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2-D Materials

Two-headed molecules template layered crystals

Self-assembled materials can combine properties of layers or yield new ones

by Sam Lemonick
September 18, 2021 | A version of this story appeared in Volume 99, Issue 34


Illustration of crystal structure of 2D material, depicting stacked, alternating layers of red and yellow polygonal prisms with ball-and-stick templating organic molecules between the layers.
Credit: Hemamala I. Karunadasa
Perovskite (blue) and Pb-Cl (yellow) layers self-assembled in water thanks to a new technique that uses bifunctional organic molecules (inset) as templates.

Two-dimensional materials’ unique electronic and optical properties lead to a number of possible applications, like perovskite solar cells or graphene semiconductors. Stacking different 2D materials like a layer cake can create new materials with useful combinations of properties or even brand-new ones. But stacking materials that are only an atom or two thick can be tricky and time-consuming. Hemamala I. Karunadasa of Stanford University and her colleagues are demonstrating a new approach that allows ions to assemble themselves into those layered crystals in water (Nature 2021, DOI: 10.1038/s41586-021-03810-x). The researchers borrowed a method for making perovskites: using organic molecules with positively charged functional groups, such as ammonium, as templates. Their innovation was adding a second functional group—for instance, a negatively charged sulfonate—on the molecule’s other end as template for a second layer. The organic molecules remain embedded in the crystal structure and can hold layers closer or farther apart in ways that affect the materials’ properties. The group demonstrated nine heterostructured materials using the method, which they characterized using X-ray diffraction. One example (shown) stacks perovskite and lead-chloride layers and has optical and electronic properties not seen in pure perovskites. Karunadasa says many combinations should be possible by changing the template functional groups, and the technique should work for large-scale manufacturing, but has not been optimized yet.


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