Advanced Denoising for X-ray Ptychography

The success of ptychographic imaging experiments strongly depends on achieving high signal-to-noise ratio. This is particularly important in nanoscale imaging experiments when diffraction signals are very weak and the experiments are accompanied by significant parasitic scattering (background), outliers or correlated noise sources. It is also critical when rare events such as cosmic rays, or bad frames caused by electronic glitches or shutter timing malfunction take place. In this paper, we propose a novel iterative algorithm with rigorous analysis that exploits the direct forward model for parasitic noise and sample smoothness to achieve a thorough characterization and removal of structured and random noise. We present a formal description of the proposed algorithm and prove its convergence under mild conditions. Numerical experiments from simulations and real data (both soft and hard X-ray beamlines) demonstrate that the proposed algorithms produce better results when compared to state-of-the-art methods.Comment: 24 pages, 9 figure

Axial Changes of Catalyst Structure and Temperature in a Fixed-Bed Microreactor During Noble Metal Catalysed Partial Oxidation of Methane

The catalytic partial oxidation of methane (CPO) over flame-made 2.5%Rh-2.5%Pt/Al2O3 and 2.5%Rh/Al2O3 in 6%CH4/3%O2/He shows the potential of in situ studies using miniaturized fixed-bed reactors, the importance of spatially resolved studies and its combination with infrared thermography and on-line mass spectrometry. This experimental strategy allowed collecting data on the structure of the noble metal (oxidation state) and the temperature along the catalyst bed. The reaction was investigated in a fixed-bed quartz microreactor (1-1.5mm diameter) following the catalytic performance by on-line gas mass spectrometry (MS). Above the ignition temperature of the catalytic partial oxidation of methane (310-330°C), a zone with oxidized noble metals was observed in the inlet region of the catalyst bed, accompanied by a characteristic hot spot (over-temperature up to 150°C), while reduced noble metal species became dominant towards the outlet of the bed. The position of both the gradient in oxidation state and the hot spot were strongly dependent on the furnace temperature and the gas flow (residence time). Heating as well as a higher flow rate caused a migration of the transition zone of the oxidation state/maximum in temperature towards the inlet. At the same time the hydrogen concentration in the reactor effluent increased. In contrast, at low temperatures a movement of the transition zone towards the outlet was observed at increasing flux, except if the self-heating by the exothermic methane oxidation was too strong. The results indicate that in the oxidized zone mainly combustion of methane occurs, whereas in the reduced part direct partial oxidation and reforming reactions prevail. The results demonstrate how spatially resolved spectroscopy can help in understanding catalytic reactions involving different reaction zones and gradients even in micro scale fixed-bed reactor

Röntgenblick in der Nanowelt

For investigations of ordered nanometre structures and for high-precision measurement of the atomic order in crystalline solids, X-ray diffraction is the most important tool. With the aid of X-ray microscopy, unordered and even living objects can also be imaged directly. The resolution of these images, applying different contrast techniques representing the state of the art, reaches the nanometre range. In the future, a synthesis of both methods could yield a shift of the spectral resolution down to the atomic level.Für die Aufklärung geordneter Nanometer-Strukturen und die hochpräzise Vermessung der Atomanordnung in kristallinen Festkörpern ist die Röntgenstrahlbeugung das wichtigste Werkzeug. Mit Hilfe der Mikroskopie mit Röntgenstrahlung können auch ungeordnete und sogar lebende Objekte direkt abgebildet werden. Die Auflösung dieser Abbildungen unter Nutzung verschiedener Kontraste erreicht nach dem Stand der Technik gerade die Grenze des Nanometerbereiches. Für die Zukunft wird eine Synthese beider Methoden und so auch die weitere Verschiebung der Grenze der erreichbaren Auflösung erwartet

Concentration dependent effects of urea binding to poly(N-isopropylacrylamide) brushes: a combined experimental and numerical study

The binding effects of osmolytes on the conformational behavior of grafted polymers are studied in this work. In particular, we focus on the interactions between urea and poly(N-isopropylacrylamide) (PNIPAM) brushes by monitoring the ellipsometric brush thickness for varying urea concentrations over a broad temperature range. The interpretation of the obtained data is supported by atomistic molecular dynamics simulations, which provide detailed insights into the experimentally observed concentration-dependent effects on PNIPAM-urea interaction. In particular, in the low concentration regime (c(u) = 2 mol L-1, the lower T-tr is explained by the favorable replacement of water molecules by urea, which can be regarded as a cross-linker between adjacent PNIPAM chains. Significant effects of the concentration-dependent urea binding on the brush conformation are noticed: at c(u) <= 0.5 mol L-1, although urea is loosely embedded between the hydrated polymer chains, it enhances the brush swelling by excluded volume effects. Beyond 0.5 mol L-1, the stronger interaction between PNIPAM and urea reduces the chain hydration, which in combination with cross-linking of monomer units induces the shrinkage of the polymer brush.DFG, EXC 310, SimulationstechnikDFG, SFB 716, Dynamische Simulation von Systemen mit großen TeilchenzahlenDFG, GRK 1524, Self-Assembled Soft-Matter Nanostructures at Interface