Lu’s team discovered that by immersing vertically grown carbon nanotube bundles into a liquid organic solvent and allowing them to dry, the nanotubes pull close together into a dense bundle. Lu attributes the densification process to capillary coalescence, which is the same physical principle that allows moisture to move up a piece of tissue paper that is dipped into water.
The process boosts the density of these carbon nanotube bundles by five to 25 times. The higher the density, the better they can conduct electricity, Lu said. Several factors, including nanotube height, diameter, and spacing, |
Images of the experiment are more striking than any “before and after” photos of the latest fad diet. In one instance, Liu started with a carbon nanotube bundle 500 micrometers in diameter, shaped somewhat like a marshmallow, and dipped it into a bath of isopropyl alcohol. As the alcohol dried and evaporated, capillary forces drew the nanotubes closer together. Van Der Waals forces, the same molecular bonds that boost the adhesion of millions of setae on gecko toes and help the lizard defy gravity, ensure the nanotubes retain their tightly packed form.
The resulting bundle shrunk to a diameter of 100 micrometers, with a 25-fold increase in density. Instead of a marshmallow, it looked more like a carpenter’s nail.
“It’s a significant and critical step toward the realization of carbon nanotube interconnects with better performance than copper,” Lu said of his research findings. “But there’s still a lot of work to do before this technology can be integrated into industrial applications.”
Despite his initial successes, Lu said the density results obtained are not ideal and carbon nanotubes would have to be further compacted before they can outperform copper as a conductor. A close-up photo, taken using a scanning electron microscope, reveals there are still large empty spaces between densified nanotubes. The research team is exploring various methods to achieve ever-higher density and higher quality of carbon nanotube bundles, he said.
Lu is confident that these densified carbon nanotubes, with their high conductivity, ability to carry high current density, and resistance to electromigration, will be key to the development of 3-D computer chips. Chips used today can only shrink so much smaller, as their flat surface must have enough room to accommodate scores of different components. But the semiconductor industry and academia are looking at ways to layer chip components into a vertical stack, which could dramatically shrink the size of the overall chip.
Densified carbon nanotubes, with their ends trimmed and polished, can be the basic building blocks for interconnects that would link the stacked layers of a 3-D computer chip, Lu said.
“Carbon nanotubes are one of the most promising materials for interconnects in 3-D integration,” he said. Other potential applications of the densified nanotubes are high surface area electrodes for supercapacitors, fuel cell electrodes for hydrogen storage, heat dissipation materials for thermal conductors, and other situations that require high electrical, thermal, or mechanical performance. ###
News from Rensselaer Polytechnic Institute June 6, 2007 news.rpi.edu
Lu’s group closely collaborates with a research group led by Pulickel Ajayan, the Henry Burlage Professor of Materials Science and Engineering at Rensselaer, who provided Lu with the grown nanotubes. Along with Liu, who leads the carbon nanotube densification process investigation, the research team includes research associates Navdeep Bajwa, Lijie Ci, Swastik Kar, and Sang Hwui Lee.
The team has filed a disclosure and is moving forward toward a patent for their densification process.
6/6/2007 - Lu’s research is supported by Defense Advanced Research Projects Agency (DARPA), the Microelectronics Advanced Research Corporation (MARCO), and the New York State Office of Science, Technology and Academic Research (NYSTAR) through the Interconnect Focus Center (IFC). The IFC is a multi-university research center focusing on the discovery and invention of new solutions that will enable the U.S. semiconductor industry to transcend known limits on interconnects that would otherwise decelerate or halt the rate of progress toward future terascale system integration.
For more information on Lu’s research into 3-D integration, go to: rpi.edu/research/magazine/.
Contact: Michael Mullaney mullam@rpi.edu 518-276-6161 Rensselaer Polytechnic Institute
About Rensselaer: Rensselaer Polytechnic Institute, founded in 1824, is the nation’s oldest technological university. The university offers bachelor’s, master’s, and doctoral degrees in engineering, the sciences, information technology, architecture, management, and the humanities and social sciences. Institute programs serve undergraduates, graduate students, and working professionals around the world.
Rensselaer faculty are known for pre-eminence in research conducted in a wide range of fields, with particular emphasis in biotechnology, nanotechnology, information technology, and the media arts and technology. The Institute is well known for its success in the transfer of technology from the laboratory to the marketplace so that new discoveries and inventions benefit human life, protect the environment, and strengthen economic development.
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