99 research outputs found
Bragg Coherent Modulation Imaging for Highly Strained Nanocrystals-A Numerical Study
Bragg coherent diffraction imaging (BCDI) is a unique and powerful method for
tracking three-dimensional strain fields non-destructively. While BCDI has been
successfully applied to many scientific research fields and receives high
demands, the reconstructed results for highly strained crystals are still
subject to big uncertainties. Here, the progress in improving the suitability
of BCDI for general samples by exploiting wavefront modulation is reported.
Extensive numerical simulations demonstrate that significant improvements over
the current method for reconstructing highly strained model nanocrystals can be
achieved. The proposed method highly suppresses the appearance of ambiguous
solutions and exhibits fast convergence and high robustness in phase retrieval.
Possible experimental difficulties in implementing this method are discussed in
detail
Ultrafast Structural Dynamics of Photo-Reactions Revealed by Model-Independent X-ray Cross-Correlation Analysis
We applied angular X-ray Cross-Correlation analysis (XCCA) to scattering
images from a femtosecond resolution LCLS X-ray free-electron laser (XFEL)
pump-probe experiment with solvated PtPOP
([Pt(POH)]) metal complex molecules. The molecules
were pumped with linear polarized laser pulses creating an excited state
population with a preferred orientational (alignment) direction. Two time
scales of ps and ps were revealed by model-independent
XCCA, associated with an internal structural changes and rotational dephasing,
respectively. Our studies illustrate the potential of XCCA to reveal hidden
structural information in a model independent analysis of time evolution of
solvated metal complex molecules.Comment: 8 pages, 5 figures, 50 reference
Classification of diffraction patterns in single particle imaging experiments performed at X-ray free-electron lasers using a convolutional neural network
Single particle imaging (SPI) is a promising method for native structure
determination which has undergone a fast progress with the development of X-ray
Free-Electron Lasers. Large amounts of data are collected during SPI
experiments, driving the need for automated data analysis. The necessary data
analysis pipeline has a number of steps including binary object classification
(single versus multiple hits). Classification and object detection are areas
where deep neural networks currently outperform other approaches. In this work,
we use the fast object detector networks YOLOv2 and YOLOv3. By exploiting
transfer learning, a moderate amount of data is sufficient for training of the
neural network. We demonstrate here that a convolutional neural network (CNN)
can be successfully used to classify data from SPI experiments. We compare the
results of classification for the two different networks, with different depth
and architecture, by applying them to the same SPI data with different data
representation. The best results are obtained for YOLOv2 color images linear
scale classification, which shows an accuracy of about 97% with the precision
and recall of about 52% and 61%, respectively, which is in comparison to manual
data classification.Comment: 23 pages, 6 figures, 3 table
Local structure of semicrystalline P3HT films probed by nanofocused coherent x-rays
We present results of an x-ray study of structural properties of
semicrystalline polymer films using nanofocused x-ray beam. We applied the
x-ray cross-correlation analysis (XCCA) to scattering data from blends of
poly(3-hexylthiophene) (P3HT) embedded with gold nanoparticles (AuNPs).
Spatially resolved maps of orientational distribution of crystalline domains
allow us to distinguish sample regions of predominant face-on morphology,with a
continuous transition to edge-on morphology. The average size of crystalline
domains was determined to be of the order of 10 nm. As compared to pristine
P3HT film, the P3HT/AuNPs blend is characterized by substantial ordering of
crystalline domains, which can be induced by Au nanoparticles. The
inhomogeneous structure of the polymer film is clearly visualized on the
spatially resolved nanoscale 2D maps obtained using XCCA. Our results suggest
that the observed changes of the polymer matrix within crystalline regions can
be attributed to nanoconfinement in the presence of gold nanoparticles.Comment: 10 pages, 6 figures, 53 reference
Gas-induced segregation in Pt-Rh alloy nanoparticles observed by in-situ Bragg coherent diffraction imaging
Bimetallic catalysts can undergo segregation or redistribution of the metals
driven by oxidizing and reducing environments. Bragg coherent diffraction
imaging (BCDI) was used to relate displacement fields to compositional
distributions in crystalline Pt-Rh alloy nanoparticles. 3D images of internal
composition showed that the radial distribution of compositions reverses
partially between the surface shell and the core when gas flow changes between
O2 and H2. Our observation suggests that the elemental segregation of
nanoparticle catalysts should be highly active during heterogeneous catalysis
and can be a controlling factor in synthesis of electrocatalysts. In addition,
our study exemplifies applications of BCDI for in situ 3D imaging of internal
equilibrium compositions in other bimetallic alloy nanoparticles
Probing the Surface Polarization of Ferroelectric Thin Films by X-ray Standing Waves
Understanding the mechanisms underlying a stable polarization at the surface
of ferroelectric thin films is of particular importance both from a fundamental
point of view and to achieve control of the surface polarization itself. In
this study, it is demonstrated that the X-ray standing wave technique allows
the polarization near the surface of a ferroelectric thin film to be probed
directly. The X-ray standing wave technique is employed to determine, with
picometer accuracy, Ti and Ba atomic positions near the surface of three
differently strained thin films grown on scandate
substrates, with a film as bottom electrode. This technique
gives direct access to atomic positions, and thus to the local ferroelectric
polarization, within the first 3 unit cells below the surface. By employing
X-ray photoelectron spectroscopy, a detailed overview of the oxygen-containing
species adsorbed on the surface, upon exposure to ambient conditions, is
obtained. The combination of structural and spectroscopic information allows us
to conclude on the most plausible mechanisms that stabilize the surface
polarization in the three samples under study. The different amplitude and
orientation of the local ferroelectric polarizations are associated with
surface charges attributed to the type, amount and spatial distribution of the
oxygen-containing adsorbates
Angular X-ray cross-correlation analysis applied to the scattering data in 3D reciprocal space from a single crystal
An application of angular X-ray cross-correlation analysis (AXCCA) to the scattered intensity distribution measured in 3D reciprocal space from a single-crystalline sample is proposed in this work. Contrary to the conventional application of AXCCA, when averaging over many 2D diffraction patterns collected from different randomly oriented samples is required, the proposed approach provides an insight into the structure of a single specimen. This is particularly useful in studies of defect-rich samples that are unlikely to have the same structure. The application of the method is shown on an example of a qualitative structure determination of a colloidal crystal from simulated as well as experimentally measured 3D scattered intensity distributions
Single Alloy Nanoparticle X-Ray Imaging during a Catalytic Reaction
The imaging of active nanoparticles represents a milestone in decoding
heterogeneous catalysts dynamics. We report the facet resolved, surface strain
state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent
x-ray diffraction imaging under catalytic reaction conditions. Density
functional theory calculations allow us to correlate the facet surface strain
state to its reaction environment dependent chemical composition. We find that
the initially Pt terminated nanoparticle surface gets Rh enriched under CO
oxidation reaction conditions. The local composition is facet orientation
dependent and the Rh enrichment is non-reversible under subsequent CO
reduction. Tracking facet resolved strain and composition under operando
conditions is crucial for a rational design of more efficient heterogeneous
catalysts with tailored activity, selectivity and lifetime.Comment: 15 pages, 4 figures, 32 reference
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