Fullerene

Fullerenes represent a class of materials that are composed of pure carbon. The most well-known materials composed completely of carbon are diamonds and graphite. Although all three materials are essentially the same, the carbon atoms are arranged differently, giving them different chemical, mechanical, electrical, and physical properties.

In diamonds, carbon atoms are linked in a tetrahe-dral lattice arrangement. This is a solid network, which forms the basis for their unique properties of having high melting points and being among the hardest known natural materials. In graphite, carbon atoms are bonded in planar hexagonal rings, and the planar sheets are stacked together to form the layered compound. Graphite is a good conductor and is a valued dry lubricant for industrial applications.

In fullerenes, pentagonal (and sometimes heptagonal) rings are inserted into a sheet of linked hexagonal rings to form nonplanar cage-like structures, such as hollow spheres (buckyballs), ellipsoids, tubes (carbon nanotubes or buckytubes), and so on. Fullerenes are generally stable, and their unique structure gives them novel chemical and electrical properties at the nanoscale, for example, as molecular carriers that can trap atoms, and electron-accepting materials for polymer photovoltaic devices.

Fullerenes were discovered unintentionally in 1985 by Harry W. Kroto (University of Sussex, UK), Robert F. Curl, Richard E. Smalley (both from Rice University), and their colleagues, while they were performing a series of experiments that sought to simulate the conditions in which carbon nucleates in the atmospheres of cool red giant stars. They suspected that such stars might be the sources of long-chain carbon molecules (such as cyanopolyynes, HCnN (n = 5–11)) in interstellar spaces. In their experiments, they used lasers to vaporize carbon atoms from graphite into a helium carrier gas. Carbon clusters were formed in the carrier gas before being expanded in a supersonic molecular beam and analyzed for their compositions.

To their surprise, their results not only confirmed that long-chain carbon molecules were formed, but also displayed an unknown carbon cluster with 60 carbon atoms. These clusters were stable and were produced in a relatively high abundance. After careful investigation, the scientists proposed a spheroidal closed cage model to visualize the clusters. Carbon atoms were arranged in the shape similar to a soccer ball or “truncated icosahedron,” a polygon with 60 vertices (carbon atoms) and 32 faces, 12 of which are pentagonal, and the rest hexagonal.

They named the newly discovered molecules “buck-minsterfullerenes,” in honor of Richard Buckminster Fuller, an American architect who invented the geodesic dome with a similar geometry. Their findings were reported in Nature in November 1985, titled “C60: Buckminsterfullerene,” a paper that unveiled an unprecedented area of nanotechnology.

In the following years, more evidence was reported to confirm that fullerenes actually assembled into a cage structure and that they were extremely stable. Later, [Page 263]new materials with similar molecular structures were introduced into fullerene family, including “baby” C20, the smallest fullerene, composed of 12 pentagons and no hexagons. With this simple structure, C20 molecules are thermodynamically unstable, compared to C60, the earliest discovered and most stable fullerene with the highest symmetry. The C70 molecule followed, and larger fullerene structures are being explored, including the artificially synthesized C540 and C960 molecules. Research event found that fullerenes existed long before laboratory discovery; they were actually common compositions of soot and interstellar material.

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