Chemists from the Nationwide College of Singapore (NUS) imaged the dynamic meeting of bilayer covalent natural frameworks (COFs) in resolution, offering new insights into moiré superlattice formation and managed stacking. The examine was revealed in Nature Chemistry.
When one lattice is rotated relative to a different in a stacked construction, a brand new correlated electron section can emerge. The moiré superlattice is a key idea within the area of “twistronics.” In a correlated electron section, the interactions between electrons considerably have an effect on their properties, fairly than behaving as particular person particles. This will result in distinctive phenomena similar to ferromagnetism or superconductivity.
Pure natural crystals are much less prone to exhibit moiré superlattice formation than pure inorganic supplies. One purpose is that for moiré superlattice imaging utilizing conventional microscopy methods, the fabric should be extraordinarily skinny and crystalline—traits which are troublesome to realize in natural supplies.
Two-dimensional covalent natural frameworks (2D COFs) are extremely porous natural supplies with vital potential for functions in gasoline storage, vitality storage, and catalysis. These constructions are made up of covalently bonded layers stacked by van der Waals forces and electrostatic interactions. Nonetheless, the transition from a monolayer to a bilayer is poorly understood as a result of complicated interactions of bonding forces similar to van der Waals, electrostatic, and hydrogen bonding.
The second layer in these supplies should be exactly aligned, as misalignment can cut back the crystallinity of the fabric. Producing single COF crystals bigger than a millimeter is troublesome as a consequence of potential bonding errors in each the horizontal (x-y) and vertical (z) dimensions.
Crystallinity points usually come up from misalignment throughout stacking, particularly when rotational misalignments happen between layers. Observing the stacking course of throughout progress is crucial to understanding this mechanism, however that is difficult as a result of the method happens in resolution.
Poor crystallinity is a results of random stacking and bond formation throughout hydrothermal synthesis, which regularly leads to crystal domains smaller than tens of microns. A greater understanding of layer stacking may enhance synthesis strategies and allow the manufacturing of bigger COF crystals.
Whereas the synthesis of monolayer 2D polymers (2DP) has made vital progress, the event of bilayer 2DP stacks continues to be in its early levels. Since 2D supplies will be stacked or twisted to create new supplies with properties distinct from the person layers, this space has nice potential. Twistronics has led to breakthroughs in inorganic supplies however stays largely unexplored in 2D natural supplies.
Breakthrough in Bilayer COF Synthesis and Imaging
A staff from the NUS Division of Chemistry, underneath the path of Professor Loh Kian Ping, has created a method for creating large-area two-layer 2D COFs on the liquid-substrate interface. Chemical molecules had been straight condensed, and the staff used scanning tunneling microscopy (STM) in resolution to picture the molecular meeting course of and seize the formation of each the monolayer and bilayer.
The staff additionally demonstrated how solvent mixtures and molecular constructions affect bilayer stacking modes, displaying that twisted bilayer stacking can produce large-area moiré superlattices underneath sure circumstances.
COFs current a major problem for STM imaging in air or ultra-high vacuum (UHV) circumstances as a consequence of their extremely porous and natural nature. Solvents usually fill the pores of COFs, and residues can grow to be trapped on their surfaces, complicating atomic-scale imaging. By imaging COFs straight in resolution, the place the floor is cleaner than when uncovered to air, the staff overcame these challenges.
Performing STM in resolution permits us to check the dynamic self-assembly means of molecular frameworks in real-time.
Loh Kian Ping, Professor and Research Lead, Division of Chemistry, Nationwide College of Singapore
Moiré Superlattices and Managed Twist Angles
When two layers of periodic constructions, similar to 2D supplies, are stacked with slight misalignment or at completely different angles, a moiré superlattice is fashioned. This misalignment produces a brand new periodic sample that’s absent in both of the unique layers. A easy analogy is stacking two units of paper strips and barely rotating them, leading to a brand new sample.
Moiré superlattices are vital in condensed matter physics and supplies science as a result of they’ll induce distinctive digital behaviors and properties not discovered within the particular person layers.
The researchers demonstrated that they might exactly management the twist angle of stacked COF layers to create a moiré superlattice by designing particular precursor molecules. In contrast to inorganic 2D supplies, the place twist angles are sometimes unpredictable and troublesome to manage, 2D COFs allow twist angle regulation by means of the design of molecular precursors.
The researchers in contrast two distinct monomer isomers: pyrene-2,7-diboronic acid (27-PDBA) and pyrene-1,6-diboronic acid (16-PDBA). The second layer of 27-PDBA could possibly be twisted relative to the primary layer or aligned in an AA-stacked configuration. In distinction, 16-PDBA solely fashioned a uniform moiré superstructure. This distinction arises from variations in electrostatic potentials. In contrast to 16-PDBA, which has a flatter electrostatic potential, 27-PDBA options concentrated unfavorable cost lobes on its boroxine rings, which can hinder the formation of twisted phases.
Implications and Future Instructions
The managed synthesis of ultra-thin porous natural movies, as skinny as two-unit cell layers, is now higher understood. These movies, with exactly regulated channel constructions, can perform as ultra-thin filtration layers in nanofiltration functions. Furthermore, the power to regulate the twist angle in stacked COFs opens up new potentialities for controlling the section and polarization of sunshine propagation.
Sooner or later, the researchers purpose to increase this idea to a broader vary of molecular precursors with completely different linkage chemistries. Their aim is to realize deterministic management over twist angles in bilayer COF stacking, which may unlock additional functions in filtration and optical supplies.
Journal Reference:
Zhan, G., et al. (2025) Moiré two-dimensional covalent natural framework superlattices. Nature Chemistry. doi.org/10.1038/s41557-025-01748-5
Supply:
