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Free-standing homochiral 2D monolayers by exfoliation of molecular crystals

Abstract

Two-dimensional materials with monolayer thickness and extreme aspect ratios are sought for their high surface areas and unusual physicochemical properties1. Liquid exfoliation is a straightforward and scalable means of accessing such materials2, but has been restricted to sheets maintained by strong covalent, coordination or ionic interactions3,4,5,6,7,8,9,10. The exfoliation of molecular crystals, in which repeat units are held together by weak non-covalent bonding, could generate a greatly expanded range of two-dimensional crystalline materials with diverse surfaces and structural features. However, at first sight, these weak forces would seem incapable of supporting such intrinsically fragile morphologies. Against this expectation, we show here that crystals composed of discrete supramolecular coordination complexes can be exfoliated by sonication to give free-standing monolayers approximately 2.3 nanometres thick with aspect ratios up to approximately 2,500:1, sustained purely by apolar intermolecular interactions. These nanosheets are characterized by atomic force microscopy and high-resolution transmission electron microscopy, confirming their crystallinity. The monolayers possess complex chiral surfaces derived partly from individual supramolecular coordination complex components but also from interactions with neighbours. In this respect, they represent a distinct type of material in which molecular components are all equally exposed to their environment, as if in solution, yet with properties arising from cooperation between molecules, because of crystallinity. This unusual nature is reflected in the molecular recognition properties of the materials, which bind carbohydrates with strongly enhanced enantiodiscrimination relative to individual molecules or bulk three-dimensional crystals.

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Fig. 1: Top-down fabrication of chiral 2D monolayer nanosheets of supramolecular metallacycles.
Fig. 2: HR-TEM of chiral 2D nanosheets of 13.
Fig. 3: CTF-corrected HR-TEM imaging.
Fig. 4: Enantioselective recognition of chiral 2D monolayer nanosheets.

Data availability

The data that support the findings of this study are available from the corresponding authors.

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Acknowledgements

This work was financially supported by the National Science Foundation of China (grant nos 21875136, 91856204 and 91956124), the National Key Basic Research Program of China (grant nos 2021YFA1200402, 2021YFA1501501 and 2021YFA1200300), the Shanghai Rising-Star Program (grant no. 19QA1404300), the National Research Foundation Singapore (grant no. NRF-CRP14-2014-01), the National University of Singapore and the Ministry of Education of Singapore (grant no. C-261-000-207-532/C-261-000-777-532), and the University of Bristol. Y.H. acknowledges the support of the CCF grant (no. FCC/1/1972‐19) from King Abdullah University of Science and Technology.

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Authors

Contributions

Y.C. formulated and supervised the project. J.D., C.T. and D.C. performed the synthesis and purification of the chiral ultrathin 2D nanosheets of 1–3; performed the AFM, field-emission scanning electron microscopy, powder X-ray diffraction, ultraviolet–visible spectrometry, electrospray ionization mass spectrometry, FT-IR, TGA, dynamic light scattering, XPS, optical band gaps and Brunauer–Emmett–Teller analyses, and the fluorescence titration and electrospinning membrane sensing experiments. L.L. and Y.H. performed the HR-TEM imaging and analyses. J.Z. and Q.Z. performed the ultrafast TA experiments and analyses. Q.X., Z.Q. and J.J. performed the theoretical calculations and simulations. Y.L. and A.P.D. advised on the interpretation of results. J.D., A.P.D. and Y.C. wrote the manuscript. All authors contributed to the data analysis, discussion and manuscript revision.

Corresponding authors

Correspondence to Yu Han, Anthony P. Davis or Yong Cui.

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Nature thanks Jonathan Foster and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary information

Supplementary Information

Supplementary Information, including Figs. 1–77, Tables 1–4 and References.

Supplementary Video 1

Molecular dynamics simulation of l-glucose (orange colour) and d-glucose (green colour) binding to zero-dimensional discrete metallacycles of (R)-1 in acetonitrile solution.

Supplementary Video 2

Molecular dynamics simulation of l-glucose (orange colour) and d-glucose (green colour) binding to 3D bulk crystals of (R)-1 in acetonitrile solution.

Supplementary Video 3

Molecular dynamics simulation of l-glucose (orange colour) and d-glucose (green colour) binding to 2D monolayer nanosheets of (R)-1 in acetonitrile solution.

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Dong, J., Liu, L., Tan, C. et al. Free-standing homochiral 2D monolayers by exfoliation of molecular crystals. Nature 602, 606–611 (2022). https://doi.org/10.1038/s41586-022-04407-8

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