Optimised power harvesting by controlling the pressure applied to molecular junctions

A major potential advantage of creating thermoelectric devices using self-assembled molecular layers is their mechanical flexibility. Previous reports have discussed the advantage of this flexibility from the perspective of facile skin attachment and the ability to avoid mechanical deformation. In this work, we demonstrate that the thermoelectric properties of such molecular devices can be controlled by taking advantage of their mechanical flexibility. The thermoelectric properties of self-assembled monolayers (SAMs) fabricated from thiol terminated molecules were measured with a modified AFM system, and the conformation of the SAMs was controlled by regulating the loading force between the organic thin film and the probe, which changes the tilt angle at the metal-molecule interface. We tracked the thermopower shift vs. the tilt angle of the SAM and showed that changes in both the electrical conductivity and Seebeck coefficient combine to optimize the power factor at a specific angle. This optimization of thermoelectric performance via applied pressure is confirmed through the use of theoretical calculations and is expected to be a general method for optimising the power factor of SAMs

Mathematical modelling of quantum transport in nanoscale structures

Molecular electronics is a versatile test bed for investigating nanoscale thermoelectricity and for contributing to the design of new low-cost and eco-friendly organic thermoelectric materials. This thesis presents theoretical results which aid this design process, firstly through demonstrating the optimisation of thermopower in self assembled monolayers based on the pressure applied to the molecules, and secondly through a novel method of predicting thermoelectric properties based on experimental I-V curves. This thesis provides a brief introduction to the theoretical tools used, starting in chapter 2 with density functional theory and its implementation in the SIESTA code, which enables Hamiltonians and ground state wavefunctions for molecules and junction interfaces to be found. Subsequently in chapter 3 the theoretical basis for calculating electronic and heat transport is described, including Green’s function methods for obtaining the transmission coefficient of semi-infinite leads connected to a scattering region. The second tool is the quantum transport code GOLLUM. To introduce this approach, 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 a major potential advantage of creating thermoelectric devices using self-assembled monolayers (SAMs). Two anthracene based SAMs terminated with thioacetate are investigated: 9,10- di(4-(ethynyl)phenylthioacetate), and 1,5- di(4-(ethynyl)phenylthioacetate. I demonstrate that the thermoelectric properties of such molecular devices can be controlled by taking advantage of their mechanical flexibility, more specifically by tuning the optimum power via the applied pressure which alters the molecules’ tilt angle θ. Through systematic theoretical simulations, I show how varying θ increases the conductance G while decreasing thermopower S, ultimately achieving the optimum power P=G S^2 at θ ≈ 65. Excellent agreement has been obtained between my simulations and experimental measurements using conductive Atomic Force Microscopy (AFM) for both SAMs. The thermoelectric properties of SAMs fabricated from thiol terminated molecules were measured by my collaborators, with a modified AFM system, and the conformation of the SAMs was controlled by regulating the loading force between the organic thin film and the probe, which changes the tilt angle at the metal-molecule interface. The thermopower shift versus the tilt angle of the SAM was tracked and showed that changes in both the electrical conductivity and Seebeck coefficient combine to optimise the power factor P at a specific tilt angle. This optimisation of thermoelectric performance via applied pressure is confirmed through the use of my theoretical calculations and is expected to be a general method for optimising the power factor of SAMs. In chapter 5, I address the question of whether the Seebeck coefficient of a single molecule or SAM can be predicted from a measurement of I-V curves. If so, then the experimentally more difficult task of creating a set-up to measure their thermoelectric properties could be avoided, thus saving a significant amount of cost and effort. My theoretical approach begins by making a fit to measured G-V curves using three fitting parameters, denoted a,b and c, hence I refer to this method as ‘ABC’ theory. Then predicts a maximum value for the magnitude of the corresponding Seebeck coefficient. This is a useful material parameter, because if the predicted upper bound is large, then the material would warrant further investigation using a full Seebeck measurement setup. On the other hand, if the upper bound is small, then the material would not be promising and this much more technically demanding set of measurements would be avoided. Histograms of predicted Seebeck coefficients from the ‘ABC’ theory are compared with histograms of directly measured Seebeck coefficients using a Scanning Tunnelling Microscope (STM) device. This is done for six SAMs of anthracene-based molecules with different anchor groups including pristine thioether, pristine thioacetate, pristine pyridine and a mixture of thioether and pyridine. I show that excellent agreement is found in each case, both when using all three parameters a,b and c in the fitting procedure, and also when the number of parameters is reduced to two by setting c=0

Akaike Information Criterion and Fourth-Order Kernel Method for Line Transect Sampling (LTS)

Parametric and noparametric approaches were used to fit line transect data. Different parametric detection functions are suggested to compute the smoothing parameter of the nonparametric fourth-order kernel estimator. Among the different candidate parametric detection functions, the researcher suggests to use Akaike Information Criterion (AIC) to select the most appropriate one of them to fit line transect data. More specifically, four different parametric models are considered in this research. Where as two models were taken to satisfy the shoulder condition assumption, the other two do not. Once the appropriate model is determined, it can be used to select the smoothing parameter of the nonparametric fourth-order kernel estimator. As the researcher expected, this technique leads to improve the performances of the fourth-order kernel estimator. For a wide range of target densities, a simulation study is performed to study the properties of the proposed estimators which show the superiority of the resulting proposed fourth-order kernel estimator over the classical kernel estimator in most considered cases

Exploring a Graph Complement in Quadratic Congruence

In this work, we investigate essential definitions, defining G as a simple graph with vertices in Zn and subgraphs Γu and Γq as unit residue and quadratic residue graphs modulo n, respectively. The investigation extends to the degree of G, Γu, and Γq, illuminating the properties of these subgraphs in the context of quadratic congruences

Quantum correlations beyond entanglement between two moving atoms interacting with a coherent cavity

Studying the ability of atom-photon interactions, especially in two two-level atomic systems, to generate quantum information resources has recently become an important research topic in quantum information science. Therefore, this paper explores the ability of two moving atoms coupling with a coherent field through a two-photon transition to generate atomic quantum correlations by using local quantum uncertainty (LQU), local quantum Fisher information (LQFI) as well as logarithmic negativity (LN). Schrödinger equation is used to obtain the time evolution of the atom-cavity-atom interactions with an initial coherent cavity state and an initial atomic uncorrelated pure state. The generation of atomic LQU, LQFI, and LN correlations are exactly examined under the unitary interaction parameter effects, including the atom-cavity coupling strengths, the cavity field half-wave number, and the initial coherent state intensity. The atom-cavity-atom interaction parameters lead to notable changes in the amplitudes, speed, and regularity of the LQU, LQFI, and LN dynamics, which can be enhanced by increasing the initial coherent intensity. The cavity field half-wave number leads to generating atomic quantum correlations with regular oscillatory behavior. The sudden death-birth phenomenon of the logarithmic negativity depends on the atom-cavity-atom interaction and the atomic location parameter

Local quantum Fisher information and Jensen-Shannon coherence dynamics of two-spin-qubits XYZ-Heisenberg state

In this study, we investigate the impact of the XYZ-DM-Heisenberg model on quantum resources, including local quantum Fisher information, concurrence, and Jensen-Shannon coherence, in the presence of intrinsic decoherence. By exploring various spin-exchange interactions, with a focus on the role of the y-DM interaction, we reveal how these interactions affect quantum correlations and coherence dynamics. We find that the system transitions from separability to the generation of quantum properties when spin interactions are introduced. The specific parameter choices significantly influence the dynamic map of quantum functions. Notably, strong y-DM interaction, in combination with equal spin interaction, leads to pronounced fluctuations, while weak DM interaction results in sudden death and rebirth in the x direction. Additionally, ferro- and anti-ferromagnetic regimes impact the quantum functions differently. In the presence of intrinsic decoherence, fluctuations decrease, and coherence remains robust compared to the local quantum Fisher information and entanglement, reducing loss

A Novel Framework of <i>q</i>-Rung Orthopair Fuzzy Sets in Field

In this manuscript, we proposed a novel framework of the q-rung orthopair fuzzy subfield (q-ROFSF) and illustrate that every Pythagorean fuzzy subfield is a q-rung orthopair fuzzy subfield of a certain field. We extend this theory and discuss its diverse basic algebraic characteristics in detail. Furthermore, we prove some fundamental results and establish helpful examples related to them. Moreover, we present the homomorphic images and pre-images of the q-rung orthopair fuzzy subfield (q-ROFSF) under field homomorphism. We provide a novel ideology of a non-standard fuzzy subfield in the extension of the q-rung orthopair fuzzy subfield (q-ROFSF)

Erratum:Molecular-scale thermoelectricity: as simple as 'ABC' (Nanoscale Adv. (2020) 2 (5329–5334) DOI: 10.1039/D0NA00772B)

The authors regret that the name of one of the authors (Troy L. R. Bennett) was shown incorrectly in the original article. The corrected author list is as shown above. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers

Multi-component self-assembled molecular-electronic films:towards new high-performance thermoelectric systems

The thermoelectric properties of parallel arrays of organic molecules on a surface offer the potential for large-area, flexible, solution processed, energy harvesting thin-films, whose room-temperature transport properties are controlled by quantum interference (QI). Recently, it has been demonstrated that constructive QI (CQI) can be translated from single molecules to self-assembled monolayers (SAMs), boosting both electrical conductivities and Seebeck coefficients. However, these CQI-enhanced systems are limited by rigid coupling of the component molecules to metallic electrodes, preventing the introduction of additional layers which would be advantageous for their further development. These rigid couplings also limit our ability to suppress the transport of phonons through these systems, which could act to boost their thermoelectric output, without comprising on their impressive electronic features. Here, through a combined experimental and theoretical study, we show that cross-plane thermoelectricity in SAMs can be enhanced by incorporating extra molecular layers. We utilize a bottom-up approach to assemble multi-component thin-films that combine a rigid, highly conductive ‘sticky’-linker, formed from alkynyl-functionalised anthracenes, and a ‘slippery’-linker consisting of a functionalized metalloporphyrin. Starting from an anthracene-based SAM, we demonstrate that subsequent addition of either a porphyrin layer or a graphene layer increases the Seebeck coefficient, and addition of both porphyrin and graphene leads to a further boost in their Seebeck coefficients. This demonstration of Seebeck-enhanced multi-component SAMs is the first of its kind and presents a new strategy towards the design of thin-film thermoelectric materials