Thermopower Modulation by Surface Interactions, and Memristive Behaviour in Self-Assembled Monolayers

Wells, Charlie and Robinson, Benjamin and Jarvis, Samuel (2026) Thermopower Modulation by Surface Interactions, and Memristive Behaviour in Self-Assembled Monolayers. PhD thesis, Lancaster University.

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Abstract

Molecular electronics is a versatile field, capable of tackling some of the most important and pressing issues in modern society. This thesis focusses on two main aspects: thermopower and memristive behaviour. A thermoelectric material is able to generate a voltage from a heat gradient across itself. Organic thermoelectric molecules have the potential to operate effectively at room temperature, which current inorganic thermoelectric technology cannot. Such a shift in operating temperature opens up many potential applications, with the ability to recycle waste heat into electricity from ordinary devices or even body heat. This thesis focusses on how surface interactions and the quality of the surface can impact the potential thermopower of an organic molecule. Thermoelectric molecules have designs optimised for improved thermopower, but here it is shown that the conditions surrounding the molecule also play an important factor. Self-assembled monolayers (SAMs) are grown on gold substrates. It is shown that improved surface quality improves the consistency of the molecular junction formed, as well as a slight improvement in the conductance of the junction. This effect can be enhanced with a molecular template layer to form a multilayered structure. Here, such a film is grown with a combination of thermal evaporation and solution self assembly. Additionally, it is shown that electrically similar molecules with extra surface contacts can dramatically change the thermoelectric properties, changing the polarity of Seebeck coefficient from negative to positive. Amemristive material is one which can reversibly switch between conductance states and retain the set state without external power. Memristive molecules act as a form of non-volatile memory and enable the possibility of new computer architectures which can greatly increase the efficiency of computing. SAMs of an edge-fusedporphyrin dimer are grown onto gold substrates. 50% of the measured samples showed potential memristive effects in the form of hysteresis between the trace and retrace during current-voltage (I-V) sweeps. This effect was greatly enhanced with graphene capping the molecular films. The graphene provides a top layer which stabilises the junction and increases the quantity of molecules involved in transmission. It may also facilitate the memristive switching effect by virtue of its shallow work function.