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The Discovery of Tunable 2D Materials and van der Waals Heterostructures for Next-gen Technologies
No full textTo keep up with the global demand for faster, lighter, and more energy-efficient technologies, there is a pressing need to make device components thinner and smaller while at the same time enhancing their functionality. The world's largest semiconductor foundry, Taiwan Semiconductor Manufacturing Company (TSMC), is racing to do just that. However, current materials are approaching their size limits, meaning they cannot be reduced further without compromising their stability or performance. Therefore, to further drive miniaturisation, the discovery of novel materials that are stable at these sizes and have the necessary properties is not just a goal, but a requirement.
One such promising class of materials is two-dimensional (2D) materials. 2D materials are confined to the nanoscale on one dimension, giving them sheet-like structures. Besides boasting a wide range of unique, unusual and novel properties compared to their bulk counterparts, these properties have been predicted to be highly tunable by applying external stimuli, such as functionalisation, strain, or an electric field. In addition, stacking 2D materials on top of each other, like atomic building blocks, creates van der Waals heterostructures (vdWHs) that can have unique properties not possessed by the individual layers.
In recent years, the number of theoretically calculated and experimentally synthesised 2D materials has dramatically increased, and therefore, the number of possible stacking combinations has as well. A computational approach could accelerate the discovery of new materials by rapidly investigating a large number of 2D materials and their vdWHs to select promising candidates for synthesis.
This thesis employs density functional theory (DFT) to examine the structural, optical, piezoelectric, and electronic properties of monolayer 2D materials and bilayer van der Waals heterostructures and how external factors like chemical modification, applied electric field, or mechanical strain can affect these properties. The materials considered in this thesis are monolayer 2D phosphorene, 19 bilayer van der Waals heterostructures composed of CuInP2S6 combined with another 2D material (CuInP2S6/X, where X = a 2D semiconductor) and the following 6 bilayer van der Waals heterostructures, HfS2/MoTe2, HfS2/WTe2, 1T-HfS2/WTe2, TiS2/WSe2, TiS2/ZnO, and TiSe2/WTe2. The effect of functionalisation, strain, and electric field, on the properties of these systems is investigated.
2D black phosphorus (phosphorene) has been shown to have excellent flexibility and highly tunable properties, which makes it ideal for flexible nanotechnologies. In this thesis, the interplay between the chemical and mechanical tunability of phosphorene functionalised with halogens is investigated. The coverage of halogens considered was 0.042, 0.0625, 0.125 and 0.25 monolayer (ML). After functionalisation, phosphorene was calculated to have a more sensitive electron and hole-effective mass in response to strain. A switching of the electrical conduction direction (from the armchair to zigzag direction) can be achieved using a smaller strain value after functionalisation. For pristine phosphorene this occurs with a tensile strain value of 4-5%, whereas, for halogen-functionalised phosphorene, this occurs at a lower strain value of 3-4%, resulting in more affordable conductance switching. Compared to pristine phosphorene, the band gap was shown to be highly dependent on the coverage and concentration of the halogen. Functionalisation with any halogen at a low concentration (0.0417 ML or 0.0625 ML) results in a widening of the band gap by up to 0.35 eV. For higher concentrations (0.125 and 0.25 ML), the band gap was calculated to be further modified (narrowed or widened) and, for some materials, reduced to 0 eV. Functionalisation at a coverage of 0.0625, 0.125 or 0.25 ML (except for the 0.25 ML Br-phosphorene system) was calculated to induce a piezoelectric response. The largest piezoelectric constant (e22) was calculated to be 1.31 C m-2 for the I-functionalised phosphorene (0.0625 ML), which outperforms the monolayer monochalcogenides SnSe, SnS, GeSe and GeS by 40%, and phosphorene oxide by 255%.
The ferroelectric material CuInP2S6 has been proposed as a promising candidate for the creation of tunable van der Waals heterostructures. In this thesis, I present 19 new systems composed of monolayer CuInP2S6 combined with another 2D monolayer semiconductor. All 19 vdWHs were calculated to have a type-II band alignment. Due to the ferroelectric properties of CuInP2S6, switching between the ferroelectric states resulted in a range of unique effects, including changes to the band gap or band alignment, direct-indirect or indirect-direct transitions, and shifts in the peaks present in the absorption spectrum. Such differences allow these materials to be employed in a range of applications, including nanoelectronic, optoelectronic and ferroelectric memory devices.
An additional 6 vdWHs, HfS2/MoTe2, HfS2/WTe2, 1T-HfS2/WTe2, TiS2/WSe2, TiS2/ZnO, and TiSe2/WTe2 were all calculated to have a type-III band alignment. The effects of strain and electric field on the band alignment are examined. The application of compressive strain along the a-direction resulted in a widening of the band gap for HfS2/MoTe2, HfS2/WTe2 TiS2/WSe2 and TiSe2/WTe2, and a type-III to type-II transition. A similar transition was shown for HfS2/MoTe2, 1T-HfS2/WTe2, TiS2/ZnO, and TiSe2/WTe2 due to the application of a positive electric field. Based on these effects, a multifunctional tunnelling device was proposed. With no strain (HfS2/MoTe2, HfS2/WTe2 TiS2/WSe2 and TiSe2/WTe2) or no applied electric field (HfS2/MoTe2, 1T-HfS2/WTe2, TiS2/ZnO, and TiSe2/WTe2), the device could be employed as a tunnel field-effect transistor or an Esaki diode, whereas using external stimuli, the device could be employed for various type-II applications.
The findings presented in this thesis have provided key insights into the interplay of functionalisation and strain on phosphorene and its potential applications in flexible nano- and opto- electronics. In addition, this work provides a set of 25 novel vdWHs with a range of tuneable properties that make these systems suitable for many unique applications. Overall, the thesis presents a range of 2D materials and vdWHs that are versatile, having highly tunable properties, which makes them promising platforms for the miniaturisation of next-generation technologies.</p
RCN Series: Canal Fishing
No full textThis creative work output composites several sequences of this series into a single film. The film focuses on the canal surface and uses blur and layering to create a film that is not a documentary retelling of place, but a reconstitution of the experiential qualities of the canal.</p
Peer Effect of CSR Disclosure: Applications of Textual Analysis
No full textThis thesis concerns itself with the topic of corporate social responsibility disclosure (CSR disclosure) and peer effects. Borrowing from the stakeholder theory and the resource-based view, engaging in CSR activities generates firms a higher reputation in the product and labor market (Freeman, 1984), which can be seen as a competitive advantage contributing to enhanced firm value (Barney, 1991). While the merits of CSR have been criticized by some neoclassical economists, led by Friedman (1970), who argue that CSR is a waste of time for firms to search for optimal CSR strategy. This paper posits that firms are embedded in a market so that they can fit themselves into the optimal CSR disclosure policy by mimicking the practices of other successful pioneering firms. Arguably, this peer effect can significantly reduce the time and resources that firms would otherwise spend on this activity, thereby alleviating the concerns raised by the CSR opponents
Using textual analysis of CSR reports from all US-listed firms between 2000 and 2021, this thesis first examines the presence of peer effects in CSR disclosure decisions. The results from the logit model reveal a strong positive relationship between peer disclosure and a firm’s decision to disclose CSR activities. To mitigate endogeneity concerns, the study employs both instrumental variables (IV) and propensity score matching (PSM) approaches, both of which provide robust evidence supporting the adoption of voluntary CSR disclosure by a focal firm when its peers disclose. Furthermore, the thesis reveals that the peer effect on CSR disclosure is particularly pronounced in industries characterized by high competition and in sectors with significant environmental impact. The thesis also extends its scope to explore the impact of peer firms' CSR report complexity on that of the focal firms. A fixed-effects linear model uncovers a negative relationship between the complexity of peer firms' CSR reports and the readability of focal firms' reports. This indicates that as peer firms produce more complex CSR reports, focal firms may strategically opt for clearer, more accessible disclosures, potentially as a means of differentiation in the market. Finally, the study examines how product market competition influences the similarity of CSR reports among firms. It finds that increased competition generally leads to greater differentiation in CSR disclosures, as firms seek to distinguish themselves from their competitors. However, in highly polluted industries, a different pattern emerges that is higher competition is associated with greater similarity in CSR reports. This suggests that in these vi industries, regulatory pressures and the need to maintain legitimacy drive firms to adopt similar CSR practices, leading to more uniformity in their disclosures.
Overall, this thesis contributes to the understanding of CSR disclosure by highlighting the significant role that peer effects play in shaping firms' CSR disclosure strategies. It provides robust empirical evidence of peer effects on CSR disclosure decisions, and it sheds light on how competitive dynamics and industry characteristics influence the adoption and presentation of CSR disclosure.</p
Characterising Changes in the Gut-Brain Axis in Mice With Modified Neuro-Immune Pathways
No full textIndividuals with autism spectrum disorder (ASD; autism) exhibit deficits in social communication and repetitive behaviours, which are associated with changes in the nervous system. Individuals with autism are also more susceptible to inflammatory disorders and show increased systemic and brain inflammation. Gastrointestinal symptoms, increased intestinal permeability, and changes in the composition of microbial communities in the gut are also commonly experienced by these individuals. Growing evidence suggests atypical interactions between microbes, immune cells, and the brain occur in autism, however, how changes in the nervous system impact inflammatory responses remain to be clarified. Disruptions in signalling pathways between the brain and gut may alter gastrointestinal structure and function as well as behaviour, and these changes could contribute to increased neuroinflammation and gastrointestinal dysfunction in individuals with autism. This thesis investigated changes to the gut-brain axis following immune stimulation in transgenic mice relevant to autism and immune dysfunction.
To understand cross talk between immune and neural pathways, the impact of the chemical inflammatory stimulus, dextran sulphate sodium (DSS), a well-established mouse model of intestinal injury and inflammation, was studied in three mouse models. Specifically, the Neuroligin-3R451C mouse model of autism expressing a missense mutation modifying a cell adhesion protein expressed at neuronal synapses (Nlgn3R451C mice) and mice lacking an important receptor in the type-I interferon immune response (Interferon α/β receptor subunit 1 (IFNAR1-/-) mice) were studied. We also bred Nlgn3R451C mice with IFNAR1-/- to generate a novel strain of mice with a combined neuro-immune mutation (Nlgn3R451CxIFNAR1- /- mice). The interactions of these genetic mutations and inflammatory pathways were assessed using a model of acute experimental colitis in which mice were given 3% DSS in drinking water for a period of 7 days. We hypothesised that Nlgn3R451C mice exhibit increased susceptibility to DSS-induced colitis symptoms, and that this increase is attenuated by deletion of IFNAR1.
Mice were monitored and scored during each day of treatment to calculate an overall colitis- relevant disease activity index, including weight loss, faecal pellet consistency, the presence of blood in faeces, and rectal bleeding. Gut and brain tissue including colon and small intestine length, caecal and spleen weight, and the presence of gut-associated lymphoid tissue (Peyer’s and caecal patch numbers) were assessed. To determine changes in structural and functional gastrointestinal function, distal colon histopathology, faecal pellet production, and permeability of ileal and colonic tissue were assessed. To examine whether intestinal inflammation affects the brain, glial cells were studied by immunofluorescence to compare the density and morphology of microglia and astrocytes in the cortex and dentate gyrus of the hippocampus. Finally, to assess how altering neuroimmune function affects behaviour, locomotor function was examined in the open field test, and anxiety-like behaviour was observed in the open field, elevated plus maze, and light-dark box. Social behaviours were also considered in a reciprocal social interaction test where mice faced an unfamiliar mouse in the open field arena.
We found that Nlgn3R451C, IFNAR1-/-, and Nlgn3R451CxIFNAR1-/- mice exhibit resilience to DSS-induced colitis compared to WT mice across a number of measures. Mice harbouring these mutations impacting the nervous and immune system exhibited decreased severity of colitis from day 0 to 8, particularly in Nlgn3R451C and Nlgn3R451CxIFNAR1-/- mice. Furthermore, all three groups exhibited faster recovery one day after treatment cessation when compared to WT mice. Anatomically, colon shortening, a hallmark of DSS-induced colitis, persists in Nlgn3R451C and IFNAR1-/- mice. However, Nlgn3R451CxIFNAR1-/- mice were resistant to this decrease in colon length. Moreover, DSS-induced splenomegaly was reduced in Nlgn3R451C mice and absent in IFNAR1-/- and Nlgn3R451CxIFNAR1-/- mice, demonstrating a reduced immune response to the toxin.
DSS-treated WT mice had increased ileal and colonic permeability compared to SHAM-treated controls, as demonstrated by Ussing chamber experiments. DSS-treated Nlgn3R451C mice exhibited an increase in colonic permeability, whereas IFNAR1-/- and Nlgn3R451CxIFNAR1-/- mice were resilient to DSS-induced intestinal barrier dysfunction. Immunofluorescent staining of brain glial cells showed morphological changes indicative of increased reactivity of microglia in DSS-treated WT mice, but not in mice with mutations affecting NLGN3 and/or IFNAR1. Behavioural testing revealed significant locomotor dysfunction in DSS-treated WT mice, which was prevented in Nlgn3R451C, IFNAR1-/-, and Nlgn3R451CxIFNAR1-/- mice. Mice with acute experimental colitis did not exhibit increased anxiety-like behaviour. Instead, DSS- treated Nlgn3R451CxIFNAR1-/- mice exhibit increased exploratory behaviour in the elevated plus maze, while both IFNAR1-/- and Nlgn3R451CxIFNAR1-/- mice demonstrate increased social behaviour when faced with a novel mouse.
Interestingly, mice expressing the autism-relevant Nlgn3R451C mutation demonstrated decreased severity of experimental colitis, and exhibit improvements in splenomegaly and ileal barrier dysfunction compared to WT mice. Similarly, mice lacking IFNAR1 exhibited less severe colitis outcomes, together with complete rescue from splenomegaly and ileal and colonic barrier dysfunction, as well as reduced evidence of glial cell reactivity. Finally, mice possessing both mutations (Nlgn3R451CxIFNAR1-/- mice) showcased a profound and additive mitigation of physiological impairments associated with DSS-induced colitis, including the absence of colon shortening, the absence of signs indicating hunching or discomfort, and a notable enhancement in exploratory and social behaviour. These findings suggest that alteration to the expression of Nlgn3 results in impairments to immune function and the ability to mount an inflammatory response to a foreign toxin. Given evidence for expression of Nlgn3 in immune cell populations, we propose that the R451C mutation directly impacts the innate immune response. We suggest that Nlgn3R451C-evoked immune impairments operate via differing mechanisms to the dysfunction caused by the disruption to type-I IFN signalling given that we observed an additive effect in double mutant mice. This study contributes to a growing body of research highlighting a role for Nlgn3 in immune function. This work enhances the understanding of immune and gut abnormalities under pathophysiological conditions with relevance to autism.</p
Responsible Practice Principles for RMIT University Library
No full textThis document captures the library’s commitment to conducting our day-to-day business in a way that supports our own reconciliation journey and makes a positive impact on the representation and inclusiveness of Aboriginal and Torres Strait Islander communities in all that we do. It will foster a supportive and inclusive environment, and contribute to the ethical and equitable representation of Aboriginal and Torres Strait Islander knowledge within our collections, services and spaces.© RMIT University Library 2024</p
<b>Right Hemisphere Lateralisation of Face Processing Predicts Social Skills</b>
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Essays on Child Health in Ghana
No full textThe thesis is a collection of five independent but related papers on child health in Ghana. Precisely, this thesis employs various econometric strategies to examine the effect of cohabitation, famine, child work, and ethnic diversity on child health. Cohabitation has emerged as an increasingly prevalent form of union, particularly among the younger adults in many developing countries which carries significant implications for parenting, and childbearing dynamics, impacting child health. In Chapter two, we investigate the influence of a mother’s age at commencing cohabitation on various child health outcomes. Using instrumental variables strategy, we find that an increase in the age at which women commence cohabitation for the first time is associated with improved health outcomes in children. These positive effects are pronounced for children born to women with no formal education. We identify reduced domestic violence, and improved healthcare practices as the pathways through which age at cohabitation improves child health.
The third chapter combines three waves from Ghana’s Demographic Health Survey (DHS) to investigate the intergenerational health impact of the 1981–83 Ghanaian famine. We focus on children born to women who survived the famine in childhood, employing a difference-in-difference framework. Identification is based on timing and regional variations in famine intensity across Ghana. Our estimates show that children born to women exposed to the 1981–83 Ghanaian famine in childhood are more likely to have poor health outcomes evident in decreased weight-for-age, and height-for-age. The poor child health effects of the 1981–83 Ghanaian famine is mainly driven by poor health and low educational attainment of the child’s mother who survived the famine in childhood. From a policy perspective, these findings imply that the potential means to attenuate the adverse long-run health implications associated with famine is to implement policies that increase school attainment and health among famine-affected cohorts.
The fourth chapter uses Ghanaian household panel data to examine the link between ethnic diversity and child health. Using an index of ethnic fractionalization, we find that unit increase in a heterogenous ethnic group is associated with poor child health outcomes. However, the adverse effect of ethnic diversity on child health is more pronounced among children living in ethnically diverse communities located in rural areas. Poverty and lack of pre-natal healthcare services serve as the transmitting channels for the impact of ethnic diversity on child health. Our findings suggest that policies that reduce poverty and ensure that pre-natal healthcare facilities are available in ethnically diverse societies are important for promoting child health.
In the fifth chapter, we examine the impact of child work on cognitive development via transmitting channels, child health and schooling. After addressing endogeneity using several quasi-experimental methods, we find that an increase in child work is associated with low cognitive development. This negative effect is particularly pronounced among children living in low-income households and male children. Mediation analyses show that school attendance and child health are important pathways through which child work affects the cognitive development of children. Targeted interventions that ensure that child workers attend school by working for a few hours may be required to reduce child labour and its adverse health and cognitive development consequences.</p
<b>Right Hemisphere Lateralisation of Face Processing Predicts Social Skills</b>
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Integrating Decision Support Tools into Organisations for Food Waste Strategies
No full textThis PhD project, presented as a thesis with publications, explores integrating decision support tools in daily strategising activities to mitigate food loss and waste (FLW). Even with the global capacity of the food industry and the supply chain to feed everyone, significant FLW persists annually. In addressing this challenge, the research investigates the multifaceted nature of organisational decision-making to curtail surplus food production and diminish FLW, a critical component for achieving sustainable food systems. By employing strategy as practice and sociomateriality frameworks, the research demonstrates the dynamics of social and technological factors influencing decisions within organisational contexts. Through 22 in-depth interviews with representatives from 20 diverse organisations, the findings provide insights into strategic approaches for industry-wide FLW reduction while considering the social and material factors influencing organisational decision-making processes. Hence, the central finding of this thesis is that entrenched practices tend to impede the adoption of sustainable strategies, posing constraints to the food industry's innovation and resilience amid growing environmental and social imperatives. The findings of this research are relevant to the academic literature in two ways. Empirically, the thesis contributes to the emerging literature on strategic management that explores social and material agencies within strategic decision-making for FLW reduction. Theoretically, identifying practice inertia in sociomaterial strategic practices offers novel insights into the interdependencies between organisational routines and technological platforms, contributing to the understanding of the barriers and catalysts in the food industry that are likely to hinder sustainable transformation. Thus, addressing practice inertia involves a comprehensive approach, intertwining technology, strategy, and behavioural transformation to foster sustainable and resilient food industry practices. In doing so, it proposes a pathway for research that examines sociomaterial concerns which may arise in the future within strategic management and the sociomaterial ecosystems these strategies create and influence.</p
Analysis and Design of FRP-Reinforced Concrete Beams with Innovative Compression Yielding Mechanism
No full textSteel reinforced concrete structures have been fundamental in civil engineering for centuries. However, they are susceptible to deterioration over time due to the corrosive effects of environmental factors such as chemical exposure, seawater and deicing salts. Corrosion of steel reinforcement significantly reduces the capacity and reliability of the beam. Fibre reinforced polymers (FRP) bars reinforced concrete beam is a fantastic alternative to solve the corrosion issue of steel reinforcement. The main drawback preventing FRP bars from widespread application is their poor ductility. Among the methods for improving the ductility of FRP-reinforced concrete beams, introducing a compression yielding (CY) mechanism has been proven to be the most effective. However, previous studies have mainly focused on the experimental testing of CY materials and determining the flexural capacity and ductility of CY beams. A comprehensive study on the analysis of CY beams, considering different cross-section shapes (rectangular and T sections), as well as strain-hardening and strain-softening behaviours of CY material, has not yet been conducted. Additionally, a systematic reliability analysis of CY beam is not available to facilitate the practical design of CY beams.
In this study, a comprehensive investigation of the flexural capacity and ductility of CY beams with rectangular and T sections is conducted. CY materials with both strain-hardening and strain-softening behaviours are addressed. In particular, different failure modes of CY beams when CY material exhibits strain-softening behaviour are analysed, which have not been reported previously. A layered approach considering the location of CY block is then developed for section analysis. The importance of key design variables is determined using the element effect method. It is revealed that enhancing ductility performance is attainable by utilising a CY material with a post modulus ratio near 0, while an increase in the strength of the CY material significantly enhances moment capacity. The most effective strategy for concurrently improving both moment capacity and ductility performance is to increase the height of the CY block.
As CY beam design conducted through layered section analysis is complex and time-consuming, this study proposes a machine learning integrated model to estimate flexural capacity and ductility of CY beams. Specifically, various activation functions are tested in artificial neural networks to enhance the accuracy of flexural capacity estimation, while different kernel functions are employed in support vector machines with Gaussian process regression to predict CY beam ductility. The results indicate that the integrated model can provide highly accurate predictions. Additionally, a genetic algorithm (GA) is developed to identify optimal design solutions for CY beam sections. The robustness of the proposed model in optimising the design of CY beams with rectangular and T sections is demonstrated through two numerical examples.
To implement CY beams in engineering practice, it is essential to conduct reliability analyses under both the ultimate limit state (ULS) and the serviceability limit state (SLS). In this study, the model for CY beam long-term deflection, considering both creep and shrinkage, is proposed for the first time. This thesis employs rigorous statistical methods (i.e., Kolmogorov-Smirnov test and Monte Carlo simulation) to ascertain the uncertainties of variables associated with CY beams in reliability analysis. Furthermore, it utilises the First-Order-Reliability-Method (FORM) to conduct reliability analysis both under ULS and SLS. Results from the reliability analysis under ULS recommend using a flexural resistance reduction factor of 0.75 for building structural members and 0.70 for bridge members. The reliability analysis under SLS indicates that the long-term deflection of CY beams is less than that of conventional FRP-reinforced concrete beams. Furthermore, the degradation in reliability for CY beams is also less than that for conventional FRP-reinforced concrete beams, suggesting better reliability of CY beams over their service life.</p