Orientational analysis of atomic pair correlations in nanocrystalline indium oxide thin films

The application of grazing-incidence total X-ray scattering (GITXS) for pair distribution function (PDF) analysis using >50 keV X-rays from synchrotron light sources has created new opportunities for structural characterization of supported thin films with high resolution. Compared with grazing-incidence wide-angle X-ray scattering, which is only useful for highly ordered materials, GITXS/PDFs expand such analysis to largely disordered or nanostructured materials by examining the atomic pair correlations dependent on the direction relative to the surface of the supporting substrate. A characterization of nanocrystalline In2O3-derived thin films is presented here with in-plane-isotropic and out-of-plane-anisotropic orientational ordering of the atomic structure, each synthesized using different techniques. The atomic orientations of such films are known to vary based on the synthetic conditions. Here, an azimuthal orientational analysis of these films using GITXS with a single incident angle is shown to resolve the markedly different orientations of the atomic structures with respect to the planar support and the different degrees of long-range order, and hence, the terminal surface chemistries. It is anticipated that orientational analysis of GITXS/PDF data will offer opportunities to extend structural analyses of thin films by providing a means to qualitatively determine the major atomic orientation within nanocrystalline and, eventually, non-crystalline films.

Evolution of Atomic-Level Structure in Sub-10 Nanometer Iron Oxide Nanocrystals: Influence on Cation Occupancy and Growth Rates

Spinel iron oxide nanocrystals (NCs) have been reported to have atomic-level core and surface structural features that differ from those of the bulk material. Recent advances in a continuous growth synthesis of metal oxide NCs make it possible to prepare a series of NCs with subnanometer control of size with diameters below 10 nm that are well-suited for investigating size-dependent structure and reactivity. Here, we study the evolution of size-dependent structure in spinel iron oxide and determine how nanoscale structure influences the growth of NCs. We synthesized spinel iron oxide NCs via a continuous growth method that permits layer-by-layer control of size in order to monitor nanoscale structure over 16 core sizes between 3 and 10 nm. X-ray total scattering data were collected and analyzed with pair distribution function (PDF) analysis in order to refine quantitative structural features including cation occupancies that could be used to detect changes both in the oxidation state and the presence of tetrahedrally coordinated cation vacancies in the NCs. We find that the average iron oxidation state increases as core diameters decrease from 8 down to 3 nm. The trend in iron oxidation state can be explained by the oxidation of surface layers in the NCs. For samples exposed to air for several weeks, oxidation appears to cease when a volume equivalent to that of an ∼1.3 nm shell is converted to the more oxidized maghemite. The number of tetrahedrally coordinated cation vacancies also increases as the NC core size decreases. The correlation between the number of these vacancies and the faster growth for smaller NCs suggests that these reactive vacancies may be responsible for the rapid growth observed for nanocrystals with diameters smaller than 8 nm.

Mesoscale Polarization by Geometric Frustration in Columnar Supramolecular Crystals

Columnar supramolecular phases with polarization along the columnar axis have potential for the development of ultrahigh‐density memories as every single column might function as a memory element. By investigating structure and disorder for four columnar benzene‐1,3,5‐trisamides by total X‐ray scattering and DFT calculations, we demonstrate that the column orientation, and thus the columnar dipole moment, is receptive to geometric frustration if the columns aggregate in a hexagonal rod packing. The frustration suppresses conventional antiferroelectric order and heightens the sensitivity towards collective intercolumnar packing effects. The latter finding allows for the building up of mesoscale domains with spontaneous polarization. Our results suggest how the complex interplay between steric and electrostatic interactions is influenced by a straightforward chemical design of the molecular synthons to create spontaneous polarization and to adjust mesoscale domain size.

XDS: a flexible beamline for X-ray diffraction and spectroscopy at the Brazilian synchrotron

The majority of the beamlines at the Brazilian Synchrotron Light Source Laboratory (LNLS) use radiation produced in the storage-ring bending magnets and are therefore currently limited in the flux that can be used in the harder part of the X-ray spectrum (above ∼10 keV). A 4 T superconducting multipolar wiggler (SCW) was recently installed at LNLS in order to improve the photon flux above 10 keV and fulfill the demands set by the materials science community. A new multi-purpose beamline was then installed at the LNLS using the SCW as a photon source. The XDS is a flexible beamline operating in the energy range between 5 and 30 keV, designed to perform experiments using absorption, diffraction and scattering techniques. Most of the work performed at the XDS beamline concentrates on X-ray absorption spectroscopy at energies above 18 keV and high-resolution diffraction experiments. More recently, new setups and photon-hungry experiments such as total X-ray scattering, X-ray diffraction under high pressures, resonant X-ray emission spectroscopy, among others, have started to become routine at XDS. Here, the XDS beamline characteristics, performance and a few new experimental possibilities are described.

Mineralogy of iron microbial mats from loihi seamount

Extensive mats of Fe oxyhydroxides and associated Fe-oxidizing microbial organisms form in diverse geochemical settings - freshwater seeps to deep-sea vents - where ever opposing Fe(II)-oxygen gradients prevail. The mineralogy, reactivity, and structural transformations of Fe oxyhydroxides precipitated from submarine hydrothermal fluids within microbial mats remains elusive in active and fossil systems. In response, a study of Fe microbial mat formation at the Loihi Seamount was conducted to describe the physical and chemical characteristics of Fe-phases using extended X-ray absorption fine structure spectroscopy, powder X-ray diffraction, synchrotron radiation X-ray total scattering, low-temperature magnetic measurements, and Mössbauer spectroscopy. Particle sizes of 3.5-4.6 nm were estimated from magnetism data, and coherent scattering domain (CSD) sizes as small as 1.6 nm are indicated by pair distribution function (PDF) analysis. Disorder in the nanostructured Fe-bearing phases results in limited intermediate-range structural order: less than that of standard two-line ferrihydrite (Fh), except for the Pohaku site. The short-range ordered natural Fh (Fh(SRO)) phases were stable at 4°C in the presence of oxygen for at least 1 year and during 400°C treatment. The observed stability of the Fh(SRO) is consistent with magnetic observations that point to non-interacting nanoparticles. PDF analyses of total scattering data provide further evidence for Fh(SRO) particles with a poorly ordered silica coating. The presence of coated particles explains the small CSD for the mat minerals, as well as the stability of the minerals over time and against heating. The mineral properties observed here provide a starting point from which progressively older and more extensively altered Fe deposits may be examined, with the ultimate goal of improved interpretation of past biogeochemical conditions and diagenetic processes.