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Separation the nanotubes

Carbon-based nanomaterials have attracted significant attention due to their potential to enable and/or improve applications such as transistors, information technology, biotechnology, transparent conductors, solar cells, batteries, water purification systems, infrastructure materials, drug delivery, and biosensors. Additionally, they possess numerous properties amenable to practical, scalable, and economic device fabrication including abundant source material, a natural disposition for solution processing, high surface area for efficient charge transfer, and flexibility.

During the synthesis of double-walled carbon nanotubes many of the single and multi walled variety are also created. This creates a real problem for the separation of double-walled tubes from the others present in small proportion. Nanomaterials possess the unique attribute that their properties depend on physical dimensions such as diameter. This size dependence implies, however, that the physical dimensions must be exquisitely controlled in order to realize uniform and reproducible performance in devices. For example in the electronic device applications of single-wall carbon nanotubes (SWCNTs), mixed production of metal and semiconductor phases was one of the most serious problems because they show completely different transport properties.

Double-sided carbon nanotubes are highly prized for their use in solar cells and other applications, but until now, creating a supply of pure double-sided carbon nanotubes instead of a mix of single- and multi-sided ones has been a challenge to the manufacturers. Researchers from Northwestern University have outlined a process for efficiently separating out these double-walled carbon nanotubes. The technique developed works by density gradient ultracentrifugation. This is based on the exploitation of the properties such as size and density, and uses a large centrifuge. The researchers claim that for the said separation the nanotubes are coated with soap-like surfactant molecules and then subjected to tens of thousands of rotations every minute within an ultracentrifuge. Their diameters and electronic structures allows for their separation due to the effect on their buoyant density, as DWNTs were discovered to be about 44% longer than SWNTs. This leads to a 2.4 times better electrical conductivity in transparent conductors, which are better fit for obtaining enhanced spatial resolution and scanning lifetimes, as well as improved field-effect transistors, biosensing and medicine administration. The above density gradient ultracentrifugation (DGU) technique can separate metallic and semi conducting SWCNTs with high purity. However, the separation process is so complicated and the scale up is difficult because of the limited rotor capacity. For the industrial application, more effective separation method was desired.

Recently, anew separation methods using agarose gel using specific interaction between semi conducting SWCNT and agarose, has been developed. These methods realize high purity separation with low cost, high speed, and high efficiency. In the latest separation method, only agarose gel beads and two surfactants, sodium dodecyl sulfate (SDS) for metallic SWCNTs and sodium deoxycholate (DOC) for semi conducting SWCNTs were used.

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