Physical Review Materials

Diffraction is the most common method for solving unknown crystal structures, but nanoscale heterogeneity poses substantial challenges. Here we train random forest models to predict the crystal system, space group, and lattice parameters from one or more unknown 2D electron diffraction patterns. We apply this architecture to a 4D-STEM scan of over 100 gold nanoparticles, accurately predicting the crystal structure and lattice constants. Our ML architecture significantly accelerates the analysis of electron diffraction patterns, particularly in the case of unknown crystal structures at speeds amenable to live analysis. This work also releases ~360 million simulated 2D dynamical electron diffraction patterns from 36,000 materials, each with 100 unique crystal orientations and 100 specimen thicknesses.

Advances in cryogenic four-dimensional scanning transmission electron microscopy (4D-STEM) enables the capture of local convergent-beam electron diffraction patterns at cryogenic temperatures for collocated mapping of charge density wave (CDW) microstructure, polarity and atomic displacements. Using this newfound capability, the authors reveal the nature of long-wavelength incommensurate CDWs in EuAl${}_4$ in the form of periodic symmetry fluctuations, resulted from two out-of-phase sinusoidal, transverse, atomic displacement modes on the Eu and Al sublattices. The results demonstrate cryogenic 4D-STEM as a powerful method for probing atomic modulations and local symmetry.

Scanning Transmission Electron Microscopy - Differential Phase Contrast (STEM-DPC) is a 4D-STEM technique that allows quantitative characterization of electromagnetic fields down to nanoscale resolution. However, a significant challenge arises from diffractive intensity scattering in crystalline materials, which introduces distortive contrast that can obscure magnetic imaging. This work details how to minimize this issue, which the authors demonstrate using a ferromagnetic freestanding La${}_{0.67}$Sr${}_{0.33}$MnO${}_3$ thin film to quantify diffraction contrast variations under different orientations and processing conditions. All the data and scripts are made publicly available, providing a platform for others to test their own processing.

The performance of any future spintronic device based on thin films or heterostructures may be compromised by thermal effects derived from Joule heating. For example, chemical interdiffusion processes may significantly alter the interface, thus threatening macroscopic behavior. Here thermally driven phenomena are studied in graphene-based magnetic epitaxial graphene/Co/Pt heterostructures. In-situ atomic resolution electron microscopy and spectroscopy are used for direct visualization of thermally activated interdiffusion phenomena at Co/Pt interfaces in the 300 – 773 K range. Noticeable Co/Pt alloying takes place at temperatures above 523K, significantly enhancing the layer coercivity and offering a direct pathway for optimizing epitaxial systems for robust future device integration.

Surface-sensitive electron microscopy, combined with in situ capabilities, enables nanoscale observation of physicochemical processes on catalytic surfaces in real time, providing a powerful tool to investigate surface dynamics under reactive environments. In this work, it allows the authors to unravel the detailed surface reduction mechanism of NiO catalysts—an essential activation step for various catalytic reactions. The facet-dependent reduction dynamics are revealed and supported by surface and binding energies derived from first-principles calculations. Atomic-scale imaging further resolves the Ni/NiO interfacial structure after reduction. This study underscores the power of in situ surface-sensitive electron microscopy in probing surface dynamics in catalytic materials.

Spatially resolved vibrational electron energy loss spectroscopy has enabled studies of phonon behavior with atomic-to-nanometer scale resolution. However, the interpretation of local vibrational signals can be hampered by delocalized scattering contributions from surface excitations. We present an approach to disentangle surface and bulk contributions, enabling the isolation of interfacial phonons in complex oxide heterostructures. Applying this method to a YBa${}_2$Cu${}_3$O${}_{7-\delta }$/(La,Sr)(Al,Ta)O${}_3$ interface, we measured atomic-scale phonon scattering suggesting the presence of interfacial phonon modes that may be relevant to superconductivity properties of underdoped YBa${}_2$Cu${}_3$O${}_{7-\delta }$ thin films.

Atomic-resolution STEM imaging typically just images a projection of the crystal structure into two dimensions. The authors show that fitting of the azimuthal intensity distribution in the First Order Laue zone in atomic resolution 4DSTEM data produces a unique set of images displaying periodic features not seen in ADF. With the aid of simulation, they show that these can be interpreted in a robust and automated way to map 3-dimensional atomic displacements of La atoms in La${}_2$CoMnO${}_6$, which is demonstrated close to an epitaxial interface. This method provides a new direction for atomic resolution 4DSTEM imaging to reveal information beyond simple projection.

Atomic-resolution scanning transmission electron microscopy (STEM) provides unparalleled insight into nanoscale structure–property relationships in functional materials. Robust, interpretable image processing is essential for turning complex datasets into quantitative measurements. In this work, the authors present algorithms combining real- and reciprocal-space information to extract subtle atomic displacements associated with symmetry breaking distortions and quantify nanoscale strain variations. Rooted in crystallography, these approaches emphasize physical interpretability as well as associated measurement uncertainties and regimes of validity. These algorithms are collected into an open-source Python package available to the community.

What happens when two tiny gold spheres come so close that only a few atoms separate them? At this scale, classical physics gives way to quantum effects, but measuring such ultra-small gaps has remained notoriously difficult. Here, the authors introduce a 3D electron microscopy workflow combined with a robust fitting model to achieve sub-pixel accuracy. This strategy provides reliable morphology-optics correlations, paving the way toward resolving quantum effects in nanoscale light-matter interactions and enabling the rational design of plasmonic nanostructures.

Domain orientations in epitaxial films can be determined by selected-area or nanobeam electron diffraction and visualised in real space using dark-field imaging. But the interpretation can be unclear because 3D information is lost when just working with low angle diffraction spots. However, if Digital Dark Field is used with reflections in Higher Order Laue zones, these ambiguities can be resolved. This approach is used to reconstruct the 3D orientations of La atom modulations in films of La${}_2$CoMnO${}_6$ grown on two different substrates, which reveals a strain-driven reorientation invisible to low angle diffraction or conventional dark field imaging.

Combining plasmonic nanoparticles with dielectric nanostructures opens new opportunities for nanoscale control of light, yet the plasmonic field behavior in such hybrid architectures remains underexplored. Here, the authors demonstrate non-monochromatic plasmon excitations in a hybrid structure of a metal nanoparticle on a dielectric nanowire (Ag on ZnO, as a representative model). Monochromated electron energy loss spectroscopy, combined with tomography, enabled them to observe two plasmon modes with an energy difference of ~0.3 eV, whose three-dimensional near-field distributions were distinctly resolved at the nanoscale in this hybrid system. Supported by comprehensive numerical simulations, the results reveal that asymmetry in the dielectric environment surrounding the plasmonic nanoparticle governs the tunability of plasmon energies and near-field distributions.

Phase transitions at the nanoscale are strongly governed by interfacial interactions and can proceed through entirely new mechanisms compared to bulk systems. Here, the authors report a surface phase transition of crystalline α-silicon nitride (α-Si${}_3$N${}_4$) originating from amorphous silicon nitride, catalyzed by migrating Au–Pd nanodroplets. In situ transmission electron microscopy directly visualizes the stepwise catalytic cycle, which consists of the dissolution of Si into the nanodroplet and its subsequent precipitation. These dissolution–precipitation processes act as alternating “pull” and “push” forces that drive the nanocatalyst forward, thereby sustaining the solid–solid phase transition. This work demonstrates a nanoscale phase transition occurring at significantly lower temperatures than its bulk counterpart and unveils a general mechanistic pathway for catalytically mediated solid–solid phase transformations.

Electron magnetic circular dichroism (EMCD) in TEM enables access to the functional magnetic material properties at length scales beyond the reach of X-ray magnetic circular dichroism (XMCD). Here, we use EMCD to probe orbital and spin moments (mL/mS) across the first-order metamagnetic phase transition in FeRh. The evolution of the EMCD-derived magnetic signals and the mL/mS ratio are systematically measured as a function of probe convergence and size, and remain comparable to XMCD down to ~6 nm probe sizes. For more convergent probes, deviations emerge, reflecting enhanced sensitivity to instrumental effects and nanoscale magnetic nonuniformities, highlighting EMCD’s potential for locally resolved studies of functional magnetic systems.

Momentum-resolved electron energy-loss spectroscopy (q-EELS) enables the study of 2D materials by simultaneously capturing energy and momentum transfer with nanometer spatial resolution, probing excitations beyond the light cone and outside the first Brillouin zone. This technique provides unrivaled detail in the investigation of plasmons, excitons, phonons, and their coupling in 2D materials and across interfaces in heterostructures. This review covers the fundamentals of q-EELS, including resolution limits and detectability challenges. Current applications in graphene, hexagonal boron nitride, and transition metal dichalcogenides are surveyed. Emerging frontiers including magnons, cryogenic operation, and in situ capabilities promise to advance our understanding of low-dimensional materials.

Indium phosphide (InP) is a promising photocatalyst, yet its performance is often limited by fast carrier recombination and inefficient charge transport. Deng \emph{et al}. use scanning ultrafast electron microscopy (SUEM) to directly visualize photoexcited carrier dynamics on femtosecond-nanometer scales at the InP surface decorated with silver nanoparticles. SUEM reveals that Ag nanoparticles remarkably enhance light absorption via localized surface plasmon resonance, leading to stronger charge separation, suppressed recombination, and a pronounced early-time boost in lateral carrier transport. In contrast, a continuous Ag film improves separation through a Schottky junction but shows limited diffusion, underscoring the unique role of plasmonic nanoparticles.

Exchange stiffness is a fundamental micromagnetic parameter, yet its value in hard ferrites has remained uncertain due to the limitations of traditional thin-film spin-wave experiments. In this work, the authors utilize tilt-scan-averaged differential phase-contrast STEM to directly measure ${180}^{\circ }$ domain-wall widths in Sr-based and Ca-La-Co-based M-type ferrites. By combining these real-space measurements with bulk anisotropy constants, the authors quantify local exchange stiffness with high precision. This approach reveals that variations in domain-wall width are driven by magnetocrystalline anisotropy rather than changes in exchange stiffness, providing a powerful new tool for mapping magnetic properties at the nanoscale within complex microstructures.

Ultrafast transmission electron microscopy, a powerful tool for visualizing ultrafast structural dynamics, is harnessed here to reveal the full-cycle reversible dynamics of five-fold twinned silver nanowires across picosecond to microsecond timescales. On the picosecond timescale, a two-step process was identified: a fast electron-phonon coupling step and a slower step ascribed to hot electron decay due to trap states. Anisotropic phonon-phonon interactions, arising from the one-dimensional structure of the silver nanowires, drive energy transfer from the radial to the axial direction on the nanosecond scale. The subsequent recovery of the lattice occurs on the microsecond scale. These findings pave the way for advanced ultrafast device applications.

Quantitative convergent beam electron diffraction (QCBED) enables determination of aspherical valence electron (VE) distributions but requires solving a highly nonlinear inverse problem with many parameters, making it time-consuming and difficult to apply to complex systems. Here, the authors introduce a machine-learning framework that replaces traditional refinement. Using a large synthetic CBED dataset generated by Bloch-wave simulations, they train a conditional diffusion model to infer multipole density formalism for VE mapping, from CBED patterns alone. This approach enables practical, high-throughput, and in situ QCBED, opening new opportunities for real-time VE mapping and their correlation with functional responses in quantum materials.

Surface oxygen vacancies in ceria (CeO${}_2$) appear as centers of extended strain fields reaching up to the nanoscale in the cation sublattice. Using transmission electron microscopy combined with density functional theory , the authors reveal highly heterogeneous (alternating tensile/compressive) and dynamic strain (high degree of fluxionality) landscapes, with both stable and unstable vacancies coexisting within a few atomic spacings. These findings characterize the structural footprint of surface defects and highlight the critical role of fluxional strain in governing cation functionality.

Probing chirality below the diffraction limit is vital for understanding biological processes and designing chiral nanostructures. While orbital angular momentum (OAM)-resolved electron energy-loss spectroscopy (EELS) is a promising candidate for detecting nanoscale circular dichroism, experimental results remain elusive. This study presents a theoretical framework for understanding and optimizing the dichroic signal using a prototypical chiral nanostructure – a plasmonic nanohelix. By analyzing the role of various parameters, we demonstrate that the dichroism in EELS is sensitive to topological charge transfer, electron energy and sample geometry yet robust against minor experimental misalignments. These insights provide a roadmap for future nanoscale dichroism measurements.