Atom-thin walls could smash size, memory barriers in next-gen devices
Nanomaterial feature could help electronic circuits adopt benefits of
human memory

Date:
February 13, 2023
Source:
University of Nebraska-Lincoln
Summary:
For all of the still-indistinguishable-from-magic wizardry packed
into the three pounds of the adult human brain, it obeys the same
rule as the other living tissue it controls: Oxygen is a must. So it
was with a touch of irony that a scientists offered his explanation
for a technological wonder -- movable, data-covered walls mere
atoms wide -- that may eventually help computers behave more like
a brain. 'There was unambiguous evidence that oxygen vacancies
are responsible for this,' Tsymbal said.


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FULL STORY ==========================================================================
For all of the unparalleled, parallel-processing,
still-indistinguishable-from- magic wizardry packed into the three
pounds of the adult human brain, it obeys the same rule as the other
living tissue it controls: Oxygen is a must.


==========================================================================
So it was with a touch of irony that Evgeny Tsymbal offered his
explanation for a technological wonder -- movable, data-covered walls mere atoms wide -- that may eventually help computers behave more like a brain.

"There was unambiguous evidence that oxygen vacancies are responsible
for this," said Tsymbal, George Holmes University Professor of physics
and astronomy at the University of Nebraska-Lincoln.

In partnership with colleagues in China and Singapore, Tsymbal and a few
Husker alumni have demonstrated how to construct, control and explain
the oxygen- deprived walls of a nanoscopically thin material suited to
next-gen electronics.

Unlike most digital data-writing and -reading techniques, which speak
only the binary of 1s and 0s, these walls can talk in several electronic dialects that could allow the devices housing them to store even more
data. Like synapses in the brain, the passage of electrical spikes sent
via the walls can depend on which signals have passed through before,
lending them an adaptability and energy-efficiency more akin to human
memory. And much as brains maintain memories even when their users sleep,
the walls can retain their data states even if their devices turn off
-- a precursor to electronics that power back on with the speed and
simplicity of a light.

The team investigated the barrier-smashing walls in a nanomaterial,
named bismuth ferrite, that can be sliced thousands of times thinner
than a human hair. Bismuth ferrite also boasts a rare quality known
as ferroelectricity: The polarization, or separation, of its positive
and negative electric charges can be flipped by applying just a pinch
of voltage, writing a 1 or 0 in the process. Contrary to conventional
DRAM, a dynamic random-access memory that needs to be refreshed every
few milliseconds, that 1 or 0 remains even when the voltage is removed, granting it the equivalent of long-term memory that DRAM lacks.

Usually, that polarization is read as a 1 or 0, and flipped to rewrite
it as a 0 or 1, in a region of material called a domain. Two oppositely polarized domains meet to form a wall, which occupies just a fraction of
the space dedicated to the domains themselves. The few-atom thickness
of those walls, and the unusual properties that sometimes emerge in or
around them, have cast them as prime suspects in the search for new ways
to squeeze ever-more functionality and storage into shrinking devices.

Still, walls that run parallel to the surface of a ferroelectric material
- - and net an electric charge usable in data processing and storage
-- have proven difficult to find, let alone regulate or create. But
about four years ago, Tsymbal began talking with Jingsheng Chen from
the National University of Singapore and He Tian from China's Zhejiang University. At the time, Tian and some colleagues were pioneering a
technique that allowed them to apply voltage on an atomic scale, even
as they recorded atom-by-atom displacements and dynamics in real time.

Ultimately, the team found that applying just 1.5 volts to a bismuth
ferrite film yielded a domain wall parallel to the material's surface --
one with a specific resistance to electricity whose value could be read
as a data state.

When voltage was withdrawn, the wall, and its data state, remained.

When the team cranked up the voltage, the domain wall began migrating down
the material, a behavior seen in other ferroelectrics. Whereas the walls
in those other materials had then propagated perpendicular to the surface, though, this one remained parallel. And unlike any of its predecessors,
the wall adopted a glacial pace, migrating just one atomic layer at a
time. Its position, in turn, corresponded with changes in its electrical resistance, which dropped in three distinct steps -- three more readable
data states -- that emerged between the application of 8 and 10 volts.

The researchers had nailed down a few W's -- the what, the where, the
when - - critical to eventually employing the phenomenon in electronic
devices. But they were still missing one. Tsymbal, as it happened,
was among the few people qualified to address it.

"There was a puzzle," Tsymbal said. "Why does it happen? And this is where theory helped." Most domain walls are electrically neutral, possessing
neither a positive nor a negative charge. That's with good reason:
A neutral wall requires little energy to maintain its electric state, effectively making it the default. The domain wall the team identified
in the ultra-thin bismuth ferrite, by contrast, possessed a substantial
charge. And that, Tsymbal knew, should have kept it from stabilizing
and persisting. Yet somehow, it was managing to do just that, seeming
to flout the rules of condensed-matter physics.

There had to be an explanation. In his prior research, Tsymbal and
colleagues had found that the departure of negatively charged oxygen
atoms, and the positively charged vacancies they left in their wake,
could impede a technologically useful outcome. This time, Tsymbal's theory-backed calculations suggested the opposite -- that the positively charged vacancies were compensating for other negative charges
accumulating at the wall, essentially fortifying it in the process.

Experimental measurements from the team would later show that the
distribution of charges in the material lined up almost exactly with the location of the domain wall, exactly as the calculations had predicted. If oxygen vacancies turn up in other ferroelectric playgrounds, Tsymbal
said, they could prove vital to better understanding and engineering
devices that incorporate the prized class of materials.

"From my perspective, that was the most exciting," said Tsymbal, who
undertook the research with support from the university's quantum-focused EQUATE project.

"This links ferroelectricity with electrochemistry. We have some kind of electrochemical processes -- namely, the motion of oxygen vacancies --
which basically control the motion of these domain walls.

"I think that this mechanism is very important, because what most people
are doing -- including us, theoretically -- is looking at pristine
materials, where polarization switches up and down, and studying what
happens with the resistance. All the experimental interpretations of this behavior were based on this simple picture of polarization. But here,
it's not only the polarization.

It involves some chemical processes inside of it." The team detailed
its findings in the journal Nature. Tsymbal, Tian and Chen authored the
study with Ze Zhang, Zhongran Liu, Han Wang, Hongyang Yu, Yuxuan Wang,
Siyuan Hong, Meng Zhang, Zhaohui Ren and Yanwu Xie, as well as Husker
alumni Ming Li, Lingling Tao and Tula Paudel.

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========================================================================== Story Source: Materials provided by
University_of_Nebraska-Lincoln. Original written by Scott Schrage. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Zhongran Liu, Han Wang, Ming Li, Lingling Tao, Tula R. Paudel,
Hongyang
Yu, Yuxuan Wang, Siyuan Hong, Meng Zhang, Zhaohui Ren, Yanwu Xie,
Evgeny Y. Tsymbal, Jingsheng Chen, Ze Zhang, He Tian. In-plane
charged domain walls with memristive behaviour in a ferroelectric
film. Nature, 2023; 613 (7945): 656 DOI: 10.1038/s41586-022-05503-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/02/230213201035.htm

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