In an article published in Small, researchers addressed the development of covalent organic nanosheets (CONs), a class of atomically thin, two-dimensional materials distinguished by their tunable electronic properties and remarkable stability.
These nanosheets attract significant interest due to their potential use in sodium-ion batteries. However, controlling their electronic structure while maintaining their structural integrity over extended periods and under harsh conditions remains challenging.
The authors aim to design nanoscale covalent organic frameworks that can be directly synthesized into nanosheets with customizable electronic features optimized for electrochemical energy storage, enhancing their functional versatility and stability. This pursuit aligns with the broader goal of advancing nano-engineered materials that can transcend the limitations of bulk counterparts.
Background
Covalent organic frameworks (COFs) are crystalline, porous polymers constructed from organic monomers linked via covalent bonds. While bulk COFs exhibit promising properties, their nanoscale derivatives, particularly nanosheets, offer unique advantages due to increased surface area, quantum confinement effects, and enhanced electronic interactions.
Previous work has demonstrated the potential of 2D organic materials; however, many existing nanosheets suffer from inadequate stability or limited tunability of their electronic properties. Achieving nanosheets with both high stability and adjustable electronic structures remains a key challenge.
This study builds on prior efforts by introducing a novel design that integrates stable covalent linkages with flexible building blocks. This approach enables precise control over the resulting nanosheets’ electronic characteristics and performance in sodium-ion batteries.
The Current Study
In this study, the authors synthesized covalent organic nanosheets through a strategic bottom-up approach that combined controlled chemical reactions with direct nanosheet formation, rather than relying mainly on bulk exfoliation.
They began by designing specific organic monomers with conjugated structures and functional groups that support covalent bonding and electronic tunability. These monomers were polymerized via condensation reactions, forming stable, crystalline covalent frameworks.
Through this controlled synthetic design, the nanosheets were inherently obtained in two-dimensional form, ensuring stability and tunability at the molecular level. This direct design approach demonstrated that careful molecular engineering alone could yield thin, uniform nanosheets with controlled thickness, eliminating the need for post-synthetic exfoliation.
Characterization played a critical role in confirming the morphology and electronic properties of the nanosheets. Atomic force microscopy (AFM) measured thickness, ensuring the nanosheets maintained atomic or near-atomic layer dimensions. Transmission electron microscopy (TEM) provided detailed images of their lateral size and crystal structure, verifying their continuity and order at the nanoscale.
Spectroscopic techniques, including ultraviolet-visible (UV-vis) absorption and photoluminescence spectroscopy, were used to probe the electronic band structure and assess the degree of conjugation within the nanosheets. Electrochemical testing in sodium-ion batteries served as the key functional evaluation, alongside conductivity studies.
The combined controlled synthesis, morphological analysis, and electronic characterization allowed the researchers to establish the nanosheets’ properties, tunability, and robustness under various external conditions, providing a strong basis for subsequent application studies.
Results and Discussion
The results demonstrated that the synthesized covalent organic nanosheets exhibited notable stability and tunable electronic properties. Morphological analysis confirmed the successful direct formation of thin, uniform nanosheets with controlled thickness, as verified by AFM and TEM imaging.
Spectroscopic studies revealed that the electronic band structure could be modulated by altering the nanosheets’ chemical composition and conjugation length, leading to adjustable optical and electronic characteristics. Notably, the nanosheets showed enhanced stability against environmental factors such as moisture and temperature, outperforming traditional organic materials.
Most importantly, when tested as anode materials in sodium-ion batteries, the nanosheets delivered a reversible capacity of over 400 mAh g?¹, with more than 90% capacity retention after 500 cycles and nearly 100% coulombic efficiency, demonstrating their robustness under electrochemical operating conditions.
The tunability of their properties was achieved through modification of the covalent linkages and conjugation pathways, rather than relying on external doping methods, directly impacting both their intrinsic electronic band structure and battery performance. These findings suggest that the covalent nanosheets have high stability, adjustable electronic features, and excellent charge-carrier mobility. Such attributes make them promising candidates for use in next-generation sodium-ion batteries.
Unlike many prior reports emphasizing generalized electronic applications, the present study highlights their role as high-performance anode materials, where “unprecedented stability” refers to long-term cycling under demanding electrochemical conditions.
The discussion emphasizes that structural integrity and conjugation length are key factors influencing the material’s performance and that further optimization could enhance its applicability in advanced energy storage technologies. Overall, the study highlights the potential of these covalent organic nanosheets as versatile, high-performance battery electrodes.
Conclusion
This study successfully introduces a robust strategy for synthesizing covalent organic nanosheets that combine adjustable electronic properties with unprecedented stability in sodium-ion batteries.
The findings demonstrate that through deliberate molecular design and controlled synthesis, it is possible to create 2D organic materials with tailored electronic structures that withstand challenging operational environments.
These nanosheets harness the advantages of nanostructured materials, such as quantum effects and high surface area, while overcoming stability limitations. The implications of this work extend across sodium-ion energy storage and closely related technologies, positioning covalent organic nanosheets as promising candidates for next-generation nano-engineered devices.
Ultimately, the research advances the understanding of how molecular-level modifications influence nanoscale properties, providing a foundation for future explorations into customizable nanomaterials.
Journal Reference
Lee M., et al. (2025). Covalent Organic Nanosheets with a Tunable Electronic Structure to Achieve Unprecedented Stability and High-Performance in Sodium-Ion Batteries. Small 21, 36. DOI: 10.1002/smll.70313, https://onlinelibrary.wiley.com/doi/10.1002/smll.70313