PORTLAND, Ore. Quantum dots are so small10 to 50 atomsthat it's difficult to determine whether a patterning method has achieved its goal of crafting regular arrays. Now University of Michigan researchers claim to have pioneered a mapping method that should hasten the development of "designer dots" tailored for specific applications in computing, photovoltaic cells and light-emitting devices.
By mapping quantum-dot arrays with atomic-scale resolution, the researchers hope to break the bottleneck to self-assembly of chips based on the arrays.
"The tailoring of quantum dots is still in its infancy," said professor Roy Clark at the University of Michigan. "Our atomic-scale maps should help develop the field for specific applications."
Engineers have been experimenting with so-called directed assembly of nanometer-sized quantum dots, but progress has been impeded by poor imaging techniques at the required subnanoscale resolution. Clark claims to have surmounted that hurdle by harnessing the powerful X-ray beam at Argonne National Laboratory's Advanced Photon Source.
"We are using an interference method that diffracts X-rays from a substrate and any quantum dots on it, which gives us an interference pattern from which we can determine their structure," said Clark.
Clark's team was able to achieve resolution of .01 nm, thereby enabling an accurate map of every atom in the quantum dots (an atom is about a quarter of a nanometer in diameter). As a result of the new technique, researchers fabricating arrays of self-assembling quantum dots should be able to speed up their development work, according to Clark, who predicts a resurgence of progress in the field.
Clark performed the work with University of Michigan doctoral candidate Divine Kumah and fellow University of Michigan researchers Sergey Shusterman, Yossi Paltiel and Yizhak Yacoby. The team now is developing droplet epitaxy (a method of creating dense, uniform arrays of quantum dots) and directed ion-beam epitaxy, which would allow individual quantum dots to be placed at any location on a substrate.
The National Science Foundation, the U.S. Department of Energy and Argonne National Lab's Advanced Photon Source provided funding for the mapping work.