Saudi Cultural Missions Theses & Dissertations
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Item Restricted Theory and Modelling of Electron Transport in Molecular-Scale Condensed Matter(Lancaster University, 2024-05-10) Alanazi, Bashayr; Lambert, ColinFor nano- and molecular-scale applications, it is crucial to investigate and fully understand the electron transport properties of molecular junctions made up of a scattering region like a molecule coupled to metallic electrodes. The electrical properties of two different kinds of two terminal junctions are presented in the theoretical work contained in this thesis: one deals with gold electrodes, which form gold-molecule-gold structures and the other has single-layer graphene forming a gold-molecule-single-layer-graphene junction. In this thesis, the above investigations into the electrical and thermoelectric properties of molecular junctions utilize the theoretical techniques covered in chapters 2 and 3. Chapter 2 presents an introduction to the density functional theory (DFT). It is followed by an outline of transport theory in Chapter 3, based on Green’s function formalism. Chapter4 represents a study of the electron transport properties of the single-molecule/bilayer molecular junctions, formed from Zinc Tetraphenyl Porphyrin (ZnTPP), small graphene-like molecules (Gr), three derivatives with pyridine backbones, and three alkyl-chain backbones terminated with asymmetric anchor groups: amine (NH2 ), and a direct carbon (CH2 ) bond. Chapter5 studied the same core molecules, junctions with asymmetric electrodes which are gold and a single-layer graphene sheet (SLG).9 0Item Restricted Theory of quantum transport in nano scale structures(Lancaster University, 2024-01-29) Alharbi, Bader; Lambert, ColinIn the pursuit of future nano-scale applications within the field of molecular electronics, extensive investigations into electron transport through single molecules hold significant importance. As single or multiple molecules serve as crucial building blocks for designing and constructing molecular electronic devices, comprehending their electronic and transport properties becomes imperative. Countless theoretical and experimental studies have been conducted to create molecular junctions and explore their electrical performance. This thesis focuses on fundamental aspects of transport theory, employing theoretical and mathematical approaches to investigate electron transport through junctions, particularly involving a scattering region formed by a single molecule connected to metal electrodes. The research methods used are based on a combination of density functional theory, implemented within the SIESTA code, and non-equilibrium Green's function, realized using the GOLLUM code, to delve into electrical conductance on a molecular scale. The objective of this chapter is to address a puzzling paradox concerning meta connectivity, which exhibits destructive quantum interference (DQI) in a tight binding model. However, in certain instances, DQI does not manifest in a DFT calculation on the same system. To shed light on this inconsistency, a selection of molecules is examined, focusing on the distinction between meta and para connectivity. Two different types of linkers, thiol (-SH) and methyl sulphide (-SMe), are employed to couple different molecules to Au electrodes. Through this investigation, we aim to gain insights into the underlying factors that lead to the observed quantum interference behaviors. In project two, we conducted a comprehensive study, combining experimental and theoretical approaches, to explore charge transport in stacked graphene-like dimers. Our findings revealed that the interaction between room-temperature quantum interference and stacking significantly influences their highly non-classical electrical conductance. Notably, for the molecule CQI-L, the electrical conductance of the dimer exceeds that of the monomer by a remarkable factor of 25, attributed to the most energetically favorable stacking interactions. Conversely, for the molecule CQI-H, the dimer's conductance is approximately 40 times lower than that of the monomer. These results unequivocally demonstrate that precise control of connectivity to molecular cores, coupled with stacking interactions between their systems, provides a versatile avenue for modifying and optimizing charge transfer between molecules. This discovery is expected to inspire further vigorous research at both macroscopic and microscopic levels.19 0Item Restricted Theory of Quantum Transport and Molecular Electronics(2023-05-17) Al Malki, Wafa; Lambert, ColinThis thesis has investigated the electronic properties of organic molecular junctions, which form gold|single-molecule|gold structures, using a combination of density functional theory (DFT) and the equilibrium Green’s function formalism of transport theory. The first project considers the role of symmetric and asymmetric anchor groups for controlling charge transfer in molecular junctions, and how that relates to the position of the electrode Fermi level relative to the molecular resonances through various molecules. The results indicated that symmetric anchors such as thiol and pyridyl yield pinning of the electrode Fermi level to the resonances, whereas this pinning behaviour does not appear with asymmetric anchors such as pyridyl and SMe. It is found that changing the basis set has no effect on the transmission curves, when the pinning occurs, and that is likely due to the shape of the gold surface. This behaviour does not occur in the absence of the tip gold with using SAMs approach, which clarifies that the shape of the electrode plays a role in controlling the relative position of its Fermi energy. The second project concentrates on studying the effect of vibrations of the molecule on electron transport, and how it controls the value of the Seebeck coefficient. Different side groups are investigated for a series of fluorene-based molecules. The results indicated that the Seebeck coefficient of meta-connected molecules is more sensitive to the fluctuations of the side groups compared to the para case, which means that varying the pendant groups in molecular junctions can be exploited to control quantum interference (QI) and enhance the thermoelectric properties. Although C≡C and C≡N pendant groups behave similarly at room temperature, the transmission behaviour of the whole molecules different, where most of the curves in the C≡N case have this parallel shape (i.e., the anti-resonance is alleviated), leading to more enhancement in the Seebeck. In contrast, a less rigid pendant group such as C13(𝐹5)2 results in a large shift of the resonances causing broader conductance and Seebeck distributions. It is believed that the charge distribution of fluorine atoms could produce a gating environment on the backbone of the molecule leading to this large shift. The final project investigates the electron transport behaviour of para and meta connected molecules with and without the metal atoms, due to the presence of PdCl2. Introducing a metal atom into the path of the molecule creates another transmission path, which may play a role in disrupting or changing the QI. The complex structure of these molecules leads to multiple binding locations for attachment to gold electrodes and therefore multiple junction configurations can be formed. The results showed that the addition of the palladium with the chlorine atoms to the ligand molecules enhanced the conductance for both connectivities through the SMe anchor group due to the shift of the LUMO resonance to the Fermi energy. However, the different junction geometries do not enhance the Seebeck value.26 0