Pressure-induced nonmonotonic cross-over of steady relaxation dynamics in a metallic glass.

Relaxation dynamics, as a key to understand glass formation and glassy properties, remains an elusive and challenging issue in condensed matter physics. In this work, in situ high-pressure synchrotron high-energy X-ray photon correlation spectroscopy has been developed to probe the atomic-scale relaxation dynamics of a cerium-based metallic glass during compression. Although the sample density continuously increases, the collective atomic motion initially slows down as generally expected and then counterintuitively accelerates with further compression (density increase), showing an unusual nonmonotonic pressure-induced steady relaxation dynamics cross-over at ~3 GPa. Furthermore, by combining in situ high-pressure synchrotron X-ray diffraction, the relaxation dynamics anomaly is evidenced to closely correlate with the dramatic changes in local atomic structures during compression, rather than monotonically scaling with either sample density or overall stress level. These findings could provide insight into relaxation dynamics and their relationship with local atomic structures of glasses.

High-pressure X-ray photon correlation spectroscopy at fourth-generation synchrotron sources.

A new experimental setup combining X-ray photon correlation spectroscopy (XPCS) in the hard X-ray regime and a high-pressure sample environment has been developed to monitor the pressure dependence of the internal motion of complex systems down to the atomic scale in the multi-gigapascal range, from room temperature to 600 K. The high flux of coherent high-energy X-rays at fourth-generation synchrotron sources solves the problems caused by the absorption of diamond anvil cells used to generate high pressure, enabling the measurement of the intermediate scattering function over six orders of magnitude in time, from 10-3 s to 103 s. The constraints posed by the high-pressure generation such as the preservation of X-ray coherence, as well as the sample, pressure and temperature stability, are discussed, and the feasibility of high-pressure XPCS is demonstrated through results obtained on metallic glasses.

Implications of disturbed photon-counting statistics of Eiger detectors for X-ray speckle visibility experiments.

This paper reports on coherent scattering experiments in the low-count regime with less than one photon per pixel per acquisition on average, conducted with two detectors based on the Eiger single-photon-counting chip. The obtained photon-count distributions show systematic deviations from the expected Poisson-gamma distribution, which result in a strong overestimation of the measured speckle contrast. It is shown that these deviations originate from an artificial increase of double-photon events, which is proportional to the detected intensity and inversely proportional to the exposure time. The observed miscounting effect may have important implications for new coherent scattering experiments emerging with the advent of high-brilliance X-ray sources. Different correction schemes are discussed in order to obtain the correct photon distributions from the data. A successful correction is demonstrated with the measurement of Brownian motion from colloidal particles using X-ray speckle visibility spectroscopy.

In situ coherent X-ray diffraction imaging of radiation-induced mass loss in metal-polymer composite spheres.

A major limitation to the use of coherent X-ray diffraction imaging (CXDI) for imaging soft materials like polymers and biological tissue is that the radiation can cause extensive damage to the sample under investigation. In this study, CXDI has been used to monitor radiation-induced structural changes in metal-coated poly(methyl methacrylate) microspheres. Using a coherent undulator X-ray beam with 8.10 keV photon energy, 14 tomograms at a resolution of ∼30 nm were measured consecutively, which resulted in an accumulated dose of 30 GGy. The three-dimensional images confirmed that the polymer core was strongly affected by the absorbed dose, giving pronounced mass loss. Specifically, as the metal-polymer composite was exposed to the X-ray beam, a bubble-like region of reduced density grew within the composite, almost filling the entire volume within the thin metallic shell in the last tomogram. The bubble seemed to have its initiation point at a hole in the metal coating, emphasizing that the free polymer surface plays an important role in the degradation process. The irradiation of an uncoated polystyrene microsphere gave further evidence for mass loss at the free surface as the radius decreased with increased dose. The CXDI study was complemented by X-ray photon correlation spectroscopy, which proved efficient in establishing exposure dose limits. Our results demonstrate that radiation-induced structural changes at the tens of nanometer scale in soft materials can be followed as a function of dose, which is important for the further development of soft-matter technology.

Anti-Aging in Ultrastable Metallic Glasses.

As ultrastable metallic glasses (UMGs) are promising candidates to solve the stability issues of conventional metallic glasses, their study is of exceptional interest. By means of x-ray photon correlation spectroscopy, we have investigated the stability of UMGs at the atomic level. We find a clear signature of ultrastability at the atomic level that results in slower relaxation dynamics of UMGs with respect to conventional (rapidly quenched) metallic glasses, and in a peculiar acceleration of the dynamics by near T_{g} annealing. This surprising phenomenon, called here anti-aging, can be understood in the framework of the potential energy landscape. For all samples, the structural relaxation process can be described with a highly compressed shape of the density fluctuations, unaffected by thermal treatments and regardless of the ultrastability of the glass.

High-resolution coherent x-ray diffraction imaging of metal-coated polymer microspheres.

Coherent x-ray diffraction imaging (CXDI) is becoming an important 3D quantitative microscopy technique, allowing structural investigation of a wide range of delicate mesoscale samples that cannot be imaged by other techniques like electron microscopy. Here we report high-resolution 3D CXDI performed on spherical microcomposites consisting of a polymer core coated with a triple layer of nickel-gold-silica. These composites are of high interest to the microelectronics industry, where they are applied in conducting adhesives as fine-pitch electrical contacts-which requires an exceptional degree of uniformity and reproducibility. Experimental techniques that can assess the state of the composites non-destructively, preferably also while embedded in electronic chips, are thus in high demand. We demonstrate that using CXDI, all four different material components of the composite could be identified, with radii matching well to the nominal specifications of the manufacturer. Moreover, CXDI provided detailed maps of layer thicknesses, roughnesses, and defects such as holes, thus also facilitating cross-layer correlations. The side length of the voxels in the reconstruction, given by the experimental geometry, was 16 nm. The effective resolution enabled resolving even the thinnest coating layer of ∼20 nm nominal width. We discuss critically the influence of the weak phase approximation and the projection approximation on the reconstructed electron density estimates, demonstrating that the latter has to be employed. We conclude that CXDI has excellent potential as a metrology tool for microscale composites.

Wavefront metrology for coherent hard X-rays by scanning a microsphere.

Characterization of the wavefront of an X-ray beam is of primary importance for all applications where coherence plays a major role. Imaging techniques based on numerically retrieving the phase from interference patterns are often relying on an a-priori assumption of the wavefront shape. In Coherent X-ray Diffraction Imaging (CXDI) a planar incoming wave field is often assumed for the inversion of the measured diffraction pattern, which allows retrieving the real space image via simple Fourier transformation. It is therefore important to know how reliable the plane wave approximation is to describe the real wavefront. Here, we demonstrate that the quantitative wavefront shape and flux distribution of an X-ray beam used for CXDI can be measured by using a micrometer size metal-coated polymer sphere serving in a similar way as the hole array in a Hartmann wavefront sensor. The method relies on monitoring the shape and center of the scattered intensity distribution in the far field using a 2D area detector while raster-scanning the microsphere with respect to the incoming beam. The reconstructed X-ray wavefront was found to have a well-defined central region of approximately 16 µm diameter and a weaker, asymmetric, intensity distribution extending 30 µm from the beam center. The phase front distortion was primarily spherical with an effective radius of 0.55 m which matches the distance to the last upstream beam-defining slit, and could be accurately represented by Zernike polynomials.

Anisotropic de Gennes Narrowing in Confined Fluids.

The collective diffusion of dense fluids in spatial confinement is studied by combining high-energy (21 keV) x-ray photon correlation spectroscopy and small-angle x-ray scattering from colloid-filled microfluidic channels. We find the structural relaxation in confinement to be slower compared to the bulk. The collective dynamics is wave vector dependent, akin to the de Gennes narrowing typically observed in bulk fluids. However, in stark contrast to the bulk, the structure factor and de Gennes narrowing in confinement are anisotropic. These experimental observations are essential in order to develop a microscopic theoretical description of collective diffusion of dense fluids in confined geometries.

The Extremely Brilliant Source storage ring of the European Synchrotron Radiation Facility.

The Extremely Brilliant Source (EBS) is the experimental implementation of the novel Hybrid Multi Bend Achromat (HMBA) storage ring magnetic lattice concept, which has been realised at European Synchrotron Radiation Facility. We present its successful commissioning and first operation. We highlight the strengths of the HMBA design and compare them to the previous designs, on which most operational synchrotron X-ray sources are based. We report on the EBS storage ring's significantly improved horizontal electron beam emittance and other key beam parameters. EBS extends the reach of synchrotron X-ray science confirming the HMBA concept for future facility upgrades and new constructions.

X-ray cross correlation analysis uncovers hidden local symmetries in disordered matter.

We explore the different local symmetries in colloidal glasses beyond the standard pair correlation analysis. Using our newly developed X-ray cross correlation analysis (XCCA) concept together with brilliant coherent X-ray sources, we have been able to access and classify the otherwise hidden local order within disorder. The emerging local symmetries are coupled to distinct momentum transfer (Q) values, which do not coincide with the maxima of the amorphous structure factor. Four-, 6-, 10- and, most prevalently, 5-fold symmetries are observed. The observation of dynamical evolution of these symmetries forms a connection to dynamical heterogeneities in glasses, which is far beyond conventional diffraction analysis. The XCCA concept opens up a fascinating view into the world of disorder and will definitely allow, with the advent of free electron X-ray lasers, an accurate and systematic experimental characterization of the structure of the liquid and glass states.

Self-transformation of solid CaCO3 microspheres into core-shell and hollow hierarchical structures revealed by coherent X-ray diffraction imaging.

The self-transformation of solid microspheres into complex core-shell and hollow architectures cannot be explained by classical Ostwald ripening alone. Here, coherent X-ray diffraction imaging and 3D X-ray fluorescence were used to visualize in 3D the formation of hollow microparticles of calcium carbonate in the presence of polystyrene sulfonate (PSS). During the dissolution of the core made from 10-25 nm crystals, the shell developed a global spheroidal shape composed of an innermost layer of 30 nm particles containing high PSS content on which oriented vaterite crystals grew with their c axis mainly oriented along the meridians. The stabilizing role of PSS and the minimization of the intercrystal dipolar energy can explain in combination with Ostwald ripening the formation of these sophisticated structures as encountered in many systems such as ZnO, TiO2, Fe2O3, Co3O4, MnO2, Cu2O, ZnS, CaCO3 and Ca8H2(PO4)6·5H2O.

Development of a hard X-ray delay line for X-ray photon correlation spectroscopy and jitter-free pump-probe experiments at X-ray free-electron laser sources.

A hard X-ray delay line capable of splitting and delaying single X-ray pulses has been developed with the aim of performing X-ray photon correlation spectroscopy (XPCS) and X-ray pump-probe experiments at hard X-ray free-electron laser sources. The performance of the device was tested with 8.39 keV synchrotron radiation. Time delays up to 2.95 ns have been demonstrated. The feasibility of the device for performing XPCS studies was tested by recording static speckle patterns. The achieved speckle contrast of 56% indicates the possibility of performing ultra-fast XPCS studies with the delay line.

Nanoscale imaging of shale fragments with coherent X-ray diffraction.

Despite the abundance of shales in the Earth's crust and their industrial and environmental importance, their microscale physical properties are poorly understood, owing to the presence of many structurally related mineral phases and a porous network structure spanning several length scales. Here, the use of coherent X-ray diffraction imaging (CXDI) to study the internal structure of microscopic shale fragments is demonstrated. Simultaneous wide-angle X-ray diffraction (WAXD) measurement facilitated the study of the mineralogy of the shale microparticles. It was possible to identify pyrite nanocrystals as inclusions in the quartz-clay matrix and the volume of closed unconnected pores was estimated. The combined CXDI-WAXD analysis enabled the establishment of a correlation between sample morphology and crystallite shape and size. The results highlight the potential of the combined CXDI-WAXD approach as an upcoming imaging modality for 3D nanoscale studies of shales and other geological formations via serial measurements of microscopic fragments.

Cryogenic X-ray diffraction microscopy for biological samples.

X-ray diffraction microscopy (XDM) is well suited for nondestructive, high-resolution biological imaging, especially for thick samples, with the high penetration power of x rays and without limitations imposed by a lens. We developed nonvacuum, cryogenic (cryo-) XDM with hard x rays at 8 keV and report the first frozen-hydrated imaging by XDM. By preserving samples in amorphous ice, the risk of artifacts associated with dehydration or chemical fixation is avoided, ensuring the imaging condition closest to their natural state. The reconstruction shows internal structures of intact D. radiodurans bacteria in their natural contrast.

Bimodal Porosity and Stability of a TiO2 Gig-Lox Sponge Infiltrated with Methyl-Ammonium Lead Iodide Perovskite.

We created a blend between a TiO2 sponge with bimodal porosity and a Methyl-Ammonium Lead Iodide (MAPbI3) perovskite. The interpenetration of the two materials is effective thanks to the peculiar sponge structure. During the early stages of the growth of the TiO2 sponge, the formation of 5-10 nm-large TiO2 auto-seeds is observed which set the micro-porosity (<5 nm) of the layer, maintained during further growth. In a second stage, the auto-seeds aggregate into hundreds-of-nm-large meso-structures by their mutual shadowing of the grazing Ti flux for local oxidation. This process generates meso-pores (10-100 nm) treading across the growing layer, as accessed by tomographic synchrotron radiation coherent X-ray imaging and environmental ellipsometric porosimetry. The distributions of pore size are extracted before (>47% V) and after MAPbI3 loading, and after blend ageing, unfolding a starting pore filling above 80% in volume. The degradation of the perovskite in the blend follows a standard path towards PbI2 accompanied by the concomitant release of volatile species, with an activation energy of 0.87 eV under humid air. The use of dry nitrogen as environmental condition has a positive impact in increasing this energy by ~0.1 eV that extends the half-life of the material to 7 months under continuous operation at 60 °C.

Mechanical Competence and Bone Quality Develop During Skeletal Growth.

Bone fracture risk is influenced by bone quality, which encompasses bone's composition as well as its multiscale organization and architecture. Aging and disease deteriorate bone quality, leading to reduced mechanical properties and higher fracture incidence. Largely unexplored is how bone quality and mechanical competence progress during longitudinal bone growth. Human femoral cortical bone was acquired from fetal (n = 1), infantile (n = 3), and 2- to 14-year-old cases (n = 4) at the mid-diaphysis. Bone quality was assessed in terms of bone structure, osteocyte characteristics, mineralization, and collagen orientation. The mechanical properties were investigated by measuring tensile deformation at multiple length scales via synchrotron X-ray diffraction. We find dramatic differences in mechanical resistance with age. Specifically, cortical bone in 2- to 14-year-old cases exhibits a 160% greater stiffness and 83% higher strength than fetal/infantile cases. The higher mechanical resistance of the 2- to 14-year-old cases is associated with advantageous bone quality, specifically higher bone volume fraction, better micronscale organization (woven versus lamellar), and higher mean mineralization compared with fetal/infantile cases. Our study reveals that bone quality is superior after remodeling/modeling processes convert the primary woven bone structure to lamellar bone. In this cohort of female children, the microstructural differences at the femoral diaphysis were apparent between the 1- to 2-year-old cases. Indeed, the lamellar bone in 2- to 14-year-old cases had a superior structural organization (collagen and osteocyte characteristics) and composition for resisting deformation and fracture than fetal/infantile bone. Mechanistically, the changes in bone quality during longitudinal bone growth lead to higher fracture resistance because collagen fibrils are better aligned to resist tensile forces, while elevated mean mineralization reinforces the collagen scaffold. Thus, our results reveal inherent weaknesses of the fetal/infantile skeleton signifying its inferior bone quality. These results have implications for pediatric fracture risk, as bone produced at ossification centers during children's longitudinal bone growth could display similarly weak points. © 2019 American Society for Bone and Mineral Research.

Anisotropic hydrodynamic function of dense confined colloids.

Dense colloidal dispersions exhibit complex wave-vector-dependent diffusion, which is controlled by both direct particle interactions and indirect nonadditive hydrodynamic interactions mediated by the solvent. In bulk the hydrodynamic interactions are probed routinely, but in confined geometries their studies have been hitherto hindered by additional complications due to confining walls. Here we solve this issue by combining high-energy x-ray photon correlation spectroscopy and small-angle x-ray-scattering experiments on colloid-filled microfluidic channels to yield the confined fluid's hydrodynamic function in the short-time limit. Most importantly, we find the confined fluid to exhibit a strongly anisotropic hydrodynamic function, similar to its anisotropic structure factor. This observation is important in order to guide future theoretical research.

Lensless Tomographic Imaging of Near Surface Structures of Frozen Hydrated Malaria-Infected Human Erythrocytes by Coherent X-Ray Diffraction Microscopy.

Lensless, coherent X-ray diffraction microscopy has been drawing considerable attentions for tomographic imaging of whole human cells. In this study, we performed cryogenic coherent X-ray diffraction imaging of human erythrocytes with and without malaria infection. To shed light on structural features near the surface, "ghost cells" were prepared by the removal of cytoplasm. From two-dimensional images, we found that the surface of erythrocytes after 32 h of infection became much rougher compared to that of healthy, uninfected erythrocytes. The Gaussian roughness of an infected erythrocyte surface (69 nm) is about two times larger than that of an uninfected one (31 nm), reflecting the formation of protein knobs on infected erythrocyte surfaces. Three-dimensional tomography further enables to obtain images of the whole cells with no remarkable radiation damage, whose accuracy was estimated using phase retrieval transfer functions to be as good as 64 nm for uninfected and 80 nm for infected erythrocytes, respectively. Future improvements in phase retrieval algorithm, increase in degree of coherence, and higher flux in combination with complementary X-ray fluorescence are necessary to gain both structural and chemical details of mesoscopic architectures, such as cytoskeletons, membraneous structures, and protein complexes, in frozen hydrated human cells, especially under diseased states.

Light-induced structural changes in a monomeric bacteriophytochrome.

Phytochromes sense red light in plants and various microorganism. Light absorption causes structural changes within the protein, which alter its biochemical activity. Bacterial phytochromes are dimeric proteins, but the functional relevance of this arrangement remains unclear. Here, we use time-resolved X-ray scattering to reveal the solution structural change of a monomeric variant of the photosensory core module of the phytochrome from Deinococcus radiodurans. The data reveal two motions, a bend and a twist of the PHY domain with respect to the chromophore-binding domains. Infrared spectroscopy shows the refolding of the PHY tongue. We conclude that a monomer of the phytochrome photosensory core is sufficient to perform the light-induced structural changes. This implies that allosteric cooperation with the other monomer is not needed for structural activation. The dimeric arrangement may instead be intrinsic to the biochemical output domains of bacterial phytochromes.

Three-dimensional coherent X-ray diffractive imaging of whole frozen-hydrated cells.

A structural understanding of whole cells in three dimensions at high spatial resolution remains a significant challenge and, in the case of X-rays, has been limited by radiation damage. By alleviating this limitation, cryogenic coherent diffractive imaging (cryo-CDI) can in principle be used to bridge the important resolution gap between optical and electron microscopy in bio-imaging. Here, the first experimental demonstration of cryo-CDI for quantitative three-dimensional imaging of whole frozen-hydrated cells using 8 keV X-rays is reported. As a proof of principle, a tilt series of 72 diffraction patterns was collected from a frozen-hydrated Neospora caninum cell and the three-dimensional mass density of the cell was reconstructed and quantified based on its natural contrast. This three-dimensional reconstruction reveals the surface and internal morphology of the cell, including its complex polarized sub-cellular structure. It is believed that this work represents an experimental milestone towards routine quantitative three-dimensional imaging of whole cells in their natural state with spatial resolutions in the tens of nanometres.