Functional polyoxometalate assemblies: from host-guest complexes to porous frameworks

The host-guest chemistry of two sets of isopolyoxometalate clusters is investigated. In particular the binding modes of cationic alkali and alkali earth metals through coordinative interactions with the cluster anions {Mo36} and {W36} were compared and contrasted. It was shown that the ionic radii of the cations are crucial in the isolation of discrete molecules or infinite 3D frameworks. Crystal engineering allowed the introduction of organic amine guest molecules by the formation of a set of intermolecular interactions between the cluster anions and the amine cations. In addition it was shown that by engaging the ammonium guest molecules in additional supramolecular interactions, the framework assembly in the solid state could be directed. Further investigations focused on the assembly of supramolecular polyoxometalate-based framework materials where organic ammonium cations were used as hydrogen-bond donors. The structural effects of three amines were compared and contrasted and it was established that the use of rigid planar molecules resulted in the formation of 2D networks whereas the use of flexible amines gave supramolecular 1D chains. Based on these results the synthesis of a functional framework was achieved; a three-component approach allowed the formation of a chiral, porous framework which shows structural stability and reversible solvent sorption properties. In a different approach, the cross-linking of polyoxometalates using transition metal linkers in organic solvents was studied. It was shown that silver(I) cations are highly versatile linking units and allow the linking of {V10} cluster anions into 1D zigzag chains and 2D planar networks. The silver units assemble into supramolecular, trinuclear complexes which are supported by bridging organic ligands. Careful choice of the reaction conditions allowed the formation of a 3D framework based on {W12} units. The tungstate clusters are cross-linked by dinuclear {Ag2} linkers which are held together by argentophilic silver-silver interactions and result in the formation of a porous framework. The material features reversible sorption capabilities and can be used to sequester small molecules as well as transition metal cations from organic solvents

Bridging microscopy with molecular dynamics and quantum simulations: An AtomAI based pipeline

Recent advances in (scanning) transmission electron microscopy have enabled routine generation of large volumes of high-veracity structural data on 2D and 3D materials, naturally offering the challenge of using these as starting inputs for atomistic simulations. In this fashion, theory will address experimentally emerging structures, as opposed to the full range of theoretically possible atomic configurations. However, this challenge is highly non-trivial due to the extreme disparity between intrinsic time scales accessible to modern simulations and microscopy, as well as latencies of microscopy and simulations per se. Addressing this issue requires as a first step bridging the instrumental data flow and physics-based simulation environment, to enable the selection of regions of interest and exploring them using physical simulations. Here we report the development of the machine learning workflow that directly bridges the instrument data stream into Python-based molecular dynamics and density functional theory environments using pre-trained neural networks to convert imaging data to physical descriptors. The pathways to ensure the structural stability and compensate for the observational biases universally present in the data are identified in the workflow. This approach is used for a graphene system to reconstruct optimized geometry and simulate temperature-dependent dynamics including adsorption of Cr as an ad-atom and graphene healing effects. However, it is universal and can be used for other material systems

Organic growth and form in abstract painting

This doctorate explores 'Organic Growth and Form in Abstract Painting', as the focus of my studio-based research, and which has resulted in two significant series of paintings, Organica and Streaming. The accompanying exegesis addresses experiences that are realized within the studio practice, and complements the two series of paintings. In the exegesis I describe the innovative and distinctive painting processes I have developed, and explain my motivation for working this way. I cite the writing of the philosopher of science, Henri Bortoft, in particular his description of 'active' seeing, which I suggest can be understood as a kind of modeling of my processes of making the Organica and Streaming paintings. Key to my research has been an investigation into the work of the early Russian avant-garde artist, musician, theorist and teacher, Mikhail Matyushin, who promoted an 'organic' vision of painting during the early years of modernist experimentation, insisting that perception cannot be separated from the body's inherent connection with nature. I discuss how the artists in the Organic studio, led by Matyushin, tested their sensitivity to perceptual and sensory experience with controlled experiments. Philosophically, they considered their findings to be congenial with the latest scientific discoveries of their time. Although my paintings are constructed very differently from those of Matyushin, my approach to perception and interpretation in painting is in sympathy with his thinking. The constructive and perceptual approach I have taken to both series of paintings has been directly influenced by immersion in natural environments. My exegesis provides a detailed account of this working process: how I work with geometric templates for the coordination of colours, and my systematic approach to their application, leading to uncontrived 'organic' extensions in the detail. I discuss my interest in the implicit knowledge garnered through perception of colours and the connective fabric underlying surface appearances in nature. I argue that these observations are generative resources for painting, and emphasise the fact that our sensory and thinking bodies are also part of nature. - provided by Candidate

Reversing a polyhedral surface by origami-deformation

AbstractWe introduce a new variety of flexatube, a rhombotube. It is obtained from a cardboard rhombohedron consisting of six rhombi with interior angles 60∘ and 120∘, by removing a pair of opposite faces, and then subdividing the remaining four faces by pairs of diagonals. It is reversible, that is, it can be turned inside out by a series of folds, using edges and diagonals of the rhombi. To turn a rhombotube inside out is quite a challenging puzzle. We also consider the reversibility of general polyhedral surfaces. We show that if an orientable polyhedral surface with boundary is reversible, then its genus is 0, and for every interior vertex, the sum of face angles at the vertex is at least 2π. After defining the tube-attachment operation, we show that every polyhedral surface obtained from a rectangular tube by applying tube-attachment operations one after another, can be subdivided so that it becomes reversible

Ordered ground state configurations of the asymmetric Wigner bilayer system -- revisited: an unsupervised clustering algorithm analysis

We have re-analysed the rich plethora of ground state configurations of the asymmetric Wigner bilayer system that we had recently published in a related diagram of states [M. Antlanger \textit{et al.}, Phys. Rev. Lett. \textbf{117}, 118002 (2016)], comprising roughly 60~000 state points in the phase space spanned by the distance between the plates and the charge asymmetry parameter of the system. In contrast to this preceding contribution where the classification of the emerging structures was carried out ``by hand'', we have used this time machine learning concepts, notably based on a principal component analysis and a $k$-means clustering approach: using a 30-dimensional feature vector for each emerging structure (containing relevant information, such as the composition of the configuration as well as the most relevant order parameters) we were able to re-analyse these ground state configurations in a considerably more systematic and comprehensive manner than we could possibly do in the previously published classification scheme. Indeed we were now able to identify new structures in previously unclassified regions of the parameter space and could considerably refine the previous classification scheme, identifying thereby a rich wealth of new emerging ground state configurations. Thorough consistency checks confirm the validity of the newly defined diagram of states

A computational study on novel carbon-based lithium materials for hydrogen storage and the role of carbon in destabilizing complex metal hydrides

One of the major impediments in the way of the realization of hydrogen economy is the storage of hydrogen gas. This involves both the storage for stationary applications as well as that of storage onboard vehicles for transportation applications. For obvious reasons, the system targets for the automotive applications are more stringent. There are many approaches which are still being researched for the storage of hydrogen for vehicular applications. Among them are the high pressure storage of hydrogen gas and the storing of liquid hydrogen in super insulated cryogenic cylinders. While both of them have been demonstrated practically, the high stakes of their respective shortcomings is hindering the wide spread application of these methods. Thus different solid state storage materials are being looked upon as promising solutions. Metal hydrides are a class of solid state hydrogen storage materials which are formed by the reaction of metals or their alloys with hydrogen. These materials have very good gravimetric storage densities, but are very stable thermodynamically to desorp hydrogen at room temperatures. Research is going on to improve the thermodynamics and the reaction kinetics of different metal hydrides. This dissertation tries to address the problem of high thermodynamic stability of the existing metal hydrides in two ways. First, a novel carbon based lithium material is proposed as a viable storage option based on its promising thermodynamic heat of formation. Pure beryllium (Be) clusters and the carbon-beryllium (C-Be) clusters are studied in detail using the Density Functional Theory (DFT) computational methods. Their interactions with hydrogen molecule are further studied. The results of these calculations indicate that hydrogen is more strongly physisorbed to the beryllium atom in the C-Be cluster, rather than to a carbon atom. After these initial studies, we calculated the geometries and the energies of more than 100 different carbon based lithium materials with varying amounts of hydrogen. A detailed analysis of the heats of reactions of these materials using different reaction schemes is performed and based on the promising thermodynamic and gravimetric storage density, LiC4Be2H5 is divulged as a promising novel carbon based lithium material. In the later part, this dissertation performs a detailed study on the effect of carbon when it is used as a dopant in four different well known complex hydrides, lithium beryllium hydride (Li2BeH4), lithium borohydride (LiBH4), lithium aluminum hydride (LiAlH 4) and sodium borohydride (NaBH4). Initially, the unit cells of the crystal structure are fully resolved using the plane-wave pseudopotential implementation of DFT. The supercells of each of these are then constructed and optimized. Varying amounts of carbon is introduced as impurity in these crystals in different sites such as the top, subsurface and the bulk of the crystal lattice. Using the electronic structure calculations, it is established that (i) C-Be-H, C-B-H or C-Al-H compounds are formed respectively in the cases of Li2BeH4, LiBH4 and LiAlH4 when carbon is doped in them; (ii) and carbon dopant causes a decrease in the bond strengths of Be-H, B-H and Al-H in respective cases. This reduction in the bond strengths combined with the fact that there is a decrease in the ionic interaction between the cation and the anionic hydride units of these complex hydrides causes a destabilization effect