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  • ItemOpen Access
    Photophysical Studies of Active Layer Compositions in Organic Solar Cells
    Unson, Darcy
    For the last few decades, inorganic solar cells, such as gallium arsenide and silicon, have dominated research and commercial ventures. Commercial silicon-based solar cells have particularly expanded in recent years, including significant rises in small-scale home solar installations and solar farms spanning several acres. Organic solar cells (OSCs), based on organic semiconductors, have also recently attracted much attention within the Internet-of-Things community because of their ability to be integrated into low-energy off-grid devices and because of their spectral sensitivity making them good candidates for indoor solar applications. OSCs possess a unique quality in that they are solution processed. This enables them to be deposited on flexible substrates which can be scaled-up easily and cheaply. OSCs rely on a donor-acceptor configuration in the active layer. An inherent characteristic of OSCs is that they form tightly bound electron-hole pairs (excitons) following photoexcitation and as these excitons separate, they form charge-transfer (CT) states. The formation and characteristics of excitons and CT states are heavily dependent on the active layer composition and morphology, and they can affect device performance by promoting charge transport processes or loss pathways. In this thesis, we explore methods for suppressing loss pathways arising from these excited states. We explore the effectiveness of increasing spacing between donor-acceptor active layer components to suppress intermolecular interactions, via an encapsulation process. We use encapsulation to ‘sheath’ a donor polymer sub-unit which has been associated with the formation of a particular type of CT state. Morphological and photophysical techniques are used to rationalise changes in device performance following encapsulation. Macrocyclic molecules are also explored as a method to control packing and we apply this to the acceptor component of the active layer. The unique cavity that forms following macrocyclisation offers interesting packing types upon addition of a polymer. We explore the morphological and photophysical implications of this structural change in the neat material and when blended with a widely used donor material. OSCs also have a high tendency to form triplet states which contribute to non-radiative voltage losses and limit charge extraction in working solar cells. We finally propose integrating photon upconversion via triplet-triplet annihilation (TTA-UC) into an OSC to recycle the low-energy triplets. This should effectively give these excitations a second chance to contribute to the photocurrent. We show the presence of TTA-UC in an organic solar cell, propose a mechanism through which the process occurs and investigate the factors which currently limits its contribution to the overall device efficiency.
  • ItemOpen Access
    Ultrafast Dynamics and Interactions During Growth at Surfaces
    Kelsall, John
    Helium-3 spin-echo (3HeSE) is a helium atom scattering technique capable of studying nanoscopic surface dynamics in thermal equilibrium. The energy resolution, which greatly exceeds that of alternative methods, makes it possible to probe motion on picosecond to nanosecond timescales. The present thesis is concerned with 3HeSE measurements and modelling of ultrafast dynamics in surface systems with interadsorbate interactions that induce the formation of islands. To this end, we have described investigations into both O/Ru(0001) and S/Ni(111) at elevated temperatures (850 K and 400 K, respectively). In the former, although the attractive forces are weak, 3HeSE is able to resolve their influence. By optimising kinetic Monte Carlo (kMC) simulations against the experimental data and analysing the results, we are able to comment on quantities such as the island lifetime as a function of size. Such information is important for exploring the growth dynamics of substrate-supported 2D materials. S/Ni(111), by contrast, demonstrates much stronger islanding. The system is important in catalysis, as sulfur is a common poison in industrial processes. The periodicity of the clusters is different from that of Ni(111), due to the strong chemisorption of the adatoms causing a reconstruction of the first nickel layer. Even so, the islands have a clear impact on the diffusion signature, demonstrating that they are not static and thus that a dynamic equilibrium is established between the clusters and their mobile precursors. We have also studied O/Ru(0001) at high coverages (above 0.5 ML), when repulsion dominates and three-body interactions are relevant. The work constitutes the first attempt at interpreting 3HeSE measurements in terms of many-body forces, and reveals the state of the surface at the onset of oxidation. In the second half of the thesis, we turn to instrumentation. We first discuss the design and manufacture of new 3HeSE spin precession solenoids with a trapezoidal profile. The coils will dramatically improve the instrument resolution, by a factor of approximately four for the same beam energy, when combined with new power supplies. However, the aberrations of the field - defined as the deviation of the off-axis field from the on-axis value - lead to a reduction in polarisation which increases rapidly as the current rises. Much of the corresponding chapter is therefore devoted to modelling this `depolarisation', using analytic calculations and numerical simulations. The desire for an improved beam resolution was inspired by our measurements of islanding, which would have benefited from the ability to study slow cluster decay. On a similar note, we also describe and model a new design for the spin rotator coils of the instrument, which reorient the 3He spins to account for the scattering geometry. The principal improvement is a change in their position, from the centre of the scattering chamber to the incoming arm, which will facilitate the study of high-energy phonons. The upgraded spectrometer will therefore be capable of measuring the growth dynamics in a wide array of 2D materials, and able to characterise their phonons, which provide an important indication of surface quality. Finally, we describe a semi-analytic method to calculate the attenuation of the beam in the source chamber of the 3HeSE instrument, which is typical of that in many atom scattering machines. There are two components, due to (i) the background gas and (ii) atoms back-scattered from the skimmer and mount. We demonstrate that at room temperatures, the former dominates, suggesting that the design of the skimmer has an insignificant impact on room temperature beams. We derive a number of scaling relationships and analytic formulae which enable us to comment on the design of future source chambers. Our results will allow skimmer interference to be readily incorporated into future calculations of e.g. centreline intensities.
  • ItemEmbargo
    Probing the Behaviour of Antiferromagnets: Electric and Magnetic Measurements Across a Range of Length-scales
    Bowen, Richard Denzil
    The spin Hall effect (SHE) describes the generation of a spin current orthogonal to an applied charge current within a heavy metal, whilst the spin Seebeck effect (SSE) refers to the generation of a spin current due to thermal gradients within a magnetic material. Both effects have attracted significant interest in recent years as means of generating spin currents and probing the behaviour of magnetic materials, with potential spintronic applications in data storage. The simultaneous action of the SHE, and its reciprocal effect, the inverse spin Hall effect (ISHE), is termed spin Hall magnetoresistance (SMR), and allows the magnetic behaviour of adjacent magnetic materials to be explored. The SSE provides information into the magnonic excitations within the magnetic materials themselves instead. These effects were measured for a number of antiferromagnets, and compared to magnetostatic measurements. SMR is primarily dependent on the interfacial properties of adjacent magnetic materials to the heavy metal, whereas the SSE probes deeper into the magnetic material, and so it was proposed that any differences between the magnetic behaviour over these different length scales could be determined. In general, these effects have been studied previously in relatively straightforward antiferromagnets, such as MnF2. However, the complexity of behaviour displayed in other antiferromagnets could provide a rich playground with which to produce previously unexpected effects. Such complexity is observed in hematite within this thesis. Often oversimplified to a simple uniaxial antiferromagnet when spin transport is considered, by using a different orientation of crystal in this thesis to that typically considered, it was possible to investigate this complexity. It was then necessary to extend previous theories, particularly when considering the antiferromagnetic resonance of 𝑘 = 0 magnons. This was then subsequently applied to explain the SSE voltages observed, which would not have been predicted by simpler analyses. Two other classes of antiferromagnets were then investigated. KMF3 compounds had previously shown interesting ultrafast properties, but their SMR and (DC) SSE properties had not previously been investigated. Here, the magnetostatics and SMR properties of KCoF3 are obtained, but the reported theory of domain wall motion within this class of compounds could not explain the observed behaviour. It was instead necessary to consider a combination of spin flop transitions and domain wall motion, demonstrating a lack of homogeneity which had been ignored in previous experiments. Within the programme, a completely separate study into the rare earth orthoferrite, HoFeO3, was then carried out, which has a complex magnetic phase diagram on account of the temperature-dependent Fe-Fe, Fe-Ho and Ho-Ho interactions. Little is known about the magnetic behaviour of this material, and even a complete description of its magnetostatics has not been reported previously. Since SMR primarily probes the Néel vector, a quantity inaccessible to standard macroscopic magnetic measurements, it was proposed that the combination of SMR and SSE measurements could actually be used as a diagnostic tool for magnetic behaviour. In spite of experimental difficulties, sufficient progress was made to justify this approach to the characterisation of magnetic behaviour. This study has demonstrated that the use of a simplified picture of antiferromagnets may miss significant aspects of magnetic behaviour, and as such it is necessary to introduce more complex models if the true potential of these materials is to be harnessed.
  • ItemEmbargo
    Nanoscale device engineering and plasmon-enhanced light – matter interactions for the characterization of 2D materials
    Symonowicz, Joanna
    The PhD thesis introduces nanoscale tools for optoelectronic characterization of 2D materials, addressing limitations of existing techniques including destructiveness, imprecision, and vacuum requirements. Three novel characterization methods are proposed. They all employ gold nanoparticles as electrical contacts, offering both nano-sized electrodes and significant optical enhancement within the biased region, enabling the first precise, non invasive concurrent optical and electrical characterization of nanomaterials. The established methods are subsequently applied to evaluate the potential of 2D materials as nanoscale electrical switches by analysing the kinetics of their switching mechanism. First, the thesis presents a technique involving the drop-casting of gold nanoparticles onto 2D materials placed on gold substrates. Electrical contact is established by positioning an optically transparent and electrically conductive cantilever on a single nanoparticle. It is used to unveil morphological nano-processes occurring in MoS2 electrical nano-switches, known for their ultralow switching energies but poorly understood switching mechanism. Second, a novel scanning plasmonic microscopy technique is devised, utilizing a plasmonic nanoprobe with a single gold nanoparticle on an optically transparent and conductive cantilever. The nanoprobes enable precise electrode positioning on various material regions, offering user-friendliness, durability, reproducibility, and minimal material perturbation, as validated on WSe2 and MoS2 nanosheets, making them applicable to diverse materials. Third, the thesis introduces a configuration where a nanosheet is biased using patterned gold nano-discs located at the intersection of striped electrodes patterned using shadow masks. This layout eliminates the need for motorized cantilevers and the variability in the location of gold nanoparticles, as observed in the first approach. This configuration is employed to gain an understanding of the switching mechanism in nanoscale h-BN memristors, which are promising as foundational components for logic-in-memory computing systems. Therefore, this PhD thesis introduces nano-techniques for characterizing 2D materials, yielding insights into their electrical behaviour. These findings have implications for advancing and optimizing 2D electronic devices, opening new avenues for nanotechnology applications.
  • ItemEmbargo
    Terahertz Raman Spectroscopy of Organic Molecules in Plasmonic Nanogaps
    Boehmke, Alexandra
    This thesis contributes to the extension of the technique of surface-enhanced Raman spectroscopy (SERS) from the infrared to the terahertz (THz) region of the electromagnetic spectrum. SERS was first discovered in 1977, and has since become a widespread, powerful analytical tool for chemistry and has led to the development of many other surface-enhanced spectroscopies. However, its exploration of THz-frequency modes of materials was limited to measurements using low-throughput and expensive equipment until 2012. Recent advances in laser and notch filter technology have made the THz region accessible to Raman spectroscopy, accelerating the publication of THz Raman studies on a wide array of bulk materials. While THz SERS measurements are likewise possible, progress has been much slower than for bulk Raman, impeded by characteristics of the spectra that make the analysis challenging. Two predominant characteristics were observed of all the measured THz SERS spectra of several different sets of systems, varying analyte and nanostructure parameters, and of the few THz SERS spectra reported in literature: a continuum underlying molecular SERS peaks which rises super-exponentially at decreasing Raman Shifts on both sides of 0 cm-1 and broadening of molecular SERS peaks at decreasing Raman Shifts. These complicate the interpretation of THz SERS spectra. This thesis presents investigations of both. The result is an approach to analyzing SERS data. As an example, this is applied to early diagnosis of Alzheimer’s Disease (AD), the archetype of amyloid diseases. The polymerization of the peptide β-amyloid is considered to be central to the pathogenesis of AD. Because of the delocalized nature of THz-frequency molecular vibrations, THz SERS has the potential to directly distinguish different structures of the peptide.
  • ItemEmbargo
    Nanopore-based Protein Sensing with DNA Nanostructures
    Sandler, Sarah; Sandler, Sarah [0000-0001-9689-8684]
    Over two decades ago, scientists created the first nanopore- an α-hemolysin channel inserted into a lipid bilayer separating two buffered KCl-filled compartments. Today, nanopores are often associated with sequencing – likely due to the immense success of companies like Oxford Nanopore Technologies. However, nanopores can also be made of inorganic materials with tuneable diameters, presenting new opportunities in the fields of biosensing and diagnostics. In the nanopore sensing technique, molecules are driven through the nanosized opening between two chambers of salt solution using forces induced by electric fields. Upon applying an electric field across the nanopore, the voltage drives electrically charged molecules, like DNA, through the nanopore. As molecules move through the pore, the liquid containing the salt ions is displaced. The drop in liquid volume in the nanopore correlates to an increase in resistance and thus a drop in current. This current drop can provide information about the charge, molecular weight, and conformation of the analyte. Because of the this, nanopores can also act as an ideal platform for sensing proteins. The Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-Associated (Cas) protein system is sequence-specific RNA-guided protein duo that enables binding to a specific nucleic-acid sequence and consequent binding followed by cleavage. The CRISPR-Cas system has recently emerged as a revolutionary and widely employed gene editing tool. For use in diagnostic assays, we first need to benchmark the specificity of the ribonucleoprotein complexes (RNPs) – something we can do using nanopores and DNA nanotechnology. We can use the combination of nanopores and DNA nanotechnology to investigate the utility of the RNPs not only for differentiating single nucleotide changes in double-stranded DNA, but also in single stranded RNA, through the construction of RNA-DNA hybrid nanostructures. The ability to detect single nucleotide changes is important in sensing for both small non-coding RNAs and ribosomal RNA. The technique can also be expanded to investigate proteins beyond Cas. For example, we can study interactions of, alpha-synuclein oligomers, a biomarker for Parkinson’s Disease, with various small-molecule drugs. DNA nanostructures allow for highly multiplexed evaluation of the efficacy of various drugs. The combination of nanopore sensing and DNA nanostructures enables single-molecule protein sensing. This can greatly impact not only our fundamental understanding of proteins and their behaviour but can influence areas of biotechnology where protein sensing is essential to both the diagnosis of diseases as well as the evaluation of novel therapeutic approaches.
  • ItemOpen Access
    Literature mining on scientific publications of battery materials
    Huang, Shu
    The scientific literature continues to be the single most important source of critical information for accessing the latest research findings. However, the number of scientific publications is constantly increasing at an ever-increasing rate, which makes it increasingly more difficult to keep up to date with all the literature. Recent advances in natural-language-processing (NLP) techniques have enabled scientists to obtain useful information more efficiently from unstructured literature text, e.g., battery research papers. To this end, this thesis aims at achieving three goals, i.e., i) creating large-scale battery-specific databases, ii) using advanced NLP models to extract and summarise useful information from the literature, and iii) developing user-friendly literature-mining toolkits. Chapter 1 introduces the importance, opportunities, and challenges of literature-mining studies on battery materials. Chapter 2 reviews the roadmap to literature mining and the relevant methodology. Chapter 3 introduces the first auto-generated large-scale battery database extracted from the literature based on a rule-based approach using ChemDataExtractor. Chapter 4 releases the first property-specific transformer-based BatteryBERT model, which enables us to perform data extraction in a completely unsupervised fashion. The model is further embedded in the battery-aware text-mining toolkit, BatteryDataExtractor (Chapter 5), to increase user-friendliness for automated data extraction. Chapter 6 extends these literature-mining studies to automatic book generation, in the form of the released toolkit, ChemDataWriter, which was used to auto-generate books that summarise research. Chapter 7 concludes this thesis and outlines future research topics that would complement and expand the work into a broader context of literature-mining research on scientific publications of battery materials.
  • ItemOpen Access
    Strain in Halide Perovskites: Characterisation, Crystallography, Consequences
    Orr, Kieran; Orr, Kieran [0000-0001-9996-1458]
    Optoelectronic devices such as solar cells, LEDs, and X-ray detectors making use of halide perovskites as the light absorbing or emitting material have experienced a dramatic increase in efficiency in recent years. As a result, halide perovskite-based solar cells are now starting to make their way out of laboratories and into the commercial market. However, there is still room for efficiency improvements, and in many of the devices that do boast high efficiencies, long-term stability remains a thorny issue for the field. Strain is known to affect the performance of a range of semiconductors, and halide perovskites are no exception. However, the precise links between strain and optoelectronic properties in these materials appeared obscure, with a number of studies in the literature presenting conflicting results. In this thesis, after setting out my motivation in Chapter 1, I make use of a range of synchrotron-based X-ray diffraction techniques to characterise strain in halide perovskites. As introduced in Chapters 2 & 3, this crystallographic approach is combined with photoluminescence (PL) measurements as a probe of optoelectronic performance with the view to understand the consequences of strain in these materials. I primarily employ coherent diffraction techniques, for example using Bragg coherent diffraction imaging in Chapter 4 to image internal strain distributions in halide perovskite devices with nanoscale resolution. In so doing, I find intra-grain and grain-to-grain strain distributions to be remarkably heterogeneous, and I identify dislocations in full solar cell device stacks. Chapter 5 then demonstrates that *in situ* illumination induces striking dislocation migration. Extending my coherent diffraction imaging approach to Bragg ptychography in Chapter 6 allows me to spatially correlate increased tensile strain with red-shifted PL spectra and shorter PL lifetimes in a single crystal of CsPbBr3 purposely isolated from other possible causes of optoelectronic heterogeneity. Finally, pivoting to diffuse scattering measurements, in Chapter 7 I uncover dynamic domains of locally tilted lead-halide octahedra in CsPbBrxCl3−x using a novel big-box combined Bragg scattering and pair distribution function refinement procedure. Off the back of the results presented in the following pages, I have tried to indicate where the crystallographic insights can inform device fabrication methods, with the aim of realising more efficient and longer-lasting halide perovskite-based devices. However, the techniques and analysis described below can be applied to many functional materials systems and devices far beyond the realms of halide perovskites.
  • ItemOpen Access
    Search for new hadronically decaying resonances with masses down to 20 GeV in pp collisions using the ATLAS Detector at the Large Hadron Collider
    Henkelmann, Lars
    Many proposed models of physics beyond the Standard Model of particle physics (BSM) predict new resonances. Thus, a single experimental search for such new resonances can probe a multitude of BSM models. In particular, searches for resonances decaying into hadrons can probe for any resonances that can be produced at the LHC. However, such dijet searches lose sensitivity at low masses due to transverse momentum trigger thresholds. This thesis presents a search targeting events where the BSM resonance is produced back-to-back with a photon radiated from the initial state partons. The search achieves sensitivity to BSM resonance masses as low as 20 GeV for the first time using data from the ATLAS detector. This is done by triggering on the initial state radiation (ISR), which decouples the accessible resonance mass from the trigger thresholds. Trigger thresholds instead imply a minimum boost of the resonance, causing the lowest-mass resonances to produce overlapping jets that are best reconstructed as a single large-radius jet. Precise large-radius jet reconstruction down to very low masses is thus a crucial requirement of the search. However, the standard Run 2 large-radius jet definition is only calibrated down to 50 GeV. To overcome this limitation, in this thesis the resonance candidate jets are instead reconstructed by reclustering calibrated 𝑅 = 0.2 hadronic anti−𝑘T jets and inner detector tracks into so-called track-assisted reclustered (TAR) jets. A dataset corresponding to 140.1 fb−1 of proton-proton collisions at √𝑠 = 13 TeV recorded with the ATLAS detector from 2015 through 2018 is analysed. The most significant excess over the SM expectation is localized near 40 GeV of jet mass, but at 2.8 𝜎 it is not significant. Thus, upper limits are set on the cross-section for producing 𝑍′ vector resonances. The 95% CL𝑠 limits range from 0.02 pb for a 20 GeV resonance to 0.2 pb for a 225 GeV resonance.
  • ItemEmbargo
    Automatic data interpretation from scientific literatures in the portable document format with information-extraction tools for the advancement of materials discovery
    Zhu, Miao
    This thesis addresses the development of scientific literature mining software tools to automatically extract metadata, text, images, and chemistry information from scientific literatures that are in PDF (portable document format), a widely used format in academic and scientific communities, to accelerate the data-driven materials discovery. The nature of PDF files presents specific challenges for data extraction. Typically, no semantic tags are usually provided in a PDF file that is not designed to be edited or its data interpreted by software. This creates a barrier to efficiently accessing and utilising the wealth of information they contain, especially in domains like materials science and chemistry where data extraction and analysis are crucial. In the materials discovery domain, efficiently accessing and analysing chemical and property data is of paramount importance. More crucially, it emphasises the need to understand and exploit the relationships between these data points to enable data science in areas such as data-driven materials discovery, where identifying correlations between different materials and their properties can lead to significant scientific breakthroughs. By addressing these challenges, this thesis contributes to enabling data science in areas like data-driven materials discovery. It demonstrates how extracting and analysing data from scientific literature can provide new insights and accelerate the process of discovery in this field. The software tools developed as part of this thesis represent significant technical innovations. They are designed to navigate the complexities of PDF files and extract valuable data with precision. This makes the tools not only valuable for researchers in materials science but also potentially applicable in other scientific domains where data extraction from literature is a key part of the research process. Chapter 1 discusses the fundamental and history of portable document format and the current literatures on data extraction from portable document format for materials discovery. Chapter 2 introduces relevant methodologies and frameworks used throughout the thesis. Chapter 3 introduces PDFDataExtractor, a highly automated data and information extraction toolkit that can automatically detect layouts of scientific literatures that are in portable document format, from which semantic information can be extracted and interpreted to reconstruct the logical structure of articles to a machine-readable format. This tool was tested on a self-created evaluation set and key metadata are extracted with nearly 60% precision. Chapter 4 descries a complete workflow for the extraction of image, scheme, and figure with corresponding caption from scientific literatures and supplementary information that are in portable document format. This workflow also extracts chemical and property data by channelling results to ImageDataExtractor1. ImageDataExtractor1 is a toolkit that extracts quantitative data from microscopy images. Chapter 5 details a bespoke workflow for the extraction of metadata, text, image, chemistry information and property data from physics literatures. Chapter 6 concludes the work and discusses future research opportunities.
  • ItemEmbargo
    Multi and Hyperspectral Imaging of Early Detection of Disease
    Taylor-Williams, Michaela
    Multi and hyperspectral imaging (MSI/HSI) techniques provide spatial and spectral information and can be applied to detect and understand changes in biological tissue that occur with disease. This thesis has evaluated the application of MSI/HSI to two disease applications: analysing nailfold capillaries to aid in the evaluation of systemic scleroderma, and enhancing cancer detection in the gastrointestinal tract during endoscopies. The nailfold capillaries are the smallest blood vessels in the body, and deformations in these capillaries are indicators of systemic scleroderma, a rheumatic disease. A hypothesis was formed correlating these deformations with changes in oxygen levels. Two multispectral systems, capable of imaging the nailfold capillaries were designed: one based on LED illumination, and the other a snapshot detector-based imaging system. These systems were tested with microfluidic blood phantoms, which simulated varying oxygenation levels. Developing microfluidic blood phantoms proved critical for evaluating system performance. Phantoms with microfluidic depths used horse blood that was chemically oxygenated and deoxygenated to variable blood oxygenation levels. The LED-based system was subsequently used for imaging healthy nailfold capillaries in a proof-of-concept demonstration. Concurrently, the multispectral snapshot camera system was incorporated into a clinical trial at the University of Manchester. Data from the nailbed capillaries of healthy controls as well as those with systemic sclerosis were evaluated and classification was achieved. Early detection of cancer in the gastrointestinal tract can lead to curative intervention, but contrast for early lesions is low. To evaluate the potential for MSI methods to address this challenge, design optimisation was performed using pre-existing hyperspectral data from endoscopies conducted in the gastrointestinal tract (oesophagus and colon). An open-source Python-based toolbox for spectral band optimisation was developed to analyse datasets in order to design the optimal imaging bands and finalise the spatial layout of multispectral filter arrays that could be deployed in clinical settings. Disease detection accuracy was optimised by selecting subsets of spectral bands and integrating machine learning methods, such as k-nearest neighbour (kNN) classification and support vector machines (SVM). The maximum classification accuracies occurred using a kNN classifier and were 0.848 and 0.999 for the oesophagus and colon, respectively. SVM performed reasonably well with accuracies of 0.811 and 0.997 for the oesophagus and colon; while spectral angle mapping classification was a good classifier of colon tissue (accuracy of 0.995), it performed poorly on oesophageal tissue (accuracy of 0.245). The toolbox was also deployed to design filters capable of imaging blood oxygenation in tissue, to improve the detection and understanding of cancers where hypoxia plays a role. This research demonstrates the promising diagnostic capabilities of spectral imaging in measuring blood oxygenation.
  • ItemOpen Access
    Extending the Reach of Searches for Staus, Charginos and Neutralinos with the ATLAS Experiment at the Large Hadron Collider
    Jones, Dominic
    This thesis describes new approaches to extend the sensitivity reach of searches for three types of supersymmetric particles; staus, charginos and neutralinos, using the 139 fb⁻¹ of data collected with the ATLAS experiment at the Large Hadron Collider between 2015 and 2018. Searches for three different production modes of these particles are considered, with new approaches for each that highlight different methods for improving sensitivity. Firstly, a search for direct stau production, in which staus decay to tau-leptons and neutralinos, is presented. The search makes use of Boosted Decision Trees to classify the signal from the Standard Model backgrounds and in doing so significantly improves the sensitivity compared to a previous search using the same dataset. No significant differences between the observed data and the Standard Model predictions are found but new, world-leading limits on direct stau production are set. Models with mass degenerate left and right-handed staus are excluded at 95% confidence level for stau masses between 80 GeV and 500 GeV, with the low mass reach closing a previous gap in sensitivity between previous searches performed by the ATLAS experiment and searches conducted by experiments at the Large Electron Positron collider. This search also provides the first sensitivity at the ATLAS experiment to the right-handed stau only interpretation, for which models with right-handed stau masses between 100 GeV and 350 GeV are excluded for a massless neutralino-one at 95% confidence level. Secondly, new search channels for the production of charginos and neutralinos with highly compressed mass spectra are explored. In particular, a re-interpretation of a previous search for new physics performed at ATLAS in a final state with an energetic jet and large missing transverse momentum is compared to a sensitivity study of a vector boson fusion channel. The re-interpretation is found to have sensitivity to previously uncovered regions of parameter space; for a simplified wino/bino interpretation models with neutralino-two masses of 140 GeV are excluded at 95% confidence level, under the assumption that signal theoretical uncertainties have a negligible impact. The vector boson fusion channel is assessed to have comparatively limited sensitivity with differences in the signal modelling between this study and previous work on this channel being identified. However, it is shown that the use of machine learning in this channel can provide a significant gain in the sensitivity. Thirdly, a statistical combination of previous ATLAS searches for chargino pair production in three different topologies is performed for the first time, leading to a notable enhancement in the overall sensitivity.
  • ItemOpen Access
    Star-forming Galaxies and Quenched Systems throughout Cosmic Time
    Sandles, Lester; Sandles, Lester [0000-0001-9276-7062]
    This thesis delves into the investigation of star-forming galaxies and quenched systems at high redshifts, exploring their evolution and properties throughout cosmic time. Firstly, I utilise the foreground lensing of massive galaxy clusters in the Hubble Frontier Fields to probe the high-redshift evolution of the main sequence of star-forming galaxies. I use the BEAGLE SED-fitting code to derive stellar masses, SFRs and redshifts from galaxies within the ASTRODEEP catalogue. I fit a fully Bayesian hierarchical model of the main sequence over redshifts 1.25 < z < 6 while explicitly modelling the outlier distribution. My results agree with an increase in normalisation of the main sequence to high redshifts that follows the redshift-dependent rate of accretion of gas onto dark matter halos. We additionally measure the slope and intrinsic scatter of the star-forming main sequence. We find that the sampling of the SED provided by the combination of filters (Hubble + ground-based Ks-band + Spitzer 3.6 and 4.5 μm) is insufficient to constrain the stellar mass and SFR over the full dynamic range of the observed main sequence, even at the lowest redshifts studied. Whilst this filter set represented (prior to the launch of *JWST*) the best sampling of high-redshift galaxy SEDs out to z > 3, I show that measurements of the main sequence to low masses and high redshifts were still strongly dependent on the priors employed in SED fitting (as well as other fitting assumptions). In the first field targeted by JADES, the statistics were not large enough to extend the full main sequence analysis to *JWST*-based datasets. I therefore continued to study high-redshift star-forming and quenched galaxies with smaller projects, more suitable for the first deep set of spectroscopy obtained by JADES. Dust, often one of the most poorly constrained parameters in SED fitting, drives the motivation for my second project. As part of the JADES survey, utilising data obtained with the *JWST*/NIRSpec Micro-Shutter Assembly, I directly explore dust attenuation in the star forming galaxy population. This is achieved by analysing Balmer decrement measurements for a sample of 51 galaxies at redshifts 4 < z < 7. Leveraging 28-hour long exposures and the efficiency of the prism disperser (but also using information from the medium-resolution gratings), I was able to probe directly the low-mass end of the galaxy population, reaching stellar masses as low as 107 M$_\odot$. I find that the correlation between Balmer decrement and stellar mass is already established at these high redshifts, indicating a rapid build up of dust in moderately massive galaxies at such early epochs. The lowest-mass galaxies in our sample (1 - 3 x 107 M$_\odot$) display a remarkably low Balmer decrement of 2.88, consistent with case B recombination and little or no dust. I further compare the Balmer decrement to continuum-derived star-formation rates, finding tentative evidence of a correlation, which likely traces the underlying connection between SFR and the mass of cold gas. Finally, based on deep *JWST*/NIRSpec spectroscopy, I report the discovery of a quiescent galaxy at z = 2.34 with a stellar mass of only 9.5 x 108 M$_\odot$. This is the least massive quiescent galaxy found so far at high redshift. I use a Bayesian approach to model the spectrum and photometry, and find the target to have been quiescent for 0.6 Gyr with a mass-weighted average stellar age of 0.8 - 1.7 Gyr (dominated by systematics). The galaxy displays a colour gradient (redder towards the edge), consistent with outside-in, environment-driven quenching. Based on a combination of spectroscopic and robust (medium- and broad-band) photometric redshifts, I identify a galaxy overdensity near the location of the target (5-σ above the background level at this redshift). The overdensity contains three spectroscopically confirmed, massive, old galaxies. The presence of these evolved systems points to accelerated galaxy evolution in overdensities at redshifts z > 2 (in agreement with previous works). In projection, our target lies only 35 pkpc away from the most massive galaxy in this overdensity (spectroscopic redshift z = 2.349) which is located close to the overdensity’s centre. This suggests the low-mass galaxy was quenched by environment, making it the earliest evidence for environment-driven quenching to date.
  • ItemEmbargo
    Machine learning force fields for elemental sulphur
    Carare, Vlad
    The sulphur phase diagram is one of the most complex ones of all elemental systems, competing with that of carbon. The flexibility of the bonds allows for a variety of motifs: rings of 5 or more atoms in various conformations, short diradical chains and thousands-atoms long polymers to name a few; which give rise to a plethora of structures: molecular & polymeric crystals of many different symmetries, amorphous solids and molecular & polymeric liquids. Modelling transitions between such phases is a challenging task, out of the reach of any current force fields (which are too inaccurate) or quantum mechanical methods (which are too slow and expensive). However, following the footsteps of similar work done on silicon, phosphorus and carbon, surrogate machine learning models mimicking quantum methods at a fraction of the cost could achieve this feat. In this work we propose several such models, prompted by the continuous evolution of the field, and benchmark them on a series of static and dynamic tests. We successfully describe the ambient condition solid phase, melting, polymerisation and depolymerisation of sulphur: an achievement out of reach of any previous method. We also dedicate considerable effort to investigating the liquid-liquid phase transition recently reported in the experimental literature [1]. This consists of a change between low and high density liquid forms heralded by a jump in density and alterations in radial distribution functions and Raman spectra. While a simple analytical model to explain the transformation was proposed in a recent publication [2], a quantum-mechanically accurate exposition of the microscopic phenomena is desirable. Our models surpass previous length and time constraints and allow the simulation of liquid sulphur for up to hundreds of nanoseconds for thousands of atoms, which enable the reaching of thermal equilibrium and the obtaining of meaningful and precise measurements of the structure factors, cluster sizes and coordination statistics. We are able to characterise the two phases: one consisting of an almost even fraction of polymers and small rings and the other comprising mostly of tightly-packed polymers. Another important contribution of this thesis is the in-depth display of the process of building a general potential for such a complex system. We showcase several methods for creating a relevant dataset, through: manual selection, iterative training and automated selection; which could prove useful for the community. Furthermore, we investigate the effect of magnetic dipole moments on small sulphur clusters and condensed liquid phases, and put forward the first general machine-learning potential trained on spin-polarised data.
  • ItemEmbargo
    Thermoelectric Properties of Organic Polymers
    Zhu, Wenjin
    Thermoelectric devices present an enticing solution for harnessing waste thermal energy from industrial processes and converting it into electrical power. Due to their cost-effectiveness and potential in flexible electronic applications, organic thermoelectrics have attracted considerable attention. Nonetheless, further enhancing their thermoelectric performance remains a great challenge. Solutions have been proposed, encompassing molecular structure design, uniaxial alignment, and advanced doping techniques. This dissertation explores the relationship between the enhanced thermoelectric properties and the structure of organic polymers. It delves into both innovative new materials systems and established typical materials. First it introduces the background and basic experimental techniques in Chapters 1-2. Then it delves into the function of uniaxial alignment and ion exchange doping to optimize the thermoelectric properties of organic polymers in Chapter 3. Uniaxial alignment achieves anisotropic charge transport by orienting the polymer backbones, which facilitates charge movement along backbones. Ion exchange doping has demonstrated superiority over traditional molecular and electrochemical doping methods, increasing charge carrier densities. By integrating these two techniques, we've observed marked improvements in the thermoelectric attributes of some typical conjugated polymers such as poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (PBTTT) and diketopyrrolopyrrole (DPP)-based polymers. In Chapter 4, we report a new model system for better understanding the key factors governing their thermoelectric properties: aligned, ribbon-phase PBTTT doped by ion-exchange doping. Using a range of microstructural and spectroscopic methods we study the effect of controlled incorporation of tie-chains between the crystalline domains through blending of high and low molecular weight chains. The tie chains provide efficient transport pathways between crystalline domains and lead to significantly enhanced electrical conductivity, that is not accompanied by a reduction in Seebeck coefficient nor a large increase in thermal conductivity. We demonstrate respectable power factors of 172.6 μW m-1 K-2 in this model system. Our approach is generally applicable to a wide range of semicrystalline conjugated polymers and could provide an effective pathway for further enhancing their thermoelectric properties and overcome traditional trade-offs in optimization of thermoelectric performance. Furthermore, this thesis explores the relationship between molecular structure and thermoelectric properties of some specially designed novel polymers in Chapters 5-6. This includes Poly-3-hexyl-thiophene (P3HT)-based random co-polymers, P[(3HT)1-x-stat-(T)x] containing different proportions of unsubstituted thiophene units (x ranging from 0 to 0.36). It also includes naphthalene diimide (NDI)-based copolymers modified by substituting side chains, incorporating Selenium atoms, and incorporating Fluorine atoms. Finally, Chapter 7 of this thesis provides a summary and offers an outlook, highlighting how our methodology is broadly applicable across various semicrystalline polymers, which can potentially and substantially improve the thermoelectric figure-of-merit in these emerging materials. Furthermore, the techniques discussed in this thesis show great promise for expanding the applications of organic thermoelectric devices in potential future industrial scenarios.
  • ItemOpen Access
    Liquid Crystalline Elastomers as Renewable Functional Materials: From Chemistry to Application
    Gablier, Alexandra
    Liquid Crystalline Elastomers (LCEs) are thermosets that belong to the family of “smart plastics”. They combine the softness and elasticity of elastomers, with the orientational order properties of liquid crystals (LCs), resulting in the ability for these materials to undergo large reversible deformation when subjected to external stimuli. This unique ability to actuate and perform work without mechanical parts has positioned LCEs as attractive materials for applications in fields such as soft robotics, sensors, surface coatings, and tissue engineering. The mechanical performance of LCEs and their capacity for large-amplitude, reversible actuation depend on the underlying chemistry and the alignment of the LC components in the network. However, achieving specific, complex, macroscopic-scale 3D structures with a predetermined actuation behaviour remains a challenge with conventional alignment techniques. The introduction of dynamic covalent chemistry into the networks of LCEs (to form xLCEs) was a significant breakthrough in the field, promising enhanced material processing and sample alignment. However, at the outset of this PhD, the understanding and control over the exchange dynamics of xLCEs was still lacking, stemming in part from a need for a broader library of materials with a greater variety of dynamic properties. Additionally, the field suffered from a lack of LCE applications geared towards addressing real-world problems. This thesis hence aims to contribute to the advancement of the field of LCEs in three key areas: (1) investigating the mechanics of xLCEs to establish fundamental principles, (2) exploring novel network chemistries, and (3) applying the knowledge gained to develop new and practical applications. First, I build on existing bond-exchange reactions to establish wider knowledge about factors controlling the material flow on a macroscopic scale in dynamic covalent polymer systems (vitrimers). I notably demonstrate that the bond exchange reaction activation energy is a poor predictor of material flow at high temperatures, with the network elastic modulus and the concentration of reactive functions for the bond exchange having a dominant impact on flow behaviours. This enhanced understanding provides design principles for controlling material dynamic properties in xLCEs. Second, I expand the library of exchange and network chemistries available for xLCE materials. Through the use of an epoxy-thiol reaction, I introduce a new network chemistry for an established covalent exchange reaction (transesterification). The reaction is simple, utilises mild conditions, cheap starting materials, and results in true elastomer xLCE materials with a wide range of material properties accessible through the system’s modular character. I show that the LC isotropic transition temperature, the material flow at high temperature from bond exchange, and the LC mesophase can all be controlled and tailored through a simple variation of the network composition. The expansion of material properties available broadens the range of possible outcomes for transesterification-based xLCE. Another new type of network with dynamic covalent properties that is introduced in this work is a poly(thiourethane) xLCE system. Such an xLCE thermoset network, containing dynamic covalent thiourethane bonds, is strengthened by physical crosslinks (H-bonding), resulting in a unique set of material properties such as enhanced strength and a remarkably high ductility at room temperature. The material obtained is the first example of an xLCE that can be reprocessed using industrially ubiquitous methods such as injection moulding and extrusion. Lastly, an example of use of LCEs towards a real world problem is investigated, namely through the use of LCEs as a to-scale Braille soft continuum interface for dynamic Braille devices. I demonstrate that the complex and numerous moving parts of a dynamic Braille device could be replaced by a single sheet of LCEs embossed with small actuating bumps. A simple moulding procedure produces a surface patterned with at-scale Braille bumps, as a result of a precise and complex internal organisation within the elastomer sample that emerges during polymerisation (as is evidenced by theoretical modelling). Unlike in previous attempts to use LCEs for Braille technology, the millimetre-scale protruding features are generated out of the bulk of the material, resulting in structural integrity, and high resistance to compression force. The localised bump-to-flat reversible actuation occurs on a timescale of seconds. The potential of this development for application into a complete Braille dynamic display are discussed. These findings open new lines of research in multiple directions for the field, in the hopes of advancing knowledge and bringing LCEs one step closer to commercialisation.
  • ItemEmbargo
    Interplay of Spin and Photophysics in Luminescent Open-Shell Molecular Semiconductors
    Gorgon, Sebastian; Gorgon, Sebastian [0000-0002-1361-1973]
    Luminescent organic radicals are an emerging class of molecular semiconductors which exhibit many unique properties attractive for optoelectronic and spintronic devices. In this thesis, we employ optical and spin-based probes to reveal the dynamics of photogenerated excitons in a selection of novel tris(2,4,6-trichlorophenyl)-methyl (TTM)-based radicals. The first three chapters present a motivation, relevant theory and methodology. In Chapter 4, we focus on the intrinsic properties of luminescent doublet (*S*=1/2) states. We find evidence of intermolecular charge transfer excitations which drastically alter the emission spectrum and lifetime in thin films. In Chapter 5, we investigate solid state intermolecular interactions between radicals and triplet (*S*=1) states on closed-shell materials, and show their management can lead to improvements in Organic Light Emitting Diode (OLED) performance. For the first time we observe cycling between the triplet and doublet manifolds, and direct energy transfer on sub-nanosecond timescales. In Chapter 6, we present the first organic molecules which can reversibly access a quartet (*S*=3/2) excited state. This is achieved by engineering strong exchange coupling between resonant radical and triplet manifolds in covalently linked structures. The resulting high-spin states are coherently addressable with microwaves even at 295 K, with optical read-out enabled by intersystem crossing to the energetically accessible radical state. In Chapter 7, we extend these results to a luminescent biradical structure which supports a quintet (*S*=2) excited state. The light-induced cycling through this state drastically increases the strength of the exchange coupling between the two radical spins, and leads to a long-lived ground-state polarisation. The findings and models developed in this thesis open a path to few functionalities for open-shell semiconductors, as outlined in Chapter 8, ranging from improved light emission to molecular quantum information science.
  • ItemOpen Access
    Deterministic spin and photon control with a symmetry protected colour centre
    Parker, Ryan
    Quantum networking, whereby quantum mechanical entanglement is distributed through a network and used as a vector for information transfer, is an ambitious emerging technology. It requires a stationary, compute, qubit at a node in the network to interface with a flying, photonic, qubit for information distribution where these two qubits have different requirements to be technologically relevant. The stationary qubit needs long-lived internal degrees of freedom that can be coherently manipulated and optically interfaced. Whereas, the flying photonic qubit should interact minimally with the environment, to prevent decoherence from disrupting information distribution throughout the network. These two sets of independent requirements for the stationary and flying qubits impose strong engineering constraints on the underlying physical system underpinning the quantum network, and no one physical system has demonstrated a scalable solution that meets both sets of requirements. In this thesis, the negatively charged tin vacancy centre (SnV) in diamond is presented as a viable, symmetry protected, platform for quantum networking applications. The stationary qubit in the network is formed by the SnV centre's intrinsic optically addressable spin-1/2 qubit. This thesis accesses the coherence of the SnV centre's spin-manifold for the first time and is subsequently leveraged to achieve multi-axis coherent control of the SnV centre's spin at Ω/2π = 4.5(1) MHz Rabi frequencies and with 82(5)% π/2-gate fidelities. Leveraging this control reveals long-lived internal degrees of freedom yielding an inhomogeneous dephasing time of T$_{2}^*$ = 1.4(3) µs. Access to an ancillary quantum register, formed of proximate nuclear spins, is also shown, which further provides a resource for quantum networking by enabling quantum memory operations and quantum state storage to be available during network activity. The flying photonic qubit interaction channel takes the form of the optically addressable, spin-selective, transitions of the SnV centre. This thesis demonstrates, for the first time, isolation of coherent photons from the SnV centre with 99.7$_{-2.5}^{+0.3}$% purity and 63(9)% indistinguishability. The symmetry protected nature of the SnV centre's spin manifold enables up to 106 identical photons to be generated per entanglement attempt before optical coherence is lost. Full quantum control of the optical transition is further demonstrated, yielding a 77.1(8)% fidelity optical π-pulse in 1.71(1) ns. Thus, the presence of both a robust photon-photon interaction and controllable optical channel is demonstrated for quantum networking applications leveraging the SnV centre. The stationary SnV spin qubit and the flying photonic qubit are combined in a single high-efficiency packaged platform, yielding a 57(6)% collection efficiency and the observation of 5-photon states. A giant optical non-linearity conditional on the spin qubit's state is used for information transmission in a two-node directional network. Further, the presence of an ancillary quantum memory register is extended through the use of the intrinsic spin-1/2 117Sn register of the 117SnV centre. This novel resource, discovered in this thesis, enables a high-efficiency photonic interface to interact directly with the 117Sn nuclear spin degrees of freedom. Thus, nuclear initialisation to 98.6(3)% fidelity and single-shot optical nuclear spin readout with 80(1)% fidelity are achieved in an all-optical control protocol, significantly reducing the overhead per qubit needed for quantum repeater nodes. Therefore, this thesis presents the SnV centre in diamond as a novel resource for quantum networking. The symmetry of the centre enables robust coherence of both the stationary spin-qubit and the flying photonic qubit that is insensitive to nano-photonic integration. This high intrinsic coherence positions the SnV centre for class-leading integration into Purcell enhanced cavity systems. Such integration would then facilitate near unity collection efficiencies and control fidelities, thereby enabling fault-tolerant quantum networking with a single, low overhead, platform.
  • ItemOpen Access
    Ultrafast Raman Scattering in Plasmonic Nanocavities
    Jakob, Lukas
    When bound to metals, molecular vibrations play a key role in sensing, catalysis, molecular electronics and beyond, but investigating their coherence and dynamics is difficult as pulsed experiments prove very challenging. In this thesis, I study vibrations of 1-1000 molecules in a plasmonic nanocavity when driven by picosecond pulsed lasers out of the linear regime. This unravels new non-linear effects such as room-temperature vibrational pumping, giant optomechanical spring shifts, collective molecular vibrations, accelerated decay of vibrational coherence, and the generation of correlated photon pairs. In plasmonic nanocavities, optical fields are enhanced 100-fold and focused to a nanometre-thin gap. Vibrations of molecules placed in the cavity interact strongly with the optical resonances, forming a coupled optomechanical system. Using pulsed laser illumination, I find that surface-enhanced Raman scattering can significantly increase the phonon population above the thermal equilibrium. This vibrational pumping leads to non-linear anti-Stokes scattering observable at room temperature. Further, the optomechanical coupling induces a red-shift of the vibrational energy by >100 cm−1 and broadening of the Raman line at high peak laser powers (optomechanical spring shift). These non-linear effects are strongly enhanced by the excitation of collective molecular phonon modes. Further experiments show that Stokes-induced anti-Stokes scattering exhibits strong cross-frequency photon bunching. These correlated Stokes – anti-Stokes photon pairs show non-classical behaviour and could be used for applications in quantum computing and communication. To study the dynamics of molecular vibrations, I use time-resolved incoherent and coherent anti-Stokes Raman scattering. Developing a new single-photon lock-in detection technique, it is possible to simultaneously record the decay of the vibrational population and vibrational dephasing for each nanocavity. The vibrational dephasing is found to strongly accelerate depending on the exciting laser intensity. Understanding these modified vibrational dynamics on plasmonically-active substrates is crucial for improving surface-enhanced catalysis of chemical reactions and heat transfer in molecular electronics.