Cryogenic electron microscopy is a powerful tool in structural biology. In thick specimens, challenges arise as an exponentially larger fraction of the transmitted electrons lose energy from inelastic scattering and can no longer be properly focused as a result of chromatic aberrations in the post-specimen optics. Rather than filtering out the inelastic scattering at the price of reducing potential signal, as is done in energy-filtered transmission electron microscopy, we show how a dose-efficient and unfiltered image can be rapidly obtained using tilt-corrected bright-field scanning transmission electron microscopy data collected on a pixelated detector. Enhanced contrast and a 3–5× improvement in dose efficiency are observed for two-dimensional images of intact bacterial cells and large organelles using tilt-corrected bright-field scanning transmission electron microscopy compared to energy-filtered transmission electron microscopy for thicknesses beyond 500 nm. As a proof of concept for the technique’s performance in structural determination, we present a single-particle analysis map at sub-nanometer resolution for a highly symmetric virus-like particle determined from 789 particles. Tilt-corrected bright-field scanning transmission electron microscopy offers enhanced cryogenic electron microscopy contrast and substantial improvement in dose efficiency for thick samples such as bacterial cells and large organelles, while still being able to perform single-particle analysis.

Cryo-EM is a powerful tool in structural biology, providing insights through techniques like single-particle analysis (SPA) and cryogenic electron tomography (cryo-ET). In thick specimens, challenges arise as an exponentially larger fraction of the transmitted electrons lose energy from inelastic scattering and can no longer be properly focused as a result of chromatic aberrations in the post-specimen optics. Rather than filtering out the inelastic scattering at the price of reducing potential signal, as is done in energy-filtered transmission electron microscopy (EFTEM), we show how a dose-efficient and unfiltered image can be rapidly obtained using tilt-corrected bright-field scanning-TEM (tcBF-STEM) data collected on a pixelated detector. Enhanced contrast and a 3-5x improvement in collection efficiency are observed for 2D images of intact bacterial cells and large organelles using tcBF-STEM compared to EFTEM for thicknesses beyond 500 nm. As a proof of concept for the technique’s performance in structural determination, we present an SPA map at a 7 Å nominal resolution for a highly symmetric virus-like particle (VLP) with 789 particles. These findings suggest applications for tcBF-STEM in cryo-EM of thicker cellular volumes where current approaches struggle. ### Competing Interest Statement The authors have declared no competing interest.

Subangstrom resolution has long been limited to aberration-corrected electron microscopy, where it is a powerful tool for understanding the atomic structure and properties of matter. Here, we demonstrate electron ptychography in an uncorrected scanning transmission electron microscope (STEM) with deep subangstrom spatial resolution down to 0.44 angstroms, exceeding the conventional resolution of aberration-corrected tools and rivaling their highest ptychographic resolutions​. Our approach, which we demonstrate on twisted two-dimensional materials in a widely available commercial microscope, far surpasses prior ptychographic resolutions (1 to 5 angstroms) of uncorrected STEMs. We further show how geometric aberrations can create optimized, structured beams for dose-efficient electron ptychography. Our results demonstrate that expensive aberration correctors are no longer required for deep subangstrom resolution.

We demonstrate a combination of computational tools and experimental 4D-STEM methods to image the local magnetic moment in antiferromagnetic Fe$_2$As with 6 angstrom spatial resolution. Our techniques utilize magnetic diffraction peaks, common in antiferromagnetic materials, to create imaging modes that directly visualize the magnetic lattice. Using this approach, we show that center-of-mass analysis can determine the local magnetization component in the plane perpendicular to the path of the electron beam. Moreover, we develop Magnstem, a quantum mechanical electron scattering simulation code, to model electron scattering of an angstrom-scale probe from magnetic materials. Using these tools, we identify optimal experimental conditions for separating weak magnetic signals from the much stronger interactions of an angstrom-scale probe with electrostatic potentials. Our techniques should be useful for characterizing the local magnetic order in systems such in thin films, interfaces, and domain boundaries of antiferromagnetic materials, which are difficult to probe with existing methods.

Orbital angular momentum and torque transfer play central roles in a wide range of magnetic textures and devices including skyrmions and spin-torque electronics(1-4). Analogous topological structures are now also being explored in ferroelectrics, including polarization vortex arrays in ferroelectric/dielectric superlattices(5). Unlike magnetic toroidal order, electric toroidal order does not couple directly to linear external fields. To develop a mechanism that can control switching in polarization vortices, we utilize a high-energy electron beam and show that transverse currents are generated by polar order in the ballistic limit. We find that the presence of an electric toroidal moment in a ferro-rotational phase transfers a measurable torque and orbital angular momentum to the electron beam. Furthermore, we find that the complex polarization patterns, observed in these heterostructures, are microscopically chiral with a non-trivial axial component of the polarization. This chirality opens the door for the coupling of ferroelectric and optical properties.

Antiferromagnets have recently attracted significant interest for their potential use in spintronic devices. Due to their switching frequencies in the terahertz range, antiferromagnets have the potential to produce fast, dense computer memories[1]. Existing magnetic imaging techniques such as Lorentz TEM[2], electron holography[3,4], and differential phase contrast scanning transmission electron microscopy (DPC-STEM)[5,6] have been used effectively to image magnetic domains and topological structures of magnetism such as skyrmions[7] with spatial resolution of a couple of nanometers[7,8]. However, higher spatial resolution is needed to study the spin arrangements of antiferromagnets near domain boundaries, defects, and interfaces.

The study of nanoscale chiral magnetic order in polycrystalline materials with a strong Dzyaloshinkii-Moriya interaction (DMI) is interesting for the observation of magnetic phenomena at grain boundaries and interfaces. One such material is sputter-deposited B20 FeGe on Si, which has been actively investigated as the basis for low-power, high-density magnetic memory technology in a scalable material platform. Although conventional Lorentz electron microscopy provides the requisite spatial resolution to probe chiral magnetic textures in single-crystal FeGe, probing the magnetism of sputtered B20 FeGe is more challenging because the sub-micron crystal grains add confounding contrast. We address the challenge of disentangling magnetic and grain contrast by applying 4-dimensional Lorentz scanning transmission electron microscopy using an electron microscope pixel array detector. Supported by analytical and numerical models, we find that the most important parameter for imaging magnetic materials with polycrystalline grains is the ability for the detector to sustain large electron doses, where having a high-dynamic range detector becomes extremely important. Despite the small grain size in sputtered B20 FeGe on Si, using this approach we are still able to observe helicity switching of skyrmions and magnetic helices across two adjacent grains as they thread through neighboring grains. We reproduce this effect using micromagnetic simulations by assuming that the grains have distinct orientation and magnetic chirality and find that magnetic helicity couples to crystal chirality. Our methodology for imaging magnetic textures is applicable to other thin-film magnets used for spintronics and memory applications, where an understanding of how magnetic order is accommodated in polycrystalline materials is important.

Nanoscale spin textures, especially magnetic skyrmions, have attracted intense interest as candidate high-density and power-efficient information carriers for spintronic devices(1,2). Facilitating a deeper understanding of sub-hundred-nanometre to atomic-scale spin textures requires more advanced magnetic imaging techniques(3-5). Here we demonstrate a Lorentz electron ptychography method that can enable high-resolution, high-sensitivity magnetic field imaging for widely available electron microscopes. The resolution of Lorentz electron ptychography is not limited by the usual diffraction limit of lens optics, but instead is determined by the maximum scattering angle at which a statistically meaningful dose can still be recorded-this can be an improvement of up to 2-6 times depending on the allowable dose. Using FeGe as the model system, we realize a more accurate magnetic field measurement of skyrmions with an improved spatial resolution and sensitivity by also correcting the probe-damping effect from the imaging optics via Lorentz electron ptychography. This allows us to directly resolve subtle internal structures of magnetic skyrmions near the skyrmion cores, boundaries and dislocations in an FeGe single crystal. Our study establishes a quantitative, high-resolution magnetic microscopy technique that can reveal nanoscale spin textures, especially magnetization discontinuities and topological defects in nanomagnets(6). The technique's high-dose efficiency should also make it well suited for the exploration of magnetic textures in electron radiation-sensitive materials such as organic or molecular magnets(7).

Journal Article Detection Limits for Imaging Chiral Magnetic Materials with 4-Dimensional Lorentz Scanning Transmission Electron Microscopy Get access Xiyue S Zhang, Xiyue S Zhang School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Kayla X Nguyen, Kayla X Nguyen School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USADepartment of Materials Science and Engineering, University of Illinois Urbana-Champaign, Urbana, IL, USA Search for other works by this author on: Oxford Academic Google Scholar Emrah Turgut, Emrah Turgut School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USATaiwan Semiconductor Manufacturing Company, San Jose, CA, USA Search for other works by this author on: Oxford Academic Google Scholar Zhen Chen, Zhen Chen School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Celesta S Chang, Celesta S Chang School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Yu-Tsun Shao, Yu-Tsun Shao School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar Gregory D Fuchs, Gregory D Fuchs School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USAKavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA Search for other works by this author on: Oxford Academic Google Scholar David A Muller David A Muller School of Applied and Engineering Physics, Cornell University, Ithaca, NY, USAKavli Institute at Cornell for Nanoscale Science, Ithaca, NY, USA Corresponding author: David.a.muller@cornell.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 28, Issue S1, 1 August 2022, Pages 1698–1700, https://doi.org/10.1017/S1431927622006742 Published: 01 August 2022

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Understanding how spins interact at the atomic scale is key for miniaturization of real, scalable devices. Lorentz transmission electron microscopy (LTEM) [1], holography [2-3], and differential phase contrast (DPC) imaging [4-5] have been standard imaging techniques local arrangement of spins, e.g. magnetic domains and topological structures such as skyrmions [6]. These techniques have been limited to spatial resolutions of a couple of nanometers [6-8]. While new methods such as 4D-STEM might enable magnetic measurements with angstrom-scale probes, extracting magnetic signals has been challenging because the electron scattering from the atomic potential is roughly 1000-fold stronger than scattering from the local magnetic moment at the angstrom-scale [4, 8].

Understanding the deformability, flexibility, and bending mechanics of two-dimensional (2D) materials is critical for the realization of next-generation deformable electronics and nanomechanical devices.While the mechanics of few-layer graphene have been studied for more than a decade, there is still no consensus on its bending stiffness and how it scales with thickness [1-3].Conventional measurements from mechanical resonance and nanoindentation [4] are challenging because out-of-plane deformations and pre-tension strongly impact the extraction of bending stiffness.Electron microscopy provides a powerful platform for addressing this challenge by enabling measurements of the conformation and strain of 2D materials at atomic resolution.Using aberration-corrected STEM at 80 kV, below the knock-on damage threshold of graphene [5], we investigate the bending mechanics of few-layer graphene.Using low voltages and dose rates, we probe the graphene on the atomic scale while minimizing electron beam damage in order to measure the equilibrium conformation of highly curved 2D materials.Using a combination of STEM, scanning convergent beam diffraction, geometric phase analysis, and density functional theory (DFT) we show that the bending of few-layer graphene is dominated by slip and shear rather than in-plane strain.As a result, few-layer graphene exhibits unusual, curvature-dependent mechanics that can dramatically tune its bending stiffness; for example, the stiffness of trilayer graphene changes by almost 300% when it is curved from 6 to 50 degrees.

Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. For example, the recent discovery of polarization vortices and their associated complex-phase coexistence and response under applied electric fields in superlattices of (PbTiO3)n/(SrTiO3)n suggests the presence of a complex, multi-dimensional system capable of interesting physical responses, such as chirality, negative capacitance and large piezo-electric responses1-3. Here, by varying epitaxial constraints, we discover room-temperature polar-skyrmion bubbles in a lead titanate layer confined by strontium titanate layers, which are imaged by atomic-resolution scanning transmission electron microscopy. Phase-field modelling and second-principles calculations reveal that the polar-skyrmion bubbles have a skyrmion number of +1, and resonant soft-X-ray diffraction experiments show circular dichroism, confirming chirality. Such nanometre-scale polar-skyrmion bubbles are the electric analogues of magnetic skyrmions, and could contribute to the advancement of ferroelectrics towards functionalities incorporating emergent chirality and electrically controllable negative capacitance.

Negative capacitance is a newly discovered state of ferroelectric materials that holds promise for electronics applications by exploiting a region of thermodynamic space that is normally not accessible1–14. Although existing reports of negative capacitance substantiate the importance of this phenomenon, they have focused on its macroscale manifestation. These manifestations demonstrate possible uses of steady-state negative capacitance—for example, enhancing the capacitance of a ferroelectric–dielectric heterostructure4,7,14 or improving the subthreshold swing of a transistor8–12. Yet they constitute only indirect measurements of the local state of negative capacitance in which the ferroelectric resides. Spatial mapping of this phenomenon would help its understanding at a microscopic scale and also help to achieve optimal design of devices with potential technological applications. Here we demonstrate a direct measurement of steady-state negative capacitance in a ferroelectric–dielectric heterostructure. We use electron microscopy complemented by phase-field and first-principles-based (second-principles) simulations in SrTiO3/PbTiO3 superlattices to directly determine, with atomic resolution, the local regions in the ferroelectric material where a state of negative capacitance is stabilized. Simultaneous vector mapping of atomic displacements (related to a complex pattern in the polarization field), in conjunction with reconstruction of the local electric field, identify the negative capacitance regions as those with higher energy density and larger polarizability: the domain walls where the polarization is suppressed. Imaging steady-state negative capacitance in SrTiO3/PbTiO3 superlattices with atomic resolution provides solid microscale support for this phenomenon.