Ordered alloy shows its stripes

By creating atomic chains in a two-dimensional crystal, researchers at Penn State believe they have found a way to control the direction of material properties in two and three dimensional crystals. According to the researchers, this finding could prove useful for the development of sensing technologies, optoelectronics and next-generation electronics.

Whether an alloy has a random arrangement of atoms or an ordered arrangement can have large effects on its material properties. In a paper published in 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 (STEM) 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 (2D) alloy as a result of fluctuations in the amount of a particular precursor, in this case sulfur," said Alem. "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, Alem and his colleagues showed that its electronic properties are the same in every direction, but using simulations they predict that its thermal transport properties are reduced in directions 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," said Alem. "Our calculations show it was the fluctuations in the third element, sulfur, that was determining how the chains formed."

Crespi, professor of chemistry and materials science and engineering at Penn State, developed the theoretical understanding of the phenomenon. "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," he explains. "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, a PhD candidate in Alem's group and lead author, 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 at Penn State, produced samples of this ordered alloy by vaporizing powders of all three elements, called precursors, under high heat.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.