Special Reports

Diamonds may once have been a girl's best friend, but chemists know they are only lumps of graphite in disguise. Both are forms of pure carbon although they have different properties. During the past five years, chemists have amassed overwhelming evidence for a third form, known collectively as fullerenes, based on closed cages of carbon atoms. The discovery of these molecules has opened up a new field of carbon chemistry - and the race to discover them was almost as intriguing as the molecules themselves.

The most symmetrical of the fullerenes, buckminsterfullerene, is a perfect sphere of 60 carbon atoms. Chemists are now speculating that buckminsterfullerene might be formed every time we light a candle. They also suggest that it might be abundant in clouds of interstellar dust, and therefore among the oldest of molecules.

Improbably enough, the buckminsterfullerene story began in interstellar space. In 1982, Donald Huffman at the University of Arizona in Tucson was working with Wolfgang Kratschmer at the Max Planck Institute for Nuclear Physics in Heidelberg. These physicists shared an interest in the properties of soot formed by heating graphite. They were keen to find out if such soot could form in interstellar space.

As part of their studies, Huffman and Kratschmer heated graphite rods electrically in a low-pressure atmosphere of helium or argon inside a bell jar. From the resulting clouds of black smoke they collected thin layers of soot on the surfaces of quartz discs. They transferred these discs to a spectrometer and measured how the soot absorbs ultraviolet light.

Carbon soot formed conventionally - by burning coal in a normal atmosphere for example - absorbs ultraviolet light over a broad range of wavelengths with a peak absorption at about 220 nanometres. Huffman and Kratschmer noticed that their artificially produced soot showed some extra 'humps' in its absorption spectrum. Kratschmer thought these must be caused by contamination of their apparatus with oil vapour from the vacuum pump, and when they lowered the pressure inside the bell jar, the humps did indeed become less visible. Everything seemed to point to an experimental artefact, and the physicists soon turned their attention elsewhere.

Meanwhile, Harold Kroto and David Walton at the University of Sussex had for 10 years been pursuing an interest in some unusual interstellar molecules called cyanopolyynes. These molecules contain long, straight chains of carbon atoms linked by alternate single and triple bonds. In 1975, together with radio astronomers, they had found a cyanopolyyne molecule with a chain of five carbon atoms in interstellar space. This was quickly followed by their discovery of cyanopolyynes with chains of seven and nine carbon atoms.

Kroto and Walton believed that molecules with much longer chains, perhaps up to 33 atoms long, might also be formed, possibly in the atmospheres surrounding red giant stars. But the chances of finding such molecules in space seemed extremely slim, and reproducing the outer atmospheres of red giant stars in the laboratory almost impossible. Then, in 1984, Kroto heard about some new laser vaporisation equipment which might do the job.

Richard Smalley and his team of chemists at Rice University in Houston, Texas, were using a high-power laser to blast atoms from the surface of a solid target. A flowing stream of inert gas picked up the atoms and transported them through a small nozzle into a vacuum chamber. There the gas expanded, making the atoms cool rapidly and cluster into new molecules which could be studied using mass spectrometry.

Kroto saw that by combining a graphite target and the high temperatures generated by the laser (tens of thousands of degrees celsius), he might be able to make long chains of carbon atoms characteristic of cyanopolyynes. When Kroto arrived in Houston in August 1985 to do these experiments, Smalley was initially sceptical that the work would produce anything novel; after all, a team of scientists at Exxon's corporate research laboratories in New Jersey had already done similar experiments. They had found a range of carbon molecules, which always contained even numbers of atoms but with no particular size dominating the mass spectrum. But Smalley agreed to Kroto's proposal and, with his colleague Robert Curl and postgraduate students Jim Heath and Sean O'Brien, they set about looking for long carbon chains.

They soon found them, but their attention was quickly caught by some other, larger molecules they had produced. At first, their measurements on molecules containing 40 or more carbon atoms seemed identical to those reported by the Exxon group. But the signal in the mass spectrum corresponding to a molecule of exactly 60 carbon atoms behaved erratically. It was always slightly bigger than the others, but its size was unpredictable. A few frantic days later, C60, now named buckminsterfullerene, was a fact (see Box overleaf).

The chemists attributed the extraordinary stability of C60 to its structure, a closed cage of carbon atoms made up of 12 pentagons and 20 hexagons that fitted together just like those on the surface of a football. But at the time of their discovery they had no direct evidence to support this notion. Why should such a perfectly ordered structure form spontaneously out of the chaos of carbon atoms produced by the laser?

During the next few years, the group at Rice University gathered a large body of circumstantial evidence to support the football structure. For instance, they found that metal atoms bind strongly to C60 at a single position, presumed to be inside the cage. By firing a laser at such metal-C60 compounds, they could rupture the cage, which spat out two carbon atoms before closing up again, in a kind of 'shrink-wrap' reaction. They discovered that they could shrink-wrap the cage down to the theoretical minimum number of carbon atoms needed to enclose the metal atom, beyond which the molecule fell apart.

Unfortunately, no one could make enough C60 to analyse it spectroscopically and so put its structure beyond doubt. As chemists speculated about it, often amid considerable controversy, theorists calculated the properties that buckminsterfullerene ought to have. Most importantly, they worked out what its ultraviolet absorption spectrum might look like.

At a conference on interstellar dust held in California in the summer of 1988, Huffman surprised the audience (which included Kratschmer) with a remarkable suggestion. He proposed that the extra 'humps' he and Kratschmer had seen six years earlier in the ultraviolet spectrum of carbon soot were caused by buckminsterfullerene.

Kratschmer thought the idea a little far-fetched, but back in Heidelberg, with his students Bernd Wagner and Konstantinos Fostiropoulos, he measured the mass spectrum and the ultraviolet and infrared absorption spectra of the soot formed in the bell-jar. The results were fairly close to the theorists' predictions for buckminsterfullerene. They also measured the infrared spectrum of soot formed from rods of carbon-13, which is heavier by one neutron than normal carbon-12. If buckminsterfullerene was being formed, the heavier mass of carbon-13 ought to affect its infrared spectrum in a predictable way compared with buckminsterfullerene made from carbon-12. This was exactly what they found.

The news spread to Sussex in late 1989. Kroto was stunned. Together with his student Ken McKay, he had tried similar bell-jar experiments two years earlier but had had to abandon them through lack of funding. With a little support from British Gas, Kroto and his students Jonathan Hare and Amit Sarkar went back to the original experiment and found that they could reproduce the physicists' infrared results. With Ala'a Abdul-Sada, another student at Sussex, they too showed that the soot really did contain C60.

Hare, who had trained as a physicist, took the chemists' logical next step. In August 1990, he decided to try to dissolve the soot in something, and chose benzene. Carbon soot does not normally dissolve in benzene but, remarkably, he produced a red solution. C60 must be the explanation.

If they could obtain a pure sample of C60 in solution, its carbon-13 nuclear magnetic resonance (NMR) spectrum would prove that this molecule has a football structure. NMR relies on the fact that some nuclei, including carbon-13, behave like tiny magnets and will line up in the same direction if placed in a magnetic field. They can then be made to flip or 'resonate' at a particular radio frequency, called the resonance frequency. This depends on the extent to which the nucleus is 'screened' from the magnetic field by electrons in neighbouring atoms, and so depends on the precise chemical environment of the nucleus. The NMR spectrum therefore reveals how atoms of carbon-13 are arranged in the molecule.

In the highly symmetrical buckminsterfullerene, all 60 carbon atoms are arranged identically - there is only one type of carbon atom. No matter where a carbon-13 nucleus sits, it should produce the same NMR resonance frequency, and the spectrum should consist of a single line. Kroto had been dreaming of this spectrum for five years.

Meanwhile, Huffman and Kratschmer were one step ahead. By late 1989 they were using their bell jar equipment to make large amounts (measurable in milligrams) of what they now believed to be C60. In May 1990, they tried to separate C60 from the rest of the soot by subliming it - heating it in a vacuum to 300 to 400 °C and collecting the solid as it condensed from vapour. Part of the sublimed solid was soluble in benzene, and evaporating the benzene left red-brown hexagonal crystals which they called 'fullerite'. When they analysed fullerite spectroscopically, they found that it was 90 per cent C60 and 10 per cent C70.

Next, Huffman, Kratschmer, Fostiropoulos and Huffman's colleague from Tucson, Lowell Lamb, measured the crystal structure of fullerite using X-ray and electron diffraction. These measurements confirmed that C60 molecules in the solid are spherical and packed about 1.04 nanometres apart. This is consistent with both the predicted diameter of individual carbon footballs (0.7 nanometres) and the spacing of carbon layers in graphite (about 0.3 nanometres). The physicists sent their results to the journal Nature in August 1990.

Kroto and his colleagues were dismayed that they had been beaten to publication of the structural details of C60, by a matter of days, but Kroto felt that the carbon-13 NMR spectrum was still the jewel in the crown. With the help of Roger Taylor, another Sussex colleague, they managed to separate C60 from its C70 'impurity'. C60 is a mustard-coloured solid that dissolves in benzene to give a beautiful magenta solution, and C70 is a reddish-brown solid that gives solutions the colour of red wine. When they measured the carbon-13 NMR spectrum of a solution of C60, it had just one line.

This result is certainly dramatic, but their work on C70 was equally important, as it provides compelling evidence for a whole family of fullerenes. In C70, the 10 extra atoms run around the middle of the cage, producing a structure which looks like a squeezed sphere. There are five types of carbon atom in this structure, each surrounded by a slightly different arrangement of atoms. These different types of atom are present in the ratio 10:10:20:20:10. When Kroto and his colleagues measured the carbon-13 NMR spectrum of C70, they found the expected pattern of five peaks, with intensities in the right ratio. They sent the C60 and C70 NMR spectra to the journal Chemical Communications on 10 September (see also New Scientist, Science, 13 October 1990).

Meanwhile, Donald Bethune and Gerard Meijer at IBM's Almaden Research Center at San Jose in California had abandoned their attempts to make C60 using a variation of the Rice group's laser vaporisation equipment, in favour of Huffman and Kratschmer's method. Using almost every known modern spectroscopic technique, they soon confirmed the closed cage structure of C60. After working round the clock for about a week, their colleague Robert Johnson measured the carbon-13 NMR spectrum of C60. They sent their results to the Journal of the American Chemical Society on 17 September, only seven days after the Sussex group had submitted their results for publication.

Two days earlier, Bethune had arrived at San Fransisco airport from a trip abroad to find Meijer insisting that he go with him immediately to the laboratory at Almaden. Their colleague Robert Wilson had taken pictures of fullerite molecules stuck to a flat plane of gold atoms, using the new technique of scanning tunnelling microscopy. There, on the monitor screen, was a picture showing hexagonal arrays of spherical C60 molecules about 1.1 nanometres in diameter, interspersed with slightly larger and brighter molecules, probably C70. Huffman and Kra tschmer, working with physicists at the University of Missouri, produced similar pictures.

By this time no one could doubt the existence of fullerenes. The next challenge was to do some chemistry. Within a few months of the publication of the X-ray crystal structure of fullerite, Smalley's group and other scientists at Rice University adapted Huffman and Kra tschmer's method to make a new 'C60 generator'. This consists of a graphite rod, spring mounted against a graphite disc, inside a metal cylinder filled with inert gas. Forcing a high current between the rod and disc makes the graphite rod vaporise at a rate of up to 10 grams per hour, depositing soot (and the fullerite it contains) on the inside of the cylinder. The key to the success of the generator is a simple pencil sharpener: striking the arc is easier if the rod is sharpened to a point. Using the C60 generator, the group at Rice can make about a gram of fullerite in a few hours. A further separation step yields pure C60.

The group at Rice University were also the first to modify C60 chemically. They reacted C60 with lithium, liquid ammonia and tertiary-butyl alcohol to add hydrogen, making both C60H18 and C60H36. In a further reaction they re-formed C60, which implies that the closed cage structure is preserved when C60 is converted into new molecules. In another experiment, they added electrons to C60 to form negatively charged 'buckide' ions, opening up the possibility of a range of new ionic salts.

In 1985, David Walton compared the discovery of buckminsterfullerene to the discovery 120 years before of the ring structure of benzene. The idea of a six-carbon ring linked by alternate single and double bonds paved the way for organic chemists to understand and develop the whole field of aromatic chemistry. Fullerene chemistry may turn out to be equally important.

Buckminsterfullerene is more than just an interesting new molecule, however. It represents a whole new concept in molecular architecture (see also 'Molecules waiting to be made', New Scientist, 17 November 1990). Chemists at Rice University have already shown that metal atoms and positively charged metal ions can be incorporated inside the cage. In future, such modifications might also enable them to control the chemistry outside it. Making these new molecules in large quantities will require further modifications to the C60 generator, but there seems to be no major obstacles to this.

Buckminsterfullerene is already manufactured and sold as a fine chemical by at least three American chemical companies. The price varies from $1250 per gram of fullerite to $200 for enough raw soot to make one gram of fullerite. Curl believes that material 90 per cent pure might in future cost as little as 10 cents a gram. There appear to be boundless possibilities for C60, both as a starting chemical for making new products and drugs and as a new material. Scientists are already speculating on its potential as a lubricant - a collection of molecular ball bearings - as a semiconductor and as a catalyst. In May, a team of scientists from the University of California at Los Angeles described how solid C60 'doped' with potassium atoms behaves as a superconductor at the relatively high temperature of 19.3 K.

Astrophysicists are now searching for evidence that C60 is present, probably in ionised form, in interstellar space. In March, scientists at IBM and at NASA's Ames Research Center reported that they had found no fullerenes in samples taken from the Murchison and Allende meteorites, possibly because the growing carbon molecules react with hydrogen in space before C60 can form. Clearly, scientists will not find buckminsterfullerene everywhere they look. But at least they now know what to look for.

* * *

Seven days in the discovery of a molecule

Wednesday-Friday, 4-6 September 1985

Jim Heath, Sean O'Brien and Harry Kroto work on the laser vaporisation experiment in Richard Smalley's laboratory in Houston. They are intrigued to find molecules containing large numbers of carbon atoms and, like the Exxon group, observe that only molecules with even numbers of atoms are formed. The mass spectrometer signal for C60 appears slightly larger than - the rest. At first these results appear identical to those previously obtained by scientists at Exxon. Then they measure one spectrum in which the C60 signal is much larger than the others (see figure (a)).

Heath, O'Brien and Kroto all note this result and discuss it with Smalley and Robert Curl. They decide to find out why the C60 signal is so large. At Curl's suggestion, Heath agrees to carry on working on the experiment over the weekend.

Saturday-Sunday, 7-8 September

Heath plays around with the conditions under which the molecules are formed, and tries differently shaped nozzles. He succeeds in dramatically increasing the C60 signal. relative to the others. The resulting, spectrum is dominated by C60 (see figure (b)) and there is a clear signal corresponding to a slightly larger molecule, C70.

Monday, 9 September

The chemists are amazed by this spectacular result. They discuss the possible structures that might explain the special stability of C60. There are two prime candidates. The first is a sandwich structure with layers of hexagons similar to graphite and which has an overall spherical shape. But' this is expected to have reactive edges ('dangling bonds') which - would make it less stable than C60 appears to be. They believe that the alternative, a closed spherical cage, is more plausible. This has no edges at all. (In fact, the chemists had been anticipated by David Jones, writing in New Scientist as Daedalus, who nearly 20 years earlier had suggested the idea of round, hollow 'balloons' of carbon in the form of graphite.)

Kroto, an admirer of the eccentric architect Buckminster Fuller, remembers seeing the geodesic dome Fuller had designed for the US Pavilion at the Montreal Expo in 1967. Kroto initially thinks hat this structure had been made up completely of hexagons, and wonders if C60 could be a similar geodesic sphere. But at the time, none of the chemists realise that it is impossible to form a closed structure solely from hexagons, as the matehmatician Leonhard Euler showed in theh 18th century. Fuller's geodesic dome had incorporated some pentagons.

Then Kroto remebers a polyhedral 'stardome' he had once assembled for his children from a cardboard kit. He thinks that this cardboard structure has several pentagonal faces, in addition to the hexagonal ones.

Kroto, Heath, O'Brien and Smalley continue their discussion over dinner, and the possible importance of Pentagons is mentioned again. Later, Heath and his wife Carmen attempt to build a geodesic model using 60 soft sweets and toothpicks, and conclude that a closed structure cannot be made out of hexagons.

Meanwhile, Smalley settles down with paper scissors, sticky. tape and a bottle of beer, and tries to construct a closed sphere trom paper hexagons. It is impossible, even by cheating. Then he remembers the pentagons. To his delight, he discovers that he can make a sphere out of 20 hexagons and 12 pentagons. He holds his breath and counts the vertices: there are 60. Here, surely, was the answer. The exceptional stability of C60 is also accounted for when he drops his paper structure, it bounces.

Tuesday, 10 September

Smalley calls the group of researchers together in his office and tosses the paper model onto the desk. Kroto is ecstatic; the model appears identical to his half-remembered stardome. Heath. and O'Brien are also very pleased. But Curl is not satisfied until he has checked that the bonding between the 'carbon atoms makes sense.

A carbon atom has four electrons in its outermost orbitals which must be used up in bonds to other carbon atoms if the molecule is to be stable. Using sticky labels, each with two parallel lines drawn on it to represent a double bond, Kroto and Curl find that the bonding does work.

Smalley is convinced that such a symmetrical and elegant structure must already be well known. He telephones William Veech, chairman of the mathematics department at Rice, and describes it. Veech calls back. The structure is indeed familiar: ' .. what you've got there, boys, is a soccer ball.'

Kroto, Heath, O'Brien, Curl and Smalley write up the C60 discovery in a short article, which is eventually published in Nature on 14 November.

The new molecule needs a name. The chemists agree to honour Buckminster Fuller's architectural vision, settling on 'buckminsterfullerene'. But this is something of a mouthful, and the new molecule quickly becomes more familiarly known as 'buckyball'.

Jim Baggott is a freelance science writer.