Material Performs Differently in the Dark

Researchers in Japan have discovered a typically brittle, inorganic semiconductor material that shows remarkable signs of plasticity in the dark.

A team from Nagoya University in Japan has discovered that an inorganic semiconductor with which they are working—crystals of zinc sulfide—performed differently in the dark compared to in the light. While the crystals were brittle when exposed to light, they were flexible and showed remarkable plasticity when kept in the dark at room temperature.

The research shows promise for using this type of inorganic semiconductor with the next generation of flexible electronics. This application demands strong, electrically conductive materials that have more stretch than traditional semiconductors, researchers said in a press release.

Inorganic semiconductors, such as silicon and gallium arsenide, are indispensable in modern electronics because they possess tunable electrical conductivity between that of a metal and that of an insulator.

A semiconductor’s band gap—the energy difference between its valence and conduction bands—controls its electrical conductivity. A narrow band gap results in increased conductivity because it is easier for an electron to move from the valence to the conduction band.

Image of electrons and holes in lattice structure

Inorganic semiconducting crystals generally tend to fail in a brittle manner. This is true for zinc sulfide, shown here with catastrophic fracture after mechanical tests under ordinary light-exposure environments (A). However, researchers at Nagoya University in Japan found that these crystals can be plastically deformed in complete darkness even at room temperature (B). Moreover, the optical band gap of the deformed zinc sulfide crystals decreased after deformation (C). (Image source: Atsutomo Nakamura, Nagoya University)

With inorganic semiconductors, the problem that electrical engineers are running into is that they are brittle. This characteristic is incompatible with the development of the new generation of flexible electronics. Brittleness can lead to device failure and limits to the application range.

Electrons

Scientists found that the reason for the zinc sulfide’s change in performance had to do with electrons in the material, or lack thereof, explained Atsutomo Nakamura, one of the researchers from the Department of Materials Physics at Nagoya University.

“In the dark, photoexcited electrons and holes are not present in materials,” he told Design News. “The electrons and holes in the light are known to affect electric properties, but little was known about the effect on mechanical properties. We showed an intense effect of photoexcited electrons and holes on mechanical properties. As a result, we found the best mechanical performance in the dark,” Nakamura said.

The work from the Nagoya researchers now demonstrates that the inorganic semiconductors are not intrinsically brittle; this characteristic could potentially be controlled through light exposure, Nakamura said.

“This is one example in an inorganic semiconductor, but one giant leap to realize the best materials with high hardness and flexibility,” he explained. “The significance is in the plasticity, which is the ability to be deformed without fracture,” he added.

Researchers published a paper about their work in the journal Science. They plan to continue to work with the material to solve issues with the mechanisms involved in the relationship between the inorganic semiconductor’s mechanical properties and light exposure, Nakamura said. Researchers also aim to explore the same study with other materials to find the best type of inorganic semiconductors to work with in the future.

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.

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