Sabine Schlabach

Photoinduced Charge-Carrier Generation in Epitaxial MOF Thin Films: High Efficiency as a Result of an Indirect Electronic Band Gap?

For inorganic semiconductors crystalline order leads to a band structure which gives rise to drastic differences to the disordered material. An example is the presence of an indirect band gap. For organic semiconductors such effects are typically not considered, since the bands are normally flat, and the band-gap therefore is direct. Herein we show results from electronic structure calculations demonstrating that ordered arrays of porphyrins reveal a small dispersion of occupied and unoccupied bands leading to the formation of a small indirect band gap. We demonstrate herein that such ordered structures can be fabricated by liquid-phase epitaxy and that the corresponding crystalline organic semiconductors exhibit superior photophysical properties, including large charge-carrier mobility and an unusually large charge-carrier generation efficiency. We have fabricated a prototype organic photovoltaic device based on this novel material exhibiting a remarkable efficiency.

Structure and grain growth of TiO2 nanoparticles investigated by electron and x-ray diffractions and Ta181 perturbed angular correlations

Bare and coated TiO₂ nanoparticles with particle sizes d 100 nm after 1300 K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO₂. The data suggest a critical grain size of d ~10 nm for the onset of the ordering process. The spectra of coarse-grained TiO₂ are reached at a particle size d >= 30 nm. In n-TiO₂ coated with Al₂O₃ and ZrO₂ both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO₂ cores and in the ZrO₂ coating of n-TiO₂/ZrO₂ at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO₂ kernels and the coatings.

Structure of Alumina and Zirconia Nanoparticles Synthesized by the Karlsruhe Microwave Plasma Process

Applying the Karlsruhe Microwave Plasma Process, alumina and zirconia particles with particle sizes in the range from 2 to 5 nm were synthesized. Additionally, the influence of small amounts of iron(III)-oxide, chromia, and magnesia on the crystallization and phase transitions in alumina powders was investigated. It is shown that these dopants may influence the crystallization behavior significantly. Especially, the addition of 1 mol% iron(III)-oxide reduced the temperature of the formation of a-alumina from 1200 °C to 300 °C.

Tailoring magnetic frustration in strained epitaxial FeRh films

We report on a strain-induced martensitic transformation, accompanied by a suppression of magnetic order in epitaxial films of chemically disordered FeRh. X-ray diffraction, transmission electronmicroscopy, and electronic structure calculations reveal that the lowering of symmetry (from cubic to tetragonal) imposed by the epitaxial relation leads to a further, unexpected, tetragonal-to-orthorhombic transition, triggered by a band-Jahn-Teller-type lattice instability. The collapse of magnetic order is a direct consequence of this structural change, which upsets the subtle balance between ferromagnetic nearest-neighbor interactions arising from Fe-Rh hybridization and frustrated antiferromagnetic coupling among localized Fe moments at larger distances.

Nanoparticle SnO 2 films as gas sensitive membranes

The Karlsruhe Microwave Plasma Process (KMPP), a versatile gas-phase process is applied to produce SnO 2 and core shell SnO 2 /SiO 2 nanoparticles which are, respectively, deposited in-situ on preheated Si-Substrates. These substrates are already equipped with an electrode microarray. The proof of sensor concept shows, that mechanically stable, nanoscaled and nanogranular gas sensing layers can be produced. In a first step synthesis and deposition parameters of SnO 2 are elaborated, and gas-sensitivity tests are performed. Additionally, annealing experiments are done. The morphology and struc-ture of nanoparticles is characterized by X-ray diffraction and TEM-methods. The layers are in-vestigated by SEM techniques and by XPS. The sensitivity of the nanogranular layer is deter-mined in comparison with a standard microarray equipped with sputtered layers. Particles crys-tallize in the tetragonal cassiterite structure. It is found that a precursor concentration of 3×10 −6 mol/l leads to particles with crystallite size in the region of 2nm, whereas a concentration of 5.5×10 −4 mol/l results in approximately 5nm particles. With the precursor concentration, columnar porous layers of 200nm thickness are obtained after a deposition time of 1min. This thickness is comparable to the one of sputtered layers. First sensor tests show 10 times higher sensitivity to isopropanol, compared to the standard array. The time of response is equivalent. The grain growth observed for bare and core/shell nanoparticles at 300°C is marginal.

Analysis of packing microstructure and wall effects in a narrow-bore ultrahigh pressure liquid chromatography column using focused ion-beam scanning electron microscopy

Column wall effects are well recognized as major limiting factor in achieving high separation efficiency in HPLC. This is especially important for modern analytical columns packed with small particles, where wall effects dominate the band broadening. Detailed knowledge about the packing microstructure of packed analytical columns has so far not been acquired. Here, we present the first three-dimensional reconstruction protocol for these columns utilizing focused ion-beam scanning electron microscopy (FIB-SEM) on a commercial 2.1mm inner diameter×50mm length narrow-bore analytical column packed with 1.7μm bridged-ethyl hybrid silica particles. Two sections from the packed bed are chosen for reconstruction by FIB-SEM: one from the bulk packing region of the column and one from its critical wall region. This allows quantification of structural differences between the wall region and the center of the bed due to effects induced by the hard, confining column wall. Consequences of these effects on local flow velocity in the column are analyzed with flow simulations utilizing the lattice-Boltzmann method. The reconstructions of the bed structures reveal significant structural differences in the wall region (extending radially over approximately 62 particle diameters) compared to the center of the column. It includes the local reduction of the external porosity by up to 10% and an increase of the mean particle diameter by up to 3%, resulting in a decrease of the local flow velocity by up to 23%. In addition, four (more ordered) layers of particles in the direct vicinity of the column wall induce local velocity fluctuations by up to a factor of three regarding the involved velocity amplitudes. These observations highlight the impact of radial variations in packing microstructure on band migration and column performance. This knowledge on morphological peculiarities of column wall effects helps guiding us towards further optimization of the packing process for analytical HPLC columns.

Comprehensive analysis of TEM methods for LiFePO4/FePO4 phase mapping: spectroscopic techniques (EFTEM, STEM-EELS) and STEM diffraction techniques (ACOM-TEM)

Transmission electron microscopy (TEM) has been used intensively in investigating battery materials, e.g. to obtain phase maps of partially (dis)charged (lithium) iron phosphate (LFP/FP), which is one of the most promising cathode material for next generation lithium ion (Li-ion) batteries. Due to the weak interaction between Li atoms and fast electrons, mapping of the Li distribution is not straightforward. In this work, we revisited the issue of TEM measurements of Li distribution maps for LFP/FP. Different TEM techniques, including spectroscopic techniques (energy filtered (EF)TEM in the energy range from low-loss to core-loss) and a STEM diffraction technique (automated crystal orientation mapping (ACOM)), were applied to map the lithiation of the same location in the same sample. This enabled a direct comparison of the results. The maps obtained by all methods showed excellent agreement with each other. Because of the strong difference in the imaging mechanisms, it proves the reliability of both the spectroscopic and STEM diffraction phase mapping. A comprehensive comparison of all methods is given in terms of information content, dose level, acquisition time and signal quality. The latter three are crucial for the design of in-situ experiments with beam sensitive Li-ion battery materials. Furthermore, we demonstrated the power of STEM diffraction (ACOM-STEM) providing additional crystallographic information, which can be analyzed to gain a deeper understanding of the LFP/FP interface properties such as statistical information on phase boundary orientation and misorientation between domains.

Molecular Dynamics of Polymer Composites Using Rheology and Combined RheoNMR on the Example of TiO2-Filled Poly(n-Alkyl Methacrylates) and Trans-1,4-Polyisoprene

The determination of the interplay between polymeric matrices and filler particles in composites is of great interest to understand structure-property relationships and develop predictive theories. To study the molecular dynamics of polymers in composites, model systems based on poly(n-alkyl methacrylates), trans-1,4-polyisoprene (gutta percha), and titania (TiO2) were prepared and characterized using rheometry and a combined RheoNMR technique. Apparent entanglement molecular weights were obtained from small amplitude oscillatory shear (SAOS) experiments, which are related to the increasing physical cross-link density as a function of the filler content. Large amplitude oscillatory shear (LAOS) experiments were performed and analyzed within the FT-rheometry framework. The filler had a strong impact on the scaling behavior of the normalized third harmonic. A combined RheoNMR technique was used to simultaneously study the molecular dynamics via NMR and the corresponding mechanical response via rheometry. A strong correlation between the macroscopic mechanical properties and microscopic molecular dynamics was found, which might lead to a new understanding of polymer melt dynamics.

New Core/Shell Ta 2 O 5 -PMMA Nanocomposites for Applications as Polymer Waveguides

To realize ceramic/polymer nanocomposites for polymer waveguides, PMMA-coated Ta 2 O 5 nanoparticles are synthesized as core/shell particles. Therefor a gas-phase process is used: the Karlsruhe Microwave Plasma Process. The organic coating is supposed to reduce the agglomeration of the ceramic cores and should facilitate the incorporation into the polymer resin. TEM investigations of the nanoparticles exhibit crystalline and amorphous Ta 2 O 5 with sizes around 3 to 6 nm, confirmed by electron diffraction. Although the polymer coating is not visible in TEM imaging, electron energy loss spectroscopy (EELS) exhibits a significant C-edge, proofing the organic coating. The Ta 2 O 5 /PMMA nanoparticles are incorporated with different weight fractions to a maximum of 1 wt% by a dissolver stirrer into PMMA resin. The optical properties (refractive index, transmission) are determined as a function of the nanoparticle content. Compared to the pure polymer, the refractive index of the modified composite, measured at 633 nm, is increased by 0.001 and 0.004 at 0.1 wt% and 0.7 wt%, respectively. A similar tendency is observed at 1550 nm. The transmission in the near infrared (NIR) is similar to that of PMMA up to a content of 0.3 wt%. At higher nanoparticle contents transmission is reduced. The reduction in transmission is due to the presence of agglomerates larger then 1/10 of the applied wavelength, confirmed by TEM. The concept of incorporating inorganic/organic hybrid nanoparticles with intrinsic high refractive index in polymer matrices is very promising. A suitable effect in refractive index for application of ceramic nanoparticle/polymer nanocomposites as polymer waveguides could be observed even with low particle concentration.

Influence of Halides on the Luminescence of Oxide/Anthracene/Polymer Nanocomposites

Nanocomposites made of an oxide core of a wide band gap insulator, a lumophore monolayer of anthracene and an outer protecting layer of PMMA are studied regarding their luminescence properties and the influence of halides stemming either from the precursor used for synthesis or from the lumophore itself. Halide-free nanocomposites exhibit luminescence spectra resembling to that of anthracene with some significant differences concerning the intensity ratio and an additional peak at 420 nm. Nanocomposites made from chlorides show excimer-like spectra with broad maxima. In microanalysis residual chlorine can be detected. Chlorine-free oxide kernels, coated with 9, 10 dichloroanthracene exhibit luminescence spectra resembling to a superposition of the pure lumophores 9 chloro- and 9, 10 dichloroanthracene. It can be shown that the origin of the halide strongly influences, but does not quench the luminescence spectra of the powders. Suspensions of the chlorine containing nanocomposites in ethanol exhibit modified anthracene like spectra. This is a strong indication for dechlorination by proton-transfer in ethanol. Suspensions of the same material in water lead to spectra showing a superposition of exci-mer spectrum and modified anthracene spectrum. Here a partial dechlorination occurs.