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Fullerene
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- "C60" and "C-60" redirect here. For other uses, see C60 (disambiguation).
Fullerene are a family of carbon allotropes, molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, tube, or plane. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes.
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- "C60" and "C-60" redirect here. For other uses, see C60 (disambiguation).
Fullerene are a family of carbon allotropes, molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, tube, or plane. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes. Graphene is an example of a planar fullerene sheet. Fullerenes are similar in structure to graphite, which is composed of stacked sheets of linked hexagonal rings, but may also contain pentagonal (or sometimes heptagonal) rings that would prevent a sheet from being planar.
The fullerene was discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley at the University of Sussex and Rice University, who named it after Richard Buckminster Fuller, whose geodesic domes it resembles.
Prediction and discovery
With mass spectroscopy, discrete peaks were observed corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms. In 1985, Harold Kroto (then of the University of Sussex), James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley, from Rice University, discovered C60, and shortly thereafter came to discover the fullerenes. Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of compounds. C60 and other fullerenes were later noticed occurring outside the laboratory (e.g., in normal candle soot). By 1991, it was relatively easy to produce gram-sized samples of fullerene powder using the techniques of Donald Huffman and Wolfgang Krätschmer. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices. So-called endohedral fullerenes have ions or small molecules incorporated inside the cage atoms. Fullerene is an unusual reactant in many organic reactions such as the Bingel reaction discovered in 1993. The first nanotubes were obtained in 1991.
Minute quantities of the fullerenes, in the form of C60, C70, C76, and C84 molecules, are produced in nature, hidden in soot and formed by lightning discharges in the atmosphere. Recently, Buckminsterfullerenes were found in a family of minerals known as Shungites in Karelia, Russia.
The existence of C60 was predicted by Eiji Osawa of Toyohashi University of Technology in a Japanese magazine in 1970. He noticed that the structure of a corannulene molecule was a subset of a soccer-ball shape, and he made the hypothesis that a full ball shape could also exist. His idea was reported in Japanese magazines, but did not reach Europe or America.
Naming
Buckminsterfullerene (C60) was named after Richard Buckminster Fuller, a noted architectural modeler who popularized the geodesic dome. Since buckminsterfullerenes have a similar shape to that sort of dome, the name was thought to be appropriate. As the discovery of the fullerene family came after buckminsterfullerene, the shortened name 'fullerene' was used to refer to the family of fullerenes.
For illustrations of geodesic dome structures, see Montreal Biosphere, Eden Project, Missouri Botanical Garden, Science World at Telus World of Science, Mitchell Park Horticultural Conservatory, Gold Dome, Tacoma Dome, Reunion Tower, and Spaceship Earth (Epcot).
Variations
Since the discovery of fullerenes in 1985, structural variations on fullerenes have evolved well beyond the individual clusters themselves. Examples include:
- buckyball clusters: smallest member is (unsaturated version of dodecahedrane) and the most common is ;
- nanotubes: hollow tubes of very small dimensions, having single or multiple walls; potential applications in electronics industry;
- megatubes: larger in diameter than nanotubes and prepared with walls of different thickness; potentially used for the transport of a variety of molecules of different sizes;
- polymers: chain, two-dimensional and three-dimensional polymers are formed under high pressure high temperature conditions
- nano"onions": spherical particles based on multiple carbon layers surrounding a buckyball core; proposed for lubricants;
- linked "ball-and-chain" dimers: two buckyballs linked by a carbon chain;
- fullerene rings
The "Buckyball"
Buckminsterfullerene
Buckminsterfullerene (IUPAC name (C60-Ih)[5,6]fullerene) is the smallest fullerene molecule in which no two pentagons share an edge (which can be destabilizing; see pentalene). It is also the most common in terms of natural occurrence, as it can often be found in soot.
The structure of C60 is a truncated (T = 3) icosahedron, which resembles a soccer ball of the type made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.
The van der Waals diameter of a C60 molecule is about 1 nanometer (nm). The nucleus to nucleus diameter of a C60 molecule is about 0.7 nm.
The C60 molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 1.4 angstroms.
Boron buckyball
A new type of buckyball utilizing boron atoms instead of the usual carbon has been predicted and described by researchers at Rice University. The B-80 structure is predicted to be more stable than the C-60 buckyball. One reason for this given by the researchers is that the B-80 is actually more like the original geodesic dome structure popularized by Buckminster Fuller which utilizes triangles rather than hexagons. However, this work has been subject to much criticism by quantum chemists as it was concluded that the predicted Ih symmetric structure was vibrationally unstable and the resulting cage undergoes a spontaneous symmetry breaking yielding a puckered cage with rare Th symmetry (symmetry of the volleyball)
number of six rings of carbon in this molecule is 20 and number of five member rings is 12
Variations of buckyballs
Another fairly common buckminsterfullerene is C70, but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.
In mathematical terms, the structure of a fullerene is a trivalent convex polyhedron with pentagonal and hexagonal faces. In graph theory, the term fullerene refers to any 3-regular, planar graph with all faces of size 5 or 6 (including the external face). It follows from Euler's polyhedron formula, |V|-|E|+|F| = 2, (where |V|, |E|, |F| indicate the number of vertices, edges, and faces), that there are exactly 12 pentagons in a fullerene and |V|/2-10 hexagons.
| | | | | 20-fullerene
(dodecahedral graph) | 26-fullerene graph | 60-fullerene
(truncated icosahedral graph) | 70-fullerene graph |
The smallest fullerene is the dodecahedron--the unique C20. There are no fullerenes with 22 vertices. The number of fullerenes C2n grows with increasing n = 12,13,14..., roughly in proportion to n9. For instance, there are 1812 non-isomorphic fullerenes C60. Note that only one form of C60, the buckminsterfullerene alias truncated icosahedron, has no pair of adjacent pentagons (the smallest such fullerene). To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes C200, 15,655,672 of which have no adjacent pentagons.
Trimetasphere carbon nanomaterials were discovered by researchers at Virginia Tech and licensed exclusively to Luna Innovations. This class of novel molecules comprises 80 carbon atoms (C80) forming a sphere which encloses a complex of three metal atoms and one nitrogen atom. These fullerenes encapsulate metals which puts them in the subset referred to as metallofullerenes. Trimetaspheres have the potential for use in diagnostics (as safe imaging agents), therapeutics and in organic solar cells.
Carbon nanotubes
Nanotubes are cylindrical fullerenes. These tubes of carbon are usually only a few nanometres wide, but they can range from less than a micrometer to several millimeters in length. They often have closed ends, but can be open-ended as well. There are also cases in which the tube reduces in diameter before closing off. Their unique molecular structure results in extraordinary macroscopic properties, including high tensile strength, high electrical conductivity, high ductility, high resistance to heat, and relative chemical inactivity (as it is cylindrical and "planar"—that is, it has no "exposed" atoms that can be easily displaced). One proposed use of carbon nanotubes is in paper batteries, developed in 2007 by researchers at Rensselaer Polytechnic Institute. Another proposed use in the field of space technologies and science fiction is to produce high-tensile carbon cables required by a space elevator.
Carbon nanobuds
Nanobuds have been obtained by adding Buckminsterfullerenes to carbon nanotubes.
Properties
For the past decade, the chemical and physical properties of fullerenes have been a hot topic in the field of research and development, and are likely to continue to be for a long time. Popular Science has published articles about the possible uses of fullerenes in armor. In April 2003, fullerenes were under study for potential medicinal use: binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma. The October 2005 issue of Chemistry and Biology contains an article describing the use of fullerenes as light-activated antimicrobial agents.
In the field of nanotechnology, heat resistance and superconductivity are some of the more heavily studied properties.
A common method used to produce fullerenes is to send a large current between two nearby graphite electrodes in an inert atmosphere. The resulting carbon plasma arc between the electrodes cools into sooty residue from which many fullerenes can be isolated.
There are many calculations that have been done using ab-initio Quantum Methods applied to fullerenes. By DFT and TD-DFT methods one can obtain IR, Raman and UV spectra. Results of such calculations can be compared with experimental results.
Aromaticity
Researchers have been able to increase the reactivity of fullerenes by attaching active groups to their surfaces. The Buckminsterfullerene does not exhibit "superaromaticity:" that is, the electrons in the hexagonal rings do not delocalize over the whole molecule.
A spherical fullerene of n carbon atoms has n pi-bonding electrons. These should try to delocalize over the whole molecule. The quantum mechanics of such an arrangement should be like one shell only of the well-known quantum mechanical structure of a single atom, with a stable filled shell for n = 2, 8, 18, 32, 50, 72, 98, 128, etc.; i.e. twice a perfect square; but this series does not include 60. As a result, C60 in water tends to pick up two more electrons and become an anion. The nC60 described below may be the result of C60's trying to form a loose metallic bonding.
Chemistry
Fullerenes are stable, but not totally unreactive. The sp2-hybridized carbon atoms, which are at their energy minimum in planar graphite, must be bent to form the closed sphere or tube, which produces angle strain. The characteristic reaction of fullerenes is electrophilic addition at 6,6-double bonds, which reduces angle strain by changing sp2-hybridized carbons into sp3-hybridized ones. The change in hybridized orbitals causes the bond angles to decrease from about 120 degrees in the sp2 orbitals to about 109.5 degrees in the sp3 orbitals. This decrease in bond angles allows for the bonds to bend less when closing the sphere or tube, and thus, the molecule becomes more stable.
Other atoms can be trapped inside fullerenes to form inclusion compounds known as endohedral fullerenes. An unusual example is the egg shaped fullerene Tb3N@C84, which violates the isolated pentagon rule. Recent evidence for a meteor impact at the end of the Permian period was found by analyzing noble gases so preserved. Metallofullerene-based inoculates using the rhonditic steel process are beginning production as one of the first commercially-viable uses of buckyballs.
Solubility
Fullerenes are sparingly soluble in many solvents. Common solvents for the fullerenes include aromatics, such as toluene, and others like carbon disulfide. Solutions of pure Buckminsterfullerene have a deep purple color. Solutions of C70 are a reddish brown. The higher fullerenes C76 to C84 have a variety of colors. C76 has two optical forms, while other higher fullerenes have several structural isomers. Fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature.
Some fullerene structures are not soluble because they have a small band gap between the ground and excited states. These include the small fullerenes C28, C36 and C50. The C72 structure is also in this class, but the endohedral version with a trapped lanthanide-group atom is soluble due to the interaction of the metal atom and the electronic states of the fullerene. Researchers had originally been puzzled by C72 being absent in fullerene plasma-generated soot extract, but found in endohedral samples. Small band gap fullerenes are highly reactive and bind to other fullerenes or to soot particles.
Solvents that are able to dissolve buckminsterfullerene (C60) are listed below in order from highest solubility. The value in parentheses is the approximate saturated concentration.
- 1-chloronaphthalene (51 mg/mL)
- 1-methylnaphthalene (33 mg/mL)
- 1,2-dichlorobenzene (24 mg/mL)
- 1,2,4-trimethylbenzene (18 mg/mL)
- tetrahydronaphthalene (16 mg/mL)
- carbon disulfide (8 mg/mL)
- 1,2,3-tribromopropane (8 mg/mL)
- bromoform (5 mg/mL)
- toluene (3 mg/ml)
- benzene (1.5 mg/ml)
- cyclohexane (1.2 mg/ml)
- carbon tetrachloride (0.4 mg/ml)
- chloroform (0.25 mg/ml)
- n-hexane (0.046 mg/ml)
- tetrahydrofuran (0.006 mg/ml)
- acetonitrile (0.004 mg/ml)
- methanol (0.00004 mg/ml)
- water (1.3x10-11 mg/mL)
Solubility of C60 in some solvents shows unusual behaviour due to existence of solvate phases (analogues of crystallohydrates). For example, solubility of C60 in benzene solution shows maximum at about 313 K. Crystallization from benzene solution at temperatures below maximum results in formation of triclinic solid solvate with four benzene molecules C60•4C6H6 which is rather unstable in air. Out of solution, this structure decomposes into usual fcc C60 in few minutes' time. At temperatures above solubility maximum the solvate is not stable even when immersed in saturated solution and melts with formation of fcc C60. Crystallization at temperatures above the solubility maximum results in formation of pure fcc C60. Large millimetre size crystals of C60 and C70 can be grown from solution both for solvates and for pure fullerenes.
Quantum mechanics
In 1999, researchers from the University of Vienna demonstrated that wave-particle duality applied to molecules such as fullerene. One of the co-authors of this research, Julian Voss-Andreae, has since created several sculptures symbolizing wave-particle duality in fullerenes (see Fullerenes in popular culture for more detail).
Science writer Marcus Chown stated on the CBC radio show Quirks and Quarks in May 2006 that scientists are trying to make buckyballs exhibit the quantum behavior of existing in two places at once (quantum superposition).
Safety and toxicity
When considering toxicological data, care must be taken to distinguish as necessary between what are normally referred to as fullerenes: (C60, C70,...); fullerene derivatives: C60 or other fullerenes with covalently bonded chemical groups; fullerene complexes (e.g., water-solubilized with surfactants, such as C60-PVP; host-guest complexes, such as with cyclodextrin; ), where the fullerene is physically bound to another molecule; C60 nanoparticles, which are extended solid-phase aggregates of C60 crystallites; and nanotubes, which are generally much larger (in terms of molecular weight and size) compounds, and are different in shape to the spheroidal fullerenes C60 and C70, as well as having different chemical and physical properties.
The above different compounds span the range from insoluble materials in either hydrophilic or lipophilic media, to hydrophilic, lipophilic, or even amphiphilic compounds, and with other varying physical and chemical properties. Therefore any broad generalization extrapolating for example results from C60 to nanotubes or vice versa is not possible, though technically all are fullerenes, as the term is defined as a close-caged all-carbon molecule. Any extrapolation of results from one compound to other compounds must take into account considerations based on a Quantitative Structural Analysis Relationship Study (QSARS), which mostly depends on how close the compounds under consideration are in physical and chemical properties.
In 1996 and 1997, Moussa et al. studied the in vivo toxicity of C60 after intra-peritoneal administration of large doses. No evidence of toxicity was found and the mice tolerated a dose of 5 000 mg/kg of body weight (BW). Mori et al. (2006)
could not find toxicity in rodents for C60 and C70 mixtures after oral administration of a dose of 2 000 mg/kg BW and did not observed evidence of genotoxic or mutagenic potential in vitro.
Other studies could not establish the toxicity of fullerenes: on the contrary, the work of Gharbi et al. (2005) suggested that aqueous C60 suspensions failing to produce acute or subacute toxicity in rodents could also protect their livers in a dose-dependent manner against free-radical damage.
A comprehensive and recent review on fullerene toxicity is given by Kolosnjaj et al. (2007a,b, c).
These authors review the works on fullerene toxicity beginning in the early 1990s to present, and conclude that very little evidence gathered since the discovery of fullerenes indicate that C60 is toxic.
With reference to nanotubes, a recent study of Poland et al. (2008) on carbon nanotubes introduced into the abdominal cavity of mice led the authors to suggest comparisons to "asbestos-like pathogenicity". It should be noted that this was not an inhalation study, though there have been several performed in the past, therefore it is premature to conclude that nanotubes should be considered to have a toxicological profile similar to asbestos. Conversely, and perhaps illustrative of how the various classes of compounds which fall under the general term fullerene cover a wide range of properties, Sayes, et al., found that in vivo inhalation of C60(OH)24 and nano-C60 in rats gave no effect, whereas in comparison quartz particles produced an inflammatory response under the same conditions (Nano Letters, 2007, Vol. 7, No. 8, 2399-2406). As stated above, nanotubes are quite different in chemical and physical properties to C60, i.e., molecular weight, shape, size, physical properties (such as solubility) all are very different, so from a toxicological standpoint, different results for C60 and nanotubes are not suggestive of any discrepancy in the findings.
Superconductivity
After the synthesis of macroscopic amounts of Fullerenes, their physical properties could be investigated. Very soon Haddon et al., found that intercalation of alkali-metal atoms in solid C60 leads to metallic behavior. In 1991, It was revealed that potassium-doped fullerenes becomes superconducting at 18K. This was the highest transition temperature for a molecular superconductor. Since then, Superconductivity has been reported in fullerene doped with various metals as well as potassium. It is shown that the superconducting transition temperature in alkaline-metal-doped fullerene increase in the unit cell volume V. As Cesium is the largest alkali ion, Cesium doped fullerene is an important material in this Family. Recently, superconductivity at 38K is reported in bulk Cs3C60.
The increase of transition temperature with the unit-cell volume had been believed to be an evidence for the BCS mechanism of C60 solid superconductivity. Because inter C60 separation can be related to an increase in the density of states on the Fermi level, N(eF). Therefore, there have been many efforts to increasing the interfullerene separation. In particular, intercalating neutral molecules into the A3 C60 lattice to increase the interfullerene spacing while the valence of C60 is kept unchanged. However, this ammoniation technique has revealed a new aspect of fullerene intercalation compound: the Mott-Hubbard transition and the correlation between the orientation/orbital order of C60 molecules and the magnetic structure.
The C60 molecules compose a solid of weakly bound molecules. The Fullerites are therefore molecular solids, in which the molecular properties still survive. The discrete levels of a free C60 molecule are only weakly broadened in the solid, which leads to a set of essentially nonoverlapping bands with a narrow width of about 0.5 eV. For an undoped C60 solid, the 5-fold hu band is HOMO level, and the 3-fold t1u band is LUMO level(empty), and this system is a band insulator. But, when The C60 solid is doped with metal atoms, this metal atoms give electrons to the t1u band or the upper 3-fold t1g band. So, this electron partial occupation of band leads to sometimes metallic behavior. However, A4C60 is an insulator, although the t1u band is only partially filled and should be a metal according to band theory. this unpredicted behavior can be probably explained by the Jahn-Teller effect, where spontaneous deformations of high-symmetry molecules induce the splitting of degenerate levels to gain the electronic energy. The Jahn-Teller type electron-phonon interaction is strong enough in C60 solids to destroy the band picture for particular valence states.
A narrow band or strongly correlated electronic system and degenerated ground states are important points to understand superconductivity in fullerene solids. When the interelectron repulsion U is greater than bandwidth, insulating localized electron ground state is produced in the simple Mott-Hubbard model. This explain well about the absence of superconductivity at ambient pressure in Cesium doped C60 solids. electron-correlation driven localization of the t1u electrons exceeds the critical value, this leads to the Mott-insulator. The application of high pressure decreases the interfullerene spacing, therefore Cesium doped C60 solids turns to metallic and superconducting. Actually, a fully developed theory of C60 solids superconductivity is still lacking. But, it has accepted widely that strong electronic correlations and Jahn-Teller electron-phonon coupling. produce local electron pairing show high transition temperature close to the insulator-metal transition. .
Examples of fullerenes in popular culture
Examples of fullerenes in popular culture are numerous. Fullerenes appeared in fiction well before scientists took serious interest in them. In New Scientist there used to be a weekly column called "Daedalus" written by David Jones, which contained humorous descriptions of unlikely technologies. In 1966 the columnist included a description of C60 and other forms of graphite. This was meant as pure entertainment. Fullerenes are part of the plot in a science fiction novel named Decipher written by Stel Pavlou and published in 2001. Bucky Balls were also mentioned in A&E's 2008 remake of The Andromeda Strain, which is based on the Michael Crichton novel of the same name; John Ringo uses fullerene as a carrier for antimatter in his legacy of the aldenata series other novels to make mention of them include Kim Stanley Robinson's Mars Trilogy; Iron by Poul Anderson and Neal Stephenson's The Diamond Age. Buckyballs are referenced in similarity with a large spherical meteorite with the potential to unleash a virus that could kill all organic life on Earth In Clive Cussler's novel The Sacred Stone. Buckyball is the state molecule of Texas.
Fullerite (solid state)
Fullerites are the solid-state manifestation of fullerenes and related compounds and materials.
Polymerized single-walled nanotubes (P-SWNT) are a class of fullerites and are comparable to diamond in terms of hardness. However, due to the way that nanotubes intertwine, P-SWNTs do not have the corresponding crystal lattice that makes it possible to cut diamonds neatly. This same structure results in a less brittle material, as any impact that the structure sustains is spread out throughout the material. Because nanotubes are still very expensive to produce in useful quantities, uses for a material lighter and stronger than steel will have to wait until nanotube production becomes more economically viable.
Ultrahard fullerite, Buckyball
Ultrahard fullerite (C60) is a form of carbon synthesized under high pressure high temperature conditions. It is believed that fullerene molecules are three-dimensionally polymerized in this material.
See also
Further reading
External links
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