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Semiconductor studies get a rise from yeast.

Semiconductor studies get a rise from yeast

For years scientists have worked on making semiconducting crystals so tiny they begin to take on the properties of individual atoms or molecules -- but now a simple yeast has produced some of the best specimens yet. Physicists plan to use the new "crystallities" to investigate the unusual properties of very small semiconductor particles.

The finding sprang from two seemingly disparate research efforts. Physicists at AT&T Bell Laboratories in Murray Hill, N.J., had been seeking the size limits below which various materials lose their semiconductor properties and investigating what happens below those limits. At the same time, biochemists at the University of Utah Medical Center in Salt Lake City noted some curious properties in the minuscule (200- to 1,000-molecule) cadmium sulfide crystals that yeast organisms make when subjected to cadmium metal. It was not until the Utah biochemists contacted the AT&T physicists that the two groups realized the yeast had created the first known biologically produced specimens of just the sort of particles the physicists were investigating. The physicists, headed by Louis E. Brus, and the biochemists, led by Dennis R. Winge, describe their discovery in the April 13 NATURE.

An individual molecule, or even several molecules, of semiconducting material will behave differently from an ordinary "bulk" semiconductor, explains Brus. In a normal semiconductor, electrons need an energy boost to free them from atoms, allowing them to flow and thereby to conduct electricity. The free electrons may have any amount of energy within a given range. In a solitary molecule or atom, quantum mechanical rules restrict electrons to specific energy states. Somewhere in between lies the "quantum" crystallite. Like the bulk semiconductor, it contains electrons that can be freed, and like the atom, it is constrained to specific energy states.

The energy boost that frees electrons to conduct must be delivered in the form of photons with more than a certain minimum energy. When the particle is extremely small, Brus says, this thresh-old energy suddenly increases and becomes dependent on the particle size. Winge noticed that in his yeast-produced particles, the threshold energy was both size-dependent and greater than that of a bigger chunk of the material.

Another quality helping the groups characterize the crystallites was the way they absorb and emit light. Winge sent a spectrum of the specific absorbed energies to Brus, who found it corresponded to the spectrum he would expect from quantum semiconducting crystals.

Winge's yeast-created crystallities were more uniform in size than their synthetic counterparts -- a property critical for physicists trying to study the size-dependent behavior, says Brus. Winge explains that a special protein in the yeast curbs particle growth between about 17 and 23 angstroms. He says he has now extracted this protein and used it alone to halt growth at this size.

The yeast organisms convert cadmium to cadmium sulfide in order to detoxify the poisonous metal, Winge says. He suspects they may also utilize the semiconductor properties of a material, possibly for transferring energy or electrons.
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Title Annotation:quantum semiconducting crystallites
Author:Flam, F.
Publication:Science News
Date:Apr 15, 1989
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