Trading places nanostyle

11/11/04

Nanocrystals show a quick route to change

Just as the Microtechnology Age was built upon the introduction of impurities into crystals of semiconductor materials, so, too, will crystalline doping be the bedrock upon which the Nanotechnology Age is built. To advance the arrival of this next technological era at a faster pace, however, scientists need a better understanding of what happens to nano-sized crystals under the various forms of doping.

Researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley have good news for the burgeoning nanotechnology industry. They've shown that for nanocrystals, the doping process in which one type of positively charged atom, or cation, is exchanged for another, take place at a much faster rate than for crystals of extended size, and is fully reversible, something that is virtually forbidden in micro-sized crystals under the same environmental conditions.

"Our findings show that the cation exchange reaction offers a versatile route for expanding the range of nanoscale materials with diverse compositions, structures, and shapes without having to develop new synthetic methods to produce each individual nanostructure," says chemist Paul Alivisatos, the principal author of a paper reporting this research which appears in the November 5, 2004 edition of the journal Science.

Alivisatos holds a joint appointment with Berkeley Lab, as director of its Materials Sciences Division, and UC Berkeley, where he's the Chancellor's Professor of Chemistry and Materials Science. He's also director of the Molecular Foundry, a U.S. Department of Energy national user facility aimed at the development of nanotechnology.

Other co-authors of the Science paper were chemists Dong Hee Son, Steven Hughes, and Yadong Yin, all of whom hold appointments with either Berkeley Lab's Materials Science Division, UC Berkeley's Chemistry Department, or both.

Says Dong Hee Son, "Another important result from this study is that ionic nanocrystals apparently can be transformed into other materials with different physical and chemical properties but without altering their original shape, simply through an exchange of cations."

Doping a crystal to transform it into another type of material with different physical and chemical properties is a long-established practice in extended solids. This practice is being been carried over into the transformation of nanocrystals grown from inorganic materials, including metals and semiconductors. However, because nanocrystals have a high surface-to-volume ratio (meaning they are virtually all surface and no interior), their reactions to the various forms of doping can be quite different from the reactions of extended solids. For example, in extended solids, chemical reactions run very slowly because of the high activation energies required to diffuse atoms and ions. These transformations are also not reversible.

"In our study with cation exchange reactions, the nanocrystals acted more like molecules in chemical reactions than extended solids," says Son. "The speed and reversibility of the reactions demonstrates that inorganic nanocrystals are far more chemically dynamic than previously realized."

Under the leadership of Alivisatos, Son and the other authors of the Science paper worked with nanocrystals of the semiconductor cadmium-selenide (CdSe), which offer a high degree of control over size and shape. They mixed a solution of CdSe nanocrystals together with a small amount of silver nitrate at room temperatures. In less than one second, the silver cations reacted with the CdSe spheres to produce spheres of silver-selenide (Ag2Se). When these Ag2Se spheres were mixed with a solution containing an excessive amount of cadmium cations, the reaction was reversed. Though the reverse reaction took about a minute to complete, the final product was CdSe spheres.

The Berkeley researchers performed similar tests to transform hollow spheres of cadmium-sulfide into hollow spheres of silver-sulfide, and crystals of cadmium-telluride in the shape of tetrapods into tetrapod crystals of silver-telluride. Again, the transformation reactions were fast, complete, and fully reversible.

"The cation exchange reaction in the nanocrystals we investigated in this study, can easily be extended to exchange with other cations," Alivisatos says. "On the other hand, attempts to induce anion (negatively charged ions) exchange have not been successful under similar experimental conditions, possibly because the much larger size of the anions, relative to the cations, makes diffusion more difficult."

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