Theory of Electron Transport through a Single Molecule

Althobaiti, Hanan and Lambert, Colin (2023) Theory of Electron Transport through a Single Molecule. PhD thesis, Lancaster University.

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Abstract

Understanding the orbital alignment of molecules sandwiched between metal electrodes is essential in the design of applicable molecular electronic devices. Orbital alignment is determined both by the molecular backbone structure and the molecule-electrode interface. This thesis presents a series of studies into the electronic and thermoelectric properties of 6 oligo (phenylene-ethynylene) OPE-based molecules trapped between the single-layer graphene (SLG) and a gold electrode to form an asymmetric junction. This study also employs 4 different anchor groups including thiol, pyridine, thioacetate and thioether. In the first part of this thesis, the theoretical tools, employed to investigate electron-transport properties of molecular junctions, are described. In chapter 2, Density Functional Theory (DFT), which is implemented in the SIESTA code, will be discussed. This provides the ground state wave functions for molecules and the Hamiltonians for molecular junctions, which is the first step in the transport calculations. Chapter 3 presents the theoretical basis for calculating the electric and thermoelectric properties, based on the Green’s function formalism, which is implemented in the quantum transport code GOLLUM. In chapter 3, I present solutions of Green’s functions for infinite and semi-infinite chains and the transmission coefficient equation, which forms the theoretical basis of this code. Chapter 4 is the first results chapter in this thesis, which demonstrates the transport properties of the six types of asymmetric junction modelled using a combination of density-functional theory and quantum transport theory. In this study, through a combined experimental and theoretical study, I show that the control of orbital alignment can be achieved by applying an external gate to six types of OPE-based molecules, which in turn control the electron transport within the HOMO-LUMO energy gap. I also demonstrate that the shape of the gold electrode (i.e., flat versus cluster) may affect the alignment of the HOMO and LUMO levels of the junction with respect to the Fermi level of the electrodes. Atheoretical investigation into the Seebeck coefficient is performed in chapter 5 using the first principles quantum transport method. This is carried out for six OPE-based molecules with two different gold electrode geometries (i.e., flat and cluster).