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FULLERENEMaterialsThe young field offullerenes has rapidlyexpanded to encompass a broad range ofstructures that can serve as basicbuilding blocks for totally new carbon-base materials.

Ripudaman Malhotra,Rodney S. Ruoff, andDonald C. LorentsMolecular Physics LaboratorySRI InternationalMen/o Park, Calif.

Iullerenes are all-carbon cage molecules. Themost celebrated f ullerene is the soccer-ballshaped Cgo, which is composed of twenty

. hexagons and twelve pentagons. Becauseits structure is reminiscent of the geodesic domesof architect R. Buckminster Fuller, C6o is calledbuckminsterfullerene, and all the materials in thefamily are designated fullerenes.

Although carbon molecules having such struc-tures were first hypothesized in 1970, it was notuntil 1985 that the first experimental evidence fortheir existence was found. In 1990, Huffman andKratschmer showed that these molecules can beeasily produced in large quantities by vaporizingcarbon in a helium atmosphere. The fullerenes may

be extracted with a solvent such as benzene. About15% of the soot dissolves in the solvents, and massspectrometric analysis shows that this extract con-tains Cgo and C/oin a roughly 4:1 ratio. The extractalso contains a few percent of the higher fullerenessuch as C76, C ys, and C 84- The novel structure offullerenes provides the basis for the developmentof completely new carbon materials.

Huffman and Kratschmer's discovery unleashedactivity around the world as scientists exploredproduction methods, properties, and potential usesof fullerenes. Within a short period, methods fortheir production in electric arcs, plasmas, andflames were discovered, and several companiesbegan selling fullerenes to the research market.What is remarkable is that in all these methods,carbon atoms assemble themselves into cage struc-tures. The capability for self-assembly points tosome inherent stability of these structures that al-lows their formation. The unusual structure natu-rally leads to unusual properties. Among them areready solubility in solvents and a relatively highvapor pressure for a pure carbon material.

The other common forms of carbon, graphiteand diamond, are infinite lattices, and requirebreaking of several strong bonds to release anyatom. The fullerenes, on the other hand, are mol-ecular solids with only van der Waals forces be-tween the molecules. Processes such as dissolutionand evaporation in this case are simplified, becausethey do not require cleavage of any covalentcarbon-carbon bonds; instead, the entire molecular

Rj-1 — Computer simulations for the stable structures o/C6o, C70, and C$4 (one of the D2d symmetry isomers). In thesestructures the pentagons are always separated by hexagons; that is, no two pentagons share an edge.

ADVANCED MATERIALS & PROCESSES 4/95

Fullerenesare carboncagemolecules.

29

i •

Pentagonsin the

graphenelattice

introducecurvature.

Fig. 2 — (a) High resolution transmission electron mi-crographs (TEM) of material recovered from the cathodicgrowth during fullerene production shows an abundance oftubular and polyhedral structures, (b) The multilayered struc-ture of the tubes and the polyhedral particles is evident underhigher magnification; the spacing between the layers is be-tween 3.4 and 3.5A. The interlayer spacing in crystallinegraphite is 3.35 A.

cluster, for example C6oor C70, departs from thelattice intact.

The structure of fullerenes can be readily un-derstood by considering the structure of agraphene sheet, which consists of carbon atoms ina flat hexagonal lattice. In this lattice, the valenciesof the atoms in the middle are fully balanced, butthose at the edges are not, which gives rise to somedestabilization. If the lattice is very large, theoverall destabilizatioh due to edge atoms, on a percarbon basis, is smal^. Moreover, many flat latticescan assemble in layers and gain stability by the in-terplanar van der Waals interactions.

However, if the lattice consists of relatively fewatoms, the destabilization becomes significant andthe system seeks to minimize its energy by foldingonto itself. Including pentagons in the lattice is oneway to introduce curvature. Each pentagon intro-duces a curvature of 60° (n/3 solid angle). It takestwelve pentagons in a hexagonal lattice for com-plete closure (4n solid angle). One can also arriveat the same conclusion from Euler's theorem about

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the relationship between the number of edgesfaces, and vertices in a closed polyhedron that con-tains only hexagons and pentagons. Thus, C^ con-sists of 20 hexagons and 12 pentagons, C70 consistsof 25 hexagons and 12 pentagons, and C^ consistsof 32 hexagons and 12 pentagons. The relative lo-cation of the pentagons in these structures givesrise to different shapes. It follows that there can bea wide range of structures for each of these clus-ters, each having unique properties. Figure 1 showsonly the most stable structures for C60 and C70,along with one of the isomers of C^.

Fullerene applicationsFullerene-base materials are being seriously

investigated in many laboratories. Potential ap-plications range from photovoltaics to supercon-ductors, from medical applications to catalysis,and from polymeric materials to rocket fuels. Forexample, the simple face-centered cubic (fee) crys-talline form of Ceo, intercalated at the level of threealkali atoms per C6o, changes from an insulatorto a metal and becomes a superconductor at lowtemperatures. This important discovery resultedin a flood of research reminiscent of that followingthe discovery of the high Tc cuprate supercon-ductors. These M3C60 superconductors have thesecond-highest superconducting transition tem-peratures, behind the high Tc materials. They arefundamentally interesting because their isotropicstructure allows for a detailed theoretical treat-ment leading to a deeper understanding of thephysics of superconductivity. The isotropic struc-ture also means that the superconducting currentcan be carried equally in three orthogonal direc-tions, in contrast to the highly anisotropic highTc cuprate superconductors.

The Ceo fullerene and even the arc-generatedfullerene soot have been shown to catalyze hy-drogen-transfer reactions that can lead to efficientcleavage and formation of carbon-carbon bonds.The ability to chemically modify and functionalizefullerenes provides an extremely rich means tocreate materials with new properties. Severalgroups, including ours, have prepared polymersderived from functionalized CM and shown themto possess high mechanical strength and thermalstability. Other examples include organic photo-conductors prepared, for example, by dopingpoly(vinylcarbazole) with Ceo-

The central cavity in the fullerenes has attracteda lot of attention. Introduction of different atomsinside the fullerene cages provides another wayof "doping" a fullerene lattice. These compounds,called endohedrals, are expected to have novelelectronic and optical properties. Smalley hasshown that many metals can indeed be encaged inthe fullerenes by simply introducing the metal inthe carbon arc. However, the isolation of these ma-terials remains a challenge. Saunders andcoworkers used a different approach to producingendohedrals. They introduced noble-gas atoms byheating C^ under high pressure in the noble gas.Using NMR spectroscopy, they demonstrated theformation of 3He@C6o when Ceo was heated with

ADVANCED MATERIALS & PROCESSES 4/95

to 600°C (1100°F) for five hours at a pressureof 2500 atmospheres (the "@" symbol signifies "en-dohedral").

Three-dimensional structuresAn important lesson from the discovery of

fullerenes is the realization that three-dimensionalstructures can be constructed from the graphenesheet. This lesson can in some ways be likened tothe one organic chemists learned about 120 yearsago, when van't Hoff and Le Bel liberated the va-lencies of tetravalent carbon from the confines ofthe two-dimensional paper and into three-dimensional space. In the present case, scientistsbegan re-examining the notion of flat graphenesheets and soon discovered (serendipitously, some-times) several other structures based on the sameprinciple.

lijima discovered that boules (crystals) that areabundantly created on the cathodes in the arcs usedfor fullerene production contain nanoscale tubularand polyhedral structures (Fig. 2). The tubularstructures consist of many concentric tubes. Theygenerally have an outer diameter of about 10 to 30nm and are as long as several micrometers. Theends of the tubes often have conical caps, and thetapering angle is in agreement with the introduc-tion of pentagons into the rolled hexagonal lattice.The polyhedral particles also consist of nestedstructures, with each layer a giant fullerene con-taining on the order of 106 carbon atoms. These

tubes and carbon polyhedra can also be classifiedas fullerenes, because the closed structures resultfrom the introduction of exactly 12 pentagons intohexagonal graphite lattices. Plastic models as wellas computer-generated models of some highlysymmetric polyhedral particles observed in trans-mission electron microscopy (TEM) images, haveprovided convincing evidence of their fullerenegeometry.

Nanotubes are particularly interesting becausetheir ideal geometrical form, high aspect ratios,and nearly faultless structures provide them withunique mechanical, electrical, and thermal prop-erties. These tubes are expected, on the basis of the-oretical calculations, to have strongly anisotropicelectrical and thermal conductivities. Mechanicalproperties such as the Young's and bendingmoduli are expected to be very high, because ofthe cylindrical geometry and the rigidity of thehexagonal lattice. Simple scaling relationships havebeen developed for estimating the yield strengthand bending modulus of multi-layer and single-layer tubes, which account for the empty corevolume. It is possible that the specific yield strengthof ideal carbon nanotubes with a small inner corewill be 50 times as large as that of steel. A chal-lenging technical problem is the achievement ofnanotube-reinforced composites in which this in-trinsic strength may be conferred to the composite.

Most of the polyhedral nanoparticles have alarge cavity in the center. In an analogy with the

Differentatomsmaybeplaced infullerenecages.

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Nanotubesconsist of

manyconcentric

tubes.

Fig, 3 — TEM of a carbon nanopolyhedron encapsulatinga single crystal of lanthanum carbide. The lattice fringes inthe carbide phase allow us to the identify this phase as aLaC2.

Fig. 4 — TEM of the soot produced from arc vaporization of predrilled graphiterods packed with iron and nickel. The long fibers often consist of bundles of single-walled nanotubes, about 7 to 12 A in diameter and up to several micrometers long.

endohedrals, we found that single crystals of othermaterials, such as lanthanum carbide (LaC2) andgadolinium carbides (Gd2C3, GdC2), can be en-capsulated within the cavity of these nested poly-hedral particles during the arc-synthesis step (Fig.3). It is interesting to note that although many ofthese carbides hydrolyze in ambient air, the car-bide phases in the encapsulates are protected bythe carbon shell and show no signs of decompo-sition upon exposure to air. In the case of encap-sulated GdCi we have found that the particles areparamagnetic. They can be selectively extractedwith a magnet from the raw mix, which containsnonmagnetic empty particles as well. Thenanoscale dimensions of the GdC2 crystals impart

32

unique magnetic properties that place them in theclass of superparamagnetics. For example, bulkGdC2 has an antiferromagnetic ordering transi-tion at 42°K (-230°C, -380°F), but this transition isnot seen in a plot of magnetic susceptibility vstemperature. At temperatures below 42°K, theGdC2 encapsulates are superparamagnetic. An ac-tive area of research involves attempts to encap-sulate ferromagnetic particles, such as iron, nickel,and cobalt inside of carbon shells.

Formation of uniformly filled nanotubes has notbeen achieved during arc synthesis, although tech-niques are being developed by which the ends ofthe nanotubes can be opened. This may allowfilling with various metals to produce nanowires.In contrast to the multi-walled nanotubes that areformed in the boules on the cathode, single-wallednanotubes are produced in the soot when thegraphite anode rod is doped with the transitionelements iron, cobalt, or nickel. TEM examinationshows these tubes to be associated with amor-phous carbon and small metal particles (2 to 20nm). They appear to be more flexible than themulti-walled nanotubes; however they are alsopredicted to have high tensile strength.

Recent research efforts in nanotubes have fo-cused on gathering pure samples. Methods basedon density centrifugation and filtration are severelyhampered by the lack of suitable solvents to dis-perse the particles. Use of temperature-pro-grammed oxidation to preferentially burn off themore reactive amorphous carbon has been some-what successful. Chemical oxidation with per-manganate or other compounds may be promisingfor producing large quantities of pure nanotubes.With these quantities, it will be possible to makecomposites and test many of the physical and me-chanical properties.

The young fullerene field has already pro-duced a surprising array of structures for thedevelopment of carbon-base materials havingcompletely new and different properties from anythat were previously possible. Many challengesremain in the production and isolation of varioustypes of fullerenes before the promise of suchmaterials can be realized. However, the enthusi-astic activity demonstrated by the internationalresearch community instills confidence that suchchallenges will soon be met. Fullerene research isfull of exciting new discoveries from the past fouryears, and we can now expect this field to movetoward the discovery of equally exciting materialswith important applications in many areas. •

For more information: Ripudaman Malhotra, RodneyS. Ruoff, and Donald C. Lorents are researchers at theMolecular Physics Laboratory, SRI International, MenloPark, CA 94025; tel: 415/859-2805; fax: 415/859-6196.

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