For the past couple of years, researchers have been excited at the prospect of what they can make from all the possible instantiations of transition metal dichalcogenides (TMDs). 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. To date, only a small number of combinations have been tested, raising hope that there may be some as yet unidentified combination out there that will prove to be a viable alternative for silicon.
Now 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. With the method they have developed for combining the two-dimensional (2-D) versions of these materials, the researchers are testing various combinations to see what combinations might be applicable to energy storage, electronics, or wear-resistant materials.
“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 researchers believe that their work, which was published in the journal ACS Nano, will prove to be significant in the future because it provides a way to combine elemental materials in a stable compound. The Drexel team claims that each new combination should exhibit new properties. Eventually, they say, a combination will be discovered that could revolutionize a variety of different technologies, including 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.
The combining of 2-D versions of elements that resist being joined on the atomic scale has proven difficult and has required over a decade of research at Drexel.
“Due to their structure and electric charge, certain elements just don’t ‘like’ to be combined,” Anasori said. “It’s like trying to stack magnets with the poles facing the same direction—you’re not going to be very successful and you’re going to be picking up a lot of flying magnets.”
The key to making this all work was the discovery of a material called MAX phase by a Drexel researcher over two decades ago. MAX phase (the M is for transition metal, the A for "A group" metal, and the X for carbon and/or nitrogen) is described as a kind of primordial goo from which all things came—it contains all the elements but needs to be organized by the researcher.
The first order placed with MAX phase as the source material was a material developed in 2011 called “MXene.” It derived its name from the process of etching and exfoliating atomically thin layers of aluminum from MAX phases.
Two years ago, this blog covered research that showed MXenes were effective in energy storage applications.
The Drexel researchers were continuing their research into the use of MXenes in energy storage applications, working their way through the periodic table. When they reached the transition metals, they were stymied by molybdenum.
“We had reached a bit of an impasse, when trying to produce a molybdenum containing MXenes,” Anasori said in the release. “By adding titanium to the mix we managed to make an ordered molybdenum MAX phase, where the titanium atoms are in center and the molybdenum on the outside.”
Michel W. Barsoum, one of the Drexel researchers who first synthesized MXene over four years ago, added in the release: “This new layering method gives researchers an unimaginable number of possibilities for tuning materials’ properties for a variety of high-tech applications.”