Engineering news
Researchers at Pennsylvania State University claim to have found a way to control the direction of materials properties in 2D and 3D crystals by creating atomic chains in a 2D crystal, with implications in sensing, optoelectronics and next-generation electronics applications.
The research team stated that this discovery could potentially lead to creating materials with unusual properties that have never existed.
Alloys possessing a random or arranged order of atoms can have large effects on a material's properties. The university used a combination of simulations and scanning transmission electron microscopy imaging to determine the atomic structure of an ordered alloy of molybdenum, tungsten and sulphur. They determined that fluctuations in the amount of available sulphur were responsible for the creation of atomic chains of either molybdenum or tungsten.
Nasim Alem, assistant professor of materials science and engineering at Penn State, said: "We discovered how chains form in a 2D alloy as a result of fluctuations in the amount of a particular precursor, in this case sulphur. Normally, when we combine atoms of different elements, we don't know how to control where the atoms will go.
“However, we have found a mechanism to give order to the atoms, which in turn introduces control of the properties, not only heat transport, 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 sulphur alloy, the researchers 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.
Vincent H. Crespi, professor of physics, 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 sulphur is available at a given location, the edge will prefer to be either 100% molybdenum or 100% tungsten.
“As the availability of sulphur 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."
The team then produced samples of the ordered alloy by vaporising powders of all three elements, called precursors, under high heat.
The National Science Foundation and the US Army Research Office supported this work.