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2D Material Shows Ferroelectric Switching

 
Scientists at the University of Washington show that two stacked monolayers of WTe2 produce spontaneous electrical polarisation

Scientists at the University of Washington
researching the
transition metal dichalcogenide WTe2 have shown that its 2D form can undergo ferroelectric
switching.

When two monolayers of WTe2 are stacked
into a bilayer, a spontaneous electrical polarisation appears, one layer
becoming positively charged and the other negatively charged. This polarisation
can be flipped by applying an electric field.
The
results were published in the journal Nature.

"Finding ferroelectric switching in
this 2D material was a complete surprise," said senior author David
Cobden, a UW professor of physics. "We weren't looking for it, but we saw
odd behaviour, and after making a hypothesis about its nature we designed some
experiments that confirmed it nicely."

Materials with ferroelectric properties can
have applications in memory storage, capacitors, RFID card technologies and
even medical sensors.

"Think of ferroelectrics as nature's
switch," said Cobden. "The polarised state of the ferroelectric
material means that you have an uneven distribution of charges within the
material - and when the ferroelectric switching occurs, the charges move collectively,
rather as they would in an artificial electronic switch based on
transistors."

The UW team created WTe2 monolayers from
its the 3D crystalline form, which was grown by co-authors Jiaqiang Yan at Oak
Ridge National Laboratory and Zhiying Zhao at the University of Tennessee,
Knoxville. Then the UW team, working in an oxygen-free isolation box to prevent WTe2 from degrading, used Scotch Tape to exfoliate thin sheets of WTe2from the
crystal - a technique widely used to
isolate graphene and other 2D materials.

WTe2 is the first exfoliated 2D material
known to undergo ferroelectric switching. WTe2 also maintains the ferroelectric
switching at room temperature, and its switching is reliable and doesn't
degrade over time, unlike many conventional 3D ferroelectric materials,
according to Cobden. These characteristics may make WTe2 a promising material
for smaller, more robust technological applications than other ferroelectric
compounds.

"The unique combination of physical
characteristics we saw in WTe2 is a reminder that all sorts of new phenomena
can be observed in 2D materials," said Cobden.

Ferroelectric switching is the second major
discovery Cobden and his team have made about monolayer
WTe2. In a 2017 paper
in Nature Physics, the team reported that this material is also a ‘topological
insulator’ the first 2D material with this exotic property.

In a topological insulator, the electrons'
wave functions - mathematical summaries of their quantum mechanical states
- have
a kind of built-in twist. Thanks to the difficulty of removing this twist,
topological insulators could have applications in quantum computing - a field
that seeks to exploit the quantum-mechanical properties of electrons, atoms or
crystals to generate computing power that is exponentially faster than today's
technology.

The
UW team's discovery also stemmed from theories developed by David J. Thouless,
a UW professor emeritus of physics who shared the 2016 Nobel Prize in Physics
in part for his work on topology in the 2D realm.

Cobden and his colleagues plan to keep
exploring monolayer
WTe2 to see what else they can learn.

"Everything we have measured so far
about
WTe2 has some surprise in it," said Cobden. "It's exciting to
think what we might find next."


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