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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 interactions3-10. 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.
Two-dimensional (2D) materials have attracted great interest since the report of Geim and co-workers on graphene11. Ultrathin sheet morphology results in extremely high surface areas and may confer valuable electronic, physical and chemical properties1. Various methods are available for creating these materials, including mechanical exfoliation11, chemical vapour deposition12, self-assembly on surfaces13 and reactions in solution14, but for large-scale preparation and applications the most useful is probably liquid exfoliation2. Suspending a layered three-dimensional (3D) solid in a solvent, followed by sonication or chemical treatments, yields bulk dispersions of flakes with extreme aspect ratios, including monolayers approximately 1 nm in thickness and with micrometre lateral dimensions. Liquid exfoliation has been applied to graphene3; 2D polymers4; inorganic solids such as transition metal dichalcogenides5,6, hematene7 and zeolites8; and metal-organic frameworks9. In all cases the layers are maintained by continuous networks of covalent or strong...