Map helps quantum dots go ‘designer’


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, is a slice through a vertical cross-section of the dot. Click to enlarge. (Courtesy: Roy Clarke/University of Michigan)

U. MICHIGAN (US)—The creation of the first atomic-scale maps of quantum dots may speed progress 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.

“I liken it (dot mapping) to exploration in the olden days,” says Roy Clarke, professor of physics at the University of Michigan and corresponding author of the study. “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.”

Engineers are gaining the ability to manipulate the atoms in quantum dots to control their properties and behavior, through a process called directed assembly.

Until now, progress has been slowed by the lack of atomic-scale information about the structure and chemical makeup of quantum dots.

“Researchers have been able to chart the outline of these quantum dots for quite a while,” Clarke says. “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 photon beam at Argonne National Laboratory’s Advanced Photon Source. The beam acts like an X-ray microscope to reveal details about the quantum dot’s structure. Because X-rays have very short wavelengths, they can be used to create 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 says. “So it’s incredibly high resolution.”

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 says.

“Atomic-scale mapping provides information that is essential if you’re going to have controlled fabrication of quantum dots,” Clarke explains.

“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.”

The research, which was published online Sept. 27 in the journal Nature Nanotechnology, was sponsored by a grant from the National Science Foundation. The U.S. Department of Energy supported work at Argonne National Laboratory’s Advanced Photon Source.

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