51 search hits
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Proceedings of the International Conference of "Atmospheric Transport and chemistry in Forest Ecosystems" Castle of Thurnau, Germany Oct 5 to Oct 8, 2009
(2009)
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Johannes Lüers
Thomas Foken
- no abstract
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Mesoscale Circulations and Energy and GaS Exchange Over the Tibetan Plateau Documentation of the Micrometeorological Experiment, Nam Tso, Tibet
(2009)
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Tobias Biermann
Wolfgang Babel
Johannes Olesch
Thomas Foken
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Carbon dynamics under natural and manipulated meteorological boundary conditions in a forest and a fen ecosystem
(2009)
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Jan Muhr
- Current climate models predict changes in the amount, intensity, frequency and type of precipitation within this century. These changes are likely to result in an increasing frequency of severe drought periods in summer, causing irregular and extreme drought stress in well-drained soils or a lowering of the water table in water-logged soils. Due to rising temperatures precipitation is more likely to occur as rain rather than snow, resulting in reduced snowpacks in winter. In some regions, this can lead to an increasing frequency of soil frost. In summary, changes in the global water cycle can significantly affect boundary conditions within soils. This thesis investigated the impact of extreme meteorological boundary conditions on CO2 fluxes in two ecosystems in South-eastern Germany. Using a combination of field site manipulation and laboratory experiments we investigated the effects of prolonged summer drought and soil frost on soil C dynamics in a Norway spruce forest. In a minerotrophic fen located nearby, the effect of water table lowering (as a result of summer drought) on ecosystem C dynamics was quantified. Additionally, soil C dynamics at both sites were modeled under current meteorological conditions. For the Norway spruce forest, modeling indicated that soil C turnover predominantly occurred within the organic horizons. During the last decades, the soil has acted as a small sink. The possibility of altered C dynamics at the site due to undocumented liming has to be considered when comparing results presented here to results from other sites. For the fen, modeling revealed that soil C turnover was dominated by processes occurring within the uppermost 15 cm of the peat and that root biomass was a very important soil C stock. Most important, modeling indicated that the fen was turned into a net C source during the last decades, presumably because of disturbance of the hydrological conditions. Results from this fen cannot be regarded as representative for undisturbed peatlands. Soil frost was induced at the forest site by removing the snowpack in the winter of 2005/2006. On the snow removal plots, soil frost occurred down to a depth of at least 15 cm and for several weeks, in contrast to the snow-covered control plots where no soil frost occurred. Soil C losses were significantly reduced not only during the soil frost period but also in the summer of 2006. This phenomenon could be explained by changes in the composition of the microbial community due to soil frost, primarily a reduction of fungal biomass. To investigate the effect of drought on soil C dynamics we experimentally induced prolonged drought at the forest-site by excluding throughfall with a transparent roof during the summers of 2006-2008. Additionally, undisturbed soil columns from the site were subjected to drought in the laboratory. In both experiments, drought reduced total soil C losses in comparison to C losses from a control. This reduction was mainly owed to decreased soil respiration rates during the actual drought period, but water repellency also hindered rewetting of the dry soil, thus further prolonging the period of reduced soil respiration rates. In the past, mobilization of stabilized C due to drying-wetting has been repeatedly discussed as a possibility to actually enhance soil C losses. In the studies presented here, no evidence for this assumption was found. Soil C mineralization rates were reduced during drought and recovery was slow, possibly delayed by water repellency and preferential flow. At the fen site we used two approaches: (i) Experimental lowering of water tables to measure resulting C fluxes in comparison to C fluxes under natural conditions (i.e. control plots), and (ii) repeated measurements under varying natural conditions to be able to later statistically identify the main drivers of CO2 fluxes. We included measurements of C uptake and respiration by aboveground vegetation, thus being able to study ecosystem rather than soil C dynamics at the fen site. In summary, the impact of the water table on CO2 fluxes in and out of the fen ecosystem was minimal. Soil respiration was not affected at all by the manipulative lowering of the water table from ca. 15 cm down to more than 60 cm, most likely due to low substrate quality in deeper peat. Measurements of the natural C dynamics indicate that water table could have an impact on soil respiration within the uppermost 0-15 cm of the soil, but predominantly low water tables during summer under current boundary conditions make it unlikely that further lowered water tables due to climate change will markedly affect soil respiration rates at this site. In summary, CO2 fluxes at the site are presumably very resilient towards an increasing frequency of summer drought resulting in lowering of the water table.
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Structure-function relationship of archaeal rhodopsin proteins analyzed by continuum electrostatics
(2009)
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Edda Kloppmann
- Rhodopsin proteins perform two cellular key functions: signaling of external stimuli and ion transport. Examples of both functional types are found in the family of archaeal rhodopsins, namely the proton pump bacteriorhodopsin, the chloride pump halorhodopsin and the photoreceptor sensory rhodopsin II. For these three membrane proteins, high-resolution X-ray structures are available, allowing a theoretical investigation in atomic detail. In this thesis, calculations are presented based on a continuum electrostatics approach using a finite-difference discretization of the Poisson-Boltzmann equation. The results can be divided into three parts. One of the interesting features of rhodopsin proteins is the extreme range over which the absorption maximum of their chromophore retinal is tuned. This characteristic and the precision of the tuning mechanism is a fundamental requirement for color vision. Using the archaeal rhodopsins as model systems, this work aims at advancing the understanding of the inter-protein absorption shift. The presented results demonstrate that the electrostatic interactions of the protein with the retinal are a major determinant of the inter-protein shift. The differences in electrostatic potential that the proteins cause at the retinal could be assigned to seven residues. A generalized model of a quantum mechanical particle in a box including the electrostatic potential as a parameter allows a qualitative description of the absorption maxima. Bacteriorhodopsin has become one of the most important model systems in the field of bioenergetics. This is due to its relative simplicity making it amenable to experimental and theoretical studies. Here, the probability of functionally relevant protonation states is calculated to characterize the available structures. The protonation behavior of the key residues of proton transfer and the correlation between the protonation of these residues is analyzed. The results show that with respect to the protonation the bR, K, L and M1 intermediate state are well represented by the available structures, while the M2, N and O intermediate state are less well represented. An algorithm is introduced that determines a gap-free list of the lowest energy states. Such a list allows to analyze the ensemble of states accessible to a system in a certain energy range and, thus, can provide useful insight into the functional mechanism. The newly developed algorithm, termed X-DEE, is based on the dead-end elimination theorem. The X-DEE algorithm is applicable to a wide range of problems, for instance in protein design attempts. Here, X-DEE is successfully applied to bacteriorhodopsin to obtain gap-free lists of the lowest energy protonation states.
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Designing novel host materials for blue phosphorescent organic light-emitting diodes
(2009)
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Michael Rothmann
- The overall efficiency of an organic light-emitting diode (OLED) is always limited to the efficiency of its individual components. The most important component is the emission layer, where excitons are formed and light is generated. This thesis deals with the improvement of one class of OLED component, namely host materials for blue phosphorescent emitters. Three generations of 1,3,5-triazine-based materials with varying donor-substituents are presented in this work. In the first generation carbazole units are bound to the triazine core. The second generation consists of triazines with diarylamino substituents. The third generation compounds are a combination of disubtituted triazines from the first two generations and a phenoxy-carbazole unit. While the first and second generation comprise substituents that are directly bound to the triazine core, in the third generation triazines the phenylcarbazole-donor is attached via a nonconjugated ether bond. Within each generation various properties are tailored to fulfill the complex profile of requirements for host materials. Known nucleophilic substitution reactions were further improved to enable the efficient synthesis of novel host materials in very high purity and high yields. The sequential replacement of the chlorines of cyanuric chloride is dependent on temperature, actual ring substitution and the nature of the nucleophile. Effective methods were developed to yield asymmetrically substituted triazines in a controlled manner. The thermal properties, including the thermal stability to enable the processing by vapor deposition and the glass forming properties to result in a morphological stability of prepared thin emission layers, were controlled by systematic investigation of different substitution patterns. Thus, glass transition temperatures up to 170 °C are presented. Studies of the long term stability of amorphous host films, carried out for several materials, revealed its importance for long term efficient devices. The electrochemical properties of the novel compounds were investigated by cyclic voltammetry to study the energetic position of the HOMO and the LUMO as well as the stability of the material upon oxidation and reduction. Using this method the injection properties of the materials were determined. The blocking of activated positions resulted in reversible redox behavior. Furthermore the ionization potential was decreased for the third generation triazines to yield an improved hole injection into these materials. Additionally computational calculations were carried out to understand and further improve the energy levels by substituent exchange. This led directly to the development of bipolar host materials with separated hole and electron transport units within one molecule. Furthermore single carrier devices were fabricated to demonstrate the benefits of the transport bipolar characteristics. For the efficient operation of a device the triplet energy of the host material has to be higher compared to the emitter. First generation triazines exhibit triplet energies up to 2.96 eV and therefore enable the use of light and middle blue phosphorescent emitters. Second generation triazines comprise exceptionally high triplet energies up to 3.24 eV. These are amongst the highest values reported in the literature and facilitate the use of deep blue phosphorescent emitters. For hosts of the third generation the triplet energy depends on the choice of the triazine moiety. They are therefore suited for light and deep blue emitter. Extensive photo physical characterizations of all materials have been carried out in solutions, neat films and doped films. Energy transfer experiments with several emitters additionally gained valuable information about the compatibility of host and guest molecules All generations of triazines are tested as host material in OLEDs. The optimization of the device configurations was carried out by combinatorial evaporation. The sequential adaption of layer thickness and composition helped to improve the device performance. The stepwise optimization of the host material properties resulted in an enduring progression concerning the luminance and efficiency. For the third generation triazines 11.5 % external quantum efficiency and a high brightness of 33000 cd/m2 were achieved.
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Iron oxidation in (Mg,Fe)0: Calibration of the Flank method on synthetic samples and applications on natural inclusions from lower mantle diamonds
(2009)
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Micaela Longo
- (Mg,Fe)O ferropericlase is the most common mineral found in diamonds originating in the lower mantle (more than 50% of occurrences). It is well known that the Fe3+ concentration in (Mg,Fe)O is sensitive to oxygen fugacity, even at high pressures. Therefore, the determination of Fe3+/Fetot in such inclusions provides a direct method for investigating lower mantle redox conditions during diamond formation. The goal of the present research is to calibrate the “flank method” by electron microprobe using synthetic (Mg,Fe)O, and then apply the method to determine in situ Fe3+/Fetot in ferropericlase inclusions from lower mantle diamonds. Up to now a calibration of the flank method is available only for garnets. Initially, the flank method was calibrated for garnets to test the reproducibility of the method on the Jeol XA-8200 electron microprobe in use at Bayerisches Geoinstitut. Results showed that for garnets a new calibration curve needs to be established at each working session. Then the flank method was calibrated for the Jeol XA-8200 electron microprobe in use at Bayerisches Geoinstitut for a homogeneous set of (Mg,Fe)O ferropericlase crystals over a wide range of composition (xFe = 2 to 60 at.%) and Fe3+/Fetot (1 to 15%). Samples were obtained by performing high pressure high temperature experiments in a multi anvil apparatus. In order to avoid compositional effects on flank method measurements, the high sample homogeneity was essential. Moreover, the determination of the Fe3+/Fetot ratio needed to be extremely accurate. For this purpose, a more accurate procedure for fitting the Mössbauer spectra of the final set of synthetic (Mg,Fe)O was adopted. The calibration curve determined is Fe2+ = 46.238 + 8.161 * ln (Fetot) - 137.01 * (Lbeta/Lalpha) + 85.57 * (Lbeta/Lalpha)2, for a Fe compositional range between 3 and 47 wt. %. A comparison of Fe3+/Fetot determined by flank method and values determined earlier by Mössbauer spectroscopy shows that results are generally consistent between the two different methods within the experimental errors. In contrast with garnet, the calibration curve established for ferropericlase does not need to be recalibrated at each microprobe session. Therefore, the calibration curve can be considered universal for the electron microprobe in use if the spectrometer adjustments remain identical with time. To explore applications of the flank method, a set of (Mg,Fe)O samples from diffusion studies was also investigated. Three (Mg,Fe)O crystals were measured by electron microprobe in order to test the sensitivity and accuracy of the flank method for small variations of bulk (Fetot)(wt%) as well as to measure Fe3+/Fetot along diffusion profiles. In the present work it is demonstrated how the flank method can be a powerful tool to measure small variations in Fe3+ content, with a spatial resolution of only few microns (2-3 µm) and a lower detection limit of Fetot of 3 wt%. Moreover, the measurement of Fe3+ content on the micron scale enables the study of the variation of oxygen fugacity conditions along diffusion gradients. A set of (Mg,Fe)O ferropericlase inclusions from ultra deep diamonds selected worldwide were analyzed by the flank method. The data set consists of eighteen (Mg,Fe)O ferropericlase samples from Juina, Brazil, Machado River, Brazil, and Ororoo, Australia. Inclusions are between 10 and 50 µm in size, therefore they are suitable to perform flank method measurements to determine Fe3+/Fetot. For the first time Fe3+/Fetot ratios were measured directly at the electron microprobe on inclusions of less than 50 µm in size. Results for the (Mg,Fe)O inclusions show good agreement with the theoretical trend described by the synthetic samples, which confirms high phase homogeneity for most of the samples. Flank method measurements show a large range of Fe3+/Fetot values for (Mg,Fe)O inclusions, which implies a large range of oxygen fugacities based on charge balance calculations. This large range of oxygen fugacities is similar to results for a suite of much larger inclusions from Kankan, Guinea, and São Luiz, Brazil, that were studied using Mössbauer spectroscopy. The variation of oxygen fugacity seems to be correlated to the geographical distribution of the inclusions studied, showing a redox gradient with more reducing conditions at Kankan, Guinea, and São Luiz, Brazil, and more oxidized in the case of Juina and Machado River, Brazil, and Eurelia, Australia. Such a correlation may be linked to the proto-pacific subduction mechanism, and the different ages combined with the geographic variation may indicate a difference in depth correlating with the large redox variation. Inclusions recovered from the same host diamond from Eurelia shows a strong redox gradient, which suggests a drastic change in the oxygen fugacity conditions during diamond growth. In order to provide information on the mechanisms able to control the redox conditions at lower mantle depths, a multi disciplinary study is suggested for further work.
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Surface Deformations of Magnetic Continua in Homogeneous Fields
(2009)
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Christian Gollwitzer
- In this thesis, experiments with magnetic liquids and gels are presented. Ferrofluids are synthetically created suspensions of magnetic nanoparticles in a carrier liquid. By adding a gelator, such a ferrofluid can be turned into a ferrogel. The magnetic properties of these substances are similar to a usual paramagnet with the important difference, that the susceptibility of the former is higher by a factor of 10^3 to 10^6. By the application of a homogeneous field, a transformation of the shape of a magnetic sample can be induced. In this thesis, four experiments on the surface deformation in homogeneous magnetic fields are presented. Two geometric configurations are considered: a horizontally extended flat layer with a free surface as well as a spherical sample. In both cases, the application of a homogeneous magnetic field leads to changes of the shape of the free boundary. In the case of the spherical geometry, the sample is deformed into a prolate ellipsoid under the action of the field, the so called magnetodeformational effect. In case of the extended flat layer, an abrupt shape transition into a patterned state takes place, the normal field or Rosensweig instability. In contrast to the smooth deformation of the sphere, this is an instability, which breaks the translational symmetry, and the transition occurs at a certain threshold value of the magnetic induction. Each of the four experiments in this thesis is briefly summarized in the following paragraphs. Part I of the thesis considers ferrofluids. In chapter 2, the ideal geometry of an infinitely extended flat layer is intentionally reduced to a cylinder such that only a single spike in the centre exists, and the solution space becomes rotationally symmetric. This makes the problem very feasible for experimental methods and numerical simulations. Two measurement techniques are applied and compared to each other, namely an X-ray technique, where the surface deformation is extracted from radioscopic images, and a laser technique, which focuses a laser spot onto the surface. The experiments and the simulations, the latter performed in close cooperation with a group in Athens, show a convincing agreement within a few percent. It remains an open question, whether the result can be deduced in analytic form, however. In chapter 3, a highly viscous ferrofluid is utilized to study the nonlinear dynamics of the normal field instability at very low Reynolds numbers. The linear growth rate for the growth and decay of the pattern at small amplitudes is extracted from the measurements and compared with an existing theoretical model. In addition, the measurement technique provides the reconstruction of a fully nonlinear amplitude equation, which is qualitatively compared to model equations. These nonlinear amplitude equations can only describe the dynamics of the growth in the immediate vicinity of the critical point so far. For a quantitative comparison, there is a need for a model with an extended range of validity. Additionally, localized patterns are observed which arise spontaneously in the neighbourhood of the unstable solution branch, which have previously been observed with the help of an external disturbance Part II of the thesis deals with thermoreversible ferrogels. Chapter 4 studies the magnetodeformational effect. A ferrogel sphere is exposed to homogeneous magnetic field. When the field is applied suddenly, the sphere not only elongates in the direction of the field, but also vibrates about the new equilibrium. On a longer time scale, the deformation continuously increases due to the viscoelastic properties of the gel. Both phenomena can well be described by a harmonic oscillator model, where the spring constant changes with time. From the deformation parallel and perpendicular to the applied field, Poisson´s ratio can be calculated, which turns out to be close to the limit of incompressibility. The absolute values of the deformation are compared to recent theoretical models. The resulting deviation of about 10% is attributed to the viscoelastic properties of the ferrogel, which are not taken into account in the static models. In chapter 5, the normal field instability is realized for the first time with a ferrogel. A flat layer of a thermoreversible ferrogel is exposed to a homogeneous magnetic field at different temperatures, where the gel is viscoelastic. This is a consequence of the need for a very soft material, such that the growth of the pattern is not completely suppressed by the elastic forces. The magnetic field is periodically modulated in time, and the amplitude of the instability is measured, which is modulated with the same frequency. The comparison with rheological measurements reveals a scaling of the modulated amplitude with the complex viscosity of the ferrogel. A comparison with the theoretical model for a ferrogel is difficult due to the viscoelasticity of the gel.
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Crystalline Morphologies of Poly(butadiene)-b-Poly(ethylene oxide) Block Copolymers in n-Heptane
(2009)
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Adriana Mirela Mihut
- This thesis reports the development of micellar crystalline morphologies in a selective solvent. The phase diagram of solution morphologies as a function of the molecular composition of the semicrystalline poly(butadiene)-b-poly(ethylene oxide)(PB-b-PEO) block copolymers was investigated. The crystalline morphologies discussed here have been generated from selective solvent condition (70°C in n-heptane) via two thermal pathways: (A) by direct immersion into liquid nitrogen, where n-heptane becomes a poor solvent for both blocks at very low temperatures, and (B): by quenching to the crystallization temperature of the PEO, i.e., determined by the length of PEO block. In pathway B, n-heptane is a poor solvent only for the PEO block. At 70°C, the block copolymers self-assembled into micellar structures consisting of a PEO molten core and a soluble PB corona. As crystallization takes place in the PEO core, a fast quenching into liquid nitrogen results in the formation of crystalline micelles retaining the shape present in the molten state at 70°C (pathway A). In the case of pathway B, the competition between the PEO core crystallization and the self-assembly of the micellar units, is the driving force that dictates the morphological development, therefore crystallization breaks out the melt morphology. These studies, demonstrated that the PB-b-PEO block copolymers are a promising system models for developing a general route towards tunable crystalline morphologies. In a symmetric PB-b-PEO block copolymer, crystalline morphologies like spheres and meanders formed upon quenching into liquid nitrogen and at 30°C, respectively. The meander morphology consisting of branched lamellae with a crystalline PEO ribbon-like core and ellipsoidal endings was observed for the first time in solution. Investigations of the crystal development revealed that this structure formed via crystallization-induced aggregation of spherical micelles upon cooling. A systematic study of the effect of crystallization kinetics on the formed morphology upon crystallization-induced aggregation of spherical micelles of a symmetric PB-b-PEO block copolymer was discussed. We demonstrated that the resulting morphology is controlled by two competitive effects, namely, by the nucleation and growth of the PEO micellar core: at lower crystallization temperatures (Tc ≤ 30°C), a high nucleation rate leads to a meander-like morphology formation, whereas at higher crystallization temperatures (Tc > 30°C), a low nucleation rate favors the formation of twisted lamellae. For a highly asymmetric PB-b-PEO block copolymer, crystallization at -30°C induced the formation of crystalline micelles retaining the spherical shape present in the molten state at 70°C. However, a quenching into liquid nitrogen facilitated a transition to rod-like micelles caused by changes of solvent quality for the PB coronar chains. This triggers the onset of an interfacial instability, therefore the spherical micelles preferred to reorganize into a morphology with a smaller interfacial curvature. The low crystallinity of the PEO core imposed a stronger tendency of the rods to aggregate and to thicken into more stable morphologies as needle-like structures, with a preferred growth direction along the long axis. Finally, the micellar morphology diagram of the PB-b-PEO block copolymers has been studied as a function of the crystallization temperature and molecular composition of the blocks via two thermal pathways. Pathway A allowed morphological transitions from spheres to rods, worms or twisted cylinders with the increase of the crystalline content of the PEO core. In Pathway B, the sequence of spheres, cylinders, lamellae, platelets and dendrites structures is observed with the increases of the PEO block length. The aggregation number of the spherical micelles is affected by the weight fraction and crystallinity of the PEO block. Moreover, an increased chain folding was observed at a high PEO composition which reduced the lamellar thickness of the crystals. The competition between the PEO core crystallization and the aggregation of the micellar units leads to coexistence regions of lamellae with platelets and cylinders with platelets. The novelty of this thesis relies on the development of novel crystalline morphologies in a selective solvent, as well as, in the detailed analysis of the major parameters that govern morphological formation in a controlled manner.
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Direct Amino Acid Uptake by Plants related to Grassland Diversity - methodological and ecological Investigations
(2009)
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Leopold Sauheitl
- Uptake of intact amino acids by plants has been identified as an alternative nitrogen (N) source for plants in a number of ecosystems and soil types. Up to now it is assumed that this uptake strategy is of particular relevance in ecosystems with low mineral N (Nmin) contents due to insignificant microbial activity or due to poorly developed soils. However, it has also been discussed that amino acid uptake might enable plants to lower intracspecific competition for mineral N and shortcut the microbial mineralization of organic N in systems were competition is exceptionally high. The positive effect of plant diversity on plant productivity is known to induce conditions of intense N competition and thus amino acid uptake might explain how plant communities enable higher productive with increasing diversity. However, the ecological importance of organic N uptake has also been questioned due to the high competitive power of microbes in soils of the temperate zone and due to a number of flaws in the commonly used method to proof and quantify direct amino acid uptake. In this, dual labelled (13C and 15N) amino acids are injected into the soil and direct tracer uptake is quantified via bulk isotope measurement of 13C and 15N enrichment in plant tissues, which recently has been challenged to exclusively reflect direct amino acid uptake. The first objective therefore was to identify and reduce methodological influences on the direct amino acid uptake by plants. Thus the effect of changed amino acid concentrations on amino acid uptake was investigated by application of different tracer amounts. Next, the accuracy and precision of commonly used bulk isotope measurements were compared to compound specific measurements with respect to the determination of direct amino acid uptake. It was shown that the use of high tracer amounts accompanied by high Nmin release reduces direct amino acid uptake via plant internal down regulation of amino acid transporters. This corroborates the importance of minimizing tracer amounts and suggests that plants can actively increase amino acid uptake when N availability in soil is low. Bulk measurements turned out to overestimate direct amino acid uptake by a factor of up to six, as they were not able to separate uptake of intact tracer molecules from uptake of tracer fragments or inorganic carbon. At the same time compound specific isotope measurements proofed to be an accurate and precise tool to demonstrate and quantify uptake of intact amino acids. Using these optimized methods, the importance of amino acid uptake for the N-nutrition of plants with respect to changing plant diversity was investigated. The uptake of amino acids and mineral N by plants as well as the competition between plants and microbes for amino acid N was investigated in grassland communities with 1 to 16 grassland species. Microbes were superior competitors for amino acid derived nitrogen, irrespective of plant diversity and took up 54% of the applied amino acid N in average within 24 h. In contrast, plants only incorporated 2.7% of the applied N and were thus less effective by a factor of 20 in short term N acquisition than microbes. In addition, plant mineral N uptake decreased with increasing plant diversity while uptake of intact amino acids increased. Thus the contribution of amino acid uptake to the overall plant N nutrition increased from 1.5 to 7.0% in which amino acid uptake was mainly controlled by plant N concentration shoot biomass and rooting density while mineral N uptake was controlled by microbial competition. In detail amino acid uptake increased with decreasing plant N concentration while mineral N uptake decreased with increasing microbial abundance and microbial N uptake. Thus, the boosted importance of amino acid uptake for plant N nutrition has to be seen as a reaction on increased N competition with increasing plant diversity. Additionally, plant diversity stimulated microbial diversity which was most likely due to the bottom up effect of increased root exudation and litter input caused by increasing N competition and plant productivity, respectively. While the microbial community was dominated by bacteria (54.7%) the abundance of litter and soil organic matter decomposing gram positive bacteria and fungi as well as protozoan abundance increased with increasing plant diversity. Protozoa are known to stimulate turnover of bacteria which was indicated by higher tracer incorporation of this microbial group and an overall increase of deaminase activity with increasing plant diversity. As higher microbial turnover is associated with increased loss of microbial N to plants, we have to expect higher N availability for plants in the long term. The positive feedback of a plant-induced higher microbial turnover rate on N availability in soil together with an increased amino acids uptake might therefore be an important model to explain the positive effect of plant diversity on plant productivity.
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Self-interaction and charge transfer in organic semiconductors
(2009)
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Thomas Körzdörfer
- The fascinating properties of organic molecular semiconductors paved the way for a new class of electronic devices such as organic light-emitting diodes, transistors, or solar cells. Despite an inferior efficiency as compared to commonly used silicon-based technologies, organic semiconductors promise the advent of fully flexible devices for large-area displays and solar cells, printed transistors as low-cost radio frequency identification (RFID) tags, displays for electronic books, and disposable measuring instruments for medical diagnosis. Hence, the investigation of organic molecular semiconductors has emerged as a vibrant field of development both in industry and in academia, spanning a wide range of subjects from physics, chemistry, and materials science to engineering and technology. Theoretical physicists can contribute to this progress by developing methods that allow to determine the electronic properties of organic semiconductors from first principles and thus deepen our knowledge of the underlying electronic processes in organic electronic devices. The calculation of the electronic properties of molecular semiconductors issues a serious challenge to theoretical physicists and chemists. Typically, organic semiconductor molecules employ several hundreds of electrons. For systems of that size, approaches that work with model Hamiltonians are typically not accurate enough in predicting many important electronic properties. However, solving the many-particle Schrödinger-equation by employing highly accurate perturbation theory approaches is often numerically too expensive to be considered as a convenient alternative. Hence, density functional theory (DFT) naturally arises as the method of choice. However, although in theory DFT yields an exact formulation of quantum mechanics, the quality of the results obtained from DFT calculations in practice strongly depends on the used approximations to the so-called exchange-correlation functional. This work concentrates on the problem of self-interaction, which is one of the most serious problems of commonly used approximative density functionals. As a major result of this work, it is demonstrated that self-interaction plays a decisive role for the performance of different approximative functionals in predicting accurate electronic properties of organic molecular semiconductors. This is particularly true for the calculation of ionization potentials, photoelectron spectra, dissociation, and charge-transfer processes. In search for a solution to the self-interaction problem, a new concept for correcting commonly used density functionals for self-interaction is introduced and applied to a variety of systems, spanning small molecules, extended molecular chains, and organic molecular semiconductors. It is further shown that the performance of functionals that are not free from self-interaction can vary strongly for different systems and observables of interest, thus entailing the danger of misinterpretation of the results obtained from those functionals. The underlying reasons for the varying performance of commonly used density functionals are discussed thoroughly in this work. Finally, this thesis provides strategies that allow to analyze the reliability of commonly used approximations to the exchange-correlation functional for particular systems of interest. This cumulative dissertation is divided into three parts. Part I gives a short introduction into DFT and its time-dependent extension (TDDFT). Part II provides further insights into the self-interaction problem, presents a newly developed concept for the correction of self-interaction, gives an introduction into the publications, and discusses their basic results. Finally, the four publications on self-interaction and charge-transfer in extended molecular systems and organic molecular semiconductors are collected in Part III.
