X ray diffraction Publications

Thin xanthine crystal plates operate as mirrors in the ocelli (small eyes) of the insect L. rozsypali (jumping bristletail), where they form superstructures that are used to reflect missed incident light back to the light detectors. Here, we present the structure of the biogenic xanthine crystals. Structure determination is made possible by combining electron diffraction and 4D scanning transmission electron microscopy with fundamental symmetry considerations and first-principles calculations based on density functional theory. We find three possible structures, all layered, with individual planes comprised of the same planar hydrogen-bonded network. However, they are different polytypes (i.e., exhibit different stacking of the individual planes). One of these polytypes corresponds to the structure of the biogenic crystals, whereas a second polytype corresponds to the structure recently determined for synthetic xanthine crystals.

We present a three-terminal resistive switching device with a 20 mol% gadolinium-doped ceria (20GDC) thin film as the solid state electrolyte. The device features a top Ta-metal gate electrode and bottom Ta-metal source and drain electrodes, separated by a 1 mm gap filled with 20GDC. Its operation relies on the redox reaction of cerium, specifically the reduction of cerium (IV) to cerium (III) at the interface between the Ta-gate and the 20GDC electrolyte. Under positive gate bias, the Ta gate electrode undergoes oxidation, while cerium is reduced, forming a conductive layer between the source and drain electrodes. Applying a negative gate bias reverses this effect. To confirm that resistivity changes originate from interface redox reactions, we conducted cyclic voltammetry at 403 K. The results demonstrate that peak current is inversely proportional to the scan rate, a characteristic of reaction at a surface. Additionally, we demonstrated that sputtering a TaO_x blocking layer beneath the gate electrode suppresses resistive switching. While the resistance changes only by a factor of two, the proposed device operates near equilibrium, is simple to fabricate, and exhibits high robustness. These characteristics make the concept of interface oxidation/reduction appealing for further exploration.

1D multiwall tungsten disulfide nanotubes were discovered more than three decades ago, and their large-scale fabrication has been worked out systematically over the years. Building on this knowledge, reports on the more complex coreshell nanotubular structures have emerged in recent years. In the present work, coreshell tungsten disulfide and tungsten diselenide nanotubes (WS₂@WSe₂NTs) have been materialized through high-temperature selenidation of WS₂nanotubes and fine WO₃powder, yielding large quantities of epitaxially aligned heterostructures. Advanced high-resolution electron microscopy revealed the formation of complex heterointerfaces with well-defined atomic arrangements between the constituent phases. Optical characterization shows distinctive electronic properties arising from the heterostructure architecture, indicating strong interfacial coupling between the WS₂and WSe₂constituents. The unique electronic and structural characteristics of these nanotubes position them as promising candidates for enhanced photocatalytic applications and quantum device integration, where precise control over electronic states and charge transfer dynamics is critical.

The science of nuclear forensics is essential for the global effort against illicit trafficking of nuclear (or radiological) materials. One of the most common interdicted materials is uranium. Hence, combating the diverting of uranium compounds from their legal uses, such as the nuclear fuel cycle, is critical. This work describes a set of measurements to establish relations between the time elapsed since the exposure of UO₂F₂, U₃O₈ and UO₃, to different environmental conditions. It was found that these uranium compounds react differently with humidity, as a function of time, to present measurable parameters, used to estimate how long they were exposed to the specific conditions. Ion beam depth profiling utilized by ToF-SIMS on one UO₂F₂ particle as a function of exposure time, yielded significant related data. Yet, a second set of depth profile measurements on a set of 10 UO₂F₂ particles, followed by statistical analysis, did not find significant differences between the exposure periods.

How molecular-level understanding of the crystal growth mechanisms and their relation to lattice bonds informs the rational design of crystals with desired shapes and properties has remained elusive. Here we employ theophylline crystals and drive them into classical growth mode, in which the crystals grow molecule-by-molecule and new layers are generated by two-dimensional nucleation. We demonstrate that classical growth allows for controlling the crystal's shape and dimensions. We correlate the anisotropic responses to the supersaturation of the growth rates of crystal layers and crystal faces to the hydrogen and π−π stacking bond chains in the crystal lattice. The obtained insights suggest strategies to direct the crystal shape to either one-dimensional needles or flat sheets. Moreover, we show that crystals that grow by the classical mode of direct monomer incorporation have the potential to regrow and heal once a defect is introduced by mechanical cut or local thermal subliming of crystalline sections.

The branched metal-organic frameworks (MOFs) are the first superstructures of this kind, and the growth mechanism may explain crystal shapes of other materials. The mechanism of the formation of fascinating structures having a hedrite, sheaf or spherulite appearance are detailed. The branching can be controlled, resulting in crystals that either exhibit multiple generations of branching or a single generation. These structures might result from an increasing number of defects on fast-grown rods. As the basal facets become less reactive, material is added to the prism facets, leading to secondary nucleation and triangular branches. These triangular structures are connected to the rod surface, growing longer than the central rod. Electron diffraction analyses show that the sheafs are polycrystalline structures with their fantails consisting of single-crystalline nanorods deviating gradually from each other in their orientation. The crystallographic structure consists of channels with opposite handedness. The accessibility of the nanochannels and the porosity of the superstructures are demonstrated by chromophore diffusion into the channels. The confinement and alignment of the chromophores inside the channels resulted in polarized-light dependent coloration of the crystals; the polycrystallinity generated areas having different optical properties.

Herein, we obtained two supramolecular assemblies with layered structures from melamine, N-methylmelamine, and hexynyl-cyanuric acid in water. By combination of X-ray diffraction, electron microscopy, and molecular dynamics studies, we found that introducing one methyl group in melamine alters the arrangement of the layers in these structures.

Coupling between an electrochemical reaction and a functional material property has been termed electro-chemo-X, or EC-X, where X can refer to mechanical, optical, magnetic or thermal properties. Recently, our group has demonstrated a two-terminal electro-chemo-mechanical (ECM) membrane actuator operating under ambient conditions and containing a Ce_0.8Gd_0.2O_1.9 solid electrolyte layer sandwiched between two Gd-doped ceria/TiO_x nanocomposite thin films. Reducing one nanocomposite film while oxidizing the other was observed to produce reversible volume change thereby driving membrane actuator operation. Here, we use the same electrolyte and nanocomposite layer pair (the upper one as the ion reservoir and the lower, as the active layer) to further explore the EC-X effect. We demonstrate the suitability of the nanocomposite for a three-terminal, thin film-based resistivity switch. We find that application of ±6 V ( Ce⁺⁴ is similarly effective in leading to increased nanocomposite conductivity, while reduction produces the opposite effect. With the expectation that the response time can be significantly shortened, the proposed resistivity switch may be suitable for future applications such as sensors, neuromorphic computing or spintronics.

The crystallinity of polymeric materials defines their properties, in particular, the mechanical ones. High-resolution transmission electron microscopy (TEM) imaging of polymers would be critical to address intricate polymer crystallinity, yet it is challenging due to polymer sensitivity to the electron beam. We performed high-resolution TEM imaging of polycaprolactone (PCL) thin films employing low-dose focal series reconstruction (LDFSR). LDFSR enabled submolecular resolution imaging of polymer crystals. The direct imaging study was augmented by scanning nanobeam electron diffraction (NBED) using the 4D STEM technique to map micro- and nanoscale crystalline domains. Employing LDFSR combined with 4D STEM, we directly observed interacting polymer chains in the crystal lattice, elucidating the crystal structure with a high degree of precision including lattice deformations. We also imaged PCL lamella using conventional TEM. Our methodology enables long-sought insights into the polymer structure, introducing a new tool for high resolution studies of polymer crystallinity that fills a critical gap in the structural science of polymer materials.

Uranium is a high-value energy element, yet also poses an appreciable environmental burden. The demand for a straightforward, low energy, and environmentally friendly method for encapsulating uranium species can be beneficial for long-term storage of spent uranium fuel and a host of other applications. Leveraging on the low melting point (60 °C) of uranyl nitrate hexahydrate and nanocapillary effect, a uranium compound is entrapped in the hollow core of WS₂ nanotubes. Followingly, the product is reduced at elevated temperatures in a hydrogen atmosphere. Nanocrystalline UO₂ nanoparticles anchor within the WS₂ nanotube lumen are obtained through this procedure. Such methodology can find utilization in the processing of spent nuclear fuel or other highly active radionuclides as well as a fuel for deep space missions. Moreover, the low melting temperatures of different heavy metal-nitrate hydrates, pave the way for their encapsulation within the hollow core of the WS₂ nanotubes, as demonstrated herein.

Numerous bio-organisms employ template-assisted crystallization of molecular solids to yield crystal morphologies with unique optical properties that are difficult to reproduce synthetically. Here, a facile procedure is presented to deposit bio-inspired birefringent crystals of xanthine derivatives on a template of single-crystal quartz. Crystalline sheets that are several millimeters in length, several hundred micrometers in width, and 300600 nm thick, are obtained. The crystal sheets are characterized with a well-defined orientation both in and out of the substrate plane, giving rise to high optical anisotropy in the plane parallel to the quartz surface, with a refractive index difference Δn ≈ 0.25 and a refractive index along the slow axis of n ≈ 1.7. It is further shown that patterning of the crystalline stripes with a tailored periodic grating leads to a thin organic polarization-dependent diffractive meta-surface, opening the door to the fabrication of various optical devices from a platform of small-molecule based organic dielectric crystals.

Surface-guided growth has proven to be an efficient approach for the production of nanowire arrays with controlled orientations and their large-scale integration into electronic and optoelectronic devices. Much has been learned about the different mechanisms of guided nanowire growth by epitaxy, graphoepitaxy, and artificial epitaxy. A model describing the kinetics of surface-guided nanowire growth has been recently reported. Yet, many aspects of the surface-guided growth process remain unclear due to a lack of its observation in real time. Here we observe how surface-guided nanowires grow in real time by in situ scanning electron microscopy (SEM). Movies of ZnSe surface-guided nanowires growing on periodically faceted substrates of annealed M-plane sapphire clearly show how the nanowires elongate along the substrate nanogrooves while pushing the catalytic Au nanodroplet forward at the tip of the nanowire. The movies reveal the timing between competing processes, such as planar vs nonplanar growth, catalyst-selective vapor-liquid-solid elongation vs nonselective vapor-solid thickening, and the effect of topographic discontinuities of the substrate on the growth direction, leading to the formation of kinks and loops. Contrary to some observations for nonplanar nanowire growth, planar nanowires are shown to elongate at a constant rate and not by jumps. A decrease in precursor concentration as it is consumed after long reaction time causes the nanowires to shrink back instead of growing, thus indicating that the process is reversible and takes place near equilibrium. This real-time study of surface-guided growth, enabled by in situ SEM, enables a better understanding of the formation of nanostructures on surfaces.

As a major component in the nuclear fuel cycle, octoxide uranium is subjected to intensive nuclear forensics research. Scientific efforts have been mainly dedicated to determine signatures, allowing for clear and distinct attribution. The oxygen isotopic composition of octoxide uranium, acquired during the fabrication process of the nuclear fuel, might serve as a signature. Hence, understanding the factors governing the final oxygen isotopic composition and the chemical systems in which U3O8 was produced may develop a new fingerprint concerning the history of the material and/or the process to which it was subjected. This research determines the fractionation of oxygen isotopes at different temperatures relevant to the nuclear fuel cycle in the system of U3O8 and atmospheric O2. We avoid the retrograde isotope effect at the cooling stage at the end of the fabrication process of U3O8. The system attains the isotope equilibrium at temperatures higher than 300 °C. The average δ18O values of U3O8 in equilibrium with atmospheric oxygen have been found to span over a wide range, from −9.90 at 300 °C up to 18.40 at 800 °C. The temperature dependency of the equilibrium fractionation (1000 ln αU3O8atm. O2 ) exhibits two distinct regions, around −33 between 300 °C and −500 °C and −5 between 700 °C and −800 °C. The sharp change coincides with the transition from a pseudo-hexagonal structure to a hexagonal structure. A depletion trend in δ18O is associated with the orthorhombic structure and may result from the uranium mass effect, which might also play a role in the depletion of 5 versus atmospheric oxygen at high temperatures.

A protocol for successfully depositing [001] textured, 23 µm thick films of Al_0.75Sc_0.25N, is proposed. The procedure relies on the fact that sputtered Ti is [001]-textured (Formula presented.) -phase (hcp). Diffusion of nitrogen ions into the α-Ti film during reactive sputtering of Al_0.75,Sc_0.25N likely forms a [111]-oriented TiN intermediate layer. The lattice mismatch of this very thin film with Al_0.75Sc_0.25N is ~3.7%, providing excellent conditions for epitaxial growth. In contrast to earlier reports, the Al_0.75Sc_0.25N films prepared in the current study are Al-terminated. Low growth stress (

We demonstrate the formation of highly interpenetrated frameworks. An interesting observation is the presence of very large adamantane-shaped cages in a single network, making these crystals new entries in the collection of diamondoid-type metal-organic frameworks (MOFs). The frameworks were constructed by assembling tetrahedral pyridine ligands and copper dichloride. Currently, the networks degree of interpenetration is among the highest reported and increases when the size of the ligand is increased. Highly interpenetrated frameworks typically have low surface contact areas. In contrast, in our systems, the voids take up to 63% of the unit cell volume. The frameworks are chiral but formed from achiral components. The chirality is manifested by the coordination chemistry frameworks around the metal center, the structure of the helicoidal channels and the motifs of the individual networks. Channels of both handedness are present within the unit cells. This phenomenon shapes the walls of the channels, which are composed of 10, 16, or 32 chains correlated to the degree of interpenetration 10-, 16- and 32-fold. By changing the distance between the center of the ligand and the coordination moieties, we succeeded in tuning the diameter of the channels. Relatively large channels were formed, having diameters up to 31.0 Å × 14.8 Å.

Aligned growth of planar semiconductor nanowires (NWs) on crystalline substrates has been widely demonstrated during the past two decades and was used for the fabrication of a large variety of devices. However, the dependence on single-crystal substrates is a major obstacle in the way of implementing NW-based applications in today's silicon- and glass-based technologies. Here, the guided growth of semiconductor NWs is demonstrated along nanoscale-depth scratches, created in a nonlithographic process on amorphous oxidized silicon wafers and soda-lime glass. Scratches are created on the substrates in a few seconds using a robust and scalable mechanical polishing process. Growth of planar NWs of different materials (CdS, CdSe, ZnSe, and ZnO) guided by scratches on Si/SiO₂ wafers and glass is demonstrated and studied. Photoluminescence measurements from individual NWs grown along scratches show that the interaction with the substrate preserves the optical properties of the material. Crystallographic analysis indicates that all materials grow as single crystals, and the influence of the scratches on the different materials is discussed in terms of morphology, crystallinity, and crystallographic orientations. This process opens the way to large-scale integration of NWs into functional devices by guided growth for various applications including displays, polarized light sensors, and smart windows.

The most abundant mineral produced in the wood and leaves of trees is calcium oxalate monohydrate (whewellite), and after burning the wood the ash obtained is calcite. In the case of the Tamarix sp. tree, the freshly prepared ash is calcium sulfate (anhydrite). The aim of this study is to determine the calcium sulfate mineral phase in the fresh wood of Tamarix aphylla prior to burning. SEM images of the crystals show that they express smooth faces, are about 515 microns in their longest dimensions and are located in the ray cells. Fourier transform infrared spectroscopy (FTIR) and Raman microspectroscopy of the crystals in the wood and after extraction, both showed that the crystals are composed of calcium sulfate hemihydrate (bassanite). As elemental analyses of the crystals showed that in addition to calcium and sulfur, around 20 atom percent of the cations are sodium and potassium, we also obtained an X-ray powder diffraction pattern. This pattern excluded the possibility that the crystals are composed of another related mineral, and confirmed that, indeed, the crystals in the T. aphylla wood are composed of calcium sulfate hemihydrate (bassanite).

The pseudo-amorphous clay components of some of the pottery sherds that formed a surface in the firing chamber of a Late Byzantine kiln were shown by Fourier Transform Infrared Spectroscopy to have undergone almost complete recrystallization. Powder X-ray diffraction showed that the crystalline montmorillonite component of these sherds increased and kaolinite formed de novo. As this recrystallization process only occurred in the center of the firing chamber, we infer that the recrystallization process was due to repeated exposure of the sherds to high temperatures. The zeolite gonnardite was identified by X-ray diffraction. The chemical compositions of sodium-rich minerals, determined by energy dispersive X-ray spectroscopy (EDS), are consistent with the presence of gonnardite and analcime, and showed that the sodium was partially substituted by calcium and other cations. As these zeolites were also present in sherds from the upper pottery chamber, they did not form only as a result of repeated exposure to high temperatures. The demonstration that the clay mineral component of ceramics can undergo diagenetic recrystallization supports the possibility that provenience studies based on elemental analyses, especially of cooking pots that are repeatedly exposed to high temperatures, may be affected by recrystallization.