Quantum Onions Add Zing to Nanoscience

Daniel C. McCarthy

Development of quantum onions could signal the onset of salad days for nanoscientists at the University of Mainz. The crystals, so called because of their layers of semiconductor material, add further seasoning to researchers' knowledge of how electric charges become trapped in regions of semiconductors.

Incorporate a quantum well in a quantum dot and you have created a quantum onion. These nanocrystals wrap alternating layers of semiconductor material to trap electric charges inside. The two models labeled b and c1 illustrate various stages of surface modification; micrograph c2 of a quantum onion confirms the tetrahedral structure of the models. Model d1 and micrograph d2 represent a more sophisticated onion structure created through twin epitaxial growth. Image d3 contrasts a nanocrystal incorporating a layer of HgS with a crystal (d4) based solely on Cd. Courtesy of the University of Mainz.

While science has refined the ability to create quantum dots as small as 1 to 10 nm in diameter, the surfaces of these crystals manifest lattice imperfections that trap electrons and holes -- their positively charged counterparts.

"The question was if it was possible or not to confine charge carriers even within a quantum dot, which is by definition already a system with zero dimensionality," said Alf Mews, a physical chemist at the university.

Using wet-chemical techniques, Mews covered a core of cadmium sulfide (CdS) atoms within a shell of mercury sulfide (HgS) and applied a final layer of CdS. Experiments with the finished crystals supported theories that the charge carriers are mainly localized in the low-bandgap HgS material and are thus well-separated from the surface of the crystal. "In analogy to the quantum wells formed in a macroscopic material, these systems can be described as a quantum well in a quantum dot," Mews said.

Where this applies to photonics is largely in research applications where quantum dots currently are substituted for dye molecules that respond to laser energy. These applications are limited, however, by a phenomenon called "blinking," in which the dots spontaneously stop fluorescing, sometimes for as long as seconds. The problem is that the fluorescence spectrum of quantum dots depends strongly on the intensity of the coherent light, Mews explained. At high intensities, electric charges escape to the surface of the crystal, causing their fluorescence to blink.

The ability of quantum onions to confine electric charges within their inner HgS layer somehow prevents this, and eventually could lend more stability to fluorescence labeling or electronic nanocircuits.

Additionally, by altering the size of the HgS well width, quantum onions with uniform dimensions could be made to fluoresce at specified wavelengths. Quantum dots also allow this, but only by altering their size and, consequently, their chemistry.