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.
Random stacking and bond formation during hydrothermal synthesis contribute to poor crystallinity, often resulting in crystal domains smaller than tens of microns. A deeper understanding of layer stacking could enhance synthesis methods, enabling the fabrication of larger COF crystals.
While there has been substantial progress in synthesising monolayer 2D polymers (2DP), the development of bilayer 2DP stacks remains limited. This area is particularly promising, as stacking or twisting 2D materials can create new materials with properties distinct from those of the individual layers. In inorganic materials, this field, known as twistronics, has led to discoveries but remains to be explored in 2D organic materials.
Breakthrough in bilayer COF synthesis and imaging
A team led by Professor Loh Kian Ping from the NUS Department of Chemistry has developed a method for synthesising large area two-layer 2D COFs at the liquid-substrate interface. This was achieved through the direct condensation of chemical molecules. Using scanning tunnelling microscopy (STM) in solution, they successfully imaged the molecular assembly process, capturing the formation of both the monolayer and bilayer. More importantly, they show how molecular structure and solvent mixture influence the bilayer stacking modes, and how, under certain conditions, large-area moiré superlattices emerge from twisted bilayer stacking.
Due to their highly porous and organic nature, COFs present significant challenges for imaging in air or ultra-high vacuum (UHV) conditions using STM. The pores of COFs are typically filled with solvent, and their surfaces may trap residues, complicating atomic-scale imaging. To overcome these difficulties, the team focused on imaging COFs directly in solution, where the surface is cleaner than when exposed to air.
Prof Loh said, "Performing STM in solution allows us to study the dynamic self-assembly process of molecular frameworks in real-time."
The research team includes Dr Zhan Gaolei who was an NUS postdoctoral fellow at the time of research and is currently a researcher at Suzhou Institute of Nano-tech and Nano-bionics, China), Professor Steven De Feyter from KU Leuven, Belgium and Professor Zhu Yihan from Zhejiang University of Technology, China.
The research findings have been published in the journal Nature Chemistry on 20 February 2025.
Moiré superlattices and controlled twist angles
A moiré superlattice is a pattern that emerges when two layers of periodic structures, like 2D materials, are stacked on top of each other but slightly misaligned or at different angles. This misalignment creates a new, larger periodic pattern that is not present in either of the original layers. In simpler terms, it is like two sets of paper strips. If one set of paper strip is placed over another but rotated slightly, the overlapping area will create a new pattern-similar to the moiré pattern. Moiré superlattices can lead to interesting electronic properties and behaviors that are not found in the individual layers, making them a significant area of research in materials science and condensed matter physics.
The research team demonstrated that by designing specific precursor molecules, they could precisely control the twist angle of the stacked COF layers to form a moiré superlattice. Unlike inorganic 2D materials, where the twist angles are often random and difficult to control, in 2D COFs, the twist angles can be controlled by designing the molecular precursors.
The researchers compared two different monomer isomers: pyrene-2,7-diboronic acid (27-PDBA) and pyrene-1,6-diboronic acid (16-PDBA). With 27-PDBA, the second layer could either be AA-stacked or twisted in relation to the first layer. In contrast, only a moiré superstructure formed with 16-PDBA, exhibiting a uniform moiré superstructure. This difference is attributed to the subtle differences in the electrostatic potentials. 27-PDBA exhibits concentrated negative charge lobes on its boroxine rings, which may hinder the formation of twisted phases compared with 16-PDBA which has a flatter electrostatic potential.
Implications and future directions
This study provides fundamental insights into the controlled synthesis of ultra-thin porous organic films, as thin as two-unit cell layers. Such films with well-controlled channel structures can be used as ultra-thin filtration layers in nanofiltration applications. Furthermore, the ability to tune the twist angle in stacked COFs opens new possibilities for manipulating light propagation, including phase and polarisation control.
Looking ahead, the researchers plan to extend the concept to a broader class of molecular precursors with different linkage chemistries. They aim to achieve deterministic control over twist angles in bilayer COF stacking, unlocking further potential applications in filtration and optical materials.
