Günter Goerigk

Small-angle scattering investigations of magnesium hydride used as a hydrogen storage material

In this work, high-energy ball-milled magnesium hydride samples used for hydrogen storage are investigated using small- and ultra-small-angle neutron scattering (SANS) as well as small-angle X-ray scattering (SAXS). Size distributions of inhomogeneities with dimensions from 10 A up to more than 10 µm, corresponding to crystallite and particle sizes obtained by X-ray diffraction and electron microscopy, are determined as a function of milling time, milling tool material and added metal oxide catalysts in order to study the influence of the microstructure on the sorption kinetics. Significant changes of the volume fraction distributions are found for samples containing the catalyst chromium oxide, particularly when the catalyst particles are nanometre-sized. Cr2O3 is an effective agent for breaking up particles during the milling process. The comparison of SANS and SAXS curves give some of the first information about the distribution of hydrogen-containing structures. Using anomalous small-angle X-ray scattering, an energy-dependent scattering is found for an MgHx sample with 1 mol% Fe2O3. From the separated scattering curve a size distribution of hard spheres is obtained with a size range which is expected for crystallite and particle sizes of the Fe2O3 catalyst. Chemical shifts in the absorption spectra give information about the stability of the metal oxide catalysts during the milling process.

Structural and magnetic characterization of Fe/δ-Mn thin films

Abstract We report on a structural and magnetic investigation of Fe/δ-Mn multilayers. In situ LEED shows that Mn grows epitaxially on BCC Fe with a good crystallinity up to 30 A. Reflectivity and small-angle diffuse scattering of synchrotron radiation on Fe/Mn/Fe sandwiches reveal sharp interfaces with correlated roughness. From MOKE measurements, evidence for non-collinear coupling between the Fe layers was found for Mn spacer thicknesses below 13 A. With polarized neutron reflectometry and high-angle neutron diffraction we could not detect any sign of magnetic order inside the Mn layers.

Contrast variation by anomalous X-ray scattering applied to investigation of the interface morphology in a giant magnetoresistance Fe/Cr/Fe trilayer

The structural properties of an epitaxically grown Fe/Cr/Fe trilayer were studied with anomalous X-ray scattering. Two different X-ray energies have been used: (i) E1 = 5985 eV to match the maximum contrast of the Fe/Cr interface close to the Cr absorption K edge; (ii) E2 = 6940 eV where the Fe/Cr interface displays the lowest contrast. The specular reflectivity and longitudinal diffuse scans together with ω scans for both energies were measured. The simulations within the frame of the distorted-wave Born approximation allowed a quantitative description of the morphology of each interface. The roughness, Hurst parameter and the thickness of every layer, as well as an oxidation effect at the surface of the sample, are derived. The strength and limitations of the method are discussed.

Anomalous small angle X-ray scattering study of CdTe quantum dots in ZnTe

Abstract Anomalous small angle X-ray scattering (ASAXS) from the molecular beam epitaxially grown multilayers of quantum dots CdTe separated by ZnTe layers on the ZnTe/CdTe/GaAs(001) support has been observed. The first sample denoted 15 ML contained 200 layers of 2.5 ML CdTe/15 ML ZnTe and the second sample denoted 75 ML contained 200 layers of 2.5 ML CdTe/75 ML ZnTe. An enhancement of scattering by quantum dots was observed for photon energy near the Zn K absorption edge. We found that the ASAXS from the 15 ML sample was by one order of magnitude higher than that from the 5 ML sample, which means that the number of quantum dots in the 15 ML sample was one order of magnitude bigger than in the 75 ML sample. The scattering curves indicate a broad size distribution of quantum dots in both samples.

Coaggregation of Two Anionic Azo Dyestuffs: A Combined Static Light Scattering and Small-Angle X-ray Scattering Study.

The formation of azo dyestuff aggregates in dilute aqueous solution induced by the addition of Mg2+, Ca2+, Sr2+, or Ba2+ ions is followed by time-resolved static light scattering (SLS) and time-resolved small-angle X-ray scattering (SAXS). Time-dependent molar mass data of the growing aggregates is interpreted by means of a kinetic model introduced by Lomakin et al. ( Proc. Natl. Acad. Sci. U.S.A. 1996 , 93 , 1125 ) for the description of β-amyloid aggregation. This interpretation reveals significant trends within the homologous series of alkaline earth cations. The trends refer to the nucleation and the growth rate of the dyestuff fibers. Time-resolved SAXS experiments indicate that these first two stages are followed by a third one during which a network forms by partial lateral alignment of fibers. At high enough dyestuff concentrations, this network formation even leads to a gel-like phase. Anomalous SAXS (ASAXS) on such a gel phase formed upon the addition of Sr2+ revealed the extent of neutralization of the dyestuff molecules within the gel by the specifically interacting alkaline earth cations.

Three-dimensional quantum dot crystal formation in CdTe/ZnTe superlattices

The observation of both vertical and lateral ordering of quantum dots in CdTe/ZnTe superlattices with different thickness of ZnTe spacer is reported. The simultaneous presence of both types of ordering in the studied structures is evidenced by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) studies. The TEM results show that when the spacer thickness is smaller than 25 monolayers, the islands are vertically correlated along the axis tilted by 40° with respect to the growth direction. On the other hand, the SAXS spectra exhibit four intensity maxima of scattering at q = 0.2 A -1 along [100], [100], [010] and [010] crystallographic directions. We interpret them as originating from a locally formed square quantum dot lattice.