Current status of the TwinMic beamline at Elettra: a soft X‐ray transmission and emission microscopy station

The current status of the TwinMic beamline at Elettra synchrotron light source, that hosts the European twin X‐ray microscopy station, is reported. The X‐ray source, provided by a short hybrid undulator with source size and divergence intermediate between bending magnets and conventional undulators, is energy‐tailored using a collimated plane‐grating monochromator. The TwinMic spectromicroscopy experimental station combines scanning and full‐field imaging in a single instrument, with contrast modes such as absorption, differential phase, interference and darkfield. The implementation of coherent diffractive imaging modalities and ptychography is ongoing. Typically, scanning transmission X‐ray microscopy images are simultaneously collected in transmission and differential phase contrast and can be complemented by chemical and elemental analysis using across‐absorption‐edge imaging, X‐ray absorption near‐edge structure or low‐energy X‐ray fluorescence. The lateral resolutions depend on the particular imaging and contrast mode chosen. The TwinMic range of applications covers diverse research fields such as biology, biochemistry, medicine, pharmacology, environment, geochemistry, food, agriculture and materials science. They will be illustrated in the paper with representative results.     

Time‐resolved X‐ray microscopy of nanoparticle aggregates under oscillatory shear

Of all the current detection techniques with nanometre resolution, only X‐ray microscopy allows imaging of nanoparticles in suspension. Can it also be used to investigate structural dynamics? When studying the response to mechanical stimuli, the challenge lies in its application with a precision comparable with the spatial resolution. In the first shear experiments performed in an X‐ray microscope, this has been accomplished by inserting a piezo actuator driven shear cell into the focal plane of a scanning transmission X‐ray microscope. Thus shear‐induced re‐organization of magnetite nanoparticle aggregates could be demonstrated in suspension. As X‐ray microscopy proves suitable for studying structural change, new prospects open up in physics at small length scales.     

Towards tender X‐rays with Zernike phase‐contrast imaging of biological samples at 50 nm resolution

X‐ray microscopy is a commonly used method especially in material science application, where the large penetration depth of X‐rays is necessary for three‐dimensional structural studies of thick specimens with high‐Z elements. In this paper it is shown that full‐field X‐ray microscopy at 6.2 keV can be utilized for imaging of biological specimens with high resolution. A full‐field Zernike phase‐contrast microscope based on diffractive optics is used to study lipid droplet formation in hepatoma cells. It is shown that the contrast of the images is comparable with that of electron microscopy, and even better contrast at tender X‐ray energies between 2.5 keV and 4 keV is expected.     

A scanning transmission X‐ray microscope at the Pohang Light Source

A scanning transmission X‐ray microscope is operational at the 10A beamline at the Pohang Light Source. The 10A beamline provides soft X‐rays in the photon energy range 100–2000 eV using an elliptically polarized undulator. The practically usable photon energy range of the scanning transmission X‐ray microscopy (STXM) setup is from ～150 to ～1600 eV. With a zone plate of 25 nm outermost zone width, the diffraction‐limited space resolution, ～30 nm, is achieved in the photon energy range up to ～850 eV. In transmission mode for thin samples, STXM provides the element, chemical state and magnetic moment specific distributions, based on absorption spectroscopy. A soft X‐ray fluorescence measurement setup has been implemented in order to provide the elemental distribution of thicker samples as well as chemical state information with a space resolution of ～50 nm. A ptychography setup has been implemented in order to improve the space resolution down to 10 nm. Hardware setups and application activities of the STXM are presented.     

X‐ray spectromicroscopy with the scanning transmission X‐ray microscope at BESSY II

Using the scanning transmission X‐ray microscope at BESSY II, colloidal structures from a Chernozem soil have been studied with a spatial resolution around 60 nm and a spectral resolution of 1700 at the K‐absorption edge of carbon. Elemental mapping has been used to determine the distribution of organic matter within the colloidal structures. Spectra have been extracted from image stacks to obtain information about the chemical state. For the analysis of the latter, principal component analysis and cluster analysis have been applied. It was possible, for example, to discriminate clay particles against organic components.     

Poly(m‐Phenylene Isophthalamide) Ultrafine Fibers from an Ionic Liquid Solution by Dry‐Jet‐Wet‐Electrospinning

Ultrafine poly(m‐phenylene isophthalamide) (PMIA) fibers from PMIA solution in an ionic liquid via dry‐jet‐wet electrospinning technology are described. The morphology of the fibers with and without treatment in a coagulation water bath in the dry‐jet‐wet‐electrosinning process was observed by scanning electrical microscopy (SEM) and a high resolution optical microscope. The crystal structure of the fibers was analyzed by wide angle X‐ray diffraction (WAXD). The differences of morphologies and properties between the ultrafine fibers obtained by the electrospinning process and fibers from conventional wet‐spinning technology are discussed. The thermal properties of the ultrafine PMIA fibers were characterized by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).     

3D imaging of a rice pollen grain using transmission X‐ray microscopy

For the first time, the three‐dimensional (3D) ultrastructure of an intact rice pollen cell has been obtained using a full‐field transmission hard X‐ray microscope operated in Zernike phase contrast mode. After reconstruction and segmentation from a series of projection images, complete 3D structural information of a 35 µm rice pollen grain is presented at a resolution of ～100 nm. The reconstruction allows a clear differentiation of various subcellular structures within the rice pollen grain, including aperture, lipid body, mitochondrion, nucleus and vacuole. Furthermore, quantitative information was obtained about the distribution of cytoplasmic organelles and the volume percentage of each kind of organelle. These results demonstrate that transmission X‐ray microscopy can be quite powerful for non‐destructive investigation of 3D structures of whole eukaryotic cells.     

Enhancements in full‐field PIXE imaging—Large area elemental mapping with increased lateral resolution devoid of optics artefacts

The combination of a pn‐junction charge‐coupled device‐based pixel detector with a poly‐capillary X‐ray optics was installed and examined at the Helmholtz‐Zentrum Dresden‐Rossendorf. The set‐up is intended for particle‐induced X‐ray emission imaging to survey the trace elemental composition of flat/polished geological samples. In the standard configuration, a straight X‐ray optics (20 μm capillary diameter) is used to guide the emitted photons from the sample towards the detector with nearly 70 000 pixels. Their dimensions of 48 × 48 μm² are the main limitation of the lateral resolution. This limitation can be bypassed by applying a dedicated subpixel algorithm to recalculate the footprint of the photon's electron cloud in the detector. The lateral resolution is then mainly determined by the capillary's diameter. Nevertheless, images are still superimposed by the X‐ray optics pattern. The optics' capillaries are grouped in hexagonal bundles resulting in a reduced transmission of X‐rays in the boundary regions. This influence can be largely suppressed by combining a series of short measurements at slightly shifted positions using a precision stage and correcting the image data for this shifting. The use of a subpixel grid for the image reconstruction allows a further increase of the spatial resolution. This approach of image‐stacking and multiframe super‐resolution in combination with the subpixel correction algorithm is presented and illustrated with experimental data. Additionally, a flat‐field correction is shown to remove the remaining imaging inhomogeneity caused by non‐uniform X‐ray transmission. The described techniques can be used for all X‐ray spectrometry methods using an X‐ray camera to obtain high‐quality elemental images.     

Caltrop particles synthesized by photochemical reaction induced by X‐ray radiolysis

X‐ray radiolysis of a Cu(CH₃COO)₂ solution was observed to produce caltrop‐shaped particles of cupric oxide (CuO, Cu₂O), which were characterized using high‐resolution scanning electron microscopy and micro‐Raman spectrometry. X‐ray irradiation from a synchrotron source drove the room‐temperature synthesis of submicrometer‐ and micrometer‐scale cupric oxide caltrop particles from an aqueous Cu(CH₃COO)₂ solution spiked with ethanol. The size of the caltrop particles depended on the ratio of ethanol in the stock solution and the surface of the substrate. The results indicated that there were several synthetic routes to obtain caltrop particles, each associated with electron donation. The technique of X‐ray irradiation enables the rapid synthesis of caltrop cupric oxide particles compared with conventional synthetic methods.     

Fluorinated Eu‐Doped SnO2 Nanostructures with Simultaneous Phase and Shape Control and Improved Photoluminescence

Fluorinated Eu‐doped SnO₂ nanostructures with tunable morphology (shuttle‐like and ring‐like) are prepared by a hydrothermal method, using NaF as the morphology controlling agent. X‐ray diffraction, field‐emission scanning electron microscopy, high‐resolution transmission electron microscopy, X‐ray photoelectron spectroscopy, and energy dispersive spectroscopy are used to characterize their phase, shape, lattice structure, composition, and element distribution. The data suggest that Eu³⁺ ions are uniformly embedded into SnO₂ nanocrystallites either through substitution of Sn⁴⁺ ions or through formation of Eu‐F bonds, allowing for high‐level Eu³⁺ doping. Photoluminescence features such as transition intensity ratios and Stark splitting indicate diverse localization of Eu³⁺ ions in the SnO₂ nanoparticles, either in the crystalline lattice or in the grain boundaries. Due to formation of Eu‐F and Sn‐F bonds, the fluorinated surface of SnO₂ nanocrystallites efficiently inhibits the hydroxyl quenching effect, which accounts for their improved photoluminescence intensity.     

Imaging interfacial micro‐ and nano‐bubbles by scanning transmission soft X‐ray microscopy

Synchrotron‐based scanning transmission soft X‐ray microscopy (STXM) with nanometer resolution was used to investigate the existence and behavior of interfacial gas nanobubbles confined between two silicon nitride windows. The observed nanobubbles of SF₆ and Ne with diameters smaller than 2.5 µm were quite stable. However, larger bubbles became unstable and grew during the soft X‐ray imaging, indicating that stable nanobubbles may have a length scale, which is consistent with a previous report using atomic force microscopy [Zhang et al. (2010), Soft Matter, 6 , 4515–4519]. Here, it is shown that STXM is a promising technique for studying the aggregation of gases near the solid/water interfaces at the nanometer scale.     

Design and performance of an X‐ray scanning microscope at the Hard X‐ray Nanoprobe beamline of NSLS‐II

A hard X‐ray scanning microscope installed at the Hard X‐ray Nanoprobe beamline of the National Synchrotron Light Source II has been designed, constructed and commissioned. The microscope relies on a compact, high stiffness, low heat dissipation approach and utilizes two types of nanofocusing optics. It is capable of imaging with ～15 nm × 15 nm spatial resolution using multilayer Laue lenses and 25 nm × 26 nm resolution using zone plates. Fluorescence, diffraction, absorption, differential phase contrast, ptychography and tomography are available as experimental techniques. The microscope is also equipped with a temperature regulation system which allows the temperature of a sample to be varied in the range between 90 K and 1000 K. The constructed instrument is open for general users and offers its capabilities to the material science, battery research and bioscience communities.     

Full‐field X‐ray reflection microscopy of epitaxial thin‐films

Novel X‐ray imaging of structural domains in a ferroelectric epitaxial thin film using diffraction contrast is presented. The full‐field hard X‐ray microscope uses the surface scattering signal, in a reflectivity or diffraction experiment, to spatially resolve the local structure with 70 nm lateral spatial resolution and sub‐nanometer height sensitivity. Sub‐second X‐ray exposures can be used to acquire a 14 µm × 14 µm image with an effective pixel size of 20 nm on the sample. The optical configuration and various engineering considerations that are necessary to achieve optimal imaging resolution and contrast in this type of microscopy are discussed.     

Shape‐Tunable Selective Synthesis of Bismuth Fluoride Nanostructures for Versatile Applications

A facile approach for shape‐tunable synthesis of bismuth fluoride nanoparticles is reported. The approach is based on the homogeneous precipitation of precursor materials in mixed solvents (H₂O and ethylene glycol) and only ethylene glycol. The influencing factors on the morphology of the particles, i.e., solvent ratio, F/Bi ratio, and ethylenediaminetetraacetic acid, are studied in detail, and are schematically illustrated. The morphology, crystallinity, structure, and optical properties of the prepared samples are characterized by using a field‐emission scanning electron microscope, transmission electron microscope, X‐ray diffractometer, Fourier transform infrared spectrometer, and spectrofluorometer, respectively. The hollow sphere‐shaped nanoparticle doped with Eu³⁺ ions exhibit reddish orange emission under ultraviolet illumination due to the symmetric environment around the dopant ions. Subsequently, the effect of dopant concentration on the optical properties is also evaluated. The temperature‐dependent photoluminescence emission spectra reveal good thermal stability. The obtained results provide an efficient strategy for synthesizing the shape‐tunable nanoparticles with excellent optical properties.     

Three-dimensional characterization of drug-encapsulating particles using STEM detector in FEG-SEM

New drug-encapsulating particles were investigated using bright field (BF) scanning transmission electron microscopy (STEM) in a field emission gun (FEG) scanning electron microscope (SEM). Thickness characterization was done based on measuring the effective cross-section for interaction in our sample-detector configuration using calibration particles. A simplified analytical model, taking account of BF-STEM contrast and effective cross-section for interaction, was utilized for transforming projected two-dimensional BF-STEM images into three-dimensional thickness images. The three-dimensional characterization is demonstrated on a new family of biological materials composed of submicron to micron drug-free and drug-encapsulating particles. The importance of using BF-STEM in SEM, relative to other electron microscopy methods, is discussed as well as the lateral and depth resolution.     

Quantitative phase retrieval in X‐ray Zernike phase contrast microscopy

In recent years, increasing attention has been devoted to X‐ray phase contrast imaging, since it can provide high‐contrast images by using phase variations. Among the different existing techniques, Zernike phase contrast microscopy is one of the most popular phase‐sensitive techniques for investigating the fine structure of the sample at high spatial resolution. In X‐ray Zernike phase contrast microscopy, the image contrast is indeed a mixture of absorption and phase contrast. Therefore, this technique just provides qualitative information on the object, which makes the interpretation of the image difficult. In this contribution, an approach is proposed for quantitative phase retrieval in X‐ray Zernike phase contrast microscopy. By shifting the phase of the direct light by π/2 and 3π/2, two images of the same object are measured successively. The phase information of the object can then be quantitatively retrieved by a proper combination of the measured images. Numerical experiments were carried out and the results confirmed the feasibility of the proposed method. It is expected that the proposed method will find widespread applications in biology, materials science and so on.     

Fabrication of high‐aspect‐ratio Fresnel zone plates by e‐beam lithography and electroplating

The fabrication of gold Fresnel zone plates, by a combination of e‐beam lithography and electrodeposition, with a 30 nm outermost zone width and a 450 nm‐thick structure is described. The e‐beam lithography process was implemented with a careful evaluation of applied dosage, tests of different bake‐out temperatures and durations for the photoresist, and the use of a developer without methylisobutylketone. Electrodeposition with a pulsed current mode and with a specially designed apparatus produced the desired high‐aspect‐ratio nanostructures. The fabricated zone plates were examined by electron microscopy and their performances were assessed using a transmission X‐ray microscope. The results specifically demonstrated an image resolution of 40 nm.     

Imaging the Hydrated Microbe‐Metal Interface Using Nanoscale Spectrum Imaging

PdAu nanocrystals are synthesised by Geobacter sulfurreducens, a dissimilatory metal‐reducing bacterium, and the resulting bimetallic nanocrystal‐decorated microbes are imaged using a range of advanced electron microscopy techniques. Specifically, the first example of elemental mapping of fully hydrated biological nanostructures using scanning transmission electron microscope (STEM) energy dispersive X‐ray (EDX) spectrum imaging within an environmental liquid‐cell is reported. These results are combined with cryo‐TEM and ex situ STEM imaging and EDX analysis with the aim of better understanding microbial synthesis of bimetallic nanoparticles. It is demonstrated that although Au and Pd are colocalized across the cells, the population of nanoparticles produced is bimodal, containing ultrasmall alloyed nanocrystals with diameters <3 nm and significantly larger core‐shell structures (>200 nm in diameter) which show higher Pd contents and exhibit a Pd enriched shell only a few nanometers thick. The application of high‐resolution imaging techniques described here offers the potential to visualize the microbe‐metal interface during the bioproduction of a range of functional materials by microbial “green” synthesis routes, and also key interfaces underpinning globally relevant environmental processes (e.g., metal cycling).     

Relationship of Processing Conditions to Structure and Properties in PMDA‐ODA Polyimide

A combined wide angle X‐ray diffraction (WAXD)/small‐angle X‐ray scattering (SAXS)/scanning electron microscopy (SEM)/density study of structure and morphology was carried out for a large series of pyromellitic dianhydride‐oxydianiline (PMDA‐ODA) polyimide (PI) samples processed using different powder metallurgy techniques. Using a combined DSC/creep rate spectroscopy (CRS)/long‐term creep resistance (LTCR) approach, their molecular dynamics, thermal and elastic properties, and creep resistance in the temperature range from 20 to 470°C were also studied. Both a choice of the method of formation of fine PI particles and the order of applying high pressure relative to high temperature to form the monolithic samples led to the observation of significant property differences. Relationships between the processing conditions, structure, and properties were determined. As a result, the conditions for optimizing certain PMDA‐ODA polyimide properties, especially creep resistance and elastic properties at extreme temperatures, were determined.     

Hard X‐ray‐induced damage on carbon–binder matrix for in situ synchrotron transmission X‐ray microscopy tomography of Li‐ion batteries

The electrode of Li‐ion batteries is required to be chemically and mechanically stable in the electrolyte environment for in situ monitoring by transmission X‐ray microscopy (TXM). Evidence has shown that continuous irradiation has an impact on the microstructure and the electrochemical performance of the electrode. To identify the root cause of the radiation damage, a wire‐shaped electrode is soaked in an electrolyte in a quartz capillary and monitored using TXM under hard X‐ray illumination. The results show that expansion of the carbon–binder matrix by the accumulated X‐ray dose is the key factor of radiation damage. For in situ TXM tomography, intermittent X‐ray exposure during image capturing can be used to avoid the morphology change caused by radiation damage on the carbon–binder matrix.