NUS: Unlocking opportunities to create new designer 2D materials with a twist

Chemists from the National University of Singapore (NUS) have successfully imaged the dynamic assembly of bilayer covalent organic frameworks (COFs) in solution, providing new insights into controlled stacking and moiré superlattice formation. Moiré superlattice belongs to the current exciting field of “twistronics”, where a new correlated electron phase can be created when one lattice is rotated with respect to another in a stacked structure. In a correlated electron phase, the properties of electrons are significantly influenced by their interactions with each other, rather than behaving as independent particles, and they can give rise to unique form of superconductivity or ferromagnetism.

While the formation of Moiré superlattice has been seen in pure inorganic materials, it is much rarer to see them in pure organic crystals. One reason is that moiré superlattice has to be ultrathin and highly crystalline to be imaged by conventional microscopy techniques, and these properties are not easy to find in organic materials.

Two-dimensional covalent organic frameworks (2D COFs) are highly porous organic materials with significant potential in catalysis, energy storage, and gas storage. These frameworks consist of covalently bonded layers, stacked via electrostatic interactions and van der Waals forces. However, the transition from a monolayer to a bilayer remains poorly understood due to the complex interplay of bonding forces, including van der Waals, electrostatic, and hydrogen bonding.

The precise stacking of the second layer is critical, as misalignment can reduce the material's crystallinity. Currently, producing single COF crystals larger than a millimeter is challenging due to potential errors in bonding in both the horizontal (x-y) and vertical (z) dimensions. Misalignment during stacking often leads to crystallinity issues, particularly from rotational misalignments between layers. Observing the stacking process during growth is essential for understanding the mechanism, but this poses significant experimental challenges, as the process occurs in solution.