In search of the imperfect nanocrystal

The unsung heroes of the microelectronics revolution are impurities intentionally added to semiconducting materials. A sprinkling of such atoms as boron or phosphorus, for example, is pivotal to much of the electronic and optical performance that makes microchips useful.

SAVING FACE. Atoms of impurities (yellow dots) stick firmly to certain facets of a growing nanocrystal. Erwin

Nevertheless, attempts to include such impurities in tiny clumps of atoms known as semiconductor nanocrystals have often failed. A study in the July 7 Nature offers an explanation for this roadblock—and a way around it.

Experiments on the nanocrystals, also called quantum dots, find them promising as fluorescent tracers for monitoring biomolecules (SN: 8/7/04, p. 94: Available to subscribers at Quantum dots light up cancer cells in mice) and as additives to such technologies as light-emitting diodes (SN: 7/16/05, p. 43: Available to subscribers at Bright Future).

Still more could be done with impurity-containing nanocrystals, but researchers have struggled to achieve even small concentrations of impurities. Many researchers explained this difficulty by hypothesizing that the minuscule clumps are self-purifying.

A different picture emerges from calculations by physicist Steven C. Erwin and his colleagues at the Naval Research Laboratory in Washington, D.C., and from experiments by a materials science group led by David J. Norris of the University of Minnesota, Twin Cities.

The new findings indicate that when certain impurities integrate into a growing nanocrystal, some of its facets have atomic arrangements that strongly adhere to those foreign atoms. The scientists predicted and confirmed that zinc-selenide nanocrystals with certain shapes were particularly prone to incorporate manganese atoms.

The researchers then went a step farther and induced nanocrystals of cadmium-selenide, which are known to shun manganese atoms, to accept those impurities. The scientists grew the nanocrystals on a template that formed manganese—accepting facets that are ordinarily absent on cadmium—selenide crystals.

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