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Room-temperature inorganic Ag2S semiconductor achieves conductivity and ductility

By Rachel Nuwer June 1, 2018
Room-temperature inorganic
Unusual mechanical behavior of Ag2S: (a) machined cylinder for compression testing and (b) deformation under hammering. Credit: Nature Materials

Typically, semiconductors are brittle and metals are ductile. Their conductivities are also separated by six orders of magnitude. Thus far, there are no known room-temperature ductile inorganic semiconductors. But now there is an exception: silver sulfide. It straddles the divide between typical properties of semiconductors and metals, showing both high conductivity and high ductility.

This finding came as a surprise to Yuri Grin, a chemist and director of the Max Planck Institute for Chemical Physics of Solids in Dresden, Germany. “In principle, the crystal structure of this material should be completely ordered and it should also be brittle—it should cleave,” he says. “But in fact, you can hammer it or even bend it at a right angle and still it does not break.”

Seizing upon this “extremely impressive and unusual” behavior, Grin and his colleagues, Lidong Chen and Xun Shi from the Shanghai Institute of Ceramics in China, used Ag2S as the basis for creating a new inorganic semiconductor. At room temperature, the material exhibits metal-like ductility—a discovery that the researchers describe as “unprecedented” in an article they published in a recent issue of Nature Materials

Silver sulfide came as a natural research subject for Grin, who has been collaborating with colleagues in Dresden and Shanghai for years on thermoelectrics research. Thermoelectric materials are typically also semiconductors that reveal high values of the Seebeck coefficient and optimal charge carrier concentration. Most recently, the researchers worked with copper chalcogenides, a thermoelectric material. From a chemical point of view, investigations into silver sulfate were simply a next step. Silver is a chemical analogue of copper, and they belong to the same group in the periodic table.

The researchers reacted silver and sulfur in a closed dish and then annealed them together. When they examined the material’s chemical bonding using quantum chemistry techniques, they noticed a layer of free electron pairs that pillowed above the surface of the sulfate. Normally, the van der Waals interactions between such pairs are weak, meaning the electrons can be shifted back and forth to create brittle, weak areas in the material that are prone to cleaving. In this case, however, brittleness is prevented. The lone pairs of electrons on the sulfur atoms form additional bonds with irregularly distributed silver atoms, giving the material unexpected ductility, comparable with metallic materials.

“What is very new is the explanation of this behavior, which starts with some chemical bonding and ends in the mechanical property,” Grin says. “This opens up very interesting opportunities for real-world applications.”

While other thermoelectric materials such as bismuth telluride are used as inorganic room-temperature semiconductors, to Grin and his colleagues’ knowledge, none exhibit the same degree of ductility as silver sulfate.

“The work is useful and well done,” says Susan Kauzlarich, a distinguished professor of chemistry at the University of California, Davis, who was not involved in the research. “It advances the broad field and shows the important contribution of materials chemistry to the topic.” 

For real world applications, the new semiconductor could be used to make miniaturized devices or ones needed for special conditions that require extreme flexibility such as flexible electronic devices. This may include, for example, a thermoelectric jacket that harnesses current produced by the wearer’s body and can be used to charge phones and other electronics.

The semiconductor material’s biggest drawback, Grin says, is its cost. While sulfur is abundant, silver is rarer and relatively expensive. According to Wolfgang Bensch, a chemist at the University of Kiel who was not involved in the research, another challenge will be finding out how to boost the material’s thermoelectric efficiency by optimizing its charge carrier. This could likely be done by substituting part of the silver or sulfur with another material, although the effects of that substitution on the material’s mechanical behavior would have to be studied. “This will be necessary in order to judge the material’s future,” Bensch says.

Grin agrees, and adds that he and his colleagues are planning to investigate this. “The question is, what influence will this chemical substation have on ductility?” he says. “This has to be investigated.”

Read the abstract in Nature Materials.