Scanning transmission x-ray microscopy as a novel tool to probe colloidal and photonic crystals

Reflectance Confocal Microscopy: A Powerful Tool for Large Scale Characterization of Ordered/Disordered Morphology in Colloidal Photonic Structures

The development of appropriate methods to correlate the structure and optical properties of colloidal photonic structures is still a challenge. Structural information is mostly obtained by electron, X-ray, or optical microscopy methods and X-ray diffraction, while bulk spectroscopic methods and low resolution bright-field microscopy are used for optical characterization. Here, we describe the use of reflectance confocal microscopy as a simple and intuitive technique to provide a direct correlation between the ordered/disordered structural morphology of colloidal crystals and glasses, and their corresponding optical properties.

Femtosecond laser produced periodic plasma in a colloidal crystal probed by XFEL radiation

With the rapid development of short-pulse intense laser sources, studies of matter under extreme irradiation conditions enter further unexplored regimes. In addition, an application of X-ray Free-Electron Lasers (XFELs) delivering intense femtosecond X-ray pulses, allows to investigate sample evolution in IR pump - X-ray probe experiments with an unprecedented time resolution. Here we present a detailed study of the periodic plasma created from the colloidal crystal. Both experimental data and theory modeling show that the periodicity in the sample survives to a large extent the extreme excitation and shock wave propagation inside the colloidal crystal. This feature enables probing the excited crystal, using the powerful Bragg peak analysis, in contrast to the conventional studies of dense plasma created from bulk samples for which probing with Bragg diffraction technique is not possible. X-ray diffraction measurements of excited colloidal crystals may then lead towards a better understanding of matter phase transitions under extreme irradiation conditions.

Ptychographic X-Ray Imaging of Colloidal Crystals

Ptychographic coherent X-ray imaging is applied to obtain a projection of the electron density of colloidal crystals, which are promising nanoscale materials for optoelectronic applications and important model systems. Using the incident X-ray wavefield reconstructed by mixed states approach, a high resolution and high contrast image of the colloidal crystal structure is obtained by ptychography. The reconstructed colloidal crystal reveals domain structure with an average domain size of about 2 µm. Comparison of the domains formed by the basic close-packed structures, allows us to conclude on the absence of pure hexagonal close-packed domains and confirms the presence of random hexagonal close-packed layers with predominantly face-centered cubic structure within the analyzed part of the colloidal crystal film. The ptychography reconstruction shows that the final structure is complicated and may contain partial dislocations leading to a variation of the stacking sequence in the lateral direction. As such in this work, X-ray ptychography is extended to high resolution imaging of crystalline samples.

Structural colour of unary and binary colloidal crystals probed by scanning transmission X-ray microscopy and optical microscopy

Colloidal crystals composed of micro- or nano- colloids have been investigated in various fields such as photonics due to their unique optical properties. Binary colloidal crystals have an outstanding potential for fine-tuning material properties by changing the components, concentration, or size of colloids. Because of their tunable optical, electrical, magnetic, and mechanical properties, those materials attracted great attention. However, it has been hard to elucidate internal structures without fluorescent labelling or cross-sectioning. Here, we demonstrate the structural analysis of not only unary but also binary colloidal crystals using scanning transmission x-ray microscopy and compare the results with colloidal structures and optical properties observed by optical microscopy. Based on the comparison of images obtained by these two methods, the domains of colloidal crystals consisting of different structures and colours were directly identified without any additional sample preparation. Therefore, it was possible to investigate the structural colours of local domains of unary and binary colloidal crystals such as the face centred cubic (FCC) structure with different orientations, that is FCC (111) and FCC (001), and hexagonal close-packed structure, HCP (0001).

Revealing Three-Dimensional Structure of an Individual Colloidal Crystal Grain by Coherent X-Ray Diffractive Imaging

We present results of a coherent x-ray diffractive imaging experiment performed on a single colloidal crystal grain. The full three-dimensional (3D) reciprocal space map measured by an azimuthal rotational scan contained several orders of Bragg reflections together with the coherent interference signal between them. Applying the iterative phase retrieval approach, the 3D structure of the crystal grain was reconstructed and positions of individual colloidal particles were resolved. As a result, an exact stacking sequence of hexagonal close-packed layers including planar and linear defects were identified.

Nanoscale characterization of local structures and defects in photonic crystals using synchrotron-based transmission soft X-ray microscopy

For the structural characterization of the polystyrene (PS)-based photonic crystals (PCs), fast and direct imaging capabilities of full field transmission X-ray microscopy (TXM) were demonstrated at soft X-ray energy. PS-based PCs were prepared on an O₂-plasma treated Si₃N₄ window and their local structures and defects were investigated using this label-free TXM technique with an image acquisition speed of ~10 sec/frame and marginal radiation damage. Micro-domains of face-centered cubic (FCC (111)) and hexagonal close-packed (HCP (0001)) structures were dominantly found in PS-based PCs, while point and line defects, FCC (100), and 12-fold symmetry structures were also identified as minor components. Additionally, in situ observation capability for hydrated samples and 3D tomographic reconstruction of TXM images were also demonstrated. This soft X-ray full field TXM technique with faster image acquisition speed, in situ observation, and 3D tomography capability can be complementally used with the other X-ray microscopic techniques (i.e., scanning transmission X-ray microscopy, STXM) as well as conventional characterization methods (e.g., electron microscopic and optical/fluorescence microscopic techniques) for clearer structure identification of self-assembled PCs and better understanding of the relationship between their structures and resultant optical properties.

X-ray and electron microscopy studies on the biodistribution and biomodification of iron oxide nanoparticles in Daphnia magna

Biodistribution and biomodification of iron oxide (Fe3O4 and α-Fe2O3) nanoparticles (NPs) in a well-known toxicity test organism, Daphnia magna (D. magna), were investigated using transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). In addition to the morphological changes in the gut tissues of D. magna, biodistribution and biomodification of iron oxide NPs in the digestive tract of D. magna were also monitored in this study. Upon exposures to both iron oxide NPs, unique morphological changes (e.g., irregular shaped microvilli, epithelial cell protrusion, and dilatation of cytoplasmic inclusion) in the gut tissues of D. magna were observed along with bacterial colonization of the gut lumen. However, despite their heavy accumulations in the digesitive tract, TEM and STXM images confirmed us that both Fe3O4 and α-Fe2O3 NPs were not penetrating into the gut tissues of D. magna. Moreover, for the Fe3O4 NPs in direct contact with the gut microvilli of D. magna, slight but significant spectral changes were observed in their Fe L-edge X-ray absorption near edge structure (XANES) spectra, which indicated that there were biomodifications of Fe3O4 NPs, probably involving oxidative dissolution of Fe3O4 NPs followed by rapid precipitation of ferric oxide or hydroxide. However, no significant changes were observed in the Fe L-edge XANES spectra of the α-Fe2O3 NPs present in the gut lumen of D. magna. These X-ray and electron microscopic observations confirmed us that, despite similarities in core sizes and chemical compositions, NPs with different crystalline phase and dissolution rates can interact quite differently with their local environment, may result in different biodistribution and cause completely dissimilar toxicities.

3D nanoscale chemical imaging of the distribution of aluminum coordination environments in zeolites with soft X-ray microscopy

Which side are you on? Scanning transmission X-ray microscopy is used for the first time to elucidate the coordination and distribution of aluminum in industrial-relevant zeolites at the single-particle level. Extended regions of a few hundred nanometers, rich in higher aluminum coordination environments, are heterogeneously embedded within the zeolite particle, before and after a steaming post-treatment.

Three-dimensional structure and defects in colloidal photonic crystals revealed by tomographic scanning transmission X-ray microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy.

Three-dimensional structure of a single colloidal crystal grain studied by coherent x-ray diffraction

A coherent x-ray diffraction experiment was performed on an isolated colloidal crystal grain at the coherence beamline P10 at PETRA III. Using azimuthal rotation scans the three-dimensional (3D) scattered intensity from the sample in the far-field was measured. It includes several Bragg peaks as well as the coherent interference around these peaks. The analysis of the scattered intensity reveals the presence of plane defects in a single grain of the colloidal sample. We confirm these findings by model simulations. In these simulations we also analyze the experimental conditions required to phase the 3D diffraction pattern from a single colloidal grain. This approach has the potential to produce a high resolution image of the sample revealing its inner structure, with possible structural defects.

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

Water-processable nanoparticle dispersions of semiconducting polymers offer an attractive 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 to device performance, to date there have been no techniques available to directly determine their intra-particle structure, which consequently has been poorly understood. 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 segregation within 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 a PFB-rich shell. Structural modelling demonstrates that the STXM technique is capable of quantifying morphological features on a sub-10 nm length scale; below the spot size of the incident focused x-ray beam. These results have important implications for the development of water-based 'solar paints' fabricated from microemulsions of semiconducting polymers.