Nanotubes and nanowires are promising building blocks for future integrated nanoelectronic and photonic circuits, nanosensors, interconnects and electro-mechanical nanodevices. But some fundamental issues remain to be resolved-among them, how to position and manipulate the tiny tubes.
Researchers from four different institutions report measuring different friction forces when a carbon nanotube slides along its axis compared to when it slides perpendicular to its axis. This friction difference has its origins in soft lateral distortion of the tubes when they slide in the transverse direction.
The findings not only could provide a better understanding of fundamental friction issues, but from a more practical standpoint, offer a new tool for assembling nanotubes into devices and clarify the forces acting on them. Asymmetries in the friction could potentially also be used in sorting nanotubes according to their chirality, a property that is now difficult to measure with other means.
When an atomic force microscope (AFM) tip is scanned transversely across a multi-walled carbon nanotube, the amount of friction measured is twice as much as when the same tube is scanned longitudinally, along the length of the tube. The researchers attribute this difference to what they call "hindered rolling"-additional effort required to overcome the nanotube's tendency to roll as the AFM tip strokes across it rather than along it.
"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," said Elisa Riedo, co-author of the study and an associate professor in the School of Physics at the Georgia Institute of Technology. "To assemble nanotubes on a surface, you need to know how they interact and what force is needed to move them."
Molecular dynamics simulations show how a carbon nanotube responds when an AFM tip scans over it in the transverse direction. (Image: Erio Tosatti, International Centre for Theoretical Physics, Trieste)
The combined theoretical and experimental study was supported by the U.S. Department of Energy. Other institutions contributing to the project include the International Centre for Theoretical Physics, International School for Advanced Studies and CNR Democritos Laboratory-all in Trieste, Italy-and the University of Hamburg in Germany. The paper was published in advance online on September 13 by the journal Nature Materials.
Carbon nanotubes have exceptional thermal, mechanical and electrical properties that have generated considerable interest since they were first reported in 1991. Though friction has been studied before in nanotubes, this research is the first to provide detailed information about the frictional forces at work in both the longitudinal and transverse directions when the tubes interact with an AFM tip.