Engineers lay foundation for vertical nanotubes

Timothy S. Fisher, the associate professor of mechanical engineering who is leading the work with Timothy D. Sands, the Basil S. Turner Professor of Engineering explained: "Verticality gives you the ability to fit more things into the same area, so you can add more and more layers while keeping the footprint the same size or smaller" The technique starts with a thin film containing two layers of aluminum sandwiching a layer of iron using electron-beam evaporation,. The aluminium was then anodised to create an aluminium oxide layer with tiny cylindrical cavities. During the process, an electric field was used to form a precisely aligned array of nanoscopic holes. A mixture of hydrogen and methane gas was then flowed into the holes, and microwave energy was applied to break down the methane, which contains carbon. The iron layer acted as a catalyst that prompted the carbon nanotubes to assemble from carbon originating from the methane, and the tubes then grew vertically out of the cavities. "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," Sands said. The research is based at the Birck Nanotechnology Center in Purdue's Discovery Park, the university's hub for interdisciplinary research. The cavities form within seconds, and the nanotubes take several minutes to finish growing. The holes vary in width from 30-50 nm. 1nm is about as long as 10 atoms strung together. It is widely thought that carbon nanotubes, which were discovered in the early 1990s by Dr. Sumio Iijima, building on the discovery of buckyballs by Robert Curl, Harold Kroto and Richard Smalley, may enable industry to create new types of transistors and more powerful, energy-efficient computers, as well as ultra-thin "nanowires" for electronic circuits. Reaching that potential promise, however, will not be possible unless carbon nanotubes can be integrated with other parts of circuitry and devices. The experiments at Purdue yielded both single- and double-walled nanotubes, about 1nm in diameter. Researchers will continue the work in efforts to understand which conditions are needed to produce single-wall tubes versus the double-wall variety and to learn how to produce more of one or the other. Early applications are expected in wireless computer networks and radar technology. Long-term uses are possible in new types of transistors, other electronic devices and circuits. The research is funded by NASA, through the Purdue-based Institute for Nanoelectronics and Computing. More information from Timothy Fisher's web site