356 research outputs found
Many-Beam Solution to the Phase Problem in Crystallography
Solving crystal structures from electron diffraction patterns rather than
X-ray diffraction data is hampered by multiple scattering of the fast electrons
within even very thin samples and the difficulty of obtaining diffraction data
at a resolution high enough for applying direct phasing methods. This letter
presents a method by which the effect of multiple scattering is being used for
solving the phase problem, allowing the retrieval of electron structure factors
from diffraction patterns recorded with varying angle of incidence without any
assumption about the scattering potential itself. In particular, the resolution
in the diffraction data does not need to be sufficient to resolve atoms, making
this method particularly interesting for electron crystallography of
2-dimensional protein crystals and other beam-sensitive complex structures.Comment: 4 pages, 3 figure
Increasing Spatial Fidelity and SNR of 4D-STEM using Multi-frame Data Fusion
4D-STEM, in which the 2D diffraction plane is captured for each 2D scan
position in the scanning transmission electron microscope (STEM) using a
pixelated detector, is complementing and increasingly replacing existing
imaging approaches. However, at present the speed of those detectors, although
having drastically improved in the recent years, is still 100 to 1,000 times
slower than the current PMT technology operators are used to. Regrettably, this
means environmental scanning-distortion often limits the overall performance of
the recorded 4D data. Here we present an extension of existing STEM distortion
correction techniques for the treatment of 4D-data series. Although applicable
to 4D-data in general, we use electron ptychography and electric-field mapping
as model cases and demonstrate an improvement in spatial-fidelity,
signal-to-noise ratio (SNR), phase-precision and spatial-resolution
Lossy Compression of Electron Diffraction Patterns for Ptychography via Change of Basis
Ptychography is a computational imaging technique that has risen in
popularity in the x-ray and electron microscopy communities in the past half
decade. One of the reasons for this success is the development of new high
performance electron detectors with increased dynamic range and readout speed,
both of which are necessary for a successful application of this technique.
Despite the advances made in computing power, processing the recorded data
remains a challenging task, and the growth in data rate has made the size of
the resulting datasets a bottleneck for the whole process. Here we present an
investigation into lossy compression methods for electron diffraction patterns
that retain the necessary information for ptychographic reconstructions, yet
lead to a decrease in data set size by three or four orders of magnitude. We
apply several compression methods to both simulated and experimental data - all
with promising results
Investigation of the electrostatic potential of a grain boundary in Y-substituted BaZrO3 using inline electron holography
We apply inline electron holography to investigate the electrostatic
potential across an individual BaZr0.9Y0.1O3 grain boundary. With holography,
we measure a grain boundary potential of -1.3 V. Electron energy loss
spectroscopy analyses indicate that barium vacancies at the grain boundary are
the main contributors to the potential well in this sample. Furthermore,
geometric phase analysis and density functional theory calculations suggest
that reduced atomic density at the grain boundary also contributes to the
experimentally measured potential well
Various Compressed Sensing Set-Ups Evaluated Against Shannon Sampling Under Constraint of Constant Illumination
Under the constraint of constant illumination, an information criterion is
formulated for the Fisher information that compressed sensing measurements in
optical and transmission electron microscopy contain about the underlying
parameters. Since this approach requires prior knowledge of the signal's
support in the sparse basis, we develop a heuristic quantity, the detective
quantum efficiency (DQE), that tracks this information criterion well without
this knowledge. It is shown that for the investigated choice of sensing
matrices, and in the absence of read-out noise, i.e. with only Poisson noise
present, compressed sensing does not raise the amount of Fisher information in
the recordings above that of Shannon sampling. Furthermore, enabled by the
DQE's analytical tractability, the experimental designs are optimized by
finding out the optimal fraction of on-pixels as a function of dose and
read-out noise. Finally, we introduce a regularization and demonstrate, through
simulations and experiment, that it yields reconstructions attaining minimum
mean squared error at experimental settings predicted by the DQE as optimal.Comment: 18 pages, 13 figures. New Monte Carlo simulations in Figure 13
showing the behavior of the single-pixel camera under various magnitudes of
read-out nois
Probing Crystallinity and Grain Structure of 2D Materials and 2D-Like Van der Waals Heterostructures by Low-Voltage Electron Diffraction
4D scanning transmission electron microscopy (4D-STEM) is a powerful method for characterizing electron-transparent samples with down to sub-Ångstrom spatial resolution. 4D-STEM can reveal local crystallinity, orientation, grain size, strain, and many more sample properties by rastering a convergent electron beam over a sample area and acquiring a transmission diffraction pattern (DP) at each scan position. These patterns are rich in information about the atomic structure of the probed volume, making this technique a potent tool to characterize even inhomogeneous samples. 4D-STEM can also be used in scanning electron microscopes (SEMs) by placing an electron-sensitive camera below the sample. 4D-STEM-in-SEMs is ideally suited to characterize 2D materials and 2D-like van der Waals heterostructures (vdWH) due to their inherent thickness of a few nanometers. The lower accelerating voltage of SEMs leads to strong scattering even from monolayers. The large field of view and down to sub-nm spatial resolution of SEMs are ideal to map properties of the different constituents of 2D-like vdWH by probing their combined sample volume. A unique 4D-STEM-in-SEM system is applied to reveal the single crystallinity of MoS2 exfoliated with gold-mediation as well as the crystal orientation and coverage of both components of a C60/MoS2 vdWH are determined
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