Creating a Precise Atomic-Scale Map of Quantum Dots

OCTOBER 9, 2009

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An atomic-scale map of the interface between an atomic dot and its substrate. Each peak represents a single atom. The map, made with high-intensity x-rays at the APS, is a slice through a vertical cross-section of the dot. (Image courtesy of Roy Clarke, University of Michigan)

With a big assist from the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne, University of Michigan (U-M) researchers have created an atomic-scale map of quantum dots with unprecedented precision, a major step toward the goal of producing “designer dots” that can be tailored for specific applications.

Quantum dots—often called artificial atoms or nanoparticles—are tiny semiconductor crystals with wide-ranging potential applications in computing, photovoltaic cells, light-emitting devices, and other technologies. Each dot is a well-ordered cluster of atoms, 10 to 50 atoms in diameter.

Engineers are gaining the ability to manipulate the atoms in quantum dots to control their properties and behavior, through a process called “directed assembly.” But progress has been slowed, until now, by the lack of atomic-scale information about the structure and chemical makeup of quantum dots.

The new atomic-scale maps obtained by the U-M researchers will help fill that knowledge gap, clearing the path to more rapid progress in the field of quantum-dot directed assembly, said Roy Clarke, U-M professor of physics and corresponding author of a paper on the topic published in the journal Nature Nanotechnology.

Lead author of the paper is Divine Kumah of the U-M's Applied Physics Program, who conducted the research for his doctoral dissertation.

“I liken it to exploration in the olden days,” Clarke said of dot mapping. “You find a new continent and initially all you see is the vague outline of something through the mist. Then you land on it and go into the interior and really map it out, square inch by square inch.

“Researchers have been able to chart the outline of these quantum dots for quite a while. But this is the first time that anybody has been able to map them at the atomic level, to go in and see where the atoms are positioned, as well as their chemical composition. It's a very significant breakthrough.”

To create the maps, Clarke's team illuminated the dots with a brilliant x-ray beam at the X-ray Operations and Research/UNI 33-ID beamline at the APS. The beam revealed details about the quantum dots structure, and because they have very short wavelengths that match the spacing of atoms, the x-rays created super-high-resolution maps.

“We're measuring the position and the chemical makeup of individual pieces of a quantum dot at a resolution of one-hundredth of a nanometer,” Clarke said, “so it's incredibly high resolution.” (A nanometer is one-billionth of a meter.)

The availability of atomic-scale maps will quicken progress in the field of directed assembly. That, in turn, will lead to new technologies based on quantum dots. The dots have already been used to make highly efficient lasers and sensors, and they might help make quantum computers a reality, Clarke said.

“Atomic-scale mapping provides information that is essential if you're going to have controlled fabrication of quantum dots,” Clarke said. “To make dots with a specific set of characteristics or a certain behavior, you have to know where everything is, so that you can place the atoms optimally. Knowing what you've got is the most important thing of all.”

See: Divine P. Kumah1, Sergey Shusterman2, Yossi Paltiel3, Yizhak Yacoby3, and Roy Clarke1*, “Atomic-scale mapping of quantum dots formed by droplet epitaxy,” Nat. Nano., published online 27 September 2009. DOI:10.1038/nnano.2009.271

Author affiliations: 1University of Michigan, 2Soreq NRC, 3Hebrew University

Correspondence: *royc@umich.edu

The work was supported by the US National Science Foundation under Grant DMR-0606048.

Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future.

The original U-M news release can be found at: http://www.ns.umich.edu/htdocs/releases/story.php?id=7334