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Synthetic and crystallographic study of various functional low-dimensional perovskites
Get PDFThe discovery and development of new low-dimensional materials have attracted extensive attention over the past decades for the applications in functional devices. The work in this thesis focuses on the structural characterisation and thermal evolution of inorganic layered perovskites, and the design, synthesis, characterisation of new hybrid organic-inorganic lead halide 2D layered perovskites and 1D perovskite-related materials. The preliminary study of physical properties such as UV-Vis spectroscopy and photoluminescence measurements are also explored in this thesis.
The thermal evolution and high-temperature phase behavior of the ferroelectric layered perovskite, Bi₄Ti₃O₁₂, a n = 3 member of Aurivillius family, has been re-examined by high-resolution powder neutron diffraction. Both experimental and theoretical studies previously, suggested various and conflicting structural models and phase transition sequences. In Chapter 3, we confirm that Bi₄Ti₃O₁₂ undergoes two distinct structural transitions from the parent I4/mmm tetragonal phase to the room temperature ferroelectric B1a1 phase. A previously unconsidered and unusual intermediate paraelectric phase is confirmed to exist above TC, with a tetragonal symmetry of P4/mbm. This phase displays a unique type of octahedral tilting, which has been rationalised by the bonding requirements of the Bi³⁺ cations within the triple perovskite blocks.
Five new hybrid lead(Ⅱ) halide layered perovskites are presented in Chapter 4: TGPbBr₄, IGPbBr₄, T₂PbBr₄, A₂PbBr₄ and ITPbBr₄, with the organic species of 1,2,4-triazolium (T), guanidinium (G), imidazolium (I) and acetamidinium (A). Four of them adopt (110)- oriented layered perovskite structures, three of them are ʺ2 × 2ʺ (110)-oriented structure types (TGPbBr₄, IGPbBr₄ and A₂PbBr₄) and one of them is a ʺ3 × 3ʺ (110)-oriented structure type (ITPbBr₄), whereas T₂PbBr₄ adopts a (001)-oriented layered perovskite structure. The structural differences and the possible formation mechanism of these structure types in terms of size effects and H-bonding preferences are also discussed. In addition, the differences between the templating molecular species, contrasting physical properties such as bandgaps and photoluminescent behaviours of these structures are also illustrated in this Chapter.
Four new 1D chain structure type hybrid organic-inorganic lead(Ⅱ) halide compounds are presented in Chapter 5: IQPbBr₃, QPbBr₃, QPbI₃ and IQPbI₃. Templated by large organic amines, isoquinoline (IQ) and its isomer quinolone (Q). All four compounds possess the same generic formula as cubic perovskite, ABX₃, but adopt totally different structures. IQPbBr₃, adopts a 1D face-sharing single chain hexagonal perovskite structure type, and the other three, QPbBr₃, QPbI₃ and IQPbI₃, adopt a non-perovskite structure which is built from 1D edge-sharing octahedral double chains. All four compounds were carefully analysed by single crystal X-ray diffraction, powder X-ray diffraction and CHN elemental analysis. The differences between these structures in terms of templating molecular species and bandgaps of each compound were also discussed. Three of them have lower bandgaps than the other reported materials with the same structure type, indicating the promising application of photovoltaics for these types of materials.""This work was supported by China Scholarship Council (CSC) (grant number 201603780005) and the School of Chemistry at the University of St Andrews." Fundin
De-epimerizing DyKAT of β-lactones generated by isothiourea-catalysed enantioselective [2 + 2] cycloaddition
No full textThe research leading to these results has received funding from the EPSRC (KK, JD, EP/T023643/1) and the EaSI-CAT Centre for Doctoral Training (ASG). ADS thanks the EPSRC Programme Grant “Boron: Beyond the Reagent” (EP/W007517) for support. MB thanks EaStCHEM and the School of Chemistry for support. Computations were performed on a local HPC cluster maintained by Dr H. Früchtl.An enantioselective isothiourea-catalysed [2 + 2] cycloaddition of C(1)-ammonium enolates with pyrazol-4,5-diones is used to construct spirocyclic β-lactones in good yields, excellent enantioselectivity (99 : 1 er) but with modest diastereocontrol (typically 70 : 30 dr). Upon ring-opening with morpholine or alternative nucleophilic amines and alcohols β-hydroxyamide and β-hydroxyester products are generated with enhanced diastereocontrol (up to >95 : 5 dr). Control experiments show that stereoconvergence is observed in the ring-opening of diastereoisomeric β-lactones, leading to a single product (>95 : 5 dr, >99 : 1 er). Mechanistic studies and DFT analysis indicate a substrate controlled Dynamic Kinetic Asymmetric Transformation (DyKAT) involving epimerisation at C(3) of the β-lactone under the reaction conditions, coupled with a hydrogen bond-assisted nucleophilic addition to the Si-face of the β-lactone and stereodetermining ring-opening. The scope and limitations of a one-pot protocol consisting of isothiourea-catalysed enantio-determining [2 + 2] cycloaddition followed by diastereo-determining ring-opening are subsequently developed. Variation within the anhydride ammonium enolate precursor, as well as N(1) and C(3) within the pyrazol-4,5-dione scaffold is demonstrated, giving a range of functionalised β-hydroxyamides with high diastereo- and enantiocontrol (>20 examples, up to >95 : 5 dr and >99 : 1 er) via this DyKAT.Peer reviewe
Biosynthesis of the antibiotic nucleocidin in Streptomyces calvus
Get PDFFluorometabolites are a group of rare plant or bacterial natural products with around ten members. A most interesting organofluorine natural product is nucleocidin. Nucleocidin is an antibiotic isolated from Streptomyces calvus in 1957. The biosynthesis of nucleocidin involves a novel fluorination enzyme that is responsible for the 4’-C-F bond formation.
A reproducible production of nucleocidin was established from a commercial strain, S. calvus T-3018, provided by Pfizer. Isotopic labelling studies with different labelled glycerols were carried out. These were (1,2)-[²H₁]-, (1S,2)-[²H₁]-and [²H₅]- glycerols, all of which were successfully fed to S. calvus, demonstrates that both of the pro- hydrogens become incorporated into the C5’ position of nucleocidin.
The structures of two novel 4’-fluoro-3’-O-β-glucosylated metabolites produced by S. calvus were elucidated. The glucosylation and deglucosylation of these metabolites catalysed by glucosyltransferase (NucGT) and β-glucosidase (NucBGS) enzymes were explored. The deglucosylation of the novel fluorometabolites produced nucleocidin and 4’-fluoroadenosine. The close location of the genes responsible for these enzymes further validates the biosynthetic gene cluster for nucleocidin biosynthesis.
The non-fluorinated counterparts of all three fluorometabolites were also isolated from the S. calvus culture. They were used to explore the fluorination with CFE and resting cells of S. calvus.
The novel glucosyltransferase (NucGT) that is responsible for the glucosylation in S. calvus was over-expressed and assayed. It is the first glucosyltransferase able to carry out glucosylation at the 3’-O position of a nucleoside. The kinetic parameters of different substrates with NucGT were determined, and the enzyme specificity was explored
Simulating and understanding quantum processes using tensor networks
Get PDFSimulation of quantum processes is essential to both furthering our understanding of the microscopic world and also in developing quantum technologies. For accuracy we must take into consideration the environment in which they take place. In this thesis we begin by introducing an existing method that utilises tensor networks to efficiently represent the time non-local evolution of the system induced by interaction with its environment. Previously reliant on a linear interaction and a Gaussian environment, it is presented with no assumptions made on the form of interaction or environment statistics. The versatility is then showcased by first simulating the dynamics of a system coupled to a structured environment before then considering a pair of spatially separated systems coupled to the same environment. Here we see that the separation can be tuned to screen the interaction with dominant modes in the environment. Moving beyond system dynamics we show how correlation functions can be efficiently calculated and used to infer the dynamics of the bath. This result is then applied to study the dynamics of heat exchange between a system and regions of the environment; in particular a time dependent drive is employed to move heat between specific regions as desired. Finally, we see how the dynamics of a dimer coupled to both photons and phonons can be exactly captured and its non-equilibrium steady state is explored in all coupling regimes. This required extending the method to be able to combine the memory effects from each environment. The occurrence of population inversion is confirmed at weak light-matter coupling and then shown to disappear as the coupling is increased before entering a regime characterised by quantum Zeno physics."This work was supported by the CM-CDT under the EPSRC [grant number EP/L015110/1]."--Fundin
Imagining children's rights in Central Africa : francophone fiction and international children's rights law
No full textIn recent decades, questions around the nature of childhood and children’s place in society have gained new attention, notably through the enunciation of children’s rights. At the same time, humanitarian and wider public discourse has focused on the threats faced by children growing up in situations of conflict and dislocation. The fictional child protagonist adds a further strand to this discourse. This thesis brings into dialogue representations of children in four francophone African novels – Petit Piment by Alain Mabanckou (2015), Johnny chien méchant by Emmanuel Dongala (2002), Notre-Dame du Nil by Scholastique Mukasonga (2012) and L’Aîné des orphelins by Tierno Monénembo (2000) – and in two international children’s rights treaties – the UN Convention on the Rights of the Child (UNCRC) and the African Charter on the Rights and Welfare of the Child (ACRWC). The UNCRC (1989) and the ACRWC (1990) establish children as rights holding subjects within a framework of adult care, while the fictional texts position adolescent children as protagonists, deprived of adult protection amid violent conflict and social upheaval in Central Africa.
This thesis asks what conceptions of the child are revealed in the fictional and legal texts and how such discourses shed light on each other and on the idea of the child as subject and bearer of universal rights. Critical analysis of the fictional and legal texts is set within a wider discussion of theoretical understandings and material realities of childhoods. This thesis challenges the tendency to oppose literature’s expansive humanity to the supposed narrow rigidity of law. Instead, it suggests that the idea of the child is an imaginative thread which flows through both the fictional and legal texts and serves as a mechanism to confront wider questions around the nature of personhood in society, the workings of power, and the possibility of change."This work was supported by an award from the Arts and Humanities Research Council, through the Scottish Graduate School for Arts & Humanities"--Fundin
'La planta prohibida' : multi-species cosmopolitics in the Mexican desert
No full textFocusing on the Chihuahuan Desert of north-central Mexico, I engage with multiple post-humanist theoretical approaches to understand how people relate to their world and the species with whom they co-habit. I focus on a community referred to as Las Velas - in this case over five thousand hectares, which is leased predominantly by three extended families. The inhabitants gain a livelihood through herding goats and sheep and cultivating corn, beans and squash. I focus particularly on their relationship with plants, whether that be in foraging for medicinal plants, in finding pastures for the herds, in cultivation, or in simply moving through the desert. A particular cactus (la planta prohibida) however, takes precedence. It is valued in the community for its medicinal properties, yet it is a much more treasured resource in that it is central to the ritual practice the indigenous Wixarika. Consequently, countless groups of Wixarika pilgrims make a 400 - 500 km annual pilgrimage to gather it. Furthermore, pilgrims must visit certain sites in order to propitiate deities manifest in features of the landscape. As it happens, one of these sites, a small hill referred to as Bernalejo, is located in Las Velas. Consequently, pilgrims pass through this territory in great numbers. This further attracts ‘new-age’ visitors who value the cactus for its psychedelic properties. It is also used therapeutically however, bringing Christian orientated groups of people recovering from substance dependence. I build on anthropological approaches to cosmopolitics (eg. Wardle and Schaffner 2017, Latour 2004, de la Cadena 2010), and take inspiration from phenomenologically inspired approaches to human-environment relations (eg. Ingold 2000, Basso 1996, Tilly 1994, Feld and Basso 1996), as well as manifold theorists who adopt a multi-species approach (eg. Haraway 2005, 2008, Tsing 2014, 2015, Myers, 2019, 2020, Kohn 2007, 2010)
PiPar: Pipeline parallelism for collaborative machine Learning
Get PDFFunding: This work was sponsored by Rakuten Mobile, Inc., Japan.Collaborative machine learning (CML) techniques, such as federated learning, have been proposed to train deep learning models across multiple mobile devices and a server. CML techniques are privacy-preserving as a local model that is trained on each device instead of the raw data from the device is shared with the server. However, CML training is inefficient due to low resource utilization. We identify idling resources on the server and devices due to sequential computation and communication as the principal cause of low resource utilization. A novel framework PiPar that leverages pipeline parallelism for CML techniques is developed to substantially improve resource utilization. A new training pipeline is designed to parallelize the computations on different hardware resources and communication on different bandwidth resources, thereby accelerating the training process in CML. A low overhead automated parameter selection method is proposed to optimize the pipeline, maximizing the utilization of available resources. The experimental results confirm the validity of the underlying approach of PiPar and highlight that when compared to federated learning: (i) the idle time of the server can be reduced by up to 64.1×, and (ii) the overall training time can be accelerated by up to 34.6× under varying network conditions for a collection of six small and large popular deep neural networks and four datasets without sacrificing accuracy. It is also experimentally demonstrated that PiPar achieves performance benefits when incorporating differential privacy methods and operating in environments with heterogeneous devices and changing bandwidths.Peer reviewe
Stebbing and Eddington in the shadow of Bergson
Get PDFIn this paper, we argue that the French philosopher Henri Bergson was a hidden interlocutor in Susan Stebbing’s critique of Arthur Eddington in her Philosophy and the Physicists. First, we outline Stebbing’s critique of Eddington’s philosophical- physical writings with a particular emphasis on her case against Eddington’s account of the passage of time. Second, we provide evidence that Eddington’s philosophy is, at its core, Bergsonian and make the case that Eddington was directly influenced by Bergson’s philosophy of la durée. Third, we lay out Stebbing’s critique of Bergsonism in her Pragmatism and French Voluntarism and identify important similarities with her critique of Eddington over twenty years later. In doing so, we show that it is Eddington’s Bergsonism that she is attacking. Finally, we situate Stebbing’s criticisms of both Eddington and Bergson within two wider conflicts that emerged in early twentieth century British philosophy: one between an objectively describable time of physics and subjective temporal experience, and the other between science and intuition.Peer reviewe
Karian cosmopolitanism : archaeology, heritage, and identity in Southwestern Turkey
No full textThis chapter explores how Kwame Anthony Appiah’s concept of ‘rooted cosmopolitanism’ helps to understand the uses to which archaeology is set in contemporary constructions of identity in Türkiye. It focuses on the ancient region of Karia and on the modern application of ‘Karianism’, an attachment to the Karian past as both distinctive and linked to contemporary identities in heritage practices in the towns of Bodrum and Milas. Through analysis of concrete applications of Karianism in Turkish archaeology, from its uses in the tourist landscape of Bodrum to recent developments in Milas since the discovery of the tomb of Hekatomnos, in addition to its appropriation by different local heritage actors, the chapter explores the complexity of a term that, while by definition exclusive, at the same time allows for a cosmopolitan dialogue to emerge that is furthermore used by individuals to construct a historicised present-day place for themselves and their communities. Finally, the chapter discusses the incentives for forging local identities connected to the past, as well as the ethical positions taken by heritage and archaeological actors who engage with local communities. It concludes that local investment in heritage does not negate its global dimension but is deeply enmeshed in both national and cosmopolitan agendas
Chemical trends of the bulk and surface termination-dependent electronic structure of metal-intercalated transition metal dichalcogenides
Get PDFFunding: VINNOVA - 2018-04969; H2020 Research Infrastructures - 730872; Engineering and Physical Sciences Research Council - EP/L01548X/1, EP/N032128/1, EP/T02108X/1; H2020 European Research Council - 714193; Svenska Forskningsrådet Formas - 2019-02496; Leverhulme Trust - RL-2016-006, RPG-2023-253; Max-Planck-Gesellschaft.The addition of metal intercalants into the van der Waals gaps of transition metal dichalcogenides has shown great promise as a method for controlling their functional properties. For example, chiral helimagnetic states, current-induced magnetization switching, and a giant valley-Zeeman effect have all been demonstrated, generating significant renewed interest in this materials family. Here, we present a combined photoemission and density-functional theory study of three such compounds: V1/3NbS2, Cr1/3NbS2, and Fe1/3NbS2, to investigate chemical trends of the intercalant species on their bulk and surface electronic structure. Our resonant photoemission measurements indicate increased hybridization with the itinerant NbS2-derived conduction states with increasing atomic number of the intercalant, leading to pronounced mixing of the nominally localized intercalant states at the Fermi level. Using spatially and angle-resolved photoemission spectroscopy, we show how this impacts surface-termination-dependent charge transfers and leads to the formation of new dispersive states of mixed intercalant-Nb character at the Fermi level for the intercalant-terminated surfaces. This provides an explanation for the origin of anomalous states previously reported in this family of compounds and paves the way for tuning the nature of the magnetic interactions in these systems via control of the hybridization of the magnetic ions with the itinerant states.Peer reviewe
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