While essential for electronics, inorganic semiconductors are brittle — a quality that can limit their use, especially for flexible electronics. Researchers at Nagoya University have found that inorganic semiconductors become brittle when exposed to light but remain flexible when kept in the dark at room temperature. Inorganic semiconducting crystals generally tend to fail in a brittle manner. This is true for zinc sulfide (ZnS); ZnS crystals (A) show catastrophic fracture after mechanical tests under ordinary light-exposure environments (B). However, researchers found that ZnS crystals can be plastically deformed up to a deformation strain of εt = 45 percent when deformed along the [001] direction in complete darkness, even at room temperature (C). Moreover, the optical bandgap of the deformed ZnS crystals decreased by 0.6 eV after deformation. Courtesy of Atsutomo Nakamura. Researchers studied the deformation of zinc sulfide (ZnS) crystals under white light, UV light, and in complete darkness. Microscopy showed that, under the two light conditions, the inorganic semiconducting material immediately cracked when researchers attempted to deform it. In contrast, the ZnS was able to withstand substantial deformation — up to 45 percent — when kept in complete darkness. The team attributed the increased plasticity of the ZnS crystals in the dark to the high mobility of dislocations (a type of defect found in crystals) when in complete darkness. Under light exposure, the ZnS crystals were brittle because their deformation mechanism was different from that in the dark. Along the cores of the defects that occur in ZnS crystals during deformation, light causes electrons and holes to be trapped at extra energy levels, and the resulting motion from this energy and entrapment causes fracturing. In darkness, electrons do not get trapped in such a way, allowing the material to deform and return to its original configuration. The high plasticity of the ZnS crystals in the dark was accompanied by a considerable decrease in the bandgap of the deformed crystals. Thus, researchers believe that the bandgap of ZnS crystals, and in turn their electrical conductivity, could be controlled by mechanical deformation in the dark. The team proposes that the decreased bandgap of the deformed crystals was caused by deformation introducing dislocations into the crystals, which changed their band structure. Plastic deformation of materials is caused by nucleation and multiplication of dislocations under an external force (A and B). It is generally believed that brittle inorganic semiconducting materials have difficulty in formation of dislocations because of their strong chemical bonds. However, researchers found that a great number of dislocations are generated and multiplied in ZnS crystals during deformation in darkness (C), resulting in the extraordinary plasticity. Courtesy of Atsutomo Nakamura. “This study reveals the sensitivity of the mechanical properties of inorganic semiconductors to light,” researcher Katsuyuki Matsunaga said. “Our findings may allow development of technology to engineer crystals through controlled light exposure.” Results suggest that the strength, brittleness, and conductivity of inorganic semiconductors could be regulated by light exposure, opening a possible avenue to optimize the performance of inorganic semiconductors in electronics. The research was published in Science (doi:10.1126/science.aar6035).