176 research outputs found
Real time in situ x-ray diffraction study of the crystalline structure modification of Ba0.5Sr0.5TiOâ during the post-annealing
We report about an in situ study of crystalline structural changes during thermal treatment of a Ba0.5Sr0.5TiO3 (BSTO) film grown on MgO. The study covers the complete cycle of heating, annealing and cooling and reveals simultaneous phenomena of phase transitions and strain evolution, which have been characterized by in situ 2D reciprocal space mapping (2D-RSM) using high-resolution synchrotron x-ray diffraction in coplanar and grazing incidence geometries. In this way, temperature induced phase transformation from the BSTO2 to the BSTO1 phase has been monitored and the appearance of a further crystalline phase was detected. Moreover, for both BSTO phases, transitions between in-plane compressive and tensile states have been determined during thermal treatment. Furthermore, a contraction of the out-of-plane lattice components has been observed during the annealing phase while the in-plane lattice components remain leading to the change of the residual in-plane strain towards tensile state. The in situ 2D-RSM findings provide valuable and versatile insights into strain engineering and structure modification upon thermal treatment
Improved success rate and stability for phase retrieval by including randomized overrelaxation in the hybrid input output algorithm
Contrast transfer functions for Zernike phase contrast in full-field transmission hard X-ray microscopy
Full-field transmission hard X-ray microscopy (TXM) has been widely applied to study morphology and structures with high spatial precision and to dynamic processes. Zernike phase contrast (ZPC) in hard X-ray TXM is often utilized to get an in-line phase contrast enhancement for weak-absorbing materials with little contrast differences. Here, following forward image formation, we derive and simplify the contrast transfer functions (CTFs) of the Zernike phase imaging system in TXM based on a linear space-shift-invariant imaging mode under certain approximations. The CTFs in ZPC in their simplified forms show a high similarity to the one in free-space propagation X-ray imaging systems
The Bragg demagnifier: X-ray imaging with kilometer propagation distance within a meter
We introduce a new X-ray imaging technique to facilitate propagation-based
phase contrast of large, centimeter-sized samples. The diffracted X-ray
wavefield behind the sample is demagnified by asymmetric Bragg crystal optics,
thereby virtually increasing the propagation distance and thus enhancing the
image contrast. We demonstrate the significant increase in image contrast
compared to conventional phase contrast imaging at the same short physical
propagation distance. Additionally, the Bragg demagnifier enables the reduction
of image blur caused by the finite X-ray source size. In combination with a
subsequent Bragg magnifier, the method will allow for an even higher dose
efficiency, rendering this technique a potential candidate for, e.g., low-dose
(bio)medical diagnostics
Cranial shape evolution of extant and fossil crocodile newts and its relation to reproduction and ecology
The diversity of the vertebrate cranial shape of phylogenetically related taxa allows conclusions on ecology and life history. As pleurodeline newts (the genera Echinotriton, Pleurodeles and Tylototriton) have polymorphic reproductive modes, they are highly suitable for following cranial shape evolution in relation to reproduction and environment. We investigated interspecific differences externally and differences in the cranial shape of pleurodeline newts via two-dimensional geometric morphometrics. Our analyses also included the closely related but extinct genus Chelotriton to better follow the evolutionary history of cranial shape. Pleurodeles was morphologically distinct in relation to other phylogenetically basal salamanders. The subgenera within Tylototriton (Tylototriton and Yaotriton) were well separated in morphospace, whereas Echinotriton resembled the subgenus Yaotriton more than Tylototriton. Oviposition site choice correlated with phylogeny and morphology. Only the mating mode, with a random distribution along the phylogenetic tree, separated crocodile newts into two morphologically distinct groups. Extinct Chelotriton likely represented several species and were morphologically and ecologically more similar to Echinotriton and Yaotriton than to Tylototriton subgenera. Our data also provide the first comprehensive morphological support for the molecular phylogeny of pleurodeline newts
A Shack-Hartmann sensor for single-shot multi-contrast imaging with hard X-rays
An array of compound refractive X-ray lenses (CRL) with 20x20 lenslets, a
focal distance of 20 cm and a visibility of 0.93 is presented. It can be used
as a Shack-Hartmann sensor for hard X-rays (SHARX) for wavefront sensing and
permits for true single-shot multi-contrast imaging the dynamics of materials
with a spatial resolution in the micrometer range, sensitivity on nanosized
structures and temporal resolution on the microsecond scale. The object's
absorption and its induced wavefront shift can be assessed simultaneously
together with information from diffraction channels. This enables the imaging
of hierarchical materials. In contrast to the established Hartmann sensors the
SHARX has an increased flux efficiency through focusing of the beam rather than
blocking parts of it. We investigated the spatiotemporal behavior of a
cavitation bubble induced by laser pulses. Furthermore, we validated the SHARX
by measuring refraction angles of a single diamond CRL, where we obtained an
angular resolution better than 4 microrad
Quantitative analysis of time-resolved RHEED during growth of vertical nanowires
We present an approach for quantitative evaluation of time-resolved reflection high-energy electron diffraction (RHEED) intensity patterns measured during the growth of vertical, free-standing nanowires (NWs). The approach considers shadowing due to attenuation by absorption and extinction within the individual nanowires and estimates the time dependence of its influence on the RHEED signal of the nanowire ensemble as a function of instrumental RHEED parameters and the growth dynamics averaged over the nanowire ensemble. The developed RHEED simulation model takes into account the nanowire structure evolution related to essential growth aspects, such as axial growth, radial growth with tapering and facet growth, as well as so-called parasitic intergrowth on the substrate. It also considers the influence of the NW density, which turns out to be a sensitive parameter for the time-dependent interpretation of the intensity patterns. Finally, the application potential is demonstrated by evaluating experimental data obtained during molecular beam epitaxy (MBE) of self-catalysed GaAs nanowires. We demonstrate, how electron shadowing enables a time-resolved analysis of the crystal structure evolution at the top part of the growing NWs. The approach offers direct access to study growth dynamics of polytypism in nanowire ensembles at the growth front region under standard growth conditions
Combined In Situ XRD and Ex Situ TEM Studies of Thin BaSrTiO Films Grown by PLD on MgO
Dielectric barium strontium titanate films were deposited on MgO (001) substrate by pulsedâlaser deposition (PLD) and monitored in situ by means of reflection highâenergy electron diffraction and timeâresolved Xâray diffraction (TRXRD). TRXRD showed two growth periods of the BSTO film and a transformation in the crystalline structure was detected as the thickness exceeds 80 nm. The occurrence of two different crystalline regions, namely BSTO1 and BSTO2 was proved by Xâray diffraction (XRD). Ex situ transmission electron microscopy (TEM) techniques, including diffractionâcontrast as well as highâresolution TEM, nanobeam electron diffraction, and scanning TEM in combination with energyâdispersive Xâray spectroscopy reveal structural and microchemical peculiarities of the BSTO film. By these TEM analyses, the presence of the two different regions BSTO1 and BSTO2 within the PLDâgrown BSTO layer was demonstrated. Regions of phase BSTO2 were found on top of nanoscaled MgO islands formed on the substrate surface during annealing at high temperature. While the majority phase BSTO1 has a singleâcrystalline structure over wide ranges, BSTO2 regions seem to be polyâ or even nanocrystalline, and the chemical composition of the two phases is also different. The transition in the growth periods is presumably related to the occurrence of BSTO2 regions during layer growth
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