arXiv subjects

Colin Ophus

Publications and source records attributed to Colin Ophus.

At least 37 records · Page 2Linked to original sources

arXiv2025

Multi-angle precession electron diffraction (MAPED): a versatile approach to 4D-STEM precession

Precession of a converged beam during acquisition of a 4D-STEM dataset improves strain, orientation, and phase mapping accuracy by averaging over continuous angles of illumination. Precession experiments usually rely on integrated systems, where automatic alignments lead to fast, high-quality results. The dependence of these experiments on specific hardware and software is evident even when switching to non-integrated detectors on a precession tool, as experimental set-up becomes challenging and time-consuming. Here, we introduce multi-angle precession electron diffraction (MAPED): a method to perform electron diffraction by collecting sequential 4D-STEM scans at different incident beam tilts. The multiple diffraction datasets are averaged together post-acquisition, resulting in a single dataset that minimizes the impact of the curvature and orientation of the Ewald sphere relative to the crystal under study. Our results demonstrate that even four additional tilts improved measurement of material properties, namely strain and orientation, as compared to single-tilt 4D-STEM experiments. We show the versatility and flexibility of our MAPED approach with data collected on a number of microscopes with different hardware configurations and a variety of detectors.

physics.ins-det↗

arXiv2025

Neutral but Impactful: Gallium Cluster-Induced Nanopores from Beam-Blanked Gallium Ion Sources

Neutral atoms emitted from liquid metal ion sources are an often-overlooked source of contamination and damage in focused ion beam microscopy. Beyond ions and single atoms, these sources also emit atom clusters. While most studies have investigated charged clusters, here we demonstrate that neutral clusters are also emitted. These neutral clusters bypass the electrostatic beam blanking system, allowing them to impinge on samples even when the ion beam is blanked. We investigate this phenomenon using thin (<20 nm) freestanding membranes of hexagonal boron nitride, silicon, and silicon nitride as targets. Randomly dispersed nanopores that form upon neutral cluster exposure are revealed. The average nanopore diameter is ~2 nm with a narrow size distribution, suggesting that the atom clusters emitted from the source have a preferred size. Various electron microscopy techniques are used to characterize the nanopores, including high-resolution transmission electron microscopy, multislice ptychography, and electron energy-loss spectroscopy. Finally, we show how electron irradiation in the transmission electron microscope can be used to both remove any amorphous material that may clog the pores and to controllably grow the pores to specific sizes. Tunable nanopores such as these are interesting for nanofluidic applications involving size-selective membranes.

cond-mat.mtrl-sci↗

arXiv2025

3D Strain Field Reconstruction by Inversion of Dynamical Scattering

Strain governs not only the mechanical response of materials but also their electronic, optical, and catalytic properties. For this reason, the measurement of the 3D strain field is crucial for a detailed understanding and for further developments of material properties through strain engineering. However, measuring strain variations along the electron beam direction has remained a major challenge for (scanning-) transmission electron microscopy (S/TEM). In this article, we present a method for 3D strain field determination using 4D-STEM. The method is based on the inversion of dynamical diffraction effects, which occur at strain field variations along the beam direction. We test the method against simulated data with a known ground truth and demonstrate its application to an experimental 4D-STEM dataset from an inclined pseudomorphically grown Al$_{0.47}$Ga$_{0.53}$N layer.

cond-mat.mtrl-sci↗

arXiv2025

Programmable Beam Control for Electron Energy-Loss Spectroscopy and Ptychography

Programmable electron-beam scanning offers new opportunities to improve dose efficiency and suppress scan-induced artifacts in scanning transmission electron microscopy. Here, we systematically benchmark the impact of non-raster trajectories, including spiral and multi-pass sequential patterns, on two dose sensitive techniques: electron energy-loss spectroscopy (EELS) and ptychography. Using DyScO3 as a model perovskite, we compare spatial resolution, spectral fidelity, and artifact suppression across scan modes. Ptychographic phase reconstructions consistently achieve atomic resolution and remain robust to large jumps in probe position. In contrast, atomic-resolution EELS maps show pronounced sensitivity to probe motion, with sequential and spiral scans introducing non-uniform elemental contrast. Finally, spiral scanning applied under cryogenic conditions in BaTiO3 thin films improves dose uniformity and mitigates drift related distortions. These results establish practical guidelines for the implementation of programmable scan strategies in low-dose 4D-STEM and highlight the inherent resilience of ptychography to trajectory-induced artifacts.

cond-mat.mtrl-sci↗

arXiv2025

High-current p-type transistors from precursor-engineered synthetic monolayer WSe$_2$

Monolayer tungsten diselenide (WSe$_2$) is a leading candidate for nanoscale complementary logic. However, high defect densities introduced during thin-film growth and device fabrication have limited p-type transistor performance. Here, we report a combined strategy of precursor-engineered chemical vapor deposition and damage-minimizing fabrication to overcome this limitation. By converting tungsten trioxide and residual oxyselenides into reactive suboxides before growth, and precisely regulating selenium delivery during deposition, we synthesize uniform, centimeter-scale monolayer WSe$_2$ films with charged defect densities as low as $5 \times 10^{9}$ cm$^{-2}$. Transistors fabricated from these films achieve record p-type on-state current up to $888 μ$A$\cdotμ$m$^{-1}$ at $V_{\mathrm{DS}}=-1$ V, matching leading n-type devices. This leap in material quality closes the p-type performance gap without exotic doping or contact materials, marking a critical step towards complementary two-dimensional semiconductor circuits.

cond-mat.mtrl-sci↗

arXiv2025

Relaxing Direct Ptychography Sampling Requirements via Parallax Imaging Insights

Direct ptychography enables the retrieval of information encoded in the phase of an electron wave passing through a thin sample by deconvolving the interference effects of a converged probe with known aberrations. Under the weak phase object approximation, this permits the optimal transfer of information using non-iterative techniques. However, the achievable resolution of the technique is traditionally limited by the probe step size -- setting stringent Nyquist sampling requirements. At the same time, parallax imaging has emerged as a dose-efficient phase-retrieval technique which relaxes sampling requirements and enables scan-upsampling. Here, we formulate parallax imaging as a quadratic approximation to part of the direct ptychography kernel and use this insight to enable upsampling in direct ptychography. We validate our analytical results numerically using simulated and experimental reconstructions.

physics.optics↗

arXiv2025

Beyond Contrast Transfer: Spectral SNR as a Dose-Aware Metric for STEM Phase Retrieval

The contrast transfer function (CTF) is widely used to evaluate phase retrieval methods in scanning transmission electron microscopy (STEM), including center-of-mass imaging, parallax imaging, direct ptychography, and iterative ptychography. However, the CTF reflects only the maximum usable signal, neglecting the effects of finite electron fluence and the Poisson-limited nature of detection. As a result, it can significantly overestimate practical performance, especially in low-dose regimes. Here, we employ the spectral signal-to-noise ratio (SSNR), as a dose-aware statistical framework to evaluate the recoverable signal as a function of spatial frequency. Using numerical reconstructions of white-noise objects, we show that center-of-mass, parallax, and direct ptychography exhibit dose-independent SSNRs, with close-form analytic expressions. In contrast, iterative ptychography exhibits a surprising dose dependence: at low fluence, its SSNR converges to that of direct ptychography; at high fluence, it saturates at a value consistent with the maximum detective quantum efficiency predicted by recent quantum Fisher information bounds. The results highlight the limitations of CTF-based evaluation and motivate SSNR as a more accurate, dose-aware metric for assessing STEM phase retrieval methods.

physics.optics↗

arXiv2025

Atomic-scale 3D structural dynamics and functional degradation of Pt alloy nanocatalysts during the oxygen reduction reaction

Pt-based electrocatalysts are the primary choice for fuel cells due to their superior oxygen reduction reaction (ORR) activity. To enhance ORR performance and durability, extensive studies have investigated transition metal alloying, doping, and shape control to optimize the three key governing factors for ORR: geometry, local chemistry, and strain of their surface and subsurface. However, systematic optimization remains incomplete, as it requires an atomic-scale understanding of these factors and their dynamics over potential cycling, as well as their relationship to ORR activity. Here, we implement neural network-assisted atomic electron tomography to measure the 3D atomic structural dynamics and their effects on the functional degradation of PtNi alloy catalysts. Our results reveal that PtNi catalysts undergo shape changes, surface alloying, and strain relaxation during cycling, which can be effectively mitigated by Ga doping. By combining geometry, local chemistry, and strain analysis, we calculated the changes in ORR activity over thousands of cycles and observed that Ga doping leads to higher initial activity and greater stability. These findings offer a pathway to understanding 3D atomic structural dynamics and their relation to ORR activity during cycling, paving the way for the systematic design of durable, high-efficiency nanocatalysts.

cond-mat.mtrl-sci↗

arXiv2025

Quantitative structure determination from experimental four-dimensional scanning transmission electron microscopy via the scattering matrix

Considerable inroads have recently been made on algorithms to determine the sample potential from four-dimensional scanning transmission electron microscopy data from thick samples where multiple scattering cannot be neglected. This paper further develops the scattering matrix approach to such structure determination. Through simulation, we demonstrate how this approach can be modified to better handle partial spatial coherence, unknown probe defocus, and information from the dark field region. By combining these developments we reconstruct the electrostatic potential of a monolithic SrTiO$_3$ crystal showing good quantitative agreement with the expected structure.

cond-mat.mtrl-sci↗

arXiv2025

Fabrication and characterization of boron-terminated tetravacancies in monolayer hBN using STEM, EELS and electron ptychography

Tetravacancies in monolayer hexagonal boron nitride (hBN) with consistent edge termination (boron or nitrogen) form triangular nanopores with electrostatic potentials that can be leveraged for applications such as selective ion transport and neuromorphic computing. In order to quantitatively predict the properties of these structures, an atomic-level understanding of their local electronic and chemical environments is required. Moreover, robust methods for their precision manufacture are needed. Here were use electron irradiation in a scanning transmission electron microscope (STEM) at high dose rate to drive the formation of boron-terminated tetravacancies in monolayer hBN. Characterization of the defects is achieved using aberration-corrected STEM, monochromated electron energy-loss spectroscopy (EELS), and electron ptychography. Z-contrast in STEM and chemical fingerprinting by core-loss EELS enable identification of the edge terminations, while electron ptychography gives insight into structural relaxation of the tetravacancies and provides evidence of enhanced electron density around the defect perimeters indicative of bonding effects.

cond-mat.mtrl-sci↗

arXiv2025

Locating the Atoms at the Hard-soft Interface of Gold Nanoparticles

Surface structure affects the growth, shape and properties of nanoparticles. In wet chemical syntheses, metal additives and surfactants are used to modify surfaces and guide nanocrystal growth. To understand this process, it is critical to understand how the surface structure is modified. However, measuring the type and arrangement of atoms at hard-soft interfaces on nanoscale surfaces, especially in the presence of surfactants, is extremely challenging. Here, we determine the atomic structure of the hard-soft interface in a metallic nanoparticle by developing low-dose imaging conditions in four-dimensional scanning transmission electron microscopy that are preferentially sensitive to surface adatoms. By revealing experimentally the copper additives and bromide surfactant counterion at the surface of a gold nanocuboid and quantifying their interatomic distances, our direct, low-dose imaging method provides atomic-level understanding of chemically sophisticated nanomaterial surface structures. These measurements of the atomic structure of the hard-soft interface provide the information necessary to understand and quantify surface chemistries and energies and their pivotal role in nanocrystal growth.

cond-mat.mtrl-sci↗

arXiv2025

Mapping strain and structural heterogeneities around bubbles in amorphous ionically conductive Bi$_2$O$_3$

While amorphous materials are often approximated to have a statistically homogeneous atomic structure, they frequently exhibit localized structural heterogeneity that challenges simplified models. This study uses 4D scanning transmission electron microscopy to investigate the strain and structural modifications around gas bubbles in amorphous Bi$_2$O$_3$ induced by argon irradiation. We present a method for determining strain fields surrounding bubbles that can be used to measure the internal pressure of the gas. Compressive strain is observed around the cavities, with higher-order crystalline symmetries emerging near the cavity interfaces, suggesting paracrystalline ordering as a result of bubble coarsening. This ordering, along with a compressive strain gradient, indicates that gas bubbles induce significant localized changes in atomic packing. By analyzing strain fields with maximum compressive strains of 3\%, we estimate a lower bound on the internal pressure of the bubbles at 2.5 GPa. These findings provide insight into the complex structural behavior of amorphous materials under stress, particularly in systems with gas inclusions, and offer new methods for probing the local atomic structure in disordered materials. Although considering structural heterogeneity in amorphous systems is non-trivial, these features have crucial impacts on material functionalities, such as mechanical strength, ionic conductivity, and electronic mobility.

cond-mat.mtrl-sci↗

arXiv2025

Complexions at the Iron-Magnetite Interface

Synthesizing distinct phases and controlling the crystalline defects in them are key concepts in materials and process design. These approaches are usually described by decoupled theories, with the former resting on equilibrium thermodynamics and the latter on nonequilibrium kinetics. By combining them into a holistic form of defect phase diagrams, we can apply phase equilibrium models to the thermodynamic evaluation of defects such as vacancies, dislocations, surfaces, grain boundaries, and phase boundaries, placing the understanding of material imperfections and their role on properties on solid thermodynamic and theoretical grounds. In this study, we characterize an interface-stabilized phase between Fe and Fe3O4 (magnetite) with differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). This method uniquely enables the simultaneous imaging of both heavy Fe atoms and light O atoms, providing precise mapping of the atomic structure and chemical composition at this heterogeneous metal-oxide interface. We identify a well-ordered two-layer interface-stabilized phase state (referred to as complexion) at the Fe[001]/Fe3O4[001] interface. Using density-functional theory (DFT), we not only explain the observed complexion but also map out various interface-stabilized phases as a function of the O chemical potential. We show that the formation of complexions influences the properties of the interface, increasing its adhesion by 20 % and changing the charge transfer between adjacent materials, also leveraging impact on the transport properties across such interfaces. Our findings highlight the potential of tunable phase states at defects as a new asset in advanced materials design, paving the way for knowledge-based and optimized corrosion protection, catalysis, magnetism, and redox-driven phase transitions.

cond-mat.mtrl-sci↗

arXiv2024

Quantitative Phase Retrieval and Characterization of Magnetic Nanostructures via Lorentz (Scanning) Transmission Electron Microscopy

Magnetic materials phase reconstruction from Lorentz transmission electron microscopy (LTEM) measurements has traditionally been achieved using longstanding methods such as off-axis holography (OAH) and the transport-of-intensity equation (TIE). Amidst the increase in access to processing power and the development of advanced algorithms, phase retrieval of nanoscale magnetic materials with higher fidelity and resolution, potentially down to the few nanometer limit, becomes possible. Specifically, reverse-mode automatic differentiation (RMAD) and the extended electron ptychography iterative engine (ePIE) are two methods that have been utilized for high confidence phase reconstructions using LTEM through-focal series imaging and Lorentz scanning TEM (Ltz-4D-STEM), respectively. This work evaluates phase retrieval using TIE, RMAD, and ePIE in simulations consisting of an array of Permalloy (Ni80Fe20) nanoscale islands. Extending beyond simulations, we demonstrate total phase reconstructions of a NiFe nanowire using OAH and RMAD in LTEM and ePIE in Ltz-4D-STEM experiments and determine the magnetization saturation through corroborations with micromagnetic simulations. Finally, we show how the total phase shift gradient can be utilized to observe and characterize the proximity effects emanating from neighboring magnetic island interactions and an isolated NiFe nanowire.

cond-mat.mtrl-sci↗

arXiv2024

Atomic evolution of hydrogen intercalation wave dynamics in palladium nanocrystals

Solute-intercalation-induced phase separation creates spatial heterogeneities in host materials, a phenomenon ubiquitous in batteries, hydrogen storage, and other energy devices. Despite many efforts, probing intercalation processes at the atomic scale has been a significant challenge. We study hydrogen (de)intercalation in palladium nanocrystals as a model system and achieve atomic-resolution imaging of hydrogen intercalation wave dynamics by utilizing liquid-phase transmission electron microscopy. Our observations reveal that intercalation wave mechanisms, instead of shrinking-core mechanisms, prevail at ambient temperature for palladium nanocubes ranging from ~60 nm down to ~10 nm. We uncover the atomic evolution of hydrogen intercalation wave transitioning from non-planar and inclined boundaries to those closely aligned with {100} planes. Our kinetic Monte Carlo simulations demonstrate the observed intercalation wave dynamics correspond to sorption pathways minimizing the lattice mismatch strain at the phase boundary. Unveiling the atomic intercalation pathways holds profound implications for engineering intercalation-mediated devices and advancements in energy sciences.

cond-mat.stat-mech↗

arXiv2024

BEACON -- Automated Aberration Correction for Scanning Transmission Electron Microscopy using Bayesian Optimization

Aberration correction is an important aspect of modern high-resolution scanning transmission electron microscopy. Most methods of aligning aberration correctors require specialized sample regions and are unsuitable for fine-tuning aberrations without interrupting on-going experiments. Here, we present an automated method of correcting first- and second-order aberrations called BEACON which uses Bayesian optimization of the normalized image variance to efficiently determine the optimal corrector settings. We demonstrate its use on gold nanoparticles and a hafnium dioxide thin film showing its versatility in nano- and atomic-scale experiments. BEACON can correct all first- and second-order aberrations simultaneously to achieve an initial alignment and first- and second-order aberrations independently for fine alignment. Ptychographic reconstructions are used to demonstrate an improvement in probe shape and a reduction in the target aberration.

cond-mat.mtrl-sci↗

arXiv2024

Digital Dark Field -- Higher Contrast and Greater Specificity Dark Field Imaging using a 4DSTEM Approach

A new method for dark field imaging is introduced which uses scanned electron diffraction (or 4DSTEM - 4-dimensional scanning transmission electron microscopy) datasets as its input. Instead of working on simple summation of intensity, it works on a sparse representation of the diffraction patterns in terms of a list of their diffraction peaks. This is tested on a thin perovskite film containing structural ordering resulting in additional superlattice spots that reveal details of domain structures, and is shown to give much better selectivity and contrast than conventional virtual dark field imaging. It is also shown to work well in polycrystalline aggregates of CuO nanoparticles. In view of the higher contrast and selectivity, and the complete exclusion of diffuse scattering from the image formation, it is expected to be of significant benefit for characterisation of a wide variety of crystalline materials.

cond-mat.mtrl-sci↗

arXiv2024

Random Forest Prediction of Crystal Structure from Electron Diffraction Patterns Incorporating Multiple Scattering

Diffraction is the most common method to solve for unknown or partially known crystal structures. However, it remains a challenge to determine the crystal structure of a new material that may have nanoscale size or heterogeneities. Here we train an architecture of hierarchical random forest models capable of predicting the crystal system, space group, and lattice parameters from one or more unknown 2D electron diffraction patterns. Our initial model correctly identifies the crystal system of a simulated electron diffraction pattern from a 20 nm thick specimen of arbitrary orientation 67% of the time. We achieve a topline accuracy of 79% when aggregating predictions from 10 patterns of the same material but different zone axes. The space group and lattice predictions range from 70-90% accuracy and median errors of 0.01-0.5 angstroms, respectively, for cubic, hexagonal, trigonal and tetragonal crystal systems while being less reliable on orthorhombic and monoclinic systems. We apply this architecture to a 4D-STEM scan of gold nanoparticles, where it accurately predicts the crystal structure and lattice constants. These random forest models can be used to significantly accelerate the analysis of electron diffraction patterns, particularly in the case of unknown crystal structures. Additionally, due to the speed of inference, these models could be integrated into live TEM experiments, allowing real-time labeling of a specimen.

cond-mat.mtrl-sci↗