Theses - Physics

Permanent URI for this collection

https://www.repository.cam.ac.uk/handle/1810/214775

Browse

Now showing 1 - 20 of 562

Open Access
Optimised ground-based near-infrared instrumentation for robotic exoplanet transit surveys
Pedersen, Peter
In this PhD, I have advanced the study of ground-based photometric observations in the near-infrared. Specifically, I have worked in the context of robotic exoplanet transit surveys in collaboration with the SPECULOOS Southern Observatory (SSO). Here, I targeted optimising the photometric precision of observing late M, L type stars by developing correction methods and a new instrument called SPIRIT. My first original contribution was the development of a correction method for the induced effects from varying precipitable water vapour (PWV) in our atmosphere, specifically on differentially resolved light curves from SSO. This work succeeded in reducing false variability of time-series data from late M, L type stars on both long and short timescales, to the extent of removing false transit features. In parallel, I performed a feasibility study of introducing new near-infrared instrumentation to SSO. One which would permit better photometric precision than the existing CCD Si based instrumentation, and likewise minimise the induced effects of PWV variability. An InGaAs based instrument, sensitive up to 1.62 µm was identified, and a custom wide-pass filter called zYJ was designed and manufactured to form SPeculoos' Infra-Red photometric Imager for Transits (SPIRIT). It proved to be a significantly lower-cost alternative to the traditionally used HgCdTe based instrumentation, as well as being better suited to robotic observatories. On sky results of SPIRIT at SSO successfully demonstrated better photometric precision for stars below 2550 K than the existing instrumentation. It similarly demonstrated the benefit of seeing further into the infrared for minimising the observed variability of M, L type stars. Finally, the custom designed wide-pass filter, zYJ, successfully demonstrated a significantly lower sensitivity to PWV variability. These results pave a new avenue for ground-based near-infrared robotic exoplanet transit surveys, as well as similar time-series focused astronomy. I conclude my work by suggesting viable routes to further improve the photometric precision of such new instrumentation.
Embargo
In-Situ and Operando Multimodal Microscopy of Metal Halide Perovskite Optoelectronic Devices
Frohna, Kyle; Frohna, Kyle [0000-0002-2259-6154]
Metal-halide perovskites are materials at the forefront of the next generation of optoelectronic materials. Of particular interest is their remarkable power conversion efficiencies when incorporated into thin film solar cells. The properties of next-generation semiconductors such as perovskites are dominated by microscopic variations in their structure, composition and photophysics. Perovskites show extraordinary levels of disorder and this has considerable implications for their function. Gaining a microscopic understanding into how the optoelectronic quality of perovskite thin films and their interfaces with contact layers affects their performance is crucial to enabling solar cells with sufficient performance and stability to commercialise. In this thesis, I detail the development of a multi-modal microscopy toolkit to probe the optoelectronic quality of perovskite thin films and devices and spatially correlate these measurements with microscopic chemistry and structural information. In the first experimental chapter, I detail the capabilities of a hyperspectral, wide-field optical microscope, capable of measuring spatially resolved photoluminescence, reflectance and transmittance spectra with diffraction resolution. With a variety of perovskite thin film samples, I show that thin-film morphology and surface passivation play a huge role in photoluminescence intensity, spectrum and stability. The second experimental chapter applies calibration tools to the hyperspectral microscope, enabling the extraction of device relevant metrics such as the quasi-Fermi level splitting and Urbach Energy microscopically. We spatially correlate these measurements with nanoprobe X-ray diffraction and fluorescence to probe structure and chemistry. Applying this multimodal toolkit to state-of-the-art alloyed perovskites, we find that nanoscale variations in chemical composition dominate the optoelectronic properties of these perovskite films and form energetic funnels that carriers fall down and away from trap states. This study helps to explain the remarkable defect tolerance of these materials. The final experimental chapter augments the optical microscopy setup to measure voltage dependent photoluminescence maps. Voltage dependent photoluminescence allows the extraction of pseudo current-voltage curves of the devices, enabling the recombination and charge transport losses of perovskite solar cells to be mapped microscopically. I show that microscopic performance heterogeneity has a large impact on both macroscopic performance and stability. By mapping the same areas before devices before and after ageing, the microscopic effects of degradation on charge extraction can be imaged. Taken together, the results here show the important microscopic influences on performance from thin films to complete devices and the powerful multi-modal methodologies developed are widely applicable to a wide array of disordered semiconductors.
Embargo
Improving the Stability of Organic Field-Effect Transistors
Nguyen, Malgorzata
With the promise of low-temperature solution-processing, organic field-effect transistors (OFETs) have evolved from a convenient tool for semiconductor parameter analysis to a technology slowly entering the market as circuit components in flexible electronics. Since the first devices in the 1980s, OFET mobilities have increased by over six orders of magnitude and surpassed the benchmark of amorphous silicon. The bottleneck for OFETs to outperform their inorganic counterpart is their environmental, thermal, and operational stability issues. This dissertation focuses on the operational stability of OFETs under a prolonged bias-stress condition. While much effort has been made to explain ON-state bias-stress instabilities in p-type materials (also known as negative bias stress) and to minimise their effects, not much has been done to address the OFF-state bias-stress instabilities (positive bias stress in p-type materials). In fact, OFF-state bias-stress stability is arguably a more important parameter since, in most OFET applications, they stay in the OFF state for longer. We chose the high-performance donor-acceptor conjugated polymer indacenodithiophene-co-benzothiadiazole (IDT-BT) as our model system due to its low energetic disorder and high reported mobilities exceeding 1 cm²V⁻¹s⁻¹. Since stability studies require an exceptional level of reproducibility, we investigated the influence of the fabrication conditions and polymer batch-to-batch variation on the device performance. We uncovered the detrimental effects of glovebox atmosphere on the OFET characteristics and observed significant differences in various batches of the polymer, which we attributed to material contamination. We hope this work will serve as a solid starting point for understanding the intricacies of OFET fabrication. Further, we developed two methods to address the OFF-state bias-stress instabilities in OFETs, the origin of which we attributed to the trapping of electrons. The first approach involves the treatment of organic semiconductor (OSC) films with an orthogonal solvent that induces local aggregation of the polymer and results in an overall increased crystallinity. This leads to a decreased probability of electron trapping in the transport-sensitive regions in the film, and threshold-voltage shifts of less than 1 V upon application of 50 V gate-bias stress for over 10 hours. The second approach involves blending an insulating polymer matrix into the OSC layer in relatively high ratios. In addition to significant suppression of OFF-state bias-stress instabilities, we observe an increased degree of IDT-BT crystallinity, a typical lateral separation between the two polymers, and, interestingly, a higher level of charge accumulation in the insulating polymer. Our work offers simple processing strategies for achieving the reliability required for applications in flexible electronics.
Open Access
Data-driven understanding and forecasting of electrochemical systems
Jones, Penelope
Electrochemical energy systems such as batteries and supercapacitors have played an important role in society for two centuries. The rise of the consumer electronics industry has been underpinned by innovations in battery technology, and batteries are subject to more public interest now than ever before due to the uptake of electric vehicles and the demand for grid-scale batteries to stabilise an intermittent renewable energy supply. Most electrochemical energy systems are complex. A multitude of processes over different length- and time-scales drive their realised performance. One source of complexity arises from the enigmatic mechanism by which inter-particle interactions at the atomistic scale give rise to critical macroscopic behaviours. Examples of this can be found in liquid electrolytes, for which electrostatic screening lengths and ion conductivity are both observed to deviate significantly from predictions of classical theories at high concentrations. A second source of complexity is that each system is operated in a different way in practice, and the way a system is used strongly influences both short- and long- term performance, with each system following a unique degradation trajectory. This thesis will demonstrate that both of these challenges can be tackled through the combination of data, physical intuition and machine learning. Machine learning models can learn from orders of magnitude more data than humans can, and we will see that such models can be trained to make more accurate predictions about how electrochemical systems will perform under different operating conditions. In addition, machine learning can act as a “computational microscope”, offering new ways of understanding the molecular origin of macroscopic properties. I begin by addressing the enigmatic under-screening effect observed in concentrated electrolytes, and explore whether discrepancies between classical theory and experiment can be explained using the concept of ion pairing, by identifying the number of statistically distinct environments inhabited by ions. The results bring into question the validity of the ion pair hypothesis for concentrated systems, but more importantly they suggest that static properties of electrolytes, such as screening length, can be explained by studying statistical differences between local ionic environments rather than just the mean local environment as captured by the radial distribution function. Extending this, I then posit that global dynamic properties such as ion conductivity can also be decomposed into atomistic contributions that are functions of local static structure. The idea is to learn the mapping from local structural motif to a local contribution to conductivity, which effectively generalises ideas first put forward in the ion-pair hypothesis or “cluster Nernst-Einstein” theory. By studying the distributions of local conductivities across electrolytic systems we can decipher what structural motifs are correlated with enhanced or degraded conductivity. I then turn to address the system level challenge of forecasting how electrochemical systems will respond to different operating conditions. In the growing lithium-ion battery industry, this is a major challenge, since batteries of the same chemistry will respond differently to the same use conditions due to differences in internal state caused by manufacturing heterogeneity and different extents of degradation. We develop a general framework that combines electrochemical impedance spectroscopy with machine learning to predict how a battery will respond to a given use condition, which has relevance for the design of improved battery management systems. The findings of this thesis help to further our understanding of the fundamental mechanism by which inter-ion interactions give rise to screening and conduction within liquid electrolytes. More broadly, these findings can help to guide the design of novel electrolytes for next generation systems, to develop optimal fast-charging protocols that do not sacrifice battery life, and to triage batteries towards second-life applications.
Embargo
Understanding and optimizing perovskite optoelectronic devices with multi-dimensional imaging techniques
Ji, Kangyu; Ji, Kangyu [0000-0002-1278-3212]
This thesis explores the application of multi-dimensional optical imaging techniques in understanding and optimizing perovskite-based optoelectronics. Chapters 1 and 2 give the motivation behind this work and background to perovskite optoelectronics and machine learning. Chapter 3 introduces the main experimental techniques. The debated passivation strategies on perovskite solar cells (PSCs) are studied in Chapter 4 through quantitative hyperspectral imaging. Specifically, alkali metal passivation imposes distinct effects on the optical and structural properties of the devices based on different transport layers. It is shown that the formation of secondary phases, either due to the additives in the perovskite precursor or in the transport layer, leads to an increase in nonradiative recombination and local open-circuit voltage loss. This provides important guidance to the development of passivation techniques toward efficient and stable PSCs. Chapter 5 expands the capability of the latest hyperspectral microscopy technique by developing a machine-learning-based image processing algorithm that is suitable for scientific research. The proposed algorithm achieves state-of-the-art denoising performances compared to recent ML models and conventional handcrafted algorithms. It is able to strengthen signals from unknown samples under low illumination conditions by exploiting spectral information and adopting a self-learning approach. This enables fast and low-dose measurements for emerging semiconductor materials with poor stability. Chapter 6 characterizes perovskite materials for a wide range of applications using the imaging platform developed in Chapter 5. The degradation of mixed halide perovskite light-emitting diodes (LEDs) is tracked through in-situ PL and in-operando electroluminescence mapping. We reveal that lateral ion migration under device operation leads to the growth of chloride-rich defective regions that emit poorly. This is the first time lateral halide migration is observed in perovskite LEDs due to locally-varying electric fields. Finally, Chapter 7 summarizes all the findings and discusses future research directions. The research presented in this thesis, with approaches across the multidisciplinary scientific fields of physics, material science, and machine learning, paves the way for computer vision-accelerated development of emerging technologies towards commercialization and scale-up.
Embargo
Ultra-thin photovoltaic technologies with nanophotonic textures for enhanced solar energy harvesting
Camarillo Abad, Eduardo; Camarillo Abad, Eduardo [0000-0001-8617-0059]
Ultra-thin solar cells are a nascent photovoltaic strategy that holds ample advantages compared to conventional thick technologies, spanning from cost and material savings to low weight and flexibility. Solar cells on this length-scale also exhibit intrinsic tolerance to radiation damage, thus being uniquely suited for space power applications. Despite this breadth of compelling characteristics, poor solar energy harvesting remains a fundamental challenge for the advancement of ultra-thin solar cells, as device thickness is intrinsically related to the efficiency of sunlight absorption and power generation. Maximising the absorption of sunlight in ultra-thin solar cells requires careful optimisation of the full device design, tailored to minimise i) front surface reflection by means of antireflection coatings, ii) transmission losses into the substrate by means of a rear mirror, and iii) outcoupling losses by means of light-trapping textures. Simultaneous application of these strategies, within architectures possessing maximal transparency outside the active region, is imperative to target competitive photovoltaic efficiencies on such reduced length-scales. In this thesis I investigate viable platforms to achieve optimal and holistic light management in ultra-thin solar cells. I give particular attention to the development of advanced light-trapping strategies, concentrating on texture designs to exploit waveguide resonances as absorption enhancement mechanisms. For this purpose, I build a robust framework to study waveguide modes in ultra-thin devices with integrated textures. Based on this framework, comprehensive computational investigations unravel design-driven mechanisms to regulate the light-harvesting potential of these optical modes. My studies escalate into the development of novel light-trapping platforms beyond current paradigms, where the full texture design is tailored to mold its scattering profile together with the modal structure of its host device, allowing engineering of their overlap and unlocking superior waveguiding benefits. Ultimately, my investigations culminate in the identification of light-trapping designs that forecast remarkable 20% photovoltaic efficiency in an 80 nm GaAs cell. Such outstanding potential is supported by the experimental validation of computational techniques, as well as the demonstration of feasible fabrication approaches for the integration of favourable texture designs. Overall, the work presented herein provides fundamental and practical insights for the advancement of ever-thinner photovoltaic technologies.
Open Access
Synthesis and Characterisation of Singlet Fission and Open-Shell Materials for Optoelectronic Application
Purdy, Michael
Indolonaphthyridine materials for singlet fission were systematically investigated. Firstly, cibalackrot was investigated as a singlet fission material. Single crystals of cibalackrot revealed insufficient electronic coupling was inhibiting singlet fission in the solid-state. A synthetically modified aza-cibalackrot was developed that had enhanced electronic coupling and exhibited singlet fission. Throughout the synthetic journey we developed a novel crystal engineering strategy that can be used to “turn on” singlet fission. Following this, the mechanism of indolonaphthyridine intramolecular singlet fission was investigated. A series of indolonaphthyridine dimers were synthesised that were conjugated using different bridging groups. The rate of singlet fission was found to be highly dependent on the extent of electronic coupling. Charge-transfer states were also found to mediate singlet fission. We revealed the traditional design principles for singlet fission dimers cannot be applied to modern polarisable systems. The relationship between molecular symmetry and charge-transfer character was then explored. A series of novel asymmetric indolonaphthyridines were synthesised that exhibited enhanced excited state charge-transfer character. Novel, high yielding bay-annulation chemistry was developed to synthesise the asymmetric indolonaphthyridines. The structureproperty relationships introduced in this chapter could potentially be used to develop high performance optoelectronic materials. Lastly, the electronic structure and charge-transport properties of ultra-narrow band gap materials was investigated. Using novel polymerisation chemistry, indolonaphthyridine was co-polymerised with thiadiazole quinoxaline yielding a polymer with an exceptionally narrow band gap. Charge transport measurements revealed the polymer had ambipolar mobility. Interestingly, the polymer also showed open-shell character.
Open Access
Sound Waves and Turbulence in Two-dimensional Bose Gases
Galka, Maciej; Galka, Maciej [0000-0002-9456-1073]
This thesis describes two experiments carried out on a two-dimensional (2D) Bose gas cooled down to quantum degeneracy. In the first one we focus on the equilibrium properties of the gas close to the Berezinskii-Kosterlitz-Thouless (BKT) phase transition, while in the second one we study the out-of-equilibrium dynamics of the system. Our experiments are performed using a gas of $^{39}$K atoms confined in a 2D box trap, which guarantees the homogeneous gas density and allows tuneability of the interaction strength. We begin with the studies of the propagation of sound. For temperatures below the BKT critical temperature, we observe first and second sound, as predicted by the hydrodynamic two-fluid model. From the two temperature-dependent speeds of sound and the established thermodynamic properties of the ultracold 2D Bose gas, we extract the superfluid density as a function of temperature. Our results agree with the predictions of the BKT theory, including the universal jump of the superfluid phase-space-density at the transition point. The second part of the thesis concerns the emergence of the wave-turbulent cascade, as the gas is driven further and further from equilibrium. We monitor how the cascade builds up from large to small length scales starting from the microscopic dynamics of the discrete low-lying quantum states of the system. By probing the gas on all relevant length and time scales we directly observe the emergence of statistical isotropy under anisotropic forcing, and the self-similar spatio-temporal scaling of the momentum spectrum, which are two key theoretical expectations associated with the development of turbulence.
Open Access
Two leptons, jets, and missing transverse momentum in ATLAS: yet more non-evidence for supersymmetry
Tombs, Rupert
Searching for new phenomena is an essential endeavour of experimental physics, and is helped by guidance from plausible theoretical predictions. This thesis presents such a search for signals of high-energy protons scattering to massive resonances with dramatic decays, which are predicted by supersymmetric models. No new phenomena appear in the results, so they support the Standard Model in its continuing successful extrapolation to describe the most extreme behaviours of proton collisions. Supersymmetry as a concept is not falsified by this lack of new phenomena, nor by the many similar observations. But it is also not confirmed. Data are best interpreted with an understanding of their origins, so we begin by describing the theoretical and experimental background to our search. Our results also depend strongly on the interpretation of data by conventional analysis procedures, whose properties and explanations are not obvious. To explain these procedures, we also discuss their historical context and the basic theory of data analysis.
Embargo
Engineering Internal and External Molecular Pathways in Artificial Cells
Leathers, Adrian; Leathers, Adrian [0000-0003-0226-0781]
Bottom-up synthetic biology aims to engineer artificial systems exhibiting biomimetic struc- ture and functionality from the rational combination of molecular and nanoscale elements. These systems often take the form of artificial cells: micron-scale, cell-like constructs that display advanced behaviours such as communication, synthesis, storage of molecules, adap- tation and motion. Artificial cells require micro-compartmentalised architectures to host and maintain separation between the various signal-processing and functional elements underpin- ning their responses. Compartmentalisation can be achieved with different solutions, most of which rely on semi-permeable membranes. However, membrane-less implementations based on hydrogels, biomolecular condensates, or coacervates are gaining traction due to their advantages in terms of design versatility and robustness. DNA nanotechnology applies our understanding of the properties of nucleic acids to program assembly and disassembly of molecular and nanoscale complexes. This has led to the creation of intricate constructs with programmable structure and kinetic response. These advantages, along with sensitivity to environmental stimuli, aptameric targeting, and ease of chemical functionalisation, have made DNA nanosystems widely used in bottom-up synthetic biology. Among the many classes of DNA-based building blocks, branched DNA motifs have been shown to aggregate into hydrogels and crystal phases. Among possible implementations, cholesterol-modified DNA junctions have been shown to robustly self-assemble into cell-size condensates with programmable features, including pores size, chemical functionality and integration of responsive molecular circuitry. These properties make amphiphilic DNA condensates promising as scaffolds for artificial cells. In this thesis I introduce a platform for engineering artificial cells reliant on self-assembled amphiphilic DNA condensates, specifically tackling two key challenges: the establishment of internal compartmentalisation and the design of inter-cell communication pathways. To engineer internal architecture, I rely on controllable reaction-diffusion processes. I show that these can generate chemically addressable domains within the condensates, with control- lable number, shape and molecular makeup. The patterning processes can be rationalised and guided by numerical modelling. As a proof-of-concept, I apply this technique to construct a prototypical artificial cell displaying spatial separation of functionality, namely, a nucleus capable of synthesising RNA and a cytoplasm-like storage domain which can accumulate it. As a means of establishing self-sustaining long-range communication between two pop- ulations of condensates, I took steps towards the design of molecular circuits capable of non-enzymatic signal amplification. Overall, my work showcases the potential of DNA condensates as a platform for bottom-up synthetic biology, which can unlock the design of biomimetic systems with ever increasingly advanced functionalities.
Open Access
Static and Dynamic Disorder in Emerging Optoelectronic Materials
Bravic, Ivona
The digital revolution, which commenced with the invention of the germanium based transistor in 1947, provided an unprecedented boost in semiconductor research and manufacturing. Advances in the synthesis of pure semiconductors of first and second generation including silicon, germanium and gallium arsenide has not only opened doors to novel electronic technologies but also facilitated the design and manufacturing of so-called optoelectronic devices. The resulting scientific narrative that emerging materials are bearers of novel technologies has led to the discovery of manifold new semiconducting material types which significantly differ from the above materials in structure and arising charge-carrier species. It remains the task of current researchers to extend the concepts and theories of light-matter interactions and design to novel materials including Van der Waals and two-dimensional materials, organic semiconductors and to all-inorganic as well as hybrid perovskites. This work looks into some of these novel materials, focusing on the role of structure as well as static and dynamic disorder on the optoelectronic properties using first-principles electronic structure theory. We first investigate the III-V semiconductor boron arsenide, which exhibits similar absorption features to those of silicon but has a much higher room-temperature thermal conductivity. We employ density-functional theory (DFT) combined with finite differences to study, how dynamic disorder impacts its optoelectronic properties at operating temperatures of photovoltaic (PV) devices. We show, that electron-phonon coupling and electron-electron correlation have a strong impact on the temperature-dependence of the band gap, while it remains fairly robust with respect to thermal expansion. Additionally, we find that the absorption coefficient at the indirect absorption onset is six times higher than that of silicon, leading to a higher absorption cross-section and to potentially interesting PV applications. We then look into two chemically related Van der Waals materials, namely bismuth triiodide (BiI3) and bismuth oxyiodide (BiOI), that exhibit promising optoelectronic properties for PV applications. Here we use a combination of DFT and many-body theory together with finite differences as well as transient spectroscopy to show, that BiI3 is an intrinsically poor semiconductor for photovoltaics due to its strongly bound photogenerated electron-hole pair, prohibiting charge carrier separation and high charge-carrier densities. In contrast, the photoexcited carriers in BiOI are delocalised within the Van der Waals layer and, despite exhibiting strong carrier-phonon coupling, their delocalisation remains intact. The low absorption coefficient at the direct absorption onset is a result of a symmetry-forbidden optical transition and combined with nonradiative decay channels at room temperature, these properties make BiOI a rather poor material for PV devices as well. Instead, we illustrate, that its charge-carrier features make BiOI a suitable X-ray detector material. Lastly, we study the impact of static, rather than dynamic disorder in the form of chemical doping in the all-inorganic lead-halide perovskite CsPbX3 (X = Cl, Br) on its band gap and its band dispersions using DFT. We propose, that the chemical disorder in this system created by B-site substitution improves the optoelectronic properties required for efficient light-emitting diodes (LED). The overarching goal of this thesis is to find intuitive explanations to photophysical phenomena occurring in these novel materials related to structure and disorder, creating ideas about rational design of novel semiconducting materials with desirable optoelectronic properties for innovative and more environmentally sustainable technologies.
Embargo
Single-electron transport and electron-phonon interactions in graphene heterostructured self-assembled molecular solid-state devices
Ning, Shanglong
This thesis presents a scalable approach to fabricating solid-state molecular junctions, featuring large-area self-assembled monolayers (SAMs) of molecules and nanocrystals (NCs). The investigation of electrical measurements related to intrinsic molecular properties is carried out through three interconnected projects. Each junction consists of a heterostructure composed of Au as the bottom electrode, SAM and/or NCs as the middle layer, and single-layer graphene as the top electrode. The first project focuses on single-electron phenomena in finger-design and microwell devices, such as the Coulomb staircase, accompanied by three distinct types of negative differential resistance, hysteresis, and random telegraph noise. Devices were fabricated using 5 nm and 2 nm PbS nanocrystals attached to SAMs derived from alkanedithiols and a series of oligo(arylene ethynylene) (OAE) molecules. The second project involves devices with SAMs of long-chain alkanethiolates (with more than 12 carbon atoms, particularly 1-hexadecanethiol) without NCs. These devices exhibit equidistant $I$-$V$ steps and conductance peaks at liquid-helium temperature, sharing similarities with the Coulomb staircase observed in single-electron transport. A model based on strong electron-phonon coupling, involving a single spin-degenerate energy level and one vibrational mode, is proposed. Statistical analysis is performed to study the spacing, and temperature-dependent measurements are carried out to search for phonon-absorption peaks. Negative differential conductance at the onset of specific current plateaus is observed for certain gate voltages using a bias-cooling method. This method is designed to gate the samples with ionic liquid in a liquid-helium dewar. Both agreements and inconsistencies with the proposed model and other hypotheses are discussed. The third project investigates Fermi level control by examining various molecule-electrode interfaces and molecular backbone structures. Visualization of the molecule's orbital alignments relative to the Fermi level of the electrodes is achieved through ionic liquid gating at room temperature. The conductance displays a minimum, which varies between molecules with different anchoring groups, signifying their distinct orbital energies relative to the Fermi energy of the leads. In summary, the findings from these three projects contribute to the pursuit of scalability, electrostatic gating, and the simultaneous observation of inherent molecular properties in molecular electronics.
Open Access
The chemical evolution of galaxies explored through multi-object integral field spectroscopy
Hayden-Pawson, Connor; Hayden-Pawson, Connor [0000-0001-7964-1027]
Galaxies are expected to grow and evolve via a series of physical processes relating to gas flows into and out of the galaxy. Inflows of gas from the surrounding cosmic web provide fuel for star formation, which subsequently causes an enrichment of the interstellar medium (ISM) with the metals produced within stars, whilst supernovae-driven outflows drive gas out of the galaxy, re-distributing metals in the process. In this way, measurements of chemical abundances within galaxies can provide insight into the different physical processes that drive galaxy evolution. The interplay between these different processes has been well studied in the local Universe by large spectroscopic surveys that have established a number of scaling relations between stellar mass, star formation rate and gas-phase metallicity. However, the existence of such relations at earlier times in the Universe is less well studied. The aim of this thesis is to investigate the evolution of chemical abundances within galaxies across cosmic time, making use of integral field spectroscopic data obtained through the KLEVER survey. In the first part of this thesis, I compare the galaxy-integrated properties of galaxies at z=2 to those found in local galaxies, with a particular focus of the abundance of nitrogen relative to oxygen (N/O). I find that high redshift galaxies have similar N/O values to local galaxies at a fixed metallicity, but much lower N/O values than local galaxies at a fixed stellar mass. I then demonstrate that an anti-correlation exists locally between N/O and star formation rate, such that at a fixed stellar mass galaxies with higher star formation rates have lower N/O values. In light of this, I parameterise a three-dimensional relationship between stellar mass, star formation rate and N/O abundance, before demonstrating that this relationship accurately predicts the N/O ratios of galaxies at z=2 as well as those observed locally. As such, I name this relationship the fundamental nitrogen relation (FNR), in analogy to the fundamental metallicity relation (FMR). Furthermore, I show that the measured FNR is well described by a simple combination of the FMR and a non-evolving relationship between N/O and metallicity. These results suggest that the physical processes that govern the FMR must be sensitive not only to the metallicity, but also the N/O abundance. In the second part of this thesis I extend my analysis to the spatially resolved scale, studying the spatial distribution of N/O in galaxies at z=2. I present some of the first measurements of N/O gradients at z=2, finding they are generally flatter than those found locally. This is contrary to inside-out growth models, which predict steeper gradients at earlier times, however this difference may be reconciled by invoking star-formation driven feedback mechanisms that effectively mix metals within the ISM. I present observations of inverted N/O gradients, which I suggest may be a consequence of the inverted metallicity gradients also observed at high redshift. I also present evidence for negative Balmer decrement gradients within z=2 galaxies, consistent with high levels of star formation in the galaxy centre that may be associated with early bulge formation. I note that the slope of the N/O gradients is dependent on the choice of diagnostic used to determine the N/O, suggesting this may be driven by differences in the ionisation properties of sulphur relative to oxygen. Finally, in the third part of this thesis I present preliminary work analysing the scatter in the relationship between N/O and O/H for local galaxies. I present observations of a population of galaxies with low metallicities that have enhanced N/O abundances. I show that the galaxies with the highest N/O values also have higher stellar masses and star formation rates. I then investigate the possibility that these galaxies have undergone recent gas accretion, driving changes in their metallicities and N/O values whilst boosting their star formation. I compare to a simple gas mixing model, finding that the deviations of galaxies from their expected metallicities and N/O values can be well modelled by the accretion of metal rich gas with a metallicity equal to 55% of that of the galaxy. However, the models also predict that the gas fraction within the galaxy is expected to increase by between 0.64-1 dex during the accretion event, much larger than the changes in gas fraction inferred from the observed deviations from the star forming main sequence for local galaxies. I demonstrate that the expected changes in gas fraction are better matched by accretion of lower metallicity gas, however such models are unable to reproduce the observed decrease in N/O from the expected values. I conclude that improved models are needed that include prescriptions for star formation, chemical enrichment and gas outflows in order to better constrain the impact of dilution events on the N/O values and metallicities within galaxies.
Open Access
Development of Liquid Crystalline Elastomers as Soft Mechanical Actuators
Lin, Xueyan
Liquid crystalline elastomers (LCE) are a class of loosely cross-linked rubbers that combine native liquid crystal phases (e.g., orientational order) and conventional elastomeric properties (e.g., rubber elasticity). Due to their ability to undergo spontaneous and reversible shape change when subjected to external stimuli, there is a continuously growing interest in the scientific community to construct soft mechanical actuators from these materials. However, the development of LCE actuators is currently limited within the academic community and it lacks interest from the industry. Despite being studied for the past 30 years, LCEs have not yet been used in any real-world applications, which can be attributed to several problems: First, the external stimuli to trigger LCE actuation is demanding (i.e., stimuli can be difficult to deliver). Second, less desirable properties of the LCE limit their deployment (i.e., diminishing mechanical properties during their actuation). Third, industries simply need to take time to absorb the exotic features of LCE material. This thesis aims to solve these three major hurdles. Regarding the first problem, this thesis attempted to use light actuation in LCE with the assistance of upconverting nanoparticles and azobenzene dyes. However, due to time and resource limitations, they yielded no successful discovery in the end. Nevertheless, some experimental plans were made in order to inspire the reader to join the further discussion. The second problem is caused by the decline of rubber modulus in LCE (at high temperatures), which causes reduced mechanical properties thus limiting their potential as soft actuators. To solve this, the thesis discussed different strategies to improve the mechanical properties of LCEs during their actuation. Various modification solutions to the material were explored, and the best procedures were selected to construct LCEs that are resistant to temperature change. Moreover, a new type of vitrimer-enabling mesogen and their corresponding LCEs were discovered and synthesised, which added to the existing library of LCE modification and offered extra functionality in LCE reprocessing. For the third problem, this thesis aims to accelerate the technical development of LCEs by discussing various LCE fabrication techniques using the existing methods from industries. The thesis successfully used the known direct-ink-writing technique (widely used in 3D printing) to fabricate LCE with sophisticated actuation patterns. The thesis introduced a scalable fiber spinning process that can cross-link the material on the fly to achieve a massive rate of production. The thesis also demonstrated how the spun LCE fibers are suitable to be made into smart fabrics using a common weaving machine. Finally, the thesis discussed some additional experiments that can be conducted in the future to fill the oversights in this PhD work and to complete our understanding of LCE material. On the sideline, a new project was also proposed to package those failed experiments in this thesis and to explore the exciting unknown territory.
Open Access
Observing the 2D Bose glass in an optical quasicrystal
Yu, Jr-Chiun
Quasicrystals are a class of materials that are long-range ordered yet not periodic. These unique features make them a fascinating middle ground between order and disorder, thus providing an ideal platform for studying a wealth of physical phenomena. This thesis contains three experimental works that my colleagues and I achieved during my PhD period, in which we explored the properties of quasicrystals in many aspects. The first experiment (Chapter 5) focus on the long-range order in quasicrystals. We report on the first experimental realisation of a two-dimensional (2D) quasicrystalline optical lattice for ultracold atoms and conduct matter-wave diffraction experiment using shot, intense lattice pulses. We reveal the different diffraction dynamics between periodic and quasiperiodic lattices and demonstrate the capability to simulate quantum walks on four-dimensional tight-binding lattices using the diffraction dynamics of 2D quasicrystalline lattices on short timescales. The second (Chapter 6) and the third (Chapter 7) experiments concentrate on studying the disorder nature of quasicrystals. In particular, we investigate disorder-induced localisation transition in the ground state of non-interacting and weakly interacting Bose gases in a 2D quasicrystalline lattice. The second experiment is mainly performed in the non-interacting limit, in which we probe the localisation transition by employing triangular lattice pulses and studying the time scale required for adiabatic loading. We observe a localisation transition at around a critical lattice depth of Vloc ≈ 1.78 Erec for noninteracting systems. In addition, we demonstrate that weak repulsive interactions can shift transition to deeper lattices. In the third experiment, we further study the interplay between the interactions and disorder, and constitute the first experimental realisation of the 2D Bose glass, an insulating but compressible groundstate phase without long-range phase coherence. By probing the coherent properties of the system, we observe a Bose glass to superfluid transition and map out the phase diagram in the weakly interacting regime. Moreover, we reveal the non-ergodic nature of the Bose glass by probing the capability of restoring coherence. Our observations are in good agreement with recent quantum Monte Carlo predictions and pave the way for experimentally testing of the connection between the Bose glass and many-body localisation.
Embargo
Development of Active Terahertz Modulators for use with Quantum Cascade Lasers and Near-Field Imaging of their Individual Resonators
Almond, Nikita
This thesis describes the development of active terahertz modulators based on arrays of coupled resonators loaded with monolayer graphene. The structures affect the amplitude, phase, and polarisation of the transmitted and reflected radiation. These devices were simulated using finite element modelling (FEM) software, fabricated using standard cleanroom lithographic techniques and characterised using terahertz (THz) time-domain spectroscopy (TDS). Devices designed to modulate the phase and frequency of the transmitted and reflected THz radiation were fabricated and characterised operating at ∼ 1.5 THz and 3 THz. Both the 3 THz and 1.5 THz devices were coupled with THz quantum cascade lasers (QCLs). The phase modulators were integrated with a partially suppressed QCL to form an external cavity. The devices acted as optoelectronic mirrors. By changing the bias on the device, the reflected phase changed the effective length of the cavity and affected the mode competition within the laser. The same experiment was also conducted with a gold mirror on a micrometre stage, and the position was changed to vary the external cavity length. They produced commensurate results. These results were compared with simulations of the external cavity and QCL set-up using reduced rate equations, which showed good agreement. Different devices that use coupled bright and dark resonators with capacitative coupling and some that use a double layer of resonators and magnetic coupling were simulated and fabricated. They were designed to change the polarisation of a linear THz beam, changing either the angle of rotation of the linear polarisation or converting from linear to elliptical polarisation, depending on the frequency. These devices were then coupled with THz QCLs to manipulate the linear polarisation output. For further improvement and optimisation of active metasurface devices, we need to understand the behaviour of the individual elements in response to THz incident illumination and their interaction with each other, which is not possible with FEM software. In response, metasurface devices were investigated with room temperature aperture and scattering scanning near-field optical microscopes (SNOM), which can provide subwavelength resolution. Furthermore, they showed the bonding and anti-bonding modes of the sub-wavelength features. For future direct integration of metasurfaces with the QCL facet inside the cryostat, the development of a scattering SNOM (s-SNOM) operating at cryogenic temperatures (∼10 K) was started. However, this proved to have multiple unforeseen difficulties, and much work was undertaken to reduce the vibration so that topography could be taken and so that it functioned as a cryogenic atomic force microscope (AFM).
Open Access
Spectroscopic studies of star-forming galaxies and the intergalactic medium in the early Universe
Witstok, Joris; Witstok, Joris [0000-0002-7595-121X]
Current observational facilities, such as the Very Large Telescope (VLT), Hubble Space Telescope (HST), and Atacama Large Millimeter/submillimeter Array (ALMA), have enabled us to perform detailed spectroscopic analyses of distant galaxies well into the Epoch of Reionisation (EoR). This crucial phase transition witnessed baryonic matter, mostly in the form of cold, neutral hydrogen gas, being chemically enriched, ionised, and heated as a result of the formation of the first stars and galaxies. Here, I present the results of several studies aiming to shed light on the early evolutionary stages of galaxies and their contribution to Cosmic Reionisation. Using cosmological hydrodynamical simulations, I consider the prospects of mapping the intergalactic medium (IGM) in the most prominent hydrogen emission line, Lyman-α. Turning to observations, I present and analyse multiple spectroscopic datasets of individual high-redshift galaxies with the aim of understanding the process of star formation on the scale of the interstellar medium. Firstly, I show the spectroscopic measurements of a unique, strongly gravitationally lensed galaxy at redshift 5, taken by VLT/X-shooter and VLT/SINFONI, are consistent with a young, metal-poor, star-forming system with a hard radiation field. This galaxy is likely analogous to typical EoR galaxies, revealing Lyman-α and MgII emission lines that may indicate the leakage of ionising photons into the IGM. Secondly, focussing on far-infrared and rest-frame UV observations of five UV-bright, star-forming galaxies at redshift 7 obtained with ALMA and HST respectively, I show these measurements point towards similar physical properties, though there are hints of substantial metal enrichment in these systems. Constraints on the dust continuum of one source indicate the presence of a surprisingly cold and massive dust reservoir. Finally, I discuss directions for future work, in particular the synergy of existing observatories in combination with JWST, the much-anticipated near- and mid-infrared space-based observatory that has recently started acquiring spectroscopy of the most distant galaxies.
Open Access
Magneto-Optical Effects in Manganese-doped Lead Halide Perovskite
Neumann, Timo
Magnetic doping holds a great potential for tailoring material properties of semiconductors via control over exciton-dopant interactions. Hybrid metal-halide perovskites are a new class of high-performance solution-processable semiconductors which combine remarkable optoelectronic performance with tolerance to structural defects and impurities. This thesis demonstrates magnetic doping of lead halide perovskites and investigates the resulting magneto-optical effects on exciton dynamics. We synthesise manganese-doped 2D Ruddlesden-Popper perovskite phenethyl ammonium lead iodide thin films and characterise the material’s basic properties. We report qualitative evidence that, depending on doping concentrations, Mn2+ ions occupy different lattice sites, e.g., either substituting lead or incorporating interstitially, or being phase separated from the host perovskite. We find that this manganese doping induces paramagnetic properties in the otherwise diamagnetic perovskite, due to introduction of Mn2+ ions that carry magnetic moment with S = 5/2 spin state into the perovskite host semiconductor. Employing spin resonance and magneto-optical spectroscopy, we find coupling between the dopant’s magnetic moment and the exciton spin of the semiconductor. We report magnetic brightening of a dark exciton population by state mixing with the bright excitons at cryogenic temperatures. We show that manganese doping leads to a significant enhancement of this effect. For this dark exciton, we further report that manganese doping creates strongly circularly polarized luminescence, depending on the alignment of the Mn dopant’s magnetic moment. We attribute our observations to spin-dependent exciton dynamics at early times after excitation and show preliminary ultrafast spectroscopy measurements supporting this interpretation. Our results demonstrate magnetic doping as an approach to control spin physics of exciton populations in perovskites. These findings will stimulate research on this highly tuneable material platform with promise for tailored interactions between magnetic moments and excitonic states.
Open Access
Development and Characterisation of Halide Perovskite Visible Light and X-Ray Detection Devices
Moseley, Oliver
Sending and receiving information with electromagnetic radiation is fundamental to imaging and communications. Humankind’s ability to utilise this radiation is dependent on how efficiently we can detect it, and improving detectors will advance these technologies. Visible light and X-rays make up two regions of the electromagnetic spectra, both key bands for imaging in different modalities. The visible spectrum represents the energies we can detect with our eyes, while X-rays are highly penetrative and allow the inside of opaque objects to be imaged. While their applications are highly complementary, their detector technologies also share commonalities, allowing the development of both simultaneously. In this thesis, the unique properties of metal halide perovskites are utilised to advance both visible light and Xray detectors. However, other properties of perovskites, such as ion migration, can provide challenges when characterising performance, and so techniques to accurately measure their detecting ability are also developed. Metal halide perovskites are exploited to design two new detector device structures, increasing the functionality of photodetectors and overcoming the existing limitations of direct X-ray detectors. A unique photodetector, utilising the band gap tunability of perovskites, is developed to produce a multiband response that can controllably detect different regions of the visible spectrum. The resulting device is employed in a method to send communications with added encryption. Additionally, a concept for a novel X-ray detecting device is developed, using the ability of perovskites to retain impressive properties after low-temperature solution deposition. The device structure decouples the dimensions of photon absorption and charge carrier collection to retain performance across the X-ray spectrum, overcoming the limitations currently preventing the commercial success of direct X-ray detectors. The potential of perovskites as scintillators for indirect X-ray detection is investigated. The published performances are contextualised with a detailed analysis of the operating mechanism. This mechanistic insight highlights the advantages this material could bring, and we propose the applications that would benefit most from perovskite scintillators, as well as the origins of the remaining limitations. The concurrent understanding of perovskites in other optoelectronic devices is utilised to suggest pathways to overcome the remaining challenges and bring the material closer to commercialisation. These suggestions are applied, and impressive scintillation performance is demonstrated from an emerging Cs2ZrBr6 nanocrystal scintillator system. This work also highlights the specific considerations required when characterising perovskitebased detectors. The large defect density in these materials is shown to be a double-edged sword; making measurements under low light intensities prone to errors, but also acting as another lever to control detection performance. The challenges of characterising direct X-ray detectors are also discussed, alongside the development of experimental procedures to robustly measure halide perovskite devices. Overall, this thesis utilises the unique properties of perovskites to develop detectors with new functionality, whilst ensuring the same properties do not reduce the accuracy when characterising their performance. The work brings perovskite detectors one step closer to a commercial reality.
No Thumbnail Available
Metadata only
Additively Manufactured Smart Cellular Materials for Reconfigurable, Light-weight Impact Energy Absorbers
Simoes, Marlini; Simoes, Marlini [0000-0002-2237-3276]
[Restricted]

Browse

Recent Submissions