Detonating silicon wafers can ID elements

In a serendipitous discovery, chemists have found a convenient way to make silicon–the stuff of computer chips–explode on command.

Wafers of specially processed silicon might someday serve in miniature propulsion systems, ignition systems for explosives, or portable detectors for identifying water contaminants, says Michael J. Sailor of the University of California, San Diego, who led the research.

As early as 1992, chemists reported creating small explosions when they put nitric acid onto silicon wafers that had been processed to have myriad tiny pores. Since then, other researchers have found that porous silicon explodes under certain extreme conditions, including ultralow temperatures.

The new research moves the explosive properties of porous silicon from a laboratory curiosity toward applications, says Sailor. His team’s work also shows that liquids or unusual conditions are not required for producing the explosions, says Philippe Fauchet of the University of Rochester in New York.

“It’s an interesting piece of work. It adds to the whole picture of the nature of porous silicon,” comments Allen J. Bard of the University of Texas at Austin, whose lab first reported the phenomenon of exploding silicon a decade ago.

The discovery in Sailor’s lab was accidental. While trying to make magnetic sensors, postdoctoral researcher Frederic Mikulec added gadolinium nitrate to a wafer of porous silicon. When he tried to cut the wafer, it exploded.

Mikulec wasn’t injured, but the event inspired him and his colleagues to investigate further. The chemists dissolved gadolinium nitrate in ethanol and then added the solution to dime-size wafers of porous silicon. When the ethanol evaporated, microscopic particles of gadolinium nitrate remained throughout the networks of pores. The team could induce explosions in a treated wafer with a spark provided by a 9-volt battery–or just by scratching it, says Sailor.

In the first January issue of Advanced Materials, Sailor and his colleagues describe their work with exploding silicon. To determine if porous silicon could help identify elements, the chemists added metals–such as barium, strontium, and lead–to their treated wafers and then triggered explosions. In the flames, the researchers detected the emission spectra long used by chemists for identifying the elements.

Sailor suggests that chips of the new material might serve as sensors for detecting water contaminants, such as heavy metals. Since the material explodes much like gunpowder does, it might also substitute for blasting caps in conventional explosives or even operate as a tiny propulsion system for microscopic vehicles and devices, speculates Sailor. Or, he adds, treated silicon might serve as a self-destruct system for top-secret electronics.

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