REPOSITORY > RESULTS

Abstract

Layered two-dimensional materials have unique physical and chemical properties due to their highly anisotropic nature. These properties are modified and complemented by novel phenomena as their thickness is reduced to one or a few layers. Two-dimensional crystals have strong covalent or ionic bonds in the plane and a weak van der Waals interaction between planes. This allows them to be expanded into low density structures or delaminate them into a few or single-layer nanosheets. Many two-dimensional materials can also intercalate molecules or ions between their layers, thus opening up new functionalities.
Transition metal dichalcogenides form layered crystals of stoichiometry MX2 (M=transition metal, X=S, Se, or Te). The best known compound MoS2 is an earth- abundant mineral and has been used for decades as a solid lubricant and as a catalyst for petroleum desulphurization. All transition metal dichalcogenides have been studied in detail for their electronic properties. The methods of exfoliation have made huge progress recently, but are still limited to either ultrasound-mediated exfoliation in liquid phase, which requires lengthy sonication and produces nanoflakes of inferior quality, or chemical exfoliation using the strongest reducing agents – alkali metals – to intercalate the layers. A subsequent reaction with water flooding the interlayer spaces produces hydrogen that pushes them even further apart. However, most transition metal dichalcogenides also react with weaker reducing agents, and this interaction is still largely unexplored.
The purpose of this dissertation is to expand the possible uses of transition metal dichalcogenides by preparing new intercalation compounds and studying the expansion by (using) relatively mild reducing agents. The workhorse compounds of their respective classes are chosen: MoS2 as a semiconductor, TiS2 as a conductor, and TaS2 as a phase change memory material. All three are treated with hydrazine in a reaction which rapidly expands TiS2 and TaS2, but is relatively inefficient in expanding MoS2. We circumvent this obstacle by intercalating MoS2 with quaternary ammonium salts. This ambient reaction yields expandable layered crystals in many ways similar to expandable graphite.
The expansion process is investigated in detail by characterising the materials at different stages – the raw material, the intercalation compound, the expanded material – using scanning electron microscopy, thermogravimetric analysis, X-ray diffraction, X-ray photoelectron spectroscopy, and Raman spectroscopy. Intercalation does not result in any structural changes or chemical modification such as oxide formation. The intercalation compounds can be stored and remain expandable for at least a year. The expansion is studied in detail under a scanning electron microscope, which both provides the electron beam and allows real time observation of the process. The range of expansion depends on the energy and incident angle of the electrons: higher energy results in deeper penetration and in a higher exfoliation range. Crystals irradiated from the side expand significantly faster and more efficiently. This process is self-accelerating since an expanding crystal is being reduced in density and the electrons can propagate deeper. The expanded material is exfoliated to nanosheets in solution and pressed into foil as a proof of principle.