|Andersen Group||El.-Phon.||QMC||C60||GW||Resistivity saturation|
|A brief history of C60|
C60 is a molecule that consists of 60 carbon atoms,
arranged as 12 pentagons and 20 hexagons. The shape
is the same as that of a soccer ball: The black pieces of leather are the
pentagons, the hexagons are white. There are 60 different points where three
of the leather patches meet. Imagine a carbon atom sitting at each of these
points, and you have a model of the C60 molecule. That
model, however, is vastly out of scale: If the
C60 molecule were the size of a soccer ball, then the
soccer ball in turn would be roughly the size of the earth.
The most striking property of the C60 molecule is its high symmetry. There are 120 symmetry operations, like rotations around an axis or reflections in a plane, which map the molecule onto itself. This makes C60 the molecule with the largest number of symmetry operations, the most symmetric molecule.
Based on a theorem of the mathematician Leonhard Euler, one can show that a spherical surface entirely built up from pentagons and hexagons must have exactly 12 pentagons. Depending on the number of hexagons, molecules of different sizes are obtained. They are called Fullerenes, after the American architect Richard Buckminster Fuller. Fuller, who is shown here on the cover of Time Magazine of January 10, 1964, was renowned for his geodesic domes, that are based on hexagons and pentagons. An even earlier example of such a construction was the dome of the first planetarium, built by Zeiss in 1922.
It should come as no surprise that a shape as symmetric and beautiful as that of the C60 molecule, has occupied many artists and mathematicians over the centuries. Probably it was already known to Archimedes (after all, it's one of the Archimedean solids!), although no drawings seem to have survived. The oldest known picture of the soccer-ball-shape seems to be a drawing found in the Vatican library. It is from a book of the painter and mathematician Piero della Francesca and dates from the 1480s. Johannes Kepler coined the name truncated icosahedron for this shape. An example of the appearance of the truncated icosahedron in art is shown in this picture from an Italian cathedral. At the top we see an icosahedron. It is bounded by twenty equilateral triangles. At each of the 12 vertices of the icosahedron, five of the triangles meet. Cutting off ('truncating') these vertices thus replaces each of them by a pentagonal face; it also converts each of the twenty former triangular faces into a hexagon. We can see the resulting truncated icosahedron at the bottom of the picture. This is the shape of the C60 molecule.
The C60 molecule was discovered by Harold Kroto, James Heath, Sean O'Brien, Robert Curl, and Richard Smalley in 1985 (Nature 318, 162). The group actually tried to understand the absorption spectra of interstellar dust, which they suspected to be related to some kind of long-chained carbon molecules. Unfortunately they could not solve that problem. But their work was not completely unsuccessful, since in the course of their experiments they discovered the Buckyball which generated so much excitement among scientists and won Curl, Kroto, and Smalley the 1996 Nobel prize in chemistry.
Initially, C60 could only be produced in tiny amounts. So there were only a few kinds of experiments that could be performed on the material. Things changed dramatically in 1990, when Wolfgang Krätschmer, Lowell Lamb, Konstantinos Fostiropoulos, and Donald Huffman discovered how to produce pure C60 in much larger quantities (Nature 347, 354). This opened up completely new possibilities for experimental investigations and started a period of very intensive research. Nowadays it is relatively straightforward to mass-produce C60. For instructions check the Workshop on how to produce bulk quantities of pure C60 or have a look at the research page of two undergraduates who produced C60. If you don't want to do the lab work yourself, you can also buy C60 (prices).
The discovery of C60 has stimulated a large activity in chemistry. It opened up the new branch of Fullerene-Chemistry which studies the new families of molecules that are based on Fullerenes. By 1997 about 9000 Fullerene compounds were known.
C60 molecules condense to form a solid of weakly bound molecules. This crystalline state is a new form of solid carbon, besides the long known diamond and graphite. It is called Fullerite. Much of the work in physics is centered around the solid phases of C60. If certain alkali atoms (A) are added to solid C60, new compounds like A3 C60 can be formed, the alkali-doped Fullerides. If A is potassium (K) or rubidium (Rb) the compounds are superconductors. That means that below a certain temperature Tc they conduct electric currents without any resistance. For the alkali-doped Fullerides Tc is quite large (20-40 K) compared to "conventional" superconductors.
At a very early stage the British House of Lords discussed the Fullerenes. The final part of the transcript of the hearing is quite entertaining! If you are more interested in applications of C60, have a glimpse at the Fullerene Patent Database.
You can find more information on the Fullerenes in:
Max-Planck-Institut für Festkörperforschung|
Heisenbergstraße 1 D-70569 Stuttgart