Design-optimized conductive 2D conjugated metal-organic frameworks allowing sub-Ångstrom resolution HRTEM imaging

Abstract text (incl. figure legends and references)

Recent years have witnessed the rise of two-dimensional conjugated metal-organic frameworks (2D c-MOFs). 2D c-MOFs with strong in-plane π-d conjugation are particularly interesting because of their intrinsic conductivity, anisotropic charge transport, and potential (opto-) electronic applications¹. Recently, interfacial synthesis has emerged as a new production method for highly crystalline 2D c-MOFs^1–3. However, the AC-HRTEM structural determination of 2D c-MOFs, particularly down to the atomic scale, remains a formidable task. The electron radiation damage severely limits the achievable resolution ^4,5. Enhancing the intrinsic electron resilience of MOFs is vital to circumventing this physical limitation and improving achievable resolution. Previous researches provide insights into the empirical rules for electron radiation resilience of organic materials. For example, aromatic compounds are more electron resilient than aliphatic due to π electron delocalization and conjugation^6,7, suggesting higher stability of conductive 2D c-MOFs than conventional insulating ones. In addition, exchanging protium for chlorine on coronene molecules has increased the crosslinking dose by two orders of magnitude due to the reduced displacement cross-section of chlorine ^8,9, indicating a potential negative correlation between hydrogen content and electron resilience in MOFs. Nevertheless, the effects of organometallic bonds on MOF stability remain unexplored.

In this work, we investigate the electron resilience of 2D c-MOFs with systematically altered structural attributes, including hydrogen content, framework density, and organometallic bonding, the atomic structures see Fig. 1. The critical fluence, which represents the electron resilience, is quantitatively determined at 300 kV condition for each material. The experimental results are explicated by ab initio quantum calculation. The highly conductive Cu₃(BHT) demonstrates exceptional stability, allowing information transfer to the sub-Ångstrom regime in the characterization by Cs/Cc-corrected HRTEM at 80 kV (Fig. 2), the carbon atoms in the benzene ring structure can be unambiguously distinguished.

Fig. 1. (A) Atomic models of 2D c-MOFs. (B) Intensity profile of the first-order reflections in 2D c-MOFs as a function of accumulated electron dose. The critical dose is reached when the reflection intensity drops to e^-1 of the original value.

Fig. 2. C_S+ C_C-corrected atomic-resolved imaging of Cu₃(BHT) at 80 kV. (A) Experimental and simulated AC-HRTEM images of Cu₃(BHT). Acquisition dose: 3.2 × 10³ e^- Å^-2. (B) Enlarged image of A. (C) Intensity profile from the line-scan region in (B), the peak represents the center of the carbon atom, and the distance between the two peaks is 1.4 Å.

Acknowledgment

2D c-MOF samples were synthesized by the group of Prof. Xinliang Feng and Dr. Renhao Dong.

Reference

Wang, M., Dong, R. & Feng, X. Chem. Soc. Rev. 50, 2764–2793 (2021). Yu, M., Dong, R. & Feng, X. J. Am. Chem. Soc. 142, 12903–12915 (2020). Liu, J., Chen, Y., Feng, X. & Dong, R. Small Struct. 2100210 (2022). Skowron, S. T. et al. Acc. Chem. Res. 50, 1797−1807 (2017). Russo, C. J. & Egerton, R. F. MRS Bull. 44, 935–941 (2019). Alexander, P. & Charlesby, A. Nature 173, 578–579 (1954). Fryer, J. R.Ultramicroscopy 23, 321–327 (1987). Stenn, K. & Bahr, G. F. J. Ultrastructure Res. 31, 526–550 (1970). Chamberlain, T. W. et al. Small 11, 622–629 (2015).