Past presentations

12.06.2024

Zhou Xuyang (Max-Planck-Institut für Eisenforschung, Germany)

Correlated Application of Precession Electron Diffraction and Atom Probe Tomography for Crystallographic and Chemical Analysis of Nanomaterials in 3D

In this talk, I will present our recent development of a method for the 3D correlated crystallographic and chemical reconstruction of grain orientations and boundaries at the nanoscale. To achieve the 3D crystallographic reconstruction, we performed precession-assisted 4D scanning electron microscopy (4D-STEM) scans at various tilted angles on the atom probe tomography (APT) needle-shaped sample in a transmission electron microscope (TEM). Using the in-house developed software SPED3D, we automatically generated virtual dark field imaging of each grain at each tilted angle, thereby achieving the 3D crystallographic reconstruction of the grain, and subsequently, the reconstruction of all grains within the APT sample. Further quantitative analysis of the 3D chemical composition was conducted by field-

evaporating and then reconstructing the same needle sample. Such a correlative method can simultaneously achieve a spatial resolution of approximately 2 nanometers for crystallographic reconstruction and chemical sensitivity at the parts-per-million (ppm) level for chemical composition analysis, opening a new door for precisely quantifying the 3D correlation between defect structures and compositions. I will also showcase the application of this new method in studying the influence of grain boundary faceting on solute segregation behavior using the Fe-B bicrystalline thin film sample and the nanocrystalline Fe-W thin film sample.

29.05.2024

Colin Ophus (LBNL Berkeley, USA)

Analysis Pipelines for Nanobeam 4D-STEM Materials Science Experiments

24.04.2024

Christoph Hofer (University of Antwerp, Belgium)

Post acquisition aberration correction and phase quantification via ptychography

10.04.2024

Ian MacLaren (University of Glasgow, UK)

3D crystallography at atomic resolution

It has long been known that high angle electron diffraction into higher order Laue zones contains 3-dimensional information as the diffraction vectors are no longer perpendicular to the beam direction and therefore all diffraction events contain some information about the crystallography parallel to the beam.  More recently, it became possible to use this in STEM with the advent of fast direct detectors and 4DSTEM and it was quickly shown that the signal is atomically resolved and varies from site to site in a crystal.  In this seminar, after introducing the topic and explaining this background (seeing as it is still not that well known in the wider community), I will show that the atomic resolution HOLZ signal contains 3D vector information about the crystallography, which can be extracted by a simple mathematical fitting approach.  

This is used to map the strength and 3D orientation of antiparallel atom movements which are occurring in a complex oxide close to an interface showing atomic resolution detail on how the atomic movements are quenched close to that interface.  This also reveals hitherto inaccessible information about the preferred domain orientation in the sample, which was neither determined by reciprocal space mapping nor by conventional STEM imaging in previous characterisation, nor could have been resolved by conventional dark field TEM.  This work shows the advent of an entirely new paradigm for performing STEM of single crystal systems (such as epitaxial heterostructures) in which 3D crystallographic orderings are directly accessible via fitting of diffraction patterns recorded along just one major crystallographic direction.

17.01.2024

Shelly Conroy (Imperial College London, UK)

Probing the Dynamics of Charged Multiferroic Topologies at the Sub-Atomic Scale

Domain walls are functional interfaces of multiferroic materials that have unusual properties such as charge, magnetism and photonic behavior, which are different from the surrounding material. The dynamic nature of these topologies gives them the edge over other 2D systems and may lead to these entities being the next promising disruptive quantum technology. This is an area of research at its very early stages with endless possible applications. As the region of interest is atomically thin and dynamic, it is essential for the physical characterization to be at this scale spatially and time resolved.

Recently there has been a surge in interest of electron beam-based atom‐by‐atom manipulation. This presentation will focus on using the aberration corrected scanning transmission electron microscopy (STEM) probe to move multiferroic topologies, and thus investigate their dynamics while imaging at the subatomic scale. As the STEM probe can be controlled in terms of dose, probe size, direction and speed, a diverse set of experiments is possible without complicated sample preparation. Using a segmented STEM detector (or 4D-STEM CoM experimental set-up) any changes in deflection and thus the changes in polarisation for each domain, can also be investigated with controlled variants in applied field conditions. By controlling the incoming STEM probe direction, parallel domain walls could be moved around to form stable four-fold charged junctions, thus switching from a neutral to charged state. Then in each frame by quantifying the atomic displacement per unit cell using the open-source python based software package TopoTEM the local polarisation at these charged topological can also be monitored. Finally, I will show how 4D-STEM data collection can be used to quantify changes in both strain and polarisation during dynamics of these topologies at multiple length scales. 

13.12.2023

Ben Savitzky (Berkeley, USA)

Nanodiffract (verb): to image with tact to extract every fact, all in one scale-bridging experimental act

Physical properties of matter depend on structure across vastly disparate length scales, from well below the atomic to macroscopic.  In this talk, we’ll discuss scale-bridging scanning transmission electron microscopy (STEM) experiments and the algorithms used to quantify them, measuring quantities from picometer deformations of individual atomic columns in charge density wave materials under in-situ cryogenic cooling, to grain orientations of hundreds of crystallites in a single capture, to lattice parameter variations measured across the many micron lengths of LixFePO4 nanoplatelets in several stages of electrochemical cycling.

Many of these datasets are large, and integrating computation and experiment is necessary in each case.  In atom tracking with high-angle annular dark-field (HAADF)-STEM, instabilities and bubbling from the cryogen can easily spoil in-situ measurements - by combining many fast-acquisition low-signal image captures with a registration algorithm tailored to nearly uniform lattices, measuring and visualizing ~pm lattice displacements in low-temperature CDW phases is possible.  In 4D-STEM, in which a 2D image of the diffracted electrons is collected at each position of the 2D beam raster, matching algorithms to experiment remains essential to make sense of the large and information rich datasets.  Examples will be selected to highlight a range of modalities, methodologies, and applications, and will include Bragg localization, amorphous/crystal classification, phase identification, automated crystal orientation mapping, and others.

29.11.2023

Colum O'Leary (UCLA, USA)

Maximizing information transfer in electron ptychography

Electron ptychography using fast pixelated detectors has enabled high-contrast, atomic-resolution phase-imaging of light elements and radiation-sensitive materials. In this presentation, I will discuss the experimental and analytical methods for maximizing information obtainable from electron ptychography. First, I will discuss the importance of convergence angle, lens aberrations and noise models for maximizing ptychographic phase contrast. Next, I will discuss how to extract three dimensional information from single ptychographic data sets for both non-iterative (single sideband, Wigner distribution deconvolution) and iterative (ePIE, maximum likelihood) methods. Recent multislice ptychography results obtained from a twisted hBN crystal system will be presented.

25.10.2023

Cong Su (Yale, USA)

Activating color centers by a twist

The color centers are widely used in quantum technologies, and those in hexagonal boron nitride have attracted attention due to the high brightness and stability, optically addressable spin states and wide wavelength coverage discovered in its emitters. However, its application is hindered by the typically random defect distribution and complex mesoscopic environment. In this talk, I'm going to show how we achieved on-demand activation and control of color center emission at the twisted interface of two hexagonal boron nitride flakes. We found (Nature Materials 21, 896–902, 2022) that color center emission brightness can be enhanced by two orders of magnitude by tuning the twist angle (to turn on), and the activated color centers could then be suppressed by a vertical voltage (to turn off). The ab-initio GW and GW plus Bethe–Salpeter equation calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron–hole recombination. This mechanism is further exploited to draw nanoscale color center patterns using electron beams.

11.10.2023

Georgios Varnavides (Berkeley, USA)

Inverse Scattering Problems in S/TEM Using Electron Ptychography: From Three-Dimensions to Magnetic Vector Potentials to Biological Samples

An electron beam passing through a thin sample acquires a phase shift due to sample interactions, including electrostatic scattering off atomic columns and scattering against a projected magnetic vector potential. The mechanistic differences between these scattering sources and the large order of magnitude difference in the acquired phase shifts, suggests the signals are hard to separate. Reconstructing the three-dimensional and vectorial nature of various scattering sources from two-dimensional diffraction intensities is thus a high-dimensional non-convex inverse problem.

Iterative electron ptychography is a phase-retrieval technique which attempts to solve this inverse problem using the redundant information in a set of converged-beam diffraction intensities with sufficient real-space illumination overlap, e.g., using defocused-probe 4DSTEM measurements. In this talk, I will introduce a general computational framework, implemented in the open-source analysis toolkit py4DSTEM, to reconstruct common scattering sources using physically inspired forward and adjoint operators. Specifically, I will show how orthogonal tilt-series of diffraction intensities can be used to directly solve for the three-dimensional nature of scalar (electrostatic) and vector (magnetic) scattering sources. Finally, I will show how joint ptychographic-tomography of scalar scattering potentials can be extended to the case of unknown tilt orientations for applications in single-particle analysis of biological samples.

31.05.2023

Timothy Pennycook (University of Antwerp, Belgium)

The future of microscopy: 4D STEM and ptychography

Over the past decade advances in camera technology have greatly facilitated 4D scanning transmission electron microscopy (STEM) and electron ptychography. A wealth of information on the probe position dependence of the scattering can be discerned from 4D STEM data, which electron ptychography uses to provide an especially dose efficient means of directly imaging atomic structures. The dose efficiency can exceed that not only of other STEM imaging modalities but can also significantly exceed the dose efficiency of phase contrast imaging in conventional TEM. In addition ptychography fills another important deficiency of ADF imaging. Light atoms are often hidden in the strong scattering from neighboring heavy elements in the ADF signal, and ptychography can reveal these more clearly than any other method. Similarly, the greater dose efficiency should allow more sensitive examination of charge density for looking at effects from bonding, for instance. The ability to correct for the residual aberrations post collection is also a clear advantage for ptychography. Phase imaging with electron ptychography is however not entirely free from complications. Contrast reversals can occur with increasing strength of the potential of an atomic column, but in direct methods of ptychography these can be compensated for with small amounts of defocus. Ensuring that the structures reflect reality is however greatly facilitated by simultaneous Z-contrast imaging, easily available with focused probe ptychography, which with event driven camera technology can be performed without any loss of speed compared to conventional rapid scan STEM.

03.05.2023 (rescheduled)
24.05.2023

Debangshu Mukherjee (ORNL, USA)

Handling and learning from 4D-STEM datasets

This talk will focus on two parts. The first part will focus on the infrastructure that is currently being built at ORNL for handling 4D-STEM datasets, with an emphasis on microscope automation and data compression. I will give an overview of lossless compression methods being currently developed at ORNL for 4D-STEM datasets. In the second part of the talk, I will focus on a use case – specifically strain mapping. I will demonstrate strain mapping results from a single nanoparticle to an ensemble of nanoparticles and demonstrate how 4D-STEM ties in the fields of diffraction and imaging.

17.05.2023

Stephanie Ribet (LBNL, USA)

STEM beam modification for iterative phase contrast reconstructions

The phase problem is a well-known challenge in electron microscopy: While detectors collect only the intensity of the exit wave, the ability to recover phase information is crucial for efficient characterization, especially for weakly scattering samples. In the first part of this talk, I'll show recent implementations of iterative phase reconstruction algorithms in py4DSTEM, such as Differential Phase Contrast (DPC) and various flavors of ptychographic reconstructions. Careful control over the incident electron probe is required for these phase retrieval experiments, where conventionally the beam is largely tuned with defocus.

In the second part of the talk, I will discuss our design for an electrostatic phase plate to enable more complex shaping of the probe. The plate, which sits in the condenser optics in a STEM, is comprised of annular segments, each of which is an independent two-terminal device that can apply a constant or ramped phase shift to a portion of the electron beam. The dynamic nature of the programmable phase plate means that it can be used in various configurations. I will show different applications including correcting aberrations for atomic resolution imaging at low accelerating voltages and adding phase diversity into the incident probe for more efficient ptychographic reconstructions at low spatial frequencies.

19.04.2023

Philip Pelz (FAU, Germany)

Three-dimensional phase-contrast imaging from 4D-STEM measurements

Spurred by the development and widespread availability of fast pixelated direct electron detectors (DEDs), scanning transmission electron microscopy (STEM) is undergoing a computational imaging renaissance. Experiments envisaged more than ten years ago can finally be realized with modern detector technology. In this talk I will show the experimental demonstration of one such experiment: ptychographic electron tomography at atomic resolution. I will discuss the limitations of the current algorithms and show developments towards imaging much larger with 4D-STEM.

22.03.2023

Laura Clark (University of York, UK)

Phase contrast methods in 4D-STEM imaging

The advent of widely available fast, pixelated detectors in the scanning transmission electron microscope (STEM) is revolutionising high-resolution imaging of materials. We can now study these materials in a previously unprecedented degree of detail in order to understand both their structure and their functional properties. The most typical setup for 4D-STEM imaging allows the collection of a 2D diffraction pattern from each position of the localised electron probe as it rasters across a 2D array of probe positions – this leads to multiple GBs of data from less than a square nanometre of material.

Such fine-grained data allows for new materials insights – in this presentation I will review two main aspects of this: firstly, the detection of long-range (µm-scale) but weak electromagnetic fields where careful analysis of 4D-STEM data can enable sensitivity to such material properties. Secondly, I will review the more fundamental challenge of high-resolution imaging of beam-sensitive materials and how phase-contrast imaging methods can contribute to improving the dose-limited resolution of these important materials.