SMALL POLARONS IN REAL CRYSTALS - CONCEPTS AND PROBLEMS

Much of small polaron theory is based on highly idealized models, often essentially a continuum description with a single vibrational frequency. These models ignore much of the wealth of experimental data, which find interpretation in many atomistic simulations. We review here a range of properties of small polarons in real, rather than model, systems. The phenomena fall into three main classes: (i) the mechanisms and dynamics of self-trapping of polarons; (ii) static properties-the relative energies of large and small polarons, the optical transitions expected, their effect on positions of other ions and on lattice vibrations, their population in thermal equilibrium, and so on; (iii) small polaron hopping and diffusion. We discuss the key concepts and methods of calculation of polarons, and explore the properties of self-trapped holes and excitons in ionic crystals, and those of an excess electron in liquid water

Interatomic potentials: Achievements and challenges

Interactions between atoms can be formally expanded into two-body, three-body, and higher-order contributions. Unfortunately, this expansion is slowly converging for most systems of practical interest making it inexpedient for molecular simulations. This is why effective descriptions are needed for the accurate simulation of many-atom systems. This article reviews potentials designed towards this end with a focus on empirical interatomic potentials not necessitating a-priori knowledge of what pairs of atoms are bonded to each other, i.e., on potentials meant to describe defects and chemical reactions from bond breaking and formation to redox reactions. The classes of discussed potentials include popular two-body potentials, embedded-atom models for metals, bond-order potentials for covalently bonded systems, polarizable potentials including charge-transfer approaches for ionic systems and quantum-Drude oscillator models mimicking higher-order and many-body dispersion. Particular emphasis is laid on the question what constraints on materials properties ensue from the functional form of a potential, e.g., in what way Cauchy relations for elastic tensor elements can be violated and what this entails for the ratio of defect and cohesive energies. The review is meant to be pedagogical rather than encyclopedic. This is why we highlight potentials with functional forms that are sufficiently simple to remain amenable to analytical treatments, whereby qualitative questions can be answered, such as, why the ratio of boiling to melting temperature tends to be large for potentials describing metals but small for pair potentials. However, we abstain for the most part from discussing specific parametrizations. Our main aim is to provide a stimulus for how existing approaches can be advanced or meaningfully combined to extent the scope of simulations based on empirical potentials

Semiconductor Nanowires: Optical Properties and All-Optical Switching

The optical properties of semiconductor nanowires are both important from a fundamental materials physics standpoint and necessary to understand in engineering applications: nanowire photovoltaic devices, sensors, and lasers, among others, could all benefit. Unfortunately, these optical properties are not easy to ascertain. Transmission times are short, in-coupling of white probe light is difficult, and the angle-resolved measurements typically used to determine material dispersion relations in bulk materials are hindered by diffraction effects at subwavelength nanowire end facets. Here, we present a series of experimental techniques and theoretical models developed to study of the optical properties of active nanowire waveguides. Beginning with a technique for determining the waveguide dispersion of individual ZnSe nanowires, we demonstrate enhanced properties with respect to bulk material. After investigating propagation loss in individual CdS nanowires, the theoretical model was then refined to quantify the strength of light-matter coupling, where size-dependence was observed. The knowledge gained from these studies was put to use in the first demonstration of all-optical switching in individual semiconductor nanowires. The switch concept was then extended into an all-optical nanowire NAND gate. These developments highlight the importance of semiconductor nanowires as both model materials systems and novel devices

The Investigation of the Electronic Properties of Si Based Heterojucntions: a First Principle Study of a-Si:H/c-Si and GaP/Si Heterojunctions

abstract: In this dissertation, I investigate the electronic properties of two important silicon(Si)-based heterojunctions 1) hydrogenated amorphous silicon/crystalline silicon (a-Si:H/c-Si) which has already been commercialized in Heterojunction with Intrinsic Thin-layer (HIT) cells and 2) gallium phosphide/silicon (GaP/Si) which has been suggested to be a good candidate for replacing a-Si:H/c-Si in HIT cells in order to boost the HIT cell’s efficiency. In the first part, the defect states of amorphous silicon (a-Si) and a-Si:H material are studied using density functional theory (DFT). I first employ simulated annealing using molecular dynamics (MD) to create stable configurations of a-Si:H, and then analyze the atomic and electronic structure to investigate which structural defects interact with H, and how the electronic structure changes with H addition. I find that H atoms decrease the density of mid-gap states and increase the band gap of a-Si by binding to Si atoms with strained bonds. My results also indicate that Si atoms with strained bonds creates high-localized orbitals in the mobility gap of a-Si, and the binding of H atoms to them can dramatically decrease their degree of localization. In the second part, I explore the effect of the H binding configuration on the electronic properties of a-Si:H/c-Si heterostructure using density functional theory studies of models of the interface between a-Si:H and c-Si. The electronic properties from DFT show that depending on the energy difference between configurations, the electronic properties are sensitive to the H binding configurations. In the last part, I examine the electronic structure of GaP/Si(001) heterojunctions and the effect of hydrogen H passivation at the interface in comparison to interface mixing, through DFT calculations. My calculations show that due to the heterovalent mismatch nature of the GaP/Si interface, there is a high density of localized states at the abrupt GaP/Si interface due to the excess charge associated with heterovalent bonding, as reported elsewhere. I find that the addition of H leads to additional bonding at the interface which mitigates the charge imbalance, and greatly reduces the density of localized states, leading to a nearly ideal heterojunction.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

The Phosphate Vibration as a Sensor for Ion-Pair Formation Studied by Nonlinear Time-Resolved Vibrational Spectroscopy

Die Struktur und Dynamik von Biomolekülen wird durch ein komplexes Wechselspiel mit Ionen und Wassermolekülen der Hydratationshülle beeinflusst. Die Wechselwirkungen sind kaum verstanden, zum Teil weil es an experimentellen molekularen Sonden mangelt. Lokale Schwingungen des RNA-Rückgrats bieten solch nicht-invasive Sonden, empfindlich gegenüber den ersten Schichten der RNA-Solvatationshülle. Die Empfindlichkeit rührt von elektrischen Feldern auf der biomolekularen Oberfläche. Diese Dissertation nutzt die Sensitivität aus, um mit Femtosekunden-2D-IR-Spektroskopie der asymmetrischen Phosphatstreckschwingung die Rolle positiv geladener Ionen, insbesondere Magnesium, Mg2+, zu untersuchen, die negativ geladene Phosphatgruppen des Rückgrats kompensieren. Erste Experimente an Dimethylphosphat, zusammen mit theoretischen Berechnungen, zeigen eine Blauverschiebung der Phosphatmode aufgrund der Bildung von Kontaktionenpaaren. Kurze Abstände zwischen Mg2+ und der Phosphatgruppe führen zu repulsiven Austauschwechselwirkungen, die die Potentialfläche der Schwingung stören. Bei Doppelstrang-RNA zeigt sich eine starke Abhängigkeit der Phosphatschwingung von lokalen Wasserstrukturen. Frequenzverschiebungen durch den Starkeffekt führen zu drei Schwingungsbanden, die unterschiedliche lokale Geometrien widerspiegeln. Elektrische Felder von solvatisierenden Wassermolekülen beeinflussen dabei das Bindungspotential. Abschließend erlaubt es die Blauverschiebung der Phosphatmode, die Bildung von Mg2+/Phosphat Kontaktionenpaaren in Transfer-RNA quantitativ zu verfolgen. Es wird gezeigt, dass diese die Tertiärstruktur der tRNA stabilisieren, indem sie die Coulombabstoßung zwischen negativ geladenen Phosphatgruppen kompensieren, besonders in kompakten Regionen. Die Dissertation demonstriert das Potential zeitaufgelöster Schwingungsspektroskopie, kombiniert mit theoretischen Beschreibungen auf molekularer Ebene, um die komplexen Interaktionen biomolekularer Solvatationsumgebungen zu erforschen.The structure and dynamics of biomolecules are influenced by a complex interplay with ions and water molecules in the local hydration shell. The underlying interactions are poorly understood, partly because of a lack of experimental probes that can access the molecular scale. Local vibrations of the RNA backbone provide non-invasive probes sensitive to the first hydration layers of the RNA solvation shell via the imposed electric field on the biomolecular surface. This thesis exploits this sensitivity in femtosecond 2D-IR spectroscopy experiments on the asymmetric phosphate stretch vibration to investigate the role of positively charged ions, particularly the magnesium cation Mg2+, in counteracting the negatively charged phosphate backbone. Initial experiments with the model system dimethyl phosphate in combination with theoretical calculations report a frequency blue-shift due to the formation of contact ion pairs. Short distances between Mg2+ and phosphate lead to exchange repulsion interactions that perturb the vibrational potential energy surface. In double helical RNA, a strong dependence of the phosphate mode on the local hydration structure of the phosphate group is found. Three distinct vibrational peaks reflect different hydration geometries as a result of vibrational Stark shifts. Responsible for the frequency shifts are electric fields from solvating water molecules. Ultimately, the blue-shift of the phosphate mode allows to quantitatively follow the formation of Mg2+-phosphate contact pairs in transfer RNA systems. It is shown that these configurations stabilize the tertiary structure of tRNA molecules by efficiently compensating the Coulomb repulsion from negatively charged phosphate groups, particularly in highly congested regions. The thesis demonstrates the potential of time-resolved vibrational spectroscopy combined with theoretical descriptions on the molecular level to probe the complex interactions of biomolecular solvation environments

Colloidal quantum dots for guided wave photonics : from optical gain to ultrafast modulation

Silicium is gekend bij het grote publiek als de bouwsteen voor micro-elektronische circuits op 'chips'. Maar het materiaal is ook uitstekend geschikt voor het bouwen van 'fotonische' chips, waar licht in plaats van elektriciteit wordt gebruikt om informatie over te dragen. Door hetzelfde materiaal te gebruiken, kan de 'fotonica' zo reuzensprongen maken naar commercialisatie. Silicium is prima om licht geleiden en te filteren, maar schiet tekort op vlak van modulatie en versterking. In dit doctoraat worden de mogelijkheden bekeken om die tekortkomingen op te lossen met behulp van een nieuw soort materiaal: 'quantum dots' of nano-kristallen, een duizendste van een micrometer groot. Door hun waanzinnig kleine afmetingen vertonen deze bouwblokjes interessante optische eigenschappen die de limieten van silicium en de silicium-fotonica kunnen remediëren