Scientists reveal unexpected nanoscale disorder in 'perfect' materials

Scientists have discovered that supposedly pristine crystalline materials contain surprising levels of disorder at the nanoscale, indicating new pathways to optimize materials for applications from water filtration to energy storage.

Why it matters

Researchers have been working for years to develop two-dimensional covalent organic frameworks (2D COFs) for applications ranging from semiconductors to water purification membranes. This collaborative research brings together Professor William Dichtel's group at Northwestern University, who are leading experts in COF structure and design, with Associate Professor and Racheff Faculty Scholar Pinshane Huang's team from the Department of Materials Science and Engineering at The Grainger College of Engineering, University of Illinois Urbana-Champaign. These materials' presumed perfect structure has been central to their promise — but new imaging reveals they're not as orderly as scientists thought.

The big picture

The study, published in the Journal of the American Chemical Society, uses cutting-edge electron microscopy techniques to reveal previously invisible defects in these materials.

• The research upends the traditional understanding of how these layered materials stack, showing that their pore channels — critical for molecular transport — are often distorted and constricted.
• The findings have significant implications for how these materials will perform in real-world applications, particularly those that rely on efficient transport of molecules, electrons, or ions.

By the numbers

• 3.2 nanometers: Size of the hexagonal pores in the 2D COF studied.
• Half a unit cell: Maximum offset between layers (much larger than the 1.6 Å previously predicted).
• ~10 nanometers: Scale at which significant variations in stacking appear.

What they're saying

"For years, the field has been depicting 2D COFs as nearly perfectly aligned stacks with straight, open channels. But our 3D imaging reveals a messier reality — these materials contain widespread stacking disorder that distorts their pore structures. This isn't just an academic distinction — it fundamentally changes how molecules will move through these materials and how they'll perform in real applications." - Associate Professor Pinshane Huang

How they did it

The researchers used two advanced electron microscopy techniques:

• Scanning transmission electron microscopy (STEM) to capture high-resolution 2D images.
• Electron ptychography, an emerging 3D imaging method that reveals the material's structure at different depths.

Between the lines

The study focused on an imine-linked 2D COF called TAPB-DMPDA, which is one of the highest quality, most structurally perfect COFs available. If disorder exists in this material, it likely exists in other COFs as well.

What's next

The researchers suggest that new design strategies are needed to better control the 3D structures of these materials:

• Future development efforts will likely focus on methods to reduce stacking heterogeneity.
• Understanding this disorder could lead to more accurate predictions of material performance.
• New synthesis techniques may emerge to create truly aligned structures.

The bottom line

This research reveals that even seemingly perfect nanomaterials can harbor significant structural disorder. For 2D COFs to fulfill their promise in advanced applications, scientists will need to account for — or eliminate — these imperfections.

Illinois Grainger Engineering Affiliations

Pinshane Y. Huang is an Illinois Grainger Engineering associate professor in the Department of Materials Science and Engineering and serves as associate director of the Materials Research Laboratory. She holds a Racheff Faculty Scholar appointment.