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Science of the small is BIG at Purdue

Ten hydrogen atoms.

Lined up side by side, they take up a nanometer, or one-billionth of a meter - a size so small you could fit a million nanometers across the head of a pin.

Rashid Bashir
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It's on a scale far too small to see. Yet working in the nanometer realm gives experts worldwide visions of unlimited technological wonders.

At Purdue, researchers are focusing on several nanowonders: smaller, faster, more efficient computers; stronger materials; and new types of biomedical devices.

Nanotechnology differs from conventional manufacturing in its ability to control the arrangement of individual atoms. With nanotechnology, it's possible to build small devices from the bottom up, atom by atom, and finely tune their features.

"We are now trying to understand how current flows from atom to atom so that we can design useful electronic devices on a molecular scale," says Supriyo Datta, the Thomas Duncan Distinguished Professor of Electrical and Computer Engineering at Purdue.

The idea of building tiny devices from the ground up is not new: The late Richard Feynman, Nobel laureate from Caltech, presented this idea in 1959 when he noted that to build devices on the atomic scale, scientists would first have to develop ways to manipulate atoms.

The first glimpse of this new "nanoage" came in 1983, when physicists working for IBM in Zurich developed a new tool to study fundamental questions about the electronic properties of semiconductors. The instrument was later named the scanning tunneling microscope, or STM.

The researchers positioned a sharp tip above a surface so closely that electrons could jump from the surface to the tip, creating a weak electron current. They then measured how the electron current decreased as the tip was withdrawn.

Ronald Reifenberger, professor of physics at Purdue, immediately recognized the potential of their feat.

"From my previous experience in electron emission, I understood what they had demonstrated - a way to manipulate and see features at the atomic scale, which was really pretty amazing to me," he says.

After meeting one of the IBM pioneers at a scientific conference in 1985, Reifenberger began work to develop an STM. Purdue became one of the first educational institutions worldwide to have such a research tool.

Working with Ron Andres and Nick Delgass, professors of chemical engineering, Reifenberger began using the new instrument for scientific studies in December 1986.

"At that time, we were one of a half-dozen academic institutions in the nation to have such a device, and perhaps the only one in the Midwest," he says.

The first "nanotechnology" experiment at Purdue was conducted in September 1987, when the group succeeded in writing and then reading the symbol "hbar" - a symbol for Planck's constant - on a gold substrate with an STM tip. The symbol measured about 400 nanometers wide by 600 nanometers tall.

"For a short period of time, Purdue held the record worldwide for smallest symbol written," Reifenberger says.

By 1990, the scanning tunneling microscope was available commercially, and many researchers worldwide began to use it. But attempts to build functional nanostructures with the STM were hindered because conventional manufacturing processes such as photolithography - which uses light to produce tiny circuit patterns on the chips - couldn't be integrated easily with the extremely small nanostructures that were being fabricated.

"Even if they could, such tiny structures would have been too fragile to take the heat buildup that would occur when current flowed through them," Andres says.

In 1991, he teamed up with David Janes, associate professor of electrical and computer engineering, and several researchers in chemistry to investigate ways to build small devices capable of conducting electrical current. Five years later, his team at Purdue became the first to create an ultrathin film, made from tiny clusters of gold atoms, that could conduct electricity.

Andres' thin film was built using a process called "self-assembly." It's a principle familiar in biology, where the right mix of biological molecules will interact on their own to form distinctive structures, such as cells, tissues and organs.

It was about this time that research interests from a number of fields - biology, biochemistry, pharmacy and engineering - began to meet and marry.

"By the mid-1990s, electrical engineering researchers were working with devices in the range of 10 to 100 nanometers, which is comparable to the size of biological molecules," Datta says. "The fact that engineers and life science researchers are now working on the same scale creates a common ground where the two fields can learn from one another. We are now exploring new, more fundamental ways for electronics to help biology, and for biology to help electronics."

Last August, Purdue reached another milestone when an interdisciplinary group of researchers created the first protein "biochips," mating silicon computer chips with biological proteins.

Michael Ladisch, Distinguished Professor of Agricultural and Biological Engineering and professor of biomedical engineering, says that if the first real-world tests of the biochips are successful, the protein-encrusted silicon chips could appear in dozens of applications in a few years. For example, physicians could use devices containing biochips to quickly diagnose common diseases or to test the efficacy of chemotherapy.

Purdue now has about 25 research laboratories - spanning engineering, chemistry, physics, agriculture and pharmacy - working together to advance interdisciplinary efforts in nanotechnology.

Last summer, the state of Indiana awarded Purdue $1.5 million to establish the Center for Nanoscale Electronics/Biological Devices. Federal and private resources also are being sought to expand these efforts.

Photo caption:
Rashid Bashir, assistant professor of electrical and computer engineering and biomedical engineering, was among a group of Purdue scientists who created the first protein biochip. The chip mates a silicon computer chip with biological proteins.