Revolutionary Solar-friendly Form of Silicon Shines

A team of Carnegie scientists led by Timothy Strobel has synthesized an entirely new form of silicon, one that promises even greater future applications.

Silicon is the second most-abundant element in the earth’s crust. When purified, it takes on a diamond structure, which is essential to modern electronic devices—carbon is to biology as silicon is to technology. A team of Carnegie scientists led by Timothy Strobel has synthesized an entirely new form of silicon, one that promises even greater future applications. Their work is published in Nature Materials.

Although silicon is incredibly common in today’s technology, its so-called indirect band gap semiconducting properties prevent it from being considered for next-generation, high-efficiency applications such as light-emitting diodes, higher-performance transistors and certain photovoltaic devices.

Metallic substances conduct electrical current easily, whereas insulating (non-metallic) materials conduct no current at all. Semiconducting materials exhibit mid-range electrical conductivity. When semiconducting materials are subjected to an input of a specific energy, bound electrons can move to higher-energy, conducting states. The specific energy required to make this jump to the conducting state is defined as the “band gap.” While direct band gap materials can effectively absorb and emit light, indirect band gap materials, like diamond-structured silicon, cannot.

In order for silicon to be more attractive for use in new technology, its indirect band gap needed to be altered. Strobel and his team—Carnegie’s Duck Young Kim, Stevce Stefanoski and Oleksandr Kurakevych (now at Sorbonne) —were able to synthesize a new form of silicon with a quasi-direct band gap that falls within the desired range for solar absorption, something that has never before been achieved.

The silicon they created is a so-called allotrope, which means a different physical form of the same element, in the same way that diamonds and graphite are both forms of carbon. Unlike the conventional diamond structure, this new silicon allotrope consists of an interesting open framework, called a zeolite-type structure, which is comprised of channels with five-, six- and eight-membered silicon rings.

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