Fullerene

A fullerene is an allotrope of carbon whose molecules consist of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to six atoms. The molecules may have hollow sphere- and ellipsoid-like forms, tubes, or other shapes.
Fullerenes with a closed mesh topology are informally denoted by their empirical formula C_n, often written Cn, where n is the number of carbon atoms. However, for some values of n there may be more than one isomer.
The family is named after buckminsterfullerene, the most famous member, which in turn is named after Buckminster Fuller. The closed fullerenes, especially C₆₀, are also informally called buckyballs for their resemblance to the standard ball of association football. Nested closed fullerenes have been named bucky onions. Cylindrical fullerenes are also called carbon nanotubes or buckytubes. The bulk solid form of pure or mixed fullerenes is called fullerite.
Fullerenes had been predicted for some time, but only after their accidental synthesis in 1985 were they detected in nature and outer space. The discovery of fullerenes greatly expanded the number of known allotropes of carbon, which had previously been limited to graphite, diamond, and amorphous carbon such as soot and charcoal. They have been the subject of intense research, both for their chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.

Definition

IUPAC defines fullerenes as "polyhedral closed cages made up entirely of n three-coordinate carbon atoms and having 12 pentagonal and hexagonal faces, where n ≥ 20."

History

Predictions and limited observations

The icosahedral cage was mentioned in 1965 as a possible topological structure. Eiji Osawa predicted the existence of in 1970. He noticed that the structure of a corannulene molecule was a subset of the shape of a football, and hypothesised that a full ball shape could also exist. Japanese scientific journals reported his idea, but neither it nor any translations of it reached Europe or the Americas.
Also in 1970, R. W. Henson proposed the structure and made a model of it. Unfortunately, the evidence for that new form of carbon was very weak at the time, so the proposal was met with skepticism, and was never published. It was acknowledged only in 1999.
In 1973, independently from Henson, D. A. Bochvar and E. G. Galpern made a quantum-chemical analysis of the stability of and calculated its electronic structure. The paper was published in 1973, but the scientific community did not give much importance to this theoretical prediction.
Around 1980, Sumio Iijima identified the molecule of from an electron microscope image of carbon black, where it formed the core of a particle with the structure of a "bucky onion".
Also in the 1980s at MIT, Mildred Dresselhaus and Morinobu Endo, collaborating with T. Venkatesan, directed studies blasting graphite with lasers, producing carbon clusters of atoms, which would be later identified as "fullerenes."

Discovery of

In 1985, Harold Kroto of the University of Sussex, working with James R. Heath, Sean O'Brien, Robert Curl and Richard Smalley from Rice University, discovered fullerenes in the sooty residue created by vaporising carbon in a helium atmosphere. In the mass spectrum of the product, discrete peaks appeared corresponding to molecules with the exact mass of sixty or seventy or more carbon atoms, namely and. The team identified their structure as the now familiar "buckyballs".
The name "buckminsterfullerene" was eventually chosen for by the discoverers as an homage to American architect Buckminster Fuller for the vague similarity of the structure to the geodesic domes which he popularized; which, if they were extended to a full sphere, would also have the icosahedral symmetry group. The "ene" ending was chosen to indicate that the carbons are unsaturated, being connected to only three other atoms instead of the normal four. The shortened name "fullerene" eventually came to be applied to the whole family.
Kroto, Curl, and Smalley were awarded the 1996 Nobel Prize in Chemistry for their roles in the discovery of this class of molecules.

Further developments

Kroto and the Rice team already discovered other fullerenes besides C₆₀, and the list was much expanded in the following years. Carbon nanotubes were first discovered and synthesized in 1991.
After their discovery, minute quantities of fullerenes were found to be produced in sooty flames, and by lightning discharges in the atmosphere. In 1992, fullerenes were found in a family of mineraloids known as shungites in Karelia, Russia.
The production techniques were improved by many scientists, including Donald Huffman, Wolfgang Krätschmer, Lowell D. Lamb, and Konstantinos Fostiropoulos. Thanks to their efforts, by 1990 it was relatively easy to produce gram-sized samples of fullerene powder. Fullerene purification remains a challenge to chemists and to a large extent determines fullerene prices.
In 2010, the spectral signatures of C₆₀ and C₇₀ were observed by NASA's Spitzer infrared telescope in a cloud of cosmic dust surrounding a star 6500 light years away. Kroto commented: "This most exciting breakthrough provides convincing evidence that the buckyball has, as I long suspected, existed since time immemorial in the dark recesses of our galaxy." According to astronomer Letizia Stanghellini, "It's possible that buckyballs from outer space provided seeds for life on Earth." In 2019, ionized C₆₀ molecules were detected with the Hubble Space Telescope in the space between those stars.

Types

There are two major families of fullerenes, with fairly distinct properties and applications: the closed buckyballs and the open-ended cylindrical carbon nanotubes. However, hybrid structures exist between those two classes, such as carbon nanobuds — nanotubes capped by hemispherical meshes or larger "buckybuds".

Buckyballs

Buckminsterfullerene

Buckminsterfullerene is the smallest fullerene molecule containing pentagonal and hexagonal rings in which no two pentagons share an edge. It is also most common in terms of natural occurrence, as it can often be found in soot.
The empirical formula of buckminsterfullerene is and its structure is a truncated icosahedron, which resembles an association football 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 buckminsterfullerene molecule is about 1.1 nanometers. The nucleus to nucleus diameter of a buckminsterfullerene molecule is about 0.71 nm.
The buckminsterfullerene molecule has two bond lengths. The 6:6 ring bonds can be considered "double bonds" and are shorter than the 6:5 bonds. The weighted average bond length is 1.44 Å.

Other fullerenes

Another fairly common fullerene has empirical formula, but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.
The smallest possible fullerene is the dodecahedral. There are no fullerenes with 22 vertices. The number of different fullerenes C_2n grows with increasing n = 12, 13, 14, ..., roughly in proportion to n⁹. For instance, there are 1812 non-isomorphic fullerenes. Note that only one form of, buckminsterfullerene, has no pair of adjacent pentagons. To further illustrate the growth, there are 214,127,713 non-isomorphic fullerenes, 15,655,672 of which have no adjacent pentagons. Optimized structures of many fullerene isomers are published and listed on the web.
Heterofullerenes have heteroatoms substituting carbons in cage or tube-shaped structures. They were discovered in 1993 and greatly expand the overall fullerene class of compounds and can have dangling bonds on their surfaces. Notable examples include boron, nitrogen, oxygen, and phosphorus derivatives.

Carbon 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 heat conductivity, and relative chemical inactivity. One proposed use of carbon nanotubes is in paper batteries, developed in 2007 by researchers at Rensselaer Polytechnic Institute. Another highly speculative proposed use in the field of space technologies is to produce high-tensile carbon cables required by a space elevator.

Derivatives

Buckyballs and carbon nanotubes have been used as building blocks for a great variety of derivatives and larger structures, such as

Nested buckyballs proposed for lubricants;
Nested carbon nanotubes
Linked "ball-and-chain" dimers
Rings of buckyballs linked together.
Heterofullerenes and non-carbon fullerenes

After the discovery of C60, many fullerenes have been synthesized in which some or all the carbon atoms are replaced by other elements. Non-carbon nanotubes, in particular, have attracted much attention.

Boron

A type of buckyball which uses boron atoms, instead of the usual carbon, was predicted and described in 2007. The structure, with each atom forming 5 or 6 bonds, was predicted to be more stable than the buckyball. However, subsequent analysis found that the predicted I_h symmetric structure was vibrationally unstable and the resulting cage would undergo a spontaneous symmetry break, yielding a puckered cage with rare T_h symmetry. The number of six-member rings in this molecule is 20 and number of five-member rings is 12. There is an additional atom in the center of each six-member ring, bonded to each atom surrounding it. By employing a systematic global search algorithm, it was later found that the previously proposed fullerene is not a global maximum for 80-atom boron clusters and hence can not be found in nature; the most stable configurations have complex geometries. The same paper concluded that boron's energy landscape, unlike others, has many disordered low-energy structures, hence pure boron fullerenes are unlikely to exist in nature.
However, an irregular complex dubbed borospherene was prepared in 2014. This complex has two hexagonal faces and four heptagonal faces with in D_2d symmetry interleaved with a network of 48 triangles.
was experimentally obtained in 2024, i.e. 17 years after theoretical prediction by Gonzalez Szwacki et al..