Semiconductor silver sulfide stretches like metal

To build electronic devices that can bend, fold, or stretch, engineers need flexible electronic materials. Engineers can choose from a wide range of ductile metals and insulators that deform under a force without rupturing. But they have far fewer options when it comes to semiconductors. Brittle inorganic semiconductors tend to crack under strain, while ductile organic semiconductors offer relatively poor electronic performance.

Researchers in China and Germany have now found that a form of silver sulfide (α-Ag₂S) not only boasts promising electronic properties, but also is the first known inorganic semiconductor that is ductile at room temperature (Nat. Mater. 2018, DOI: 10.1038/s41563-018-0047-z). The team used density functional theory calculations to understand the atomic origins of the material’s ductility, providing a possible general approach to discover other flexible semiconductors for applications such as biosensors or optoelectronics.

The discovery came by accident when a team member tried to prepare a sample of α-Ag₂S for X-ray powder diffraction, as part of an ongoing project to develop thermoelectric materials that convert heat into electrical energy. Rather than forming a powder when pounded in a mortar, the material simply deformed like a metal. “This was quite unexpected,” says Yuri Grin at the Max Planck Institute for Chemical Physics of Solids, who was part of the team.

The researchers tested how the material responded to compression, bending, and stretching, and found that it could deform much more than typical semiconductors—by about 4% under tension, and more than 50% under compression, values similar to many metals. Bulk α-Ag₂S and thin films a few hundred nanometers thick could bend several times without significantly altering their electronic properties such as bandgap, charge mobility, and electrical resistance.

“What’s really cool is that they measured a high electron mobility,” much higher than flexible organic semiconductors, says Niko Munzenrieder, who works on flexible electronics at the University of Sussex. That property could be crucial in expanding the range of flexible electronic applications.

Flexible displays, for example, do not need semiconductors with a particularly impressive electronic performance. But sensors are more demanding, and flexible devices that transmit data wirelessly at high speeds require semiconductors with even higher electron mobility. “That’s something we can’t do effectively right now,” Munzenrieder says.

Properties like ductility ultimately stem from the pattern of chemical bonding in a material, so the team used density functional theory calculations to explain the behavior of α-Ag₂S. At room temperature, the material’s atoms arrange in stacks of wrinkled layers with sulfur atoms in one layer bonding to silver layers in the next. The calculations showed that as the layers slip past each other during compression or stretching, one set of silver-sulfur bonds gradually weaken as new silver-sulfur bonds form. “This is why the material does not cleave, but simply slips a little bit,” Grin says.

By looking for similar bonding patterns in other materials, Grin hopes that he and his colleagues could discover a range of new ductile semiconductors. He is particularly interested in using the materials in thermoelectric generators. “There is a need for flexible thermoelectric materials, to use the heat of the human body to power various applications,” he says.

Munzenrieder says that the next step is to prove that α-Ag₂S can sustain its electronic performance in a working flexible transistor. “They haven’t demonstrated any kind of electronic device yet, so we’ll have to wait and see,” he says.