Researchers at the US National Institute of Standards and Technology (NIST) have found a way to chemically align carbon nanotubes.
"Unfortunately, just mixed in a solvent, the nanotubes will clump together in a black goo," said NIST. "They can be coated with another molecule to prevent clumping - DNA is sometimes used - but spread the mixture out and dry it and you get a random tangled mat of nanotubes."
For many reasons carbon nanotubes are causing a stir amongst semiconductor scientists, but until they can be handles predictably, they will never be more than a research curiosity.
They are in many ways strongly an-isotropic, and aligning them is important to exploit these characteristic.
NIST was using a bile acid - sodium deoxycholate - to coat nanotubes to prevent clumping.
"Sodium deoxycholate is a surfactant, and like most surfactants it has a part that likes water and a part that doesn't," said NIST chemist Erik Hobbie. "This is a slightly complex surfactant because instead of having a head and a tail, it has two faces, one that likes water and one that doesn't."
Mixed in water, long thin surfactant molecules want to group together in hollow spheres with their hydrophobic tails sheltered on the inside.
However, the two-faced geometry of sodium deoxycholate makes it form hollow rod shapes.
"Conveniently, the hollow rods can house the rod-shaped nanotubes," said NIST. Over the course of about a day, the surfactant shells cause the nanotubes to line up, end to end in long strands, and then the strands begin to join together in twisted filaments.
"The discovery is a long way from a perfect solution for ordering nanotubes," cautioned Hobbie, "and a lot of development remains to be done."
Firstly, NIST has yet to find a way to remove the acid from the aligned tubes. "On the other hand, it already is an easy technique for researchers interested in studying optical properties of carbon nanotubes," said NIST. "You don't need to use any external magnetic or electrical fields, and you don't need to dry the tubes out in a polymer and heat it up and stretch it. You can get fairly significant regions of very nice alignment just spontaneously through this self assembly."
Fig 1. Single wall carbon nanotubes enclosed in bile acid shells self assembled into a sheaf of long ordered fibrils each composed of several nanotube rods. Treating the microscope slide with a hydrophobic compound causes the fibrils to cluster like this at specific sites, probably at defects in the hydrophobic surface, said US research lab NIST.
The image is 70µm wide, taken using near-infrared fluorescent microscopy.