X-ray scattering is a structural characterization tool that has impacted diverse fields of study. It is unique in its ability to examine materials in real time and under realistic sample environments, enabling researchers to understand morphology at nanometer and angstrom length scales using complementary small and wide angle X-ray scattering (SAXS, WAXS), respectively. Herein, we focus on the use of SAXS to examine nanoscale particulate systems. We provide a theoretical foundation for X-ray scattering, considering both form factor and structure factor, as well as the use of correlation functions, which may be used to determine a particle’s size, size distribution, shape, and organization into hierarchical structures. The theory is expanded upon with contemporary use cases. Both transmission and reflection (grazing incidence) geometries are addressed, as well as the combination of SAXS with other X-ray and non-X-ray characterization tools. We conclude with an examination of several key areas of research w...

Small Angle X-ray Scattering for Nanoparticle Research

Magnetic oxides show a variety of extrinsic magnetotransport phenomena: grain-boundary, tunnelling and domain-wall magnetoresistance. In view of these phenomena the role of some magnetic oxides is outstanding: these are believed to be half-metallic having only one spin-subband at the Fermi level. These fully spin-polarized oxides have great potential for applications in spin-electronic devices and have, accordingly, attracted intense research activity in recent years. This review is focused on extrinsic magnetotransport effects in ferromagnetic oxides. It consists of two parts; the second part is devoted to an overview of experimental data and theoretical models for extrinsic magnetotransport phenomena. Here a critical discussion of domain-wall scattering is given. Results on surface and interfacial magnetism in oxides are presented. Spin-polarized tunnelling in ferromagnetic junctions is reviewed and grain-boundary magnetoresistance is interpreted within a model of spin-polarized tunnelling through natural oxide barriers. The situation in ferromagnetic oxides is compared with data and models for conventional ferromagnets. The first part of the review summarizes basic material properties, especially data on the spin polarization and evidence for half-metallicity. Furthermore, intrinsic conduction mechanisms are discussed. An outlook on the further development of oxide spin-electronics concludes this review.

Extrinsic magnetotransport phenomena in ferromagnetic oxides

Upon cooling, the isolated ferromagnetic domains in thin films of La0.33Pr0.34Ca0.33MnO3start to grow and merge at the metal-insulator transition temperatureTP1, leading to a steep drop in resistivity, and continue to grow far below TP1. In contrast, upon warming, the ferromagnetic domain size remains unchanged until near the transition temperature. The jump in the resistivity results from the decrease in the average magnetization. The ferromagnetic domains almost disappear at a temperature TP2higher than TP1, showing a local magnetic hysteresis in agreement with the resistivity hysteresis. Even well above TP2, some ferromagnetic domains with higher transition temperatures are observed, indicating magnetic inhomogeneity. These results may shed more light on the origin of the magnetoresistance in these materials.

https://science.sciencemag.org/content/sci/298/5594/805.full.pdf

Direct observation of percolation in a manganite thin film

Atomic-level understanding of the active sites and transformation mechanisms under realistic working conditions is a prerequisite for rational design of high-performance photocatalysts. Here, by using correlated scanning fluorescence X-ray microscopy and environmental transmission electron microscopy at atmospheric pressure, in operando, we directly observe that the (110) facet of a single Cu2O photocatalyst particle is photocatalytically active for CO2 reduction to methanol while the (100) facet is inert. The oxidation state of the active sites changes from Cu(i) towards Cu(ii) due to CO2 and H2O co-adsorption and changes back to Cu(i) after CO2 conversion under visible light illumination. The Cu2O photocatalyst oxidizes water as it reduces CO2. Concomitantly, the crystal lattice expands due to CO2 adsorption then reverts after CO2 conversion. The internal quantum yield for unassisted wireless photocatalytic reduction of CO2 to methanol using Cu2O crystals is ~72%. Photocatalytic reduction of CO2 to methanol offers a promising route to storage of solar energy in the form of chemical fuels. Here, Wu et al. use in operando microscopy to identify the active facets for CO2 reduction on Cu2O and exploit this to obtain high conversion efficiency and selectivity to methanol.

Facet-dependent active sites of a single Cu 2 O particle photocatalyst for CO 2 reduction to methanol

The oxygen interstitials in the spacer layers that separate the superconducting CuO2 planes undergo ordering phenomena in Sr2O1+y CuO2, YBa2Cu3O6+y and La2CuO4+y that induce enhancements in the transition temperatures (Tc) with no changes in hole concentrations. It is also known that complex systems often have a scale-invariant structural organization, but hitherto none had been found in high-Tc copper oxides. Fratini et al. report that the ordering of oxygen interstitials in the La2O2+y spacer layers of La2CuO4+y high-Tc superconductors is characterized by a fractal distribution up to a maximum limiting size of 0.5 millimetres. Intriguingly, these fractal distributions of dopants appear to enhance superconductivity at high temperature. The oxygen interstitials in the layers separating the superconducting CuO2 planes undergo ordering phenomena in La2CuO4+y that enhance the transition temperature (Tc). It is also known that complex systems often have a scale-invariant structural organization, but hitherto none had been found in high-Tc materials. These authors report that the ordering of oxygen interstitials in the La2O2+y spacer layers of La2CuO4+y high-Tc superconductors is characterized by a fractal distribution up to a maximum limiting size of 400 µ. It is well known that the microstructures of the transition-metal oxides1,2,3, including the high-transition-temperature (high-Tc) copper oxide superconductors4,5,6,7, are complex. This is particularly so when there are oxygen interstitials or vacancies8, which influence the bulk properties. For example, the oxygen interstitials in the spacer layers separating the superconducting CuO2 planes undergo ordering phenomena in Sr2O1+yCuO2 (ref. 9), YBa2Cu3O6+y (ref. 10) and La2CuO4+y (refs 11–15) that induce enhancements in the transition temperatures with no changes in hole concentrations. It is also known that complex systems often have a scale-invariant structural organization16, but hitherto none had been found in high-Tc materials. Here we report that the ordering of oxygen interstitials in the La2O2+y spacer layers of La2CuO4+y high-Tc superconductors is characterized by a fractal distribution up to a maximum limiting size of 400 μm. Intriguingly, these fractal distributions of dopants seem to enhance superconductivity at high temperature.

Scale-free structural organization of oxygen interstitials in La 2 CuO 4+y

Polycrystalline gold films coated with thiol-based self-assembled monolayers (SAM) form the basis of a wide range of nanomechanical sensor platforms. The detection of adsorbates with such devices relies on the transmission of mechanical forces, which is mediated by chemically derived stress at the organic-inorganic interface. Here, we show that the structure of a single 300-nm-diameter facetted gold nanocrystal, measured with coherent X-ray diffraction, changes profoundly after the adsorption of one of the simplest SAM-forming organic molecules. On self-assembly of propane thiol, the crystal's flat facets contract radially inwards relative to its spherical regions. Finite-element modelling indicates that this geometry change requires large stresses that are comparable to those observed in cantilever measurements. The large magnitude and slow kinetics of the contraction can be explained by an intermixed gold-sulphur layer that has recently been identified crystallographically. Our results illustrate the importance of crystal edges and grain boundaries in interface chemistry and have broad implications for the application of thiol-based SAMs, ranging from nanomechanical sensors to coating technologies.

/pdf/differential-stress-induced-by-thiol-adsorption-on-facetted-4fhcw3zqts.pdf

Differential stress induced by thiol adsorption on facetted nanocrystals

Using x-ray submicrobeam, we spatially mapped the strain in epitaxial La1−xSrxMnO3 films grown on SrTiO3(001) bicrystal substrates. Our results show that there is an elastic strain gradient at the artificial grain boundary, which decays over a length scale of ∼1 μm. The tensile strain at the interior of the grain—due to the lattice mismatch between La1−xSrxMnO3 and SrTiO3—relaxes as the film nears the grain boundary, yielding a grain boundary lattice constant which approaches the value of that in bulk La1−xSrxMnO3.

Local mapping of strain at grain boundaries in colossal magnetoresistive films using x-ray microdiffraction

Cantilever arrays have been used to monitor biochemical interactions and their associated stress. However, it is often necessary to passivate the underside of the cantilever to prevent unwanted ligand adsorption, and this process requires tedious optimization. Here, we show a way to immobilize membrane receptors on nanomechanical cantilevers so that they can function without passivating the underlying surface. Using equilibrium theory, we quantitatively describe the mechanical responses of vancomycin, human immunodeficiency virus type 1 antigens and coagulation factor VIII captured on the cantilever in the presence of competing stresses from the top and bottom cantilever surfaces. We show that the area per receptor molecule on the cantilever surface influences ligand-receptor binding and plays an important role on stress. Our results offer a new way to sense biomolecules and will aid in the creation of ultrasensitive biosensors.

/pdf/decoupling-competing-surface-binding-kinetics-and-pin1lfoaxw.pdf

Decoupling competing surface binding kinetics and reconfiguration of receptor footprint for ultrasensitive stress assays.

Magnetic domain structure of La1−xSrxMnO3 films was studied using magnetic force microscopy (MFM). Evolution of the magnetic patterns was monitored as a function of temperature by using a variable temperature sample stage. The magnetic contrast in the MFM images decreases as the temperature is raised and vanishes as the system approaches the Tc of the film. At temperatures above the Tc of the film, local ferromagnetic regions with a higher Tc are detected around the grain boundaries. We attribute this variation in the local Tc to the local variation of strain in the film.

Temperature dependent phenomena in La1−xSrxMnO3 films studied by magnetic force microscopy

Although there have been dramatic advances in generating enormous magnetic fields, measuring these fields remains difficult. A non-metallic sensor based on a silver chalcogenide looks promising.

Yeong-Ah Soh

Papers

Differential stress induced by thiol adsorption on facetted nanocrystals

Local mapping of strain at grain boundaries in colossal magnetoresistive films using x-ray microdiffraction

Decoupling competing surface binding kinetics and reconfiguration of receptor footprint for ultrasensitive stress assays.

Temperature dependent phenomena in La1−xSrxMnO3 films studied by magnetic force microscopy

Applied physics: making sense of magnetic fields.