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Polyoxometalate nanoscale electronic devices
No full textElectronic memories play a crucial role in our ever more digital world. Traditional tech-nologies are reaching their limits as new computing paradigms continue to increasetheir demand for computing power, efficiency, and miniaturisation. Resistive randomaccess memories (RRAM) have emerged as promising candidates to solve these issues.Specifically, polyoxometalate (POM) -based devices show particular potential due totheir rich redox properties.This thesis explores the development of POM-based nanoscale electronic devices fornext-generation memory applications and neuromorphic computing. Throughout thiswork, I show that the phosphomolybdate POM, H3PMo12O40 is promising: it demon-strates rich redox activity especially when carefully deposited in nanogap separatedAl/Au asymmetric coplanar electrodes. By adding a thin poly(methyl methacrylate)(PMMA) layer between the metal electrodes and the POM film, device performancecan be enhanced, while addressing challenges such as device-to-device and cycle-to-cycle variations. My analysis reveals that a complex combination of factors explainsthe switching mechanism of these devices including: the POM redox reactions, envi-ronmental factors such as moisture, and device structure-related effects.I showcase the ability of these devices to mimic some operation of the biological brainand specific neural responses such as nociception, opening new possibilities for artifi-cial neural network technologies.This work advances the field of nanoelectronics by optimising both device architectureand material selection. My findings pave the way for the development of more effi-cient and tuneable bio-inspired computing systems. These POM-based devices offersolutions to overcome current technology limitations, potentially shaping future elec-tronic systems
Robust calibration of shaft and base resistance factors for piles based on multiobjective optimization
No full textResistance factors are used to account for the uncertainties associated with pile resistance in load and resistance factor design (LRFD). Current design codes and most previous studies recommend a single resistance factor applied to the total pile resistance (shaft and base resistances). However, the uncertainties associated with shaft and base resistances are significantly different. Moreover, resistance factors are generally calibrated based on the statistics of resistance bias factors derived using all data collected from different sites, whereas the variability of the statistics between various sites (i.e., cross-site variability) has been ignored in the traditional calibration approaches, which may result in the designs based on the calibrated resistance factors violating safety requirements. In this paper, a robust calibration approach is proposed to calibrate shaft and base resistance factors, explicitly considering the cross-site variability in the statistics of resistance bias factors in the calibration process. To achieve that, the feasible robustness concept is adopted to describe the probability that the design remains able to achieve the target reliability index when the statistics of resistance bias factor exhibit cross-site variability. The calibration process is implemented through a multiobjective optimization, which leads to a Pareto front that describes the trade-off relationship between shaft and base resistance factors and feasible robustness. The optimal shaft and base resistance factors are determined using the minimum distance approach. The proposed approach is demonstrated and applied to calibrate shaft and base resistance factors for three design methods, the Vesic, Meyerhof, and Nordlund methods. Results show that resistance factors are significantly affected by design methods and the ratio of shaft and base resistances.</p
Ketone monoester ingestion improves cardiac function in adults with type 2 diabetes: a double-blind, placebo controlled, randomised, crossover trial
No full textType 2 diabetes (T2D) is a metabolic disease associated with cardiovascular dysfunction. The myocardium preferentially uses ketones over free fatty acids as a more energy efficient substrate. The primary aim was to assess the effects of ketone monoester (Kme) ingestion on cardiac output index (Q̇i). Secondary aims were to assess the effects of Kme ingestion on markers of cardiac haemodynamics, muscle oxygenation and vascular function at rest, during and following step-incremental cycling.We undertook a double-blind, randomised, crossover design study in 13 adults (age, 66±10 y; BMI, 31.3±7.0 kg·m−2) with T2D. Participants completed two conditions, where they ingested a Kme (0.115 g‧kg−1) or a placebo taste-mathced drink. Cardiac function was measured using thoracic impedance cardiography and muscle oxygenation of the calf was determined via near-infrared spectroscopy. Macrovascular endothelial function was measured by flow mediated dilation (FMD) and microvascular endothelial function was measured via transdermal delivery of acetylcholine (ACh) and insulin. Circulating β-hydroxybutyrate [β-Hb] was measured throughout.Kme ingestion raised circulating β-Hb throughout the protocol (peak 1.9 mM; P=0.001 vs. placebo). Kme ingestion increased Q̇i by 0.75±0.5 L∙min−1∙m−2 (P=0.003) stroke volume index by 7.2±4.5 mL∙m−2 (P=0.001), and peripheral muscle oxygenation by 9.9±7.1% (P=0.001) and reduced systemic vascular resistance index by-420±-225 dyn∙s−1∙cm−5∙m−2 (P=0.031) compared to placebo condition. There were no differences between Kme and placebo in heart rate (P=0.995), FMD (P=0.542), ACh max (P=0.800), insulin max (P=0.242).Ingestion of Kme improved Q̇i, stroke volume index and peripheral muscle oxygenation, but did not alter macro- or microvascular endothelial function in people with T2D.<br/
Nanoscale metamaterials tailored for optical and mechanical applications
No full textMetamaterials have been exploited to show a number of exotic effects, in particular for longer wavelengths, from infrared to microwaves. Extending their response to shorter wavelengths requires structuring on the nanoscale which is made possible with increasing advances of fabrication techniques. The rigid pattern of metamaterials, however, meant that their response can only be observed for a narrow wavelength range. The aim of this project was to extend the functionality of metamaterials to manipulate visible and infrared light and to demonstrate wavelength tuneability. Utilising liquid crystals, with their attractive optical properties and easily controllable nature, was the main method towards achieving adaptive metamaterials. While typically the optical properties of liquid crystals are employed in applications, this work went beyond that and exploited their elastic properties. A liquid crystal layer, coupled to a mechanical metasurface, was shown to remove the limits of the stiction forces present at the nanoscale. New liquid crystal loaded metamaterials, made of nanostructured zigzag bridges, were fabricated and characterised to better understand the interactions taking place and to improve the functionality of future devices. The zigzag design was then explored separately in a project investigating its selective dependence on the spatial coherence of light. Computational modelling of the geometry was first completed and then compared with the experimental results. A large discrepancy between the experimental and modelled spectra for the zigzag metamaterial design was found, namely a split resonance being experimentally observed while the model predicted a single resonance peak. The split resonance was then successfully simulated for the case of incoherent incident light. In order to understand this feature further, variations of the zigzag geometry were fabricated and analysed. The final nanoscale design explored for investigating the optical properties of metamaterials included arrays of asymmetric slits. Optical activity upon reflection from a metasurface with equivalent, larger slits was demonstrated in the earlier work for microwave wavelengths. Samples with pairs of both symmetric and asymmetric slits were fabricated to obtain data from both reference and active samples. The presence of optical activity was then demonstrated for the asymmetric samples at optical wavelengths, in line with the theoretical predictions. The nanostructured metamaterials simulated, fabricated and experimentally characterised for this thesis, contribute to demonstrating the exciting potential of nanoscale metamaterials for photonic components and other groundbreaking technologies
Optimal design of experiments for MNAR data
No full textThe presence of missing data leads to biases in data analyses. To overcome these biases, it is crucial to understand the type of missing data that is present in the data. Amongst the three types of missing data (known as missing data mechanisms) that will be formally introduced in this thesis, the Missing Not At Random (MNAR) mechanism is the most complex. MNAR poses the most difficulties as it is an untestable assumption based on the current incomplete data. A recovery of some of the missing data is required to test its presence. In this research, we developed two statistical tests for testing the presence of MNAR in datasets and provide the theoretical framework of the tests. In the first test, the recovery design consists of a random sampling of the responses whose covariates lie within a particular region while the second test is based on an assignment of probabilities. We introduced techniques from Design of Experiments to improve the properties of these tests. The developed tests are compared with a random follow-up of missing responses, which will act as our benchmark design throughout. We formulate an easy and simple conjecture that uses the empirical density of the covariates to obtain the recovery region. Through simulations, the performance of thetests is evaluated. Keywords: Missing data; Missing not at random; Selection model; Recovery region;Conjecture
Development of erbium-doped and bismuth-doped optical fibres for wideband and high-performance amplifiers for telecommunication applications
No full textTo support the continuously growing demand for data-carrying capacity of optical fibres in telecommunications, developing efficient fibre amplifiers for extended wavelength bands beyond traditional C- and L-bands is crucial. The O-, E-, S-bands (1260-1530 nm), and the extended L-band (1615-1625 nm), within the low-loss transmission window of standard silica fibres, hold significant research and commercial potential. Erbium (Er)-doped fibre amplifiers (EDFAs) are well-studied for the C- and L-bands, but require enhancements in gain flatness, noise, efficiency, and bandwidth extension beyond 1615 nm. Bismuth (Bi)-doped silica fibres, with their ultra-wide near-infrared (NIR) luminescence from various Bi active centres (BACs), offer promising potential for developing wideband Bi-doped fibre amplifiers (BDFAs).In this thesis, I fabricated Er-doped fibres using the modified chemical vapour deposition (MCVD)-solution doping technique by tailoring the glass core compositions to favour L-band amplification. Fibre spectroscopic properties were characterised including refractive index profile, absorption, background loss, unsaturable loss, fluorescence, lifetime, on/off gain, and Er3+ cross-sections. Using the fabricated EDFs, I developed L-band amplifiers to achieve a high gain of ≥20 dB from 1565-1625 nm in a double-pass configuration, a flat gain ripple of ±0.7 dB from 1580-1615 nm by forming AlPO4 units in the fibre and pumped at 980 nm, an extended operating wavelength up to 1628 nm with a 13.6 dB gain by forming AlPO4 units and pumped at 1480 nm. Er/Yb co-doped fibre amplifier (EYDFA) was built up utilising 1480 nm core-pumping to shorten the L-band device length to 23 m.High-performance BDFAs were developed using BDFs fabricated by other fabricators. Using Bi-doped germano-silicate fibres (BGSFs) and phospho-silicate fibres (BPSFs), a high gain and high gain-per-unit-length BDFA was demonstrated in the E+S band with shortened device lengths (25.5 m to 48 m), and a high-gain ultra-wideband BDFA was presented in the O+E+S band with a 140 nm 20 dB-gain bandwidth for a -23 dBm input signal, respectively. Spectroscopic properties of various BACs were analysed, and a fully integrated, compact BDFA device box was developed.Moreover, robust amplifiers were developed to adapt to extreme temperatures and irradiation conditions. For EDFAs, a hybrid configuration was proposed to balance temperature stability and L-band gain, achieving low noise and temperature-insensitive gain flatness from 1585-1615 nm. Radiation tolerance was characterised in various core-composition EDFs after gamma-irradiation to develop radiation-resistant EDFAs in the C- or L-band using cerium (Ce) co-doping. In addition, a radiation-resistant E+S band BDFA was developed with a 35 dB post-irradiation gain. <br/
Regulating for the future of work: Algorithmic management and the digital transformation of work
No full textSecondary school students’ learning persistence in human-AI hybrid learning: the Supplementary Role of Parental Mediation
No full textThis mixed methods study explores the under-examined area of secondary students’ persistence in human-AI hybrid learning, with a particular emphasis on parental involvement. By integrating two forms of parental mediation—active and restrictive—into an established baseline model from prior research, a conceptual model is proposed to explain secondary students’ learning persistence in a human-AI hybrid learning environment. Using structural equation modelling to analyse data from 302 students, the proposed model accounted for 66.6% of the variance in their intention to continue hybrid learning and 27.5% of the variance in actual continuous learning behaviour. Interestingly, while restrictive mediation was found to be positively correlated with students’ continuous learning behaviour, active mediation did not show a significant association with the behaviour. This unexpected finding prompted further investigation through in-depth interviews with ten students from the same school. Thematic analysis revealed that restrictive mediation was more commonly adopted to regulate students’ recreational usage of technologies. In contrast, parents were reported to rarely use active mediation to guide students in digital learning. Additionally, the findings indicated a lack of school initiatives to involve parents in hybrid learning. These research findings highlight the need for parents to enhance their mediation strategies in human-AI hybrid settings through collaborative efforts between families and schools
Performance analysis and optimization of STAR-RIS-aided cell-free massive MIMO systems relying on imperfect hardware
No full textSimultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-aided cell-free massive multiple-input multiple-output (CF-mMIMO) systems are investigated under spatially correlated fading channels using realistic imperfect hardware. Specifically, the transceiver distortions, time-varying phase noise, and RIS phase shift errors are considered. Upon considering imperfect hardware and pilot contamination, we derive a linear minimum mean-square error (MMSE) criterion-based cascaded channel estimator. Moreover, a closed-form expression of the downlink ergodic spectral efficiency (SE) is derived based on maximum ratio (MR) based transmit precoding and channel statistics, where both a finite number of access points (APs) and STAR-RIS elements as well as imperfect hardware are considered. Furthermore, by exploiting the ergodic signal-to-interference-plus-noise ratios (SINRs) among user equipment (UE), a max-min fairness problem is formulated for the joint optimization of the passive transmitting and reflecting beamforming (BF) at the STAR-RIS as well as of the power control coefficients. An alternating optimization (AO) algorithm is proposed for solving the resultant problems, where iterative adaptive particle swarm optimization (APSO) and bisection methods are proposed for circumventing the non-convexity of the RIS passive BF and the quasi-concave power control sub-problems, respectively. Our simulation results illustrate that the STAR-RIS-aided CF-mMIMO system attains higher SE than its RIS-aided counterpart. The performance of different hardware parameters is also evaluated. Additionally, it is demonstrated that the SE of the worst UE can be significantly improved by exploiting the proposed AO-based algorithm compared to conventional solutions associated with random passive BF and equal-power scenarios
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