Harley, Sam and Robinson, Benjamin and Jarvis, Samuel (2024) Experimental Characterisation of Molecular Junctions Via Scanning Probe Microscopy. PhD thesis, Lancaster University.
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Abstract
The experimental characterisation of molecular junctions is perhaps the most important class of measurement within molecular electronics; as interest in the field has grown throughout the 21st century, SPM and break-junction methods for characterising electrical and thermal transport through molecular junctions have become increasingly advanced, providing insight into a diverse range of quantum transport phenomena. This work details the development of several bespoke instrumentation and data analysis systems to facilitate precise, repeatable measurement of the electrical and thermoelectric properties of molecular junctions. Novel fabrication strategies and careful design were employed to attain state-of-the-art performance at low-cost. A bespoke STM break junction (STMBJ) system for single-molecule conductance measurements is presented, featuring a novel approach to the fabrication of an SPM head using steel-reinforced epoxy granite (SREG) composite. In order to extract additional information beyond the most-probable conductance from the large datasets produced by STMBJ, a detailed investigation of autoencoder-based feature extraction and clustering algorithms was conducted. Notably, the role of feature disentanglement in STMBJ data segmentation was studied for the first time. Additionally, a high-precision thermovoltage AFM (ThAFM) module was developed which enables characterisation of the thermopower of multi-molecule junctions using a commercial AFM. The key challenges of ThAFM are outlined, and corresponding instrumentation design principles are identified and implemented to ensure optimal performance. The performance is demonstrated by measurements of the Seebeck coefficients of C60 and ODT monolayers with precision exceeding the best reported measurements to date. Finally, the system is used to produce the first reported measurement of the Seebeck coefficient of a ZnPc monolayer.