Discovery of new method called density gradient ultracentrifugation to cleanly and easily separate the double-walled nanotubes (DWNTs) from the single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) to reliably produce and sort out double-walled carbon nanotubes could lower the cost of this dynamic material. Photo Credit: Mark Hersam, Northwestern UniversityDecember 23, 2008, (Sawf News) - Discovery of new method called density gradient ultracentrifugation to cleanly and easily separate the double-walled nanotubes (DWNTs) from the single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs) to reliably produce and sort out double-walled carbon nanotubes could lower the cost of this dynamic material.
In a paper published today in the online edition of the journal Nature Nanotechnology, two researchers, Professor Mark C. Hersam and a graduate student Alexander A. Green of Northwestern University outline a new process for efficiently gathering up these coveted double-walled carbon nanotubes.
The sorting method works by exploiting subtle differences in the buoyant densities of the nanotubes as a function of their size and electronic behavior.
In recent years, the possible applications for double-walled carbon nanotubes have excited scientists and engineers, particularly those working on developing renewable energy technologies. These tiny tubes, just two carbon atoms thick, are thin enough to be transparent, yet can still conduct electricity. This combination makes them well-suited for advanced solar panels, sensors and a host of other applications.
Up until now, the problem with double-walled carbon nanotubes has been being able to produce a homogeneous supply of them. When double-walled carbon nanotubes are synthesized, the process also creates many of the single- and multi-walled variety. Given their small size, sorting the valuable double-walled tubes from the other types has posed a real challenge.
Using the Northwestern method, carbon nanotubes first are encapsulated in water by soap-like molecules called surfactants. The surfactant-coated nanotubes then are sorted in density gradients that are spun at tens of thousands of rotations per minute in an ultracentrifuge. Each nanotube’s diameter and electronic structure help determine the nanotube’s buoyant density, which enables the method to separate DWNTs from the SWNTs and MWNTs.
The double-walled nanotubes, the researchers discovered, were approximately 44 percent longer than the single-walled nanotubes. This longer length of the DWNTs results in a factor of 2.4 improvement in the electrical conductivity of transparent conductors.
Double-walled nanotubes also enable improved spatial resolution and longer scanning lifetimes as tips for atomic force microscopes and are useful in field-effect transistors, biosensing and drug delivery.
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