Buckyballs – a new sphere of science

Key text

Carbon is well-studied: an entire scientific discipline, organic chemistry, is based on it. Yet despite all the research, it was only in 1985 that one of its most extraordinary features was discovered.

Up until then, scientists knew of only two forms in which pure carbon occurred: diamond and graphite. Both these substances consist entirely of carbon atoms, but differ greatly in their structure and physical properties. In diamond, each carbon atom is bound to four other carbon atoms in a pattern of tetrahedrons. This structure makes diamond extremely hard.

In graphite, the carbon atoms form sheets of linked hexagons, giving the appearance of chicken wire. Each carbon atom within a sheet forms strong bonds to three other carbon atoms, but the stacked sheets are only held together by weak bonds. This means that the sheets can slide past each other, giving graphite its soft and greasy feel.

diamond

graphite

The discovery

In 1985, British chemist Harry Kroto was puzzling over strange chains of carbon atoms that could be detected billions of kilometres away in space by radiotelescopes. He thought that these chains might form in conditions that are found near red giant stars.

Kroto visited the US laboratory of Richard Smalley and Robert Curl, who were studying 'clusters' – aggregates of atoms that only exist briefly. Together they attempted to create high-temperature conditions in the laboratory, conditions similar to those near red giants. They vaporised graphite with a powerful laser in an atmosphere of helium gas.

When they analysed the resulting carbon clusters, they found many previously unknown carbon molecules. These varied in size, but the most common molecule contained 60 carbon atoms.

The structure of this molecule was not immediately apparent, although Curl, Kroto and Smalley knew that it was extremely stable. Only a spherical molecule, they reasoned, could produce this stability. After considerable debate, they worked out that the only geometric shape that could combine 60 carbon atoms into a spherical structure was a set of interlocking hexagons and pentagons (Box 1: Finding the molecular structure of buckyballs). Incidentally, an Australian theoretician at the University of California at Berkeley, Tony Haymet, published a paper at about this time predicting the existence of such a compound (he called it 'footballene').

Kroto and his colleagues then discovered that the combination of hexagons and pentagons also formed the basis of a geodesic dome designed by the architect and engineer, R. Buckminster Fuller, for the 1967 Montreal World Exhibition. So they decided to name the new molecule buckminsterfullerene (these days shortened to fullerene or buckyball). Chemists write it as C₆₀ .

The announcement of the discovery in the prestigious scientific journal Nature created quite a stir in the scientific community – people quickly realised that buckyballs could be very useful substances (Box 2: The many potential uses of fullerenes). In 1996 Curl, Kroto and Smalley were awarded the Nobel Prize for their discovery.

The soccerball effect

You could go to Montreal to get an idea of what a buckyball looks like. But perhaps an easier way is to look at a soccer ball: you will see that it consists of 20 hexagons (the white patches of leather) and 12 pentagons (the black patches), exactly the same pattern as that of the new molecule.

Buckyballs have more in common with soccerballs than just their looks. They spin (much faster than a soccerball – at more than 100 million times per second). If they are squeezed and then released they spring back to their original shape. And they bounce if they are hurled against a hard surface such as steel.

Other shapes

The C₆₀ buckyball is the most famous of the fullerenes but by no means the only one. In fact, scientists have now discovered hundreds of different combinations of these interlocking pentagon/hexagon formations. Examples include

• ‘buckybabies’ – spheroid carbon molecules containing fewer than 60 carbon atoms,
• ‘fuzzyballs’ – C₆₀ buckyballs with 60 hydrogen atoms attached,
• ‘giant fullerenes’ – fullerenes containing hundreds of carbon atoms, and
• C₇₀ – molecules with 70 carbon atoms, shaped a bit like a rugby ball or an Australian Rules football.

Buckyballs in bulk

Curl, Kroto and Smalley were the first to make and identify buckyballs in the laboratory but they were only able to produce tiny quantities. The race was on to manufacture buckyballs in large enough quantities for detailed investigation of their properties and potential.

In 1990, five years after the first synthesis of buckyballs, German and American scientists independently made larger quantities of buckyballs. They heated graphite rods to a high temperature by passing an electric current between them, then separated the fullerenes from other carbon compounds in the resulting soot (fine carbon particles). About 75 per cent of the crystals were C₆₀ molecules, 23 per cent were C₇₀ and the rest were larger molecules. Soon after this manufacturing breakthrough, dozens of groups around the world were making fullerenes. It wasn't long before research papers began to appear at the rate of almost one a day.

The not-so-new form of carbon

But it turns out that we've actually been making fullerenes unknowingly for thousands of years – whenever we burn a candle or an oil lamp. The candle's flickering flame vaporises wax molecules containing carbon, hydrogen and oxygen. Some of these molecules burn instantly in the blue heart of the flame. Others move upwards into the yellow tip where the temperature is great enough to split them apart. The result is carbon-rich soot particles that glow, giving off gentle yellow light. Amid this soot are buckyballs.

Buckyballs also exist in interstellar dust and in geological formations on Earth. So while they are new to science they are reasonably common in nature.

Chemical and physical properties of buckyballs

Buckyballs and other fullerenes intrigue scientists because of their chemistry and their unusual hollow, cage-like shape. Buckyballs are extremely stable and can withstand very high temperatures and pressures. The carbon atoms of buckyballs can react with other atoms and molecules, leaving the stable, spherical structure intact. Researchers are interested in creating new molecules by adding other molecules to the outside of a buckyball and also in the possibility of trapping smaller molecules inside a buckyball.

Carbon tubes

As well as carbon spheres in many sizes, scientists have discovered tubes of carbon. These nanotubes, or buckytubes, are created in a similar way to buckyballs: by passing an electric current between graphite rods. Nanotubes formed in this way are a series of tubes packed inside each other. When a tiny dose of cobalt, nickel or iron catalyst is added during manufacture, the result is an empty nanotube with a wall just one atom thick.

You can imagine an empty nanotube as being formed from a flat sheet of graphite. The sheet, like a length of chicken wire, is rolled into a cylinder with the opposite edges forming a perfect join. Nanotubes can be extremely long (eg, a nanotube might contain 1,000,000 carbon atoms).

Nanotubes exhibit some peculiar characteristics. For example, experiments suggest that they are incredibly tough. Other properties – such as electrical conductivity – seem to vary with the particular geometry of the tube. This means it could be possible to have two concentric nanotubes, one inside the other, the outer one acting as an insulator and the inner one conducting a current.

Scientific fun and games

The emergence of the buckyball and its cousins has been a stimulus to both scientific research and the human imagination, although we are yet to see any practical applications (Box 2). One day, perhaps, they will have a major impact on our lives. In the meantime, hundreds or even thousands of chemists, physicists and molecular biologists in laboratories around the world continue to play molecular football with these most intriguing of structures.

Box 1. Finding the molecular structure of buckyballs

Toothpicks, bubblegum and computers

To test this idea, Kroto, Curl and Smalley made models. One was a sphere formed from 60 bubble gum balls and toothpicks. In a separate attempt, Smalley used his home computer to try to make a ball from a series of hexagons. None of the models worked. The scientists simply could not make a regularly shaped ball from 60 'atoms'.

As a last resort, Smalley cut a sheet of paper into five-sided and six-sided shapes. After a little experimentation, he used sticky tape to join together 12 of the pentagons with 20 hexagons to make a ball. Much to Smalley's delight, the neatly shaped ball had 60 corners. It even bounced!

Related site

• Designs on C₆₀ (Chemistry in Britain, September 1996)

Box 2. The many potential uses of fullerenes

Chemical sponges

Medical researchers believe that fullerenes could be put to work as tiny chemical sponges, mopping up dangerous chemicals from injured brain tissue. Excess production of free radicals (eg, peroxide) in the brain following a head injury or a stroke destroys nerve cells. Buckyballs, made soluble in water, appear able to ‘swallow’ and hold free radicals, thereby reducing the damage to tissue.

Nanotubes in microscopes

Buckyball discoverer Richard Smalley and colleagues have used nanotubes as chemical probes in a scanning-force microscope. The microscope relies on a tiny tip that detects and skims the surface of target molecules. The great resilience of fullerenes means that the tube springs back into its original shape when bent.

Buckyballs in miniature circuits

A supercomputer the size of a paperback is the ambition of European researchers who have managed to attach a single buckyball to a sheet of copper. The scientists compressed the buckyball by 15 per cent, improving electrical conductivity by more than 100 times compared to the undisturbed molecule. A tiny electronic component like this could make miniature circuits feasible.

Lubricants, catalysts and superconductors

Other exciting potential uses of fullerenes include buckyballs behaving as 'molecular ball bearings' allowing surfaces to glide over one another. Fullerenes with metal atoms attached to them might function as catalysts, increasing the rate of important chemical reactions. Scientists know that buckyball compounds with added potassium act as superconductors at very low temperatures.

Molecular sieves

Because of the way they stack, buckyballs could act as molecular sieves, trapping particles of particular sizes while leaving others unaffected. Scientists talk of designing sieve-like membranes from buckyballs that allow biological materials to pass through, but not larger particles such as viruses. This would be useful for handling transplant organs, for example.

Buckycopiers?

In the United States, Xerox owns patents for using buckyballs to improve resolution of photocopies. They are 1000 times smaller than the particles used in conventional photocopier toner.

Related sites

• Harold Kroto contemplates applications of Nobel-winning fullerenes (The Scientist, 6 January 1997)

• Have buckminsterfullerenes been put to any practical uses? (Ask the experts, Scientific American)

Activities

Science NetLinks (American Association for the Advancement of Science)
• Structure matters – explores the molecular structure of matter and how it can affect the physical characteristics of a specific material.
• Microworlds: Exploring the structure of materials (Lawrence Berkeley National Laboratory, University of California, USA)
  • Is carbon hard or soft?

• Schlumberger Excellence in Educational Development – SEED (Schlumberger Ltd, USA)
  • Build a buckyball – provides patterns for a paper model of the structure of a buckyball.

• Chemistry Department, University of Wisconsin (USA)
  • Problems for buckyball, diamond and graphite topic. (These problems are designed for tertiary students.)

Further reading

Useful sites

A brief introduction to the new form of carbon – C₆₀ .
http://www.nottingham.ac.uk/~ppzjld/what.htm

A brief history of C₆₀ (Max-Planck-Institute Stuttgart, Germany)

An overview (with external links) of the structure and discovery of C₆₀ .
http://www.fkf.mpg.de/andersen/fullerene/intro.html

The allotropes of carbon (IN-VSEE, USA)

Covers different forms of carbon – diamond, graphite, buckyballs and fullerenes – and includes structural diagrams of each form. Discusses possible uses, physical properties and bonding of different forms of carbon.
http://invsee.asu.edu/Modules/carbon/question.htm

Nobelprize.org (Sweden)

• The discovery of carbon atoms bound in the form of a ball is rewarded
  The announcement by the Swedish Academy of the awarding of the 1996 Nobel Prize in Chemistry to Professors Curl, Kroto and Smalley.
  http://www.nobelprize.org/chemistry/laureates/1996/press.html

• The Nobel Prize in chemistry 1996
  This poster describes the work on buckyballs that won a Nobel Prize.
  http://nobelprize.org/chemistry/educational/poster/1996/index.html

Fullerenes (Kim Allen, USA)

A PhD student's site about fullerenes, buckyballs and nanotubes. Written in a friendly style.
http://kimallen.sheepdogdesign.net/Fuller/index.html

Buckytubes (AZoM.com, Australia)

• Introduction and basic facts
  Provides an introduction to bucky tubes.
  http://www.azom.com/details.asp?ArticleID=1295#_Ropes

• Properties
  Provides information on the properties of buckytubes.
  http://www.azom.com/details.asp?ArticleID=1294

• Applications
  Provides information on some of the applications for buckytubes.
  http://www.azom.com/details.asp?ArticleID=1293

Glossary

electron. A negatively charged particle that is a constituent of an atom. Electrons can move from atom to atom. When they do, they produce an electric current.

free radical. A molecule that is unstable and highly reactive because it contains at least one unpaired electron. Free radicals combine with molecules to generate further unpaired electrons, thereby starting off chain reactions. Free radicals can damage cell membranes and DNA, eventually causing cancer and other diseases.

nanotubes. Extremely small tubes made from pure carbon. For more information see IPE nanotube primer (Institut de Physique des Nanostructures, Switzerland).

red giant star. An old star with a very large radius and a relatively low surface temperature. The colour of a star is a guide to its surface temperature – blue-white is the hottest and red is the coolest.

superconductor. A substance that has no resistance to the flow of an electric current. Superconductors currently require very low temperatures to function. They can be used for energy storage, storing and retrieving digital information, medical imaging machines and friction free transport. For more information see What is superconductivity? (How Stuff Works, USA) and Superconductor information for the beginner (Superconductors.org).