Gecko Grip Given to Any Surface

A graduate student at Rice University has devised a way to transfer the gravity-defying grip of a gecko – so strong it can stick to even glass – from any surface to another in a matter of minutes.

What makes a gecko foot stick is the attractive force – the van der Waals force – between millions of microscopic hairs on its feet and the surface. Graduate student Cary Pint came up with a way to transfer synthetic gecko "hairs" – forests of strongly aligned, single-walled carbon nanotubes (SWNTs) – from one surface to another quickly. The template used to grow the nanotubes, with its catalyst particles still intact, can be used repeatedly to grow more nanotubes, almost like inking a rubber stamp.

Pint's method also can quickly and easily determine the range of diameters in a batch of nanotubes grown through chemical vapor deposition (CVD). Common spectroscopic techniques are poor at seeing tubes bigger than 2 nm in diameter – or most of the nanotubes in the CVD "supergrowth" process.

Close-up of the underside of a gecko's foot as it walks on a glass wall. Van der Waals force interactions between the finely divided setae (hairs on the toes) and the glass enable the gecko to stay in place and to walk on the seemingly smooth glass. (Wikipedia Commons photo by Bjørn Christian Tørrissen)

"This is important, since all of the properties of the nanotubes – electrical, thermal and mechanical – change with diameter," he said. "The best thing is that nearly every university has an FTIR (Fourier transform infrared) spectrometer sitting around that can do these measurements, and that should make the process of synthesis and application development from carbon nanotubes much more precise."

Pint and other students and colleagues of Robert Hauge, a Rice distinguished faculty fellow in chemistry, also are investigating ways to take printed films of SWNTs and make them all-conducting or all-semiconducting – a process Hauge refers to as "Fermi-level engineering" for its ability to manipulate electron movement at the nanoscale.

Combined, the techniques represent a huge step toward a nearly limitless number of practical applications that include sensors, highly efficient solar panels and electronic components.

A Rice University graduate student has devised a way to transfer the stickiness of a gecko's foot to forests of strongly aligned single-walled carbon nanotubes that can be quickly transferred from one surface to another. (Image: Rice University)

"A big frontier for the field of nanoscience is in finding ways to make what we can do on the nanoscale impact our everyday activities," Hauge said. "For the use of carbon nanotubes in devices that can change the way we do things, a straightforward and scalable way of patterning aligned carbon nanotubes over any surface and in any pattern is a major advance."

Pint said an afternoon of "experimenting with creative ideas" as a first-year graduate student turned into a project that held his interest through his time at Rice. "I realized early on it may be useful to transfer carbon nanotubes to other surfaces," he said. "I started playing around with water vapor to clean up the amorphous carbons on the nanotubes. When I pulled out a sample, I noticed the nanotubes actually stuck to the tweezers.

"I thought to myself, 'That's really interesting."

Water turns out to be the key. After growing the nanotubes, Pint etches them with a mix of hydrogen gas and water vapor, which weakens the chemical bonds between the tubes and the metal catalyst. When stamped, the nanotubes lie down and adhere, via van der Waals, to the new surface, leaving all traces of the catalyst behind.

Transferring patterns of strongly aligned, single-walled carbon nanotubes from a substrate to another surface is now possible (Image: Rice University)

Pint, who hopes to defend his dissertation in August, developed a steady enough hand to deposit nanotubes on a range of surfaces – "anything I could lay my hands on" – in patterns that could easily be replicated and certainly enhanced by industrial processes. A striking example of his work is a crisscross film of nanotubes made by stamping one set of lines onto a surface and then reusing the catalyst to grow more tubes and stamping them again over the first pattern at a 90° angle. The process took no more than 15 minutes.

"I'll be honest – that was a little bit of luck, combined with the skill of having done this for a few years," he said of the miniature work of art. "But if I were in industry, I would make a machine to do this for me."

Pint believes that industries will take a hard look at the technique, which he said could be scaled up easily, for embedding nanotube circuitry into electronic devices.

His own goal is to develop the process to make a range of highly efficient sensing devices. He also is investigating doping techniques that will take the guesswork out of growing metallic (conducting) or semiconducting SWNTs.

The research is reported this week in the online version of the journal ACS Nano; Pint is primary author.

For more information, visit: www.rice.edu