a carbon nanotube

Take a single sheet of graphene — that one-atom-thick honeycomb of carbon — and roll it up into a seamless cylinder, like rolling a sheet of chicken wire into a pipe and welding the edge. The result is a carbon nanotube: a hollow tube whose wall is a single layer of carbon atoms, typically about 1 nm across but able to run thousands of times longer than it is wide. The ends are often closed by a domed cap that is half of a fullerene. It is, in effect, a rolled-up 2D crystal that has become a 1D object.

There are two families. A single-walled nanotube is one rolled sheet, a wall just one atom thick. A multi-walled nanotube is several tubes nested one inside another like the rings of a tree, each held to the next by weak van der Waals forces across a gap of about 0.34 nm — the same spacing as graphite's layers. Because the wall is graphene, the carbon-carbon bonds are the same short, strong 0.142 nm bonds, which is why nanotubes are among the stiffest and strongest materials known, and why they conduct heat and electricity so well along their length. Confined around the circumference but free along the axis, a nanotube is a near-ideal quantum wire.

The single most important structural fact is that how you roll the sheet matters. Roll it straight, or on a slant, and you get tubes that differ in diameter and in the tilt of the honeycomb around the tube — a property called chirality — and that rolling choice decides whether the very same carbon atoms form a metal or a semiconductor. So a nanotube's electronic character is set purely by its geometry. Nanotubes appear in strong composites, conductive films, and nanoscale electronics, always with structure firmly in the driver's seat.