Stacking together different 2D materials into thin heterostructures can both improve the performance of materials in certain applications and reveal new, unexpected properties arising from interactions at layer interfaces. The process of assembling these heterostructures from layers just one atom thick can be a slow and laborious process that does not always produce reproducible results. Researchers at the SLAC National Accelerator Laboratory have presented a new method that improves the speed, scalability and reproducibility of 2D-layered material assembly by utilizing organic molecules that direct chemical “ingredients” to layer themselves in an orderly fashion.
The process involves placing a combination of atoms in water with the barbell-shaped molecules that serve as a “template,” directing the atoms to form a 2D layer at each end. These molecules also link the layers together as they crystallize, ultimately forming one large crystal with the different 2D materials neatly interleaved together. The team used this method to produce six self-assembled perovskite heterostructures, layering atoms of the mineral with metal halides or metal sulfides. These structures were subsequently examined through X-ray diffraction.
In most of the structures, the barbell-shaped molecules held the layers slightly apart, but in one structure, the layers were in direct contact and able to form chemical bonds. This interface allowed the researchers to study the behavior of electrons distributed across the different layers. The study was published in Nature.
“Rather than manipulating materials one layer at a time, we’re just throwing the ions into a pot of water and letting the ions assemble the way they want to assemble. We can make grams of this stuff, and we know where the atoms are in the crystals,” said Stanford professor and corresponding author Hemamala Karunadasa. “This level of precision allows me to know what the interfaces between the layers really look like, which is important for determining the material’s electronic structure – how its electrons behave.”
This new method provides opportunities for fast, scalable material assembly and reproducible structures for consistent quality and research. The researchers are now exploring other structures that could be produced using this method.
Photo: A diagram shows how layers of two 2D materials – a perovskite (blue) and a metal halide (yellow) – assemble themselves out of chemicals tumbling around in water (left). The assembly is directed by linker molecules that look like barbells. Each end of a barbell (blue or yellow) carries a chemical template for growing one of the layers, and as the layers grow, the barbells link them together in the correct order. The new self-assembly technique, developed by SLAC and Stanford researchers, produces large crystals with a wide range of electronic properties. Credit: Jiayi Li/Stanford University