“The ability to observe the resonant behavior at
room temperature with synthesized 2D materials rather than exfoliated, stacked
flakes is exciting as it points toward the possibilities for scalable device
fabrication methods that are more compatible with industrial interests. The
challenge we now must address includes improving the grown 2D materials further
and obtaining better performance for future device applications”
Dr. Robert
Wallace of the University of Texas at Dallas and Co-Author on the Research
paper that developing a 2D Resonant Tunneling
Diode demonstrating NDR (Negative Differential Resistance) a part of Quantum
Transport Effect Theory
Despite the advances in Materials Sciences, most
Telecommunications electronic circuits are 3D in nature. So image a
breakthrough that would one day make it possible to have 2D Electronic
components as thin as a sheet of paper.
That's what Researchers at Penn State University,
University of Texas at Dallas have done by creating the first working example
of a synthetic 2D Material that demonstrated the Quantum Transport Effect
Theory at Room Temperatures as reported in the article “Diode
a few atoms thick shows surprising quantum effect”, published June 23,
2015, Physorg.
Their research was published in the Journal Nature Communication on Friday June 19th
2015 under the title “Atomically
Thin Resonant Tunnel Diodes Built from Synthetic Van Der Waals Heterostructures”.
The team consisted of the usual rogue’s gallery of PhD and graduate students
who have been doing time at Penn State University and University of Texas at
Dallas to make this unique device:
1. Penn
State Assistant Professor, Dr. Joshua Robinson
2. Materials
science and Engineering student at Penn State, Ms. YuChuan Lin,
3. Penn
State Professor of Electrical Engineering, Dr. Suman Datta
4. University
of Texas at Dallas Dr. Robert Wallace
The team made the new material by using vapor
deposition techniques to create layers of semiconductor material a few atoms
thick. These three (3) layers of molybdenum disulfide (MoS2),
molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2),
also known as Van Der Waals materials, were layered onto a base of graphene.
They then applied a voltage to these 2D layers of
semiconductor material and observe a phenomenon called NDR (Negative
Differential Resistance). This structure is as shown below for illustration
purposes.
In plain English, this is like having a Diode in
reverse bias, effectively implying that the team had made a 2D Diode. So what’s the big deal about making a 2D Diode?
Think printable Electronic Radio Tattoos!
Penn
State University and University of Texas at Dallas 2D Diode - What is the
Quantum Transport Effect and Negative Differential Resistance
The team from Penn State University and University
of Texas at Dallas decided to make this 2D Material after realizing that
stacking semiconductor materials in such thin layers would invoke the Quantum
Transport Effect to quote Penn State Assistant Professor, Dr. Joshua Robinson:
“Theory suggests that stacking twodimensional layers of different materials
one atop the other can lead to new materials with new phenomena”.
However, there is a catch; the layers of the
semiconductor material molybdenum disulfide (MoS2), molybdenum
diselenide (MoSe2) and tungsten diselenide (WSe2),
sitting on a base of graphene have to be a few atoms thick, effectively pulling
the scientists into the Nanoscopic world.
This world is a strange one, where atoms or
molecules that are usually stable in a macromolecular structure suddenly become
more reactive when separate into small clumps of only tens or hundreds atoms at
a time.
At these levels, the forces that typically hold
macromolecular structure begin to disappear, namely:
1. Covalent
Bonding
2. Hydrogen
Bonding
3. Ionic
Bonding
4. Metallic
Bonding
5. Van
Der Waal Forces
With no forces of attraction based on permanent
charge dipole moment to stabilize them, atoms and molecules of almost any
compound become very reactive. They may even exhibit transitioning to higher
vibrational, rotational and quantum energy levels via the introduction of small
amounts of energy, be it in the form of radiation, sound, heat or even magnetic
fields.
The research of Professor of Mechanical Engineering
at Ohio State Dr. Joseph Heremans on using magnetic fields to reduce a heating
effect in diamagnetic materials implies that they are connected as reported in
my blog article
entitled “Ohio
State University and Heat Reduction using Magnetic Fields - How Heat, Sound,
Radiation and Magnetism in Paramegnetic and Diamagnetic materials are related”.
However, a very curious phenomenon occurs when you
have such isolated groups of atoms or molecules. The Metallic Bonding, Covalent
Bonding and Ionic Bonding usually create stability in macromolecular
structures.
But when atoms or molecules from these macromolecular
structures are separated into groupings of 20 to 200, these forces disappears
only to be replaced by much weaker Hydrogen Bonding and Van Der Waal Forces.
These Hydrogen Bonding and Van Der Waal Forces, which are based on temporary
dipole moment forces electrostatic, become stronger, especially when the
molecule has a strong dipole moment.
This is exactly what's happening with the three (3)
layers of molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2)
and tungsten diselenide (WSe2), sitting on a base of graphene. Van
Der Waal forces begin to dominate these nanoscopic structures, both within each
layer as well as between the layers.
The usually small dipole moments that are usually
insignificant become magnified resulting in electrons being able to flow freely
between the three (3) layers of molybdenum disulfide (MoS2),
molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2)
as there are no restrictions to their movement.
However, the flow of electrons is biased in one
direction, from layers where the electrons are free to move and are in higher
concentrations to layers where movement is a little less free and have lower
concentration. This means that electron flow will occur when an external e.m.f
is applied in forward bias but the current will be smaller in the reverse bias,
effectively becoming a kind of Diode or Transistor.
In fact, movement of electrons might even occurs
without an external e.m.f., as radiation, heat, sound or magnetic field might
cause electron flow in both forward and reverse bias between the three (3)
layers of molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2)
and tungsten diselenide (WSe2).
The phenomenon, called NDR (Negative Differential
Resistance), is the physical manifestation of the Quantum Transport Effect
Theory that the teams from Penn State University and University of Texas at
Dallas observed at room temperature.
How
Quantum Transport Effect Theory is enabled - Layers of Semi-Conductors Material
So Fresh and so Clean
To create this NDR effect in the three (3) layers of
molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2)
and tungsten diselenide (WSe2), sitting on a base of graphene
requires that the interfaces closely fit each other, molecule by molecule.
To achieve this, vapour deposition techniques were
used to literally apply a layer of the molybdenum diselenide (MoSe2)
and tungsten diselenide (WSe2) onto the Graphene substrate.
In the case of molybdenum disulfide (MoS2),
molybdenum oxide (MoO2(g))was vaporized in the presence of Sulphur
(S2(g)) vapour to react and grow the molybdenum disulfide (MoS2)
layer as follows:
MoO2(g) + S2(g) - MoS2(g)
+ O2(g)
Apparently this reaction occurs because the
vapourized Sulphur (S2(g)), which is also a Group VI element like
Oxygen (O2(g)) was more reactive as it more easily gives up its
electrons to form Covalent bonds that are increasingly ionic in nature. The
resulting layers appear as shown in this enhanced photograph.
These three (3) layers of molybdenum disulfide (MoS2),
molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2),
because of this method of growing them atom by atom means that the surfaces
were closely bonded.
This was a drastic departure from previous methods
used by other researchers who did papers on the Quantum Transport Effect, as
they'd resorted to cutting very thin veneers of the semiconductor material from
a larger sample and then using vacuum heat fusion to bond the layers together,
to quote Dr. Joshua Robinson: “This is the first time these vertical
heterostructures have been grown like this. People typically use exfoliated
materials that they stack, but it has been extremely difficult to see this
phenomenon with exfoliated layers, because the interfaces are not clean. With
direct growth we get pristine interfaces where we see this phenomenon every
time”.
Using Vapour depositing and growing techniques
resulted in no rough surfaces. The result was a clean contact that enhanced the
Van Der Waals forces between the layers and thus enableing the Quantum
Transport Effect.
2D
Resonant Tunneling Diode - How Sticker Radios and Paper Thin Smartphones
powered by our bodies may soon be possible
However, despite this knowledge, they had no idea
what to make of the result from the graphs below when an e.m.f. was applied at
from temperature. The graph below shows Current-voltage curves of single
junction (green) Van Der Waals solid (no NDR) and multifunction (red, orange) Van
Der Waals solids (NDR).
The sharp peak and valley in their Electrical
measurements was not the smooth slope they'd expected in a typical Diode, whether
in forward or reverse bias.
So they sought the help of an expert in nanoscale
Electronics, Penn State Professor of Electrical Engineering, Dr. Suman Dutta,
who told them the unthinkable; they'd created a resonant Tunneling Diode. Dr.
Suman Dutta consulted with postdoctoral researcher Ram Krishna Ghosh, whose
ran some numbers and came up with values that mimicked the experimental
results.
For those with a background in electronics, this is
the equivalent of a Schottky Diode which is used in high-frequency Resonant
circuits because of its fast switching times. Only this time the Resonant Tunneling
Diode is a 2D device at a nanoscopic level meaning its properties can be
controlled by changing the composition of the three (3) layers of molybdenum
disulfide (MoS2), molybdenum diselenide (MoSe2) and
tungsten diselenide (WSe2).
This is as soon as the researchers can figure out
how to make other 2D components on a nanoscopic level, like resistors,
capacitors and inductors as Dr. Suman Dutta has pointed out, quote: “The take
home message is that this gives us a nugget that we as device and circuit
people can start playing around with and build useful circuits for 2D
electronics”.
With this Diode, they can build high-frequency
resonant circuits but at a nanoscopic level to quote Dr. Suman Dutta: “Resonant
tunnel Diodes are important circuit components. Resonant Tunneling Diodes with
NDR can be used to build high frequency oscillators. What this means is we have
built the world's thinnest resonant Tunneling Diode, and it operates at room
temperature”.
So we might just be looking at the developement of
Radios and resonant antennas that are as thin as a sheet of paper that use
little or no voltage, as with this technique, which is factory scalable, it
will be possible to build a transmitter as flat as a sheet of stickers. Then
when pasted onto human skin, it could that person’s Electrical charge to power
itself and act as a transmitter, sending data on the person’s location and
allowing them to be trackable.
Still, more benign usages, such as making
smartphones and radio equiptment even small and thinner, are on the horizon in
a few years time.
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