“We could potentially create new materials with unusual properties that have never existed,”
By creating atomic chains in a two-dimensional crystal, researchers at Penn State believe they have found a way to control the direction of materials properties in two and three dimensional crystals with implications in sensing, optoelectronics and next-generation electronics applications.
Whether an alloy has a random arrangement of atoms or one that is ordered can have large effects on a material’s properties. In a new paper published online in the journal Nano Letters, Nasim Alem, assistant professor of materials science and engineering, and colleagues at Penn State used a combination of simulations and scanning transmission electron microscopy imaging to determine the atomic structure of an ordered alloy of molybdenum, tungsten and sulfur. They determined that fluctuations in the amount of available sulfur were responsible for the creation of atomic chains of either molybdenum or tungsten.
“We discovered how chains form in a two-dimensional alloy as a result of fluctuations in the amount of a particular precursor, in this case sulfur,” Alem said. “Normally, when we combine atoms of different elements, we don’t know how to control where the atoms will go. But we have found a mechanism to give order to the atoms, which in turn introduces control of the properties, not only heat transport, as is the case in this work, but also electronic, chemical or magnetic properties in other alloy cases. If you know the mechanism, you can apply it to arrange the atoms in a wide range of alloys in 2D crystals across the Periodic Table.”
In the case of the molybdenum, tungsten and sulfur alloy, they showed that the electronic properties were the same in every direction, but using simulations they predict that the thermal transport properties are smaller perpendicular to the chains or stripes.
“We didn’t know why this crystal forms an ordered structure, so we worked with my colleague Dr. Vin Crespi to understand the underlying physics that causes order in this crystal. Our calculations show it was the fluctuations in the third element, sulfur, that was determining how the chains formed,” Alem said.
Vincent H. Crespi, Distinguished Professor of Physics, and professor of chemistry and materials science and engineering who developed the theoretical understanding of the phenomenon, said, “Although the interior of the flake is indifferent to whether molybdenum or tungsten occupies any site in the crystal lattice, the edge of the growing crystal does care: Depending on how much sulfur is available at a given location, the edge will prefer to be either 100% molybdenum or 100% tungsten. So as the availability of sulfur randomly varies during growth, the system alternately lays down rows of molybdenum or tungsten. We think this may be a general mechanism to create stripe-like structures in 2D materials.”
Amin Aziz, who is a Ph.D. candidate in Alem’s group and lead author on the Nano Letters paper, produced the STEM imaging and spectroscopy that showed the fine atomic structure of the alloy samples and their electronic properties.
“When we are able to directly image constitutive atoms of a substance, see how they interact with each other at the atomic level and try to understand the origins of such behaviors, we could potentially create new materials with unusual properties that have never existed,” said Azizi.
A team led by Mauricio Terrones, professor of physics, produced samples of this ordered alloy by vaporizing powders of all three elements, called precursors, under high heat.
Small balloons made from one-atom-thick material graphene can withstand enormous pressures, much higher than those at the bottom of the deepest ocean, scientists at The University of Manchester report.
This is due to graphene’s incredible strength – 200 times stronger than steel.
The graphene balloons routinely form when placing graphene on flat substrates and are usually considered a nuisance and therefore ignored. The Manchester researchers, led by Professor Irina Grigorieva, took a closer look at the nano-bubbles and revealed their fascinating properties.
These bubbles could be created intentionally to make tiny pressure machines capable of withstanding enormous pressures. This could be a significant step towards rapidly detecting how molecules react under extreme pressure.
Writing in Nature Communications, the scientists found that the shape and dimensions of the nano-bubbles provide straightforward information about both graphene’s elastic strength and its interaction with the underlying substrate.
The researchers found such balloons can also be created with other two-dimensional crystals such as single layers of molybdenum disulfide (MoS2) or boron nitride.
They were able to directly measure the pressure exerted by graphene on a material trapped inside the balloons, or vice versa.
To do this, the team indented bubbles made by graphene, monolayer MoS2 and monolayer boron nitride using a tip of an atomic force microscope and measured the force that was necessary to make a dent of a certain size.
“One can now start thinking about creating these baloons intentionally to change materials or study the properties of atomically thin membranes under high strain and pressure.”
Sir Andre Geim
These measurements revealed that graphene enclosing bubbles of a micron size creates pressures as high as 200 megapascals, or 2,000 atmospheres. Even higher pressures are expected for smaller bubbles.
Ekaterina Khestanova, a PhD student who carried out the experiments, said: “Such pressures are enough to modify the properties of a material trapped inside the bubbles and, for example, can force crystallization of a liquid well above its normal freezing temperature’.
In an advance that helps pave the way for next-generation electronics and computing technologies—and possibly paper-thin gadgets —scientists with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) developed a way to chemically assemble transistors and circuits that are only a few atoms thick.
What’s more, their method yields functional structures at a scale large enough to begin thinking about real-world applications and commercial scalability.
They report their research online July 11 in the journal Nature Nanotechnology.
The scientists controlled the synthesis of a transistor in which narrow channels were etched onto conducting graphene, and a semiconducting material called a transition-metal dichalcogenide, or TMDC, was seeded in the blank channels. Both of these materials are single-layered crystals and atomically thin, so the two-part assembly yielded electronic structures that are essentially two-dimensional. In addition, the synthesis is able to cover an area a few centimeters long and a few millimeters wide.
“This is a big step toward a scalable and repeatable way to build atomically thin electronics or pack more computing power in a smaller area,” says Xiang Zhang, a senior scientist in Berkeley Lab’s Materials Sciences Division who led the study.