arXiv subjects

Lewys Jones

Publications and source records attributed to Lewys Jones.

At least 19 recordsLinked to original sources

arXiv2026

Predictive drift compensation of multi-frame STEM via live scan modification

Scanning transmission electron microscopy (STEM) is widely used tool for materials characterisation. However, being a scanned technique, STEM is susceptible to sample, stage or beam drift, manifesting as distortions within images or movement in the field-of-view during multi-frame imaging. Often this is corrected post-acquisition using image registration of multiple frames, but drift reduces the usable area common to all frames. Here we present a method to mitigate sample drift by analysing past frames to predict the sampling-grid points for the immediately future frame. We present this correction across two time-scales and two lengthscales. By offsetting the scan-grid framewise we remove long-range drift, and offsetting pixelwise we minimise intra-image warping. Examples are presented for both atomic-resolution imaging and lower-magnification in-situ video capture. The framework is general to raster, serpentine, interlaced and other scan patterns, as well as sequential or scan-rotation series STEM.

physics.ins-det↗

arXiv2025

Relaxation approach to quantum-mechanical modeling of ferroelectric and antiferroelectric phase transitions

Ferroelectrics and antiferroelectrics are the electric counterparts of ferromagnets and antiferromagnets. These materials undergo temperature- and electric-fieldinduced phase transitions that give rise to their characteristic hysteresis loops. Modeling such hysteresis loops and associated phase transitions enables both a deeper fundamental understanding and reliable property predictions for this important class of materials. To date, modeling has largely relied on classical approaches, often remaining qualitative and/or empirical. Traditional interpretation of these transitions rests on two assumptions: (i) they are activated Arrhenius-type processes and (ii) they occur well within the classical regime. Here, we demonstrate that a model can instead be built on two \orthogonal" assumptions: (i) the phase transitions are relaxational processes and (ii) they require a quantum mechanical treatment. Applying this model to both antiferroelectrics and ferroelectrics overcomes the limitations of traditional models and enables efficient first-principles simulations of phase transitions. The success of our unconventional approach highlights the significance of quantum mechanics in transitions long regarded as purely classical. We anticipate that this framework will be applicable to a broad range of phase transitions, including magnetic, elastic, multiferroic, and electronic, along with modeling of quantum tunneling, rates of chemical reactions, and others.

cond-mat.mtrl-sci↗

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

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

The Dublin Lens: A Cc=1.0 mm Objective Lens Intended for CryoEM at 100 keV

We have designed, fabricated and tested a lens with chromatic aberration coefficient (Cc) of 1.0 mm, a 4.0 mm pole-gap and 2.0 mm bore that is wide enough to accommodate an anti-contamination system and an objective aperture. This lens extends the temporal-coherence envelope of the electron microscope beyond 2 Angstrom, using a low-cost Schottky FEG. We hope that this lens design can be used to improve all 100 keV electron microscopes designed for single-particle electron cryomicroscopy (cryoEM).

physics.ins-det↗

arXiv2025

Defects in the $\beta$-Ga$_2$O$_3$($\bar201$)/HfO$_2$ MOS system and the effect of thermal treatments

We have investigated the properties of the $\beta$-Ga$_2$O$_3$($\bar201$)/HfO$_2$/Cr/Au MOS (metal-oxide-semiconductor) system after annealing (450$^\circ$C) in different ambient conditions (forming gas, N$_2$ and O$_2$). Defect properties have been analyzed using an approach combining experimental impedance measurements with physics-based simulations of the capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics of $\beta$-Ga$_2$O$_3$/HfO$_2$ MOS capacitors. This approach enabled us to detect two defect bands in HfO$_2$ characterized by thermal ionization energies of ~1.1eV (acceptor-like) and ~2eV (donor-like) attributed to a polaronic self-trapping state and an oxygen vacancy in HfO$_2$, respectively. This study demonstrates how thermal treatments affect the energy distributions and densities of the observed defects. The adopted methodology also enabled the extraction of the spatial distribution of defects across the HfO$_2$ thickness and Cr/HfO$_2$ interface. The high concentration of oxygen vacancies close to the Cr/HfO$_2$ interface extracted from experimental and simulated electrical data is confirmed by in-situ XPS analysis which shows how Cr is scavenging oxygen from the HfO$_2$ and creating the donor band confined near the Cr/HfO$_2$ interface. This donor band density is observed to be reduced after annealing as per simulation and unchanged for different annealing conditions. We speculate this may be due to the formation of dense films and polyforms of HfO$_2$ under different ambient as revealed by high-resolution TEM images.

cond-mat.mtrl-sci↗

arXiv2024

Ultra-fast Digital DPC Yielding High Spatio-Temporal Resolution for Low-Dose Phase Characterisation

In the scanning transmission electron microscope, both phase imaging of beam-sensitive materials and characterisation of a material's functional properties using in-situ experiments are becoming more widely available. As the practicable scan speed of 4D-STEM detectors improves, so too does the temporal resolution achievable for both differential phase contrast (DPC) and ptychography. However, the read-out burden of pixelated detectors, and the size of the gigabyte to terabyte sized data sets, remain a challenge for both temporal resolution and their practical adoption. In this work, we show that a high-fidelity DPC phase reconstruction can be achieved from both annular segmented detectors or pixelated arrays with relatively few elements using signal digitisation. Unlike conventional analog data, even at the fastest scan speeds, phase reconstructions from digitised DPC-segment images yield reliable data. Finally, dose fractionation by fast scanning and multi-framing allows for post-process binning of frame streams to balance signal-to-noise ratio and temporal resolution for low-dose phase imaging for in-situ experiments.

cond-mat.mtrl-sci↗

arXiv2024

Ferroelectricity at the extreme thickness limit in the archetypal antiferroelectric PbZrO$_3$

Size-driven transition of an antiferroelectric into a polar ferroelectric or ferrielectric state is a strongly debated issue from both experimental and theoretical perspectives. While critical thickness limits for such transitions have been explored, a bottom-up approach in the ultrathin limit considering few atomic layers could provide insight into the mechanism of stabilization of the polar phases over the antipolar phase seen in bulk PbZrO$_3$. Here, we use first-principles density functional theory to predict the stability of polar phases in Pt/PbZrO$_3$/Pt nanocapacitors. In a few atomic layer thick slabs of PbZrO$_3$ sandwiched between Pt electrodes, we find that the polar phase originating from the well established R3c phase of bulk PbZrO$_3$ is energetically favorable over the antipolar phase originating from the Pbam phase of bulk PbZrO$_3$. The famous triple-well potential of antiferroelectric PbZrO$_3$ is modified in the nanocapacitor limit in such a way as to swap the positions of the global and local minima, stabilizing the polar phase relative to the antipolar one. The size effect is decomposed into the contributions from dimensionality reduction, surface charge screening, and interfacial relaxation, which reveals that it is the creation of well-compensated interfaces that stabilizes the polar phases over the antipolar ones in nanoscale PbZrO$_3$.

cond-mat.mtrl-sci↗

arXiv2023

3D-imaging of Printed Nanostructured Networks using High-resolution FIB-SEM Nanotomography

Networks of solution-processed nanomaterials are important for multiple applications in electronics, sensing and energy storage/generation. While it is known that network morphology plays a dominant role in determining the physical properties of printed networks, it remains difficult to quantify network structure. Here, we utilise FIB-SEM nanotomography to characterise the morphology of nanostructured networks. Nanometer-resolution 3D-images were obtained from printed networks of graphene nanosheets of various sizes, as well as networks of WS2 nanosheets, silver nanosheets and silver nanowires. Important morphological characteristics, including network porosity, tortuosity, pore dimensions and nanosheet orientation were extracted and linked to network resistivity. By extending this technique to interrogate the structure and interfaces within vertical printed heterostacks, we demonstrate the potential of this technique for device characterisation and optimisation.

cond-mat.mes-hall↗

arXiv2022

How Fast is Your Detector? The Effect of Temporal Response on Image Quality

With increasing interest in high-speed imaging should come an increased interest in the response times of our scanning transmission electron microscope (STEM) detectors. Previous works have previously highlighted and contrasted performance of various detectors for quantitative compositional or structural studies, but here we shift the focus to detector temporal response, and the effect this has on captured images. The rise and decay times of eight detectors' single electron response are reported, as well as measurements of their flatness, roundness, smoothness, and ellipticity. We develop and apply a methodology for incorporating the temporal detector response into simulations, showing that a loss of resolution is apparent in both the images and their Fourier transforms. We conclude that the solid-state detector outperforms the photomultiplier-tube (PMT) based detectors in all areas bar a slightly less elliptical central hole and is likely the best detector to use for the majority of applications. However, using tools introduced here we encourage users to effectively evaluate what detector is most suitable for their experimental needs.

physics.ins-det↗

arXiv2022

A Retrofittable Photoelectron Gun Proposal for Improved Low Voltage Imaging

Low voltage scanning electron microscopy is a powerful tool for examining surface features and imaging beam sensitive materials. Improving resolution during low voltage imaging is then an important area of development. Decreasing the effect of chromatic aberration is one solution to improving the resolution and can be achieved by reducing the energy spread of the electron source. Our approach involves retrofitting a light source onto a thermionic Lanthanum Hexaboride (LaB6) electron gun as a cost-effective low energy spread photoelectron emitter. The energy spread of the emitter's photoelectrons is predicted to be between 0.11-0.38eV depending on the photon energy of the UV light source. Proof of principle images have been recorded using this retrofitted photoelectron gun and an analysis of its performance is presented.

physics.ins-det↗

arXiv2022

An Accessible Method for Simulating Charged-Particle Optics, with Examples for Transmission Electron Microscopy

The transmission electron microscope (TEM) has become an essential tool for innovation in nanoscience, material science, and biology. Despite these instruments being widely used across both industry and academia, academics may hesitate to propose substantial modifications to the optical setup due to the instrument's significant purchase price, fear of voiding the service contract, or downtime being unacceptable in shared user facilities. For instruments found in industry, similarly the risk-reward balance makes substantive modifications untenable. This limits the development of radically new optical geometries, and with the performance of the TEM largely being dictated by the specification of the objective lens pole-piece, exploring novel designs may be valuable. Alternatively, potential lens designs can be analyzed rapidly and inexpensively using finite element analysis multiphysics simulation packages. Several are available, but here COMSOL Multiphysics was used, which is readily available in many universities. Changes to the geometry or materials of the lens can be investigated without any need to disassemble, reassemble, and realign the TEM column. Here we demonstrate an intuitive and accessible method to simulate charged particle optics using this 'digital twin' approach, with the hope that this encourages new creative and sustainable grassroots innovation in TEM lens design and microscope modification.

physics.ins-det↗

arXiv2022

Correcting for probe wandering by precession path segmentation

Precession electron diffraction has in the past few decades become a powerful technique for structure solving, strain analysis, and orientation mapping, to name a few. One of the benefits of precessing the electron beam, is increased reciprocal space resolution, albeit at a loss of spatial resolution due to an effect referred to as 'probe wandering'. Here, a new methodology of precession path segmentation is presented to counteract this effect and increase the resolution in reconstructed virtual images from scanning precession electron diffraction data. By utilizing fast pixelated electron detector technology, multiple frames are recorded for each azimuthal rotation of the beam, allowing for the probe wandering to be corrected in post-acquisition processing. Not only is there an apparent increase in the resolution of the reconstructed images, but probe wandering due to instrument misalignment is reduced, potentially easing an already difficult alignment procedure.

physics.ins-det↗

arXiv2022

Interlacing in atomic resolution scanning transmission electron microscopy

Fast frame-rates are desirable in scanning transmission electron microscopy for a number of reasons: controlling electron beam dose, capturing in-situ events or reducing the appearance of scan distortions. Whilst several strategies exist for increasing frame-rates, many impact image quality or require investment in advanced scan hardware. Here we present an interlaced imaging approach to achieve minimal loss of image quality with faster frame-rates that can be implemented on many existing scan controllers. We further demonstrate that our interlacing approach provides the best possible strain precision for a given electron dose compared with other contemporary approaches.

physics.ins-det↗

arXiv2021

Cost & Capability Compromises in STEM Instrumentation for Low-Voltage Imaging

Low voltage transmission electron microscopy (<=80 kV) has many applications in imaging beam-sensitive samples, such as metallic nanoparticles, which may become damaged at higher voltages. To improve resolution, spherical aberration can be corrected for in a Scanning Transmission Electron Microscope (STEM), however chromatic aberration may then dominate, limiting the ultimate resolution of the microscope. Using image simulations, we examine how a chromatic aberration corrector, different objective lenses, and different beam energy-spreads each affect the image quality of a gold nanoparticle imaged at low voltages in a spherical aberration-corrected STEM. Quantitative analysis of the simulated examples can inform the choice of instrumentation for low-voltage imaging. We here demonstrate a methodology whereby the optimum energy spread to operate a specific STEM can be deduced. This methodology can then be adapted to the specific sample and instrument of the reader, enabling them to make an informed economical choice as to what would be most beneficial for their STEM in the cost-conscious landscape of scientific infrastructure.

physics.ins-det↗

arXiv2021

Using Your Beam Efficiently: Reducing Electron-dose in the STEM via Flyback Compensation

In the scanning transmission electron microscope, fast-scanning and frame-averaging are two widely used approaches for reducing electron-beam damage and increasing image signal-noise ratio which require no additional specialised hardware. Unfortunately, for scans with short pixel dwell-times (less than 5 ${\mu}$s), line flyback time represents an increasingly wasteful overhead. Although beam exposure during flyback causes damage while yielding no useful information, scan-coil hysteresis means that eliminating it entirely leads to unacceptably distorted images. In this work, we reduce this flyback to an absolute minimum by calibrating and correcting for this hysteresis in postprocessing. Substantial improvements in dose-efficiency can be realised (up to 20 %), while crystallographic and spatial fidelity is maintained for displacement/strain measurement.

physics.ins-det↗

arXiv2021

Procedure for 3D atomic resolution reconstructions using atom-counting and a Bayesian genetic algorithm

We introduce a Bayesian genetic algorithm for reconstructing atomic models of nanoparticles from a single projection using Z-contrast imaging. The number of atoms in a projected atomic column obtained from annular dark field scanning transmission electron microscopy (ADF STEM) images serves as an input for the initial three-dimensional (3D) model. The novel algorithm minimizes the energy of the structure while utilizing a priori information about the finite precision of the atom-counting results and neighbor-mass relations. The results show excellent prospects for obtaining reliable reconstructions of beam-sensitive nanoparticles during dynamical processes from images acquired with sufficiently low incident electron doses.

cond-mat.mtrl-sci↗

arXiv2021

Increasing Spatial Fidelity and SNR of 4D-STEM using Multi-frame Data Fusion

4D-STEM, in which the 2D diffraction plane is captured for each 2D scan position in the scanning transmission electron microscope (STEM) using a pixelated detector, is complementing and increasingly replacing existing imaging approaches. However, at present the speed of those detectors, although having drastically improved in the recent years, is still 100 to 1,000 times slower than the current PMT technology operators are used to. Regrettably, this means environmental scanning-distortion often limits the overall performance of the recorded 4D data. Here we present an extension of existing STEM distortion correction techniques for the treatment of 4D-data series. Although applicable to 4D-data in general, we use electron ptychography and electric-field mapping as model cases and demonstrate an improvement in spatial-fidelity, signal-to-noise ratio (SNR), phase-precision and spatial-resolution.

physics.ins-det↗