Under ordinary light-exposure environments, ZnS crystals (A) show catastrophic fracture after mechanical tests (B). In the dark, however, ZnS crystals can be plastically deformed up to a strain of 45% (C); moreover, the optical band gap of the deformed ZnS crystals decreased by 0.6eV after deformation. Image: Atsutomo Nakamura.
Under ordinary light-exposure environments, ZnS crystals (A) show catastrophic fracture after mechanical tests (B). In the dark, however, ZnS crystals can be plastically deformed up to a strain of 45% (C); moreover, the optical band gap of the deformed ZnS crystals decreased by 0.6eV after deformation. Image: Atsutomo Nakamura.

Inorganic semiconductors such as silicon are indispensable in modern electronics because they possess tunable electrical conductivity between that of a metal and an insulator. The electrical conductivity of a semiconductor is controlled by its band gap, which is the energy difference between its valence and conduction bands. A narrow band gap results in increased conductivity because it is easier for an electron to move from the valence to the conduction band. But inorganic semiconductors are brittle, which can lead to device failure and limits their application range, particularly in flexible electronics.

A group at Nagoya University in Japan has now discovered that inorganic semiconductors can behave differently in the dark compared with the light. They found that crystals of zinc sulfide (ZnS), a representative inorganic semiconductor, were brittle when exposed to light but flexible when kept in the dark at room temperature. The group reports its findings in a paper in Science.

"The influence of complete darkness on the mechanical properties of inorganic semiconductors had not previously been investigated," says study co-author Atsutomo Nakamura. "We found that ZnS crystals in complete darkness displayed much higher plasticity than those under light exposure."

The ZnS crystals in the dark deformed plastically without fracture under a large strain of 45%. The team attributed the increased plasticity of the ZnS crystals to the high mobility of dislocations in complete darkness – dislocations are a type of defect found in crystals and are known to influence crystal properties. When exposed to light, the ZnS crystals became brittle because they adopted a different deformation mechanism.

The high plasticity of the ZnS crystals in the dark was accompanied by a considerable decrease in the band gap of the deformed crystals. Thus, the band gap of ZnS crystals and, in turn, their electrical conductivity may be controlled by mechanical deformation in the dark. The team proposed that the decreased band gap of the deformed crystals was caused by the deformation introducing dislocations into the crystals, which changed their band structure.

"This study reveals the sensitivity of the mechanical properties of inorganic semiconductors to light," says co-author Katsuyuki Matsunaga. "Our findings may allow development of technology to engineer crystals through controlled light exposure." The researchers' results suggest that the strength, brittleness and conductivity of inorganic semiconductors may be regulated by light exposure, opening an interesting avenue to optimize the performance of inorganic semiconductors in electronics.

This story is adapted from material from Nagoya University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.