Researchers from Seoul National University and Sungkyunkwan in South Korea have developed a technique for aligning nanotubes over large areas based on the flow of a nanotube-containing solution through nanochannels. Not being able to align carbon nanotubes in a cheap and simple way has been a roadblock to making the superior commercial electronic devices that should be possible with carbon nanotubes.
This technique is especially attractive because of its simplicity; no external stimuli such as the application of an electric field or syringe pumping are required to align the nanotubes.
This novel approach for aligning carbon nanotubes is based on the simple flow of a nanotube solution through a nanochannel fabricated from a charged polymeric mold. The nanotubes are ordered within the channels by the influence of the capillary force existing within the confines of the channel. When the channels are of the correct geometry, aqueous solutions containing nanotubes enter from both ends, and upon evaporation leave behind dense and highly oriented arrays of nanotubes. Suh cautions that the mechanical properties and surface chemistry of the polymeric mold used for making the nanochannels are of paramount importance. “The stiffness of the polymer has to be just right”, says Suh, “it has to be rigid enough to keep the nanochannels from collapsing but flexible enough to bond well with the substrate over a large area”. Good adhesion is required between the nanochannel and the substrate to prevent the polymer nanochannels from coming unstuck upon the introduction of the aqueous nanotube solution. The researchers have found that polyethylene glycol diacrylate has the right combination of properties for use as the polymer mold. It is negatively charged and facilitates conformal contact with the substrate. Moreover, it is hydrophilic and thus the nanotube solution is able to enter and flow through the channels without need for additional pumping.