“A serious drawback in any application of nanotubes is their tendency to cluster and entangle,” Ingo Dierking of the University of Manchester told nanotechweb.org. “Our aim was to align nanotubes along a preferred direction and at the same time still be able to change this direction dynamically, co-operatively re-orienting nanotubes dispersed in a matrix.”
Dierking explained that in liquid crystals the long axes of elongated organic molecules spontaneously align along a common direction. “It is this property of self-organization that we wanted to exploit to align carbon nanotubes,” he said. “The basic idea is that the spontaneous orientation of the liquid crystal will be transferred onto the dispersed nanotubes.”
To prepare the liquid crystal-nanotube dispersions the scientists chopped the nanotubes into lengths of about 600 nm. They also used ultrasound to disperse the tubes in commercially available E7 liquid crystal and prevent them clustering together. The scientists believe that the concentration of nanotubes in the liquid crystal was less than 1%.
To change the preferred orientation of the liquid crystal Dierking and colleagues applied an electric field. They reckon elastic interactions between the liquid crystal molecules and the nanotubes produced a torque on the nanotubes, causing the tubes to re-orient into the new liquid crystal direction. By altering the applied voltage the researchers were able to select liquid crystal directions through angles between 0 and 90°.
“On switching the electric field off, the liquid crystal as well as the nanotubes co-operatively re-orient back into their original orientation, although on different time scales,” said Dierking.
For multi-walled nanotubes the orientational order parameter was about 0.9, a value the scientists say is exceptionally high and demonstrates very good parallel alignment of the nanotubes. “The real significance of our work lies in the ability to select and vary dynamically the nanotubes’ direction through application of external fields of varying amplitude,” explained Dierking.
The liquid crystal-nanotube dispersions could have applications in the field of externally controlled molecular switches, as well as sensors responding to electrical, magnetic, mechanical or even optical external stimuli. The researchers say they have demonstrated electrically controlled OFF-ON and ON-OFF nanotube switches based on the positive and negative dielectric anisotropy of the respective liquid crystal host.
Now the scientists are concentrating their efforts on nanotube dispersions in ferroelectric liquid crystals, and on making optically controlled devices. “Suitable device geometries have already been designed and we now hope for the necessary financial support,” said Dierking.
The researchers reported their work in Advanced Materials.