124 research outputs found
Cryo-Electron Ptychography: Applications and Potential in Biological Characterisation
There is a clear need for developments in characterisation techniques that
provide detailed information about structure-function relationships in biology.
Using electron microscopy to achieve high resolution while maintaining a broad
field of view remains a challenge, particularly for radiation sensitive
specimens where the signal-to-noise ratio required to maintain structural
integrity is limited by low electron fluence. In this review, we explore the
potential of cryogenic electron ptychography as an alternative method for
characterisation of biological systems under low fluence conditions. Using this
method with increased information content from multiple sampled regions of
interest, potentially allows 3D reconstruction with far fewer particles than
required in conventional cryo-electron microscopy. This is important for
achieving higher resolution for systems where distributions of homogeneous
single particles are difficult to obtain. We discuss the progress, limitations
and potential areas for future development of this approach for both single
particle analysis and in applications to heterogeneous large objects
Direct detection of electron backscatter diffraction patterns.
We report the first use of direct detection for recording electron backscatter diffraction patterns. We demonstrate the following advantages of direct detection: the resolution in the patterns is such that higher order features are visible; patterns can be recorded at beam energies below those at which conventional detectors usefully operate; high precision in cross-correlation based pattern shift measurements needed for high resolution electron backscatter diffraction strain mapping can be obtained. We also show that the physics underlying direct detection is sufficiently well understood at low primary electron energies such that simulated patterns can be generated to verify our experimental data
Detectors—The ongoing revolution in scanning transmission electron microscopy and why this important to material characterization
Detectors are revolutionizing possibilities in scanning transmission electron microscopy because of the advent of direct electron detectors that record at a high quantum efficiency and with a high frame rate. This allows the whole back focal plane to be captured for each pixel in a scan and the dataset to be processed to reveal whichever features are of interest. There are many possible uses for this advance of direct relevance to understanding the nano- and atomic-scale structure of materials and heterostructures. This article gives our perspective of the current state of the field and some of the directions where it is likely to go next. First, a wider overview of the recent work in this area is given before two specific examples of its application are given: one is imaging strain in thin films and the other one is imaging changes in periodicity along the beam direction as a result of the formation of an ordered structure in an epitaxial thin film. This is followed by an outlook that presents future possible directions in this rapidly expanding field
SIM-STEM Lab: Incorporating Compressed Sensing Theory for Fast STEM Simulation
Recently it has been shown that precise dose control and an increase in the
overall acquisition speed of atomic resolution scanning transmission electron
microscope (STEM) images can be achieved by acquiring only a small fraction of
the pixels in the image experimentally and then reconstructing the full image
using an inpainting algorithm. In this paper, we apply the same inpainting
approach (a form of compressed sensing) to simulated, sub-sampled atomic
resolution STEM images. We find that it is possible to significantly sub-sample
the area that is simulated, the number of g-vectors contributing the image, and
the number of frozen phonon configurations contributing to the final image
while still producing an acceptable fit to a fully sampled simulation. Here we
discuss the parameters that we use and how the resulting simulations can be
quantifiably compared to the full simulations. As with any Compressed Sensing
methodology, care must be taken to ensure that isolated events are not excluded
from the process, but the observed increase in simulation speed provides
significant opportunities for real time simulations, image classification and
analytics to be performed as a supplement to experiments on a microscope to be
developed in the future.Comment: 20 pages (includes 3 supplementary pages), 15 figures (includes 5
supplementary figures), submitted to Ultramicroscop
Simultaneous High-Speed and Low-Dose 4-D STEM Using Compressive Sensing Techniques
Here we show that compressive sensing allow 4-dimensional (4-D) STEM data to
be obtained and accurately reconstructed with both high-speed and low fluence.
The methodology needed to achieve these results compared to conventional 4-D
approaches requires only that a random subset of probe locations is acquired
from the typical regular scanning grid, which immediately generates both higher
speed and the lower fluence experimentally. We also consider downsampling of
the detector, showing that oversampling is inherent within convergent beam
electron diffraction (CBED) patterns, and that detector downsampling does not
reduce precision but allows faster experimental data acquisition. Analysis of
an experimental atomic resolution yttrium silicide data-set shows that it is
possible to recover over 25dB peak signal-to-noise in the recovered phase using
0.3% of the total data
A Targeted Sampling Strategy for Compressive Cryo Focused Ion Beam Scanning Electron Microscopy
Cryo Focused Ion-Beam Scanning Electron Microscopy (cryo FIB-SEM) enables
three-dimensional and nanoscale imaging of biological specimens via a slice and
view mechanism. The FIB-SEM experiments are, however, limited by a slow
(typically, several hours) acquisition process and the high electron doses
imposed on the beam sensitive specimen can cause damage. In this work, we
present a compressive sensing variant of cryo FIB-SEM capable of reducing the
operational electron dose and increasing speed. We propose two Targeted
Sampling (TS) strategies that leverage the reconstructed image of the previous
sample layer as a prior for designing the next subsampling mask. Our image
recovery is based on a blind Bayesian dictionary learning approach, i.e., Beta
Process Factor Analysis (BPFA). This method is experimentally viable due to our
ultra-fast GPU-based implementation of BPFA. Simulations on artificial
compressive FIB-SEM measurements validate the success of proposed methods: the
operational electron dose can be reduced by up to 20 times. These methods have
large implications for the cryo FIB-SEM community, in which the imaging of beam
sensitive biological materials without beam damage is crucial.Comment: Submitted to ICASSP 202
Atomic Structure and Dynamics of Single Platinum Atom Interactions with Monolayer MoS
We have studied atomic level interactions between single Pt atoms and the surface of monolayer MoSâ‚‚ using aberration-corrected annular dark field scanning transmission electron microscopy at an accelerating voltage of 60 kV. Strong contrast from single Pt atoms on the atomically resolved monolayer MoSâ‚‚ lattice enables their exact position to be determined with respect to the MoSâ‚‚ lattice, revealing stable binding sites. In regions of MoSâ‚‚ free from surface contamination, the Pt atoms are localized in S vacancy sites and exhibit dynamic hopping to nearby vacancy sites driven by the energy supplied by the electron beam. However, in areas of MoSâ‚‚ contaminated with carbon surface layers, the Pt atoms appear at various positions with respect to the underlying MoSâ‚‚ lattice, including on top of Mo and in off-axis positions. These variations are due to the Pt bonding with the surrounding amorphous carbon layer, which disrupts the intrinsic Pt-MoSâ‚‚ interactions, leading to more varied positions. Density functional theory (DFT) calculations reveal that Pt atoms on the surface of MoSâ‚‚ have a small barrier for migration and are stabilized when bound to either a single or double sulfur vacancies. DFT calculations have been used to understand how the catalytic activity of the MoSâ‚‚ basal plane for hydrogen evolution reaction is influenced by Pt dopants by variation of the hydrogen adsorption free energy. This strong dependence of catalytic effect on interfacial configurations is shown to be common for a series of dopants, which may provide a means to create and optimize reaction centers
Quantifying the Performance of a Hybrid Pixel Detector with GaAs:Cr Sensor for Transmission Electron Microscopy
Hybrid pixel detectors (HPDs) have been shown to be highly effective for
diffraction-based and time-resolved studies in transmission electron
microscopy, but their performance is limited by the fact that high-energy
electrons scatter over long distances in their thick Si sensors. An advantage
of HPDs compared to monolithic active pixel sensors (MAPS) is that their sensor
does not need to be fabricated from Si. We have compared the performance of the
Medipix3 HPD with a Si sensor and with a GaAs:Cr sensor using primary electrons
in the energy range of 60 - 300keV. We describe the measurement and calculation
of the detectors' modulation transfer function (MTF) and detective quantum
efficiency (DQE), which show that the performance of the GaAs:Cr device is
markedly superior to that of the Si device for high-energy electrons.Comment: 15 pages + references, 13 figure
Atomic resolution HOLZ-STEM imaging of atom position modulation in oxide heterostructures
It is shown that higher order Laue zone (HOLZ) rings in high energy electron diffraction are specific to individual columns of atoms, and show different strengths, structure and radii for different atom columns along the same projection in a structure. An atomic resolution 4-dimensional STEM dataset is recorded from a <110> direction in a perovskite trilayer, where only the central LaFeO3 layer should show a period doubling that gives rise to an extra HOLZ ring. Careful comparison between experiment and multislice simulations is used to understand the origins of all features in the patterns. A strong HOLZ ring is seen for the La-O columns, indicating strong La position modulation along this direction, whereas a weaker ring is seen along the O columns, and a very weak ring is seen along the Fe columns. This demonstrates that atomic resolution HOLZ-STEM is a feasible method for investigating the 3D periodicity of crystalline materials with atomic resolution
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