Layered Nickelates and Rare Earth Elements Could Beat Silicon in Energy Efficiency

September 08, 2020 by Luke James

A joint research team based at the University of Geneva has successfully controlled the properties of artificial electronic structures, dramatically improving energy efficiency.

It’s no secret that silicon-based electronics consume a large amount of energy. And as devices get more powerful and smaller, they burn through more power.

This is problematic not only because using more energy translates to higher operating costs, but it also leads to higher CO2 emissions. Naturally, it’s something that has scientists and design engineers scrambling for a solution, and one area that is currently being touted as a potential solution is alternative materials to silicon. 

Now, researchers from the University of Geneva (UNIGE), 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, claim to have made a new discovery in a material made up of thin layers of nickelates.


A "Super Sandwich" of Rare Earth Elements

Nickelates, which can switch from an insulating to conductive state when temperature surpasses a certain threshold, are formed from a nickel oxide and an atom belonging to one of the rare earth elements ("a set of 17 elements from the Periodic Table"), such as neodymium.

The temperature at which nickelate's state changes depends entirely on the rare earth element in question. This, in turn, dictates which applications nickelates can be used in. When the rare earth element is samarium, for example, the state change takes place at 130°C. When it’s neodymium, it takes place at -73°C. 

The published research paper attributes this to the addition of rare earth elements, which somewhat disrupt and deform the material’s crystal structure. 


An image of a superlattice taken with a scanning transmission electron microscope

An image of a superlattice taken with a scanning transmission electron microscope. It consists of an alternating sequence of neodymium nickelate (blue) and samarium nickelate (yellow). Image used courtesy of Bernard Mundet/EPFL


The researchers analyzed several layers of samarium nickelate atop of layers of neodymium nickelate with the atoms perfectly arranged to form what the team termed a "super sandwich." In this arrangement of thinly-stacked layers (each no bigger than eight atoms wide), the whole sample began functioning as a single material. 


Controlling the Properties of Artificial Electronic Structures

This newly-observed physical phenomenon in the material could be leveraged to control the electronic properties of nickelates—such as the transition from an insulating to a conductive state, and vice-versa—and lead to electronic devices and systems that are far more energy-efficient than those available today. 

According to the research, nickelates’ layers behave independently when they’re sufficiently thick with each one maintaining its own transition temperature. But when they’re thinner, they behave like a single material with one large jump in conductivity taking place at an intermediate transition temperature. 


Schematic rendering of the superlattice heterostructures

Schematic rendering of the superlattice heterostructures, including layers of samarium nickelate atop of layers of neodymium nickelate. Image used courtesy of Nature

The researchers carried out an analysis of a sample under an electron microscope and found that the propagation of the crystal structure’s deformations at the materials’ interfaces only takes place in two or three atomic layers. It therefore cannot be the crystal structure’s distortion that causes the physical phenomenon.

The scientists also found that the thin layers can behave in a less energy-intensive way and become either a totally metallic or insulating material with a common transition temperature. This is achieved without changing the material’s crystal structure, something which the study calls “unprecedented” in research. 


More Energy Efficient Piezoelectric Transistors?

In theory, the UNIGE joint research study could provide a new way to control the properties of artificial electronic structures. In this study, it’s the jump in conductivity in nickelates that can be controlled.

Given that nickelates hold promise for valuable applications such as piezoelectric transistors, the study could represent a huge step forward for future device development if they can be used as a replacement for silicon.