Sorting Nanotubes Through Friction-Force Differences

According to experts at the Georgia Institute of Technology (Georgia Tech), positioning and manipulating tiny carbon nanotubes is one of the last standing major issues that need tackling in the field of nanotechnology. Integrated nanoelectronic and photonic circuits, nanosensors, interconnects and electro-mechanical nanodevices will all become possible as soon as scientists develop an effective way of sorting, griping and handling the tubes into their correct or desired position.

In this week's issue of the respected scientific journal Nature Materials, four research groups published studies dealing with the properties of friction forces acting on the small tubes. Apparently, when a carbon nanotube slides along its axis, it experiences a different friction force than when it slides perpendicular to its axis. Scientists hope to be able to use this difference to their advantage, in a manner that will allow them both to sort the tubes, and to assemble them into the desired structures as well.

Physicists believe that the different forces that act on the tubes are generated by their soft lateral distortions. The good news is that the distortions have no negative effect when the material is placed in larger structures, and also that asymmetries in the distortion intensity can yield clues as to the chirality, or spin, of the molecules inside the tubes. This is something that is very difficult to measure with other existing means. The friction-force differences were also proven by atomic force microscope (AFM) scans. The US Department of Energy (DOE) provided the funds for the new research.

“Because the energy required to move in one direction is twice as much as required to move in the other direction, this could be an easy way to control the assembly of carbon nanotubes for nanoelectronics, sensors and other applications. To assemble nanotubes on a surface, you need to know how they interact and what force is needed to move them,” Georgia Tech School of Physics Associate Professor Elisa Riedo, also the co-author of one of the journal papers, says.

“Understanding the basic mechanism of friction in carbon nanotubes will help us in designing devices with them in the future. We have shown an anisotropy in the friction coefficient of carbon nanotubes in the transverse and longitudinal directions, which has its origin in the soft lateral distortion of tubes when the tip-tube contact is moving in the transverse direction. 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,” she concludes.