Atomic-scale Analysis Research Articles

Strain induced second-order Jahn–Teller reconstruction and magnetic moment modulation at monovacancy in graphene

Using density functional theory simulations, we examine the electronic structure of an isolated monovacancy defect in graphene under symmetry-breaking deformation. Results show that the defect experiences a second-order Jahn–Teller reconstruction at a critical strain of 1.7%. It stabilizes the orientation of the JT bond relative to the loading direction and breaks the threefold degeneracy of the defect structure. We call it Jahn–Teller re-reconstruction (JTRR), and it is mechanically reversible. The reversibility and stabilization of the orientation depend on the direction cosine between the JT bond and the loading direction. Also, a change in the loading direction by 90° can change the orientation of the JT bond by 120°. An atomic-scale analysis suggests that the maximum bond force arising from “the derivative of the kinetic energy of electrons” defines the critical strain. JTRR alters the electron occupation in the individual electronic orbitals at the defect site. The electronic charge redistribution and the density of states at the defective sites reveal that the pz orbitals dominate the reconstruction process. Furthermore, JTRR changes the magnitude of the magnetic moment at the defective site from 1.36 μB to 1.22 μB. This unravels a new way of controlling the magnetic behavior of monovacancy by applying symmetry-breaking mechanical strain. Results also show that passivation of the dangling bond can subside or eliminate the reconstruction process depending on the number of valence electrons available in the passivating atom.

Antiphase-Boundary-Engineered Domain Switching in a (110)-Oriented BiFeO3 Film

A ferroelectric domain wall memory device exploits the presence (or absence) of domain walls as the basis for information storage. Although structural defects strongly influence ferroelectric domain wall nucleation and growth, this correlation is far from being fully understood. For example, a single defect can play opposing roles: it can act as a nucleation site to initiate domain switching or, conversely, serve as a pinning center inhibiting domain wall motion. This is particularly the case for antiphase boundaries (APBs), whose influence on switching remains relatively unexplored despite being ubiquitous in oxide perovskite ferroelectrics. Here, we use aberration-corrected scanning transmission microscopy and in situ transmission electron microscopy under an applied bias to extensively characterize the influence of an APB on the switching behavior in a (110)-oriented BiFeO3 thin film. We demonstrate that deterministic nucleation occurs in the proximity of a vertically oriented APB, following which domain growth occurs laterally in a symmetric fashion. Atomic-scale structural analysis reveals that the shear operation vector for the APB is 1/2[110]; this half-unit-cell shift extends over a few nanometers, implying that the APB plane is inclined toward the imaging direction. The symmetric switching observation is attributed to local inhomogeneous lattice strain and a reduction of polarization caused by the APB. These results demonstrate that an extended structural defect can be used as an active switching element rather than a stumbling block in reprogrammable domain-wall-based devices. Ultimately, this will expand the capabilities of ferroelectric thin films for future electronic devices based on ferroelectric domain walls.

Influence of crystalline defects on magnetic nanodomains in a rare-earth-free magnetocrystalline anisotropic alloy

A complex interplay between magnetic domain structure and crystalline imperfections, here twins, is revealed in a rare-earth-free MnAl bulk magnet. The magnetic domains are observed to be in the nanometer range for a large part of the magnetic structure and to scale with the number density of twins formed during thermal processing. We explain this phenomenon by a reduction in domain-wall energy at the twinned regions as proven by ab initio calculations. In addition, our atomic-scale analysis reveals that the twin boundaries contain excess Mn atoms that reduce the local magnetization, serving as an obstacle for domain wall motion. These insights can help guide the strategic design of magnetic materials by controlling the initial phase distribution to tailor the twin density and hence, the distribution of domains.

Influence of flexoelectric effects on domain switching in ferroelectric films

Flexoelectricity has been shown to be an effective strategy to modulate the polarization configurations, domain structures, and physical properties in nanoscale ferroelectric thin films. However, the relations between the domain switching processes and flexoelectric effects remain elusive, which is essential for the design of nanoscale ferroelectric electric devices. In this work, strain-gradient and normal PbTiO3 films are fabricated and investigated to resolve this elusive relationship. By using large-scale and local piezoelectric force microscopy characterization, the ferroelectric domain switching in strain-gradient PbTiO3 films is found to be hard and hindered under applied electric fields compared with the normal ones. Successive atomic-scale scanning transmission electron microscopy imaging analysis manifests that the domains in the strain-gradient PbTiO3 films are stabilized by an additional effective strain gradient-induced flexoelectric field, which was introduced by negative pressure originated from vertically distributed Pb-rich anti-phase domains. This study proposes an effective method to stabilize the ferroelectric polarization in nanoscale ferroelectric films, thus facilitate improving the reliability of ferroelectric electronic devices.

Unit-cell-thick domain in free-standing quasi-two-dimensional ferroelectric material

Understanding the domain structure of two-dimensional materials is of paramount importance for the design of next generation microelectronic devices. Here, the authors employ a Dion--Jacobson layered oxide as a model system to study the ferroelectric domain structure with atomic scale analysis. They reveal the existence of a unit-cell-thick ferroelectric domain size as well as both 180\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} domain walls in a free-standing ferroelectric oxide. This may suggest ways to achieve unit-cell-thick domain structures and shed light on promising material solutions for emerging nonvolatile high-density memories and synaptic devices.

Stabilization of an iridium oxygen evolution catalyst by titanium oxides

The anodic oxygen evolution reaction (OER) has significant importance in many electrochemical technologies. In proton exchange membrane water electrolyzers it plays a pivotal role for electrochemical energy conversion, yet sluggish kinetics and the corrosive environment during operation still compel significant advances in electrode materials to enable a widespread application. Up-to-date Iridium is known as the best catalyst material for the OER in acidic media due to its relatively high activity and long-term stability. However, scarcity of iridium drives the development of strategies for its efficient utilization. One promising way would be the formation of mixtures in which the noble catalyst element is dispersed in the non-noble matrix of more stable metals or metal oxides. A promising valve metal oxide is TiOx, yet the degree to which performance can be optimized by composition is still unresolved. Thus, using a scanning flow cell connected to an inductively coupled plasma mass spectrometer, we examined the activity and stability for the OER of an oxidized Ir–Ti thin film material library covering the composition range from 20–70 at.% of Ir. We find that regardless of the composition the rate of Ir dissolution is observed to be lower than that of thermally prepared IrO2. Moreover, mixtures containing at least 50 at.% of Ir exhibit reactivity comparable to IrO2. Their superior performance is discussed with complementary information obtained from atomic scale and electronic structure analysis using atom probe tomography and x-ray photoelectron spectroscopy. Overall, our data shows that Ir–Ti mixtures can be promising OER catalysts with both high activity and high stability.

Solid-state NMR spectroscopy as a powerful tool to investigate the location of fluorinated lipids in highly porous hybrid organic-inorganic nanoparticles.

Nanosized metal-organic frameworks (nanoMOFs) have emerged as a new class of biodegradable and nontoxic nanomaterials of high interest for biomedical applications thanks to the possibility to load large amounts of a wide variety of therapeutic molecules in their porous structure. The surface of the highly porous nanoMOFs is usually engineered to increase their colloidal stability, to tune their interactions with the biological environment, and to allow targeting specific cells or organs. However, the atomic-scale analysis of these complex core-shell materials is highly challenging. In this study, we report the investigation of aluminum-based nanoMOFs containing two fluorinated lipids by solid-state NMR spectroscopy, including 27 Al, 1 H and 19 F MAS NMR. The ensemble of NMR data provides a better understanding of the localization and conformation of the fluorinated lipids inside the pores or on the nanoMOF surface.

Oxygen adsorption on (100) surfaces in Fe\u2013Cr alloys

The adsorption of oxygen on bcc Fe–Cr(100) surfaces with two different alloy concentrations is studied using ab initio density functional calculations. Atomic-scale analysis of oxygen–surface interactions is indispensable for obtaining a comprehensive understanding of macroscopic surface oxidation processes. Up to two chromium atoms are inserted into the first two surface layers. Atomic geometries, energies and electronic properties are investigated. A hollow site is found to be the preferred adsorption site over bridge and on-top sites. Chromium atoms in the surface and subsurface layers are found to significantly affect the adsorption properties of neighbouring iron atoms. Seventy-one different adsorption geometries are studied, and the corresponding adsorption energies are calculated. Estimates for the main diffusion barriers from the hollow adsorption site are given. Whether the change in the oxygen affinity of iron atoms can be related to the chromium-induced charge transfer between the surface atoms is discussed. The possibility to utilize the presented theoretical results in related experimental research and in developing semiclassical potentials for simulating the oxidation of Fe–Cr alloys is addressed.

The role of Ti addition on the evolution and stability of γ/γ′ microstructure in a Co-30Ni-10Al-5Mo-2Ta alloy

The paper explores the influence of Ti addition to the Ta stabilized γ/γ′ two-phase Co-based superalloys having the compositions of Co-30Ni-10Al-5Mo-2Ta-xTi (x = 0, 1, 2 and 4 at.%). The microstructure of the alloys after solutionizing (1300 °C for 25 h) and quenching show fine distribution of γ′ precipitates with spherical shape within the γ matrix. Aging of these alloys at 900 °C results in a morphological transition from spheroidal to cuboidal with a concomitant increase in γ/γ′ lattice misfit as compared to the solutionized alloys. However, the γ/γ′ lattice misfit of cuboidal particles decreases with an increase in Ti in the alloys. Atomic-scale compositional analysis by atom probe tomography across γ/γ′ interface for the aged alloys indicates a preferential partitioning of Ti to the γ′. This can be correlated with the observed increase in the γ′ solvus temperature up to 1166 °C for alloys with 4 at.% Ti addition. The increase in Ti content also decreases the partitioning of Mo to the γ′. Despite the rise in solvus temperature, the coarsening study reveals an escalation in the γ′ coarsening rate at 900 and 950 °C with a maximum for the alloy containing the highest Ti content. The initial stages of γ′ coarsening follow diffusion-controlled LSW kinetics. However, during later stages, a propensity for particle coalescence could be observed. An addition of 3 and 4 at.% Ti is shown to improve the high-temperature strength with 0.2% proof stress at 1000 °C of 360 and 400 MPa, respectively.

Elucidating the origin of planar defects that enhance critical current density in CaKFe4As4 single crystals

CaKFe4As4 is a new member of the 1144-type iron-based superconductors (IBSs) which is expected to show remarkable superconducting properties because its crystal structure is similar to that of 122-type IBSs. Recently, substantial anisotropy between the in-plane and out-of-plane critical current densities (J C), with peculiar temperature dependence of in-plane J C at high fields, has been reported in CaKFe4As4 single crystals. The anisotropy of J C was attributed to the presence of planar defects, which were shown to be confined to the ab plane. However, the origin of these defects remains unclear. To elucidate the origin of these planar defects, we herein present our results on atomic-scale microstructure analysis of high-quality CaKFe4As4 single crystals. Using high-resolution scanning transmission electron microscopy (STEM), along with electron energy loss spectroscopy, we have demonstrated that these planar defects consist of one- or two-layer KFe2As2 step network, which is spread over the periodically ordered KFe2As2 and CaFe2As2 monolayers. Further, we report that the dark contrast regions observed via conventional STEM are local strain distributions, which originate from the substitution of Ca by K in a unit cell.

Heptagons in the Basal Plane of Graphene Nanoflakes Analyzed by Simulated X-ray Photoelectron Spectroscopy.

The performance of graphene-based electronic devices depends critically on the existence of topological defects such as heptagons. Identifying heptagons at the atomic scale is important to completely understand the electronic properties of these materials. In this study, we report an atomic-scale analysis of graphene nanoflakes with two to eight isolated or connected heptagons, using simulated C 1s X-ray photoelectron spectroscopy (XPS) to estimate the XPS profiles depending on the density and the position of the heptagons. The introduction of up to 24% of isolated heptagons shifted the peak position toward high binding energies (284.0 to 284.3 eV), whereas the introduction of up to 39% of connected heptagons shifted the calculated peak position toward low binding energies (284.0 to 283.5 eV). The presence of heptagons also influenced the full width at half-maximum (FWHM). The introduction of 24% of isolated heptagons increased the FWHMs from 1.25 to 1.50 eV. However, the introduction of connected heptagons did not increase the FWHMs above 1.40 eV. The FWHMs increased to 1.40 eV for 19% of connected heptagons, but did not increase further as the percentage of connected heptagons increased to 39%. Based on the calculated results, the XPS profiles of graphene nanoflakes containing heptagons with different densities and positions can be obtained. Our precise identification of heptagons in graphene nanoflakes by XPS lays the groundwork for the analysis of graphene.

Interface Structure and Transport Properties of Thin-Film Solid-State Batteries with an Epitaxial Cathode

A problem of solid-state Li ion batteries is a relatively low ionic conductivity at the interface between electrodes and electrolytes that limits the charging/discharging rate. For designing the solid-state batteries, understanding the mechanism behind the low interface conductivity is crucial. Investigation of the atomic structure of the interfaces and its relation to the ionic conductivity would be helpful for this purpose. However, atomic-scale observation of the buried interfaces is often difficult in practical batteries which consist of grain aggregates. Recent progress in thin-film deposition techniques of electrodes and electrolyteshas opened a path to study the interface quantitatively. By using an all-in-vacuum battery fabrication system [1], we succeeded in fabricating a very low solid-electrolyte/electrode interface resistance in thin film batteries with an epitaxial cathode. The well-defined crystalline interfaces allow the analysis of atomic-scale structure and its transport properties. For the structure analysis, we used X-ray crystal truncation rod (CTR) scattering which enables the atomic-scale analysis in a non-destructive manner.The analysis of two interfaces will be presented: a solid-electrolyte/electrode interface and an electrode/current collector interface. In the former case, we studied Li(negative electrode)/Li3PO4(solid electrode)/LiCoO2(positive electrode) thin-film Li batteries with low and high (more than 10 times) interface resistances. It turned out that the low-resistance interface has an atomically well-ordered LiCoO2 surface and a disordered surface CoO2 layer can results in the high-resistance interface, demonstrating the crucial importance of the structural ordering in the interfacial layer for realizing a low-resistance interface [2]. In the second topic, we focused on electrode/current collector interface that can also affect the battery performance. We found a drastic improvement of the charging/discharging property by inserting 1.2 nm-thick band insulator LaAlO3 into the LiCoO2 (electrode)/Nb-doped SrTiO3 (current collector) interface. The CTR analysis revealed that a formation of electric dipole at the LaAlO3/Nb-doped SrTiO3 interface, which causes a energy band alignment preferable for the electronic conductance between the electrode and the current collector.

Atomistic origin of high-concentration Ce3+ in {100}-faceted Cr-substituted CeO2 nanocrystals

Improving the potential of promising CeO2-based nanocatalysts in practical applications requires an atomic-scale analysis of the effects of active dopants on the distribution of Ce valence states and the formation of oxygen vacancies (VOs). In this study, a Cr dopant is introduced into the cubic {100}-faceted CeO2 nanocrystals (NCs) with an average size of 7.8 nm via supercritical water. The Cr dopants substitute Ce sites in the amount of approximately 3 mol%. Based on the aberration-corrected STEM-EELS, the effects of Cr dopant on the distribution of cerium valence states are investigated layer by layer across the ultrafine Cr-substituted CeO2 NC perpendicular to the {100} exposed facet. It is found that an increased amount of Ce3+ cations is present in Cr-substituted CeO2 NCs, particularly in the internal atomic layers, compared to the pristine CeO2 NCs. The atomic-scale analysis of the local structure combined with theoretical calculations demonstrates that Cr dopant reduces the formation energy of VOs and increases the mobility of oxygen atoms for the nano-sized CeO2. These effects, in principle, result in an improved oxygen storage capacity and provide a fundamental understanding of role of the dopant in the formation and distribution of VOs in the doped CeO2 NCs.

WITHDRAWN: Application of atom probe tomography in understanding high entropy alloys: 3D local chemical compositions in atomic scale analysis

This article has been withdrawn due to the incorrect Volume assignment. The Publisher apologizes for any inconvenience this may cause. The full Elsevier Policy on Article Withdrawal can be found at https://www.elsevier.com/about/our-business/policies/article-withdrawal .

Spinodal decomposition versus classical γ′ nucleation in a nickel-base superalloy powder: An in-situ neutron diffraction and atomic-scale analysis

Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by their thermo-mechanical treatments during processing. Here, the influence of a thermal exposure to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated γ only; from this, the evolution of γ′ volume fraction and lattice misfit was assessed. The initial powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent γ to γ′ phase transformations. Subsolvus heat treatments yielded a unimodal γ′ distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same γ′ population during heating, but dissolves as the temperature increases further. The γ′ then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the γ′ formed during heating or cooling.

Atomic scale analysis of defect clustering and predictions of their concentrations in UO2+x

The physical properties of uranium dioxide vary greatly with stoichiometry. Oxidation towards hyperstoichiometric UO2 − UO2+x – might be encountered at various stages of the nuclear fuel cycle if oxidative conditions are met; the impact of stoichiometry changes upon physical properties should therefore be properly assessed to ensure safe and reliable operations. These physical properties are intimately linked to the arrangement of atomic defects in the crystalline structure. The evolution of the defect concentration with environmental parameters – oxygen partial pressure and temperature – were evaluated by means of a point defect model where the reaction energies are derived from atomic-scale simulations. To this end, various configurations and net charge states of oxygen interstitial clusters in UO2 have been calculated. Various methodologies have been tested to determine the optimum cluster configurations and a rigid lattice approach turned out to be the most useful strategy to optimize defect configuration structures. Ultimately, results from the point defect model were discussed and compared to experimental measurements of stoichiometry dependence on oxygen partial pressure and temperature.

Atomic-scale Analysis of Mechanical Response of SrTiO3 by MEMS-based in Situ STEM Mechanical Testing

Atomic-scale Analysis of Mechanical Response of SrTiO₃ by MEMS-based in Situ STEM Mechanical Testing

Achieving lower mass density with high strength in Nb stabilised γ/γ′ Co–Al–Mo–Nb base superalloy by the replacement of Mo with V

We report new compositions of γ/γ′ Co base super alloys with low mass density where the strengthening γ′ precipitates Co3(Al0.45V0.33Nb0.21) are stabilised with a systematic replacement of Mo with V in Co–10Al–5Mo–2Nb (all in at.%) superalloy. The γ/γʹ microstructure is found to be metastable in nature that decomposes into equilibrium phases after 200 h of aging at 800 °C. Diffraction and atomic-scale compositional analysis reveal that phases are a γ-Co solid solution with composition Co-6.9Al–5V-1.4Nb, a B2 ordered Co(Al,V) phase with composition Co-34.6Al-5.6V and a DO19 ordered Co3Nb phase with composition Co-23.4Nb-1.6Al. Addition of 30 at.% Ni to the Co–10Al–5V–2Nb superalloy increases the γ′-solvus temperature to 1017 °C with improved high temperature strength and stability, concomitantly achieving a yield strength of 701 ± 9 MPa at 770 °C. The γ/γʹ microstructure of the alloy is found to be stable at 800 °C and 900 °C till 1000 h. Vanadium addition, therefore, have marked beneficial effect in cobalt base superalloys indicating good potential for future alloy development.

Investigation on vanadium carbide transformations in M23C6 carbide by experimental observation and crystallographic analysis

An investigation based on high-resolution transmission microscopy (HRTEM) and crystallographic analysis was performed to gain insights into vanadium (V) carbide transformations in M23C6 carbide. Vanadium carbides including V6C5, V8C7 and VC were observed in or adjacent to M23C6 carbides, and the orientation relationships for V6C5/M23C6, VC/ M23C6 and V8C7/VC were revealed. Energy dispersive spectrometry (EDS) and atomic-scale configuration analysis on M23C6 contained a certain amount of V indicated that VV atomic pair and local V-rich domain were gradually formed in M23C6 cell as the diffusion of V in M23C6, contributing to formation of vanadium carbides in M23C6. Crystallographic analysis on vanadium carbides observed in M23C6 clarified that the mechanism of vanadium carbide transformations in M23C6 was fulfilled by decreasing of metal atom number in the carbon coordination polyhedron from eight to six, as well that V6C5 and V8C7 containing carbon vacancies precipitated preferentially in M23C6.

Enhanced doping effects of multielement on anisotropic thermal expansion in ZrO2 with new compositions

AbstractCoefficient of thermal expansion (CTE) of a solid material plays a critical role for a variety of high temperature applications such as thermal barrier coating (TBC) systems during the thermal cycling process. Ceramics contain ionic bonds; hence they tend to exhibit lower CTE values than alloys/metals. Developing new ceramic thermal barrier materials using promising dopants and compositions that have higher CTE values than the conventional 6‐8 wt% Y2O3 stabilized ZrO2(8YSZ) will contribute to the decrease in thermal expansion mismatch between a typical ceramic 8YSZ (10 ~ 11 × 10−6°C−1) top coat and a metal alloy based bond coat such as NiCrAlY (14 ~ 17×10−6°C−1, Padture et al., Science, 2002;296:280–4; Liang et al., J Mater Sci Technol, 2011;27(5):408–14), which is highly desirable. This work reports design, modeling, synthesis, and characterization of promising new compositions based on Dy3+, Al3+, and Ce4+‐doped YSZ that consist of the tetragonal structure and have an enhanced thermal expansion than 8YSZ. The intrinsic CTE at the atomic level has been investigated via molecular dynamics (MD) simulation. The atomic scale analysis provides new insights into the enhanced doping effects of multiple trivalent and tetravalent cations on the lattice structure, lattice energy, and thermal expansion in ZrO2. The calculated lattice energy becomes smaller with the incorporation of Dy3+, Al3+, and Ce4+ions, which corresponds strongly to the increase in CTE. The crystalline size is reduced due to the incorporation of the Al3+ and Ce4+, whereas the sintering resistance is enhanced ascribed to the addition of Dy3+ and Al3+. Doping Dy3+, Al3+, and Ce4+ cations to YSZ increased the CTE value of YSZ and for Dy0.03Y0.075Zr0.895O1.948, the CTE is 12.494 × 10−6°C−1 at 900°C, which has an 11% increase, as compared with that of 8YSZ.