Dec 5 2017
“Two-dimensional (2D) materials” — materials placed in layers that are just a few atoms thick — are favorable for high-performance electronics as well as flexible, transparent electronics that could be layered onto physical surfaces to render computing ubiquitous.
A slight mismatch between the hexagonal structures of molybdenum disulfide and tungsten diselenide creates a strain that can be released by the formation of a "5|7 dislocation," in which two hexagons collapse to form a pentagon and a heptagon. Reactions with molybdenum disulfide in the environment cause the dislocation to move deeper into the tungsten diselenide, drawing a nanowire of molybdenum disulfide behind it. (Courtesy of the researchers)
The most popular 2D material is graphene, which is a form of carbon, but of late researchers have been exploring other 2D materials, such as molybdenum disulfide, which have their own, unique benefits.
Creating useful electronics, however, requires combining many 2D materials in the same plane, which is a difficult challenge. In 2015, researchers at King Abdullah University in Saudi Arabia developed a method for placing molybdenum disulfide (MoS2) adjacent to tungsten diselenide (WSe2), with a very clean junction between the two materials. With a variation of the method, researchers at Cornell University then discovered that they could induce long, straight wires of MoS2 — just a few atoms in diameter — to extend into the WSe2, while maintaining the clean junction.
The researchers reached out to Markus Buehler, the McAfee Professor of Engineering in MIT’s Department of Civil and Environmental Engineering, who specializes in atomic-level models of crack propagation, to see if his team could help clarify this odd phenomenon.
In the recent issue of Nature Materials, the King Abdullah, Cornell, and MIT researchers teamed with colleagues at Academia Sinica, the Taiwanese national research academy, and Texas Tech University to describe both the material deposition technique and the mechanism underlying the creation of the MoS2 nanowires, which the MIT researchers were successfully modeled computationally.
The manufacturing of new 2D materials still remains a challenge. The discovery of mechanisms by which certain desired material structures can be created is key to moving these materials toward applications. In this process, the joint work of simulation and experiment is critical to make progress, especially using molecular-level models of materials that enable new design directions.
Markus Buehler, The McAfee Professor of Engineering, Department of Civil and Environmental Engineering, MIT
Wired up
The ability to form long, thin MoS2 channels in WSe2 could have several applications, the researchers say.
“Based on [the materials’] electrical properties and optical properties, people are looking at using MoS2 and WSe2 for solar cells or for water splitting based on sunlight,” says Gang Seob Jung, an MIT graduate student in civil and environmental engineering and a co-author on the new paper. “Most of the interesting stuff happens at the interface. When you have not just the one interface — if there are many nanowire interfaces — it could improve the efficiency of a solar cell, even if it’s quite random.”
But the theoretical explanation of the molecular mechanism fundamental to the nanowires’ formation also increases the hope that their formation could be manipulated, to enable the assembly of atom-scale electronic components.
Two-D materials, one of the most promising candidates for future electronics, ultimately need to beat silicon-based devices, which have achieved a few nanometers in size alread. Two-D materials are the thinnest in the vertical direction but still span a quite large area in the lateral dimensions. We made the thinnest dislocation-free channels in 2D materials, which is a big step toward sub-nanometer electronic devices out of 2D materials.
Yimo Han, Chemistry Graduate Student, Cornell and First Author of the paper
Propagating polygons
In a 2D crystal, both MoS2 and WSe2 naturally position themselves into hexagons in which the constituent elements — molybdenum and sulfur or tungsten and selenium — alternate. Together, these hexagons form a honeycomb pattern.
The Cornell researchers’ fabrication method maintains this honeycomb pattern across the junction between materials, an exceptional feat and one that is highly useful for electronics applications. Their method uses chemical vapor deposition, in which a substrate — here, sapphire — is exposed to gases containing chemicals that react to create the desired materials.
The natural sizes of the WSe2 and MoS2 hexagons are somewhat different, however, so their mixing puts a strain on both crystals, mainly near their junction. If a pair of WSe2 hexagons right at the MoS2 junction convert into a hexagon matched with a heptagon (a seven-sided polygon), it is free from the strain.
This so-called 5|7 dislocation forms a site at which a MoS2 particle can attach itself. The resulting reaction inserts a molybdenum atom into the pentagon, creating a hexagon, and breaks the heptagon open. Sulfur atoms then attach to the heptagon to make another 5|7 dislocation. As this process recurs, the 5|7 dislocation moves deeper into WSe2 territory, with a nanowire extending behind it. The pattern in which the strain on the uneven hexagons relaxes and recurs ensures that the dislocation advances along a straight line.
The Office of Naval Research and the Department of Defense supported the research.