A combination of two two-dimensional materials is showing promise for high-performance electronics and flexible, transparent electronics that could be layered onto physical surfaces.
Building electronics in 2D materials has been difficult because of the need to have conductors and insulators within the same plane. In 2015, researchers at King Abdullah University of Science and Technology (KAUST) in Saudi Arabia developed a technique for depositing molybdenum disulfide (MoS2) next to tungsten diselenide (WSe2), with a very clean junction between the two materials. With a variation of the technique, researchers at Cornell University then found that they could induce long, straight wires of MoS2 — only a few atoms in diameter— to extend into the WSe2, while preserving the clean junction.
These teams contacted Markus Buehler, from MIT’s Department of Civil and Environmental Engineering, who specialises in atomic-level models of crack propagation, to describe both the material deposition method and the mechanism underlying the formation of the MoS2 nanowires, which the MIT researchers were able to model computationally.
“The manufacturing of new 2D materials still remains a challenge,” Buehler said.
“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.”
In a 2D crystal, both MoS2 and WSe2 naturally arrange themselves into hexagons in which the constituent elements — molybdenum and sulfur or tungsten and selenium — alternate. Together, these hexagons produce a honeycomb pattern.
The Cornell researchers’ fabrication technique preserves this honeycomb pattern across the junction between materials, a rare feat and one that’s very useful for electronics applications. Their technique uses chemical vapour deposition, in which a substrate — in this case, sapphire — is exposed to gases carrying chemicals that react to produce the desired materials.
The natural sizes of the MoS2 and WSe2 hexagons are slightly different, however, so their integration puts a strain on both crystals, particularly near their junction. If a pair of WSe2 hexagons right at the MoS2 junction convert into a pentagon matched with a heptagon, it releases the strain.
This so-called 5|7 dislocation creates a site at which an MoS2 particle can attach itself. The resulting reaction inserts a molybdenum atom into the pentagon, producing a hexagon, and breaks the heptagon open. Sulphur atoms then attach to the heptagon to form another 5|7 dislocation. As this process repeats, the 5|7 dislocation moves deeper into WSe2 territory, with a nanowire extending behind it. The pattern in which the strain on the mismatched hexagons relaxes and recurs ensures that the dislocation progresses along a straight line.
[An illustration of how the 5|7 dislocation moves through the tungsten diselenide, drawing a nanowire of molybdenum disulphide behind it. Image: MIT]