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PbTe Quantum Dots Assembled into Superlattices and Glassy Films

Photonics Spectra
Apr 2006
Daniel S. Burgess

A team of investigators from IBM’s T.J. Watson Research Center in Yorktown Heights, N.Y., Michigan State University in East Lansing and Columbia University in New York has developed techniques for the solution-phase synthesis of PbTe quantum dots that yield either grams of one size of the dots or milligrams of the structures with increasing sizes. Using the quantum dots prepared with these methods, which should be suitable for the production of other types of lead chalcogenide quantum dots, they fabricated highly ordered superlattices and glassy films that may have applications in various near-infrared devices.

Jeffrey J. Urban, a postdoctoral researcher at T.J. Watson’s Nanoscale Materials and Devices Group, explained that lead chalcogenide quantum dots are of particular interest because the IV-VI semiconductors possess relatively large excitonic Bohr radii — from 20 nm in PbS to 46 nm in PbTe, compared with 1 to 10 nm in most II-VI and III-V semiconductors. This facilitates the synthesis of structures with physical dimensions that are much smaller than the Bohr radius so that quantum confinement in the dots is described by the extreme limits of the strongly confined regime.

Moreover, the electronic band structure of the lead chalcogenides makes them particularly suited for use in fundamental studies of phenomena such as phonon confinement and exciton-photon coupling. And their bandgaps render them appropriate for near-IR photovoltaic, light-harvesting and telecommunications applications, he said.

In their work, the investigators demonstrated that they could control the size of the synthesized PbTe quantum dots by manipulating the ratio of oleic acid to lead acetate in the reaction, producing structures with uniform diameters of 4 to 14 nm. By manipulating the reaction time, they found that they could produce dots with different diameters.

They further discovered that the solvent used in the controlled evaporation of a concentrated solution of these quantum dots determined the packing properties of the resulting film. Trichloroethylene or pentane yielded hexagonal close-packed and face-centered cubic superlattices. Chloroform or a mixture of hexane and octane yielded glassy films with only local ordering.

This was surprising, Urban said. The scientists had expected that employing solvents with a high vapor pressure, such as pentane, would produce glassy films and that employing those with a low vapor pressure, such as octane, would produce superlattices.

The investigators have begun to characterize the low-temperature electronic properties of the PbTe quantum dot films, he said, to explore their potential applications in thermoelectric, thermophotovoltaic and other direct energy conversion devices. They also are collaborating with a theory group to better understand the phonon modes in the materials.

Journal of the American Chemical Society, online Feb. 14, 2006, doi:10.1021/ja058269b.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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