Tech Briefs Magazine - August 2022 - 36

Optics
Laser Processing Method for Optoelectronic Devices
This method increases the efficiency of light-emitting diodes and other optical elements.
Naval Research Laboratory, Washington, DC
A
new method was developed to passivate
defects in next-generation optical
materials. The new photocatalytic reaction
enables the integration of high-quality,
optically active, atomically thin material
in a variety of applications such as
electronics, electro-catalysts, memory, and
quantum computing.
The laser processing technique significantly
improves the optical properties
of monolayer molybdenum disulphide
(MoS2
) - a direct gap semiconductor -
with high spatial resolution. The process
produces a 100-fold increase in the material's
optical emission efficiency in the
areas " written " with the laser beam.
Atomically thin layers of transition
metal dichalcogenides (TMDs), such as
MoS2
, are promising components for
flexible devices, solar cells, and optoelectronic
sensors due to their high optical
absorption and direct band gap. The
semiconducting materials are particularly
advantageous in applications where
weight and flexibility are a premium;
however, their optical properties are often
highly variable and non-uniform,
making it critical to improve and control
the optical properties of the TMD
materials to realize reliable, high-efficiency
devices.
Defects are often detrimental to the ability
of monolayer semiconductors to emit
light. These defects act as nonradiative
trap states, producing heat instead of light;
therefore, removing or passivating these
defects is an important step toward high-efficiency
optoelectronic devices.
In a traditional LED, approximately
90 percent of the device is a heat sink to
improve cooling. Reduced defects enable
smaller devices to consume less
power, which results in a longer operational
lifetime
for distributed sensors
and low-power electronics.
Water molecules passivate the MoS2
only
when exposed to laser light with an energy
above the band gap of the TMD. The result
is an increase in photoluminescence with
no spectral shift. Treated regions maintain
a strong light emission compared to the
untreated regions that exhibit much a
weaker emission. This suggests that the laser
light drives a chemical reaction between
the ambient gas molecules and the MoS2.
For more information, contact the NRL
Technology Transfer Office at techtran@
nrl.navy.mil.
Flexible Micro LEDs for Wearables
These micro LEDs can be folded, twisted, cut, and stuck to different surfaces.
University of Texas, Dallas, TX
U
sed in products ranging from brake
lights to billboards, LEDs are ideal
components for backlighting and displays
in electronic devices because they are
lightweight, thin, energy-efficient, and
visible in different types of lighting. Micro
LEDs, which can be as small as 2 micrometers
and bundled to be any size, provide
higher resolution than other LEDs. Their
size makes them a good fit for small devices
such as smartwatches but they can be
bundled to work in flat-screen TVs and
other larger displays. LEDs of all sizes,
however, are brittle and typically can only
be used on flat surfaces.
36
Researchers have developed a method
to create micro LEDs that can be
folded, twisted, cut, and stuck to different
surfaces. The research helps pave
the way for the next generation of flexible,
wearable technology. The detachable
LED can be transferred onto
clothing or even rubber and can survive
even if it is wrinkled. It also can be
cut to use half of the LED.
The flexible LED was created through
a technique called remote epitaxy, which
involves growing a thin layer of LED crystals
on the surface of a sapphire crystal
wafer, or substrate. Typically, the LED
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would remain on the wafer. To make it
detachable, researchers added a nonstick
layer to the substrate, which acts
similarly to the way parchment paper
protects a baking sheet and allows for
the easy removal of cookies, for instance.
The added layer, made of a one-atomthick
sheet of carbon called graphene,
prevents the new layer of LED crystals
from sticking to the wafer.
The graphene does not form chemical
bonds with the LED material, so it
adds a layer that allows the LEDs to be
peeled from the wafer and stuck to any
surface. Laboratory tests of LEDs were
Tech Briefs, August 2022
Time
(Top) Illustration of a water molecule bonding
at a sulfur vacancy in the MoS2
upon laser light
exposure. (Bottom) Photoluminescence (PL) increase
observed during laser light exposure in
ambient. (Inset) Fluorescence image showing
brightened regions spelling out " NRL. " (Image:
U.S. Naval Research Laboratory)
Photoluminescence
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Tech Briefs Magazine - August 2022

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