Artifact-suppressing reconstruction of strongly interacting objects in X-ray near-field holography without a spatial support constraint.

The phase problem is a well known ill-posed reconstruction problem of coherent lens-less microscopic imaging, where only the squared magnitude of a complex wavefront is measured by a detector while the phase information of the wave field is lost. To retrieve the lost information, common algorithms rely either on multiple data acquisitions under varying measurement conditions or on the application of strong constraints such as a spatial support. In X-ray near-field holography, however, these methods are rendered impractical in the setting of time sensitive in situ and operando measurements. In this paper, we will forego the spatial support constraint and propose a projected gradient descent (PGD) based reconstruction scheme in combination with proper preprocessing and regularization that significantly reduces artifacts for refractive reconstructions from only a single acquired hologram without a spatial support constraint. We demonstrate the feasibility and robustness of our approach on different data sets obtained at the nano imaging endstation of P05 at PETRA III (DESY, Hamburg) operated by Helmholtz-Zentrum Hereon.

Live Iterative Ptychography.

We demonstrate live-updating ptychographic reconstruction with the extended ptychographical iterative engine, 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 are 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.

3D and Multimodal X-Ray Microscopy Reveals the Impact of Voids in CIGS Solar Cells.

Small voids in the absorber layer of thin-film solar cells are generally suspected to impair photovoltaic performance. They have been studied on Cu(In,Ga)Se2 cells with conventional laboratory techniques, albeit limited to surface characterization and often affected by sample-preparation artifacts. Here, synchrotron imaging is performed on a fully operational as-deposited solar cell containing a few tens of voids. By measuring operando current and X-ray excited optical luminescence, the local electrical and optical performance in the proximity of the voids are estimated, and via ptychographic tomography, the depth in the absorber of the voids is quantified. Besides, the complex network of material-deficit structures between the absorber and the top electrode is highlighted. Despite certain local impairments, the massive presence of voids in the absorber suggests they only have a limited detrimental impact on performance.

Culling a Self-Assembled Quantum Dot as a Single-Photon Source Using X-ray Microscopy.

Epitaxially grown self-assembled semiconductor quantum dots (QDs) with atom-like optical properties have emerged as the best choice for single-photon sources required for the development of quantum technology and quantum networks. Nondestructive selection of a single QD having desired structural, compositional, and optical characteristics is essential to obtain noise-free, fully indistinguishable single or entangled photons from single-photon emitters. Here, we show that the structural orientations and local compositional inhomogeneities within a single QD and the surrounding wet layer can be probed in a screening fashion by scanning X-ray diffraction microscopy and X-ray fluorescence with a few tens of nanometers-sized synchrotron radiation beam. The presented measurement protocol can be used to cull the best single QD from the enormous number of self-assembled dots grown simultaneously. The obtained results show that the elemental composition and resultant strain profiles of a QD are sensitive to in-plane crystallographic directions. We also observe that lattice expansion after a certain composition-limit introduces shear strain within a QD, enabling the possibility of controlled chiral-QD formation. Nanoscale chirality and compositional anisotropy, contradictory to common assumptions, need to be incorporated into existing theoretical models to predict the optical properties of single-photon sources and to further tune the epitaxial growth process of self-assembled quantum structures.

Multimodal imaging of cubic Cu2O@Au nanocage formation via galvanic replacement using X-ray ptychography and nano diffraction.

Being able to observe the formation of multi-material nanostructures in situ, simultaneously from a morphological and crystallographic perspective, is a challenging task. Yet, this is essential for the fabrication of nanomaterials with well-controlled composition exposing the most active crystallographic surfaces, as required for highly active catalysts in energy applications. To demonstrate how X-ray ptychography can be combined with scanning nanoprobe diffraction to realize multimodal imaging, we study growing Cu2O nanocubes and their transformation into Au nanocages. During the growth of nanocubes at a temperature of 138 °C, we measure the crystal structure of an individual nanoparticle and determine the presence of (100) crystallographic facets at its surface. We subsequently visualize the transformation of Cu2O into Au nanocages by galvanic replacement. The nanocubes interior homogeneously dissolves while smaller Au particles grow on their surface and later coalesce to form porous nanocages. We finally determine the amount of radiation damage making use of the quantitative phase images. We find that both the total surface dose as well as the dose rate imparted by the X-ray beam trigger additional deposition of Au onto the nanocages. Our multimodal approach can benefit in-solution imaging of multi-material nanostructures in many related fields.

The synchrotron radiation source PETRA III and its future ultra-low-emittance upgrade PETRA IV.

PETRA III at DESY is one of the brightest synchrotron radiation sources worldwide. It serves a broad international multidisciplinary user community from academia to industry at currently 25 specialised beamlines. With a storage-ring energy of 6 GeV, it provides mainly hard to high-energy X-rays for versatile experiments in a very broad range of scientific fields. It is ideally suited for an upgrade to the ultra-low emittance source PETRA IV, owing to its large circumference of 2304 m. With a targeted storage ring emittance of 20 × 5 pm 2 rad 2 , PETRA IV will reach spectral brightnesses two to three orders of magnitude higher than today. The unique beam parameters will make PETRA IV the ultimate in situ 3D microscope for biological, chemical, and physical processes helping to address key questions in health, energy, mobility, information technology, and earth and environment.

Rapid aberration correction for diffractive X-ray optics by additive manufacturing.

Diffraction-limited hard X-ray optics are key components for high-resolution microscopy, in particular for upcoming synchrotron radiation sources with ultra-low emittance. Diffractive optics like multilayer Laue lenses (MLL) have the potential to reach unprecedented numerical apertures (NA) when used in a crossed geometry of two one-dimensionally focusing lenses. However, minuscule fluctuations in the manufacturing process and technical limitations for high NA X-ray lenses can prevent a diffraction-limited performance. We present a method to overcome these challenges with a tailor-made refractive phase plate. With at-wavelength metrology and a rapid prototyping approach we demonstrate aberration correction for a crossed pair of MLL, improving the Strehl ratio from 0.41(2) to 0.81(4) at a numerical aperture of 3.3 × 10-3. This highly adaptable aberration-correction scheme provides an important tool for diffraction-limited hard X-ray focusing.

Imaging Cu2O nanocube hollowing in solution by quantitative in situ X-ray ptychography.

Understanding morphological changes of nanoparticles in solution is essential to tailor the functionality of devices used in energy generation and storage. However, we lack experimental methods that can visualize these processes in solution, or in electrolyte, and provide three-dimensional information. Here, we show how X-ray ptychography enables in situ nano-imaging of the formation and hollowing of nanoparticles in solution at 155 °C. We simultaneously image the growth of about 100 nanocubes with a spatial resolution of 66 nm. The quantitative phase images give access to the third dimension, allowing to additionally study particle thickness. We reveal that the substrate hinders their out-of-plane growth, thus the nanocubes are in fact nanocuboids. Moreover, we observe that the reduction of Cu2O to Cu triggers the hollowing of the nanocuboids. We critically assess the interaction of X-rays with the liquid sample. Our method enables detailed in-solution imaging for a wide range of reaction conditions.

Multi-beam X-ray ptychography using coded probes for rapid non-destructive high resolution imaging of extended samples.

Imaging large areas of a sample non-destructively and with high resolution is of great interest for both science and industry. For scanning coherent X-ray diffraction microscopy, i. e., ptychography, the achievable scan area at a given spatial resolution is limited by the coherent photon flux of modern X-ray sources. Multibeam X-ray ptychography can improve the scanning speed by scanning the sample with several parallel mutually incoherent beams, e. g., generated by illuminating multiple focusing optics in parallel by a partially coherent beam. The main difficulty with this scheme is the robust separation of the superimposed signals from the different beams, especially when the beams and the illuminated sample areas are quite similar. We overcome this difficulty by encoding each of the probing beams with its own X-ray phase plate. This helps the algorithm to robustly reconstruct the multibeam data. We compare the coded multibeam scans to uncoded multibeam and single beam scans, demonstrating the enhanced performance on a microchip sample with regular and repeating structures.

Tracking dynamic structural changes in catalysis by rapid 2D-XANES microscopy.

Many processes and materials in heterogeneous catalysis undergo dynamic structural changes depending on their chemical environment. Monitoring such dynamic changes can be challenging using conventional spectroscopic characterization tools, due to the high time resolution required. Here, a high-resolution 2D X-ray camera operating at 50 Hz full-frame rate was synchronized with a QEXAFS monochromator, enabling rapid spectro-microscopic imaging with chemical contrast over individual pixels. This was used to monitor chemical gradients within a model Pt/Al2O3 catalyst during catalytic partial oxidation of methane to synthesis gas. The transition from methane combustion (partly oxidized Pt) to combustion-reforming and partial oxidation (fully reduced Pt) was observed by a characteristic reduction front, which progressed from the end of the catalyst bed towards its beginning on the second time scale. The full-field QEXAFS imaging method applied here allows acquisition of entire XANES spectra `on the fly' in a rapid and spatially resolved manner. The combination of high spatial and temporal resolution with spectroscopic data offers new opportunities for observing dynamic processes in catalysts and other functional materials at work. The methodology is flexible and can be applied at beamlines equipped with a QEXAFS or other fast-scanning monochromators and a suitable sample environment for gas phase analytics to allow for catalytic studies at the same time.

Planar refractive lenses made of SiC for high intensity nanofocusing.

We report on the manufacturing and testing of the first nanofocusing refractive lenses made of single-crystal silicon carbide. We introduce the fabrication process based on lithography, followed by deep isotropic etching. The lenses were characterized at the energy of 12 keV at the beamline P06 of the synchrotron radiation source PETRA III. A focal spot of 186 nm×275 nm has been achieved with a lens working distance of 29 mm.

Nanofocusing with aberration-corrected rotationally parabolic refractive X-ray lenses. Corrigendum.

A correction in the paper by Seiboth et al. [(2018). J. Synchrotron Rad. 25, 108-115] is made.

Erratum: Ossig, C., et al. Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In, Ga)Se2 Solar Cell. Materials 2021, 14, 228.

In the original version of our article [...].

Single-pulse phase-contrast imaging at free-electron lasers in the hard X-ray regime.

X-ray free-electron lasers (XFELs) have opened up unprecedented opportunities for time-resolved nano-scale imaging with X-rays. Near-field propagation-based imaging, and in particular near-field holography (NFH) in its high-resolution implementation in cone-beam geometry, can offer full-field views of a specimen's dynamics captured by single XFEL pulses. To exploit this capability, for example in optical-pump/X-ray-probe imaging schemes, the stochastic nature of the self-amplified spontaneous emission pulses, i.e. the dynamics of the beam itself, presents a major challenge. In this work, a concept is presented to address the fluctuating illumination wavefronts by sampling the configuration space of SASE pulses before an actual recording, followed by a principal component analysis. This scheme is implemented at the MID (Materials Imaging and Dynamics) instrument of the European XFEL and time-resolved NFH is performed using aberration-corrected nano-focusing compound refractive lenses. Specifically, the dynamics of a micro-fluidic water-jet, which is commonly used as sample delivery system at XFELs, is imaged. The jet exhibits rich dynamics of droplet formation in the break-up regime. Moreover, pump-probe imaging is demonstrated using an infrared pulsed laser to induce cavitation and explosion of the jet.

Multi-slice ptychography enables high-resolution measurements in extended chemical reactors.

Ptychographic X-ray microscopy is an ideal tool to observe chemical processes under in situ conditions. Chemical reactors, however, are often thicker than the depth of field, limiting the lateral spatial resolution in projection images. To overcome this limit and reach higher lateral spatial resolution, wave propagation within the sample environment has to be taken into account. Here, we demonstrate this effect recording a ptychographic projection of copper(I) oxide nanocubes grown on two sides of a polyimide foil. Reconstructing the nanocubes using the conventional ptychographic model shows the limitation in the achieved resolution due to the thickness of the foil. Whereas, utilizing a multi-slice approach unambiguously separates two sharper reconstructions of nanocubes on both sides of the foil. Moreover, we illustrate how ptychographic multi-slice reconstructions are crucial for high-quality imaging of chemical processes by ex situ studying copper(I) oxide nanocubes grown on the walls of a liquid cell.

Four-Fold Multi-Modal X-ray Microscopy Measurements of a Cu(In,Ga)Se2 Solar Cell.

Inhomogeneities and defects often limit the overall performance of thin-film solar cells. Therefore, sophisticated microscopy approaches are sought to characterize performance and defects at the nanoscale. Here, we demonstrate, for the first time, the simultaneous assessment of composition, structure, and performance in four-fold multi-modality. Using scanning X-ray microscopy of a Cu(In,Ga)Se2 (CIGS) solar cell, we measured the elemental distribution of the key absorber elements, the electrical and optical response, and the phase shift of the coherent X-rays with nanoscale resolution. We found structural features in the absorber layer-interpreted as voids-that correlate with poor electrical performance and point towards defects that limit the overall solar cell efficiency.

Hard X-ray wavefront correction via refractive phase plates made by additive and subtractive fabrication techniques.

Modern subtractive and additive manufacturing techniques present new avenues for X-ray optics with complex shapes and patterns. Refractive phase plates acting as glasses for X-ray optics have been fabricated, and spherical aberration in refractive X-ray lenses made from beryllium has been successfully corrected. A diamond phase plate made by femtosecond laser ablation was found to improve the Strehl ratio of a lens stack with a numerical aperture (NA) of 0.88 × 10-3 at 8.2 keV from 0.1 to 0.7. A polymer phase plate made by additive printing achieved an increase in the Strehl ratio of a lens stack at 35 keV with NA of 0.18 × 10-3 from 0.15 to 0.89, demonstrating diffraction-limited nanofocusing at high X-ray energies.

PtyNAMi: ptychographic nano-analytical microscope.

Ptychographic X-ray imaging at the highest spatial resolution requires an optimal experimental environment, providing a high coherent flux, excellent mechanical stability and a low background in the measured data. This requires, for example, a stable performance of all optical components along the entire beam path, high temperature stability, a robust sample and optics tracking system, and a scatter-free environment. This contribution summarizes the efforts along these lines to transform the nanoprobe station on beamline P06 (PETRA III) into the ptychographic nano-analytical microscope (PtyNAMi).

Unraveling the Spatial Distribution of Catalytic Non-Cubic Au Phases in a Bipyramidal Microcrystallite by X-ray Diffraction Microscopy.

Tuning of crystal structures and shapes of submicrometer-sized noble metals have revealed fascinating catalytic, optical, electrical, and magnetic properties that enable developments of environmentally friendly and durable nanotechnological applications. Several attempts have been made to stabilize Au, knowing its extraordinary stability in its conventional face-centered cubic (fcc) lattice, into different lattices, particularly to develop Au-based catalysis for industry. Here, we report the results from scanning X-ray diffraction microscopy (SXDM) measurements on an ambient-stable penta-twinned bipyramidal Au microcrystallite (about 1.36 µm in length and 230 nm in diameter) stabilized in noncubic lattice, exhibiting catalytic properties. With more than 82% of the crystal volume, the majority crystallite structure is identified as body-centered orthorhombic (bco), while the remainder is the standard fcc. A careful analysis of the diffraction maps reveals that the tips are made up of fcc, while the body contains mainly bco with very high strain. The reported structural imaging technique of representative single crystallite will be useful to investigate the growth mechanism of similar multiphase nano- and micrometer-sized crystals.

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.