New perovskite LED emits a circularly polarized glow

Date:
March 12, 2021
Source:
University of Utah
Summary:
LEDs led to the high-definition viewing experience we've come to
expect from our screens. A new type of LED that utilizes spintronics
could take displays to the next level.



FULL STORY ========================================================================== Light-emitting diodes (LEDs) have revolutionized the displays
industry. LEDs use electric current to produce visible light without
the excess heat found in traditional light bulbs, a glow called electroluminescence. This breakthrough led to the eye-popping,
high-definition viewing experience we've come to expect from our
screens. Now, a group of physicists and chemists have developed a new
type of LED that utilizes spintronics without needing a magnetic field, magnetic materials or cryogenic temperatures; a "quantum leap" that
could take displays to the next level.


==========================================================================
"The companies that make LEDs or TV and computer displays don't want
to deal with magnetic fields and magnetic materials. It's heavy and
expensive to do it," said Valy Vardeny, distinguished professor of physics
and astronomy at the University of Utah. "Here, chiral molecules are self-assembled into standing arrays, like soldiers, that actively spin
polarize the injected electrons, which subsequently lead to circularly polarized light emission. With no magnetic field, expensive ferromagnets
and with no need for extremely low temperatures. Those are no-nos for
the industry." Most opto-electronic devices, such as LEDs, only control
charge and light and not the spin of the electrons. The electrons possess
tiny magnetic fields that, like the Earth, have magnetic poles on opposite sides. Its spin may be viewed as the orientation of the poles and can
be assigned binary information -- an "up" spin is a "1," a "down" is a
"0." In contrast, conventional electronics only transmit information
through bursts of electrons along a conductive wire to convey messages in
"1s" and "0s." Spintronic devices, however, could utilize both methods, promising to process exponentially more information than traditional electronics.

One barrier to commercial spintronics is setting the electron
spin. Presently, one needs to produce a magnetic field to orient the
electron spin direction.

Researchers from the University of Utah and the National Renewable
Energy Laboratory (NREL) developed technology that acts as an active
spin filter made of two layers of material called chiral two-dimension metal-halide perovskites.

The first layer blocks electrons having spin in the wrong direction,
a layer that the authors call a chiral-induced spin filter. Then
when the remaining electrons pass through the second light-emitting
perovskite layer, they cause the layer to produce photons that move in
unison along a spiral path, rather than a conventional wave pattern,
to produce circular polarized electroluminescence.

The study was published in the journal Science on March 12, 2021.

Left-handed, right-handed molecules The scientists exploited a property
called chirality that describes a particular type of geometry. Human
hands are a classic example; the right and left hands are arranged
as mirrors of one another, but they will never perfectly align,
no matter the orientation. Some compounds, such as DNA, sugar and
chiral metal-halide perovskites, have their atoms arranged in a chiral symmetry. A "left-handed" oriented chiral system may allow transport
of electrons with "up" spins but block electrons with "down" spins,
and vice versa.



==========================================================================
"If you try to transport electrons through these compounds, then the
electron spin becomes aligned with the chirality of the material,"
Vardeny said. Other spin filters do exist, but they either require
some kind of magnetic field, or they can only manipulate electrons in
a small area. "The beauty of the perovskite material that we used is
that it's two-dimensional -- you can prepare many planes of 1 cm2 area
that contain one million of a billion (1015) standing molecules with the
same chirality." Metal-halide perovskite semiconductors are mostly used
for solar cells these days, as they are highly efficient at converting
sunlight to electricity. Since a solar cell is one of the most demanding applications of any semiconductor, scientists are discovering other uses
exist as well, including spin-LEDs.

"We are exploring the fundamental properties of metal-halide
perovskites, which has allowed us to discover new applications beyond photovoltaics," said Joseph Luther, a co-author of the new paper
and NREL scientist. "Because metal-halide perovskites, and other
related metal halide organic hybrids, are some of the most fascinating semiconductors, they exhibit a host of novel phenomena that can be
utilized in transforming energy." Although metal-halide perovskites are
the first to prove the chiral-hybrid devices are feasible, they are not
the only candidates for spin-LEDs. The general formula for the active
spin filter is one layer of an organic, chiral material, another layer
of an inorganic metal halide, such as lead iodine, another organic layer, inorganic layer and so on.

"That's beautiful. I'd love that someone will come out with another 2-
D organic/inorganic layer material that may do a similar thing. At this
stage, it's very general. I'm sure that with time, someone will find
a different two- dimensional chiral material that will be even more
efficient," Vardeny said.

The concept proves that using these two dimensional chiral-hybrid systems
gain control over spin without magnets and has "broad implications for applications such as quantum-based optical computing, bioencoding and tomography," according to Matthew Beard, a senior research fellow and
director of Center for Hybrid Organic Inorganic Semiconductors for Energy.

Vardeny and Xin Pan from the Department of Physics & Astronomy at the University of Utah co-authored the study. The other co-authors from
NREL are Beard, Young-Hoon Kim, Yaxin Zhai, Haipeng Lu, Chuanxiao Xiao,
E. Ashley Gaulding, Steven Harvey and Joseph Berry. All are part of
CHOISE collaboration, an Energy Frontier Research Center (EFRC) funded
by the Office of Science within DOE.

Funding for the research came from CHOISE.

========================================================================== Story Source: Materials provided by University_of_Utah. Original written
by Lisa Potter.

Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Young-Hoon Kim, Yaxin Zhai, Haipeng Lu, Xin Pan, Chuanxiao Xiao, E.

Ashley Gaulding, Steven P. Harvey, Joseph J. Berry, Zeev Valy
Vardeny, Joseph M. Luther, Matthew C. Beard. Chiral-induced
spin selectivity enables a room-temperature spin light-emitting
diode. Science, 2021; 371 (6534): 1129 DOI: 10.1126/science.abf5291 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/03/210312121330.htm

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