Simulations of charge transport in organic compounds

To aid the design of organic semiconductors, we study the charge transport properties of organic liquid crystals, i.e. hexabenzocoronene and carbazole macrocycle, and single crystals, i.e. rubrene, indolocarbazole and benzothiophene derivatives (BTBT, BBBT). The aim is to find structure-property relationships linking the chemical structure as well as the morphology with the bulk charge carrier mobility of the compounds. To this end, molecular dynamics (MD) simulations are performed yielding realistic equilibrated morphologies. Partial charges and molecular orbitals are calculated based on single molecules in vacuum using quantum chemical methods. The molecular orbitals are then mapped onto the molecular positions and orientations, which allows calculation of the transfer integrals between nearest neighbors using the molecular orbital overlap method. Thus we obtain realistic transfer integral distributions and their autocorrelations. In case of organic crystals the differences between two descriptions of charge transport, namely semi-classical dynamics (SCD) in the small polaron limit and kinetic Monte Carlo (KMC) based on Marcus rates, are studied. The liquid crystals are investigated solely in the hopping limit. To simulate the charge dynamics using KMC, the centers of mass of the molecules are mapped onto lattice sites and the transfer integrals are used to compute the hopping rates. In the small polaron limit, where the electronic wave function is spread over a limited number of neighboring molecules, the Schroedinger equation is solved numerically using a semi-classical approach. The results are compared for the different compounds and methods and, where available, with experimental data. The carbazole macrocycles form columnar structures arranged on a hexagonal lattice with side chains facing inwards, so columns can closely approach each other allowing inter-columnar and thus three-dimensional transport. When taking only intra-columnar transport into account, the mobility is orders of magnitude lower than in the three-dimensional case. BTBT is a promising material for solution-processed organic field-effect transistors. We are able to show that, on the time-scales of charge transport, static disorder due to slow side chain motions is the main factor determining the mobility. The resulting broad transfer integral distributions modify the connectivity of the system but sufficiently many fast percolation paths remain for the charges. Rubrene, indolocarbazole and BBBT are examples of crystals without significant static disorder. The high mobility of rubrene is explained by two main features: first, the shifted cofacial alignment of its molecules, and second, the high center of mass vibrational frequency. In comparsion to SCD, only KMC based on Marcus rates is capable of describing neighbors with low coupling and of taking static disorder into account three-dimensionally. Thus it is the method of choice for crystalline systems dominated by static disorder. However, it is inappropriate for the case of strong coupling and underestimates the mobility of well-ordered crystals. SCD, despite its one-dimensionality, is valuable for crystals with strong coupling and little disorder. It also allows correct treatment of dynamical effects, such as intermolecular vibrations of the molecules. Rate equations are incapable of this, because simulations are performed on static snapshots. We have thus shown strengths and weaknesses of two state of the art models used to study charge transport in organic compounds, partially developed a program to compute and visualize transfer integral distributions and other charge transport properties, and found structure-mobility relations for several promising organic semiconductors.Um die Herstellung organischer Halbleiter zu erleichtern, untersuchen wir den Ladungstransport in organischen Fluessigkristallen, wie Hexabenzocoronen und Carbazolringen, und Einkristallen, wie Rubren, Indolocarbazol und Benzothiophenderivaten. Zielsetzung ist es, Zusammenhaenge zwischen der Struktur und der Ladungstraegermobilitaet zu finden. Zu diesem Zweck werden Molekulardynamiksimulationen (MD) durchgefuehrt, welche realistische, equilibrierte Morphologien liefern. Partialladungen und Elektronenorbitale werden an Einzelmolekuelen im Vakuum mit quantenchemischen Methoden berechnet. Die Orbitale werden dann auf die Positionen und Orientierungen der Molekuele abgebildet, was die Berechnung von Transferintegralen zwischen naechsten Nachbarn mit der Molecular-Orbital-Overlap-Methode ermoeglicht. Dies ergibt realistische Transferintegralverteilungen und dazugehoerige Autokorrelationen. Fuer organische Kristalle untersuchen wir zusaetzlich den Unterschied zwischen zwei Methoden zur Simulation von Ladungstransport, naemlich Semi-Classical-Dynamics (SCD) und Kinetic-Monte-Carlo (KMC) mit Marcusraten. Um den Ladungstransport mit KMC zu simulieren, werden die Schwerpunkte der Molekuele mit Gitterpunkten identifiziert, waehrend die Transferintegrale zur Berechnung der Sprungraten dienen. Im Grenzfall kleiner Polaronen (SCD), indem die Wellenfunktion der Elektronen ueber mehrere Molekuele ausgedehnt ist, wird die Schroedingergleichung semi-klassisch geloest. Die Ergebnisse fuer die unterschiedlichen Materialien und Methoden werden verglichen, sofern vorhanden auch mit experimentellen Daten. Die Carbazolringe bilden hexagonal angeordnete Saeulen, deren Seitenketten nach innen gerichtet sind, was eine dichte Annaeherung und somit interkolumnaren Ladungstransport ermoeglicht. Beruecksichtigt man nur intrakolumnaren Transport, ist die Mobilitaet um mehrere Groeßenordnungen niedriger als im dreidimensionalen Fall. BTBT ist ein vielversprechendes Material fuer in Loesung hergestellte Feldeffekttransistoren. Wir zeigen, dass auf der Zeitskala von Ladungstransport statische Unordnung aufgrund langsam fluktuierender Seitenketten die Mobilitaet erheblich beeinflusst. Die resultierenden Transferintegralverteilungen modifizieren die Konnektivitaet des Systems, jedoch verbleiben genug Perkolationswege fuer schnellen Ladungstransport. Rubren, Indolocarbazol und BBBT sind Kristalle, in denen statische Unordnung eine untergeordnete Rolle spielt. Die hohe Mobilitaet in Rubren wird durch zwei Eigenschaften erklaert: erstens die planare, verschobene Anordnung der Molekuele, und zweitens deren hohe Schwingungsfrequenz. Im Vergleich zu SCD ist KMC basierend auf Marcusraten in der Lage, Nachbarn mit niedrigen Transferraten und dreidimensionale statische Unordnung zu beschreiben. Diese Methode ist daher fuer Systeme, die von statischer Unordnung bestimmt werden, zu bevorzugen. Allerdings ist sie ungeeignet fuer den Fall hoher Transferraten und unterschaetzt die Mobilitaet in gut geordneten Kristallen. SCD ist zwar eindimensional, aber dennoch gut geeignet, um stark gekoppelte Materialien mit wenig Unordnung zu beschreiben. Zusaetzlich ermoeglicht es die Beschreibung dynamischer Effekte, wie intermolekularer Schwingungen. Ratenbasierende Methoden sind dazu nicht in der Lage, da sie auf statischen Momentaufnahmen basieren. Wir haben damit die Vor- und Nachteile zweier aktueller Modelle zur Untersuchung von Ladungstransport in organischen Molekuelen gezeigt, an einem Programm zur Berechnung und Visualisierung von Transferintegralen und Transporteigenschaften mitgearbeitet und Zusammenhaenge zwischen Struktur und Eigenschaften vielversprechnder organischer Halbleiter gefunden

Interaction of short cracks with the local microstructure

AbstractIncreasing the resistance of a material against fatigue crack growth by optimizing the microstructure is one of the major tasks of modern materials science. Thereby grain and phase boundaries are microstructural elements which can decelerate the propagation rate especially of short cracks. However, in different materials cracks and grain boundaries interact differently. For instance in some steels the blocking effect was only found for large angle grain boundaries while small angle boundaries showed nearly no effect. On the other hand in nickel based superalloys a retardation of cracks was found even for small angle boundaries when the crack was propagating in stage I. Even in front of the same grain boundary, the blocking effect varies for different cracks. To investigate this behaviour systematically focused ion beam (FIB) initiated cracks were used. By this method of artificial crack initiation the crack parameters like crack length and distance to the obstacle can be varied separately. Additionally special grain boundaries can be selected after a microstructural characterisation by electron backscatter diffraction (EBSD). Finally FIB tomography helps to understand the process how a crack overcomes a grain boundary

Hard multilayered thin films of metal–intermetallic Ni/Ni3Al

Metal-intermetallic Ni/Ni3Al multilayered thin films were synthesized by the magnetron sputtering technique. The synthesized films possessed high hardness that could be compared with intermetallic Ni3Al films. The constituent layers of Ni and Ni3Al were fully adherent to one another at the multilayered boundaries. The fracture surface of the multilayered films on bending showed the characteristics of local ductile fracture. This novel type of multilayered thin films is expected to be used as hard coatings and miniaturized parts of apparatus in micro-electromechanical systems.published_or_final_versio

Mid-infrared interferometric observations of the high-mass protostellar candidate NGC 3603 IRS 9A

We investigate the question of how high-mass stars form by combining mid-infrared observations from the worlds largest ground- and space-based telescopes and interferometers. The target of this study is IRS 9A, a promising candidate for the rare class of very young, high-mass protostars. In the first part of this work we present the immediate results and implications of the individual observations, and in the second part we try to devise a model for IRS 9A and its circumstellar structure that can account for these observations. We also make use of a publicly available grid of spectral energy distributions which has been calculated for a large number of protostellar objects. We find that neither geometrical models of the brightness distribution nor simple one-dimensional models for the density structure can explain IRS 9A's appearance in the mid-infrared. However, using radiative transfer models that comprise circumstellar disks and envelopes, we are able to simultaneously reproduce all our observational data. Moreover, the comparison with the grid of protostellar objects independently confirms IRS 9A to be a high-mass protostar. Hence our study provides further support to the idea that high-mass stars form in a similar manner to their low- and intermediate-mass counterparts

Fractography, elastic modulus and oxidation resistance of novel metal-intermetallic Ni/Ni 3Al multilayer films

Novel metal–intermetallic Ni/Ni3Al multilayer films are synthesized by a magnetron sputtering technique. The fractography, elastic modulus, and the oxidation resistance of the multilayer films are studied by a series of experimental tests. The scanning electron microscopy fractography of the films shows that both Ni and Ni3Al layers fracture with the appearance of ductile metal failure. No metal–intermetallic delamination appears in the multilayered films. Fluted dimpling in each Ni and Ni3Al layer is evident and continuous, layer through layer, illustrating very good adherence among the constituent layers. Such adherence makes the toughness of the Ni layers capable of transferring into the Ni3Al layers. Young’s modulus of the Ni/Ni3Al film is found to be 226 and 253 ± 10 GPa by nanoindentation and laser acoustic techniques, respectively. The continuity of elastic modulus between the two phases is revealed by nanoindentation test. The modulus continuity indicates an excellent integration of the constituent layers with similar crystal structure and close lattice constants. This integration makes the multilayers unsurpassed in comprehensive mechanical properties. Sheet resistance measurements show a good protective ability of the Ni/Ni3Al multilayers during high temperature oxidation. X-ray photoelectron spectroscopy spectra suggest that crystallized Al2O3 /Ni scales formed during the deposition and subsequent annealing processes are apparently responsible for the stability of these films under oxidative conditions. The appearance of the crystallized Al2O3 /Ni thin scales on the top of Ni3Al layers provides the Ni/Ni3Al multilayers good thermal oxidation resistance without lowering the fracture toughness.published_or_final_versio

Информационная система поддержки принятия решений в задачах организации учебного процесса 2-го уровня

Объектом исследования является (ются) характеристики и деятельность студента во время обучения, влияющая на его учебно-научный потенциал Цель работы – создание ИСППР для оценки учебно-научного потенциала студентовThe subject is characteristics and activity of students which affects their progress and achievements in science during their education The goal of the thesis is to create the informational decision support system for estimation scientific and educational potential of student

Étude de l'activité plastique dans des bi-cristaux métalliques: modélisation et expériences

Des contraintes d’incompatibilité et des rotations de réseau peuvent se développer dans les bi-cristaux en raison des anisotropies élastique et plastique qui existent en lien avec les différentes orientations cristallines présentes de part et d’autre du joint de grains. Récemment, un modèle a été développé qui tient compte pleinement des effets couplés entre élasticité et plasticité hétérogènes. Ce modèle fournit les expressions analytiques explicites des champs de contrainte et de désorientation du réseau dans un bi-cristal en supposant une interface plane infinie et une élasticité et une plasticité uniformes par morceaux. Ces expressions permettent de prédire les valeurs des cissions résolues sur tous les systèmes de glissement d’un bi-cristal. Dans cette étude, le cas général d'une fraction volumique de cristaux quelconque a également été pris en compte. Considérant un chargement uniaxial en élasticité pure, ce modèle permet de retomber sur deux autres théories classiques. Les cissions résolues par système calculées à partir du modèle de Hook-Hirth sont retrouvées en supposant une élasticité hétérogène isotrope et en négligeant l'effet de Poisson. En considérant une élasticité homogène dans le bi-cristal, les « facteur de Schmid » classiques sont retrouvés. A partir d’une cartographie EBSD de Ni pur, ce modèle a été appliqué à la prédiction des possibles systèmes de glissement actifs dans des bi-cristaux suite à des essais de compression parallèle aux joints de grains. L'étude s´est concentrée sur les bi-cristaux où la nouvelle approche conduit à des prédictions différentes concernant l’entrée en plasticité par rapport aux modèles de Schmid et Hook-Hirth. Expérimentalement, les essais de compression uniaxiale sont réalisés par nanoindentation sur des micropilliers fabriqués au FIB. Enfin, les propriétés élastiques effectives de tous les bi-cristaux sont calculées et comparées à celles obtenues à partir des approches de Voigt, Reuss et Hook-Hirth, ainsi qu’aux mesures expérimentales

Mid-infrared interferometry of massive young stellar objects

The very inner structure of massive young stellar objects (YSOs) is difficult to trace. With conventional observational methods we identify structures still several hundreds of AU in size. However, the (proto-)stellar growth takes place at the innermost regions (<100 AU) where the actual mass transfer onto the forming high-mass star occurs. We present results from our programme toward massive YSOs at the VLTI, utilising the two-element interferometer MIDI. To date, we observed 10 well-known massive YSOs down to scales of 20 mas (typically corresponding to 20 - 40 AU for our targets) in the 8-13 micron region. We clearly resolve these objects which results in low visibilities and sizes in the order of 30-50 mas. For two objects, we show results of our modelling. We demonstrate that the MIDI data can reveal decisive structure information for massive YSOs. They are often pivotal in order to resolve ambiguities still immanent in model parameters derived from sole SED fitting.Comment: 6 pages, 5 figures, necessary style files iopams.sty, jpconf11.clo, and jpconf.cls included; contribution for the conference "The Universe under the Microscope" (AHAR 2008), held in Bad Honnef (Germany) in April 2008, to be published in Journal of Physics: Conference Series by Institute of Physics Publishing, R. Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.

Mid-infrared interferometry of the massive young stellar object NGC3603 - IRS 9A

We present observations and models for one of these MYSO candidates, NGC3603 IRS 9A. Our goal is to investigate with infrared interferometry the structure of IRS 9A on scales as small as 200AU, exploiting the fact that a cluster of O and B stars has blown away much of the obscuring foreground dust and gas. Observations in the N-band were carried out with the MIDI beam combiner attached to the VLTI. Additional interferometric observations which probe the structure of IRS 9A on larger scales were performed with an aperture mask installed in the T-ReCS instrument of Gemini South. The spectral energy distribution (SED) is constrained by the MIDI N-band spectrum and by data from the Spitzer Space Telescope. Our efforts to model the structure and SED of IRS 9A range from simple geometrical models of the brightness distribution to one- and two-dimensional radiative transfer computations. The target is resolved by T-ReCS, with an equivalent (elliptical) Gaussian width of 330mas by 280mas (2300 AU by 2000 AU). Despite this fact, a warm compact unresolved component was detected by MIDI which is possibly associated with the inner regions of a flattened dust distribution. Based on our interferometric data, no sign of multiplicity was found on scales between about 200AU and 700AU projected separation. A geometric model consisting of a warm (1000 K) ring (400 AU diameter) and a cool (140 K) large envelope provides a good fit to the data. No single model fitting all visibility and photometric data could be found, with disk models performing better than spherical models. While the data are clearly inconsistent with a spherical dust distribution they are insufficient to prove the existence of a disk but rather hint at a more complex dust distribution.Comment: 8 pages, 11 figures. Accepted for publication in A&