The Catalogue of Two-Dimensional Materials Grows
The simple arithmetic of transition metal dichalcogenides (TMDs) has promised that the world of 2D materials will continue to multiply. TMDs are combinations of one of 15 transition metals such as molybdenum or tungsten, with one of the three members of the chalcogen family, which includes sulfur, selenium and tellurium.
Researchers have only just begun to test all the different combinations of these TMDs to see what attractive properties they might hold in two dimensions. Meanwhile there is a kind of tacit hope that hidden among these various combinations could be a superior replacement for silicon.
But the expansion of the 2D universe is not limited to just TMDs, there’s black phosphorous, sometimes referred to as phosphorene, silicene and germanene and a host of other silly names waiting for materials that have not yet been fabricated into 2D thin films.
The sheer volume of 2D possibilities would seem to engender hope that some of them make actually lead to new devices and products in the not-too-distant future.
New Method Enables Sandwiching of Transition Metals
Researchers at Drexel University have multiplied the number of potential silicon replacements by demonstrating that they can combine two transition metals—in this case molybdenum and titanium—using carbon atoms as the glue bonding the two together.
“By sandwiching one or two atomic layers of a transition metal like titanium between monoatomic layers of another metal, such as molybdenum, with carbon atoms holding them together, we discovered that a stable material can be produced,” said Babak Anasori, the post-doctoral researcher who led the research, in a press release. “It was impossible to produce a 2-D material having just three or four molybdenum layers in such structures, but because we added the extra layer of titanium as a connector, we were able to synthesize them.”
The Drexel researchers are confident that their research, which was published in the journal ACS Nano, will provide a platform for combining elemental materials in a stable compound and claim that each new combination should provide new properties. Their expectations are that the properties of these various combinations will likely have applications in thermoelectrics, batteries, catalysis, solar cells, electronic devices, and structural composites.
“While it’s hard to say, at this point, exactly what will become of these new families of 2-D materials we’ve discovered, it is safe to say that this discovery enables the field of materials science and nanotechnology to move into an uncharted territory,” Anasori said in the release.
Molybdenum Ditelluride Begins to Make a Splash
Transition metal dichalcogenides (TMDs) remain at the top of most people’s list as the place we are most likely to find a replacement for silicon. However, we already know that two TMDs, namely molybdenum disulfide and tungsten diselenide, do not really provide good electrical contacts between them and metal, which would seem to be a strike against them.
Now researchers from Germany and Japan are looking at a material that was first synthesized in the 1960s that may be a viable alternative to silicon: molybdenum ditelluride. The international team has reported in the journal Science that they have fabricated a transistor in which the channel consists of layers of 2D versions of molybdenum ditelluride.
While the researchers have conceded that it’s more difficult to produce 2D versions of molybdenum ditelluride, even compared to exfoliating graphene from graphite, the semiconductor has such a narrow band gap that it’s hard to ignore it for electronic applications.
First Transistor Made From Black Phosphorus
Okay, if you follow the increasing amount of research in black phosphorus, you’re going to have to get accustomed to variations on headlines like “Black Phosphorus is the new Black” that will become as boring as variations on “Nanotechnology Is the Next Big Thing”.
But as lame as these headlines may be they do offer up a bit of accurate reporting. Black phosphorus, or phosphorene, is a big deal for electronic applications like field-effect transistors because of its inherent band gap and it is one of the few 2-D materials to be a natively p-type semiconductor and this summer researchers from Germany and the US were the first to successfully fabricate a field-effect transistor from the material.
Researchers from the Technical University of Munich (TUM) and the University of Regensburg in Germany and the University of Southern California (USC) reported in the journal Advanced Materials the development of a new method for synthesizing black-arsenic phosphorous that doesn’t require the high pressure typically needed, lowering energy requirements for the process and thereby costs.
The key to this new process was the replacement of phosphorus atoms with arsenic so that when the material reaches a concentration of 83 percent arsenic it gets an extremely small band gap of only 0.15 electron volts.
Of course, if you don’t want a band gap that small, you can simply adjust the amount of arsenic concentration to control the width of the band gap.
"This allows us to produce materials with previously unattainable electronic and optical properties in an energy window that was hitherto inaccessible," said Tom Nilges, head of the research group at TUM, in a press release.
Black Phosphorous Takes Square Aim at CMOS
The basis of today’s digital logic--CMOS (Complimentary Metal-Oxide Semiconductor—requires both n-type (excess electrons) semiconductors, and p-type (excess holes) transistors.
Now researchers at the Institute for Basic Science Center for Integrated Nanostructure Physics at Sungkyunkwan University (SKKU) in South Korea have manipulated black phosphorus so it behaves as either an n-type semiconductor or a p-type, or as if it were ambipolar (both n- or p-type) simply by changing its thickness and its bandgap or by using a different metal to contact it with.
As a result of this manipulation, the researchers reported in the journal Nature Communications the fabrication of transistors made from the material that can operate at lower voltages than a silicon-based transistor.
The SKKU team found that by simply adjusting the thickness of the material they could change its band gap. They also discovered that if aluminum was used as a contact, 13-nanometer-thick black phosphorus sheets displayed ambipolar properties similar to graphene. If they used thinner 3-nm flakes they operated as unipolar n-types with on/off current ratios greater than 105. The thinner they can make the material, the better its switching performance.
“The driving force in black phosphorus is the carrier mobility,” said David J. Perello, one of the researchers and authors of the article, in a press release. Carrier mobility is the speed with which charge can move through a material. While the mobility of silicon can compete with the today’s experimental devices made from black phosphorous, the potential carrier mobility of black phosphorus is much higher than silicon. “Everything centers around that,” said Perello. “The fact that the band gap changes with thickness also gives us flexibility in circuit design. As a researcher it gives me a lot of things to play with.”
But the material has a long way to go. “I don’t think it can compete with silicon at the moment, that’s a dream everybody has,” said Perello in the release. “Silicon is cheap and plentiful and the best silicon transistors we can make have mobilities that are similar to what I was able to make in these black phosphorus devices.”
Perello added: “The fact that it was so simple to make such an excellent transistor without having access to state of the art commercial growth, fabrication, and lithography facilities means that we could make it significantly better.”