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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

MR spektroskopie srdce in vivo / MR spectroscopy of the heart in vivo

Drobný, Miloslav January 2013 (has links)
The presented thesis deals with the use of MR spectroscopy methods for the study of heart metabolism, namely the determination of the lipid content in the heart tissue. The aim of the thesis was the optimization of the examination protocol for the use in the routine examination of patients. The heart examination by means of MR spectroscopy is due to movements caused by the heart rhythm and breathing very challenging. We have, therefore, proposed the measurement protocol, when the measurement is controlled both by ECG signal and the signal monitoring breathing of the examined subject. To achieve sufficient quality of MR spectra we also proposed the use of an advanced method (GREshim) for the adjustment of the homogeneity of the static magnetic field. Using the optimized examination protocol we have examined a group of 14 healthy volunteers. An average content of lipids in the myocardium equal to 0,7 % as determined from the performed measurements is in agreement with published data.
2

The AGN-host galaxy connection : new insights from the extended ionised gas

Husemann, Bernd January 2011 (has links)
Active Galactic Nuclei (AGN) are powered by gas accretion onto supermassive Black Holes (BH). The luminosity of AGN can exceed the integrated luminosity of their host galaxies by orders of magnitude, which are then classified as Quasi-Stellar Objects (QSOs). Some mechanisms are needed to trigger the nuclear activity in galaxies and to feed the nuclei with gas. Among several possibilities, such as gravitational interactions, bar instabilities, and smooth gas accretion from the environment, the dominant process has yet to be identified. Feedback from AGN may be important an important ingredient of the evolution of galaxies. However, the details of this coupling between AGN and their host galaxies remain unclear. In this work we aim to investigate the connection between the AGN and their host galaxies by studying the properties of the extendend ionised gas around AGN. Our study is based on observations of ~50 luminous, low-redshift (z<0.3) QSOs using the novel technique of integral field spectroscopy that combines imaging and spectroscopy. After spatially separating the emission of AGN-ionised gas from HII regions, ionised solely by recently formed massive stars, we demonstrate that the specific star formation rates in several disc-dominated AGN hosts are consistent with those of normal star forming galaxies, while others display no detectable star formation activity. Whether the star formation has been actively suppressed in those particular host galaxies by the AGN, or their gas content is intrinsically low, remains an open question. By studying the kinematics of the ionised gas, we find evidence for non-gravitational motions and outflows on kpc scales only in a few objects. The gas kinematics in the majority of objects however indicate a gravitational origin. It suggests that the importance of AGN feedback may have been overrated in theoretical works, at least at low redshifts. The [OIII] line is the strongest optical emission line for AGN-ionised gas, which can be extended over several kpc scales, usually called the Narrow-Line Region (NLR). We perform a systematic investigation of the NLR size and determine a NLR size-luminosity relation that is consistent with the scenario of a constant ionisation parameter throughout the NLR. We show that previous narrow-band imaging with the Hubble Space Telescope underestimated the NLR size by a factor of >2 and that the continuum AGN luminosity is better correlated with the NLR size than the [OIII] luminosity. These affects may account for the different NLR size-luminosity relations reported in previous studies. On the other hand, we do not detect extended NLRs around all QSOs, and demonstrate that the detection of extended NLRs goes along with radio emission. We employ emission line ratios as a diagnostic for the abundance of heavy elements in the gas, i.e. its metallicity, and find that the radial metallicity gradients are always flatter than in inactive disc-dominated galaxies. This can be interpreted as evidence for radial gas flows from the outskirts of these galaxies to the nucleus. Recent or ongoing galaxy interactions are likely responsible for this effect and may turn out to be a common prerequisite for QSO activity. The metallicity of bulge-dominated hosts are systematically lower than their disc-dominated counterparts, which we interpret as evidence for minor mergers, supported by our detailed study of the bulge-dominated host of the luminous QSO HE 1029-1401, or smooth gas accretion from the environment. In this line another new discovery is that HE 2158-0107 at z=0.218 is the most metal poor luminous QSO ever observed. Together with a large (30kpc) extended structure of low metallicity ionised gas, we propose smooth cold gas accretion as the most likely scenario. Theoretical studies suggested that this process is much more important at earlier epochs of the universe, so that HE 2158-0107 might be an ideal laboratory to study this mechanism of galaxy and BH growth at low redshift more detailed in the furture. / Aktive Galaxienkerne (AGN) entstehen durch die Akkretion von Gas auf massive Schwarze Löcher, welche im Zentrum jeder Galaxie mit einer spherodialen Komponente vermutet werden. Die Leuchtkraft eines AGN kann die seiner gesamten Muttergalaxie um Größenordnungen übersteigen. In diesem Fall werden AGN oft als Quasi-Stellare Objekte (Quasare) bezeichnet. Spezielle Mechanismen müssen für das Auslösen dieser Kernaktivität in Galaxien verantwortlich sein. Verschiedene Prozesse wurden bereits identifiziert, aber der entscheidende Mechanismus wurde bisher noch nicht entdeckt. Die Wechselwirkung mit einem AGN könnte außerdem einen entscheidenden Einfluss auf die Entwicklung von Galaxien haben. Es ist noch unklar wie diese Wechselwirkung genau abläuft und ob es die Sternentstehung in Galaxien beeinflusst. In dieser Arbeit studieren wir die Eigenschaften des ausgedehnten ionisierten Gases in AGN-Muttergalaxien, um mögliche Wechselwirkungen zu untersuchen. Wir benutzen dazu eine Stichprobe von ~50 Quasaren bei geringer Rotverschiebung (z<0.3), die mit der neuartigen Technik der Integralfeld-Spektroskopie beobachtet wurden. Diese Technik kombiniert bildgebende und spektroskopische Verfahren. Wir können mit unserer Analyse zeigen, dass die spezifische Sternentstehungsrate in einigen Scheiben-dominierten AGN-Muttergalaxien vergleichbar mit denen von normalen Galaxien ohne Kernaktivität ist. Allerdings können wir in einigen AGN-Muttergalaxien keine Anzeichen von Sternentstehung feststellen. Ob Sternentstehung in diesen Galaxien momentan durch die Wechselwirkung mit dem AGN unterdrückt wird, ist daher nicht eindeutig. Hinweise auf Gasausflüsse liefert die Kinematik des ionisierten Gases für einige wenige Objekte, doch die Kinematik für die meisten AGN-Muttergalaxien kann allein durch das Wirken der Gravitation erklärt werden. Daraus schließen wir, dass der Einfluss von AGN auf ihre Muttergalaxien geringer sein könnte als theoretisch angenommen wird. Die [OIII] Emissionslinie ist die stärkste optische Linie für AGN-ionisiertes Gas und kann sich über eine Region von mehreren kpc vom Kern erstrecken, die als "Narrow-Line Region" (NLR) bezeichnet wird. Durch eine systematische Untersuchung der NLR-Ausdehnung können wir eine Beziehung zwischen NLR-Radius und AGN-Leuchtkraft bestimmen. Diese Relation ist konsistent mit einem konstanten Ionisationsparameter über die gesamte Ausdehnung der NLR. Frühere Studien mit dem Hubble Weltraumteleskop unterschätzten die Größe der NLR um mehr als einen Faktor 2. Andererseits können wir nicht für alle Quasare eine ausgedehnte NLR nachweisen, wobei eine NLR-Detektion bei einer höheren Radioleuchtkraft des Quasars wahrscheinlicher ist. Dies deutet auf eine Wechselwirkung eines Radio-Jets mit dem kernumgebenden Gas hin. Wir benutzen Emissionslinien des ionisierten Gases, um den Anteil von schweren Elementen im Gas, die so genannte Metallizität, zu bestimmen. Dabei finden wir, dass die radialen Metallizitätsgradienten in Scheiben-dominierten AGN-Muttergalaxien deutlich flacher sind als in vergleichbaren Galaxien ohne Kernaktivität, was wir als Anzeichen für radialen Gastransport vom Rand der Galaxien zum Kern interpretieren. Dies könnte durch kürzliche oder immer noch andauernde gravitative Wechselwirkungen zwischen Nachbargalaxien entstanden sein und stellt eventuell eine Voraussetzung für Kernaktivität dar. Sehr interessant ist unser Ergebnis, dass die ellptischen AGN-Muttergalaxien eine geringere Metallizität aufweisen als die Spiralgalaxien. Dies könnte z.B. durch das Verschmelzen mit kleinen Nachbargalaxien induziert werden, welche eine intrinsisch geringe Metallizität aufweisen. Am Beispiel der elliptischen Muttergalaxie des Quasars HE 1029-1401 können wir durch eine detaillierte Analyse des ionisierten Gases verschiedene Indizien für einen solchen Prozess nachweisen. Eine weiteres Resultat dieser Arbeit ist die Entdeckung eines leuchtkräftigen Quasars mit der geringsten Metallizität, die bisher für solche Objekte nachgewiesen werden konnte. Wir interpretieren die geringe Metallizität und die Ausdehnung des ionisierten Gases über 30kpc als deutliche Indizien für die Akkretion von intergalaktischem Gas. Dieser Prozess findet viel häufiger im frühen Universum statt. HE 2158-0107 könnte daher ein ideales Objekt sein, um diesen Prozess im nahen Universum detaillierter studieren zu können.
3

Interactions of Quercetin-Uranium Complexes with Biomembranes and DNA

Attia, Enas 05 August 2014 (has links) (PDF)
Uranium decontamination gains a great importance with the spread of nuclear waste in both soil and water systems across the planet. All known remediation methods of uranium can be exclusively based either on synthetic materials with high adsorbent power and known physical chemistry or life organisms by which the uranium eventually accumulated inside their tissues. In the present thesis, it was attempted to design a rational approach for uranyl removal primarily from waters using the reducing potential of quercetin, which is a plant-derived small organic molecules, along with its photochemical activities. Such approach, which is neither a fully synthetic nor an organism-based approach, was chosen here to avoid disadvantages with both traditional strategies. Here, complexation experiments were designed to assess the use of uranyl-quercetin complexes for the photoreduction of water-soluble U(VI) to insoluble U(IV) by comparing absorption properties of uranyl-quercetin complexes in acetone, water, and hydrophobic bilayer lipid vesicles. The UV-vis data show that uranyl quercetin complex can form in both hydrophobic and hydrophilic environments. In both cases the B-ring band in quercetin structure becomes reduced, red shifted and a pronounced absorption arises in the 400-500 nm range. Such data suggests that U(VI) binds at the 3-OH and 4-carbonyl of ring C of quercetin. Interestingly, the results of UV-Vis spectroscopy part hint at a crucial role of a stable or transiently ionized hydroxyl for the efficient uranyl-dependent photodegradation of quercetin. FTIR spectroscopy absorption changes further demonstrates that the UV-vis-spectroscopic changes are indeed accompanied by changes in the chemical structure of the complex as expected for a uranyl-dependent photodegradation. IR data thus suggest that U(VI) becomes reduced by the photoreaction, rather than merely changing its coordination shell. The frequency shifts in the C=C and C=O absorption range on the other hand are consistent with changes in force constants rather than bond breakage. Upon illumination condition, uranyl quercetin complex in water forms a dark precipitate. Uranyl precipitation and the disappearance of U(VI) IR absorption bands upon illumination further demonstrate that uranyl acts as a redox partner rather than a catalyst in the photoreaction of quercetin. The formation of uranyl-quercetin complexes in the presence of lipidic phases has been addressed experimentally. The complex is partitioned into the hydrophilic/hydrophobic interface of liposomes. Its electronic absorption properties are influenced by the degree of hydrophobicity provided by the adjacent lipid headgroups. The preference of quercetin to associate with hydrophobic microenvironments can thus be exploited to transfer uranyl to the lipid water biomolecular interface. Illumination of the uranyl-quercetin complex in the presence of different liposomes has been performed in this study for the first time, to the best of my knowledge. The data provide evidence that again uranyl is a redox partner for the photodegradation of quercetin also in this microenvironment. Uranyl in an oxidation state smaller than VI is unsoluble in water. Therefore, its quercetin-mediated photoreduaction of uranium provides a method to transfer soluble uranium to the liposome and stabilize the reduced photoproduct. Thereby, uranyl could be removed from solution in an insoluble form using cheap natural compounds. The binding site assignment of uranyl-quercetin complex in acetone have been verified here using NMR spectra and DFT theory. NMR Spectra showed that the observations of broadened and narrow bands in the NMR spectra of quercetin, upon complexation with uranyl, support an intramolecular exchange or site exchange within the quercetin molecule. Moreover, the complexation takes place around the carbonyl group with U(VI) exhibiting two possibly coordination modes, involving the carbonyl and the adjacent O(H) groups. This has been also confirmed from the DFT calculations. Finally, interaction experiments of uranyl-quercetin complex with DNA have been performed to assess an alternative uranyl-trapping and photoreduction system. The data show that consecutive addition of quercetin and uranyl destabilizes DNA. However, a preformed uranyl quercetin complex has very little effect on DNA structure. On the other hand, quercetin and uranyl appear to bind to DNA as a preformed complex in the loop portion of hairpin DNA. Therefore, also HP DNA is expected to be a suitable but less effective trapping system for the uranyl quercetin complex and its potential photoproducts.
4

MR spektroskopie pacientů s diabetem mellitus / MR spectroscopy in patients with diabetes mellitus

Šedivý, Petr January 2013 (has links)
This thesis deals with in vivo MR spectroscopy. Measurements in this thesis were performed on whole-body MR tomograph at the Institute for Clinical and Experimental Medicine in Prague. The objective of the thesis was to study differences in the biochemical processes and energy metabolism in the muscle tissue under physical workload between the groups of healthy subjects and patients with type 1 diabetes mellitus (DM1). We used phosphorous spectroscopy in combination with ergometer. The thesis is divided into five chapters. The first chapter describes theoretical introduction to in vivo 1 H and 31 P MR spectroscopy and muscle metabolism, the second chapter deals with the description of the experimental equipment and measurement, results of the thesis are reported in the third chapter and the fourth chapter is a discussion of results. Main result of this work is summarized in conclusion; we found differences between the metabolism of patients with DM1 and healthy volunteers.
5

Fluorescence půdních a lignitických huminových látek / Fluorescence of soil and lignite humic substances

Konečná, Soňa January 2009 (has links)
The purpose the diploma thesis was to determine main fluorophores of soil and lignite humic substances using methods of 2D synchronous fluorescence spectroscopy (SFS) and 3D SFS. These fluorescence methods are frequently used for characterization multifluorophore systems. Measured synchronous spectra were compared with standards IHSS. Next to that the aim was to find differences between chosen samples of humic and fulvic acids.
6

Biofyzikální a funkční charakterisace aspartátových proteas z rodiny proteinů podobných Ddi-1, zapojených do odpovědi na replikační stres / BIOPHYSICAL AND FUNCTIONAL CHARACTERIZATION OF DDI1-LIKE ASPARTIC PROTEASES INVOLVED IN REPLICATION STRESS RESPONSE

Svoboda, Michal January 2021 (has links)
Accurate, timely replication of a DNA molecule is a pivotal moment in the life cycle of every living organism. Any temporal or spatial defect putting the fine-tuned replication machinery off balance causes the so-called replication stress. As the replication machinery consists mainly of enzymes and other proteins, it is not surprising that many of the obstacles most severely blocking the replication machinery progress are of protein origin. Therefore, specialized proteases responsible for relieving replication stress matured during evolution. However, neither the full repertoire of proteolytic enzymes and their particular substrates taking place in countering the DNA replication stress nor detailed molecular mechanisms involved remain unknown. This thesis describes how conserved putative aspartic proteases of the Ddi1-like family engage in countering DNA replication stress via a proteolysis dependent mechanism. We structurally and biophysically characterized yeast and human members of the Ddi1-like family, explored their interactions with ubiquitin and polyubiquitin chains, and identified hypersensitivity to DNA replication inhibitor hydroxyurea in a yeast strain double deleted for DDI1 gene together with a DNA dependent metalloprotease WSS1. Detailed analysis of the DDI1 role in hydroxyurea...
7

Interactions of Quercetin-Uranium Complexes with Biomembranes and DNA

Attia, Enas 21 July 2014 (has links)
Uranium decontamination gains a great importance with the spread of nuclear waste in both soil and water systems across the planet. All known remediation methods of uranium can be exclusively based either on synthetic materials with high adsorbent power and known physical chemistry or life organisms by which the uranium eventually accumulated inside their tissues. In the present thesis, it was attempted to design a rational approach for uranyl removal primarily from waters using the reducing potential of quercetin, which is a plant-derived small organic molecules, along with its photochemical activities. Such approach, which is neither a fully synthetic nor an organism-based approach, was chosen here to avoid disadvantages with both traditional strategies. Here, complexation experiments were designed to assess the use of uranyl-quercetin complexes for the photoreduction of water-soluble U(VI) to insoluble U(IV) by comparing absorption properties of uranyl-quercetin complexes in acetone, water, and hydrophobic bilayer lipid vesicles. The UV-vis data show that uranyl quercetin complex can form in both hydrophobic and hydrophilic environments. In both cases the B-ring band in quercetin structure becomes reduced, red shifted and a pronounced absorption arises in the 400-500 nm range. Such data suggests that U(VI) binds at the 3-OH and 4-carbonyl of ring C of quercetin. Interestingly, the results of UV-Vis spectroscopy part hint at a crucial role of a stable or transiently ionized hydroxyl for the efficient uranyl-dependent photodegradation of quercetin. FTIR spectroscopy absorption changes further demonstrates that the UV-vis-spectroscopic changes are indeed accompanied by changes in the chemical structure of the complex as expected for a uranyl-dependent photodegradation. IR data thus suggest that U(VI) becomes reduced by the photoreaction, rather than merely changing its coordination shell. The frequency shifts in the C=C and C=O absorption range on the other hand are consistent with changes in force constants rather than bond breakage. Upon illumination condition, uranyl quercetin complex in water forms a dark precipitate. Uranyl precipitation and the disappearance of U(VI) IR absorption bands upon illumination further demonstrate that uranyl acts as a redox partner rather than a catalyst in the photoreaction of quercetin. The formation of uranyl-quercetin complexes in the presence of lipidic phases has been addressed experimentally. The complex is partitioned into the hydrophilic/hydrophobic interface of liposomes. Its electronic absorption properties are influenced by the degree of hydrophobicity provided by the adjacent lipid headgroups. The preference of quercetin to associate with hydrophobic microenvironments can thus be exploited to transfer uranyl to the lipid water biomolecular interface. Illumination of the uranyl-quercetin complex in the presence of different liposomes has been performed in this study for the first time, to the best of my knowledge. The data provide evidence that again uranyl is a redox partner for the photodegradation of quercetin also in this microenvironment. Uranyl in an oxidation state smaller than VI is unsoluble in water. Therefore, its quercetin-mediated photoreduaction of uranium provides a method to transfer soluble uranium to the liposome and stabilize the reduced photoproduct. Thereby, uranyl could be removed from solution in an insoluble form using cheap natural compounds. The binding site assignment of uranyl-quercetin complex in acetone have been verified here using NMR spectra and DFT theory. NMR Spectra showed that the observations of broadened and narrow bands in the NMR spectra of quercetin, upon complexation with uranyl, support an intramolecular exchange or site exchange within the quercetin molecule. Moreover, the complexation takes place around the carbonyl group with U(VI) exhibiting two possibly coordination modes, involving the carbonyl and the adjacent O(H) groups. This has been also confirmed from the DFT calculations. Finally, interaction experiments of uranyl-quercetin complex with DNA have been performed to assess an alternative uranyl-trapping and photoreduction system. The data show that consecutive addition of quercetin and uranyl destabilizes DNA. However, a preformed uranyl quercetin complex has very little effect on DNA structure. On the other hand, quercetin and uranyl appear to bind to DNA as a preformed complex in the loop portion of hairpin DNA. Therefore, also HP DNA is expected to be a suitable but less effective trapping system for the uranyl quercetin complex and its potential photoproducts.
8

Homogeneity and elemental distribution in self-assembled bimetallic Pd–Pt aerogels prepared by a spontaneous one-step gelation process

Schmidt, Thomas Justus, Oezaslan, Methap, Liu, W., Nachtegaal, Maarten, Frenkel, Anatoly I., Rutkowski, B., Werheid, Matthias, Herrmann, Anne-Kristin, Laugier-Bonnaud, C., Yilmaz, H.-C., Gaponik, Nikolai, Czyrska-Filemonowicz, A., Eychmüller, Alexander 06 April 2017 (has links) (PDF)
Multi-metallic aerogels have recently emerged as a novel and promising class of unsupported electrocatalyst materials due to their high catalytic activity and improved durability for various electrochemical reactions. Aerogels can be prepared by a spontaneous one-step gelation process, where the chemical co-reduction of metal precursors and the prompt formation of nanochain-containing hydrogels, as a preliminary stage for the preparation of aerogels, take place. However, detailed knowledge about the homogeneity and chemical distribution of these three-dimensional Pd–Pt aerogels at the nano-scale as well as at the macro-scale is still unclear. Therefore, we used a combination of spectroscopic and microscopic techniques to obtain a better insight into the structure and elemental distribution of the various Pd-rich Pd–Pt aerogels prepared by the spontaneous one-step gelation process. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectroscopy (EDX) were employed in this work to uncover the structural architecture and chemical composition of the various Pd-rich Pd–Pt aerogels over a broad length range. The Pd80Pt20, Pd60Pt40 and Pd50Pt50 aerogels showed heterogeneity in the chemical distribution of the Pt and Pd atoms inside the macroscopic nanochain-network. The features of mono-metallic clusters were not detected by EXAFS or STEM-EDX, indicating alloyed nanoparticles. However, the local chemical composition of the Pd–Pt alloys strongly varied along the nanochains and thus within a single aerogel. To determine the electrochemically active surface area (ECSA) of the Pd–Pt aerogels for application in electrocatalysis, we used the electrochemical CO stripping method. Due to their high porosity and extended network structure, the resulting values of the ECSA for the Pd–Pt aerogels were higher than that for a commercially available unsupported Pt black catalyst. We show that the Pd–Pt aerogels possess a high utilization of catalytically active centers for electrocatalytic applications based on the nanostructured bimetallic framework. Knowledge about the homogeneity and chemical distribution of the bimetallic aerogels can help to further optimize their preparation by the spontaneous one-step gelation process and to tune their electrocatalytic reactivity.
9

Stability of polarization in organic ferroelectric metal-insulator-semiconductor structures

Kalbitz, René January 2011 (has links)
Organic thin film transistors (TFT) are an attractive option for low cost electronic applications and may be used for active matrix displays and for RFID applications. To extend the range of applications there is a need to develop and optimise the performance of non-volatile memory devices that are compatible with the solution-processing fabrication procedures used in plastic electronics. A possible candidate is an organic TFT incorporating the ferroelectric co-polymer poly(vinylidenefluoride-trifluoroethylene)(P(VDF-TrFE)) as the gate insulator. Dielectric measurements have been carried out on all-organic metal-insulator-semiconductor structures with the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) as the gate insu-lator. The capacitance spectra of MIS devices, were measured under different biases, showing the effect of charge accumulation and depletion on the Maxwell-Wagner peak. The position and height of this peak clearly indicates the lack of stable depletion behavior and the decrease of mobility when increasing the depletion zone width, i.e. upon moving into the P3HT bulk. The lack of stable depletion was further investigated with capacitance-voltage (C-V) measurements. When the structure was driven into depletion, C-V plots showed a positive flat-band voltage shift, arising from the change in polarization state of the ferroelectric insulator. When biased into accumulation, the polarization was reversed. It is shown that the two polarization states are stable i.e. no depolarization occurs below the coercive field. However, negative charge trapped at the semiconductor-insulator interface during the depletion cycle masks the negative shift in flat-band voltage expected during the sweep to accumulation voltages. The measured output characteristics of the studied ferroelectric-field-effect transistors confirmed the results of the C-V plots. Furthermore, the results indicated a trapping of electrons at the positively charged surfaces of the ferroelectrically polarized P(VDF-TrFE) crystallites near the insulator/semiconductor in-terface during the first poling cycles. The study of the MIS structure by means of thermally stimulated current (TSC) revealed further evidence for the stability of the polarization under depletion voltages. It was shown, that the lack of stable depletion behavior is caused by the compensation of the orientational polarization by fixed electrons at the interface and not by the depolarization of the insulator, as proposed in several publications. The above results suggest a performance improvement of non-volatile memory devices by the optimization of the interface. / Organische Transistoren sind besonders geeignet für die Herstellung verschiedener preisgünstiger, elektronischer Anwendungen, wie zum Beispiel Radio-Frequenz-Identifikations-Anhänger (RFID). Für die Erweiterung dieser Anwendung ist es notwendig die Funktion von organischen Speicherelementen weiter zu verbessern. Das ferroelektrische Polymer Poly(vinylidene-Fluoride-Trifluoroethylene) (P(VDF-TrFE)) eignet sich besonders gut als remanent polarisierbarer Isolator in Dünnschich-Speicherelementen. Um Schalt- und Polarisationsverhalten solcher Speicherelemente zu untersuchen, wurden P(VDF-TrFE)-Kondensatoren und Metall-Halbleiter-Isolator-Proben sowie ferroelektrische Feld-Effekt-Transistoren (Fe-FET) aus dem Halbleiter Poly(3-Hexylthiophene) (P3HT) und P(VDF-TrFE) hergestellt und dielektrisch untersucht. Die Charakterisierung der MIS-Strukturen mittels spannungsabhängiger Kapazitätsspektren machte deutlich, dass es nicht möglich ist, einen stabilen Verarmungzustand (Aus-Zustand) zu realisieren. Kapazität-Spannungs-Messungen (C-V) an MIS-Proben mit uni/bi-polaren Spannungszyklen zeigten eine stabile ferroelektrische Polarisation des P(VDF-TrFE)-Films. Eine Depolarisation des Isolators durch den Mangel an Minoritäts-Ladungsträgern konnte als Grund für die Instabilität des Verarmungs-Zustandes ausgeschlossen werden. Die C-V-Kurven wiesen vielmehr auf die Existenz fixierter, negativer Ladungsträger an der Grenzfläche hin. Zusammenfassend kann festgestellt werden: die Ursache der Ladungsträgerinstabilitäten in organischen ferroelektrischen Speicherelementen ist auf die Kompensation der ferroelektrischen Orientierungspolarisation durch "getrappte"(fixierte) negative Ladungsträger zurückzuführen. Dieses Ergebnis liefert nun eine Grundlage für die Optimierung der Isolator/Halbleiter-Grenzfläche mit dem Ziel, die Zahl der Fallenzustände zu minimieren. Auf diesem Wege könnte die Stabilität des Speicherzustandes in organischen Dünnschichtspeicherelementen deutlich verbessert werden.
10

Correlation between anatomical/chemical wood properties and genetic markers as a means of wood certification

Rana, Rumana 06 June 2008 (has links)
No description available.

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