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Quantum Dots: The finish line in high-speed computing

In the full-throttle quest to make smaller, faster and better computer chips, UW-Madison Professor Max G. Lagally (Departments of Physics and Materials Science and Engineering) is exploring what may be the final frontier: Building them one atom at a time.

Working on a scale that would make dust specks look like boulders, Lagally and his colleagues have created tiny atomic structures called "quantum dots." The tiny crystals look like atomic versions of Egyptian pyramids, and represent some of the smallest materials structures ever created.

Max G. Lagally

Max G. Lagally (large image)

Although it's a distant prospect, the structures could become the semiconducting guts for a future generation of wireless computing, Lagally says. Some have speculated the computers could be fast enough to create life-like interactions between humans and computers, or instantly translate the spoken word into another language.

Lagally and researcher Zhenyu Zhang, of Oak Ridge National Laboratory, summarize the basic mechanisms of this process in a recent issue of the journal Science. Lagally says the article may help direct more research in nanometer-scale materials science, where scientists are trying to manipulate and control structures at the atomic scale. A nanometer is one billionth of a meter, the width of about three atoms.

The process of creating quantum materials is the exact opposite of computer chips, he says. Rather than etching precise lines into silicon for current flow, Lagally's process actually "grows" them, in the same manner that nature grows crystals or snowflakes. By controlling the deposition of atoms on a material, the researchers can actually produce ridges and capable of holding electrical charges.

"We are putting things together that nature would not ordinarily produce," Lagally says. "At the atomic scale, you can achieve the ultimate control materials by controlling the way in which atoms are put together to form structures."

Lagally and colleagues achieve this through a process called thin-film deposition. By heating materials such as germanium and silicon, the resulting "steam" of evaporated material is composed of individual atoms. Those atoms can be layered over the surface of materials in the desired depth and pattern, giving the material useful new chemical and electrical properties.

Thin films have been used for decades, Lagally says, in products such as architectural glass and corrosion-resistant coatings. But the new line of research is looking at how the atoms actually organize themselves on a surface, to optimize their chemical bonding.

One result in Lagally's lab was tiny pyramids assembled from several thousand germanium atoms. The pyramid crystals, perfectly shaped and uniform across the surface, can hold a single charge. First created by accident in 1989, Lagally's lab has since been working to understand the physics behind their creation and increase their density in thin films.

Lagally says computer chips will continue to shrink in size and grow in power through conventional approaches for at least another 15 years. But quantum dot materials would become useful when the conventional approach ultimately hits the wall. He says the technology could be useful for other devices such as sensors and communication tools.

"I'm very excited about this field and it's certainly futuristic," Lagally says. "It's the last frontier in the sense that we're working with things at the atomic scale. We're a long way from a potential device, but it's bound to lead to some form of computational tool."

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