Scientists at Motorola Labs are researching ways to improve control in the growth of carbon nanotubes (CNTs) that can make transistors smaller and faster and chemical/biological detectors ultra-sensitive.
CNTs represent a relatively new form of molecular carbon that was discovered only twelve years ago. Each nanotube is formed with one or more seamless concentric cylinders consisting of a rolled-up sheet of graphite. At the Tempe, Arizona facility of Motorola Labs, researchers Ruth Zhang, Islamshah Amlani, John Tresek, and Larry Nagahara are focusing on a category known as single-walled carbon nanotubes. As the name implies, these consist of only a single cylinder of carbon atoms and each nanotube has a diameter as small as 1-5 nanometers or about one ten-thousandth the diameter of a human hair.
Early research indicates that CNTs have very promising mechanical and electrical characteristics. Scientists studying CNTs have reported that they can be as much as 100 times stronger than steel at a fraction of the weight. Other studies showed that the metallic variety of CNTs has a current carrying capability more than 100 times that of copper, while the semiconducting variety can potentially be used for transistors that are smaller and faster than conventional ones made with silicon.
One of the technical challenges associated with CNT-based device technology is in controlling the placement of nanotubes with specific properties on a substrate. The Motorola Labs team has developed a technique using chemical vapor deposition (CVD) that can position individual single- walled carbon nanotubes at pre-determined locations on a substrate in a highly parallel manner that is compatible with conventional semiconductor processing techniques. Furthermore, the team found that almost 90% of these nanotubes are semiconducting in nature. Such results provide an important step forward in developing arrays of ultra-small transistors using CNTs. Details of this work were published by the Motorola Labs researchers in Nano Letters (2003, volume 3, 731-735), and also presented at the 2003 Electronics Materials Conference.
"CNTs can play a very significant role in the emergent technological area known as nanotechnology," said Ray Tsui, who leads the Nanoelectronics research effort at Motorola Labs. "With diameters in the nanometer regime and a very high length-to-diameter ratio, CNTs have unique properties that can exploited to achieve extraordinary performance in a number of applications. Having a high degree of control in the type and positioning of CNTs is critical in moving this technology forward," Tsui added. In addition to researching advanced transistors, the Motorola Labs team also is collaborating with researchers at Arizona State University to develop ultra-sensitive chemical/biological sensors based on single-walled carbon nanotubes. Another group at Motorola Labs is researching the use of CNTs for high-brightness flat panel displays.
To further improve growth control for the CVD technique, the Motorola Labs team also collaborated with researchers led by Professor Jian-Min Zuo from the University of Illinois at Urbana-Champaign to develop atomic resolution imaging via a novel electron diffraction technique. The results were discussed in papers that appeared in Science (2003, volume 300, 1419-1421) as well as in Applied Physics Letters (2003, volume 82, 2703-2705). With this technique, researchers can now view CNTs three dimensionally with a resolution down to 0.1 nanometer, thus resulting in a very accurate picture of its structural properties. This provides a powerful tool to help monitor and optimize the growth process of CNTs.