Theory and Modelling of Electron Transport in Molecular-Scale Condensed Matter Asma Mohammed Alajmi Ph.D. Thesis in Physics Department of Physics, Lancaster University, UK 2025 This Thesis is submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy

Abstract

Studying electron transport properties in molecular junctions, comprising a molecule as the scattering region coupled to metallic electrodes is critical for nano- and molecular-scale applications. This thesis focuses on the thermoelectric properties of a specific type of two- terminal molecular junction involving gold electrodes, forming gold-molecule-gold structures. The investigation into these molecular junctions' electrical and thermoelectric behaviour relies on theoretical approaches detailed in Chapters 2 and 3. Chapter 2 introduces the principles of density functional theory DFT, while Chapter 3 provides an overview of transport theory, emphasizing Green's function formalism. Chapter 4 examines the electron transport properties of cross-linked molecular junctions, consisting of a dimer formed by two OPE3 molecules connected through two different bridges. These junctions feature six connection points with thiol groups (SH). To validate the findings, three theoretical approaches are employed: the orbital product rule OPR, the tight-binding model TBM, and density functional theory DFT calculations. Additionally, these cross-linked systems' electrical conductance and Seebeck coefficient are analysed using DFT simulations. Chapter 5 explores the impact of the bridge type on electron transport properties by studying another two different bridge configurations in the junctions. Finally, this thesis is concluded with a summary and future works in chapter 6.