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Researchers from NIST, Rice Probe Low-Cost SWNT Self-Assembly Technique

by Editor1 last modified January 14, 2009 - 17:08

U.S. National Institute of Standards and Technology (NIST) and Rice University nanoresearchers say they are unlocking a new and inexpensive process to improve nanotube self-assembly. The process could speed commercialization of electronic, material and biomedical offerings.

Researchers from NIST, Rice Probe Low-Cost SWNT Self-Assembly Technique

SWNTs enclosed in bile acid shells self assembled into a sheaf of long ordered fibrils -- each composed of several nanotube rods when treated with hydrophobic compound.

Breakthroughs that CNT-based nanowires could offer to a new class of electronics and materials rely on the ability to efficiently line the nanotubes up in some organized arrangement. Unfortunately, nanotubes often clump together or spread into random, tangled mats. To solve the problem, researchers have been pursuing techniques to encourage SWNT/CNT self-assembly.

For NIST/Rice researchers, their focus was to explore new ways to better sort and purify SWNT/CNTs. The idea was that with more attention to preparation of a “standard sample” of SWNT/CNT material, more orderly and predictable self-assembly might result. The researchers used a bile acid to coat the nanotubes to prevent clumping.

“Bile acids,” says NIST research chemist Erik Hobbie, “are biological surfactants, and like most surfactants they have a part that likes water and a part that doesn’t. This is a slightly complex surfactant because instead of having a head and a tail, the usual geometry, it has two faces, one that likes water and one that doesn’t.”

Mixed in water, such hydrophobic/hydrophilic molecules normally want to group together in hollow spheres with their hydrophobic “tails” sheltered on the inside, Hobbie explains, but the two-faced geometry of this bile acid makes it form hollow rod shapes instead. Conveniently, the hollow rods can house the rod-shaped nanotubes.

Moreover, after about a day in the bile acid, the SWNT/CNTs develop a uniform shell and line up end-to-end, in long strands. Further, the strands begin to join together in twisted filaments, like a length of twisted copper wire. Treating SWNT/CNTs on a slide with a hydrophobic compound causes the fibrils to cluster at specific sites, probably at defects in the hydrophobic surface.

The discovery, researchers add, is a long way from a perfect solution for ordering nanotubes, but opens an encouraging line of exploration. One area for further work, Hobbie said, is to find ways that the bile acid shells could be removed after the nanotubes are in their ordered positions. So far, this has proven difficult.

Another complication: The bile acid surfactant is toxic to living cells, which precludes most biomedical applications unless it is removed.

On the bright side, researchers said the approach has already led to an easy and extremely inexpensive technique for researchers interested in studying CNT attributes, such as their optical properties. “It gives a recipe for how to create ordered, aligned arrangements of individual carbon nanotubes. 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.”