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Lurking in the dark toughens up semiconductor material

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Researchers based at Nagoya University, Japan, have discovered an intriguing property of a semiconductor; it is brittle in the light, and becomes flexible in the dark. This could pave the way for the development of new flexible electronics.

Inorganic semiconductors are crucial in electronics due to the fact that scientists can “tune” their electrical conductivity between that of an insulator and a metal. A semiconductor’s conductivity is controlled by its band gap (the small jump in energy separating the valence band and conduction band), across which electrons must pass in order to generate a current. A smaller band gap makes it easier for electrons to move between these bands, resulting in higher conductivity, while a larger band gap results in lower conductivity.

While inorganic semiconductors – such as silicon and gallium arsenide – are in some ways excellent candidates for new applications in electronics, they tend to be very brittle. This can lead to them snapping and causing devices to fail, and makes them unfeasible for use in flexible and some wearable electronics.

Now, a team of scientists based at Nagoya University have discovered that an inorganic semiconductor – zinc sulphide (ZnS) crystals – behaves differently in the dark than it does in the light. While the semiconductor is brittle under light, in darkness it becomes flexible.

Under the cover of complete darkness, the material was able to deform without breaking under considerable strain. According to the scientists behind this discovery, this flexibility could arise due to the increased mobility of dislocations – defects which affect the mechanical properties of crystals – under darkness.

“The influence of complete darkness on the mechanical properties of inorganic semiconductors had not previously been investigated,” said Professor Atsutomo Nakamura, a co-author of the Science paper describing the findings. “We found that ZnS crystals in complete darkness displayed much higher plasticity than those under light exposure.”

An inorganic semiconductor that can be as flexible as metals could have applications in emerging fields of electronics, such as flexible electronics. The Nagoya team believe that this property of ZnS and other inorganic semiconductors could be tuned by controlling light exposure.

“This study reveals the sensitivity of the mechanical properties of inorganic semiconductors to light,” said Professor Katsuyuki Matsunaga, co-author of the study. “Our findings may allow development of technology to engineer crystals through controlled light exposure.”

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