Spin control: New technique sorts nanotubes...( Researchers at the National Institut...)

Researchers at the National Institute of Standards and Technology (NIST) have reported* a new technique to sort batches of carbon nanotubes by length using high-speed centrifuges. Many potential applications for carbon nanotubes depend on the lengths of these microscopic cylinders, and one of the most important features of the new technique, say the scientists, is that it should be easily scalable to produce industrial quantities of high-quality nanotubes.

So-called single wall carbon nanotubes (SWCNTs) are essentially sheets of carbon atoms only one atom thick that have rolled themselves into tubes with a diameter of approximately one nanometer. They have unique combinations of thermal, mechanical, optical and electronic properties that suggest a wide variety of uses, including circuit elements in molecular electronics, fluorescent tags for diagnostic and therapeutic applications in medicine and light sources for compact, efficient flat-panel displays, among many others.

Unfortunately, the methods for manufacturing carbon nanotubes always create a large percentage of nanojunk in the mixclumps of carbon, ordinary soot, particles of metal used as a catalystand nanotubes come in an enormous range of lengths, from a few tens or hundreds, up to thousands of nanometers. Refining the lot is essential for most uses. For many potential applications, nanotubes need to be separated by length. In biomedical applications, for example, it has been shown that whether or not nanotubes are taken up in cells depends critically on length (see Study: Cells Selectively Absorb Short Nanotubes.) Nanotubes used as components in future microcircuits obviously need to fit in place, and in optical applications, a nanotubes length determines how strongly it will absorb or emit light (see Longer is Better for Nanotube Optical Properties.)

In 2006, researchers found that you could separate nanotubes by chirality (a measure of the twist in the carbon atom s
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