There are dozens of types of SWNTs, each with a characteristic atomic arrangement. These variations, though slight, can lead to drastically different properties: Some nanotubes are like metals, and others are semiconductors. While materials scientists are anxious to use SWNTs in everything from bacteria-sized computer chips to geostationary space elevators, most applications require pure compounds. Since all nanotube production methods, including the industrial-scale system Smalley invented in the 1990s, create a variety of 80-odd types, the challenge of making mass quantities of pure tubes – which Smalley referred to as "SWNT amplification" – is one of the major, unachieved goals of nanoscience.
The nanotube seeds are about 200 nanometers long and one nanometer wide – length-to-diameter dimensions roughly equal to a 16-foot garden house. After cutting, the seeds underwent a series of chemical modifications. Bits of iron were attached at each end, and a polymer wrapper was added that allowed the seeds to stick to a smooth piece of silicon oxide. After burning away the polymer and impurities, the seeds were placed inside a pressure-controlled furnace filled with ethylene gas. With the iron acting as a catalyst, the seeds grew spontaneously from both ends, growing to more than 30 times their initial length – imagine that 16-foot water hose growing by more than 500 feet – in just a few minutes