The team wanted to do this by stitching different fabric-like, three-atom-thick crystals. “Usually these are grown in stages under very different conditions; grow one material first, stop the growth, change the condition, and start it again to grow another material,” said Jiwoong Park, Professor of Chemistry in the James Franck Institute and the Institute for Molecular Engineering and a lead author on the study.
Instead, they developed a new process to find the perfect window that would work for both materials in a constant environment, so they could grow the entire crystal in a single session.
The resulting single-layer materials are the most perfectly aligned ever grown, Park said. The gentler transition meant that at the points where the two lattices meet, one lattice stretches or grows to meet the other—instead of leaving holes or other defects.
The atomic seams are so tight, in fact, that when they looked up close using scanning electron microscopes, they saw that the larger of the two materials forms ripples around the joint.
To probe the energetics governing ripple formation in these strained materials, the team performed simulations using Mira, the 10-petaflops supercomputer at the Argonne Leadership Computing Facility (ALCF), a U.S. Department of Energy Office of Science User Facility. The simulations were in excellent agreement with their experimental findings.
“We utilized a model system comprised of nearly 150,000 atoms to explore the flat-rippled conformational space,” said Robert A. DiStasio Jr., Assistant Professor in the Department of Chemistry and Chemical Biology at Cornell University and one of the paper’s lead authors. “The computational resources provided by the ALCF enabled us to perform the large-scale calculations required to validate this discovery.”
The researchers tested the material’s performance in one of the most widely used electronic devices: a diode. Two different kinds of material are joined, and electrons are supposed to be able to flow one way through the “fabric,” but not the other
The diode lit up. “It was exciting to see these three-atom-thick LEDs glowing. We saw excellent performance—the best known for these types of materials,” said Saien Xie, a graduate student and first author on the paper.
The discovery opens up some interesting ideas for electronics. Devices like LEDs are currently stacked in layers—3D versus 2D, and are usually on a rigid surface. But Park said the new technique could open up new configurations, like flexible LEDs or atoms-thick 2D circuits that work both horizontally and laterally.
He also noted that the stretching and compressing changed the optical properties—the color—of the crystals due to the quantum mechanical effects. This suggests potential for light sensors and LEDs that could be tuned to different colors, for example, or strain-sensing fabrics that change color as they’re stretched.
“This is so unknown that we don’t even know all the possibilities it holds yet,” Park said. “Even two years ago it would have been unimaginable.”
This work was carried out in collaboration with co-lead author David A. Muller at Cornell University. Other coauthors included University of Chicago postdoctoral scholars Kibum Kang and Chibeom Park and graduate student Preeti Poddar, as well as Cornell University postdoctoral scholar Ka Un Lao and graduate students Lijie Tu, Yimo Han, and Lujie Huang
Abstract – Coherent Atomically-Thin Superlattices with Engineered Strain
Epitaxy forms the basis of modern electronics and optoelectronics. We report coherent atomically-thin superlattices, in which different transition metal dichalcogenide monolayers–despite large lattice mismatches–are repeated and integrated without dislocations. Grown by a novel omnidirectional epitaxy, these superlattices display fully-matched lattice constants across heterointerfaces while maintaining a surprisingly isotropic lattice structure and triangular symmetry. This strong epitaxial strain is precisely engineered via the nanoscale supercell dimensions, thereby enabling broad tuning of the optical properties and producing photoluminescence peak shifts as large as 250 meV. We present theoretical models to explain this coherent growth as well as the energetic interplay governing the flat-rippled configuration space in these strained monolayers. Such coherent superlattices provide novel building blocks with targeted functionalities at the atomically-thin monolayer limit.