Production and Characterisation of Thermoelectrically Efficient Thin Films for Green Energy Applications

Penhale-Jones, Becky and Robinson, Benjamin (2024) Production and Characterisation of Thermoelectrically Efficient Thin Films for Green Energy Applications. PhD thesis, Lancaster University.

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Abstract

Molecular electronics focuses on using molecules or their monolayers as electrical devices. Within this, the field of molecular thermoelectronics hones in on using these same building blocks for improvement of thermoelectric properties, or conversion of waste heat into useful electricity. Specifically designing molecules enables fine tuning of these properties to produce devices that are capable of effectively harnessing previously wasted energy. For example, adding side chains to a molecular backbone will impact phonon transport within it, as well as changing the packing density once the molecule forms into a monolayer. Furthermore, altering where the electrodes in a junction contact these molecules and their monolayers will influence quantum interference (QI) effects. These characteristics consequently shift other properties of the materials such as electrical conductance and figure of merit (FOM). This, in turn, enables advancements in the devices utilising such materials to produce useful products with greater efficiency. In this research, monolayers were formed via Langmuir-Blodgett (LB) deposition and selfassembled monolayers (SAMs) from specially designed molecules for improved QI and thermoelectric efficiency. Techniques including atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) were used to determine whether these formed uniform monolayers with reasonable electrical conductance. In this research, monolayers were formed via Langmuir-Blodgett (LB) deposition and selfassembled monolayers (SAMs) from specially designed molecules for improved QI and thermoelectric efficiency. Techniques including atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) were used to determine whether these formed uniform monolayers with reasonable electrical conductance. Graphene-coated AFM probes were wear-tested. Standard non-conductive AFM probes were coated in graphene via Langmuir-Schaefer (LS), in an attempt to prolong their lifetime. Conductive probes have been coated previously, with the conclusion that this improves conductance for tailored molecules in a junction. However, this study viewed the potential benefit of increased usability of the probes with minimal reduction in attainable detail. Furthermore, tetrapodal molecules were investigated, building on previous work on the possibility of decoupling the electrical conductance of the molecular backbone from its anchors, creating stable yet conductive monolayers. The aim was to understand if varying the ‘tail’ group of the molecule influenced its interaction within the molecular junction, therefore changing its electrical conductance. Finally, several dithiolenates were studied, with one anchor point and two thiol groups for substrate binding. Thiols have been extensively studied due to their affinity for gold, however, the role of dithiolenates is largely unknown. Two molecules were tested, both with protected and deprotected forms. Greater electrical conductance was anticipated for the deprotected forms than their protected counterparts. This thesis concludes that LS and self-assembly produce good monolayers with the desired properties per their molecular design. As such, the techniques present promising methods for fabricating useful thermoelectrically efficient thin films.