulos, D. R. Huffman,

Nature 347, 354(1990).3. T. W. Ebbesen, Ann. Rev. Mater. Sci. 24, 235 (1994).4. J. W. Mintmire, B. I. Dunlap, C. T. White, Phys. Rev. Lett. 68,

ELECTRICAL AND MAGNETIC CONDUCTANCE, a: Electricalconductance of a single multiwalled 20-nm nanotube as afunction of temperature at the indicated magnetic fields. Thecolored curve separates the contributions to themagnetoconductance from the Landau levels and the weaklocalization, b: Plot of magnetoconductance as a function ofmagnetic field, showing the appearance of universalconductance fluctuations at low temperatures. (From L.Langer, V. Bayot, E. Grivei, J.-P. Issi, J. P. Heremans, C. H.Oik, L. Stockman, C. Van Haesendonck, Y. Bruynseraede,Phys. Rev. Lett. 76, 479, 1996.) FIGURE 5

631(1992).5. A. Oberlin, M. Endo, T. Koyama, J. Cryst. Growth 32, 335

(1976).6. S. Iijima, Nature 354, 56 (1991).7. N. Hamada, S. Sawada, A. Oshiyama, Phys. Rev. Lett. 68,1579

(1992). R. Saito, M. Fujita, G. Dresselhaus, M. S. Dresselhaus,Appl. Phys. Lett. 60, 2204 (1992). K. Tanaka, K. Okahara, M.Okada, T. Yamabe, Chem. Phys. Lett. 191, 469 (1992).

8. T. W. Ebbesen, P. M. Ajayan, Nature 358, 220 (1992).9. R. E. Smalley, Mater. Sci. Eng. B 19, 1 (1993).

10. E. G. Gamaly, T. W. Ebbesen, Phys. Rev. B 52, 2083 (1995).11. D. S. Bethune, C. H. Kiang, M. S. de Vries, G. Goreman, R.

Savoy, J. Vazquez, R. Beyers, Nature 363, 606 (1993). S.Iijima, T. Ichihashi, Nature 363, 603 (1993).

12. S. C. Tsang, P. J. F. Harris, M. L. H. Green, Nature 362, 520(1993). P. M. Ajayan, T. W. Ebbesen, T. Ichihashi, S. Iijima, K.Tanigaki, H. Hiura, Nature 362, 522 (1993).

13. M. Liu, J. M. Cowley, Mater. Sci. Eng. A185, 131 (1994).14. E. Dujardin, T. W. Ebbesen, H. Hiura, K. Tanigaki, Science

265, 1850(1994).15. P. M. Ajayan, O. Stephan, P. Redlich, C. Colliex, Nature 375,

564 (1995). P. M. Ajayan, S. Iijima, Nature 361, 333 (1993).16. S. C. Tsang, Y. K. Chen, P. J. F. Harris, M. L. H. Green, Nature

372, 159(1994).17. A. G. Rinzler, J. H. Hafner, P. Nikolaev, L. Lou, S. G. Kim, D.

Tomanek, P. Nordlander, D. C. Colbert, R. E. Smalley, Science269, 1550 (1995). W. A. De Heer, A. Chatelain, D. Ugarte,Science 270, 1179 (1995). L. A. Chernozatonskii, Yu. V.Gulyaev, Z. Ja. Kosakovskaja, N. I. Sinitsyn, G. V. Tbrgashov,Yu. F. Zakharchenko, E. A. Fedorov, V. P. Val'chuk, Chem.Phys. Lett. 233,63(1995).

18. K. Fossheim, E. D. Tuset, T. W. Ebbesen, M. M. J. Treacy, J.Schwartz, Physica C 248, 195 (1995).

19. R.Tenne.Adv. Mater. 7,965(1995). •

32 JUNE 1996 PHYSICS TODAY