Electron and Phonon Transport in the Molecular Nanostructures

Alahdal, Angham and Lambert, Colin (2023) Electron and Phonon Transport in the Molecular Nanostructures. PhD thesis, Lancaster University.

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Abstract

Understanding the electron and phonon transport properties of molecular junctions, which are formed from a single molecule coupled to metallic electrodes, is crucial for nano- and molecular-scale applications. In this thesis, a number of theoretical studies are presented in Chapters 2 and 3 that investigate the electrical and thermoelectric properties of molecular junctions. In Chapter 2, density functional theory (DFT) is introduced and in chapter 3, an overview of transport theory is provided, based on the formalism of Green's functions. Chapter 4 aims to investigate the role that anchor groups play in determining intra-molecular quantum interference features and their related thermoelectric behaviour. I tested seven anchor groups, connected to two different (flurorene and flurenone) cores, with either para or meta connectivity. I also examined the role of the angle between the pyramidal electrode tip and the terminal groups, by considering two possibilities, denoted linear or non-linear. My findings indicate that switching from para to meta connectivity causes a marked decrease in the electronic transmission coefficient within the HOMO-LUMO gap, whereas switching from linear to non-linear binding has a less significant effect, at least in the para case. Finally, I show that the anchor groups play a crucial role in controlling the precise location of the Fermi energy within the HOMO-LUMO gap. In Chapter 5, I present a study of Verdazyl radicals and show how the symmetry of the molecule controls quantum interference. Overall, these molecules are predicted to show multiple conductance values and the differences between the number of conductance values and their magnitudes can be attributed to quantum interference features deriving from the relative positions of the connecting anchor groups. Most current research is focused on understanding the behaviour of electron transport in molecular systems. However, these studies provide only part of the fundamental knowledge needed to understand and optimise such materials at the molecular scale, because they ignore the crucial role of phonons. In chapter 6, I introduce a theoretical study of phonon transport for a series of molecules containing pendant groups and show how changing the mass of atoms in the pendant group can modify the thermal conductance. I find that the addition of the side branch would lead to an enhancement of the thermoelectric figure of merit ZT and this can guide the development of organic electronics in the future.