Materials
Fullerenes and C60
For most of the twentieth century carbon was known in a few solid forms, chief among them graphite and diamond. In 1985 a new form turned up in a laser experiment: a closed cage of sixty atoms shaped like a football. The fullerenes, as the family came to be called, helped launch modern nanoscience.
What a fullerene is
The Wikipedia article on fullerenes defines a fullerene as an allotrope of carbon whose molecules are closed cages of carbon atoms, usually arranged in five- and six-membered rings, and which may be spherical, ellipsoidal or tubular. The family is named after its best-known member, buckminsterfullerene or C60, which in turn is named after the architect Buckminster Fuller, whose geodesic domes it resembles. Closed fullerenes, especially C60, are informally called buckyballs.
The geometry has a neat rule behind it. The article quotes the IUPAC definition of fullerenes as closed polyhedral cages of three-coordinate carbon atoms having exactly 12 pentagonal faces and a number of hexagonal faces that grows with the size of the cage. The twelve pentagons are what allow a sheet of hexagons to close into a ball; add more hexagons and the cage grows larger or longer, which is one way of seeing the kinship between fullerenes and nanotubes.
C60 in detail
The Wikipedia article on buckminsterfullerene describes C60 as a cage of twenty hexagons and twelve pentagons in the shape of a truncated icosahedron, with each of its sixty atoms bonded to three neighbours. In bulk it is a black solid that dissolves in hydrocarbon solvents to give a purple solution. The article also notes that C60 exists in space: the neutral molecule has been observed in planetary nebulae and several types of star, and its ionised form has been identified in the interstellar medium.
A predicted molecule, found by accident
The fullerene story has a long prehistory of predictions that nobody followed up. The fullerene article records that the icosahedral cage was mentioned as a possible structure in 1965, that Eiji Osawa predicted C60 in 1970 in Japanese journals that did not reach Western readers, and that two other chemists published a quantum-chemical analysis of its stability in 1973. None of this drew much attention at the time.
The buckminsterfullerene article describes what happened next. In 1984 a group using a laser to vaporise carbon in a supersonic helium beam produced the molecule without recognising it. In 1985 the experiment was repeated at Rice University by Harold Kroto, James R. Heath, Sean C. O'Brien, Robert Curl and Richard Smalley, who recognised the structure of C60 and proposed the closed-cage explanation. Part of the motivation, the article notes, came from astrophysics: the team was interested in carbon clusters that might explain infrared emission from carbon-rich stars.
From traces to grams
For five years fullerenes could be made only in tiny quantities in molecular beams, which limited what anyone could do with them. According to the buckminsterfullerene article, the change came in 1989 and 1990, when the physicists Wolfgang Krätschmer, Konstantinos Fostiropoulos and Donald R. Huffman studied soot made by an electric arc between graphite electrodes in helium. They found infrared bands matching those predicted for C60, then extracted a benzene-soluble material from the soot whose electron microscopy, X-ray analysis and mass spectra were consistent with C60 molecules. The article describes the arc method as simple and efficient enough to make the material in gram amounts per day, and says it greatly boosted fullerene research.
That is why the fullerene literature has two distinct phases. The late 1980s literature is mainly about gas-phase clusters and their mass spectra; the 1990s literature, once gram quantities were available, is about the chemistry of the cages, their crystals, the metals and molecules that can be trapped inside them, and their electronic properties.
The 1996 Nobel Prize
The Royal Swedish Academy of Sciences press release for the 1996 Nobel Prize in Chemistry announced the award to Robert F. Curl Jr. and Richard E. Smalley of Rice University and Harold W. Kroto of the University of Sussex for their discovery of fullerenes. The release explains that fullerenes form when vaporised carbon condenses in an inert gas, and that the number of atoms in the shell can vary, so that many new carbon structures became known. It also notes that before the discovery six crystalline forms of carbon had been recognised, including two kinds of graphite and two kinds of diamond.
Why fullerenes matter to nanoscience
Few large-scale applications of fullerenes have emerged; the buckminsterfullerene article itself says that the substance has received intense study but that few real-world applications have been found. Their importance lies elsewhere. They showed that carbon could form discrete, nanometre-sized molecules with well-defined shapes, they trained a generation of researchers in making and characterising such objects, and they led directly to the search for other carbon nanostructures. The history of the idea of nanotechnology counts their discovery as one of the events that brought the field into being, and the tube and sheet forms that followed are described on the carbon nanotubes and graphene pages.