The anisotropy of perovskite nanoplatelets (PeNPLs) opens up many opportunities in optoelectronics, including enabling the emission of linearly polarized light. But the limited stability of PeNPLs is a pressing challenge, especially for red-emitting CsPbI3. Herein, we address this limitation by alloying formamidinium (FA) into the perovskite cuboctahedral site. Unlike Cs/FA alloying in bulk thin films or nanocubes, FA incorporation in nanoplatelets requires meticulous control over the reaction conditions, given that nanoplatelets are obtained in kinetically-driven growth regimes instead of thermodynamically-driven conditions. Through in-situ photoluminescence (PL) measurements, we find that excess FA leads to uncontrolled growth, where phase impurities and nanoplatelets of multiple thicknesses co-exist. Restricting the FA content to up to 25

High-resolution transmission electron microscopy (HRTEM) is an important method for imaging beam sensitive materials often under cryo conditions. Electron ptychography in the scanning transmission electron microscope (STEM) has been shown to reconstruct low-noise phase data at a reduced fluence for such materials. This raises the question of whether ptychography or HRTEM provides a more fluence-efficient imaging technique. Even though the transfer function is a common metric for evaluating the performance of an imaging method, it only describes the signal transfer with respect to spatial frequency, irrespective of the noise transfer. It can also not be well defined for methods, such as ptychography, that use an algorithm to form the final image. Here we apply the concept of detective quantum efficiency (DQE) to electron microscopy as a fluence independent and sample independent measure of technique performance. We find that, for a weak-phase object, ptychography can never reach the efficiency of a perfect Zernike phase imaging microscope but that ptychography is more robust to partial coherence.

Integrated Centre-of-Mass (iCOM) is a widely used phase-contrast imaging method based on Centre-of-Mass (COM), which makes use of a 4D Scanning Transmission Electron Microscopy (STEM) dataset using an in-focus probe. In this paper, we introduce a novel approach that combines Single-Side Band (SSB) ptychography with COM and iCOM, termed Side Band masked Centre-of-Mass (SBm-COM) and integrated Centre-of-Mass (SBm-iCOM) which is applicable to weak-phase objects. This method compensates for residual aberrations in 4DSTEM datasets while also reducing the noise contribution up to the 2 α $2\alpha $ resolution limit. The aberration compensation and noise filtering features make the SBm-(i)COM suitable for samples that are difficult to focus or those that require minimal electron fluence. SBm-iCOM transfers the same information as SSB ptychography but results in an intrinsic transfer function that enhances low-frequency information.

The rich information of electron energy-loss spectroscopy (EELS) comes from the complex inelastic scattering process whereby fast electrons transfer energy and momentum to atoms, exciting bound electrons from their ground states to higher unoccupied states. To quantify EELS, the common practice is to compare the cross-sections integrated within an energy window or fit the observed spectrum with theoretical differential cross-sections calculated from a generalized oscillator strength (GOS) database with experimental parameters. The previous Hartree-Fock-based and DFT-based GOS are calculated from Schrödinger's solution of atomic orbitals, which does not include the full relativistic effects. Here, we attempt to go beyond the limitations of the Schrödinger solution in the GOS tabulation by including the full relativistic effects using the Dirac equation within the local density approximation, which is particularly important for core-shell electrons of heavy elements with strong spin-orbit coupling. This has been done for all elements in the periodic table (up to Z = 118) for all possible excitation edges using modern computing capabilities and parallelization algorithms. The relativistic effects of fast incoming electrons were included to calculate cross-sections that are specific to the acceleration voltage. We make these tabulated GOS available under an open-source license to the benefit of both academic users and to allow integration into commercial solutions.

Understanding Li+ ion diffusion pathways in Li-rich layered transition metal (TM) oxides is crucial for understanding the sluggish kinetics in anionic O2- redox. Although Li diffusion within the alkali layers undergoes a low-barrier octahedral-tetrahedral-octahedral pathway, it is less clear how Li diffuses in and out of the TM layers, particularly given the complex structural rearrangements that take place during the oxidation of O2-. Here, we develop simultaneous electron ptychography and annular dark field imaging methods to unlock the Li migration pathways in Li1.2Ni0.13Mn0.54Co0.13O2 associated with structural changes in the charge-discharge cycle. At the end of TM oxidation and before the high-voltage O oxidation plateau, we show that the Li migrating out of the TM layers occupies the alkali-layer tetrahedral sites on opposite sides of the TM layers, forming Li-Li dumbbell configurations, consistent with the density functional theory calculations. Also occurring are the TM migration and phase transition from O3 to O1 stacking, leading to unstable tetrahedral Li and the absence of Li contrast in imaging. Upon further Li deintercalation to 4.8 V, most of the tetrahedral Li are removed. After discharging to 2 V, we did not identify the reformation of tetrahedral Li but observed permanently migrated TMs at the alkali-layer sites, disfavoring the Li occupying the tetrahedral sites for diffusion. Our findings suggest a landscape of Li diffusion pathways in Li-rich layered oxides and strategies for minimizing the disruption of Li diffusion.

Ni-rich layered cathodes deliver high energy densities, but uncertainties remain about their charge compensation mechanisms and the degradation processes that limit their cycle life. Recent studies have identified trapped molecular O2 within LiNiO2 at high states of charge, as seen for Li-rich cathodes where excess capacity is associated with reversible O2−/O2 redox. Here we show that bulk redox in LiNiO2 occurs by Ni-O rehybridization, lowering the electron density on O sites, but without significant involvement of molecular O2. Trapped O2 is predominantly associated with surface degradation through contact with the electrolyte, and is accompanied by Ni reduction. O2 is removed on discharge, but an excess of Ni2+ remains forming a reduced surface layer that impedes Li transport. Our findings highlight the importance of separating bulk redox from near-surface degradation, and implicate the instability of delithiated LiNiO2 in contact with the electrolyte in surface degradation through O2 formation.

Steve Pennycook is a pioneer in the application of high-resolution scanning transmission electron microscopy (STEM) and in particular the use of annular dark-field (ADF) imaging. Here we show how a general framework for 4D STEM allows clear links to be made between ADF imaging and the emerging methods for reconstructing images from 4D STEM data sets. We show that both ADF imaging and ptychographical reconstruction can be thought of in terms of integrating over the overlap regions of diffracted discs in the detector plane. This approach allows the similarities in parts of their transfer functions to be understood, though we note that the transfer functions for ptychographic imaging cannot be used as a measure of information transfer. We also show that conditions of partial spatial and temporal coherence affect ADF imaging and ptychography similarly, showing that achromatic interference can always contribute to the image in both cases, leading to a robustness to partial temporal coherence that has enabled high-resolution imaging.

Segmented and pixelated detectors on scanning transmission electron microscopes enable the complex specimen transmission function to be reconstructed. Imaging the transmission function is key to interpreting the electric and magnetic properties of the specimen, and as such four-dimensional scanning transmission electron microscopy (4D-STEM) imaging techniques are crucial for our understanding of functional materials. Many of the algorithms used in the reconstruction of the transmission function rely on the multiplicative approximation and the (weak) phase object approximation, which are not valid for many materials, particularly at high resolution. Herein, we study the breakdown of simple phase imaging in thicker samples. We demonstrate the behavior of integrated center of mass imaging, single-side band ptychography, and Wigner distribution deconvolution over a thickness series of simulated GaN 4D-STEM datasets. We further give guidance as to the optimal focal conditions for obtaining a more interpretable dataset using these algorithms.

Journal Article Combine 4D STEM and EELS Using a Fast Pixelated Direct Detector with Center Hole Get access M Huth, M Huth PNDetector GmbH, München, Germany Corresponding author: bjoern.eckert@pndetector.de Search for other works by this author on: Oxford Academic Google Scholar B Eckert, B Eckert PNDetector GmbH, München, GermanyUniversity of Siegen, Department of Physics, Siegen, Germany Corresponding author: bjoern.eckert@pndetector.de Search for other works by this author on: Oxford Academic Google Scholar S Aschauer, S Aschauer PNDetector GmbH, München, Germany Search for other works by this author on: Oxford Academic Google Scholar E Hedley, E Hedley University of Oxford, Department of Materials, Oxford, United Kingdom Search for other works by this author on: Oxford Academic Google Scholar P Nellist, P Nellist University of Oxford, Department of Materials, Oxford, United Kingdom Search for other works by this author on: Oxford Academic Google Scholar P Majewski, P Majewski PNDetector GmbH, München, Germany Search for other works by this author on: Oxford Academic Google Scholar L Strüder, L Strüder University of Siegen, Department of Physics, Siegen, GermanyPNSensor GmbH, München, Germany Search for other works by this author on: Oxford Academic Google Scholar H Soltau H Soltau PNDetector GmbH, München, Germany Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 401–402, https://doi.org/10.1093/micmic/ozad067.188 Published: 22 July 2023

Inelastic excitation as exploited in Electron Energy Loss Spectroscopy (EELS) contains a rich source of information that is revealed in the scattering process. To accurately quantify core-loss EELS, it is common practice to fit the observed spectrum with scattering cross-sections calculated using experimental parameters and a Generalized Oscillator Strength (GOS) database [1]. The GOS is computed using Fermi’s Golden Rule and orbitals of bound and excited states. Previously, the GOS was based on Hartree-Fock solutions [2], but more recently Density Functional Theory (DFT) has been used [3]. In this work, we have chosen to use the Dirac equation to incorporate relativistic effects and have performed calculations using Flexible Atomic Code (FAC) [4]. This repository contains a tabulated GOS database based on Dirac solutions for computing double differential cross-sections under experimental conditions. We hope the Dirac-based GOS database can benefit the EELS community for both academic use and industry integration. Database Details: - Covers all elements (Z: 1-108) and all edges - Large energy range: 0.01 - 4000 eV - Large momentum range: 0.05 -50 Å-1 - Fine log sampling: 128 points for energy and 256 points for momentum - Data format: GOSH [3] Calculation Details: - Single atoms only; solid-state effects are not considered - Unoccupied states before continuum states of ionization are not considered; no fine structure - Plane Wave Born Approximation - Frozen Core Approximation is employed; electrostatic potential remains unchanged for orthogonal states when - core-shell electron is excited - Self-consistent Dirac–Fock–Slater iteration is used for Dirac calculations; Local Density Approximation is assumed for electron exchange interactions; continuum states are normalized against asymptotic form at large distances - Both large and small component contributions of Dirac solutions are included in GOS - Final state contributions are included until the contribution of the previous three states falls below 0.1%. A convergence log is provided for reference. Version 1.1 release note: - Update to be consistent with GOSH data format [3], all the edges are now within a single hdf5 file. A notable change in particular, the sampling in momentum is in 1/m, instead of previously in 1/Å. Great thanks to Gulio Guzzinati for his suggestions and sending conversion script. Version 1.2 release note: - Add “File Type / File version” information [1] Verbeeck, J., and S. Van Aert. Ultramicroscopy 101.2-4 (2004): 207-224. [2] Leapman, R. D., P. Rez, and D. F. Mayers. The Journal of Chemical Physics 72.2 (1980): 1232-1243. [3] Segger, L, Guzzinati, G, & Kohl, H. Zenodo (2023). doi:10.5281/zenodo.7645765 [4] Gu, M. F. Canadian Journal of Physics 86(5) (2008): 675-689.

Metal halide perovskite semiconductors have shown great performance in solar cells, and including an excess of lead iodide (PbI 2 ) in the thin films, either as mesoscopic particles or embedded domains, often leads to improved solar cell performance. Atomic resolution scanning transmission electron microscope micrographs of formamidinium lead iodide (FAPbI 3 ) perovskite films reveal the FAPbI 3 :PbI 2 interface to be remarkably coherent. It is demonstrated that such interface coherence is achieved by the PbI 2 deviating from its common 2H hexagonal phase to form a trigonal 3R polytype through minor shifts in the stacking of the weakly van‐der‐Waals‐bonded layers containing the near‐octahedral units. The exact crystallographic interfacial relationship and lattice misfit are revealed. It is further shown that this 3R polytype of PbI 2 has similar X‐ray diffraction (XRD) peaks to that of the perovskite, making XRD‐based quantification of the presence of PbI 2 unreliable. Density functional theory demonstrates that this interface does not introduce additional electronic states in the bandgap, making it electronically benign. These findings explain why a slight PbI 2 excess during perovskite film growth can help template perovskite crystal growth and passivate interfacial defects, improving solar cell performance.