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Title: Theory of electron transport through single molecules
Author: Al-Jobory, Alaa
ISNI:       0000 0004 6498 6614
Awarding Body: Lancaster University
Current Institution: Lancaster University
Date of Award: 2018
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The theoretical work carried out in this thesis presents the electrical properties of two different types of two terminal nanojunctions: one dealing with gold electrodes which form gold|molecule|gold structures and the other has carbon nanotube (CNT) electrodes forming CNT|molecule|CNT junctions. The theoretical tools employed are firstly, density functional theory (DFT). Chapter 2 presents an introduction to the theoretical concept of DFT and in this work the implemented version used, namely the SIESTA code. The second tool is the quantum transport code GOLLUM. To introduce this technique 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. The first topic I investigate is quantum interference based connectivity dependence in a series of molecular wires. Two isomeric series have been obtained with 4-ethynylpyridine units linked to the core either at para-para positions or meta-meta positions. A combined experimental and computational study is described, in which my work provides the theoretical understanding of the experiment. The conductance of these molecules is measured using a mechanically controlled break junction and density functional theory calculations, demonstrates consistently higher conductance in the para series compared to the meta series: this is in agreement with increased conjugation of the π-system in the para series. Within the para series conductance increases in the order of decreasing heteroaromaticity (dibenzothiophene < carbazole < dibenzofuran). However, the sequence is very different in the meta series, where dibenzothiophene . dibenzofuran < carbazole. Excellent agreement between theoretical and experimental conductance values is obtained. This study is presented in chapter 4 and establishes that both quantum interference and heteroaromaticity in the molecular core units play important and inter-related roles in determining the conductance of single molecular junction. Secondly, the electrical properties (band structure, open channel and transmission coefficient) was studied for different types of carbon nanotubes which act as electrodes, and two asymmetric molecules attached to the carbon nanotubes to form a nanojunction. There are two strategies used in this study: first the established DFT method and the second a parametrized tight binding approach. A four-orbital tight binding model (sp3) is used to construct the Hamiltonian and using Gollum to calculate transmission coefficients I find very good agreement with the transmission coefficient calculated by a DFT Hamiltonian. However, the tight binding approach is limited to carbon atoms only, at least in this work but it offers a more efficient calculation method and opens up the possibility to study the how different orbitals control transport and quantum interference. Also in chapter 5 using the DFT method, I compute the transmission coefficient and then IV curves to investigate rectification. In the chapter 6, I present a theoretical study of the conductance perpendicular to the plane of a series of polycyclic aromatic hydrocarbons. The smaller members of the oligoacenes up to and including anthracene are found to be insulators or semi-conductors but those with eighteen carbon atoms (tetracene) and over are found to be conductors which is in stark contrast to previous calculation of in- plane conductance and experiment which generally predict them to be insulators or semiconductors. The number of open conductance channels increases as the number of aromatic rings increases for the variants studied and these trends are found to persist for more complex geometries. Features in the electrical conductance around the Fermi energy suggest possible candidates for future thermoelectric devices.
Supervisor: Lambert, Colin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral