Embedded Ribbons of Graphene Allotropes: An Extended Defect Perspective

Four fundamental dimer manipulations can be used to produce a variety of localized and extended defect structures in graphene. Two-dimensional templates result in graphene allotropes, here viewed as extended defects, which can exhibit either metallic or semiconducting electrical character. \emph{Embedded allotropic ribbons}--i.e. thin swaths of the new allotropes--can also be created within graphene. We examine these ribbons and find that they maintain the electrical character of their parent allotrope even when only a few atoms in width. Such extended defects may facilitate the construction of monolithic electronic circuitry.Comment: 24 pages, 21 figure

First principles studies of Si-C alloys

This study involves the investigation of silicon-carbon systems using ab initio techniques. It was motivated by the search for off-50:50 alloys and a way to quantify the strengths of 2D silicon-carbon materials. The study also predicts some under-reported properties for three previously proposed hypothetical allotropes of carbon. Preferably stable off-50:50 structures are identified from a set of trial structures for silicon-rich and carbon-rich candidates and their conditions of stability and physical properties are identified. A two-dimensional equation of state is introduced and applied to analyze the relative strengths of various 2D silicon-carbon materials. Of the possible off-50:50 alloy combinations and candidate structures considered, only the pyrite-FeS2, glitter-SiC2 and t-BC2 structures for SiC2 are elastically and dynamically stable. Analysis of the instability of Si2C reveals that it seems likely that carbon rich alloys are more favorable to their silicon-rich counterparts due to the smaller size of the carbon atoms and the more compact carbon-carbon bonds which result in less distorted bonding that is less metallic. The stiffness of the silicon dicarbide structures rank, in increasing order with 3C-SiC included for comparison, as glitter --> pyrite --> 3C-SiC --> t-SiC2. The moduli values for t-SiC2 are very comparable to 3C-SiC since for both materials, all atoms are four-fold coordinated with t-SiC2 having similar but slightly distorted, strong covalent tetrahedral bonding. The pyrite and glitter structures exhibit metallic character whereas t-SiC2 is a semi-conductor. Not only has this work demonstrated that, in principle, off-50:50 alloys of carbon and silicon are plausible, it has also provided information on how the strength and elastic properties of these materials are effected by increased silicon content. This has filled in a significant lack of knowledge about these bulk systems. For 2D systems, an equation of state is proposed that equates in-plane pressure with a change in surface area. It extracts the layer modulus as one of its fit parameters, which measures a material's resilience to hydrostatic stretching and predicts the material's intrinsic strength. Graphene is the most resilient to stretching with the highest intrinsic strength of all structures considered followed by SiC. Buckled Si is the least resilient with the lowest strength. An off-50:50 planar alloy, called silagraphene, differs elastically from SiC but has a comparable strength due to the similarity of their layer modulus. The novel 2D equation of state presented here opens up new ways to study and compare the strength properties of mono or multi-layered 2D materials, especially how their resilience to isotropic stretching responds to in-plane pressure.Thesis (PhD)--University of Pretoria, 2013.Physicsunrestricte

Currents in Carbon and Heterocyclic Networks

Current density maps are calculated within the ipsocentric approach for a variety of systems, to determine the nature of their aromatic magnetic response, and to probe the underlying principles governing ring current aromaticity. The first chapter briefly discusses the history of the term aromaticity, from the discovery of benzene, to ring current theory, the modern quantum mechanical ipsocentric approach, and the simple but powerful selection rules that are derived from it. Examples of systems displaying aromatic, antiaromatic and localised, non aromatic responses are provided in Chapter 2 to demonstrate the utility of the method, and the in depth analysis of aromaticity that it permits. Chapter 3 explores the possibility of designing tailored ring current responses on finite nanographene flakes via functionalisation by examining a variety of nanographene/nanographane hybrids derived from coronene and ovalene. This idea is extended in Chapter 4, by consideration of substitution of C6 cycles for borazine like B3N3 cycles, creating benzenoid/borazinoid hybrids. Chapter 5 investigates how BN heteroannulenes can be successfully aromatised by alteration of electronic charge. The approaches for altering current response introduced in Chapters 3 to 5 are unified in Chapter 6 in a case study of pyrene and structures derived from it by variation of charge, substitution, and functionalisation. Chapter 7 further examines how changing the electronic environment by substitution of carbon centres for heteroatoms alters ring current patterns, using linear polyacenes as the example systems. Chapter 8 moves away from the methods of controlling ring current by chemical manipulation and considers the effects of geometric change on aromaticity of the homotropenylium cation and the extended family of N homoannulenes. A brief discussion of the possibilities of designing aromatic systems using extended non standard ring architectures concludes the thesis

2d Materials For Energy Applications

Accelerated energy demands, together with unprecedented CO2 emissions, aggravate the global energy and climate change crises, endangering the sustainable development of society in a perpetuity way. The ability to find, extract, and use energy in an effective and clean way is pivotal to the energy paradigm shift, where a large percentage of global energy demand is expected to be met through sustainable energy resources. Research in materials science is contributing towards such a sustainable future by addressing bottleneck questions in energy storage and conversion, which are two main parts of energy sustainability. In particular, recently discovered two-dimensional (2D) materials exhibit extraordinary mechanical, chemical, electronic, optical, and magnetic properties that are promising to break through current material limitations in energy applications. The main goal of this thesis is to examine the possibility of using 2D materials in improving current energy applications, in particular, battery electrodes and hydrogen evolution reaction (HER) catalysts, and to elucidate the mechanisms and guiding principles in tuning 2D materials using combinatorial simulation techniques that bridge different length scales. Representative and promising 2D material systems, including graphene-like materials, MXenes, transition metal dichalcogenides (TMDs), layered covalent-organic framework (COF), and oxides are studied. To evaluate the performance of 2D materials in battery electrodes, we employ the density functional theory (DFT) simulations to investigate the adsorption of different metal ions onto 2D MXenes and 2D graphene-like materials, and hence quantify the enhanced theoretical capacities and rate-performance. Moreover, we find the origin of such improvements and summarize guiding principles in tuning 2D materials for similar applications in batteries beyond lithium. We also show that 2D TMDs are capable of improving hydrogen production efficiency. The role of defects and electronic coupling between substrate and MoS2 catalysts is investigated, followed by a study of using the Janus asymmetry as a feasible way to activate basal plane catalytic activity. Finally, we present a multiscale modeling method that bridges different length scales, and show several successful examples in applying this method in energy applications. This thesis provides new understandings of 2D materials in energy applications. Such understandings may be used to accelerate the realization of future energy plan

Structure and Reactivity of Aromatic Molecules on Metal Single-Crystal Surfaces and at Metal/Organic Interfaces

Low-dimensional carbon-based nanostructures are considered for the fabrication of modern electronic devices. For the realization of such devices, it is of utmost importance to achieve a high control over the structural quality. As a result, the field of on-surface synthesis, which aims at producing well-defined structures from tailor-made molecular precursors, has grown rapidly over the past decade. The reaction most frequently used to conduct on-surface synthesis is the Ullmann coupling reaction. Although a lot of work has already been invested, the fundamental principles determining the outcome of this reaction have not fully been understood to date. One prototypical case for such a situation is the product formation on the basis of precursor molecules that can either form long oligomer chains or macrocycles. This cumulative dissertation thesis contains a number of articles investigating the reaction products of different precursor molecules bearing these characteristics. They are investigated on metal single-crystal surfaces by scanning tunneling microscopy and complementary surface science techniques such as X-ray photoelectron spectroscopy or angle-resolved photoemission spectroscopy, accompanied by Monte Carlo simulations. The ring/chain ratio formed by the model system 1,3-dibromoazulene on Cu(111) was studied. By this means new insights on how the ring/chain ratio can be tunedby variation of coverage and temperature were gained based on fundamental physicochemical considerations. An alternative approach to steer the reaction outcome was used by applying a surface template, i.e., a vicinal Ag surface, to exclusively form long, perfectly aligned oligomer chains from the 4,4''-dibromo-1,1':3',1''-terphenyl precursor. Furthermore, the 2,6-dibromoazulene precursor, which can exclusively form chains, was used to generate nanoribbons of the non-alternant graphene allotropes phagraphene and tetra-penta-hepta-graphene on Au(111). The structures of these species have been unambiguously elucidated by non-contact atomic force microscopy experiments carried out in a collaboration project. As a last project, the structural polymorphism of the pure self-assembly of 1,1':3',1'':4'',1'''-quaterphenyl-4,4'''-dicarbonitrile on the Ag(111) surface was investigated. This molecule shows an adsorbate structure containing flat-lying and upright-standing molecules. Such a structure had not been reported so far. Along with the structures formed, the performance of organic-electronic devices is also crucially dependent on the interactions between the substrate and the organic layer itself. To contribute to this field of research, studies on different model systems, i.e., porphyrins, corroles, and the non-alternant aromatic molecule azulene, have been performed in collaboration projects mostly involving synchrotron radiation beamtimes. In addition to the results already published in scientific journals, some unpublished results are part of this thesis. These are the investigation of the 1,3-dibromoazulene precursor on the Ag(111) surface with co-deposited Cu atoms and the successful initial operation of a commercially available atomic layer injection device. The experimental results are supplemented by the development and construction of technical instrumentation, which expands the capabilities of the measurement setup in the laboratory of the Gottfried group in Marburg

Strukturchemie und Phasenbeziehungen der intermetallischen Phasen des Zweistoffsystems Iridium-Zink und ternärer Substitutionsvarianten mit Magnesium

Bericht über die erstmalige Synthese und Charakterisierung von intermetallischen Phasen des Zweistoffsystems Iridium-Zink, sowie einiger ternärer Substitutionsvarianten mit Magnesium. Die Synthese erfolgte mittels Hochtemperaturmethoden aus den Elementen. Zur Strukturaufklärung wurden Röntgenbeugungsexperimente an Einkristallen und Pulvern durchgeführt, ergänzt um transmissionselektronenmikroskopische Studien zur Realstruktur. Ausgewählte Präparate wurden hinsichtlich ihrer physikalischen Eigenschaften untersucht (thermisches Verhalten, Magnetismus, elektrische Leitfähigkeit). Diskutiert werden die Strukturchemie und Phasenbeziehungen, mit einem Schwerpunkt auf der struktursystematischen Einordnung der Befunde. Bemerkenswert sind das Auftreten einer zweidimensional inkommensurabel modulierten Phase (IrZn3), einer Phase mit dreidimensional ausgedehnter diffuser Streuung (IrZn2), sowie ganz allgemein der hohe Grad an struktureller Ausdifferenzierung und Komplexität, der sich an den binären Phasen - ihren Kristallstrukturen und Phasenbeziehungen - beobachten läßt

Pentaheptite Modifications of the Graphite Sheet

Pentaheptites (three-coordinate tilings of the plane by pentagons and heptagons only) are classified under the chemically motivated restriction that all pentagons occur in isolated pairs and all heptagons have three heptagonal neighbors. They span a continuum between the two lattices exemplified by the boron nets in ThMoB4 (cmm) and YCrB4 (pgg), in analogy with the crossover from cubic-close-packed to hexagonalclose-packed packings in 3D. Symmetries realizable for these pentaheptite layers are three strip groups (periodic in one dimension), p1a1, p112, and p111, and five Fedorov groups (periodic in two dimensions), cmm, pgg, pg, p2, and p1. All can be constructed by simultaneous rotation of the central bonds of pyrene tilings of the graphite sheet. The unique lattice of cmm symmetry corresponds to the previously proposed pentaheptite carbon metal. Analogous pentagon-heptagon tilings on other surfaces including the torus, Klein bottle, and cylinder, face-regular tilings of pentagons and b-gons, and a full characterization of tilings involving isolated pairs and/or triples of pentagons are presented. The Kelvin paradigm of a continuum of structures arising from propagation of two original motifs has many potential applications in 2D and 3D. 1