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Precise atom manipulation.

Research groups are concentrating their efforts on building nanostructures by moving atoms one at a time, trying to reliably position these atoms in precise locations. Unfortunately, such efforts are based on time-consuming trial and error approaches. Looking to streamline this atomic-scale work are Joseph Stroscio and Robert Celotta at the National Institute of Standards and Technology (NIST), Gaithersburg, Md., who demonstrated new methods for placing individual atoms at specific positions on a crystal surface.

Atom hopping

With a custom-built, cryogenic scanning tunneling microscope (STM), the two researchers used atom manipulation to create a tunable chemical bond between the STM tip's end and a single cobalt atom on the surface of a bed of closely packed copper atoms. This interaction allowed them to move the cobalt atom to a desired location.

The NIST researchers found that the STM tip could be used to reshape the energy environment surrounding the cobalt atom. Minor increases in the current flowing through the probe tip to the cobalt atom allowed Stroscio and Celotta to heat up and vibrate the lattice sites. This weakened the cobalt-copper bonds, allowing the scientists to regulate the amount of time the cobalt atom spent in one of the lattice sites. Consequently, they were able to trap the cobalt atom with the probe tip in a lattice site with unfavored energy. When the tip was moved back and forth with the atom trapped underneath, a recurring scratching sound was "heard" as the cobalt atom hopped between the unfavored and closest, energy-favored site.

"We know when the atom jumps by monitoring the current passing through the atom, as well as 'listening' to these current changes--atom hops--by passing the current signal through a loud speaker," says Stroscio.

Stroscio and Celotta also varied the probe tip-cobalt atom interaction to alter the extent of the atom's motion under the tip by adjusting the current. "This allowed us to study the mechanisms for the switching, which ultimately gives us insight into how to efficiently control the atom manipulation process," says Stroscio.

The results obtained at NIST stand to impact nanoscale work on different levels. "First, it shows how to use single atoms as sensors to obtain new information about the nanoscopic world--we name this approach Atom-Based Metrology," says Stroscio. "Second, by understanding the atom manipulation process in greater detail, we will be able to more efficiently build nanostructures atom by atom, and ultimately build perfect atomic scale devices."

Developers: Joseph Stroscio and Robert Celotta at NIST.

What's new: A method for placing atoms at precise locations on a crystal surface and using electronic noise emitted by the atoms as they move between bonding sites to confirm their movement.

How it works: A custom-built, cryogenic STM was used to move a cobalt atom on a bed of closely packed copper atoms to desired locations. The STM tip was used to trap the cobalt atom in a lattice site that it usually avoids, forcing it to hop back and forth between nearby bonding sites, giving rise to a "hip hop" scratching sound.

Applications: The fabrication of more efficient nanostructures, atom by atom.

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Title Annotation:Emerging Technologies
Publication:R & D
Date:Nov 1, 2004
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