9,837 research outputs found
Structural identification of cubic iron-oxide nanocrystal mixtures: X-ray powder diffraction versus quasi-kinematic transmission electron microscopy
Two novel (and proprietary) strategies for the structural identification of a
nanocrystal from either a single high-resolution (HR) transmission electron
microscopy (TEM) image or a single precession electron diffraction pattern are
proposed and their advantages discussed in comparison to structural
fingerprinting from powder X-ray diffraction patterns. Simulations for cubic
magnetite and maghemite nanocrystals are used as examples. This is an expanded
and updated version of a conference paper that has been published in Suppl.
Proc. of TMS 2008, 137th Annual Meeting & Exhibition, Volume 1, Materials
Processing and Properties, pp. 25-32.Comment: 7 pages, 3 figures, 1 table, expanded and updated version of a
conference paper that has been published in Suppl. Proc. of TMS 2008, 137th
Annual Meeting & Exhibition, Volume 1, Materials Processing and Properties,
pp. 25-3
py4DSTEM: a software package for multimodal analysis of four-dimensional scanning transmission electron microscopy datasets
Scanning transmission electron microscopy (STEM) allows for imaging,
diffraction, and spectroscopy of materials on length scales ranging from
microns to atoms. By using a high-speed, direct electron detector, it is now
possible to record a full 2D image of the diffracted electron beam at each
probe position, typically a 2D grid of probe positions. These 4D-STEM datasets
are rich in information, including signatures of the local structure,
orientation, deformation, electromagnetic fields and other sample-dependent
properties. However, extracting this information requires complex analysis
pipelines, from data wrangling to calibration to analysis to visualization, all
while maintaining robustness against imaging distortions and artifacts. In this
paper, we present py4DSTEM, an analysis toolkit for measuring material
properties from 4D-STEM datasets, written in the Python language and released
with an open source license. We describe the algorithmic steps for dataset
calibration and various 4D-STEM property measurements in detail, and present
results from several experimental datasets. We have also implemented a simple
and universal file format appropriate for electron microscopy data in py4DSTEM,
which uses the open source HDF5 standard. We hope this tool will benefit the
research community, helps to move the developing standards for data and
computational methods in electron microscopy, and invite the community to
contribute to this ongoing, fully open-source project
Reconstructing Detailed Line Profiles of Lamellar Gratings from GISAXS Patterns with a Maxwell Solver
Laterally periodic nanostructures were investigated with grazing incidence
small angle X-ray scattering (GISAXS) by using the diffraction patterns to
reconstruct the surface shape. To model visible light scattering, rigorous
calculations of the near and far field by numerically solving Maxwell's
equations with a finite-element method are well established. The application of
this technique to X-rays is still challenging, due to the discrepancy between
incident wavelength and finite-element size. This drawback vanishes for GISAXS
due to the small angles of incidence, the conical scattering geometry and the
periodicity of the surface structures, which allows a rigorous computation of
the diffraction efficiencies with sufficient numerical precision. To develop
dimensional metrology tools based on GISAXS, lamellar gratings with line widths
down to 55 nm were produced by state-of-the-art e-beam lithography and then
etched into silicon. The high surface sensitivity of GISAXS in conjunction with
a Maxwell solver allows a detailed reconstruction of the grating line shape
also for thick, non-homogeneous substrates. The reconstructed geometrical line
shape models are statistically validated by applying a Markov chain Monte Carlo
(MCMC) sampling technique which reveals that GISAXS is able to reconstruct
critical parameters like the widths of the lines with sub-nm uncertainty
Mapping Atomic Motions with Electrons: Toward the Quantum Limit to Imaging Chemistry
Recent advances in ultrafast electron and X-ray diffraction have pushed imaging of structural dynamics into the femtosecond time domain, that is, the fundamental time scale of atomic motion. New physics can be reached beyond the scope of traditional diffraction or reciprocal space imaging. By exploiting the high time resolution, it has been possible to directly observe the collapse of nearly innumerable possible nuclear motions to a few key reaction modes that direct chemistry. It is this reduction in dimensionality in the transition state region that makes chemistry a transferable concept, with the same class of reactions being applicable to synthetic strategies to nearly arbitrary levels of complexity. The ability to image the underlying key reaction modes has been achieved with resolution to relative changes in atomic positions to better than 0.01 Ã…, that is, comparable to thermal motions. We have effectively reached the fundamental space-time limit with respect to the reaction energetics and imaging the acting forces. In the process of ensemble measured structural changes, we have missed the quantum aspects of chemistry. This perspective reviews the current state of the art in imaging chemistry in action and poses the challenge to access quantum information on the dynamics. There is the possibility with the present ultrabright electron and X-ray sources, at least in principle, to do tomographic reconstruction of quantum states in the form of a Wigner function and density matrix for the vibrational, rotational, and electronic degrees of freedom. Accessing this quantum information constitutes the ultimate demand on the spatial and temporal resolution of reciprocal space imaging of chemistry. Given the much shorter wavelength and corresponding intrinsically higher spatial resolution of current electron sources over X-rays, this Perspective will focus on electrons to provide an overview of the challenge on both the theory and the experimental fronts to extract the quantum aspects of molecular dynamics
Linear chemically sensitive electron tomography using DualEELS and dictionary-based compressed sensing
We have investigated the use of DualEELS in elementally sensitive tilt series tomography in the scanning transmission electron microscope. A procedure is implemented using deconvolution to remove the effects of multiple scattering, followed by normalisation by the zero loss peak intensity. This is performed to produce a signal that is linearly dependent on the projected density of the element in each pixel. This method is compared with one that does not include deconvolution (although normalisation by the zero loss peak intensity is still performed). Additionaly, we compare the 3D reconstruction using a new compressed sensing algorithm, DLET, with the well-established SIRT algorithm. VC precipitates, which are extracted from a steel on a carbon replica, are used in this study. It is found that the use of this linear signal results in a very even density throughout the precipitates. However, when deconvolution is omitted, a slight density reduction is observed in the cores of the precipitates (a so-called cupping artefact). Additionally, it is clearly demonstrated that the 3D morphology is much better reproduced using the DLET algorithm, with very little elongation in the missing wedge direction. It is therefore concluded that reliable elementally sensitive tilt tomography using EELS requires the appropriate use of DualEELS together with a suitable reconstruction algorithm, such as the compressed sensing based reconstruction algorithm used here, to make the best use of the limited data volume and signal to noise inherent in core-loss EELS
HAADF-STEM block-scanning strategy for local measurement of strain at the nanoscale
Lattice strain measurement of nanoscale semiconductor devices is crucial for
the semiconductor industry as strain substantially improves the electrical
performance of transistors. High resolution scanning transmission electron
microscopy (HR-STEM) imaging is an excellent tool that provides spatial
resolution at the atomic scale and strain information by applying Geometric
Phase Analysis or image fitting procedures. However, HR-STEM images regularly
suffer from scanning distortions and sample drift during image acquisition. In
this paper, we propose a new scanning strategy that drastically reduces
artefacts due to drift and scanning distortion, along with extending the field
of view. The method allows flexible tuning of the spatial resolution and
decouples the choice of field of view from the need for local atomic
resolution. It consists of the acquisition of a series of independent small
subimages containing an atomic resolution image of the local lattice. All
subimages are then analysed individually for strain by fitting a nonlinear
model to the lattice images. The obtained experimental strain maps are
quantitatively benchmarked against the Bessel diffraction technique. We
demonstrate that the proposed scanning strategy approaches the performance of
the diffraction technique while having the advantage that it does not require
specialized diffraction cameras
HAADF-STEM Image Resolution Enhancement Using High-Quality Image Reconstruction Techniques: Case of the Fe3O4(111) Surface
From simple averaging to more sophisticated registration and restoration strategies, such as super-resolution (SR), there exist different computational techniques that use a series of images of the same object to generate enhanced images where noise and other distortions have been reduced. In this work, we provide qualitative and quantitative measurements of this enhancement for high-angle annular dark-field scanning transmission electron microscopy imaging. These images are compared in two ways, qualitatively through visual inspection in real and reciprocal space, and quantitatively, through the calculation of objective measurements, such as signal-to-noise ratio and atom column roundness. Results show that these techniques improve the quality of the images. In this paper, we use an SR methodology that allows us to take advantage of the information present in the image frames and to reliably facilitate the analysis of more difficult regions of interest in experimental images, such as surfaces and interfaces. By acquiring a series of cross-sectional experimental images of magnetite (Fe3O4) thin films (111), we have generated interpolated images using averaging and SR, and reconstructed the atomic structure of the very top surface layer that consists of a full monolayer of Fe, with topmost Fe atoms in tetrahedrally coordinated sites
Fast pixelated detectors in scanning transmission electron microscopy. Part II: post acquisition data processing, visualisation, and structural characterisation
Fast pixelated detectors incorporating direct electron detection (DED)
technology are increasingly being regarded as universal detectors for scanning
transmission electron microscopy (STEM), capable of imaging under multiple
modes of operation. However, several issues remain around the post acquisition
processing and visualisation of the often very large multidimensional STEM
datasets produced by them. We discuss these issues and present open source
software libraries to enable efficient processing and visualisation of such
datasets. Throughout, we provide examples of the analysis methodologies
presented, utilising data from a 256×256 pixel Medipix3 hybrid DED
detector, with a particular focus on the STEM characterisation of the
structural properties of materials. These include the techniques of virtual
detector imaging; higher order Laue zone analysis; nanobeam electron
diffraction; and scanning precession electron diffraction. In the latter, we
demonstrate nanoscale lattice parameter mapping with a fractional precision
≤6×10−4 (0.06%)
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