October 5, 2009 – Researchers from the U. of Michigan have come up with the first atomic-scale maps of quantum dots, seen as a first step to producing and tailoring them for specific applications.

Their work, published online in the journal Nature Nanotechnology, derived sub-ånström resolution maps of quantum dots — clusters of atoms (10-50nm wide) that form semiconducting crystals — crystallized from indium droplets exposed to antimony and the interface with a GaAs (100) substrate. The dots were illuminated with a brilliant X-ray photon beam at Argonne National Laboratory’s Advanced Photon Source. The research was sponsored by a grant from the National Science Foundation; the US Department of Energy supported the work at Argonne.

From the paper abstract:

We find that the QDs form coherently and extend a few unit cells below the substrate surface. This facilitates a droplet–substrate exchange of atoms, resulting in core–shell structures that contain a surprisingly small amount of [indium].

The new maps will push forward the general understanding of quantum dots’ structure and chemical makeup, a first step in figuring out how to control their properties and behavior via directed assembly, according to Roy Clarke, U-M professor of physics and corresponding author of the paper, who likens this quantum-dot-charting work to discovering a new continent. "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," he said. Similarly, "this is the first time that anybody has been able to map [quantum dots] 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."

Already used for lasers and sensors, quantum dots could be helped along by this work into applications such as quantum computing.

Atomic-scale map of the interface between an atomic dot and its substrate, sliced through a vertical cross-section of the dot. Each peak represents a single atom. (Source: U. of Michigan)