Ralph Wagner

Phase separation and nanocrystal formation in GeO

The temperature-induced phase separation (disproportionation) and Ge nanocrystal formation in bulk amorphous germanium monoxide (a-GeOx,x≈1) are studied both in situ and ex situ by measurements of the x-ray absorption near edge structure at the Ge K-edge and x-ray diffraction. The considerable amount of suboxides contained in the native a-GeO samples decreases with increasing annealing temperature. The phase separation sets in at a temperature of 260±20 °C and is almost completed at a temperature of 450±18 °C before nanocrystal formation occurs. Ge nanocrystals of a few nanometers in diameter are observed for an annealing temperature of 509±15 °C. The time dependence of the phase separation and the effect of different annealing procedures are discussed. The presented results provide important information for the production of Ge nanocrystals embedded in amorphous oxide matrices which are relevant for optoelectronic applications.

The materials science X-ray beamline BL8 at the DELTA storage ring

The hard X-ray beamline BL8 at the superconducting asymmetric wiggler at the 1.5 GeV Dortmund Electron Accelerator DELTA is described. This beamline is dedicated to X-ray studies in the spectral range from approximately 1 keV to approximately 25 keV photon energy. The monochromator as well as the other optical components of the beamline are optimized accordingly. The endstation comprises a six-axis diffractometer that is capable of carrying heavy loads related to non-ambient sample environments such as, for example, ultrahigh-vacuum systems, high-pressure cells or liquid-helium cryostats. X-ray absorption spectra from several reference compounds illustrate the performance. Besides transmission measurements, fluorescence detection for dilute sample systems as well as surface-sensitive reflection-mode experiments have been performed. The results show that high-quality EXAFS data can be obtained in the quick-scanning EXAFS mode within a few seconds of acquisition time, enabling time-resolved in situ experiments using standard beamline equipment that is permanently available. The performance of the new beamline, especially in terms of the photon flux and energy resolution, is competitive with other insertion-device beamlines worldwide, and several sophisticated experiments including surface-sensitive EXAFS experiments are feasible.

The multi‐purpose hard X‐ray beamline BL10 at the DELTA storage ring

The layout and the characteristics of the hard X-ray beamline BL10 at the superconducting asymmetric wiggler at the 1.5 GeV Dortmund Electron Accelerator DELTA are described. This beamline is equipped with a Si(111) channel-cut monochromator and is dedicated to X-ray studies in the spectral range from ∼4 keV to ∼16 keV photon energy. There are two different endstations available. While X-ray absorption studies in different detection modes (transmission, fluorescence, reflectivity) can be performed on a designated table, a six-axis kappa diffractometer is installed for X-ray scattering and reflectivity experiments. Different detector set-ups are integrated into the beamline control software, i.e. gas-filled ionization chambers, different photodiodes, as well as a Pilatus 2D-detector are permanently available. The performance of the beamline is illustrated by high-quality X-ray absorption spectra from several reference compounds. First applications include temperature-dependent EXAFS experiments from liquid-nitrogen temperature in a bath cryostat up to ∼660 K by using a dedicated furnace. Besides transmission measurements, fluorescence detection for dilute sample systems as well as surface-sensitive reflection-mode experiments are presented.

X-ray near-edge absorption study of temperature-induced low-spin-to-high-spin change in metallo-supramolecular assemblies.

X-ray absorption near the iron K edge (XANES) was used to investigate the characteristics of temperature-induced low-spin-to-high-spin change (SC) in metallo-supramolecular polyelectrolyte amphiphile complexes (PAC) containing FeN(6) octahedra attached to two or six amphiphilic molecules. Compared to the typical spin-crossover material Fe(phen)(2) (NCS)(2) XANES spectra of PAC show fingerprint features restricted to the near-edge region which mainly measures multiple scattering (MS) events. The changes of the XANES profiles during SC are thus attributed to the structure changes due to different MS path lengths. Our results can be interpreted by a uniaxial deformation of FeN(6) octahedra in PAC. This is in agreement with the prediction that SC is originated by a structural phase transition in the amphiphilic matrix of PAC, but in contrast to Fe(phen)(2) (NCS)(2), showing the typical spin crossover being associated with shortening of all the metal-ligand distances.

Influence of hydrogen on thermally induced phase separation in GeO/SiO2 multilayers.

The influence of the annealing atmosphere on the temperature induced phase separation of Ge oxide in GeO(x)/SiO(2) multilayers (x≈1), leading to size controlled growth of Ge nanocrystals, is explored by means of x-ray absorption spectroscopy at the Ge K-edge. Ge sub-oxides contained in the as-deposited multilayers diminish with increasing annealing temperature, showing complete phase separation at approximately 450 °C using inert N(2) ambient. The use of reducing H(2) in the annealing atmosphere influences the phase separation even at an early stage of the disproportionation. In particular, the temperature regime where the phase separation occurs is lowered by at least 50 °C. At temperatures above 400 °C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO(2) by H(2).

XAS at the materials science X-ray beamline BL8 at the DELTA storage ring

The hard X-ray beamline BL 8 at the 1.5 GeV electron storage ring DELTA is described, and experimental data of different fields of research are presented. Making use of the intense X-ray beam emitted by a superconducting wiggler, the beamline is dedicated to X-ray absorption experiments. Three different monochromator crystal pairs are permanently available for experiments in the spectral range from about 1 keV to ca. 25 keV photon energy. Results of reference materials show that high quality EXAFS data can be obtained using Si(111) and Si(311) monochromators. First measurements in the energy range between 1.2 and 5 keV have been accomplished using YB66(400) monochromator crystals. The experimental hutch accommodates a unique 6-axis diffractometer which is well suited for all kinds of diffraction and absorption experiments, including the grazing incidence geometry. The diffractometer can carry heavy loads related to non-ambient sample environments such as e.g. ultrahigh vacuum sample stages or cryostats. Here we present typical results obtained at BL8 in different areas of materials science including investigations of dilute alloys by fluorescence mode EXAFS and the study of the structural changes associated with temperature induced spin transitions of metallo-supramolecular polyelectrolyte-amphiphile-complexes.

In situ surface-sensitive X-ray investigations of thin quench condensed bismuth films

In situ X-ray reflectometry, grazing-incidence X-ray absorption spectroscopy and simultaneous electrical resistivity measurements were used to investigate the microstructure of (5.9±0.1) nm thick bismuth films quench condensed at (20±1) K onto float-glass substrates. A subsequent annealing process induces an irreversible transformation of the film structure from an amorphous to a highly disordered nanocrystalline structure above (42±2) K. This transformation is accompanied by an irreversible increase of the film resistivity by a factor of more than 2.7. Different negative temperature coefficients of resistance were found directly after deposition and after the annealing process. No changes of the film density, which is always less than the density of the bulk material, or of the film roughness, are observed as a result of the heat treatment.

Ex-situ and in-situ investigations of thermal anti-oxidation treatments of stainless steels by reflection mode EXAFS

Different thermal treatments were performed for the anti-oxidation of steel surfaces, and the processes were investigated ex-situ and in-situ by surface sensitive reflection mode EXAFS experiments at the Cr and Fe K-edges. While the samples for the ex-situ studies were heat-treated in a conveyor belt furnace at temperatures between 600 and 900 °C in inert carrier gases (N2 or Ar) using different additives such as hydrogen (H2) and monosilane (SiH4), the in- situ anti-oxidation treatments have been performed in a high vacuum environment (p < 10-6 mbar). While the ex-situ experiments suggest that SiH4-additives are needed for the reduction of the steel to a metallic state, the in-vacuum treatments appear successful for temperatures above approx. 900 °C.

The materials science X‐ray beamline BL8 at the DELTA storage ring

The instrumentation of the hard X‐ray beamline BL8 at the 1.5 GeV Dortmund Electron Accelerator DELTA is described, and the properties of the X‐ray optical elements are presented together with experimental data of different fields of research. The beamline, which makes use of the photons emitted by a superconducting asymmetric wiggler, is dedicated to X‐ray studies—especially X‐ray absorption spectroscopy (XAS)—in the spectral range from about 1 keV to ca. 25 keV photon energy using three different monochromators, namely YB66(400), Si(111) and Si(311). The endstation comprises a 6‐axis diffractometer that is capable of carrying heavy loads related to non‐ambient sample environments such as high pressure cells, cryostats or even complete ultrahigh vacuum systems. X‐ray absorption spectra from several reference compounds illustrate the performance of the beamline optics concerning e.g. energy range, energy resolution and beam stability. Fluorescence detection enables the investigation of dilute sample syste...

In-situ investigation of Bi thin film condensation by surface sensitive X-ray absorption spectroscopy at cryogenic temperatures

Reflection mode grazing incidence X-ray absorption spectroscopy at the Bi L3-edge and simultaneous electrical resistivity measurements were used to investigate the microstructure of quench condensed thin Bi metal films on float-glass substrates at temperatures from 20 K to 300 K. While thin films of ≈ 6 nm thickness appear to be amorphous after deposition at 20 K, thicker films of about 18 nm show the well-known rhombohedral structure of bulk Bi. During a subsequent heat treatment, the amorphous structure of the thin films transforms irreversibly into the crystalline form for temperatures at (42 ± 2) K. This crystallisation is accompanied by an irreversible increase of the film resistivity by a factor of more than 2.7. The film density and roughness do not change within detection limits during the heat treatment.