Tech Briefs Magazine - August 2021 - 20

Electrical/Electronics
D. To determine the most efficient route,
the race logic circuit evaluates each possible
segment of the trip such as A-B and AD.
If A-B takes more time to travel than AD,
whether it's because the path is longer
or has more traffic, A-B will be assigned a
longer delay time. In the team's design,
the longer time delay is implemented by
adding additional resistance to the slower
segment.
Race logic does involve a race but in
this contest, all the truck drivers initially
drive in different directions. To determine
which route to the final destination
is fastest, they race over all possible
routes through the different intermediate
delivery points. In the new circuit,
the researchers inserted a group of timeencoded
signals at the starting point,
each acting as a different driver that
speeds through the team's simulated
hardware circuit.
Whenever a driver arrives at an intermediate
destination point in the race,
the model system sends out new drivers
(new time signals) who fan out in different
directions to the remaining destinations.
If a driver arrives at a destination
that another driver has already been to,
that driver drops out because the path is
no longer competitive. The winner of
the race - the first driver to arrive at the
Rolled 2D Heterostructures
The technology potentially enables a new generation of miniaturized electronic and
optoelectronic devices.
The Pennsylvania State University, University Park
T
he recent synthesis of one-dimensional
van der Waals heterostructures
- a type of heterostructure made by layering
two-dimensional materials that are
one atom thick - may lead to new,
miniaturized electronics that are currently
not possible.
Engineers commonly produce heterostructures
to achieve new device properties
that are not available in a single
material. A van der Waals hetero structure
is one made of 2D materials stacked
directly on top of each other like a sandwich.
The van der Waals force, which is
an attractive force between uncharged
molecules or atoms, holds the materials
together. The one-dimensional van der
Waals heterostructure produced by the
researchers is different from the van der
Waals heterostructures engineers have
produced thus far.
It looks like a stack of 2D-layered ma -
terials that are rolled up in a perfect
cylinder. In this way, the 2D materials
still contact each other in a desired vertical
heterostructure sequence. The re -
search suggests that all 2D materials
could be rolled into these one-dimenA
heterotube diode: This device contains a MoS2 semiconductor shell (blue), the insulator hBN shell
(purple), the carbon nanotube core (green) of the heteronanotube, covered with gold electrodes
(yellow). (Image: Elizabeth Flores-Gomez Murray/Penn State)
20
Cov
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sional heterostructure cylinders, known
as heteronanotubes, which can work as
extremely small diodes with high performance
despite their size.
In regular, flat van der Waals hetero -
structures, confirming existence or ab -
sence of some layers can be done easily
because they are flat and have a large
area. This means a researcher can use various
type microscopies to collect a lot of
signal from the large, flat areas, so they
are easily visible. When researchers roll
them up, like in the case of a one-dimensional
van der Waals heterostructure, it
becomes a very thin wire-like cylinder that
is hard to characterize because it gives off
little signal and be comes practically invisible.
In addition, in order to prove the
existence of insulating layer in the semiconductor-insulator-semiconductor
junction
of the di ode, one needs to resolve not
just the outer shell of the heteronano tube
but the middle one, which is completely
shadowed by the outer shells of a molybdenum
sulfide semiconductor.
To solve this, the team used a scattering
scanning near-field optical microscope
that can " see " the objects of nano scale size
and determine their material's optical
properties. A special method of analysis of
the data, known as hyperspectral optical
imaging with nanometer resolution, can
distinguish different materials and test the
structure of the one-di mensional diode
along its entire length.
For more information, contact Ben Man ning
at bam58@psu.edu; 814-863-4713.
Tech Briefs, August 2021
end of the circuit - indicates the solution
to the particular puzzle that the
hardware was programmed to solve.
The design, which has not yet been
incorporated into a working device, can
handle a much broader class of networks,
enabling race logic to tackle a
wider variety of computational puzzles.
These puzzles include finding the best
alignment between two proteins or two
strings of nucleotides - the molecules
that form the building blocks of DNA -
and determining the shortest path be -
tween two destinations in a network.
For more information, contact Mark D.
Stiles at mark.stiles@nist.gov; 301-975-3745.
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Tech Briefs Magazine - August 2021

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