By Peter Weiss
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.
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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.