University of Siegen

Permanent URI for this communityhttps://dspace.ub.uni-siegen.de/handle/ubsi/7

Browse

Settings

End date: 2009Subject: search.filters.subject.530 Physik

Search Results

Now showing 1 - 10 of 48
  • Some of the metrics are blocked by your 
    consent settings
    Publication
    Investigation of the surface relief grating formation on amorphous azobenzene polymer films
    (2009)
    Veer, Padmanabh
    The most staggering effect in photosensitive organic thin films is the formation of surface relief gratings (SRGs) onto thin azobenzene polymer films. The phenomenon was reported independently by two different groups in 1995. These gratings can be inscribed onto thin films of azobenzene polymer films using intensity or polarisation modulations resulting from an interference pattern. Using a wavelength corresponding to the absorption maxima of the azobenzene moiety, grating of several hundreds of nanometre heights, depending upon initial film thickness, can be easily written well below the glass transition temperature. Several models have been proposed to describe the process of grating formation and to explain the origin of the force, forming these gratings but none of these theories could explain the complete process depending upon various parameters like polarisation, photo-induced softening and the origin of the inscribing force. The scope of this thesis work is to find out the origin of the stress which leads to the formation of grating onto the azobenzene polymer film and to support the experimental data with an appropriate model. From all the theories and previously reported scientific papers, it has been seen that trans-cis-trans photo-isomerization is the initiative of the orientation phenomenon within azobenzene polymer. Considering this fact, the interaction of azobenzene chromophores with the polarised light leads to the orientation of the whole polymer chain along with the azobenzene chromophores was our assumption about the formation process. The orientation of azo chromophores leads to the generation of the mechanical stress necessary for the formation of the gratings. To verify the orientation mechanism, I studied the formation of surface relief grating by continuous and pulsed exposure techniques with respect to temperature. The temperature dependent continuous exposure measurements have revealed the ordering of chromophores at room temperature and disordering at higher temperature whereas the temperature dependent pulsed exposure measurements have revealed the orientation and relaxation of chromophores. The experimental results were found to support the assumption of orientation mechanism. The orientation mechanism was also studied in the context of the dipole moment of the two different azobenzene polymers namely pDR1M and pMEA having dipole moments µ=7.0D and µ=0.05D respectively. From this study it was found that the orientation rate is higher for high dipole moment pDR1M compared to low dipole pMEA. Most of the theories support the assumption of softening of the azobenzene polymer film under visible irradiation, at least three orders of magnitude less compared to the room temperature as found near the glass transition temperature of the polymer and hence predict a light induced stress of σ<1Kpa for the inscription of grating. The logical consequence of this assumption is that inscription of surface relief gratings should become more effective upon approaching the glass transition region of the azobenzene polymer. The temperature dependent continuous and pulsed exposure results are contradictory to the above statement. The grating formation diminishes approaching the glass transition temperature of the polymer. Also in this study, It was possible to predict the magnitude of the stress generated during the orientation mechanism of azobenzene and its relation to the laser power. The magnitude of the stress is found to be enough to deform the glassy polymer at room temperature during the process of grating formation. The stress is higher than the yield stress of the typical polymer like pDR1M, (σY=50MPa). The obtained data of the temperature dependent measurements can be very well explained in frame of the viscoelastic model. Further the decomposition of the total external stress obtained from the simulation data into yield stress and viscoplastic stress can lead to the up- gradation of viscoelastic model into viscoplastic model which is found to be the appropriate model for the description of deformation of a polymers in its glassy state. Finally, the alteration of the mechanical properties of the polymer film during the process of grating formation have been reported. The ex-situ investigation was carried out by spatially resolved hardness measurements using atomic force microscope (AFM) to clarify the process at microscopic level. It can be concluded that the polymer chains are uniformly arranged with respect to each other at the crests of the grating. This uniformity and higher density increases the hardness of the polymer material at crests compared to the non uniform arrangement at troughs.
    Source Type:Doctoral Thesis
      1  13
  • Some of the metrics are blocked by your 
    consent settings
    Publication
    Reconstruction of surface morphology from coherent scattering of "white" synchrotron radiation in hard x-ray regime
    (2009)
    Sant, Tushar
    The present third generation synchrotron sources provide intense x-ray radiation in hard x-ray regime to perform coherent x-ray scattering experiments. In 90´s several attempts have been made in performing speckle experiments with x-ray radiation at various surfaces which can provide sub-nm spatial resolution. However, the method is not well established as a routine characterization technique. Several details of the experimental technique and data analysis are not solved. In this work I measured the coherent reflectivity data from various surfaces at EDR (Energy Dispersive Reflectometry) beamline at the bending magnet of BESSY II synchrotron. White x-rays in the useful energy range of 5
    Source Type:Doctoral Thesis
      411  147
  • Some of the metrics are blocked by your 
    consent settings
    Publication
    Non-leptonic B meson decays as a probe of new physics
    (2009)
    Jung, Martin
    Despite the tremendous success of the Standard Model, the arguments for the necessity of an extension are compelling. The corresponding energy scale is expected to be O(TeV); it should lead therefore to visible effects in high-precision flavour observables. While no conclusive effect is seen there up to now, the data reveal certain puzzles when compared to Standard Model expectations based on a global fit of the CKM unitarity triangle and general theoretical expectations. The discussion of these tensions in the channels B->J/psi K, B->phi K, and B->pi K, and the deduced constraints for New Physics operators of the class b->sqq form the first project discussed in this thesis. On the other hand, hadronic uncertainties within the Standard Model are still not well understood. Therefore the opposite assumption of large hadronic Standard Model effects in B->J/psi K is made in the second project, allowing in addition for a New Physics phase in B meson mixing. Finally, the necessity of reliable SM predictions is addressed by developing a framework for the model-independent inclusion of corrections to U-spin symmetry predictions.
    Source Type:Doctoral Thesis
      1  6
  • Some of the metrics are blocked by your 
    consent settings
    Publication
    Analysis of the charge collection process in solid state X-ray detectors
    (2008)
    Kimmel, Nils
    Physics with X-rays spans from observing large scales in X-ray astronomy down to small scales in material structure analyses with synchrotron radiation. Both fields of research require imaging detectors featuring spectroscopic resolution for X-rays in an energy range of 0.1keV to 20.0keV. Originally driven by the need for an imaging spectrometer on ESA's X-ray astronomy satellite mission XMM-Newton, X-ray pnCCDs were developed at the semiconductor laboratory of the Max-Planck-Institute. The pnCCD is a pixel array detector made of silicon. It is sensitive over a wide band from near infrared- over optical- and UV-radiation up to X-rays. This thesis describes the dynamics of signal electrons from the moment after their generation until their collection in the potential minima of the pixel structure. Experimentally, a pinhole array was used to scan the pnCCD surface with high spatial resolution. Numerical simulations were used as a tool for the modeling of the electrical conditions inside the pnCCD. The results predicted by the simulations were compared with the measurements. Both, experiment and simulation, helped to establish a model for the signal charge dynamics in the energy range from 0.7keV to 5.5keV. More generally, the presented work has enhanced the understanding of the detector system on the basis of a physical model. The developed experimental and theoretical methods can be applied to any type of array detector which is based on the full depletion of a semiconductor substrate material.
    Source Type:Doctoral Thesis
      2  10
  • Some of the metrics are blocked by your 
    consent settings
    PublicationAccess status: Open Access ,
    Fundamental studies of structure and crystallinity of low and high molecular weight poly(3-hexylthiophene) P3HT by means of synchrotron X-ray diffraction
    (2008)
    Joshi, Siddharth
    Conjugated polymers are the one of the most promising candidates for the active layer of low-cost Organic-field-effect transistors (OFETs). The charge carrier mobility of these conjugated polymers is the key material property, limiting the performance of the devices. The structural ordering along the whole thickness of film, especially near to interface region play an important role of controlling such electrical properties which ultimately gives high device performance. In the present work using complementary techniques I have studied structural properties of P3HT films as a function of molecular weight and film thickness. Detailed structural studies have been done to understand this polythiophene families internal structural ordering and consequently its correlation with charge carrier mobility. This work focussed on the dependence of the charge transport on morphology of the best known polythiophene group member i.e. regioregular poly(3-hexylthiophene). P3HT films (thin as well as bulk) with different molecular weights provided an ideal system for correlating morphological changes in conjugated polymers to resulting changes in charge transport seeing that the charge carrier mobility in OFETs was found to increase up to four orders-of-magnitude as the molecular weight (Mw) of P3HT is increased from 2500 g/mol to 30,000 g/mol. Grazing incidence X-ray scattering (GIXS) with combination of its complementary techniques like Atomic Force Microscopy (AFM) and Transmission Electron Microscopy (TEM) were used to measure changes in the crystallinity and crystal orientation associated with varying the annealing conditions, substrate surface treatment at a constant Mw. In particular, X-ray grazing-incidence diffraction (GID) used for depth-resolved structural analysis. We have noticed the several diffraction peaks which are associated with crystalline ordering within the films for low as well as HMW fractions (LMW, HMW) of P3HT besides the amorphous scattering from disordered part of the polymer sample. Both molecular weight fractions display a well pronounced periodicity normal to the surface due to stacking of main chains. The inter-planar distances for such stacked sheets were found to be 1.5 and 1.6 nm for low and high molecular weight fractions, respectively. These films are formally amorphous having the crystalline domains dispersed in amorphous matrix. Here, a textured and strong orientation effects are observed. On the other hand, for the low molecular weight fraction, the X-ray diffraction data are different at the surface and for the bulk samples, where the CCD images show a randomly orientated powder without any preferential orientation of nanocrystallites. Our results for LMW P3HT indicate the overall higher crystallinity, better in-plane stacking and the concentration of highly oriented crystals, but the mobility is more than a factor of 100 lower than HMW P3HT. These counterintuitive results indicate the charge carrier mobility of conjugated polymers is coupled to several different aspects of the morphology. In the case of the LMW films, the ordered regions are embedded in amorphous matrix which isolates the crystallites from their next neighbours. Whereas in HMW films, the long chains connect the small ordered regions and provide a smooth pathway for charges to move through the film. The molecular structure and morphology of an organic semiconductor are important key factors which control the properties of the interface between the organic film and the insulator, thus a part of research has also focused on interface engineering. We have modified the interface layer interaction by varying the dielectric layers (HMDS, OTS). Our GIXS results on samples with a chemically modified surface showed highly oriented crystals that were nucleated from the substrate and correlate with variations in charge transport for the first 5-10 layers. Correlation between the structural, thermal properties with the OFET performance gives strong evidence that the transport properties of layers prepared from both fractions of poly(3-hexylthiophene) is largely determined by the crystallinity of the samples and in part, responsible for the strong dependence of the OFET mobilities for polymer OFETs on the preparation conditions. Regarding the comparison of electronic and structural properties for HMW fraction, we have found that the mobility remains constant over a wide range of film thicknesses. A clear interface region found for thick HMW P3HT films and it is responsible for charge transport in the OFET measurements. The existence of amorphous regions in between highly-crystalline lamellae (or nanofibrils) for both short and long chain P3HT samples, controls the charge carrier mobility of such semi-crystalline system. Therefore control of this amorphous region could hold the answer for high charge carrier mobility of any semicrystalline polymers.
    Source Type:Doctoral Thesis
      2  5
  • Some of the metrics are blocked by your 
    consent settings
    PublicationAccess status: Open Access ,
    Zwei-Photon-Beiträge zur elastischen Elektron-Nukleon-Streuung im Skyrme-Modell
    (2008)
    Kuhn, Markus
    The most important goal of this work is the calculation of the contribution arising from the anomaly of QCD to the inelastic electron-proton scattering. Based on the Skryme model the protons are realized as soliton solutions in effective chiral theories. The next to leading order contribution to the cross section is given by the interference between the one- and two-photon exchange. The latter contains an ultraviolet divergence, which is renormalized by a local effective counterterm. This counterterm contributes to the width of the neutral pion decay which determines the finite part of the counterterm coefficient. The affect of the anomaly to the Rosenbluth separation of the electromagnetic form factors as well as the discrepance regarding the polarization measurements is extensively discussed within the Skyrme model. In addition, the correction as expected from the effective two-photon coupling of the non-linear-sigma model is estimated.
    Source Type:Doctoral Thesis
      1  3
  • Some of the metrics are blocked by your 
    consent settings
    PublicationAccess status: Open Access ,
    Vergleichende Analyse von computerunterstützten Messwerterfassungssystemen für den Physikunterricht
    (2008)
    Bernshausen, Henrik
    Computer und deren Anwendungen sind in den letzten Jahrzehnten Standardwerkzeuge in Wissenschaft und Technik bis hinein ins alltägliche Leben geworden. Auch in die Schullandschaft hat die elektronische Datenverarbeitung Einzug gefunden. Die denkbaren Nutzungsmöglichkeiten von Computern im Unterricht sind vielfältig. Das Erfassen von Messgrößen aus Experimenten gehört zu den Grundlagen naturwissenschaftlicher Erkenntnisgewinnung. In einem experimentzentrierten Unterricht kommt Computerunterstützten Messwerterfassungssystemen als technisches Hilfsmittel speziell zur Aufnahme und Weiterverarbeitung der Messwerte daher eine große Bedeutung zu. In diesem Bereich hat die Entwicklung der jüngsten Zeit zu einer nahezu unüberschaubaren Vielzahl von Soft- und Hardwarepaketen geführt. Dabei variieren die existierenden Systeme von einfachen Anwendungen der Standard-Computerperipherie bis hin zu komplexen, baugruppenartig erweiterbaren Schnittstellensystemen. Sowohl Funktionsprinzip als auch Funktionsumfang und Bedienung unterscheiden sich zum Teil beträchtlich. Eine wichtige Aufgabe der gegenwärtigen fachdidaktischen Forschung ist, die Vergleichbarkeit Computerunterstützter Messwerterfassungssysteme im Physikunterricht zu untersuchen. Dabei muss ihrer Nutzbarkeit – zumindest als technisches Hilfsmittel zur Messdatenerfassung – analysiert werden. Die vorliegende Arbeit leistet hierzu einen Beitrag. Ihr Ziel ist es, einen Katalog möglichst objektiv erfassbarer Kriterien zu finden, anhand deren die „Nutzbarkeit“ von Computerunterstützten Messwerterfassungsystemen im Physikunterricht komparativ analysiert werden kann. Eine Reihe solcher Messwerterfassungsysteme ist in den Bestand des Medienlabors des Fachbereichs Physik – Didaktik der Physik – der Universität Siegen integriert und unter unterrichtspraktischen Bedingungen auf ihre „Nutzbarkeit“ verglichen worden. Basierend auf den gewonnenen Ergebnissen wurde ein Analysebogen entwickelt, der es Lehrenden und Lernenden nun erleichtert ermöglicht, Computerunterstützte Messwerterfassungssysteme zielgerichtet zu erproben und zu beurteilen.
    Source Type:Doctoral Thesis
      4  4
  • Some of the metrics are blocked by your 
    consent settings
    PublicationAccess status: Open Access ,
    Eine Implementierung bis "Next to Leading Order" von b-Quarks in PHOJET
    (2008)
    Lammers, Stefan
    This works aim is primarily the implementation of b-quarks into the event generator PHOJET (respectively into DPMJET III containing PHOJET). Following a method of S. Frixione and B.R. Webber the implementation includes "next to leading order" in a perturbative expansion on partonic level. A typical problem of such a calculation in event generators that include initial and final state radiation is "double counting". To overcome this error Frixione and Webber developed a scheme involving subtraction terms. In this thesis this elaborate method is adapted and implemented in DPMJET III by means of recalculation of this subtraction term to match DPMJET III needs (chapter 4 and sections 5.4, 5.5 as well as the section 6.3. The correct outcome of this matching is shown in chapter 7 by means of production of vector bosons. In contrast to the event generator HERWIG it is found, that DPMJET III is able to reproduce the correct partonic behavior in the "soft"' and "collinear"' limits, when one additional parton is emitted. After a short description of the method of event generation this work presents, beginning with chapter 9 the results of the calculation which are then compared to available experimental data provided by the CDF collaboration (RUN I and II). A moderate adjustment of the two relevant scales (factorization and renormalization scales) leads to good agreement between model prediction and the CDF data. Subsequently the predictions for the transverse spectra of bottom mesons at LHCb (section 10.1) are presented. As the adjusted factorization and renormalization scales were kept, the results can be expected to be quite reliable. DPMJET III and PHOJET are highly checked out and complete event generators that can reliably reproduce the background part of the events. It is therefore possible to address more technical questions like the identification of B-mesons in realistic events to DPMJET III. To investigate this task in a generic way a small program was built to identify daughter particles of specific bottom decays with suitable cuts on an event.
    Source Type:Doctoral Thesis
      4  3
  • Some of the metrics are blocked by your 
    consent settings
    PublicationAccess status: Open Access ,
    B-Zerfallsformfaktoren aus QCD-Summenregeln
    (2008)
    Offen, Nils
    In the Standard Model of particle physics there is only one source of CP-violation. Namely, a single complex phase in the unitary 3 Ã 3 CKM-Matrix governing flavor transitions in the weak interaction. The unitarity is usually visualized by a triangle in the complex ρ - η-plane. Therefore testing this framework comes down to measuring weak decays, relating observables to sides and angles of this so called Unitarity Triangle(UT). Particular interest in this respect is payed to decays of mesons containing a heavy b -quark, giving the opportunity to alone determine all parameters of the UT. Doing this is far from easy. Besides tedious experimental measurements the theoretical calculations are plagued by hadronic quantities which cannot be determined by perturbation theory. In this work several of these quantities so called form factors are computed using the well known method of light cone sum rules(LCSR). Two different setups have been used. One, established in this work, utilizing a correlation function with an on-shell B-Meson and one following the traditional calculation by taking the light meson on-shell. Both using light cone expansion in the respective on-shell mesons distribution amplitudes. While the first approach allows to calculate a whole bunch of phenomenologically interesting quantities by just changing Dirac-structures of the relevant currents it has the drawback that it does not have access to the well developed twist expansion of the latter. To incorporate higher Fock-state contributions the first models for three-particle distribution amplitudes of the B-Meson have been derived. α s -corrections remain out of the scope of this work. Nevertheless does a comparison with more sophisticated methods show an encouraging numerical agreement. In the second setup all known corrections especially the never verified α s -corrections to Twist three terms have been recalculated and a competitive result for the CKM-matrixelement |V ub | was obtained. Abstract (PDF-Dokument)
    Source Type:Doctoral Thesis
      3  20
  • Some of the metrics are blocked by your 
    consent settings
    PublicationAccess status: Open Access ,
    Characterization of physical properties of polymers using AFM force-distance curves
    (2007)
    Kaliappan, Senthil Kumar 
    A novel analysis method based on Hertz theory was used to determine the mechanical properties from force-distance curves obtained over a wide range of temperatures and frequencies on poly(n-butyl methacrylate) (PnBMA) and two polystyrene (PS) samples, having different molecular weight and hence different glass transition temperature Tg. The analysis technique extends the elastic continuum contact theories to the plastic deformations and permitted to calculate the stiffness in the plastic regime of deformation, the yielding force, the parameters of the WLF and Arrhenius equations, and the Young's modulus. The Young's modulus and the shift coefficients of the polymers determined through AFM measurements were in excellent agreement with the values from DMA measurements and/or the literature values. Force-distance curves were also acquired on a model polymer blend of PS/PnBMA at different temperatures. The analysis method was used to determine the Young's modulus of PS and PnBMA away from the interface and close to the interface with a resolution of 800 nm. The differences in Tg of the two polymers resulted in different viscoelastic behavior. The modulus of PnBMA and PS was in excellent agreement with the DMA and AFM data from the measurements on individual films. The morphology of the PS/PnBMA blend was characterized using the Young's modulus of the constituting polymers. A several µm long transition region was observed in the vicinity of the interface, where the modulus of PnBMA decreased from the value on PS to the value on PnBMA away from the interface. This experiment shows the capability of AFM of surveying local mechanical properties and studying heterogeneous samples. Such spatially resolved measurements cannot be achieved with any other technique.
    Source Type:Doctoral Thesis
      6  15