Füredi, Máté; (2025) Ellipsometry-based in-situ and in-operando characterisation of nanoscale electrode and sensor materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis aims to advance the understanding of the relationship between architectural attributes of thin films with nanoscale features and their behaviour as sensors and nanoelectrodes for energy storage. The primary characterisation techniques employed throughout this work are based on spectroscopic ellipsometry, covering a broad spectrum from ultraviolet to infrared. A particular focus is placed on ellipsometric porosimetry, a method that combines vapour sorption with in-situ ellipsometric measurements to explore nanopores (with diameters <100 nm). This includes innovative use of vacuum porosimetry setups - enabling measurements with multiple adsorptives tailored to different surface chemistries. The first part of the results sections (Chapters 4-6) explores a wide range of materials, including polymers, sol-gel-derived oxides, and anodic alumina, with pore sizes spanning 0.7 nm to 80 nm. The chapters establish improved methodologies for the characterisation of these materials, thereby enhancing the understanding of ellipsometric porosimetry and its limitations. Chapter 6 introduces new adsorptives, potentially extending the technique's reach into the macroporous regime (>50 nm). The second part of the results sections (Chapters 7-8) shifts focus to the practical application of in-situ ellipsometry in designing nanomaterials for high-performance sensors and electrochemical energy storage. Mesoporous thin films, fabricated using block copolymer co-assembly and sol-gel chemistry, are investigated for structural control. As practical example, transparent humidity sensors with rational pore design are fabricated this way. A key finding is the correlation between pore size and the linear relative humidity sensing range in resistive humidity sensors, providing insights for the development of broad-spectrum devices. Finally, operando ellipsometry is introduced to track pseudocapacitive charge/discharge processes in a sol-gel derived Li-ion battery anode, with future work outlined in exploring templated, porous nanoelectrodes, complemented by ellipsometric porosimetry. Overall, this thesis highlights how precise structural characterisation can guide the design of functional materials tailored for their specific applications.

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