Thermoelectric Properties of Organic Thin Films Fabricated by Thermal Sublimation and Solution Self-Assembly

Au-Yong, Sophie (2023) Thermoelectric Properties of Organic Thin Films Fabricated by Thermal Sublimation and Solution Self-Assembly. PhD thesis, Lancaster University.

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Abstract

As the nanotechnology industry continues to find ways to further the miniaturisation of electronic components a single chip, alternative methods and materials are being explored. This poses new challenges for the scientific community. Due to their size which allows for alternative transport mechanisms to occur, molecular electronics have gained much traction in the community and in particular, their thermoelectric properties have gained attention due to their unusual phenomena, showing their potential to substitute typical semiconductor materials. Organic molecules were carefully selected to be deposited on an Au(111) surface either under an ultra-high vacuum (UHV) environment or in solution self-assembly (SA) to form self-assembled monolayers (SAMs). These two methods were proven to produce high-quality films. The advantages of these two methods are highlighted throughout this thesis with the support of the topography obtained through atomic force microscopy (AFM). AFM in combination with X-ray photoelectron (XPS) can provide vital information about the structural composition of the monolayers and their interaction with the surface, allowing the conformation to be deduced. AFM can be adapted to study the conductive properties of "softer" (C-AFM) materials without damaging the conformation and properties. This thesis studies a series of dialkynylferrocenes which exhibit attractive electronic and rotational properties. Previously, work on these molecules was carried out on single molecular junctions whereas here, properties of the SAM were investigated and deposited via solution. The single molecular junctions showed that the molecules adopt an `open' conformation as opposed to the self-assembled monolayer where it preferentially forms a `hairpin' conformation (both alkynes pointing to the surface). Using the same deposition method, a series of parallel anthracene molecules were explored. These molecules offer the potential to harvest energy at room temperature and their thermoelectric properties can be tuned due to their quantum interference effects. These `sticky' anthracene bases are bound to the metal electrodes through their anchor groups and underwent further deposition to form a 3D architecture using zinc porphyrin as its `slippery' linker. Taking inspiration from the `slipper linkers' concept, the following chapter shows zinc porphyrin deposited via thermal sublimation to form a highly pristine base monolayer. Two derivatives of bipyridine were studied as the potential for a linker to the zinc centre of the porphyrin. Deposition methods of the second layer were attempted with both thermal sublimation and solution self-assembly. The findings presented in this research demonstrate that well-ordered self-assembled monolayers can be formed using two different methods of deposition: thermal sublimation and solution self-assembly. It is found that these molecular structures and their interactions with the surface can greatly alter the thermoelectrical and electrical properties leading to exciting and novel ways of fabrication methods highlighting the importance of molecular structures within a monolayer.