Liquid phase exfoliation (LPE) has become an important production technique giving access to single and few-layered nanosheets in colloidal dispersion. It is applicable to a whole host of inorganic crystals with the nanosheet morphology being defined by the in plane and out of plane binding strength in the parent crystal.[1] While LPE can be applied to layered crystalline organic sheet stacks, the mechanism of the exfoliation is less understood than in the case of inorganic materials. In particular, it is not clear which factors govern the exfoliability. This is mostly because post-exfoliation processing can have an impact in the nanosheet dimensionality making comparisons challenging.

Here, we apply our learning from the exfoliation of layered inorganic materials to 2D crystals. In particular, we suggest that the exfoliability can be assessed by statistical AFM analysis to determine the length/thickness aspect ratio or characteristic monolayer length. First, we applied this methodology to 2D polymers synthesized by the single crystal to single crystal transformation. We found that crystalline nanosheets are produced after LPE of charge-neutral polymers with yield and sheet morphology similar to graphite exfoliation giving nanosheets with average length/thickness aspect ratios of ~60.[2] In contrast, LPE of metal organic frameworks (MOFs) yields relatively small and thick sheets. Using a Zr-MOF as model system, an average length/thickness aspect ratio of ~6 was determined which is significantly lower than for most layered inorganic materials.[3] In a comparative study using a range of Ga, Sc and Zr-based MOFs we find that the aspect ratio varies between 4-10 when performing LPE in aqueous surfactant. The length/thickness aspect ratio cannot be rationalized on the basis of intersheet binding strength and we suggest that porosity has a positive impact on the exfoliability.

Finally, we were intrigued by the question whether molecular crystals with only noncovalent bonds between discrete molecules could be exfoliated using LPE. With the realisation that the binding strength anisotropy governs the (average) shape of the LPE nanomaterials, it should be possible to obtain nanomaterials with distinct shape from exfoliation of organic molecular crystals. To test this, orthorhombic and triclinic single crystals of the organic semiconductor rubrene were used in LPE.[4] Distinct nanorods and nanobelts of rubrene with only a few molecular layers are formed, stabilised against aggregation in aqueous sodium cholate solution and isolated by liquid cascade centrifugation.