Focused X-ray Beam Research Articles

Highly resolved synchrotron-based investigations related to nuclear waste disposal

ABSTRACTSynchrotron-based X-ray techniques are used increasingly to characterize actinide element speciation in heterogeneous media related to nuclear waste disposal safety. Especially techniques offering added temporal, spatial and energy resolved information are advancing our understanding of f-element physics and chemistry in general and of actinide element waste disposal in particular. Examples of investigations of uranium containing systems using both highly (energy) resolved X-ray emission spectroscopy (HRXES) techniques and spatially resolved techniques with focused X-ray beams are presented in this paper: polarization dependent partial fluorescence yield X-ray absorption near edge structure (PD-PFY-XANES) spectroscopic studies of a single Cs2UO2Cl4 crystal, which experimentally reveal a splitting of the σ, π, and δ components of the 6d valence states [1], and characterization of UO2/Mo thin films prepared on different substrates using a combination of techniques (2D and 3D micro- and nano-X-ray fluorescence, XANES and both holographic and ptychographic tomography).

Experimental investigation of beam heating in a soft X-ray scanning transmission X-ray microscope

A variable temperature sample holder with an operational range of 15 to 200 °C and an accuracy of ±1 °C has been fabricated for scanning transmission X-ray microscopes (STXM). Here we describe the device, and use it to image the polycrystalline morphology of solid stearic acid and palmitic acid at temperatures near their respective melting points as a means of checking for possible sample heating caused by the focused X-ray beam. The melting points observed in STXM were identical to those observed by conventional methods within measurement uncertainty, even under the most extreme, high dose rate imaging conditions investigated. The beam-induced temperature rise in the sample is inferred to be below 1 °C for dose rates of up to 2.7 GGy/s.

Characterization of InGaN and InAlN Epilayers by Microdiffraction X-Ray Reciprocal Space Mapping

ABSTRACTWe report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that “seed” InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of “seeds” on strain fields generated by thea-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth.

Controlled hydrogel photopolymerization inside live systems by X-ray irradiation

We successfully demonstrate the in situ formation by X-ray irradiation induced polymerization of a three-dimensional (3D) hydrogel based on poly(ethylene glycol) (PEG) diacrylate (PEG DA), with the option of heparin incorporation to enhance cell adhesion. The polymerization reaction can be completed in tens of seconds, thanks to the fast and uniform X-ray induced process. Eliminating polymerization initiators and the residual unreacted monomers improves the biocompatibility and stability of the hydrogel. The photopolymerization mechanism can be locally controlled by using a trimmed or a focused X-ray beam to allow precise in vivo administration deep in the tissue, without surgery. We also demonstrate that photopolymerized hydrogels containing polyethylenimine (PEI)/nucleic acid (DNA or small interfering RNA, siRNA) nanoparticles (NPs) can deliver such nanoparticles. The hydrogel is biocompatible with mice and is not toxic to epithelial cells.

High-pressure X-ray diffraction studies of potassium chlorate

Two static high-pressure X-ray diffraction (XRD) studies of potassium chlorate have been performed at pressures of up to ∼14.3 GPa in a diamond anvil cell at ambient temperature using the 16 ID-B undulator beamline at the Advanced Photon Source for the X-ray source. The first experiment was conducted to ascertain decomposition rates of potassium chlorate as a function of pressure. Below 2 GPa, the sample was observed to decompose rapidly in the presence of the X-ray beam and release oxygen. Above 2 GPa (near the phase I → phase II transition), the decomposition rate dramatically slowed so that good quality XRD patterns could be acquired. This suggests a phase-dependent decomposition rate. In the second study, X-ray diffraction spectra were collected at pressures from 2 to 14.3 GPa by aligning virgin portions of the sample into the focused X-ray beam at each pressure. The results suggest the co-existence of mixed monoclinic (I) and rhombohedral (II) phases of potassium chlorate near 2 GPa. At pressures beyond 4 GPa, the XRD patterns show a very good fit to KClO3in the rhombohedral phase with space groupR3m, in agreement with earlier studies. No further phase transitions were observed with pressure. Decompression of the sample to ambient pressure indicated mixed phases I and II coupled with a small amount of synchrotron X-ray-induced decomposition product. The equation of state within this pressure regime has been determined.

Diffraction of a focused x-ray beam from La3Ga5SiO14 crystal modulated by surface acoustic waves

X-ray Bragg diffraction on an acoustically modulated La3Ga5SiO14 crystal (langasite, LGS) was studied in the saggital geometry where a surface acoustic wave (SAW) serves as diffraction grating of normal incidence. A set of parabolic compound Be refracting lenses was used to focus x-ray radiation in order to obtain good angular and spatial resolution of diffraction satellites. X-ray diffraction spectra analysis provided information on the acoustic properties of the LGS crystal and enabled the determination of SAW amplitudes and wavelengths.

In situ imaging of orthoclase–aqueous solution interfaces with x-ray reflection interface microscopy

The use of x-ray reflection interface microscopy (XRIM) to image molecular-scale topography at the aqueous–solid interface, in situ, is described. Specifically, we image interfacial topography of the orthoclase-(001)–aqueous solution interface at room temperature and describe the challenges associated with in situ XRIM imaging. The measurements show that the reflectivity signal for in situ XRIM measurements is substantially smaller than that for ex situ measurements, because of both intrinsic and extrinsic factors. There is also a systematic temporal reduction in the image intensity with increasing x-ray dose, revealing that interaction of the focused x-ray beam with the orthoclase interfaces leads to interfacial perturbations, presumably in the form of surface roughening. This image fading is localized to the x-ray beam footprint, suggesting that the primary damage mechanism is initiated by photoelectrons produced by x-ray beam absorption near the substrate–electrolyte interface. Finally, the role of aqueous solution composition in controlling the sensitivity of the orthoclase surface to x-ray beam-induced effects is explored. A substantial increase in the orthoclase (001) surface stability was observed in solutions having elevated ionic strength, apparently as a result of the reduced lifetime of radiation chemistry products at these conditions.

InGaN epilayer characterization by microfocused x-ray reciprocal space mapping

We report the use of microfocused three-dimensional x-ray reciprocal space mapping to study InGaN epilayers with average InN content ∼20%-22%. Analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused x-ray beam, allowed us to gain valuable information about the nanostructure of InN-rich InGaN epilayers. It is found that “seed” InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain-free. The initial stages of InGaN-on-GaN epitaxial growth, therefore, conform to the Volmer-Weber growth mechanism with “seeds” nucleated on strain fields generated by the a-type edge dislocations.

Towards high-resolution ptychographic x-ray diffraction microscopy

Ptychographic x-ray diffraction microscopy is a lensless imaging technique with a large field of view and high spatial resolution, which is also useful for characterizing the wavefront of an x-ray probe. The performance of this technique is degraded by positioning errors due to the drift between the sample and illumination optics. We propose an experimental approach for correcting the positioning errors and demonstrate success by two-dimensionally reconstructing both the wavefront of the focused x-ray beam and the complex transmissivity of the weakly scattering objects at the pixel resolution of better than 10 nm in the field of view larger than 5 $\ensuremath{\mu}$m. This method is applicable to not only the observation of organelles inside cells or nano-mesoscale structures buried within bulk materials but also the characterization of probe for single-shot imaging with x-ray free electron lasers.

Scanning transmission x-ray microscopy of polymer nanoparticles: probing morphology onsub-10 nm length scales

Water-processable nanoparticle dispersions of semiconducting polymers offer anattractive approach to the fabrication of organic electronic devices since they offer:(1) control of nanoscale morphology and (2) environmentally friendly fabrication.Although the nature of phase segregation in these polymer nanoparticles is critical todevice performance, to date there have been no techniques available to directlydetermine their intra-particle structure, which consequently has been poorlyunderstood. Here, we present scanning transmission x-ray microscopy (STXM)compositional maps for nanoparticles fabricated from poly(9,9-dioctyl-fluorene-2,7-diyl-co-bis-N, N′-(4-butylphenyl)-bis-N, N′-phenyl-1,4-phenylenedi-amine) (PFB) and poly(9,9-dioctylfluorene-2,7-diyl-co-benzothiadiazole) (F8BT) 1:1 blend mixtures. The images show distinct phase segregationwithin the nanoparticles. The compositional data reveals that, within these nanoparticles,PFB and F8BT segregate into a core–shell morphology, with an F8BT-rich core and aPFB-rich shell. Structural modelling demonstrates that the STXM technique is capable ofquantifying morphological features on a sub-10 nm length scale; below the spot size ofthe incident focused x-ray beam. These results have important implications forthe development of water-based ‘solar paints’ fabricated from microemulsions ofsemiconducting polymers.

Studying single nanocrystals under high pressure using an x-ray nanoprobe

In this report, we demonstrate the feasibility of applying a 250-nm focused x-ray beam to study a single crystalline NbSe(3) nanobelt under high-pressure conditions in a diamond anvil cell. With such a small probe, we not only resolved the distribution and morphology of each individual nanobelt in the x-ray fluorescence maps but also obtained the diffraction patterns from individual crystalline nanobelts with thicknesses of less than 50 nm. Single crystalline diffraction measurements on NbSe(3) nanobelts were performed at pressures up to 20 GPa.

Coherent x-ray diffraction imaging of ZnO nanostructures under confined illumination

Coherent x-ray diffraction imaging has been used to study a single ZnO nanorod in a confined illuminating condition. The focused beam size is smaller than the length of the nanorod, and the diffraction intensity is strongly dependent on the illumination position. The density maps show that the nanorod width in the radial direction is around 210 nm and has a length of 1.5 μm, in agreement with the scanning electron microscope measurement. Reconstructed phase maps show a maximum phase change of 0.8 radians. The reconstructed direct space structures reveal the exit wavefront profile, which includes that of the focused x-ray beam. The beam profile presents in reconstructions some ‘hill and valley’ surface features with a typical size of a few tens of nanometres and are attributed to the noise due to the slow variation of the focused beam intensity along the boundary. A single ZnO tetrapod has been investigated with the same method to recover the beam profile in the horizontal direction.

Hard X-ray microbeam lithography using a Fresnel zone plate with a long focal length

Focused hard X-ray microbeams for use in X-ray nanolithography have been investigated. A 7.5 keV X-ray beam generated at an undulator was focused to about 3 µm using a Fresnel zone plate fabricated on silicon. The focused X-ray beam retains a high degree of collimation owing to the long focal length of the zone plate, which greatly facilitates hard X-ray nanoscale lithography. The focused X-ray microbeam was successfully utilized to fabricate patterns with features as small as 100 nm on a photoresist.

From ensemble average to single (nano-) objects properties by X-ray microdiffraction: a short review on structure determination (local strain, composition, ...) and objects manipulation (AFM-coupled)

In standard diffraction experiments, ensembles of objects are characterized yielding averaged, statistical properties (meaningful only if the ensemble is monodisperse). Focused x-ray beams are used here to localize single nanostructures, identifying and probing individual objects one by one. In a scanning mode, a 2-dimensional image of the sample is recorded, which allows the reproducible alignment of a specific nanostructure for analysis. The x-ray scattered signal is analyzed and modelled, to give access to the shape, strain and composition inside the single object with sub-micron resolution. Combination of x-ray microdiffraction technique with other micro-probe experiments on the very same individual object (simultaneous coupling of x-ray diffraction measurements with atomic force microscopy (AFM)) is also shown; we prove the possibility to interact with the objects and to address elastic properties for individual nano-structures out of an ensemble.

Polarization effects in thallium bromide x-ray detectors

We present the results of a detailed experimental study of polarization effects in thallium bromide planar x-ray detectors. Measurements were carried out in the range 10–100 keV by scanning a highly focused x-ray beam, 50 μm in diameter, from a synchrotron source across the detector. Above a certain radiation threshold the detector response showed a systematic degradation of its spectroscopic characteristics, peak channel position, peak height, and energy resolution. Using a pump-and-probe technique, we studied the dynamics of spectral degradation, the spatial extent and relaxation of the polarized region, and the dependence of the detector response on bias voltage and temperature. For comparison, we modeled polarization effects induced by the charging of traps by both electrons and holes using a model based on recent theoretical work of Bale and Szeles. We calculated the charge collection efficiency and spectral line shapes as functions of exposure time, beam position, count rate, and photon energy, and obtained credible agreement with experimental results.

Magnetic microstructures of neodymium in Nd2Fe14B permanent magnet by hard x-ray magnetic-circular dichroism using focused x-ray beam

Magnetic microstructures of neodymium (Nd) in a Nd–Fe–B magnet were investigated by the hard x-ray magnetic circular dichroism (HXMCD) with a focused synchrotron circular-polarized beam (<2 μm). Magnetic domain and Nd concentration were simultaneously acquired by measuring Nd HXMCD and Nd Lα intensity. The magnetic momentum of Nd was high for Nd2Fe14B microdomains but was nearly zero at Nd-rich precipitates. This clearly indicates the magnetic momentum distributions are well correlated with the local concentration of Nd. Our experiments demonstrate the feasibility of studying magnetic microstructures by HXMCD mapping, which thus enables a discussion of the bulk magnetic behavior of Nd–Fe–B magnets.

Diffraction imaging of crystals with focused x-ray beams

We describe an imaging technique based on diffraction of a focused x-ray beam in crystals. A focused beam is formed by a zone plate and Bragg diffracted from a crystalline sample positioned between the zone plate and the focus. The intensity pattern is recorded by a high-resolution charge-coupled-device detector placed in the focus. Diffraction images recorded from perfect Si and GaAs crystals for various reflections demonstrate the broadening of the focused beam due to a finite scattering length. The images from semiconductor epitaxial films and heterostructures show additional peaks originating from the interfaces with their spatial position corresponding to the depth from the surface. Diffraction images from isolated defects in Si crystal demonstrate capabilities to study bulk defects. Theoretical simulations for perfect crystals show excellent agreement with experiments. We demonstrate that the new imaging technique is depth sensitive and combines structural sensitivity of traditional x-ray topography methods with spatial in-plane resolution provided by focusing.

Imaging the displacement field within epitaxial nanostructures by coherent diffraction: a feasibility study

We investigate the feasibility of applying coherent diffraction imaging to highly strained epitaxial nanocrystals using finite-element simulations of SiGe islands as input in standard phase retrieval algorithms. We discuss the specific problems arising from both epitaxial and highly strained systems and we propose different methods to overcome these difficulties. Finally, we describe a coherent microdiffraction experimental setup using extremely focused x-ray beams to perform experiments on individual nanostructures.

Application of kinoform lens for X-ray reflectivity analysis

In this paper the first practical application of kinoform lenses for the X-ray reflectivity characterization of thin layered materials is demonstrated. The focused X-ray beam generated from a kinoform lens, a line of nominal size approximately 50 microm x 2 microm, provides a unique possibility to measure the X-ray reflectivities of thin layered materials in sample scanning mode. Moreover, the small footprint of the X-ray beam, generated on the sample surface at grazing incidence angles, enables one to measure the absolute X-ray reflectivities. This approach has been tested by analyzing a few thin multilayer structures. The advantages achieved over the conventional X-ray reflectivity technique are discussed and demonstrated by measurements.

Individual GaAs nanorods imaged by coherent X-ray diffraction

Using scanning X-ray diffraction microscopy with a spot size of 220 x 600 nm, it was possible to inspect individual GaAs nanorods grown seed-free through circular openings in a SiN(x) mask in a periodic array with 3 microm spacing on GaAs[111]B. The focused X-ray beam allows the determination of the strain state of individual rods and, in combination with coherent diffraction imaging, it was also possible to characterize morphological details. Rods grown either in the centre or at the edge of the array show significant differences in shape, size and strain state.