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2-D Materials

2-step synthesis yields large 2-D COF crystals

Separating crystal nucleation and growth processes is key to controlling crystal dimensions

by Mitch Jacoby
June 22, 2018 | A version of this story appeared in Volume 96, Issue 26


This reaction scheme depicts a two-step process for making covalent organic framework crystals.
Credit: Adapted from Science
Polymerizing a triphenylene derivative (left, green) and a bisboronic acid compound (left, blue) forms colloidal COF seed crystals (structure shown, center). Adding monomer in a second step enlarges the crystals, if done slowly (top right), or forms more tiny seed crystals, if added quickly (bottom right).

Organic polymers don’t readily form large, two-dimensional single crystals. If they did, their properties could be tuned via organic chemistry and they could join the growing family of 2-D materials being studied for use in electronics and sensing. Some covalent organic frameworks (COFs) do take on a 2-D shape, but they typically form insoluble polycrystalline powders with tiny crystalline domains—less than 50 nm. Northwestern University scientists Austin M. Evans, William R. Dichtel, and coworkers attribute the problem to poorly controlled nucleation and growth of COF crystals. To bypass those problems, they devised a two-step synthesis strategy that separates the nucleation and growth processes (Science 2018, DOI: 10.1126/science.aar7883). For the nucleation step, the team reacted a bisboronic acid compound and a triphenylene derivative. The reaction formed a colloidal suspension of COF-5 nanoparticles that remained stable because of the presence of acetonitrile in a custom-blended solvent. Then they slowly added additional monomer, which caused the colloidal seed crystals to grow, forming 2-D single crystals with domains measuring up to 1,500 nm wide. In contrast, adding monomer quickly led to nucleation of more nanocrystals.


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