Fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds so as to form a closed or partially closed mesh, with fused rings of five to seven atoms.
The molecule may be a hollow sphere, ellipsoid, tube, or many other shapes and sizes.
Since each carbon atom is connected to only three neighbors, instead of the usual four, it is customary to describe those bonds as being a mixture of single and double covalent bonds.
Graphene (isolated atomic layers of graphite), which is a flat mesh of regular hexagonal rings, can be seen as an extreme member of the family.
There are two major families of fullerenes, with fairly distinct properties and applications: the closed buckyballs and the open-ended cylindrical carbon nanotubes. However, hybrid structures exist between those two classes, such as carbon nanobuds — nanotubes capped by hemispherical meshes or larger “buckybuds”.
Buckminsterfullerene is the smallest fullerene molecule containing pentagonal and hexagonal rings in which no two pentagons share an edge (which can be destabilizing, as in pentalene). It is also most common in terms of natural occurrence, as it can often be found in soot.
The empirical formula of buckminsterfullerene is C60 and its structure is a truncated icosahedron, which resembles an association football ball of the type made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge.
Another fairly common fullerene has empirical formula C70, but fullerenes with 72, 76, 84 and even up to 100 carbon atoms are commonly obtained.
The smallest possible fullerene is the dodecahedral C20. There are no fullerenes with 22 vertices.
Buckyballs and carbon nanotubes have been used as building blocks for a great variety of derivatives and larger structures, such as
- Nested buckyballs (“carbon nano-onions” or “buckyonions”)proposed for lubricants;
- Nested carbon nanotubes (“carbon megatubes”)
- Linked “ball-and-chain” dimers (two buckyballs linked by a carbon chain)
- Rings of buckyballs linked together.
Fullerenes had been predicted for some time, but only after their accidental synthesis in 1985 were they detected in nature and outer space. The discovery of fullerenes greatly expanded the number of known allotropes of carbon, which had previously been limited to graphite, diamond, and amorphous carbon such as soot and charcoal.
They have been the subject of intense research, both for their chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.
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