Structure retrieval at atomic resolution in the presence of multiple scattering of the electron probe

The projected electrostatic potential of a thick crystal is reconstructed at atomic-resolution from experimental scanning transmission electron microscopy data recorded using a new generation fast- readout electron camera. This practical and deterministic inversion of the equations encapsulating multiple scattering that were written down by Bethe in 1928 removes the restriction of established methods to ultrathin ($\lesssim 50$ {\AA}) samples. Instruments already coming on-line can overcome the remaining resolution-limiting effects in this method due to finite probe-forming aperture size, spatial incoherence and residual lens aberrations.Comment: 6 pages, 3 figure

The mechanism of twin thickening and the elastic strain state of TWIP steel nanotwins

A Twinning Induced Plasticity (TWIP) steel with a nominal composition of Fe-16.4Mn-0.9C-0.5Si-0.05Nb-0.05V was deformed to an engineering strain of 6\%. The strain around the deformation twins were mapped using the 4D-STEM technique. Strain mapping showed a large average elastic strain of approximately 6\% in the directions parallel and perpendicular to the twinning direction. However, the large average strain comprised of several hot spots of even larger strains of up to 12\%. These hot spots could be attributed to a high density of sessile Frank dislocations on the twin boundary and correspond to shear stresses of 1--1.5 GPa. The strain and therefore stress fields are significantly larger than other materials known to twin and are speculated to be responsible for the early thickness saturation of TWIP steel nanotwins. The ability to keep twins extremely thin helps improve grain fragmentation, \textit{i.e.} the dynamic Hall-Petch effect, and underpins the large elongations and strain hardening rates in TWIP steels

Characterization of Ordering in A-Site Deficient Perovskite Ca1-xLa2x/3TiO3 Using STEM/EELS

The vacancy ordering behavior of an A-site deficient perovskite system, Ca1-xLa2x/3TiO3, was studied using atomic resolution scanning transmission electron microscopy (STEM) in conjunction with electron energy-loss spectroscopy (EELS), with the aim of determining the role of A-site composition changes. At low La content (x = 0.2), adopting Pbnm symmetry, there was no indication of long-range ordering. Domains, with clear boundaries, were observed in bright-field (BF) imaging, but were not immediately visible in the corresponding high-angle annular dark-field (HAADF) image. These boundaries, with the aid of displacement maps from A-site cations in the HAADF signal, are shown to be tilt boundaries. At the La-rich end of the composition (x = 0.9), adopting Cmmm symmetry, long-range ordering of vacancies and La3+ ions was observed, with alternating La-rich and La-poor layers on (001)p planes, creating a double perovskite lattice along the c axis. These highly ordered domains can be found isolated within a random distribution of vacancies/La3+, or within a large population, encompassing a large volume. In regions with a high number density of double perovskite domains, these highly ordered domains were separated by twin boundaries, with 90° or 180° lattice rotations across boundaries. The occurrence and characteristics of these ordered structures are discussed and compared with similar perovskite systems

Observation of room-temperature polar skyrmions

peer reviewe

Examining the impact of digital technologies on students’ Higher Education outcomes:The case of the Virtual Learning Environment and social media

Digital natives is a term used to describe current higher education (HE) students, whose lives are proliferated by digital technologies. To cater to the needs of this new generation of students, HE institutions increasingly adopt digital tools such as virtual learning environments (VLE) and social media (SM). Little is known, however, about the impact of these digital technologies on students’ HE outcomes. Drawing from service productivity theories, this study aims to address this gap. Through exploratory sequential mixed research methods, we identify five HE outcomes and reveal that Learning-Oriented Outcomes are the most important in HE even when digital technologies are not used; and these outcomes are further enhanced when students use VLE. Learning-Oriented Outcomes, however, are the least important when SM is used in HE; students tend to prioritise outcomes related to Knowledge Transfer instead. Our research findings derive theoretical and practical contributions and open up avenues for future research

Tensile behavior of Al1-xMox crystalline and amorphous thin films

The exceptional strength and distinct deformation physics exhibited by pure ultrafine-grained and nanocrystalline metals in comparison to their microcrystalline counterparts have been ascribed to the dominant influence of grain boundaries in accommodating plastic flow. Such grain-boundary-mediated mechanisms can be augmented by additional strengthening in nanocrystalline alloys via solute and precipitate interactions with dislocations, although its potency is a function of the changes in the elastic properties of the alloyed material. In this study, we investigate the elastic and plastic properties of Al1-xMox alloys (0 <= x <= 0.32) by tensile testing of sputter-deposited freestanding thin films. Isotropic elastic constants and strength are measured over the composition range for which three microstructural regimes are identified, including solid solutions, face-centered cubic and amorphous phase mixtures and body-centered cubic (bcc)/amorphous mixtures. Whereas the bulk modulus is measured to follow the rule of mixtures over the Mo composition range, the Young's and shear moduli do not. Poisson's ratio is non-monotonic with increasing Mo content, showing a discontinuous change at the onset of the bcc/amorphous two-phase region. The strengthening measured in alloyed thin films can be adequately predicted in the solid solution regime only by combining solute strengthening with a grain boundary pinning model. The single-step co-sputtering procedure presented here results in diversity of alloy compositions and microstructures, offering a promising avenue for tailoring the mechanical behavior of thin films.close0

A systematic neutron reflectometry study on hydrogen absorption in thin Mg1-xAlx alloy films

Nous montrons ici comment la R\ue9flectom\ue9trie Neutronique (NR) peut permettre une \ue9tude plus approfondie des propri\ue9t\ue9s d\u2019absorption et de d\ue9sorption de mat\ue9riaux de stockage d\u2019hydrog\ue8ne commercialement prometteurs. NR utilise la grande longueur de diffusion n\ue9gative des atomes d\u2019hydrog\ue8ne qui change substantiellement la courbe de r\ue9flectivit\ue9 de telle sorte que NR peut d\ue9terminer non seulement la quantit\ue9 totale d\u2019hydrog\ue8ne stock\ue9e, mais \ue9galement la distribution de l\u2019hydrog\ue8ne selon la normale au film, avec une pr\ue9cision du nanom\ue8tre. Pour pouvoir utiliser NR, les films doivent avoir une surface lisse et leur \ue9paisseur doit se trouver entre 10 et 200 nm. Nous avons \ue9tudi\ue9 syst\ue9matiquement des films minces d\u2019alliage Mg1-xAlx (x = 0,2, 0,3, 0,4, 0,67) recouverts d\u2019une couche de Pd comme catalyseur. Nos mesures montrent que Mg0,7Al0,3 est l\u2019alliage optimum qui permet le plus haut stockage d\u2019hydrog\ue8ne et la plus basse temp\ue9rature de d\ue9sorption. Tous les films minces gonflent d\u2019environ 20 % suivant l\u2019absorption d\u2019hydrog\ue8ne qui est stock\ue9 uniquement dans la couche de MgAl, sans hydrog\ue8ne dans le Pd.Nous montrons ici comment la R\ue9flectom\ue9trie Neutronique (NR) peut permettre une \ue9tude plus approfondie des propri\ue9t\ue9s d\u2019absorption et de d\ue9sorption de mat\ue9riaux de stockage d\u2019hydrog\ue8ne commercialement prometteurs. NR utilise la grande longueur de diffusion n\ue9gative des atomes d\u2019hydrog\ue8ne qui change substantiellement la courbe de r\ue9flectivit\ue9 de telle sorte que NR peut d\ue9terminer non seulement la quantit\ue9 totale d\u2019hydrog\ue8ne stock\ue9e, mais \ue9galement la distribution de l\u2019hydrog\ue8ne selon la normale au film, avec une pr\ue9cision du nanom\ue8tre. Pour pouvoir utiliser NR, les films doivent avoir une surface lisse et leur \ue9paisseur doit se trouver entre 10 et 200 nm. Nous avons \ue9tudi\ue9 syst\ue9matiquement des films minces d\u2019alliage Mg1-xAlx (x = 0,2, 0,3, 0,4, 0,67) recouverts d\u2019une couche de Pd comme catalyseur. Nos mesures montrent que Mg0,7Al0,3 est l\u2019alliage optimum qui permet le plus haut stockage d\u2019hydrog\ue8ne et la plus basse temp\ue9rature de d\ue9sorption. Tous les films minces gonflent d\u2019environ 20 % suivant l\u2019absorption d\u2019hydrog\ue8ne qui est stock\ue9 uniquement dans la couche de MgAl, sans hydrog\ue8ne dans le Pd.Peer reviewed: YesNRC publication: Ye

Scattering Matrix Determination in Crystalline Materials from 4D Scanning Transmission Electron Microscopy at a Single Defocus Value.

Recent work has revived interest in the scattering matrix formulation of electron scattering in transmission electron microscopy as a stepping stone toward atomic-resolution structure determination in the presence of multiple scattering. We discuss ways of visualizing the scattering matrix that make its properties clear. Through a simulation-based case study incorporating shot noise, we shown how regularizing on this continuity enables the scattering matrix to be reconstructed from 4D scanning transmission electron microscopy (STEM) measurements from a single defocus value. Intriguingly, for crystalline samples, this process also yields the sample thickness to nanometer accuracy with no a priori knowledge about the sample structure. The reconstruction quality is gauged by using the reconstructed scattering matrix to simulate STEM images at defocus values different from that of the data from which it was reconstructed