Chemical Doping of Liquid Exfoliated 2D-Materials

Nanomaterials play an important role in the flourishing field of nanoscience. Size reduction of materials results in a broad range of outstanding physical and chemical properties as well as a wealth of potential applications. A particularly interesting class of low-dimensional nanostructures are two-dimensional (2D) materials, i.e. individual layers of so-called van der Waals crystals. The research was triggered in 2014 by Geim and Novoselov through the isolation and characterization of graphene, a single layer of two-dimensionally arranged sp2 hybridised carbon atoms. 2D nanomaterials can be obtained by various methods including bottom-up approaches such as chemical vapour deposition and top-down approaches such as liquid phase exfoliation (LPE) and mechanical exfoliation. In recent years, LPE has gained increasing attention due to the high production rates and broad applicability to a range of structures beyond graphene including transition metal dichalcogenides (TMDs), hexagonal boron nitride, metal phosphorus trisulfides and many more. In LPE, high energy and shear forces (e.g. through sonication) are applied to reduce the dimensions of the crystal and the resulting nanosheets are stabilized in the liquid medium through appropriate solvents and surfactant systems. The resultant nanosheets are extremely polydisperse in lateral size and thickness so that LPE is typically coupled with size selection, for example through centrifugation. Due to this additional processing step, it is difficult to assess the impact of the stabilizer on for example the optical properties of the nanosheets which will be a function of both size and stabilizer. In addition, the number of pure organic solvents suitable to prevent reaggregation is very limited which is a bottleneck for further processing and deposition. The goal of the work conducted within the scope of this thesis is to establish protocols to make high quality 2D nanosheets from LPE accessible in a range of liquid media and to achieve a deeper understanding of the impact of the stabilizer on the optical properties of the nanomaterial. To this end, tungsten disulphide (WS2), a semiconducting transition metal dichalcogenide was chosen as model substance due to unique optical fingerprints of the monolayers (e.g. narrow linewidth photoluminescence from exciton only in WS2 monolayers). Throughout this thesis, monolayer-rich dispersions of WS2 nanosheets were prepared by sonication-assisted LPE in a common detergent solution in combination with liquid cascade centrifugation for size selection. In the first part, a protocol was developed to transfer these nanosheets into a range of additive/solvent systems. The advantage over a direct exfoliation in this systems is that dispersions containing nanosheets of the same size/thickness can be compared. This allowed to assess the impact of various chemical environments on the optical properties and to study effects associated with the dielectric screening of excitons (e.g. changes in exciton energy and width). With this foundation established, the nanosheets were transferred into a range of common pure organic solvents using a modified protocol. This is more challenging due to aggregation taking place. Nonetheless, this broad screening made it possible to relate the changes in exciton response to physical parameters such as refractive index and dielectric constant. Importantly, it was confirmed that monolayers can be stable in solvents that are not suitable for the exfoliation itself greatly expanding the choice of solvent for further processing. The third part focuses on precise deposition of the nanosheets on substrates using spin coating. Experimental difficulties such as aggregation and restacking of nanosheets in solvents are addressed in detail together with solutions to improve the colloidal stability of the nanomaterials. In the optimized samples, monolayer properties, such as exciton photoluminescence, are retained after deposition. At last, a new route for transferring nanosheets from water-based WS2 dispersions into different media is introduced which greatly facilitates deposition. In this approach, water-insoluble polymers are added to the aqueous surfactant solution prior to sonication. Through hydrophpobic interaction, the polymer is driven to the interface between the hydrophobic part of the detergent and the nanomaterial. This polymer coating on the nanomaterial reduces aggregation after transfer to hydrophobic organic solvents, suitable for thin-film processing. Such techniques for nanomaterial processing are highly demanded for the integration of these materials into functional devices