Penn State University and University of Texas at Dallas 2D Resonant Tunneling Diode - How Sticker Radios and Paper Thin Smartphones may soon be possible

“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 19^th 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 (MoO_2(g))was vaporized in the presence of Sulphur (S_2(g)) vapour to react and grow the molybdenum disulfide (MoS₂) layer as follows:

MoO_2(g) + S_2(g) - MoS_2(g) + O_2(g)

Apparently this reaction occurs because the vapourized Sulphur (S_2(g)), which is also a Group VI element like Oxygen (O_2(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.