Liquid phase exfoliation and interfacial assembly of two-dimensional nanomaterials

Liquid phase exfoliation (LPE) has been demonstrated to be a powerful and versatile technique for scalable production of two-dimensional (2D) nanomaterials, such as graphene and molybdenum disulfide (MoS2) which allows for their processing into a wide range of structures. LPE can be understood in terms of the chemical physics of the interactions of the liquid with the nanosheets. Here, it is shown that the prototypical solvent for LPE of 2D materials, N-methyl- 2-pyrrolidone, undergoes chemical modification during exfoliation which gives rise to increased absorption and photoluminescence, making it particularly unsuitable for dispersion of photoluminescent nanomaterials such as MoS2. A subsequent study identifies the influence of solvent properties on the exfoliation process and presents a model which allows for consistent size selection of few-layer nanosheets from any chosen solvent. Using this understanding, applications-driven solvent selection can be used to identify alternative solvents which facilitate processing of liquid-exfoliated nanosheets into composite and thin film structures. This approach allows for exfoliation into water-immiscible solvents to enable assembly of liquid-exfoliated 2D materials can be assembled at the interface between two immiscible liquids as solid-stabilised emulsions where the nanosheets act as both stabiliser and functional material. An understanding of the chemical physics of these emulsions is developed in terms of surface energies which allows for both measurement of the surface properties of the stabilising nanosheets and design of emulsions for potential applications as inks, composites, sensors and energy storage devices. In addition, Langmuir deposition can be used to assemble densely-packed ultrathin films at the air/water interface. This method is used to prepare few-layer MoS2 nanosheet networks, which exhibit interesting spectroscopic properties. Furthermore, these films exhibit high conductivity which is attributed to doping by nanosheet edges. The combination of nanoscale film thickness and increased conductivity highlights their potential for optoelectronic devices. As such, this study demonstrates that, through understanding of exfoliation and size selection, interfacial assembly represents a promising approach for realisation of functional composites and thin films, enabled by ultra-thin interfacial films of 2D nanosheets.