Insights into the exfoliation mechanism of pyrene-assisted liquid phase exfoliation of graphene from lateral size-thickness characterisation

Abstract

Assisted Liquid-Phase Exfoliation (LPE) with pyrene derivatives has been shown to be very effective at producing stable and concentrated 2D material dispersions in water. However, the microscopic exfoliation mechanism is still not completely known. Initial models have proposed the intercalation of pyrene molecules between the layers, but it is unclear if these molecules remain trapped or not and if they can affect the morphology of the nanosheets. In this work we show an extensive atomic force microscopy study on the size and thickness of graphene, MoS2 and WS2 nanosheets produced by LPE in water using different pyrene derivatives. Our results show that the nanosheet lateral size scales with thickness and layer number by a power-law regardless of stabiliser type or material class. By using the equipartition model, we could rule out the presence of intercalated molecules between the layers; remarkably, we found that the pyrene stabiliser does affect the nanosheets morphology as it preferentially drives tearing over peeling during exfoliation. Finally, we determine the apparent thickness of a single layer to be ∼1–1.5 nm.