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

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

Influence of gas atmospheres and ceria on the stability of nanoporous gold studied by environmental electron microscopy and <em>In</em> <em>situ</em> ptychography

A novel complementary approach of electron microscopy/environmental TEM and in situ hard X-ray ptychography was used to study the thermally induced coarsening of nanoporous gold under different atmospheres, pressures and after ceria deposition. The temperature applied during ptychographic imaging was determined by IR thermography. While using elevated temperatures (room temperature – 400 °C) and realistic gas atmospheres (1 bar) we achieved for the first time a spatial resolution of about 20 nm during hard X-ray ptychography. The annealing of pure and ceria stabilized nanoporous gold in different atmospheres revealed that the conditions have a tremendous influence on the coarsening. The porous structure of the samples was stable up to approximately 800 °C in vacuum, whereas pronounced changes and coarsening were observed already at approximately 300 °C in oxygen containing atmospheres. A layer of ceria on the nanoporous gold led to an improvement of the stability, but did not alleviate the influence of the gas atmosphere. Different behaviors were observed, such as coarsening and even material loss or migration. The results suggest that additional mechanisms beyond surface diffusion need to be considered and that microscopic studies aimed at more realistic conditions are important to understand the behavior of such materials and catalysts

Ab initio nonrigid X-ray nanotomography

Abstract: Reaching the full potential of X-ray nanotomography, in particular for biological samples, is limited by many factors, of which one of the most serious is radiation damage. Although sample deformation caused by radiation damage can be partly mitigated by cryogenic protection, it is still present in these conditions and, as we exemplify here using a specimen extracted from scales of the Cyphochilus beetle, it will pose a limit to the achievable imaging resolution. We demonstrate a generalized tomographic model, which optimally follows the sample morphological changes and attempts to recover the original sample structure close to the ideal, damage-free reconstruction. Whereas our demonstration was performed using ptychographic X-ray tomography, the method can be adopted for any tomographic imaging modality. Our application demonstrates improved reconstruction quality of radiation-sensitive samples, which will be of increasing relevance with the higher brightness of 4th generation synchrotron sources

Live Iterative Ptychography

We demonstrate live-updating ptychographic reconstruction with ePIE, an iterative ptychography method, during ongoing data acquisition. The reconstruction starts with a small subset of the total data, and as the acquisition proceeds the data used for reconstruction is extended. This creates a live-updating view of object and illumination that allows monitoring the ongoing experiment and adjusting parameters with quick turn-around. This is particularly advantageous for long-running acquisitions. We show that such a gradual reconstruction yields interpretable results already with a small subset of the data. We show simulated live processing with various scan patterns, parallelized reconstruction, and real-world live processing at the hard X-ray ptychographic nanoanalytical microscope PtyNAMi at the PETRA III beamline

The Phase-Contrast Imaging Instrument at the Matter in Extreme Conditions Endstation at LCLS

We describe the Phase-Contrast Imaging instrument at the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source. The instrument can image phenomena with a spatial resolution of a few hundreds of nanometers and at the same time reveal the atomic structure through X-ray diffraction, with a temporal resolution better than 100 femtosecond. It was specifically designed for studies relevant to High-Energy-Density Science and can monitor, e.g., shock fronts, phase transitions, or void collapses. This versatile instrument was commissioned last year and is now available to the MEC user community

Tomographic reconstruction with a generative adversarial network

This paper presents a deep learning algorithm for tomographic reconstruction (GANrec). The algorithm uses a generative adversarial network (GAN) to solve the inverse of the Radon transform directly. It works for independent sinograms without additional training steps. The GAN has been developed to fit the input sinogram with the model sinogram generated from the predicted reconstruction. Good quality reconstructions can be obtained during the minimization of the fitting errors. The reconstruction is a self-training procedure based on the physics model, instead of on training data. The algorithm showed significant improvements in the reconstruction accuracy, especially for missing-wedge tomography acquired at less than 180° rotational range. It was also validated by reconstructing a missing-wedge X-ray ptychographic tomography (PXCT) data set of a macroporous zeolite particle, for which only 51 projections over 70° could be collected. The GANrec recovered the 3D pore structure with reasonable quality for further analysis. This reconstruction concept can work universally for most of the ill-posed inverse problems if the forward model is well defined, such as phase retrieval of in-line phase-contrast imaging

Restructuring of Ag catalysts for methanol to formaldehyde conversion studied using in situ X-ray ptychography and electron microscopy

Dynamic restructuring of silver catalysts during the industrial conversion of methanol to formaldehyde leads to surface faceting and pinhole formation. Subsequent sintering under reaction conditions, followed by increased pressure drop and decreased catalyst activity requires catalyst bed replacement after several months of operation. This necessitates a comprehensive understanding of the bulk catalyst restructuring under exposure to different gas environments. In this work, Ag restructuring was studied at elevated temperatures under different reactive and inert gas environments. Bubble formation within catalysts of 5-8 µm thickness was visualized in real-time using in situ X-ray ptychography. Stepwise heating up to 650 °C in combination with imaging was used to determine the effect of temperature on silver restructuring. Dynamic changes within the catalyst were further quantified in terms of relative changes in mass on selected regions at a constant temperature of 500 °C. Quantitative assessment of dynamic changes in the catalyst resulting from bubble growth and movement revealed the influence of temperature, time, and gas environment on the degree of restructuring. Post-mortem scanning electron microscopy with energy-dispersive X-ray spectroscopy mapping confirmed the redistribution of material as a consequence of bubble rupture and collapse. The formation of pores and cavities under reaction environments was additionally confirmed using a fixed bed reactor, and subsequent examination using focused-ion beam milling, providing detailed analysis of the surface structure.This study demonstrates the unique advantage of correlative hard X-ray and electron microscopy for quantitative morphological studies of industrial catalysts