New type of silicon could find use in solar cells and LEDs

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A view through the channels of the new zeolite-type allotrope of silicon (Image: Timothy Strobel)

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You probably wouldn't be reading this if it weren't for silicon. It's the second most-abundant element in the Earth's crust as well as the key to modern technology – used in the integrated circuits that power such electronics as computers, mobile phones, and even some toasters and refrigerators. It's also used in compound form in building, ceramics, breast implants, and many other areas. And now the ubiquitous element may have a plethora of new applications, thanks to a team of Carnegie scientists who synthesized an allotrope (new/different physical form) with the chemical formula Si24.

The diamond-structured form of silicon normally used in technology applications has a semiconducting property called an indirect band gap, which differs from a direct band gap in that it requires an extra step to excite bound electrons into a free state so that they can participate in electrical conduction. Direct band gap semiconductors need only two entities to intersect; a photon imparts momentum on an electron. But indirect band gap semiconductors require a third entity – a lattice vibration called a phonon – because the minimum energy state of the conduction band and the maximum energy state of the valence band occur at different values of momentum.

This new form of silicon is a quasi-direct band gap material, which means not only that it can conduct electricity more efficiently than diamond-structured silicon but also that it can absorb and emit light – a property never before achieved. (I say quasi-direct because it is technically a very small and almost flat indirect band gap.) These properties make it ripe for use in next-generation solar cells, LEDs, and other semiconductor technologies.

To create Si24, the researchers first formed a polycrystalline compound of silicon and sodium (Na4Si24) with help from a tantalum capsule, very high temperature, and a 1,500 ton multi-anvil press that gradually reached a pressure of 10 gigapascals (1,450,377 pounds per square inch). This compound was then "degassed" in a vacuum at 400 Kelvin (260 F) for eight days, after which they had pure Si24 in an open framework called a zeolite-type structure.

The structure is comprised of five-, six-, and eight-membered silicon rings through which small atoms and molecules could spread, with potential applications in electrical energy storage and molecular-scale filtering, among other things.

Si24 could be just the tip of the iceberg for desirable new materials formed at high pressure, the researchers suggest. Lead researcher Timothy Strobel has gone so far as to call high-pressure precursor synthesis "an entirely new frontier in novel energy materials" that goes above and beyond silicon. And the stability of the new structures at atmospheric pressure means that low-pressure methods such as chemical vapor deposition could potentially allow large-scale production.

A paper describing the research was published in the journal Nature Materials.

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