University of Nebraska Engineers Work on Technique that Could Aid Scalability of Next-Gen Electronics

For the better part of a decade, engineers have been crafting and testing recipes for so-called van der Waals heterostructures: stacks of atomically thin crystal layers that can be sequenced just so. Compared with a homostructure — the nanoscopic equivalent of a slab of ham — a heterostructure might feature slices of pastrami, pepperoni and pepper jack, all held together by the weak van der Waals forces among neighboring atomic layers.

Engineers soon discovered that the diversity could cultivate technologically interesting properties, often in the regions where two different materials meet, that are otherwise difficult or impossible to recreate. Then, a few years ago, researchers began exploring the effects of rotating the layers within van der Waals stacks. That misalignment of layers, they found, could also yield interesting results — turning a material into a superconductor, for instance, or changing how a semiconductor emits light.

Yet the achievement came in the face of a considerable challenge: Despite the weakness of van der Waals forces, adjacent layers strongly prefer to remain aligned. Manually stacking layers one by one can overcome the issue but demands extreme precision and, more importantly, time that large-scale manufacturers of small-scale technology just don’t have.

So the Sutters, alongside colleagues from Aalto University and the University of Wyoming, decided to try a different tactic: directly synthesizing twisted stacks. Managing that, though, meant overcoming a foundational principle of thin-film growth: the tendency for each added layer to inherit its orientation from the underlying crystal.

They started with a support of tin disulfide, a compound that features two sulfur atoms for every tin atom and is useful as a layered semiconductor. After growing an atomically thin layer of tin monosulfide — one sulfur atom, one tin — on the tin disulfide base, the team saturated that tin monosulfide with a sulfur vapor.

As expected, the tin monosulfide spontaneously transformed into tin disulfide. But because tin monosulfide crystals grow in a rectangular lattice — as opposed to the hexagonal configuration of tin disulfide — the lattice of the newly transformed second layer adopted a 30-degree twist relative to the supporting crystal. And when the researchers repeated the process, the third layer took its cue from the second, rotating 30 degrees relative to it and 60 degrees relative to the first.

To demonstrate the generalizability of the approach, the team achieved the same feat after replacing the tin disulfide substrate with two other van der Waals semiconductors, molybdenum disulfide and tungsten disulfide.

Read the full story from the University of Nebraska-Lincoln here.