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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.
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 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.
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.”
originally posted by: TEOTWAWKIAIFF
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.
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)!
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.
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.
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.
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.
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.
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."
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."
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."
Have you ever walked into a meadow and seen an apple tree with mangoes hanging from it?
-Swastik Kar (same source)