Monolayer Polymers grown on Surfaces via Ullmann Coupling and Plasma Polymerisation

Quadrelli, Alessio and Jarvis, Samuel (2023) Monolayer Polymers grown on Surfaces via Ullmann Coupling and Plasma Polymerisation. PhD thesis, Lancaster University.

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Abstract

Surfaces are the interfaces between materials and the surrounding environment and, thus, play a significant role in technological applications, ranging from catalysis to medical implants, electronics, and fundamental studies. The properties of surfaces can be tailored by functionalising them without affecting the bulk material, resulting in enhanced performances or new properties. Among the many available options, the use of organic molecules for surface functionalisation offers unique advantages. First, the functional groups of organic molecules can be engineered, allowing exquisite control over their chemical, electronic, and optical properties down to the single-atom level. Additionally, organic molecules can be stabilised by covalent bonds into polymers that are robust against a wide variety of stimuli, such as heat, light, pressure, and chemicals, and thus suitable for real-life applications. This thesis focuses on studying the growth of nanoscaled polymers on surfaces through on-surface catalytic reactions and plasma polymerisation. Plasma polymerisation is a technologically relevant process to coat surfaces with thin functional films of organic molecules with a host of applications, ranging from biomaterials to energy materials. The fundamental understanding of plasma polymerisation, however, lags behind its applications. In particular, the role of the surface in the formation of the polymer is still underexplored. This thesis investigates the effect of different surfaces on the formation of nitroxide-containing polymers using a monomer with anti-microbial activity, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), employing surface-sensitive techniques such as atomic force microscopy (AFM) and X-ray photoemission spectroscopy (XPS). The results reveal that, contrary to the widespread assumption of surface independence, the chemical and morphological properties of the plasma polymers depend on the surface in the early stages of plasma polymerisation. Finally, the anti-bacterial properties of TEMPO are investigated by showing that the use of TEMPO plasma polymers as a matrix to host TEMPO molecules improves the performance. One of the most studied on-surface polymerisation reactions, Ullmann coupling, is usually carried out on noble metal surfaces that catalyse it. The intended applications for 1D and 2D surface polymers typically require insulators or semiconductors as substrate materials. Developing a method for polymerisation on insulators is, thus, essential. However, molecules sooner desorb from insulating surfaces before the reaction can be thermally activated. This thesis investigates the use of atomic quantum clusters (AQCs) as catalysts for Ullmann coupling on non-metal surfaces with AFM and XPS. First, the AQCs are characterised and then their role as catalysts in the polymerisation of halogenated porphyrin molecules on non-metal surfaces is investigated. Silver and copper AQCs are found to catalyse the on-surface polymerisation. These results represent a major step towards growing 2D polymers directly on technologically relevant surfaces, such as silicon wafers.