Simulating unusual phenomena in low dimensional systems

"This thesis deals with physics phenomena in low dimensional systems, i.e., systems that range between few atoms and the bulk. The work deals with well studied (in this sense, classical) phenomena such as quantum tunneling and the enhancement of tunneling, surface reconstructions on surfaces, stability of structures vs. shape and size of nanoparticles, the mechanical properties studied in classical elastic theory for metals (such as hardness, ductility, malleability, etc.). However, althouh we have used and applied "classical" theories from both quantum and mechanical physics, we studied new, unusual phenomena in low dimensional systems. Thus, the reader will find in this thesis work, theoretical developments, hypothesis and results that advance explanations in unusual physical phenomena. In quantum physics there are phenomena which do not have analogs in classical physics. One of these phenomena is quantum tunneling. The manifestation of this process is penetration of a particle or more complicated object through a potential barrier. Such process is forbidden by classical mechanics. Besides tunneling through static potential barriers there are many phenomena in labs and nature when tunneling occurs across nonstationary barriers. There are many examples of such processes, ionization of atoms by a nonstationary electric filed, ac electric current in junctions of metals or semiconductors, dissociation of molecules by a high-frequency fields, field emission from metals by the action of an ac field, decay of current states in Josephson junctions, alpha decay of nuclei initiated by an external flux of protons, etc. These processes of tunneling through nonstationary barriers require an adequate theoretical description. Regarding studies for nanoparticles, we have faced the shape, structure and mechanical properties of metallic nanoparticles. First, we studied a particular truncation operated on the regular icosahedron gives rise to a new (not reported before) particle with five-fold symmetry and external decahedral shape termed the decmon motif. These truncated icosahedra exposes internal facets (100) and (111)â˘AS¸ additional to the external (111) facets of the regular icosahedron. The uncovering of additional facets to the icosahedron gives rise to phenomena such as surface reconstructions and a delicate competition in the energy contribution coming from the new facets (100) and (111) to the total surface energy.

Studies of Metal-Organic Polyhedra: Synthesis and Applications in Gas Storage and Separation

Metal-organic polyhedra (MOPs) are a class of porous materials, comprised of metal ions and organic ligands, which form discrete cage-like structures. Possessing large internal void volumes, the physical and chemical properties of MOPs can be tuned by appropriate selection of the metal and organic building units. The work presented in this thesis investigates the application of MOPs to environmentally significant gas storage and separation. The first chapter introduces MOPs as an emerging class of promising materials and discusses their historical development since the first examples of synthetic supramolecular structures. As microporous materials, MOPs demonstrate strong interactions with small molecules giving them promise as a medium for selective gas adsorption. These attributes are of interest in the areas of H2 storage and the capture of CO2. As such, Chapter 1 discusses the benefits and challenges of utilising porous materials in these areas. Chapter 2 presents synthetic routes to MOPs containing both internal and external functionalisation by modifying both the metal nodes and the organic components which constitute the supramolecular structure. Presented herein are the first permanently porous examples of this class of materials to incorporate two different metal elements into a single paddlewheel unit in a controlled manner. These bimetallic MOPs demonstrate a strong binding affinity as well as an impressive volumetric capacity for H2 gas furthering development of a suitable H2 storage solution. Chapter 3 explores the decoration of MOPs with various external functionality and the effect these motifs have on MOP packing in the crystalline matrix. The synthesis of several new organic pro-ligands and subsequent MOPs is also detailed within. A method of modelling single crystal X-ray diffraction data is described that provides insight into the interactions between MOP structures which are dominated by the chemistry of the MOP exteriors. The tendency of these large structural entities to reduce porosity is further studied using X-ray diffraction data and computational methods. The work in Chapter 4 focuses on the use of MOPs in a composite system rather than as a singular material. By combining MOPs with a highly permeable polymer, poly[1- (trimethylsilyl)-1-propyne] (PTMSP), a series of mixed matrix membranes are synthesised for application in separating N2 and CO2 gases, a challenge of global significance from both an environmental and economic perspective. The compatibility of these constituents is shown to have a significant impact on the physical properties as well as the gas separation performance of the resulting composites.Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 201

Towards a National 3D Mapping Product for Great Britain

Knowing where something happens and where people are located can be critically important to understand issues ranging from climate change to road accidents, crime, schooling, transport and much more. To analyse these spatial problems, two-dimensional representations of the world, such as paper or digital maps, have traditionally been used. Geographic information systems (GIS) are the tools that enable capture, modelling, storage, retrieval, sharing, manipulation, analysis, and presentation of geographically referenced data. Three-dimensional geographic information (3D GI) is data that can represent real-world features as objects in 3D space. 3D GI offers additional functionality not possible in 2D, including analysing and querying volume, visibility, surface and sub-surface, and shadowing. This thesis contributes to the understanding of user requirements and other data related considerations in the production of 3D geographic information at a national level. The study promotes Ordnance Survey’s efforts in developing a 3D geographic product through: (1) identifying potential applications; (2) analysing existing 3D city modelling approaches; (3) eliciting and formalising user requirements; (4) developing metrics to describe the usefulness of 3D data and; (5) evaluating the commerciality of 3D GI. A review of current applications of 3D showed that visualisation dominated as the main use, allowing for better communication, and supporting decision-making processes. Reflecting this, an examination of existing 3D city models showed that, despite the varying modelling approaches, there was a general focus towards accurate and realistic geometric representation of the urban environment. Web-based questionnaires and semi-structured interviews revealed that while some applications (e.g. subsurface, photovoltaics, air and noise quality) lead the field with a high adoption of 3D, others were laggards due to organisational inertia (e.g. insurance, facilities management). Individuals expressed positive views on the use of 3D, but still struggled to justify the value and business case. Simple building geometry coupled with non-building thematic classes was perceived to be most useful by users. Several metrics were developed to quantify and compare the characteristics of thirty-three 3D datasets. Results showed that geometry-based metrics such as minimum feature length or Euler characteristic can be used to provide additional information as part of fitness-for-purpose evaluations. The metrics can also contribute to quality control during data production. An investigation into the commercial opportunities explored the economic value of 3D, the market size of 3D data in Great Britain, as well as proposed a number of opportunities within the wider business context of Ordnance Survey

PHASE BEHAVIOR OF STAR-SHAPED DNA NANO-STRUCTURES

In this thesis we studied the collective behavior of limited valence DNA particles. By exploiting the selectivity of Watson-Crick pairing, we synthesized star-shaped DNA particles having either three or four arms, each arm terminating in a sticky overhang sequence that provides interactions between individual particles. Each nano-star can thus be viewed as limited valence particle whose valence number f is dictated by the number of star arms. Solutions of such structures are found to exhibit liquid-vapour-like phase separation. Our results show that by reducing the valence of the structures, the coexistence region is greatly shrunk both in temperature and in concentration, thereby confirming for the first time recent theoretical predictions. As the temperature of the system is reduced, and the critical point approached from above, the dynamic behavior slows down and becomes characterized by a two-step relaxation process. The two characteristic times behave differently: the faster one (\u3c4f) changes only very mildly while the slower one (\u3c4s) slows down by more than three orders of magnitude in an Arrhenius fashion, without any noticeable divergence as Tc is approached. Quite remarkably, \u3c4s does not show the power-law divergence expected for critical slowing down. The colloidal system here proposed makes use of DNA not only to introduce mutual interactions between individual particles, but to model their geometry controlling internal interactions at the nanoscale level. This work proves that DNA is a powerful tool to produce particles with directional interactions, and can be used to design complex structures as colloidal molecules at the nanoscale

Collection of abstracts of the 24th European Workshop on Computational Geometry

International audienceThe 24th European Workshop on Computational Geomety (EuroCG'08) was held at INRIA Nancy - Grand Est & LORIA on March 18-20, 2008. The present collection of abstracts contains the 63 scientific contributions as well as three invited talks presented at the workshop

Asymptotics, Geometry, and Soft Matter

This dissertation is concerned with two problems that lie at the interface of soft-matter physics, geometry, and asymptotic analysis, but otherwise have no bearing on one another. In the first problem, I consider the equilibrium thermal fluctuations of deformable mechanical frameworks. These frameworks have served as highly idealized representations of mechanical structures that underlie a plethora of soft, few-body systems at the submicron scale such as colloidal clusters and DNA origami. When the holonomic constraints in a framework cease to be linearly independent, singularities can appear in its configuration space, where it becomes energetically softer. Consequently, the framework\u27s free-energy landscape becomes dominated by the neighborhoods of points corresponding to these singularities. In the second problem, I study the localization of elastic waves in thin elastic structures with spatially varying curvature profiles, using a curved rod and a uniaxially-curved shell as concrete examples. Waves propagating on such structures have multiple components owing to the curvature-mediated coupling of the tangential and normal components of the displacement field. Here, using the semiclassical approximation, I show that these waves form localized, bound states around points where the absolute curvature of the structure has a minimum. Both these problems exemplify the subtle interplay between the mechanical properties of soft materials and their geometry, which further sets the stage for many interesting consequences

Percolation, statistical topography, and transport in random-media

A review of classical percolation theory is presented, with an emphasis on novel applications to statistical topography, turbulent diffusion, and heterogeneous media. Statistical topography involves the geometrical properties of the isosets (contour lines or surfaces) of a random potential psi(x). For rapidly decaying correlations of psi, the isopotentials fall into the same universality class as the perimeters of percolation clusters. The topography of long-range correlated potentials involves many length scales and is associated either with the correlated percolation problem or with Mandelbrot's fractional Brownian reliefs. In all cases, the concept of fractal dimension is particularly fruitful in characterizing the geometry of random fields. The physical applications of statistical topography include diffusion in random velocity fields, heat and particle transport in turbulent plasmas, quantum Hall effect, magnetoresistance in inhomogeneous conductors with the classical Hall effect, and many others where random isopotentials are relevant. A geometrical approach to studying transport in random media, which captures essential qualitative features of the described phenomena, is advocated.Physic

A comparative geological study of some major kimberlite pipes in the Northern Cape and Orange Free State

In the first part of this thesis the petrological status of 'kimberlite' is defined and mineralogical and textural classifications which allow systematic documentation and collation of these complex rocks are evaluated. Quantitative mineralogical classification based on the modal abundances of the matrix minerals is advocated. A textural classification is adopted within which different textures are related to different modes and conditions of kimberlite emplacement. In the subsequent and major part of the thesis the results of a comparative geological study of six diamondiferous kimberlite pipes are reported. The pipes concerned are the De Beers, Wesselton, Dutoitspan, Bultfontein, Finsch and Koffiefontein occurrences (collectively termed the KIMFIK pipes)