24 Photonics Spectra June 2016 www.photonics.com
TECH pulse • • • • • • • •
Optoelectronics could benefit from monolayer semiconductors
OAK RIDGE, Tenn. — Optoelectronics applications may soon be built using
monolayered 2D heterostructures made
from materials using mismatched lattices.
In a study led by the U.S. Department
of Energy’s Oak Ridge National Laboratory (ORNL), scientists synthesized a
stack of atomically thin monolayers of
two lattice-mismatched semiconductors.
Where the two layers met, they formed an
atomically sharp heterostructure, which
generated a photovoltaic response by
separating electron-hole pairs that were
generated by light.
“Because the two layers had such a
large lattice mismatch between them, we
did not expect them to grow on each other
in an orderly way — but they did,” said
researcher Xufan Li. “It’s a new, poten-
tial building block for energy-efficient
The creation of an atomically thin solar
cell may facilitate synthesis of mismatched
layers to enable new families of functional
2D materials. The research broadens the
number of materials that can be combined,
thus creating potential for a wider range of
atomically thin electronic devices.
“These new 2D mismatched layered het-
Novel liquid crystal mix expands LCD operating temperature range
erostructures open the door to novel build-
ing blocks for optoelectronic applications,”
said researcher Kai Xiao. “They will allow
us to study new physics properties which
cannot be discovered with other 2D hetero-
structures with matched lattices.”
For the study, Li first grew a monolayer
of molybdenum diselenide, and then grew
a layer of gallium selenide on top. This
technique, called van der Waals epitaxy,
is named for the weak attractive forces
that hold dissimilar layers together. “With
van der Waals epitaxy, despite big lattice
mismatches, you can still grow another
layer on the first,” Li said. Using scanning
transmission electron microscopy, the
team characterized the atomic structure of
the materials and revealed the formation
of Moiré patterns.
ORLANDO, Fla. — An experimental
liquid crystal (LC) mixture has been
shown to function properly in the temperature range of − 40 to about 100 °C,
an improvement over current technologies
used in automobile displays, smartphones
and televisions, for example, which grow
blurry and sluggish in extreme temperatures.
A team of researchers led by professor
Shin-Tson Wu at the University of Central
Florida formulated three new LC mixtures with a wide nematic range, small
visco-elastic coefficient and low activation energy.
In addition, the LC pixels are able to
change their brightness level about 20
times faster than required by European automotive standards, the researchers said.
The LC mixes used four major compounds; homologues of compound 1
showed high birefringence and large
dielectric anisotropy; however, their viscoelastic coefficient and activation energy
were relatively large.
To reduce viscosity, the researchers
doped about 50 percent nonpolar diluters
(compound 2). Compounds 3 and 4 (about
30 percent) were added to obtain high
clearing point and wide nematic range.
Phase transition temperatures were
measured by differential scanning calorimetry.
The team said these LCs could greatly
improve the performance of different
display devices in a car, such as a head-up
projection using liquid-crystal-on-silicon
with an average gray-to-gray (GTG) re-
sponse time less than 1 millisecond (ms)
at an elevated temperature. The average
GTG response time was maintained at
about 10 ms for fringing field switching
LCD at 0 °C, and also about 10 ms for
twisted nematic LCD at − 20 °C.
Wu previously contributed to the
development of LCDs that are readable
in sunlight for smartphones and other de-
vices, and is currently working on a smart
brightness-control film that has applica-
tions for automobiles, planes, eyewear,
windows and more.
The study was published in Optical
Materials Express (doi: 10.1364/
Professor Shin-Tson Wu and doctoral students work on liquid crystal mixtures in his lab at the University
of Central Florida’s College of Optics & Photonics. From left, Fenglin Peng, Yuge ‘Esther’ Huang, Wu and
Fangwang ‘Grace’ Gou.