Rice University graduate student Cary Pint looked at the tweezers he was using to pull a sample; they were coated with carbon nanotubes. “That’s really interesting….” In fact, precisely what he was researching – how to make carbon nanotubes stick to various surfaces. Light bulb time. The Eureka! moment. Perhaps not exactly, but at the time Pint was experimenting with using water vapor to clean unwanted ‘amorphous’ carbon from nanotubes. He wondered if water had anything to do with making the nanotubes stick to the metal of the tweezers. As it turned out, it does.

Pint grew his carbon nanotubes using the chemical vapor deposition (CVD) technique, which starts with a layer of catalytic metal nanoparticles. The size of these particles helps determine the size of the carbon nanotubes. The layer of catalyst (a substrate) is heated to 700C and two gasses are introduced, one is a process gas (ammonia, nitrogen, or hydrogen) and the other a carbon containing gas (acetylene, ethylene, ethanol, or methane). From these gases, carbon nanotubes grow at the sites of the metal catalyst. Typically the catalyst metal stays attached to the tips of the growing nanotubes, or remains at the base. However, Pint found that by etching the nanotubes with a mixture of hydrogen gas and water vapor, the bonds of the metal catalyst are loosened. When the nanotubes are ‘stamped’ (pressed onto another surface), they adhere to the new surface, leaving behind the catalyst.

The nanotubes stick to a surface thanks to something called the Van der Waals forces. Named after the Dutch scientist Johannes van der Waals, these forces are the attraction or repulsion between molecules, or within a molecule, at the quantum level. This relatively weak bond is different than chemical bonds created by shared ions, or bonds created by electron sharing covalence. Weak though they may be, Van der Waals forces have interesting effects, for example, the gecko (a lizard) can climb glass windows because it has millions of microscopic hairs on its feet, which generate a Van der Waals bond with surfaces it climbs. Similarly with carbon nanotubes, they ‘adhere’ to surfaces with Van der Waals forces.

Pint’s work with the CVD technique has led to the research paper on the use of metallic nanoparticles of controlled size for the carbon nanotube catalyst. The discovery of a technique for making nanotubes adhere to surfaces has led to the ability to easily create patterns – rows, crosses – of nanotubes. This is a key property in nanotube manufacturing processes and should be readily scalable for commercial purposes.

Pint is primary author of the research paper, which also details a way to 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 two nanometers in diameter – or most of the nanotubes in the CVD “supergrowth” process.

“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.”

[Source: EurekAlert]

From the standpoint of the ongoing (or is it onrushing?) advance of practical applications for nanotechnology, really interesting indeed.