Search

In 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 interferencebehaviors. 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 signi

In recent years, heterojunctions and devices consisting of two-dimensional material stacks held together through van der Waals (vdW) forces have gained a lot of attention. In particular, graphene is a promising 2D material for use in single-molecule junctions, due to its high mechanical strength and robust chemical stability at room temperature. In the case of conventional metal electrode junctions, the molecules are attached to the metal electrodes through anchor groups at both ends. Through this configuration, electrons are transported along the molecular backbone, which means they travel from the left electrode to the right electrode via anchors attached to the molecule. Due to this, the size of the device corresponds to the length of the molecule. In contrast, in this thesis, a series of selected molecules was successfully sandwiched between two-dimensional graphene electrodes via vdW interactions to create single-molecule two-dimensional van der Waals heterojunctions (M-2D-vdWHs). In this case, electrons are transported between two graphene electrodes in a cross-plane manner, and the size of the device is determined by the molecule's thickness, rather than it’s length. This thesis investigates cross-plane charge transport in graphene-based single-molecule van der Waals heterojunctions (M-2D-vdWHs). The results presented in this thesis are computed using SIESTA, which is a density functional theory (DFT) code that solves the Kohn-Sham self-consistent equations. This is then combined with the Gollum code to obtain electron transport properties. The resulting predictions are compared with the experimental results obtained using a newly developed cross-plane break junction (XPBJ) technique. In this thesis, two collaborative research projects have been undertaken, whose results are summarized below. First, I have investigated charge transport through three well-defined molecular bilayer-graphenes (MBLGs), which consist of two vertically stacked graphene nanoflakes b

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

This 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 l

An important issue with photocatalysts in producing renewable energy such as hydrogen is the low efficiency in harvesting solar energy. Fortunately, the deposition of small metal clusters such as silver (Ag5) could enhance the photocatalysts’ absorption range to match visible light. In this thesis, I simulated different photocatalysts formed from Ag5 atomic quantum clusters (AQCs) on anatase TiO2 (101), rutile TiO2 (110), and CeO2 (111). Using the Vienna ab initio simulation package (VASP), I employed the generalised gradient approximation (GGA) and a hybrid functional combined with Hartree Fock (HF) theory to systematically study their geometric and electronic structures. The Hubbard term (U) is considered for all the calculations to account for the strongly interacted and localised d and f electrons. The first focus of this thesis is to elucidate the photocatalytic activities of rutile and anatase TiO2 surfaces. An in-depth investigation of stoichiometric and reduced rutile TiO2 (110) and anatase TiO2 (101) decorated with bipyramidal and trapezoidal Ag5 atomic quantum clusters (AQCs) is carried out. It is found that the silver AQCs donate electrons to both stoichiometric and reduced TiO2 surfaces resulting in the formation of a single polaron at either a five-fold coordinated (Ti5c) atom or a six-fold coordinated (Ti6c) atom, indicating improved surface activity. Furthermore, depositing Ag5 AQCs on both TiO2 surfaces can produce mid-gap states within the band gap of the bulk, thereby improving the optical response of the composite in the visible and infrared. As expected, the number of mid-gap energy states increases further when a single oxygen vacancy is introduced into the studied surfaces, which reveals that Ag5 AQCs and oxygen vacancies can reinforce each other, leading to higher efficiency photocatalytic activity. We also find that upon adsorption of Ag5 AQCs on an anatase TiO2 (101) surface, the energy required to form an oxygen vacancy is lower than that o

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 clu

For 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).

In developing future nano-scale applications in the molecular electronics field, studies of electron transport through single molecules are of fundamental interest. Since single or multiple molecules are considered essential building blocks to design and construct these molecular electronics devices, understanding their electronic and transport properties is required. Enormous theoretical and experimental studies were carried out to make the molecular junctions and explore their electrical performance. Within this framework, this thesis addresses some of the fundamental aspects of transport theory, involving the theoretical and mathematical approaches to investigate the electron transport via junctions, including a scattering region formed from a single molecule connected to metal electrodes. The methods used in this research are based on a combination of density functional theory and its realisation within the SIESTA code, and non-equilibrium Green’s function embodied in the GOLLUM code to study electrical conductance on a molecular scale. In this thesis, I focus on various N-Heterocyclic carbenes (NHC) complexes of double NHCanchored single-molecule junctions and investigate the mechanism of charge transport through their molecular junctions. I also study their electronic structure properties, such as the wave function plot and their binding energetics to electrodes. Experimental measurements and my DFT simulations revealed a high electrical conductance of monomer and dimer NHC molecular junctions. Consequently, my simulations provide a novel strategy for designing high conductance molecular devices using NHCs and are a step along with the roadmap toward future integration of molecular electronic devices.

As a result of recent developments, alkane derivatives have been used in many applications, including sensors for safety and security. This thesis provides a theoretical contribution to these developments by exploring molecular junctions formed from nearly 40 alkane derivatives including linear chains and rings. This study also employs 4 different anchor groups such as amine, thiol, direct carbon and thiomethyl. Within this thesis, I will provide a simple introduction to the theoretical tools used to explore electron transport through single-moelcule junctions. In Chapter 2, Density Functional Theory (DFT) and its applications within the SIESTA code will be discussed. This allows computation of ground state wave functions for molecules and provides the underlying Hamiltonians for molecular junctions which are used as a starting point for transport calculations. Chapter 3, provides the theoretical framework that is used to calculate electron transport properties such as electrical conductance and thermopower . This is based on Green’s and Dyson’s equation and is embodied in the GOLLUM code, which is the second major tool used during in this thesis. In chapter 3, I present some solutions of Green’s functions for infinite and semi-infinite chains and the transmission coefficient equations that implemented in the GOLLUM code. Chapter 4 is the first results chapter in this thesis, which provides thorough and vigorous theoretical investigations about series of alkane chains using different anchor groups including amine (NH2), thiol (S), direct carbon contact (C), and thiomethyl (SMe). Thus, I demonstrate the impact of using different terminal groups on the electrical conductance of alkane molecules. As expected, the conductance of alkane chains decreases exponentially with length, regardless of the type of anchor groups. However, the precise value of the conductance differs from one anchor to another. The theoretical predictions of the four anchors are checked against the

The focus of this thesis is both to improve the characterisation of hyperelastic materials and to develop a simple hyperelastic constitutive model for different composites materials, including woven fabric reinforcements with a hyperelastic matrix. Physical tests are performed on PVC/nitrile elastomer with woven continuous nylon reinforcement composite sheet under loading under uniaxial extension, pure shear, picture frame and bulge tests achieved via wide strip tension testing. Through the novel use of an advanced non-contact optical strain measurement technique, the hyperelastic material behaviour of the composite is investigated, and materials parameters reported for both the warp and the weft directions of reinforcement fibre alignment. To characterise the materials, an appropriate constitutive model is determined by fitting experimental shear and uniaxial tension data. The non-contact technique is used to acquire normal and shear strains at the surface of the composite sheet material when loaded to tensile strains (stretches). Directly measured shear strains are compared to those derived from the normal strain outputs of an optical rectangular strain rosette array, where the two measures are in close agreement. The measured mechanical behaviour under loading is used to determine an approximate strain energy function for the composite via ABAQUS software hyperelastic materials modelling curve fitting, with the Ogden and Yeoh hyperelastic models showing reasonable agreement to experimental data. A simple hyperelastic constitutive model is developed to investigate nonlinear mechanical properties of composites (loaded to large deformations) made of an elastomeric matrix containing biased woven fabric reinforcement. The strain energy function of the developed constitutive model is decomposed into four parts via a series of strain energy contributions. These include the strain energy from the matrix, the tensile energy from fibre elongation in the warp and weft direc

This thesis investigates the fundamental aspects of the molecular-scale junctions and their electrical properties. Experimental and theoretical studies assessed the importance of finding ways to get consistent and reproducible improvements in electronics devices fabrications techniques. In this context, I will start this thesis by introducing a general discussion about using the density functional theory (DFT), and Green's function to study transport calculations at a molecular scale. Then, I will present the theoretical and experimental benzo-(bis)imiadzol molecular studies in chapter 4. This study focusses on changes in the conductance as a consequence of chemical stimuli. I will demonstrate that benzo-bis(imidazole) conductance switching upon protonation depends on the lateral functional groups. The protonated H-substituted molecule shows a higher conductance than the neutral one (Gpro>Gneu), while the opposite (Gneu>Gpro) is observed for a molecule functionalized by amino-phenyl groups. Based on theoretical calculations, I conclude that these opposite behaviours depend on the electronic coupling between molecules and electrodes. Furthermore, quantum interference properties have recently attracted excessive interest in electron transport studies at the single-molecule scale. Within this framework, myself and collaborators have aimed to improve the efficiency of pi-stacked molecules in controlling quantum interference by carefully designing them. Chapter 5 introduces a novel strategy for designing folded carbazoles with low conductance in different structures. This strategy highlights the presence or absence of destructive quantum interference in different configurations. This project is part of a collaborations with the experimental group at the University of Madrid, which is ongoing at present.

This thesis addresses the fundamental aspects of controlling transport through organic molecules by presenting a series of studies in the electronic properties of molecular junctions. The exploration and understanding of the electronic characteristics of single molecules connected to electrodes is an essential part in the application of electronics. Here, I implemented transport calculations based on the Landauer formula combined with Kohn–Sham orbitals extracted from density functional theory (DFT). Chapter 4 elucidates the validity of a ‘curly arrow rule’, which has been used widely by chemists and physicists to predict the electronic properties of molecular junctions. Anthraquinone is found to break this rule in the case of meta connectivity to electrodes. This is significant, because changing the redox state of meta-connected dihydroxyanthracene to meta-connected anthraquinone, increases the conductance by a couple of orders of magnitude, due to the transition from constructive to destructive QI, which can help in the design of the QI based single-molecule switches such as data storage elements. Finally, chapter 5 presents a theoretical investigation of electron transport through dimethyldihydropyrene (DHP) and Cyclophanediene (CPD) systems focusses on changes in the conductance as a consequence of photochemical stimuli. These molecules could be exploited in the function of electronic devices, when responding to external stimuli

Understanding the electronic transport properties of junctions consisting of a scattering region such as a nanoscale region or molecule connected two electrodes is of fundamental interest. 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 with graphene sheet electrodes forming graphene | molecule | graphene junctions. Chapter 2 presents an introduction to the theoretical concept of density functional theory (DFT) and its implemented in this thesis, via the SIESTA code. The second tool is the quantum transport code Gollum which is based on Green’s function-based scattering theory. 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 original topic I investigate in chapter 4 addresses anti-resonance features of destructive quantum interference in single-molecule thiophene junctions. This study is a collaborative work with experimentalists in Xiamen University, China. Controlling the electrical conductance and in particular the occurrence of quantum interference in single-molecule junctions through gating effects, has potential for the realization of high-performance functional molecular devices. In this work, I demonstrate the underling science behind the tunable electronic structure of thiophene-based molecular junctions using electrochemically-gated. This is explained by destructive quantum interference (DQI) features in these molecules. Using electrochemical gating the Fermi energy is moved towards the DQI feature leading to two orders of magnitude changes in electrical conductance. This is a promising strategy for obtaining improved in-situ control over the electrical performance of interference-based molecular devices. In

The study presents the Landauer formula and Green’s function approach for analysing the scattering processes in a system attached to infinite one-dimensional leads. The study involves the calculation of the retarded Green’s function in which the simple formula of a one-dimensional tight binding chain in presented. The study involves mathematical aspects of solving the Schrodinger equation in open systems with a view to developing new conceptual approaches to scattering theory. Efficient schemes to obtain scattering matrices from mean-field Hamiltonians are developed and these are implemented in new numerical codes.

In this thesis we use a combination of density functional theory and equilibrium Green’s function to study thermoelectricity in molecular scale. We have aimed to improve the efficiency of single molecules in converting heat to electricity by carefully designing them. We introduced a novel strategy for designing molecules with low thermal conductance. This strategy states that adding side branches of different length to the backbone of a molecule can eliminate phonon transport through the backbone over a wide range of frequencies. Moreover, we demonstrate that chemical modification of thiophene molecular wire to ethylenedioxy-thiophene molecular wire can improve their thermoelectric efficiency. Furthermore, to demon- strate that the adopted molecular designs will be effective when scaled up to a self-assembled monolayer (SAM) of molecules, we model three independent exper- iments on gold-SAM-graphene vertical transport devices. The agreement between our calculations and the experiments elucidates the survival of quantum interfer- ence effect on a single molecular level in SAM devices. The work presented in this thesis has received considerable attention from many experimental groups in the UK and overseas stimulating novel experimental studies which are ongoing at present.

A molecular junction consists of a single molecule or self-assembled monolayer (SAM) placed between two electrodes. It has varieties of functionalities due to quantum interference in nanoscale. Although there exist issues, advantages could still appeal to scientists who wish to investigate transport properties in many aspects such as electronics, thermoelectronics, spintronics, and optotronics. Recent studies of single-molecule thermoelectricity have identified families of high-performance molecules. However, controlled scalability might be used to boost electrical and thermoelectric performance over the current single-junction paradigm. In order to translate this discovery into practical thin-film energyharvesting devices, there is a need for an understanding of the fundamental issues arising when such junctions are placed in parallel. As a first step in this direction, we investigate here the properties of two C60 molecules placed in parallel and sandwiched between top and bottom graphene electrodes. It is found that increasing the number of parallel junctions from one to two can cause the electrical conductance to increase by more than a factor of 2 and furthermore, the Seebeck coefficient is sensitive to the number of parallel molecules sandwiched between the electrodes, whereas classically it should be unchanged. This non-classical behaviour of the electrical conductance and Seebeck coefficient are due to interjunction quantum interference, mediated by the electrodes, which leads to an enhanced response in these vertical molecular devices. Except the study of thermoelectricity, on the other hand, stable, single-molecule switches with high on-off ratios are an essential component for future molecularscale circuitry. Unfortunately, devices using gold electrodes are neither complementary metal-oxide-semiconductor (CMOS) compatible nor stable at room temperature. To overcome these limitations, several groups are developing electroburnt graphene electrodes for singl

In recent years, driven by the need to downsize to the molecular level, advances in technology has made the manufacture of nanoscale devices possible. In this thesis I will investigate the theoretical electronic and thermal properties of a carbon based class of nanoscale materials by examining the possibility of using fullerenes and decachlorofullerenes as building blocks towards viable molecular scale devices. In particular I have looked at ways to enhance the electronic communication between fullerenes through introducing exohedral varients, which I found to have a positive effect on the electronic and thermoelectric properties. The methods used in this work are based on density functional theory, combined with quantum transport calculations using Greens functions. Fullerenes are promising building blocks for nano-scale electronics, as they have relatively large spherical surface area and are geometrically symmetric. If fullerenebased thermoelectricity is to become a viable technology, then fullerenes exhibiting both positive and negative Seebeck coefficients will be needed and therefore I also calculate the thermoelectric properties of the naturally occurring fullerene C60 known as the buckyball together with an exohedral example namely C50Cl10. C60 is known to have a negative Seebeck coefficient of S which varies in the range of - 18 to -23 µV/K and therefore in this thesis I address the challenge of identifying a fullerene with a positive-Seebeck-coefficient. I investigated the thermoelectric properties of single-molecule junctions of the exohedral fullerene C50Cl10 connected to gold electrodes and found that it has a positive Seebeck coefficient. Furthermore, in common with C60, the Seebeck coefficient can be increased by placing more than one C50Cl10 in series. For a single C50Cl10, I find S=+8 µV/K and for two C50Cl10’s in series I find S=+30 µV/K. I also find that the C50Cl10 monomer and dimer have power factors of 0.5×10-5 W/m.K2 and 6.0×10-5 W/m.K2 respec

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

Optoelectronic properties of metallo-porphyrins play a central role in photosynthesis and are therefore crucial to life on earth. This thesis presents a series of studies into the electronic and thermoelectric properties of various families of molecular junction of metalloporphryins Two main techniques will be included in the theoretical approach; Density Functional Theory, which is implemented in the SIESTA code, and the Green’s function formalism of electron transport (Chapter 2), which is implemented in the GOLLUM code. Both techniques are used extensively to study a family of metallo-porphyrin molecules. In this thesis, I cover three main results in the areas of electical and thermoelectrical properties of metallo-porphyrin molecular wires, in which a Co, Ni, Cu, or Zn metal ion in the center of the porphyrin skeleton is coordinated to pyridyl moieties attached to gold electrodes and demonstrate that the current-perpendicular-to-the-plane (CPP) electrical conductances of the series of Ni, Co, Cu or Zn-5,15-diphenylporphyrins increase with the atomic weight of the divalent metal ion. This supramolecularly wired arrangement with the aromatic plane perpendicular to the current is stable at room temperature and provides a unique family of high-conductance molecular wires, whose electrical transport properties can be tuned by metal substitution. I deal with the thermoelectric properties of the same metallo-porphyrin junction (CPP) in chapter four, where I demonstrate that varying the transition metal-centre of a porphyrin molecule allows the molecular energy levels to be tuned relative to the Fermi energy of the electrodes thereby creating the ability to optimise the thermoelectric properties of metallo-porphryins. In chapter six I compare thermoelectric properties of three zinc porphyrin (ZnP) dimers and a ZnP monomer. The results show that the “edge-over-edge” dimer formed from stacked ZnP rings possesses a highest room temperature ZT ever reported for an organic m

This thesis presents a series of studies into the electronic and thermoelectric properties of molecular junction single organic molecules: They include perylene Bisimide (PBIs), naphthalenediimide (NDI), metallo-porphryins and a large set of symmetric and asymmetric molecules. Two main techniques will be included in the theoretical approach, which are Density Functional Theory, which is implemented in the SIESTA code [1], and the Green’s function formalism of elctron transport (Chapter 2), which is implemented in the GOLLUM code [2], it is a next-generation code, born out of the non-equilibrium transport code SMEAGOL code [3]. Both techniques are used to extensively to study a family of perylene bisimide molecules (PBIs) (Chapter 3) to understand the potential of these molecules for label-free sensing of organic molecules by investigating a change in the electronic properties of PBI derivatives. Also, these techniques are used to simulate electrochemical gating of a single molecule naphthalenediimide (NDI) junction (Chapter 4) using a strategy to control the number of electrons on the molecule by modelling different forms of charge double layers comprising positive and negative ions. Chapter 5 will deal with the thermoelectric properties of the single organic molecule. I will demonstrate that varying the transition metal-centre of a porphyrin molecule over the family of metallic atoms allows the molecular energy levels to be tuned relative to the Fermi energy of the electrodes and that leads to the ability to tune the thermoelectric properties of metallo-porphryins. Chapter 6 will present our new approach to materials discovery for electronic and thermoelectric properties of single-molecule junctions. I will deal with a large set of symmetric and asymmetric molecules to demonstrate a general rule for molecular-scale quantum transport, which provides a new route to materials design and discovery. The rule of this approach that “the conductance of an asymmetric molecu

The theoretical work carried out in this thesis presents the electrical properties of two different types of two terminal molecular junctions: one dealing with gold electrodes which form gold |molecule| gold structures and the other with a graphene sheet and gold electrodes forming gold |molecule| graphene junctions. The theoretical tools employed are firstly, density functional theory (DFT). Chapter 2 presents an introduction to the theoretical concept of DFT and the implementation used in this work, 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 main results of this thesis are as follows: The first topic I identify Fano resonances in the transport properties of carbine-metal- amides and demonstrate that their energetic location and magnitude can be controlled by varying the connectivity of the core to external electrodes and by rotating the pendant moiety connected to the current-carrying core. The Fano resonances can be suppressed by rotating the pendant group and increasing the linkages to electrodes. Secondly, I compute the transmission coefficient, electrical conductance and thermoelectric properties of structures formed from terthiophene with tetracyanoethylene and terthiophene with dinitrotoluene. A theoretical investigation into the Seebeck coefficient in stacked molecular junctions is performed using a first principles quantum transport method. I show that the quantum interference produces Fano resonance in the gap between the HOMO and LUMO and the stacking geometry can control the position of this quantum interference feature. The shifting of this resonance enhances the thermopower as expected when the junction is tuned through a node in the transmission function. I also found that supramolecular interactions betw

This thesis addresses the question of the time spent by a transmitted wave packet within a scattering region. The study involves mathematical aspects of solving the Schrodinger equation in open systems with a view to developing new conceptual approaches to scattering theory. Efficient schemes to obtain scattering matrices from mean-field Hamiltonians are developed and these are implemented in new numerical codes. The relationship between the phase of S-matrix elements and Wigner delay times is also elucidated. I consider the scattering problem in a tight-binding lattice, as a simple way to understand the relation between M-functions and Greens functions and to investigate the connectivity dependence of Wigner delay times. To analyse delay times in bipartite lattices, tight binding calculations are used and a new computer code is developed to verify analytical predictions. In particular, Green’s functions and a mid-gap theory are used to calculate Wigner delay times for different connectivities in graphene like molecules. One interesting and counterintuitive result is that in the weak coupling limit at the middle of HOMO and LUMO gap, the Wigner delay time does not depend on the distance between the connections to external reservoirs.

In recent years, electron transport through single molecules has attracted huge attention, since the molecules are promising building blocks for the next generation of electronic devices. To create molecular junctions and probe their electrical properties, intense experimental efforts and theoretical studies are underway. For single molecule electronic applications, an important property is their electrical conductance. In this context, I start my thesis by introducing a general discussion about some basic topics related to single molecule transport theory. Quantum interference effects have recently attracted great interest in studies of the charge transport at the single molecule scale. Within this framework, I study the single molecule conductances of five-membered ring compounds to investigate the effect of molecular symmetry and quantum interference on the charge transport through single molecule junctions. This theoretical and experimental study highlights the presence of destructive quantum interference and more importantly reveals that the control of molecular asymmetry via the heteroatom substitution allows the tuning of destructive quantum interference. Theoretically, I identify similar features over some range of energies using different anchoring groups and elucidate the impact of the anchoring groups on the charge transport through single molecule. Moreover, I find that molecular symmetry has a slight effect on the binding energies of the 1-8 compounds seen in Figure 3.1. Within the phase coherent regime, electron transport through a single molecule junction is described by the transmission coefficient, which describes how electrons pass through a molecule from one electrode to the other. Predicting features related to the transmission coefficient is a powerful tool for probing the electronic structure of molecular systems. In this connection, mid-gap transport theory is considered an efficient and easy method, which utilizes a magic ratio rule (MRR) to

The behaviour of two graphene based structures has been theoretically investigated and analysed by using one or more tools. This tool kit consists firstly of SIESTA, a density functional theory software package, secondly Gollum, a non-equilibrium Green’s function code, and finally the tight binding approach. The first project considers the variation of the thermoelectric properties of a graphene-graphene junction functionalised by the amino-silane molecule. The second project studies the mechanical properties of the interface between a silicon-carbide substrate and monolayer graphene. The results of these two projects are summarised in the next paragraphs. The calculation of the thermoelectric properties of a graphene-silane-graphene junction reveals a number of interesting results. The most important result is that silane hinders the cross-plane electron transmission and thermal conductance. Such properties have effective applications through controlling the heat flow in the electronic chip. Furthermore, the silane molecule enhances the figure of merit of the junction which refers to the ability to convert heat. To sum up, silane-functionalized graphene has an improved heat mediation over a non-functionalised junction. The second project analyses the mechanical properties of the silicon-carbide/graphene junction. The study of this junction focuses on the trends in terms of stiffness and work function as the hydrogen concentration intercalating the interface and the number of graphene sheets on top of the silicon-carbide substrate varies. As a result of this study I have found that the effect of increasing the number of penetrating hydrogen atoms is to reduce the stiffness and to enhance the work function. The same situation is found for the stiffness when the number of graphene layers is increased. However the work functions shows two completely opposing behaviours; the first one can be seen in the quasi-free standing graphene layer type 1 and type 2, where the wor