Celesta S. Chang

High Dynamic Range Pixel Array Detector for Scanning Transmission Electron Microscopy

We describe a hybrid pixel array detector (electron microscope pixel array detector, or EMPAD) adapted for use in electron microscope applications, especially as a universal detector for scanning transmission electron microscopy. The 128×128 pixel detector consists of a 500 µm thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit. The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition system, and preliminary results from experiments with 80-200 keV electron beams.

Topological Defects in Hexagonal Manganites: Inner Structure and Emergent Electrostatics

Diverse topological defects arise in hexagonal manganites, such as ferroelectric vortices, as well as neutral and charged domain walls. The topological defects are intriguing because their low symmetry enables unusual couplings between structural, charge, and spin degrees of freedom, holding great potential for novel types of functional 2D and 1D systems. Despite the considerable advances in analyzing the different topological defects in hexagonal manganites, the understanding of their key intrinsic properties is still rather limited and disconnected. In particular, a rapidly increasing number of structural variants is reported without clarifying their relation, leading to a zoo of seemingly unrelated topological textures. Here, we combine picometer-precise scanning-transmission-electron microscopy with Landau theory modeling to clarify the inner structure of topological defects in Er1-xZrxMnO3. By performing a comprehensive parametrization of the inner atomic defect structure, we demonstrate that one primary length scale drives the morphology of both vortices and domain walls. Our findings lead to a unifying general picture of this type of structural topological defects. We further derive novel fundamental and universal properties, such as unusual bound-charge distributions and electrostatics at the ferroelectric vortex cores with emergent U(1) symmetry.

Reconstruction of Polarization Vortices by Diffraction Mapping of Ferroelectric PbTiO3 / SrTiO3 Superlattice Using a High Dynamic Range Pixelated Detector

Kayla X. Nguyen, Prafull Purohit, Ajay Yadav, Mark W. Tate, Celesta S. Chang, Ramamoorthy Ramesh, Sol M. Gruner, David A. Muller School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Department of Physics, Cornell University, Ithaca, NY, USA Department of Material Science and Engineering, University of California, Berkeley Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY USA

An Electron Microscope Pixel Array Detector as a Universal STEM Detector

Complete information about the scattering potential of a sample is in principle encoded in the distribution of scattered electrons from a localized beam propagating through it. A new generation of high speed imaging detectors brings us closer to this goal and will allow us to explore practical limits and identify the most promising methods of analysis. We have recently developed an electron microscope pixel array detector (EMPAD) that functions as a compact and high-speed, high dynamic range electron diffraction camera (Figure 1a). It has single electron sensitivity with a signal/noise ratio of 140 for a single electron at 200 keV [1,2]. It has a dynamic range of 10 6 for primary electrons– i.e a pixel can detect from 1 to 1,000,000 electrons, and reads out an image frame in 0.86 ms. These properties allow us to record essentially an image of all the transmitted electrons, from the unscattered beam to out beyond the HOLZ lines, and do so for every probe position in a real-space, atomic resolution image. Not only does this allow quantitative and simultaneous annular dark and bright field signals on an absolute scale, but from the analysis of the spatially-resolved diffraction patterns we can extract thickness, strain and tilt, octahedral rotations, polarity and even electric and magnetic fields.

Quantitative Imaging of Probability Current Flow in Real and Momentum Space

A new generation of momentum-resolving detectors offers both new physical information about the sample, and also new opportunities to visualize and understand the propagation of the probe electron beam through the sample. We have developed a new electron microscope pixel array detector (EMPAD), with a 1,000,000:1 dynamic range, single electron sensitivity and sub-millisecond frame time [1]. This has allowed us to record the full, unsaturated, diffraction pattern for an atomic-resolution beam scanned across a sample. From this we reconstruct quantitative bright field, dark field and generalized imaging modes that can be compared directly to theory. The detector also allows us to study beam propagation through the sample by measuring probability current flow, which is calculated from the 1 moment of the diffraction pattern p [2]. Here we focus on the physical meaning of the first and second moment generated images ( p , p! ), that also gives quantitative insight to differential phase contrast (DPC) imaging, and the optimal imaging conditions by determining effective cutoff angles – a physical realization of strong vs weak quantum measurements.

Lorentz-STEM imaging of Fields and Domains using a High-Speed, High-Dynamic Range Pixel Array Detector at Atomic Resolution

Kayla X. Nguyen, Robert Hovden, Mark W. Tate, Prafull Purohit, John Heron , Celesta Chang, Sol M. Gruner, David A. Muller Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY, USA School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Department of Physics, Cornell University, Ithaca, NY, USA Department of Materials Science and Engineering, Cornell University, Ithaca, NY USA Kavili Institute for Nanoscale Science, Cornell University USA

Measuring ferroelectric order parameters at domain walls and vortices in hexagonal manganites with atomic resolution STEM

Hexagonal manganites exhibit a rich variety of topological defects, including vortices and neutral and charged domain walls. These topological defects are promising avenues for information carriers in nextgeneration memory devices. Here, we explore the atomic scale structure underlying ferroelectricity in hexagonal ErMnO3 (Fig 1a) with STEM. We quantify the local polarization by mapping the picometer scale Er buckling across ferroelectric domain walls and vortices. We measure the change in order parameter at charged and neutral walls and at vortices, and find a common length scale that is intrinsic to all of the topological defects within the Landau theory framework.

Depth-Dependent Contrast in Probability-Current Imaging from Channeling in Crystalline Materials

Differential phase contrast (DPC) imaging is a useful technique for studying magnetic or electric field distributions in materials at both the nanoscale and atomic resolution [1,2]. A new generation of fast high dynamic range pixel array detectors provides the possibility of obtaining the whole electron diffraction pattern at each probe position in scanning transmission electron microscopy (STEM) [3]. These 4D-STEM datasets enable quantitative and momentum-resolved DPC, or center of mass (CoM), imaging which potentially provides more physical information about the specimen compared to DPC images from traditional segmented detectors. Furthermore, CoM images can be understood as the quantum mechanical probability current flow [4] and give direct measurements of the electron beam’s lateral propagation within the specimen. Within the strong phase object approximation, which is only valid if the real-space probe shape is unchanged by scattering in the specimen, CoM images can be expressed as the gradient of the specimen potential [5,6]. Here, we demonstrated a probability current flow oscillation within a crystal beyond the strong phase object approximation [7], using SrTiO3 as an example.

Imaging Local Polarization and Domain Boundaries with Picometer-Precision Scanning Transmission Electron Microscopy

Megan E. Holtz, Julia A. Mundy, Celesta S. Chang, Jarrett A. Moyer, Charles M. Brooks, Hena Das, Alejandro F. Rebola, Robert Hovden, Elliot Padgett, Craig J. Fennie, Peter Schiffer, Dennis Meier, Darrell G. Schlom, David A. Muller 1. School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA 2. Department of Physics and Materials Research Institute, University of Illinois at Urbana-Champaign, Urbana, IL, USA 3. Department of Materials Science and Engineering, Cornell University, Ithaca, NY, USA 4. Department of Materials, ETH Zürich, CH-8093 Zürich, Switzerland 5. Kavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA