This diagram shows how an electrical voltage can be used to modify the oxygen concentration, and therefore the phase and structure, of strontium cobaltites. Pumping oxygen in and out transforms the material from the brownmillerite form (left) to the perovskite form (right).
This diagram shows how an electrical voltage can be used to modify the oxygen concentration, and therefore the phase and structure, of strontium cobaltites. Pumping oxygen in and out transforms the material from the brownmillerite form (left) to the perovskite form (right).

Two researchers from Massachusetts Institute of Technology (MIT) have developed a thin-film material whose phase and electrical properties can be switched between metallic and semiconducting simply by applying a small voltage. The material then stays in its new configuration until switched back by another voltage. This discovery could pave the way for a new kind of ‘nonvolatile’ computer memory chip that retains information when the power is switched off, while the material could also have various energy conversion and catalytic applications.

The findings, reported in a paper by MIT materials science graduate student Qiyang Lu and associate professor Bilge Yildiz in Nano Letters, involve a thin-film material called strontium cobaltite (SrCoOx).

Usually, Yildiz says, the structural phase of a material is controlled by its composition, temperature and pressure. "Here for the first time," she says, "we demonstrate that electrical bias can induce a phase transition in the material. And in fact we achieved this by changing the oxygen content in SrCoOx."

"It has two different structures that depend on how many oxygen atoms per unit cell it contains, and these two structures have quite different properties," Lu explains. One of the configurations of the molecular structure is called perovskite, while the other is called brownmillerite. When more oxygen is present, it forms the tightly-enclosed, cage-like crystal structure of perovskite, whereas a lower concentration of oxygen produces the more open structure of brownmillerite.

The two forms have very different chemical, electrical, magnetic and physical properties, and Lu and Yildiz found that the material can be flipped between the two forms with the application of a very tiny amount of voltage – just 30 millivolts. And, once changed, the new configuration remains stable until it is flipped back by a second application of voltage.

Strontium cobaltites are just one example of a class of materials known as transition metal oxides, which are considered promising for a variety of applications. Examples include as electrodes in fuel cells, membranes that allow oxygen to pass through for gas separation, and electronic devices such as memristors, a form of nonvolatile, ultrafast and energy-efficient memory device. The ability to trigger such a phase change through the use of just a tiny voltage could open up many uses for these materials, the researchers say.

The basic principle of switching strontium cobaltite between the two phases by changing the oxygen concentration in the surrounding gas atmosphere was developed within the past year by scientists at Oak Ridge National Laboratory, but that is inherently a much slower and more difficult process to control. "While interesting, this is not a practical means for controlling device properties in use," says Yildiz. "So our idea was, don't change the atmosphere, just apply a voltage," says Lu.

"Voltage modifies the effective oxygen pressure that the material faces," Yildiz adds. To take advantage of that effect, the researchers deposited a very thin film of the material (in the brownmillerite phase) onto a substrate made from yttrium-stabilized zirconia.

In this setup, applying a voltage drives oxygen atoms into the material; applying the opposite voltage has the reverse effect. To observe and demonstrate that the material did indeed go through this phase transition when the voltage was applied, the team used a technique called in-situ X-ray diffraction at MIT's Center for Materials Science and Engineering.

In addition to memory devices, the material could ultimately find applications in fuel cells and electrodes for lithium ion batteries, Lu says. "Our work has fundamental contributions by introducing electrical bias as a way to control the phase of an active material, and by laying the basic scientific groundwork for such novel energy and information processing devices," Yildiz adds.

In ongoing research, the team is working to enhance their understanding of the electronic properties of the material in its different structures, and to extend this approach to other oxides of interest for memory and energy applications.

This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.