Chemists from the National University of Singapore (NUS) have achieved a remarkable breakthrough in materials science by successfully imaging the dynamic assembly of bilayer covalent organic frameworks (COFs) in solution. This advancement provides significant insights into the complex mechanisms of controlled stacking and the formation of moiré superlattices—an intriguing phenomenon that falls under the emerging area of research known as "twistronics." Moiré superlattices manifest unique correlated electron phases when layered materials are rotated with respect to one another, presenting potential for novel materials with unique superconducting and ferromagnetic properties.
The significance of moiré superlattices is underscored by their rarity in organic crystal formations, in stark contrast to their presence in inorganic structures. Achieving such formations requires the materials to be ultrathin and highly crystalline—characteristics that are notoriously challenging to realize in organic substances. The research team’s focus on bilayer COFs is particularly noteworthy as it addresses the intrinsic difficulties associated with imaging organic materials using traditional microscopy techniques, which often fall short when applied to such delicate structures.
Covalent organic frameworks encapsulate a vibrant landscape of possibilities, specifically in applications like catalysis, energy storage, and gas storage. These structures are comprised of covalently bonded layers aggregated through electrostatic interactions and van der Waals forces. However, despite their utility, the transition from a monolayer to a bilayer configuration exemplifies a poorly understood aspect of their synthesis, primarily due to intermolecular bonding complexities. Information regarding the precise alignment and stacking of layers is paramount in determining the resultant material’s crystallinity and overall performance characteristics.
The research undertaken by Professor Loh Kian Ping and his team illuminates the intricate interplay of bonding forces involved in COF assembly, including van der Waals, electrostatic, and hydrogen bonds. Despite previous advancements in producing monolayers, challenges persist in synthesizing single COF crystals exceeding millimeter dimensions due to potential bonding error accumulations in both horizontal and vertical stacking processes. This misalignment can lead to significant complications regarding the crystallinity of layered materials and real-time observation of the stacking process presents an additional hurdle, particularly when dealing with the fluid dynamics involved in solution-based growth.
The research highlights that random stacking tendencies and bond formations during hydrothermal synthesis frequently hinder crystallinity, resulting in crystal domains significantly smaller than expected sizes. Gaining an in-depth understanding of the stacking mechanisms could dramatically enhance the synthesis protocols, possibly enabling the development of larger COF crystals with improved properties. The present advancements particularly in 2D polymers are exciting; however, many opportunities lie within the yet-untapped area of bilayer 2D polymer (2DP) stacks—a field promising exceptional advances through careful control of stacking and twisting of 2D materials.
Loh’s team employed a significant methodological leap that allowed them to synthesize large-area bilayer 2D COFs directly at the liquid-substrate interface. By utilizing a direct condensation technique during synthesis, they adhered to the layered structure’s integrity. Their implementation of scanning tunneling microscopy (STM) in solution was revolutionary, as it permitted real-time observation of the molecular assembly during bilayer formation. This method was crucial in revealing how solvent composition and molecular structure influenced bilayer stacking modes, leading to the spectacular emergence of large-area moiré superlattices.
The technical challenges posed by COFs, given their organic and highly porous nature, complicate imaging under traditional conditions. The scenarios necessitating ultra-high vacuum (UHV) or air-exposed conditions often contribute to the degradation of quality essential for atomic-scale imaging. However, by adapting their imaging methods to directly observe COFs while they remain in solution, the research team was able to circumvent many of these obstacles. Prof. Loh expressed the advantage of conducting STM in a liquid medium, remarking that it creates cleaner surfaces than those typically seen when materials are subjected to air.
In pursuit of characterizing the fundamental aspects of twisted bilayers, the research team dedicated significant attention to comparing different isomers, namely pyrene-2,7-diboronic acid (27-PDBA) and pyrene-1,6-diboronic acid (16-PDBA). They discovered that the second layer’s stacking behavior was influenced considerably by the variations in the precursor molecular architecture. Specifically, with 27-PDBA, the stacking could result in either an AA-stacked configuration or a twisted formation, showcasing the potential scalability of tunable properties. Conversely, 16-PDBA yielded a consistent moiré structure without the emergence of dwellings eliciting twist differences, demonstrating the complexity arising from the distinct electrostatic properties of the constituent molecules.
The implications of this research are far-reaching and suggest profound potential applications across various fields. With a foundation built upon controlled synthesis and the ability to manipulate twist angles, the opportunities for tailored materials are vast. The enhancement of ultra-thin porous structures paves the way for innovations in nanofiltration technologies—they could serve as functional barriers and frameworks with tuned channel geometries. Moreover, opportunities for developments that enable optimized light propagation, including manipulation of phase and polarization, are emerging as a critical avenue for further exploration.
Looking towards the future, the research group aims to leverage their foundational knowledge to elaborate upon a broader array of molecular precursors characterized by diverse linkage chemistries. Achieving deterministic control over the twist angles in subsequent bilayer COF systems could unlock previously unimagined applications, further contributing to the rapidly evolving field of organic electronics and nanomaterials. This ambitious initiative signals a promising horizon for researchers and industries alike, as they pursue novel applications driven by understanding and manipulating the molecular architecture of layered organic frameworks.
The intersection of advanced materials science and innovative imaging technologies heralds exciting prospects in the development of next-generation materials. With substantial evidence showcasing the practical applications and a clarified framework for future endeavors, the research conducted at the National University of Singapore establishes itself as a cornerstone in the ongoing quest toward functionalized, smart materials that blur the lines between traditional chemistry and advanced engineering.
Participants in this collaborative research included notable figures from various institutions, extending the impact of their findings across the global scientific community. The collective effort underscores the importance of cross-institutional collaboration in tackling complex challenges and pushing the frontiers of what is achievable in the field of materials science.
The research findings were disseminated through a formal publication in the esteemed journal, "Nature Chemistry," currently heralding significant interest in the scientific community. The implications of these discoveries are poised to inspire an extensive array of future studies exploring the intricate properties and applications of twisted bilayers in diverse scientific domains.
Given the evolving landscape of materials science and the potential for novel innovations to emerge, this research not only contributes to the current body of knowledge but also ignites curiosity for unexplored avenues in bilayer COFs and moiré superlattices. As researchers continue to unravel the complexities within these organic frameworks, we anticipate further revelations and advancements that could redefine technological applications and foster sustainable solutions within our increasingly material-driven world.
Subject of Research: Covalent Organic Frameworks and Moiré Superlattices
Article Title: Moiré two-dimensional covalent organic framework superlattices
News Publication Date: 20-Feb-2025
Web References: Link to Nature Chemistry
References: DOI 10.1038/s41557-025-01748-5
Image Credits: National University of Singapore
Keywords
Superlattices, Discovery Research, Two Dimensional Materials, Covalent Organic Frameworks, Scanning Tunneling Microscopy, Molecular Structure.
