Putting the heat on semimetals

In the early twentieth century, the very notion of a semiconductor—a material that behaves somewhere between an electrical conductor and insulator—was something even the leading physicists of the time thought improbable and unappealing. Yet we now find that semiconductors have become the bedrock of modern electronics. Semimetals are similarly unclassifiable as either metal or non-metal, and like the early days of semiconductors, their utility in electronics has so far been limited. Shintaro Ishiwata and colleagues from the RIKEN Center for Emergent Matter Science (CEMS), in collaboration with researchers from the University of Tokyo, have now found a layered semimetal compound that provides unprecedented efficiency of heat to electric current conversion at room temperature¹, greatly expanding the possible applications of semimetals as thermoelectric materials.

Ishiwata, previously from the CEMS and now with the University of Tokyo, had led a collaborative team in research on novel ‘spintronic’ devices—systems in which the magnetic properties of electrons are used rather than their electrical charge. Through those studies, the team discovered that a byproduct produced during their research, silver selenide (Ag₂Se), displayed some interesting properties. The electrical resistance of the material was found to change remarkably depending on its interaction with magnetic fields. This effect, known as giant magnetoresistance (GMR), is used widely in modern electronic devices to read information from magnetic hard disk drives. Furthermore, the material also displayed high thermoelectric efficiency, generating a heat gradient when it passes an electrical current or generating an electrical current when cooled on one side and heated on the other. The combination of GMR and high thermoelectric efficiency is unusual, and the occurrence of GMR, in particular, shows that electrons can flow very easily through the material.

These behaviors had already been reported before for Ag₂Se, but Ishiwata and his colleagues were spurred to look deeper. “We started studying related materials and found that one of them, CuAgSe, shows stronger GMR effects and better thermoelectric efficiency,” he says. CuAgSe is a multilayered semimetal consisting of alternating layers of silver (Ag) and copper selenide (CuSe) (Fig. 1).

Although Ishiwata notes that his research on CuAgSe began by “accident,” it turned out the material presented some exciting possibilities. The team’s work showed that the electrons in CuAgSe are particularly mobile, increasing its usefulness for device applications. More interesting, however, was the discovery that the electron mobility was increased when the material had been ‘doped’ with nickel atoms. Atomic impurities interrupt the regular atomic lattice of the host material and can be used to manipulate a material’s bulk electronic properties. Impurity doping, for example, is a conventional method used to control the conductive properties of semiconductors, and also the thermal and electrical conductive behavior of thermoelectric materials. However, the introduction of impurities generally has the unwelcome side effect of reducing the material’s electrical conductivity, thereby impairing its overall performance. Contrary to this typical behavior, Ishiwata’s team found that this is not the case in CuAgSe. “After the addition of nickel to CuAgSe, the electron mobility increased, even if we created a chemically ‘dirty’ system,” says Ishiwata.

By overcoming the traditional trade-off between ‘polluting’ the material in order to improve the thermoelectric performance and reduce electron mobility, the team has created a promising material that opens up the possibility of a range of interesting and new applications. One use of the technology the researchers have in mind involves applying the material as a Peltier cooling element (Fig. 2), in which electrical current is used to transfer heat from one side of the device to the other.

“So far, thermoelectric materials that work at low temperatures are limited to bismuth-based materials. Materials based on silver and copper are now promising candidates that offer better thermoelectric performance near or below room temperature,” says Ishiwata. The increase in electron mobility achieved by doping also enhances the GMR performance, although GMR in this material only appears at very low temperatures, making it less promising for practical applications.