Transition Metal Dichalcogenide

Wikipedia: Transition Metal Dichalcogenide

Transition metal dichalcogenide (TMD) monolayers are atomically thin semiconductors of the type MX2, with M a transition metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms.

“2D” is not the dimension like “rectangles represent a 2D object and atoms are spheres so they cannot be 2D”. That is not what is being referred to when atomically thin materials are being discussed. The 2D in material science is the “degrees of freedom” an electron can travel along the surface. In atomically thin materials, that is “up/down” and “left/right”, hence the use of 2D to describe the material.

When it is in single element form, they also get a new ending: -ene

They can be combined in various ways to make other 2D materials.

Quantum Mechanics (QM)

Electrons (in QM there is no such thing, I know. But this is not that argument) orbit the nucleus of an atom. The fill in these orbits, or “shells”, in a specific order, to preserve their lowest value state. The first shell consists of the orbital electron. The next shell is called the “s shell” and consists of two electrons. The next shell is the “p shell” which consists of 6 electrons.

The next shell is the “d shell” and consists of 10 electrons.

See, Wikipedia: Electron Shell

Wikipedia TMD (same link as above):

The IUPAC definition defines a transition metal as "an element whose atom has a partially filled d sub-shell, or which can give rise to cations with an incomplete d sub-shell".

A cation is just a positively charged ion.

Chalcogen are the oxygen family of elements in group VI of the periodic table. Oxygen is the only non-ore version which is one of the reasons why it is sometime not included in the list; leaving it out, that leaves: sulfur, selenium, tellurium, and polonium.

Since polonium is radioactive (this is what the Curies studied to discover radioactivity) I doubt that there is a real need to create atomically thin layers of the stuff!

The typical forms of each vary. They have crystal structures but only two have the flat, hexagonal crystal form: selenium and tellurium.

- Pt. 1-

Just remember that TMDs can also be made various other metals related to TM list in the sandwiched layers. It is kind of confusing when other metals, like ruthenium, appears, but there will always be the chalcogen present.

The news for TMDs has been fast and furious.

A study led by Rice materials scientist Pulickel Ajayan and lead author Sruthi Radhakrishnan details a new method to transform tungsten disulfide from a semiconductor to a metallic state.

Other labs have achieved the transformation by adding elements to the material – a process known as doping – but the change has never before been stable. Tests and calculations at Rice showed fluorinating tungsten disulfide locks in the new state, which has unique optical and magnetic properties.

Phys.org, Oct. 16, 2018 – Fluorine flows in, makes material metal.

A transistor based on the 2-D material tungsten ditelluride (WTe2) sandwiched between boron nitride can switch between two different electronic states — one that conducts current only along its edges, making it a topological insulator, and one that conducts current with no resistance, making it a superconductor — researchers at MIT and colleagues from four other institutions have demonstrated.

Using four-probe measurements, a common quantum electronic transport technique to measure the electronic behavior of materials, the researchers plotted the current carrying capacity and resistance characteristics of the two-dimensional tungsten ditelluride transistor and confirmed their findings across a range of applied voltages and external magnetic fields at extremely low temperatures.

“This is the first time that the exact same material can be tuned either to a topological insulator or to a superconductor,” says Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT. “We can do this by regular electric field effect using regular, standard dielectrics, so basically the same type of technology you use in standard semiconductor electronics.”

news.mit.edu, Oct. 31, 2018 – First two-dimensional material that performs as both topological insulator and superconductor.

Also, other news and references,

Wikipedia: Two-dimensional materials (includes TMDs and layered 2D materials as well as a list of known materials)

phys.org - Scientists creating an atomic 'Lego set' of 2-D wonder materials.

About layering 2D materials together.

Oct. 3, 2018 - Atomically thin semiconductors could make computers run a million times faster

The byline reads: Transition metal dichalcogenides or TMDCs could be the key to radically faster computers, claim researchers.

Phys.org, Oct. 29, 2018 – Scientists form flat tellurium.

Trying to find a substitute to tungsten ditelluride, they accidently created tellurene!

It is heavier than graphene, more stable, and has a higher melting point (same source).

I am sure there will be more TMD news! This is just something I have been seeing lately and figured a share was in order. I hope this makes sense but the “news” does not always contain enough “science” to explain what has been accomplished in the article. Plus there was no single one place all this information was located (it is mentioned across several threads).

We are beginning to enter of 2D materials and useful electronics. As these 2D materials are simulated upon computers then demonstrated in the lab (the MIT folks lucked out when another lab re-created their results at nearly the same time proving it was not luck), uses only dreamed of will be realized.

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Looks like it all fit in the first two posts!

This was from October 15 of this year. It is about being able to stamp out 2D materials like TMDs.

Toward Practical Heterostructures of 2-D Materials.

I have been seeing more news stories so figured there needs to be a "hey check this one out" thread (for those interested).

The MIT news is just insane! They are gating a "white graphene" sandwiched TMD at supercritical temperatures with magnetic fields [Correction: a voltage] to create either an insulator (eta: along the edge) or superconductor (eta: in the interior)! That was going to be my OP until I realized it would probably need some 'splaining.

Science is...

originally posted by: TEOTWAWKIAIFF
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Looks like it all fit in the first two posts!

This was from October 15 of this year. It is about being able to stamp out 2D materials like TMDs.

Toward Practical Heterostructures of 2-D Materials.

I have been seeing more news stories so figured there needs to be a "hey check this one out" thread (for those interested).

The MIT news is just insane! They are gating a "white graphene" sandwiched TMD at supercritical temperatures with magnetic fields [Correction: a voltage] to create either an insulator (eta: along the edge) or superconductor (eta: in the interior)! That was going to be my OP until I realized it would probably need some 'splaining.

Science is...

Nano scale efficiency control of energy conversion is what I see as the most interesting byproduct of your post material. (Furthermore atomic scale control is unreal)

Fantastic! By zigzagging this material, new types of diodes will be possible, not only that, but the physical structure could be adjusted to control the input and output at the atomic scale. Finally, a potential reality for near maximum efficiency solar panel designs. (I'd bet 90%+ solar will be enabled by this.)

There's no reason to build an atom gate, if you can make atom hallways that force single file in and out anyway! Incredible! If they can hook complex molecules to the input or output gate areas this creates the possibility for machines and gate control that can act like FPGA's but at a magnitude of efficiency silicon chips can't even dream of.

Imagine being able to attach fullerenes to the end of this and microcontrol the flow of Noble gases. This could make laboratory devices that enable chemists, biologists, or even physicists to perform atomic level experiments that are impossible with current equipment.

This is one of those Willard Whyte secrets isn't it?

The MIT news is just insane! They are gating a "white graphene" sandwiched TMD at supercritical temperatures with magnetic fields [Correction: a voltage] to create either an insulator (eta: along the edge) or superconductor (eta: in the interior)!

Now we need to find out how to do both simultaneous...
The skin effect is going to be like "I can´t work with this material!".

a reply to: Archivalist & verschickter

@Archivalist, that is kind of what the MIT news hinted at. Being able to "squeeze" the boundries together of the superconductor/insulator to create virtual QM particles and trap them.

A particular area where this new capability may be useful is the realization of Majorana modes at the interface of topologically insulating and superconducting materials. First predicted by physicists in 1937, Majorana fermions can be thought of as electrons split into two parts, each of which behaves as an independent particle. These fermions have yet to be found as elementary particles in nature but can emerge in certain superconducting materials near absolute zero temperature.

“It is interesting by itself from a fundamental physics point of view, and in addition, it has prospects to be of interest for topological quantum computing, which is a special type of quantum computing,” Jarillo-Herrero says.

The uniqueness of Majorana modes lies in their exotic behavior when one swaps their positions, an operation that physicists call “braiding” because the time dependent traces of these swapping particles look like a braid. The braiding operations can’t change the quantum states of regular particles like electrons or photons, however braiding Majorana particles changes their quantum state completely. This unusual property, dubbed “non-Abelian statistics,” is the key to realizing topological quantum computers. A magnetic gap is also needed for pinning the Majorana mode at a location.

from MIT.edu link

@Both, the only thing missing is the magnetic gap but if you are playing with 2D materials near absolute zero, I am sure there some other material that can fit the bill. And since it is running at the QM scale the state doesn't have to last long. 1,000s of calculations can be done in picoseconds! A second is an enternity at the QM scale!

@verschickter, whatever you do, don't sneeze! lol

a reply to: Cauliflower

One of the places mentioned in one of the links where research was being done was Wright Patterson where a certain famous crashed object from New Mexico was allegedly housed. So, maybe!

To fashion the films, the team employed spin-spray layer-by-layer processing (SSLbL), a method Taylor pioneered in 2012. The system employs mounted spray heads above a spin coater that deposit sequential nanometer-thick monolayers of oppositely charged compounds on a component, producing high quality films in much less time than by traditional methods, such as dip coating.

The process allowed them to fashion flexible, semi-transparent EMI-shielding film comprising hundreds of alternating layers of carbon nanotube (CNT), an oppositely charged titanium carbide called MXene—a family of carbide flakes first engineered by Gogotsi—and polyelectrolytes. Taylor explained that those charge characteristics confer benefits beyond EMI shielding.

phys.org, Nov. 1, 2018 - New tech delivers high-tech film that blocks electromagnetic interference

A little learning from the previous day and what used to be gobbledygook begins to make sense!

It would be nice to have EMI coating built into devices. It would protect our precious bodily fluids!

Here I go, just starting a new thread on TMDs and researchers jump into the fray of combining two different TMDs with another 2D material, hexagonal boron nitride (hBN) to create what is known as a heterostructure by layering them on top of each other.

Transition metal dichalcogenides (TMDs), such as the inorganic compounds molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2), are a class of layered 2-dimensional (2-D) materials akin to graphene. Novel heterostructures can be fabricated by stacking single monolayers of these materials and the properties can be tailored by the choice and sequence of these monolayers.

"Based on the experimental findings, we developed a new model of the interaction between these materials that has wide ranging implications for how they behave and how they can be used," said Dr. Aubrey Hanbicki, research physicists and lead author of the study. "We show how the interaction between the layers can alter their behavior to create a new composite system."

The research at NRL used several advanced fabrication processes to stack and align single layer MoSe2 flakes onto single layer WSe2. The MoSe2-WSe2 stack was further encapsulated by ultra-smooth hexagonal boron nitride (hBN) layers and then "cleaned" using a novel flattening technique recently developed by NRL scientists.

As a result, the ultraclean hBN/MoSe2-WSe2/hBN stack exhibits this unique interlayer exciton [ILE] even at room temperature. At low temperatures, the ILE emission feature splits into two peaks providing the first clear resolution of this splitting, and enabling insight into the origin of the ILE itself. In particular, because the ILE peaks have nearly equal intensity, but opposite polarization, theoretical calculations can pinpoint the origin of the ILE.

phys.org, Nov. 5, 2018 - Researchers observe unique interlayer state in a bilayer heterostructure.

This is how we are going to make a "tricorder"!!

The ILEs will be catalogued into a database of known objects. Pass a device with the heterostructure over it, it, like Siri, queries the database of previously sampled substances, and you get back what the material is made of!

You ugly bag of mostly water!

Nice article explaining what all this layering of atomically thin materials is trying to achieve.

They do a review of what a "heterostructure" is...

The idea to stack layers of different materials to make [...] heterostructures goes back to the 1960s, when semiconductor gallium arsenide was researched for making miniature lasers—which are now widely used.

Today, heterostructures are common and are used very broadly in semiconductor industry as a tool to design and control electronic and optical properties in devices.

That is/was where things were in the semiconductor era. Now, add in 2D materials (the "dimension" in a "2D" material refers to electron transport: electrons can forward/backward, or, left/right, and are simply called "2D materials"), and it becomes complicated due to each material bringing its own set of rules to the table...

More recently in the era of atomically thin two-dimensional (2-D) crystals, such as graphene, new types of heterostructures have emerged, where atomically thin layers are held together by relatively weak van der Waals forces.

The new structures nicknamed 'van der Waals heterostructures' open a huge potential to create numerous 'meta'-materials and novel devices by stacking together any number of atomically thin layers. Hundreds of combinations become possible otherwise inaccessible in traditional three-dimensional materials, potentially giving access to new unexplored optoelectronic device functionality or unusual material properties.

In the study researchers used van der Waals heterostructures made out of so-called transition metal dichalcogenides (TMDs), a broad family of layered materials. In their three-dimensional bulk form they are somewhat similar to graphite—the material used in pencil leads—from where graphene was extracted as a single 2-D atomic layer of carbon.

The researchers found that when two atomically thin semiconducting TMDs are combined in a single structure their properties hybridise.

But wait! There's more!!

Unlike semiconductors, 2D materials interact not just layer on top of layer, but also how they twist together! That is because of the interference between the two materials. When looked at under STEM (scanning transmission electron microscope), a moiré pattern emerges. That is what the article (link below) is setting up.

Professor Alexander Tartakovskii, from the Department of Physics and Astronomy at the University of Sheffield, said: "The materials influence each other and change each other's properties, and have to be considered as a whole new 'meta'-material with unique properties—so one plus one doesn't make two.

"We also find that the degree of such hybridisation is strongly dependent on the twist between the individual atomic lattices of each layer.

"We find that when twisting the layers, the new supra-atomic periodicity arises in the heterostructure—called a moiré superlattice.

"The moiré superlattice, with the period dependent on the twist angle governs how the properties of the two semiconductors hybridise."

And he then adds this comment to explain why all this stacking and twisting matters...

Professor Tartakovskii added: "The more complex picture of interaction between atomically thin materials within van der Waals heterostructures emerges. This is exciting, as it gives the opportunity to access an even broader range of material properties such as unusual and twist-tunable electrical conductivity and optical response, magnetism etc. This could and will be employed as new degrees of freedom when designing new 2-D-based devices."

That is where the whole train is headed, new devices, with new properties, and the real interesting items are: electric conductivity, optical response, and magnetism. And because these are a few atomic layers (even if they stack them hundreds deep), all of these devices will be smaller than anything doing the same amount of work!

For a good, long, review and overview, of TMD and 2D materials see:

phys.org, March 6, 2019 - 1 + 1 does not equal 2 for graphene-like 2-D materials.

Plus you have all kinds of cool TLAs (Three Letter Acronyms) floating about and cool, techy words like: van der Waals heterostructures, and moiré superlattice, now, at least "seen" by your vocabulary!!

In a paper published in Nanoscale, the researchers described a way to make electrons do something entirely new: Distribute themselves evenly into a stationary, crystalline pattern.

"I'm tempted to say it's almost like a new phase of matter," Kar says. "Because it's just purely electronic."

The phenomenon appeared while the researchers were running experiments with crystalline materials that are only a few atoms thick, known as 2-D materials. These materials are made up of a repeating pattern of atoms, like an endless checkerboard, and are so thin that the electrons in them can only move in two dimensions.

Stacking these ultra-thin materials can create unusual effects as the layers interact at a quantum level.

Kar and his colleagues were examining two such 2-D materials, bismuth selenide and a transition metal dichalcogenide, layered on top of each other like sheets of paper. That's when things started to get weird.

Electrons should repel one another—they're negatively charged, and move away from other negatively charged things. But that's not what the electrons in these layers were doing. They were forming a stationary pattern.

Now, physicists at Northeastern have discovered a new way to manipulate electric charge. And the changes to the future of our technology could be monumental.

"When such phenomena are discovered, imagination is the limit," says Swastik Kar, an associate professor of physics. "It could change the way we can detect and communicate signals. It could change the way we can sense things and the storage of information, and possibilities that we may not have even thought of yet."

phys.org, Feb. 27, 2020 - Physicists may have accidentally discovered a new state of matter.

Besides having the coolest name ever, "Swastik Kar" (if he isn't, he should be in a Swedish death metal band! "I wants to learn how to playz ze guitar like you Swastik Kar"!!), they were fooling around with van der Waals heterostructures, and trying the "twisted" interference patterns (Moiré). The article states that at first they thought they had some kind of measurement error. So they tried again. And again. And yet again. The darn thing wouldn't go away! So they called in a theorist to help explain what they were seeing.

TMDs, unlike graphene, can be engineered very easily to have a band gap which is what you need to do things like create a semiconductor. That is why they are important and the reason for this thread.

Thanks to QM, electrons are seeking to find their ground state. When trapped in a 2D material (i.e., the electrons can only go forward/backwards or side-to-side. Hence the use of "2D" to describe how the material carries charge; not its shape in the world), or this case, between two sheets of 2D materials, when twisted, the electrons line up in a crystalline pattern shared between the two materials.

This has never been seen before!!

Have you ever walked into a meadow and seen an apple tree with mangoes hanging from it?

-Swastik Kar (same source)

That is how shocking it is to find something new in the world for the first time. The mind boggles!!