Introduction to Nanotechnology

110 CARBONNANOSTRUCTURES
molecules, like those shown in Fig. 5.4, in outer space. He was interested in how
these chains came to be, and had speculated that such molecules might be created in
the outer atmosphere of a type of star called a “red giant.” In order to test his
hypothesis, he wanted to re-create the conditions of the outer atmosphere of the star
in a laboratory setting to determine whether the linear carbon chains might be
formed. He knew that high-powered pulsed lasers could simulate the conditions of
hot carbon vapor that might exist in the outer surface of red giants. He contacted
Professor Richard Smalley of Rice University in Houston, who had built the
apparatus depicted in Fig. 4.2, to make small clusters of atoms using high-powered
pulsed lasers. In this experiment a graphite disk is heated by a high-intensity laser
beam that produces a hot vapor of carbon. A burst of helium gas then sweeps the
vapor out through an opening where the beam expands. The expansion cools the
atoms and they condense into clusters. This cooled cluster beam is then narrowed by
a skimmer and fed into a mass spectrometer, which is a device designed to measure
the mass of molecules in the clusters. When the experiment was done using a
graphite disk, the mass spectrometer yielded an unexpected result. A mass number
of 720 that would consist of 60 carbon atoms, each of mass 12, was observed.
Evidence for a c60 molecule had been found! Although the data from this
experiment did not give information about the structure of the carbon cluster, the
scientists suggested that the molecule might be spherical, and they built a geodesic
dome model of it.
5.3.3. Structure of c60 and Its Crystal
The c60 molecule has been named fullerene after the architect and inventor R.
Buckminister Fuller, who designed the geodesic dome that resembles the structure of
c60. Originally the molecule was called buckminsterjiullerene, but this name is a bit
unwieldy, so it has been shortened to fullerene. A sketch of the molecule is shown in
Fig. 5.6. It has 12 pentagonal (5 sided) and 20 hexagonal (6sided) faces symme-
trically arrayed to form a molecular ball. In fact a soccer ball has the same geometric
configuration as fullerene. These ball-like molecules bind with each other in the
solid state to form a crystal lattice having a face centered cubic structure shown in
Fig. 5.7. In the lattice each c60 molecule is separated from its nearest neighbor by
1 nm (the distance between their centers is 1 nm), and they are held together by weak
forces called van der Waals forces that were discussed in the previous chapter.
Because c60 is soluble in benzene, single crystals of it can be grown by slow
evaporation from benzene solutions.
5.3.4. Alkali-Doped c60
In the face-centered cubic fullerene structure, 26% of the volume of the unit cell is
empty, so alkali atoms can easily fit into the empty spaces between the molecular
balls of the material. When c60 crystals and potassium metal are placed in evacuated
tubes and heated to 400°C, potassium vapor diffuses into these empty spaces to form
the compound K&O. The c60 crystal is an insulator, but when doped with an alkali