Nanotubes sorted, aligned to reduce friction

PORTLAND, Ore. — An international team studying the effects of friction on carbon nanotubes claims that it can be cut in half when carbon nanotubes are aligned lengthwise rather than transversely.

"The energy required to move in one direction [transversely] is twice as much as required to move in the other direction [longitudinally]," said professor Elisa Riedo of the Georgia Institute of Technology. "Our findings could help in developing better strategies for chirality sorting, large-scale, self-assembling of nanotubes on surfaces and designing nanotube adhesives and nanotube-polymer composite materials."

Georgia Tech, the International Centre for Theoretical Physics, the International School for Advanced Studies, the CNR Democritos Laboratory and the University of Hamburg contributed to the work. The research was sponsored by the U.S. Energy Department, which wants to find ways to conserve energy by reducing friction.

Deformation doubles friction between a carbon nanotube and an atomic force microscope tip sliding transversely across the grain. Source: Christian Klinke, University of Hamburg

Carbon nanotubes are as small as a single nanometer in diameter, making it impossible to accurately pick-and-place them with robots. This obstacle prompted scientists and engineers to search for alternative methods for assembling nano devices.

The researchers used an atomic force microscope (AFM) to characterize the coefficient of friction for carbon nanotubes with different grain orientations. Called chirality, the different orientations allow nanotubes to exhibit different semiconducting characteristics when they are rolled with the grain, against the grain or somewhere in between.

The researchers did not measure electrical characteristics, but rather investigated the physical mechanism behind the different amounts of friction encountered when manipulating nanotubes with an AFM. Their conclusion was that higher friction in the transverse direction was the result of soft lateral distortion in the tubes' shape, which they called "hindered rolling."

"To assemble nanotubes on a surface, you need to know how they interact and what force is needed," said Riedo. "This could be an easy way to control the assembly of carbon nanotubes for nanoelectronics, sensors and other applications."

Using Van der Waals forces to hold the tubes in place on a substrate, the researchers claim their study is the first to accurately characterize the frictional forces at work in both the longitudinal and transverse directions using an AFM tip.

The nanotube assembly techniques could eventually become the building blocks for integrated nano electronics, photonics, interconnects and nanoelectromechanical systems, according to the researchers.