Session N06: Keithley Award Symposium

Prize Talk: Joseph F. Keithley Award For Advances in Measurement

Electron microscopes use electrons with wavelengths of a few picometers and are potentially capable of imaging individual atoms in solids at a resolution ultimately set by the atoms themselves. Even with the development of aberration-corrector technology the best achievable resolution was more than an order magnitude worse than this, limited by both residual aberrations in the electron lenses and multiple scattering of the incident beam inside the sample. However, with recent advances in detector technology [1] to collect all scattered electrons and ptychographic algorithms to unscramble their multiple scattering, the resolution of the electron microscope is now limited only by the dose to the sample, and thermal vibrations of the atoms themselves.  At high doses, these approaches have allowed us to image the detailed vibrational envelopes of individual atom columns [2].   Solving the multiple scattering problem also recovers three-dimensional information about the sample, including surface relaxations and revealing interstitial dopant atoms that would be hidden by channeling of the probe with conventional imaging modes.  Even the location of all atoms in thin amorphous films now seems within reach. These approaches have also allowed us to image the internal structures of both magnetic and ferroelectric vortices, skyrmions and merons, including their singular points that are critical for accurately describing the topological properties of these field textures.  The reduced sensitivity to chromatic aberrations also makes these ptychographic approaches of interest for thick biological samples.  However, as the dose is reduced, so is our ability to robustly reconstruct the object. Challenges in characterizing and predicting performance will be discussed.   

Invited Talk: Sol M. GrunerAbstract Title: EMPAD, an integrating scanning transmission electron microscope pixel array detector

Modern scanning transmission electron microscopes are powerful tools to probe the physical properties of thin sheet materials. However, extracting maximum information from the electrons transmitted through the sample imposes numerous constraints on the detector. The Cornell electron microscope pixel array detector (EMPAD) developed by a collaboration of the Cornell Detector Group and David Muller’s electron microscope group meets many of these constraints, including: (1) A detector sensor of sufficient thickness to convert all of the incident electron energy to a measured current of electron or hole carriers without unduly compromising image spatial resolution.  (2) An array of pixel size and shape well matched to the width of the carrier cloud produced in the sensor. (3) A dynamic range that spans from low-noise detection of single incident electrons to high currents (e.g., few hundred pA) per pixel per exposure, at electron energies ranging from ~60 to 300 keV. (4) Very rapid image acquisition to limit deleterious effects of sample movement over time. (5) A detector configuration that is robust, stable, and resistant to radiation damage. (6) A detector configuration that accommodates relatively easily into common STEM platforms. This talk will discuss the technical development of the EMPAD from conception to the devices now implemented in many STEM laboratories.   

Optimizing Parameters for High-resolution and Low-dose Ptychography

Ptychography is an increasingly popular technique in scanning transmission electron microscopy and has demonstrated sub-angstrom spatial resolution and high dose-efficiency, with applications ranging from single dopant detection to imaging of magnetic skyrmions. In practice, obtaining accurate reconstructions requires optimization of numerous types of experimental and algorithm parameters that are highly data-dependent and often selected by trial and error. To reduce human involvement and biases, we leverage physics-informed Bayesian optimization with Gaussian processes (BO-GP) to develop automatic data analysis and simulation workflows in ptychography. Our method assumes minimal prior knowledge about experimental datasets and can consistently produce high-resolution reconstructions similar or even superior to those processed by human experts. We also used BO-GP to explore the ideal experimental conditions for low-dose ptychography – a challenging task for humans due to the complex trade-offs between various physical requirements. The workflow was recently utilized to optimize high-order aberrations when designing probe structures that further push the limit of electron ptychography.   

Pushing atomic-scale detection limits with 4D STEM and ptychography

A new generation of fast, high dynamic range pixelated electron detectors is enabling new research across physics and materials science. Here, we leverage the new capabilities of pixelated detectors in two projects: for atomic scale magnetic imaging and for sub-angstrom imaging in an uncorrected scanning transmission electron microscope (STEM). First, we demonstrate 4D-STEM methods to image the local magnetic moments in antiferromagnetic Fe2As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks combined with center-of-mass analysis to directly visualize the magnetic lattice. We illustrate how to robustly isolate the weak magnetic signals, a key challenge for atomic-scale magnetic imaging methods in the electron microscope. Second, we demonstrate sub-angstrom resolution electron ptychography of 2D materials in an uncorrected scanning transmission electron microscope. Counterintuitively, we find that geometric aberrations produce structured beams that are more dose-efficient for ptychography than focused, aberration-free probes. Most importantly, our results demonstrate that expensive aberration correctors are no longer required for atom-by-atom imaging, a significant step towards broadening access to high-end electron microscopy.