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.
University of Michigan physicists have created the first atomic-scale maps of quantum dots, 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.
Quantum dots (QDs) have applications in optoelectronic devices quantum information processing and energy harvesting. Although the droplet epitaxy fabrication method allows for a wide range of material combinations to be used, little is known about the growth mechanisms involved. Here we apply direct X-ray methods to derive sub-ångström resolution maps of QDs crystallized from indium droplets exposed to antimony, as well as their interface with a GaAs (100) substrate. We find that the QDs form coherently15 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 In. The work provides the first atomic-scale mapping of the interface between epitaxial QDs and a substrate, and establishes the usefulness of X-ray phasing techniques for this and similar systems.