Ultrafast Three-Dimensional X-ray Imaging of Deformation Modes in ZnO Nanocrystals.

Imaging the dynamical response of materials following ultrafast excitation can reveal energy transduction mechanisms and their dissipation pathways, as well as material stability under conditions far from equilibrium. Such dynamical behavior is challenging to characterize, especially operando at nanoscopic spatiotemporal scales. In this letter, we use X-ray coherent diffractive imaging to show that ultrafast laser excitation of a ZnO nanocrystal induces a rich set of deformation dynamics including characteristic "hard" or inhomogeneous and "soft" or homogeneous modes at different time scales, corresponding respectively to the propagation of acoustic phonons and resonant oscillation of the crystal. By integrating the 3D nanocrystal structure obtained from the ultrafast X-ray measurements with a continuum thermo-electro-mechanical finite element model, we elucidate the deformation mechanisms following laser excitation, in particular, a torsional mode that generates a 50% greater electric potential gradient than that resulting from the flexural mode. Understanding of the time-dependence of these mechanisms on ultrafast scales has significant implications for development of new materials for nanoscale power generation.

Ultrafast Three-Dimensional Integrated Imaging of Strain in Core/Shell Semiconductor/Metal Nanostructures.

Visualizing the dynamical response of material heterointerfaces is increasingly important for the design of hybrid materials and structures with tailored properties for use in functional devices. In situ characterization of nanoscale heterointerfaces such as metal-semiconductor interfaces, which exhibit a complex interplay between lattice strain, electric potential, and heat transport at subnanosecond time scales, is particularly challenging. In this work, we use a laser pump/X-ray probe form of Bragg coherent diffraction imaging (BCDI) to visualize in three-dimension the deformation of the core of a model core/shell semiconductor-metal (ZnO/Ni) nanorod following laser heating of the shell. We observe a rich interplay of radial, axial, and shear deformation modes acting at different time scales that are induced by the strain from the Ni shell. We construct experimentally informed models by directly importing the reconstructed crystal from the ultrafast experiment into a thermo-electromechanical continuum model. The model elucidates the origin of the deformation modes observed experimentally. Our integrated imaging approach represents an invaluable tool to probe strain dynamics across mixed interfaces under operando conditions.

Speciation of Soil Phosphorus Assessed by XANES Spectroscopy at Different Spatial Scales.

Precise management of soil phosphorus (P) to meet competing demands of agriculture and environmental protection can benefit from more comprehensive characterization of P speciation in soils. Our objectives were to provide spatial context for spectroscopic analyses of soil P speciation in relation to molecular-scale species and landscape-scale management of P, and to compare soil P-species diversity from spectroscopic measurements at submicron and millimeter scales. The spatial range of ∼26 orders of magnitude between atomic and field scales presents a challenge to upscaling and downscaling information from spectroscopic analyses of soils. Scanning fluorescence X-ray microscopy images of a 50-µm × 45-µm area of an organic soil sample showed heterogeneous distributions of P, Al, and Si. Microscale X-ray absorption near edge structure (µ-XANES) spectra collected at the P K-edge from 12 spots on the soil sample exhibited diverse features that indicated variations in highly localized P speciation. Linear combination fitting analysis of the µ-XANES spectra included various proportions of three standards that appeared in fits for most spots and five standards that appeared in fits for one spot each. The fit to a bulk-soil spectrum was dominated by two of the common standards in the µ-XANES fits, and a fit to the sum of µ-XANES spectra included four of the standards. These results illustrate a gain in P species sensitivity from spatially resolved XANES analysis. Integrating spectroscopic analyses from multiple scales determines soil P species diversity and will ultimately help connect speciation to the chemical reactivity and mobility of P in soils.

Hard X-ray polarizer to enable simultaneous three-dimensional nanoscale imaging of magnetic structure and lattice strain.

Recent progress in the development of dichroic Bragg coherent diffractive imaging, a new technique for simultaneous three-dimensional imaging of strain and magnetization at the nanoscale, is reported. This progress includes the installation of a diamond X-ray phase retarder at beamline 34-ID-C of the Advanced Photon Source. The performance of the phase retarder for tuning X-ray polarization is demonstrated with temperature-dependent X-ray magnetic circular dichroism measurements on a gadolinium foil in transmission and on a Gd5Si2Ge2 crystal in diffraction geometry with a partially coherent, focused X-ray beam. Feasibility tests for dichroic Bragg coherent diffractive imaging are presented. These tests include (1) using conventional Bragg coherent diffractive imaging to determine whether the phase retarder introduces aberrations using a nonmagnetic gold nanocrystal as a control sample, and (2) collecting coherent diffraction patterns of a magnetic Gd5Si2Ge2 nanocrystal with left- and right-circularly polarized X-rays. Future applications of dichroic Bragg coherent diffractive imaging for the correlation of strain and lattice defects with magnetic ordering and inhomogeneities are considered.

Single-view phase retrieval of an extended sample by exploiting edge detection and sparsity.

We propose a new approach to robustly retrieve the exit wave of an extended sample from its coherent diffraction pattern by exploiting sparsity of the sample's edges. This approach enables imaging of an extended sample with a single view, without ptychography. We introduce nonlinear optimization methods that promote sparsity, and we derive update rules to robustly recover the sample's exit wave. We test these methods on simulated samples by varying the sparsity of the edge-detected representation of the exit wave. Our tests illustrate the strengths and limitations of the proposed method in imaging extended samples.

Core-shell strain structure of zeolite microcrystals.

Zeolites are crystalline aluminosilicate minerals featuring a network of 0.3-1.5-nm-wide pores, used in industry as catalysts for hydrocarbon interconversion, ion exchangers, molecular sieves and adsorbents. For improved applications, it is highly useful to study the distribution of internal local strains because they sensitively affect the rates of adsorption and diffusion of guest molecules within zeolites. Here, we report the observation of an unusual triangular deformation field distribution in ZSM-5 zeolites by coherent X-ray diffraction imaging, showing the presence of a strain within the crystal arising from the heterogeneous core-shell structure, which is supported by finite element model calculation and confirmed by fluorescence measurement. The shell is composed of H-ZSM-5 with intrinsic negative thermal expansion whereas the core exhibits a different thermal expansion behaviour due to the presence of organic template residues, which usually remain when the starting materials are insufficiently calcined. Engineering such strain effects could have a major impact on the design of future catalysts.

Ptychographic overlap constraint errors and the limits of their numerical recovery using conjugate gradient descent methods.

Ptychographic coherent x-ray diffractive imaging is a form of scanning microscopy that does not require optics to image a sample. A series of scanned coherent diffraction patterns recorded from multiple overlapping illuminated regions on the sample are inverted numerically to retrieve its image. The technique recovers the phase lost by detecting the diffraction patterns by using experimentally known constraints, in this case the measured diffraction intensities and the assumed scan positions on the sample. The spatial resolution of the recovered image of the sample is limited by the angular extent over which the diffraction patterns are recorded and how well these constraints are known. Here, we explore how reconstruction quality degrades with uncertainties in the scan positions. We show experimentally that large errors in the assumed scan positions on the sample can be numerically determined and corrected using conjugate gradient descent methods. We also explore in simulations the limits, based on the signal to noise of the diffraction patterns and amount of overlap between adjacent scan positions, of just how large these errors can be and still be rendered tractable by this method.

Fresnel coherent diffractive imaging of elemental distributions in nanoscale binary compounds.

We report quantitative determination of elemental distribution in binary compounds with nano meter scale spatial resolution using x-ray Fresnel coherent diffractive imaging (FCDI). We show that the quantitative magnitude and phase values of the x-ray wave exiting an object determined by FCDI can be utilized to obtain full-field atomic density maps of each element independently. The proposed method was demonstrated by reconstructing the density maps of Pt and NiO in a Pt-NiO binary compound with about 18 nm spatial resolution.

Dichroic coherent diffractive imaging.

Understanding electronic structure at the nanoscale is crucial to untangling fundamental physics puzzles such as phase separation and emergent behavior in complex magnetic oxides. Probes with the ability to see beyond surfaces on nanometer length and subpicosecond time scales can greatly enhance our understanding of these systems and will undoubtedly impact development of future information technologies. Polarized X-rays are an appealing choice of probe due to their penetrating power, elemental and magnetic specificity, and high spatial resolution. The resolution of traditional X-ray microscopes is limited by the nanometer precision required to fabricate X-ray optics. Here we present a novel approach to lensless imaging of an extended magnetic nanostructure, in which a scanned series of dichroic coherent diffraction patterns is recorded and numerically inverted to map its magnetic domain configuration. Unlike holographic methods, it does not require a reference wave or precision optics. In addition, it enables the imaging of samples with arbitrarily large spatial dimensions, at a spatial resolution limited solely by the coherent X-ray flux, wavelength, and stability of the sample with respect to the beam. It can readily be extended to nonmagnetic systems that exhibit circular or linear dichroism. We demonstrate this approach by imaging ferrimagnetic labyrinthine domains in a Gd/Fe multilayer with perpendicular anisotropy and follow the evolution of the domain structure through part of its magnetization hysteresis loop. This approach is scalable to imaging with diffraction-limited resolution, a prospect rapidly becoming a reality in view of the new generation of phenomenally brilliant X-ray sources.

An Efficient Epilepsy Prediction Model on European Dataset With Model Evaluation Considering Seizure Types.

This paper develops a computationally efficient model for automatic patient-specific seizure prediction using a two-layer LSTM from multichannel intracranial electroencephalogram time-series data. We decrease the number of parameters by employing a smaller input size and fewer electrodes, thereby making the model a viable option for wearable and implantable devices. We test the proposed prediction model on 26 patients from the European iEEG dataset, which is the largest epileptic seizure dataset. We also apply an automatic preprocessing technique based on a common average reference to remove artifacts from this dataset. The simulation results show that the model with its simple structure in conjunction with the mean post-processing procedure performed the best, with an average AUC of 0.885. This study is the first that utilizes the European database for epilepsy prediction application and the first that analyzes the effect of the seizure type on the system performance and demonstrates that the seizure type has a considerable impact.

Quantitative 3D elemental microtomography of Cyclotella meneghiniana at 400-nm resolution.

X-ray fluorescence tomography promises to map elemental distributions in unstained and unfixed biological specimens in three dimensions at high resolution and sensitivity, offering unparalleled insight in medical, biological, and environmental sciences. X-ray fluorescence tomography of biological specimens has been viewed as impractical-and perhaps even impossible for routine application-due to the large time required for scanning tomography and significant radiation dose delivered to the specimen during the imaging process. Here, we demonstrate submicron resolution X-ray fluorescence tomography of a whole unstained biological specimen, quantifying three-dimensional distributions of the elements Si, P, S, Cl, K, Ca, Mn, Fe, Cu, and Zn in the freshwater diatom Cyclotella meneghiniana with 400-nm resolution, improving the spatial resolution by over an order of magnitude. The resulting maps faithfully reproduce cellular structure revealing unexpected patterns that may elucidate the role of metals in diatom biology and of diatoms in global element cycles. With anticipated improvements in data acquisition and detector sensitivity, such measurements could become routine in the near future.

A hard X-ray nanoprobe beamline for nanoscale microscopy.

The Hard X-ray Nanoprobe Beamline (or Nanoprobe Beamline) is an X-ray microscopy facility incorporating diffraction, fluorescence and full-field imaging capabilities designed and operated by the Center for Nanoscale Materials and the Advanced Photon Source at Sector 26 of the Advanced Photon Source at Argonne National Laboratory. This facility was constructed to probe the nanoscale structure of biological, environmental and material sciences samples. The beamline provides intense focused X-rays to the Hard X-ray Nanoprobe (or Nanoprobe) which incorporates Fresnel zone plate optics and a precision laser sensing and control system. The beamline operates over X-ray energies from 3 to 30 keV, enabling studies of most elements in the periodic table, with a particular emphasis on imaging transition metals.

Internal structure of an intact Convallaria majalis pollen grain observed with X-ray Fresnel coherent diffractive imaging.

We have applied Fresnel Coherent Diffractive Imaging (FCDI) to image an intact pollen grain from Convallaria majalis. This approach allows us to resolve internal structures without the requirement to chemically treat or slice the sample into thin sections. Coherent X-ray diffraction data from this pollen grain-composed of a hologram and higher resolution scattering information-was collected at a photon energy of 1820 eV and reconstructed using an iterative algorithm. A comparison with images recorded using transmission electron microscopy demonstrates that, while the resolution of these images is limited by the available flux and mechanical stability, we observed structures internal to the pollen grain-the intine/exine separations and pore dimensions-finer than 60 nm. The potential of this technique for further biological imaging applications is discussed.

Selectivity in biomineralization of barium and strontium.

The desmid green alga Closterium moniliferum belongs to a small number of organisms that form barite (BaSO(4)) or celestite (SrSO(4)) biominerals. The ability to sequester Sr in the presence of an excess of Ca is of considerable interest for the remediation of (90)Sr from the environment and nuclear waste. While most cells dynamically regulate the concentration of the second messenger Ca(2+) in the cytosol and various organelles, transport proteins rarely discriminate strongly between Ca, Sr, and Ba. Herein, we investigate how these ions are trafficked in C. moniliferum and how precipitation of (Ba,Sr)SO(4) crystals occurs in the terminal vacuoles. Towards this goal, we simultaneously visualize intracellular dynamics of multiple elements using X-ray fluorescence microscopy (XFM) of cryo-fixed/freeze-dried samples. We correlate the resulting elemental maps with ultrastructural information gleaned from freeze-fracture cryo-SEM of frozen-hydrated cells and use micro X-ray absorption near edge structure (micro-XANES) to determine sulfur speciation. We find that the kinetics of Sr uptake and efflux depend on external Ca concentrations, and Sr, Ba, and Ca show similar intracellular localization. A highly ion-selective cross-membrane transport step is not evident. Based on elevated levels of sulfate detected in the terminal vacuoles, we propose a "sulfate trap" model, where the presence of dissolved barium leads to preferential precipitation of (Ba,Sr)SO(4) due to its low solubility relative to SrSO(4) and CaSO(4). Engineering the sulfate concentration in the vacuole may thus be the most direct way to increase the Sr sequestered per cell, an important consideration in using desmids for phytoremediation of (90)Sr.

Phosphorus K-edge XANES spectroscopy of mineral standards.

Phosphorus K-edge X-ray absorption near-edge structure (XANES) spectroscopy was performed on phosphate mineral specimens including (a) twelve specimens from the apatite group covering a range of compositional variation and crystallinity; (b) six non-apatite calcium-rich phosphate minerals; (c) 15 aluminium-rich phosphate minerals; (d) ten phosphate minerals rich in either reduced iron or manganese; (e) four phosphate minerals rich in either oxidized iron or manganese; (f) eight phosphate minerals rich in either magnesium, copper, lead, zinc or rare-earth elements; and (g) four uranium phosphate minerals. The identity of all minerals examined in this study was independently confirmed using X-ray powder diffraction. Minerals were distinguished using XANES spectra with a combination of pre-edge features, edge position, peak shapes and post-edge features. Shared spectral features were observed in minerals with compositions dominated by the same specific cation. Analyses of apatite-group minerals indicate that XANES spectral patterns are not strongly affected by variations in composition and crystallinity typical of natural mineral specimens.

Dynamic sample imaging in coherent diffractive imaging.

As the resolution in coherent diffractive imaging improves, interexposure and intraexposure sample dynamics, such as motion, degrade the quality of the reconstructed image. Selecting data sets that include only exposures where tolerably little motion has occurred is an inefficient use of time and flux, especially when detector readout time is significant. We provide an experimental demonstration of an approach in which all images of a data set exhibiting sample motion are combined to improve the quality of a reconstruction. This approach is applicable to more general sample dynamics (including sample damage) that occur during measurement.

X-ray diffraction microscopy of magnetic structures.

We report the first proof-of-principle experiment of iterative phase retrieval from magnetic x-ray diffraction. By using the resonant x-ray excitation process and coherent x-ray scattering, we show that linearly polarized soft x rays can be used to image both the amplitude and the phase of magnetic domain structures. We recovered the magnetic structure of an amorphous terbium-cobalt thin film with a spatial resolution of about 75 nm at the Co L3 edge at 778 eV. In comparison with soft x-ray microscopy images recorded with Fresnel zone plate optics at better than 25 nm spatial resolution, we find qualitative agreement in the observed magnetic structure.

Phase-diverse coherent diffractive imaging: high sensitivity with low dose.

This Letter demonstrates that coherent diffractive imaging (CDI), in combination with phase-diversity methods, provides reliable and artefact free high-resolution images. Here, using x rays, experimental results show a threefold improvement in the available image contrast. Furthermore, in conditions requiring low imaging dose, it is demonstrated that phase-diverse CDI provides a factor of 2 improvement in comparison to previous CDI techniques.

Evolution of dealloying induced strain in nanoporous gold crystals.

We studied the evolution of dealloying-induced strain along the {111} in a Ag-Au nano-crystal in situ, during formation of nanoporous gold at the initial stage of dealloying using Bragg coherent X-ray diffractive imaging. The strain magnitude with maximum probability in the crystal doubled in 10 s of dealloying. Although formation of nano-pores just began at the surface, the greatest strain is located 60-80 nm deep within the crystal. Dealloying induced a compressive strain in this region, indicating volume shrinkage occurred during pore formation. The crystal interior showed a small tensile strain, which can be explained by tensile stresses induced by the non-dealloyed region upon the dealloyed region during volume reduction. A surface strain relaxation developed, attributed to atomic rearrangement during dealloying. This clearer understanding of the role of strain in the initial stages of formation of nanoporous gold by dealloying can be exploited for development of new sensors, battery electrodes, and materials for catalysis.

Domain morphology, boundaries, and topological defects in biophotonic gyroid nanostructures of butterfly wing scales.

Many organisms in nature have evolved sophisticated cellular mechanisms to produce photonic nanostructures and, in recent years, diverse crystalline symmetries have been identified and related to macroscopic optical properties. However, because we know little about the distributions of domain sizes, the orientations of photonic crystals, and the nature of defects in these structures, we are unable to make the connection between the nanostructure and its development and functionality. We report on nondestructive studies of the morphology of chitinous photonic crystals in butterfly wing scales. Using spatially and angularly resolved x-ray diffraction, we find that the domains are highly oriented with respect to the whole scale, indicating growth from scale boundaries. X-ray coherent diffractive imaging reveals two types of crystalline domain interfaces: abrupt changes between domains emerging from distinct nucleation sites and smooth transitions with edge dislocations presumably resulting from internal stresses during nanostructure development. Our study of the scale structure reveals new aspects of photonic crystal growth in butterfly wings and shows their similarity to block copolymer materials. It opens new avenues to exploration of fundamental processes underlying the growth of biological photonic nanostructures in a variety of species.