|A transmission electron microscope took images of the nanotubes emerging vertically from the cavities in the thin film. The nanotubes self-assemble once the process is in motion, says Eric Stach, Purdue associate professor of materials engineering. The cavities form within seconds, and the nanotubes take several minutes to grow. The holes vary in width from 30 to 50 nm. The NASA funded research is featured in the July 11 issue of the journal Nanotechnology.|
This may one day let electrical engineers build advanced electronics, wireless devices and sensors with even smaller footprints. But these developments won't be possible unless carbon nanotubes can be integrated with other parts of circuitry and devices, says Purdue Engineering Professor, Timothy D. Sands.
"Verticality gives the ability to fit more circuits and components into the same area," adds Timothy S. Fisher, associate professor of mechanical engineering. "But before we can even think about using nanotubes in electronics, we must learn how to put them where we want them."
The Purdue process first sandwiches an ultrathin layer of iron (using electron-beam evaporation, a standard process employed in the semiconductor industry) between two layers of aluminum. Thin-film is selectively anodized to oxidize tiny cylindrical cavities in the aluminum surface. This turns the film into a "porous anodic alumina template" with a thickness less than 1/100th the width of a human hair. Applying an electric field forms a precisely aligned array of nanoscopic holes and turns the aluminum into porous alumina - the oxidized form of aluminum also known as aluminum oxide.
Next a mixture of hydrogen and methane gas flows into the template's holes. Microwave energy then breaks down the methane, which contains carbon. The iron layer acts as a catalyst that prompts the carbon nanotubes to assemble from carbon originating in the methane. The nanotubes grow from the sidewall of the pore and then extend vertically.
"You get a single nanotube in each pore. And that's important because we can start to think about controlling how and where to put nanotubes to vertically integrate them for future electronic devices and sensing technologies," says Sands.
Other researchers previously have made the templates, but the Purdue researchers are the first to add a layer of iron. Future work will focus on determining conditions needed to produce single-wall tubes versus double-wall and to develop methods to produce more of one type than the other. Early applications are most likely in wireless computer networks and radar technology. Long-term uses are possible in new types of transistors, other electronic devices, and circuits.