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).
Courtesy of the
researchers
(January 21, 2016) Small
voltage can flip thin film between two crystal states — one metallic, one
semiconducting.
Two MIT researchers 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. The discovery
could pave the way for a new kind of “nonvolatile” computer memory chip that
retains information when the power is switched off, and for energy conversion and
catalytic applications.
The findings, reported in the journal Nano Letters in a
paper by MIT materials science graduate student Qiyang Lu and associate
professor Bilge Yildiz, involve a thin-film material called a strontium
cobaltite, or 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 these configurations of the molecular structure is
called perovskite, and 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.