ARTIFICIAL MATERIALS New artificial materials discovery could lead to more efficient devices

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A research team at the University of Geneva - in collaboration with the Swiss Federal Institute of Technology in Lausanne (EPFL), the University of Zurich, the University of Liège, and the Flatiron Institute of New York - has discovered a new phenomenon in an artificial material that could enable the control of its electronic properties.

Today’s silicon-based electronics, which include everything from the phones in our hands to the advanced power systems in electric vehicles, consume a large and constantly increasing amount of energy. This has researchers and design engineers understandably nervous; it’s having a significant impact on the environment and lots of alternative solutions are being explored as a result.

One area that is currently being heavily researched is that of materials that are more complex than silicon, but because of their properties are more promising for tomorrow’s electronic devices and consume less energy.

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Now, in keeping with this area of researchers, a joint team led by the University of Geneva (UNIGE) claims to have made a new discovery concerning an artificial material made up of very thin layers of nickelates. The research team observed a physical phenomenon in the artificial material that could be exploited to accurately control the material’s electronic properties, such as the transition from a conductive to an insulating state. Among other things, it could also be used to develop devices that are more energy efficient.

Nickelates

"Nickelates are known for a special characteristic: they suddenly switch from an insulating state to that of an electrical conductor when their temperature rises above a certain threshold," says UNIGE professor Jean-Marc Triscone. He goes on to add that this transition temperature varies according to the material’s composition.

Nickelates are formed from a nickel oxide with the addition of an atom belonging to the rare earth elements. When this rare earth element is samarium (Sm), for example, the metal-insulator jump takes place at around 130°C. However, when it’s neodymium (Nd), this threshold changes to -73°C. This difference may be explained by the material’s crystal compound deforming with the addition of rare earth elements, which controls the transition temperature.

Behaving as a single material

According to the research, the layers behave independently when they’re thicker, with each one keeping its own transition temperature. When they’re thinner (no larger than eight atoms) “the entire sample began behaving like a single material, with only one large jump in conductivity at an intermediate transition temperature,” says Claribel Dominguez, the study’s first author.

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A detailed analysis was carried out under an electron microscope at EPFL. This showed that the propagation of the crystal structure’s deformations at the interfaces between the materials only happens in two or three atomic layers. Therefore, it’s not this distortion that can explain the observed physical phenomenon.

The study shows that it costs a lot of energy to maintain an interface between a conductive and an insulation region. However, when the layers are thin enough, they can behave in a much less energy-intensive way, either becoming a totally metallic or totally insulating single material with a common transition temperature. This can be achieved without altering the material’s crystal structure, which is currently unprecedented in research.

In theory, this research provides a new way of controlling the properties of artificial electronic structures which, in this instance, is the jump in conductivity in composite nickelate. This represents a huge step forward for developing electronic devices, as nickelates could be used in several power electronics applications such as piezoelectric transistors.

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