The development of biodegradable plastics from sustainable sources is at the forefront of chemical research. One such example is the production of polylactide (PLA) via the ring-opening polymerisation (ROP) of the cyclic ester lactide (LA). Current industrial metal initiators utilised for the ROP of LA do not allow control over the stereochemistry of the resulting product. This thesis will investigate various initiators containing a variety of ligand sets for the ROP of rac-LA. Chapter 1 introduces the ROP of rac-LA, the mechanisms utilised and the methods employed for characterisation of PLA. A review of the current literature of recent developments in the production of PLA via various metal initiators is also included. Chapter 2 reports the development of a series of group (IV) complexes containing various amine tris(phenolate) ligands, where the sterics and electronics have been varied. Such complexes were trialled for the ROP of rac-LA as well as the ROP of trimethylene carbonate (TMC). The ability of such initiators to produce copolymers of rac-LA/TMC and rac-LA/isosorbide was also investigated and discussed. Chapter 3 describes the synthesis of a range of group (IV) complexes containing Salalen ligands. The sterics of the ligands have been varied and the ability of the initiators to initiate the ROP of rac-LA in a stereocontrolled fashion has been investigated. Furthermore, the complexes have been trialled for the degradation of PLA into methyl lactate, an important starting material in the production of LA. Chapter 4 investigates the development of Al(III) Salalen complexes for the ROP of rac-LA, where the sterics and electronics of the ligand have been varied. Kinetic investigations have been carried out to aid the understanding of the polymerisation process. Chapter 5 provides details of the reaction procedures for the synthesis of ligands, complexes and polymers. Kinetic procedures are also reported together with details of the analytical techniques employed.

abstract: ABSTRACT This thesis focuses on structural characterizations and optical properties of Si, Ge based semiconductor alloys. Two material systems are characterized: Si-based III-V/IV alloys, which represent a possible pathway to augment the optical performance of elemental silicon as a solar cell absorber layer, and Ge-based Ge1-ySny and Ge1-x-ySixSny systems which are applicable to long wavelength optoelectronics. Electron microscopy is the primary tool used to study structural properties. Electron Energy Loss spectroscopy (EELS), Ellipsometry, Photoluminescence and Raman Spectroscopy are combined to investigate electronic band structures and bonding properties. The experiments are closely coupled with structural and property modeling and theory. A series of III-V-IV alloys have been synthesized by the reaction of M(SiH3)3 (M = P, As) with Al atoms from a Knudsen cell. In the AlPSi3 system, bonding configurations and elemental distributions are characterized by scanning transmission electron microscopy (STEM)/EELS and correlated with bulk optical behavior. The incorporation of N was achieved by addition of N(SiH3)3 into the reaction mixture yielding [Al(As1-xNx)]ySi5-2yalloys. A critical point analysis of spectroscopic ellipsometry data reveals the existence of direct optical transitions at energies as low as 2.5 eV, well below the lowest direct absorption edge of Si at 3.3 eV. The compositional dependence of the lowest direct gap and indirect gap in Ge1-ySny alloys extracted from room temperature photoluminescence indicates a crossover concentration of yc =0.073, much lower than virtual crystal approximation but agrees well with large atomic supercells predictions. A series of Ge-rich Ge1-x-ySixSny samples with a fixed 3-4% Si content and progressively increasing Sn content in the 4-10% range are grown and characterized by electron microscopy and photoluminescence. The ternary represents an attractive alternative to Ge1-ySny for applications in IR optoelectronic technologies. / Dissertation/Thesis / Ph.D. Physics 2014

No / Group IV metal complexes have previously shown promise as novel anticancer agents. Here, we discuss the mechanistic and cytotoxic nature of a series of group IV b-diketonate coordination complexes. Clear evidence that the ligands are exchangeable on the metal centre and that the b-diketonate ligands can act as potential drug delivery vehicles of the group IV metal ions was obtained. When evaluated for the cytotoxicity against human colon adenocarcinoma (HT-29) and human breast adenocarcinoma (MCF-7) cell lines, a general trend of decreasing potency down the group IV metals was observed. The most promising results obtained were for the hafnium complexes, with the tris diphenyl b-diketonate hafnium complex exhibiting IC50 values of 4.9 0.9 mm and 3.2 0.3 mm against HT-29 and MCF-7, respectively, which are comparable with the activity of cisplatin against the same cell lines. This tri b-diketonate hafnium complex is the first to show potent in vitro cytotoxic activity. The results reported show that ligand design has a significant effect on the cytotoxic potential of the complexes, and that these group IV complexes warrant further evaluation as novel metal-containing anticancer agents.

abstract: The work described in this thesis explores the synthesis of new semiconductors in the Si-Ge-Sn system for application in Si-photonics. Direct gap Ge1-ySny (y=0.12-0.16) alloys with enhanced light emission and absorption are pursued. Monocrystalline layers are grown on Si platforms via epitaxy-driven reactions between Sn- and Ge-hydrides using compositionally graded buffer layers that mitigate lattice mismatch between the epilayer and Si platforms. Prototype p-i-n structures are fabricated and are found to exhibit direct gap electroluminescence and tunable absorption edges between 2200 and 2700 nm indicating applications in LEDs and detectors. Additionally, a low pressure technique is described producing pseudomorphic Ge1-ySny alloys in the compositional range y=0.06-0.17. Synthesis of these materials is achieved at ultra-low temperatures resulting in nearly defect-free films that far exceed the critical thicknesses predicted by thermodynamic considerations, and provide a chemically driven route toward materials with properties typically associated with molecular beam epitaxy. Silicon incorporation into Ge1-ySny yields a new class of Ge1-x-ySixSny (y>x) ternary alloys using reactions between Ge3H8, Si4H10, and SnD4. These materials contain small amounts of Si (x=0.05-0.08) and Sn contents of y=0.1-0.15. Photoluminescence studies indicate an intensity enhancement relative to materials with lower Sn contents (y=0.05-0.09). These materials may serve as thermally robust alternatives to Ge1-ySny for mid-infrared (IR) optoelectronic applications. An extension of the above work is the discovery of a new class of Ge-like Group III-V-IV hybrids with compositions Ga(As1–xPx)Ge3 (x=0.01-0.90) and (GaP)yGe5–2y related to Ge1-x-ySixSny in structure and properties. These materials are prepared by chemical vapor deposition of reactive Ga-hydrides with P(GeH3)3 and As(GeH3)3 custom precursors as the sources of P, As, and Ge incorporating isolated GaAs and GaP donor-acceptor pairs into diamond-like Ge-based structures. Photoluminescence studies reveal bandgaps in the near-IR and large bowing of the optical behavior relative to linear interpolation of the III-V and Ge end members. Similar materials in the Al-Sb-B-P system are also prepared and characterized. The common theme of the above topics is the design and fabrication of new optoelectronic materials that can be fully compatible with Si-based technologies for expanding the optoelectronic capabilities of Ge into the mid-IR and beyond through compositional tuning of the diamond lattice. / Dissertation/Thesis / Doctoral Dissertation Chemistry 2018

abstract: The realization of Silicon based photonic devices will enable much faster data transmission than is possible today using the current electronics based devices. Group IV alloys germanium tin (GeSn) and silicon germanium tin (SiGeSn) have the potential to form an direct bandgap material and thus, they are promising candidates to develop a Si compatible light source and advance the field of silicon photonics. However, the growth of the alloys is challenging as it requires low temperature growth and proper strain management in the films during growth to prevent tin segregation. In order to satisfy these criteria, various research groups have developed novel chemical vapor deposition (CVD) reactors to deposit the films. While these reactors have been highly successful in depositing high crystal quality high Sn concentration films, they are generally expensive set-ups which utilize several turbomolecular/cryogenic pumps and/or load-lock systems. An more economical process than the state-of-the art to grow group IV materials will be highly valuable. Thus, the work presented in this dissertation was focused on deposition of group IV semiconductor thin films using simplified plasma enhanced CVD (PECVD) reactors. Two different in-house assembled PECVD reactor systems, namely Reactor No. 1 and 2, were utilized to deposit Ge, GeSn and SiGeSn thin films. PECVD technique was used as plasma assistance allows for potentially depositing the films at growth temperatures lower than those of conventional CVD. Germane (GeH4) and Digermane (Ge2H6) were used as the Ge precursor while Disilane (Si2H6) and tin chloride (SnCl4) were used as the precursors for Si and Sn respectively. The growth conditions such as growth temperature, precursor flow rates, precursor partial pressures, and chamber pressure were varied in a wide range to optimize the growth conditions for the films. Polycrystalline Ge films and SiGeSn films with an Sn content upto 8% were deposited using Reactor No. 1 and 2. Development of epitaxial Ge buffers and GeSn films was accomplished using a modified Reactor No. 2 at temperatures <400oC without the aid of ultra-high vacuum conditions or a high temperature substrate pre-deposition bake thereby leading to a low economic and thermal budget for the deposition process. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

In den letzten Jahren gibt es großes Interesse am SiGeSn Materialsystem aufgrund seines Potentials für die Verwendung in der Optoelektronik, Elektronik und Photovoltaik. Während jedoch die binären Verbindungshalbleiter Si(x)Ge(1-x) und Ge(1-y)Sn(y) schon intensiv untersucht wurden, sind die Materialeigenschaften des ternären Verbindungshalbleiters Ge(1-x-y)Si(x)Sn(y) und Nanostrukturen basierend auf diesem Verbindungshalbleiter noch weitgehend unbekannt. In dieser Arbeit werden drei theoretische/theoretisch-experimentelle Studien zur Untersuchung des SiGeSn Materialsystems vorgestellt. In einer Studie wird die Abhängigkeit der Größe der direkten Bandlücke von der Zusammensetzung des Ge(1-x-y)Si(x)Sn(y) Verbindungshalbleiters untersucht. Basierend auf Messungen der Rutherford Rückstreuung, Röntgenbeugung und Photolumineszenz (PL) von Ge(1-x-y)Si(x)Sn(y) Proben mit an Ge angepassten Gitterkonstanten wird die Abhängigkeit von Größe der direkten Bandlücke und der Materialkomposition mit einer quadratischen Gleichung beschrieben. Weiterhin wird die Bandanordnung der elementaren Halbleiter Si, Ge und Sn an Grenzflächen untersucht. Anhand von Kohn-Sham basierter Density Functional Theory (DFT) in Kombination mit Local Density Approximation (LDA) berechneten Bandstrukturen von Grenzflächen zwischen Elementarhalbleitern wird der Versatz im Valenzband zwischen Si, Ge und Sn untersucht. Es wird gezeigt, dass aufgrund zu kleiner Bandlücken resultierend aus dem Kohn-Sham-Ansatz in Verbindung mit der LDA ein unphysikalischer „Broken Gap“ Versatz zwischen Ge und Sn Bändern entsteht. In einer dritten Studie werden die PL-Spektren von Ge Quantentöpfen mit Si Barrieren untersucht. Um die Abhängigkeit der PL-Spektren von Anregungsintensität und Temperatur zu verstehen, wird ein selbstkonsistentes Effektives-Massen-Model entwickelt. Mit diesem Model ist es möglich den Einfluss von Temperatur und Bandauffüllung auf das PL-Spektrum zu untersuchen. / The SiGeSn semiconductor material system has recently attracted great interest due to its prospective potential for use in optoelectronics, electronics, and photovoltaics. While the binary alloy Si(x)Ge(1-x) and Ge(1-y)Sn(y) have already been well studied, the properties of bulk and heterostructures involving the Ge(1-x-y)Si(x)Sn(y) ternary alloy are largely unknown. In this thesis, we present the results of three theoretical/experimental-theoretical investigations concerning the SiGeSn material system. First, we investigate the compositional dependence of the direct band-gap of Ge(1-x-y)Si(x)Sn(y) alloys. Based on Rutherford backscattering, x-ray diffraction, and photoluminescence (PL) measurement of Ge(1-x-y)Si(x)Sn(y) alloys lattice-matched to Ge, we describe the compositional dependence of the band gap using a quadratic equation. We predict Ge(1-x-y)Si(x)Sn(y) alloys lattice-matched to Ge to be direct-band-gap semiconductors for Sn concentrations larger than 12%. Secondly, we investigate the band alignment between the elemental semiconductors Si, Ge, and Sn. Performing bulk and interface calculations using density functional theory (DFT) in combination with the local density approximation (LDA), we attempt to calculate the valence band offset between the elemental semiconductors. We find that the Kohn-Sham based DFT-LDA calculations are flawed by the underestimation of the band-gaps of the elemental semiconductors, which leads to a false broken gap band alignment between Ge and Sn. Third, we study the PL of ultrathin Ge multiple quantum well (multiple-QW) structures grown on Si. To understand the excitation density and temperature related shifts of the PL spectra of the sample, we develop a self-consistent multivalley effective mass model. Using second-order perturbation theory, we calculate the indirect phonon-assisted radiative spontaneous recombination rate together with the no-phonon peak energy and compare our results to the experimental results.

No description available.

In den letzten Jahren hat sich maschinelles Lernen als hilfreiches Werkzeug zur Vorhersage von simulierten Materialeigenschaften erwiesen. Somit können aufwendige Berechnungen mittels Dichtefunktionaltheorie umgangen werden und bereits bekannte Materialien besser verstanden oder sogar neuartige entdeckt werden. Eine zentrale Rolle spielt dabei der Deskriptor, ein möglichst interpretierbarer Satz von Materialkenngrößen. Diese Arbeit präsentiert einen Ansatz zur Auffindung von Deskriptoren für periodische Multikomponentensysteme, deren Eigenschaften durch die genaue atomare Anordnung mitbeinflusst wird. Primäre Features von Einzel-, Paar- und Tetraederclustern werden über die Superzelle gemittelt und weiter algebraisch kombiniert. Aus den so erzeugten Kandidaten wird mittels Dimensionalitätsreduktion ein geeigneter Deskriptor identifiziert. Zudem stellt diese Arbeit Strategien vor bei der Modellfindung Kreuzvalidierung einzusetzen, sodass stabilere und idealerweise besser generalisierbare Deskriptoren gefunden werden. Es werden außerdem mehrere Fehlermaße untersucht, die die Qualität der Deskriptoren bezüglich Genauigkeit, Komplexität der Formeln und Berücksichtung der atomaren Anordnung charakterisieren. Die allgemeine Methodik wurde in einer teilweise parallelisierten Python-Software implementiert. Als konkrete Problemstellungen werden Modelle für die Gitterkonstante und die Mischenergie von ternären Gruppe-IV Zinkblende-Legierungen "gelernt", mit einer Genauigkeit von 0.02 Å bzw. 0.02 eV. Datenbeschaffung, -analyse, und -bereinigung werden im Hinblick auf die Zielgrößen als auch auf die primären Features erläutert, sodass umfassende Analysen und die Parametrisierung der Methodik an diesem Testdatensatz durchgeführt werden können. Als weitere Anwendung werden Gitterkonstante und Bandlücken von binären Oktett-Verbindungen vorhergesagt. Die präsentierten Deskriptoren werden mit den Fehlermaßen evaluiert und ihre physikalische Relevanz wird abschließend disktutiert. / In the last years, machine learning methods have proven as a useful tool for the prediction of simulated material properties. They may replace effortful calculations based on density functional theory, provide a better understanding of known materials or even help to discover new materials. Here, an essential role is played by the descriptor, a desirably interpretable set of material parameters. This PhD thesis presents an approach to find descriptors for periodic multi-component systems where also the exact atomic configuration influences the physical characteristics. We process primary features of one-atom, two-atom and tetrahedron clusters by an averaging scheme and combine them further by simple algebraic operations. Compressed sensing is used to identify an appropriate descriptor out from all candidate features. Furthermore, we develop elaborate cross-validation based model selection strategies that may lead to more robust and ideally better generalizing descriptors. Additionally, we study several error measures which estimate the quality of the descriptors with respect to accuracy, complexity of their formulas and the capturing of configuration effects. These generally formulated methods were implemented in a partially parallelized Python program. Actual learning tasks were studied on the problem of finding models for the lattice constant and the energy of mixing of group-IV ternary compounds in zincblende structure where an accuracy of 0.02 Å and 0.02 eV is reached, respectively. We explain the practical preparation steps of data acquisition, analysis and cleaning for the target properties and the primary features, and continue with extensive analyses and the parametrization of the developed methodology on this test case. As an additional application we predict lattice constants and band gaps of octet binary compounds. The presented descriptors are assessed quantitatively by the error measures and, finally, their physical meaning is discussed.