33 research outputs found
Atomistic Investigation of Grain Boundary Fracture in Alumina
Grain boundary (GB) fracture is a major mechanism of
material failure
in polycrystalline ceramics. However, the intricate atomic arrangements
of GBs have impeded our understanding of the atomistic mechanisms
of these processes. In this study, we investigated the atomic-scale
crack propagation behavior of an α-Al2O3 ∑13 grain boundary, using a combination of in situ transmission electron microscopy (TEM) and scanning TEM. The atomic-scale
fracture path along the GB core was directly determined by the observation
of the atomic structures of the fractured surfaces, which is consistent
with density functional theory calculations. We found that the GB
fracture can be attributed to the weaker local bonds and a smaller
number of bonds along the fracture path. Our findings provide atomistic
insights into the mechanisms of crack propagation along GBs, offering
significant implications for GB engineering and the toughening of
ceramics
Atomistic Investigation of Grain Boundary Fracture in Alumina
Grain boundary (GB) fracture is a major mechanism of
material failure
in polycrystalline ceramics. However, the intricate atomic arrangements
of GBs have impeded our understanding of the atomistic mechanisms
of these processes. In this study, we investigated the atomic-scale
crack propagation behavior of an α-Al2O3 ∑13 grain boundary, using a combination of in situ transmission electron microscopy (TEM) and scanning TEM. The atomic-scale
fracture path along the GB core was directly determined by the observation
of the atomic structures of the fractured surfaces, which is consistent
with density functional theory calculations. We found that the GB
fracture can be attributed to the weaker local bonds and a smaller
number of bonds along the fracture path. Our findings provide atomistic
insights into the mechanisms of crack propagation along GBs, offering
significant implications for GB engineering and the toughening of
ceramics
Patterning Oxide Nanopillars at the Atomic Scale by Phase Transformation
Phase transformations in crystalline
materials are common in nature and often modify dramatically properties
of materials. The ability to precisely control them with a high spatial
precision represents a significant step forward in realizing new functionalities
in confined dimensions. However, such control is extremely challenging
particularly at the atomic scale due to the intricacies in governing
thermodynamic conditions with a high spatial accuracy. Here, we apply
focused electron beam of a scanning transmission electron microscope
to irradiate SrNbO<sub>3.4</sub> crystals and demonstrate a precise
control of a phase transformation from layered SrNbO<sub>3.4</sub> to perovskite SrNbO<sub>3</sub> at the atomic scale. By purposely
squeezing O atoms out of the vertex-sharing NbO<sub>6</sub> octahedral
slabs, their neighboring slabs zip together, resulting in a patterning
of SrNbO<sub>3</sub> nanopillars in SrNbO<sub>3.4</sub> matrix. Such
phase transformations can be spatially manipulated with an atomic
precision, opening up a novel avenue for materials design and processing
and also for advanced nanodevice fabrication
Stable Magnetic Skyrmion States at Room Temperature Confined to Corrals of Artificial Surface Pits Fabricated by a Focused Electron Beam
Stable
confinement of elemental magnetic nanostructures, such as
a single magnetic domain, is fundamental in modern magnetic recording
technology. It is well-known that various magnetic textures can be
stabilized by geometrical confinement using artificial nanostructures.
The magnetic skyrmion, with novel spin texture and promise for future
memory devices because of its topological protection and dimension
at the nanometer scale, is no exception. So far, skyrmion confinement
techniques using large-scale boundaries with limited geometries such
as isolated disks and stripes prepared by conventional microfabrication
techniques have been used. Here, we demonstrate an alternative technique
confining skyrmions to artificial nanostructures (corrals) built from
surface pits fabricated by a focused electron beam. Using aberration-corrected
differential phase contrast scanning transmission electron microscopy,
we directly visualized stable skyrmion states confined at a room temperature
to corrals made of artificial surface pits on a thin plate of Co<sub>8</sub>Zn<sub>8</sub>Mn<sub>4</sub>. We observed a stable single-skyrmion
state confined to a triangular corral and a unique transition into
a triple-skyrmions state depending on the perpendicular magnetic field.
Furthermore, we made an array of stable single-skyrmion states by
using concatenated triangular corrals. Artificial control of skyrmion
states with the present technique should be a powerful way to realize
future nonvolatile memory devices using skyrmions
Atomic-Scale Tracking of a Phase Transition from Spinel to Rocksalt in Lithium Manganese Oxide
For
the intercalation type cathode in lithium-ion batteries, the
structural framework of electrode is expected to remain unchanged
during lithium insertion and extraction. Unfavorable phase transition
in electrode materials, which has been frequently observed, modifies
the structural framework, which leads to capacity loss and voltage
decay. Here, we track atoms motion/shift in lithium manganese oxide
during a phase transition from spinel to rocksalt by using atomically
resolved aberration corrected scanning transmission electron microscopy
and spectroscopy. We find that when given energy, the transition metal
cation can readily hop between oxygen tetrahedral and octahedral sites
in oxygen deficient lithium manganese oxide similar to lithium diffusion
behavior, which leaves the anion structure framework almost unchanged.
During this phase transition, the intermediate state, migration length,
and atomic structure of phase boundaries are revealed, and the mechanism
is discussed. Our observations help us to understand the past experimental
phenomena and provide useful information to stabilize the structure
of electrode materials and thus improve the cycling life of lithium-ion
batteries
Direct Observation of Impurity Segregation at Dislocation Cores in an Ionic Crystal
Dislocations,
one-dimensional lattice defects, are known to strongly interact with
impurity atoms in a crystal. This interaction is generally explained
on the basis of the long-range strain field of the dislocation. In
ionic crystals, the impurity–dislocation interactions must
be influenced by the electrostatic effect in addition to the strain
effect. However, such interactions have not been verified yet. Here,
we show a direct evidence of the electrostatic impurity–dislocation
interaction in α-Al<sub>2</sub>O<sub>3</sub> by visualizing
the dopant atom distributions at dislocation cores using atomic-resolution
scanning transmission electron microscopy (STEM). It was found that
the dopant segregation behaviors strongly depend on the kind of elements,
and their valence states are considered to be a critical factor. The
observed segregation behaviors cannot be explained by the elastic
interactions only, but can be successfully understood if the electrostatic
interactions are taken into account. The present findings will lead
to the precise and quantitative understanding of impurity induced
dislocation properties in many materials and devices
High Electron Mobility of Nb-Doped SrTiO<sub>3</sub> Films Stemming from Rod-Type Sr Vacancy Clusters
Achieving high electron mobility in SrTiO<sub>3</sub> films is of significant interest, particularly in relation to technological applications such as oxide semiconductors, field-induced superconductors, and thermoelectric generators. One route to achieving high electron mobility is growth of high quality SrTiO<sub>3</sub> films with low defect concentrations. Another approach for mobility enhancement is applying a strain to the crystal. However, the maximum mobilities obtainable by these approaches are limited both by external and internal factors (currently available fabrication techniques, and maximum crystal strain, for example). In this paper, we demonstrate a unique crystal engineering approach to alter the strain in Nb-doped SrTiO<sub>3</sub> films based on the deliberate introduction of Sr vacancy clusters. Nb-doped SrTiO<sub>3</sub> films produced in this manner are found to exhibit remarkably enhanced electron mobilities (exceeding 53 000 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>). This method of defect engineering is expected to enable tuning and enhancement of electron mobilities not only in SrTiO<sub>3</sub> films, but also in thin films and bulk crystals of other perovskite-type materials
Atomic-Scale Identification of Individual Lanthanide Dopants in Optical Glass Fiber
Various dopants are added in commercially available optical glass fibers. The specific atomic species and charge state of lanthanide dopants are known to significantly influence the fiber’s optical properties. For understanding the role of dopants on the optical properties, atomic-scale identification of the lanthanide dopants in the optical fiber is crucial. Aberration-corrected scanning transmission electron microscopy (STEM) is especially powerful for visualizing individual atoms of heavy elements buried in a matrix composed of light elements. Here, we apply aberration-corrected high-angle annular dark field (HAADF)-STEM to directly visualize individual erbium (Er) dopants buried in the optical glass fiber. Molecular dynamics and image simulations are used to interpret the experimental images and draw quantitative conclusions. The visibility of the buried Er atoms in the amorphous glass is strongly dependent on the defocus and specimen thickness, and only Er atoms in very thin regions can be reliably identified
Atomic-Scale Observations of (010) LiFePO<sub>4</sub> Surfaces Before and After Chemical Delithiation
The ability to view
directly the surface structures of battery materials with atomic resolution
promises to dramatically improve our understanding of lithium (de)Âintercalation
and related processes. Here we report the use of state-of-the-art
scanning transmission electron microscopy techniques to probe the
(010) surface of commercially important material LiFePO<sub>4</sub> and compare the results with theoretical models. The surface structure
is noticeably different depending on whether Li ions are present in
the topmost surface layer or not. Li ions are also found to migrate
back to surface regions from within the crystal relatively quickly
after partial delithiation, demonstrating the facile nature of Li
transport in the [010] direction. The results are consistent with
phase transformation models involving metastable phase formation and
relaxation, providing atomic-level insights into these fundamental
processes
Full Determination of Individual Reconstructed Atomic Columns in Intermixed Heterojunctions
Heterojunctions
offer a tremendous opportunity for fundamental as well as applied
research, ranging from the unique electronic phases in between oxides
to the contact issues in semiconductor devices. Despite their pivotal
roles, determining individual building atom of matter in heterojunctions
is still challenging, especially for those between highly dissimilar
structures, in which breaking of symmetry, chemistry, and bonds may
give rise to complex reconstruction and intermixing at the junction.
Here, we combine electron microscopy, spectroscopy, and first-principles
calculations to determine individual reconstructed atomic columns
and their charge states in a complex, multicomponent heterojunction
between the delafossite CuScO<sub>2</sub> and spinel MgAl<sub>2</sub>O<sub>4</sub>. The high resolution enables us to demonstrate that
the reconstructed region can accommodate a highly selective intermixing
of Cu cations at specific Sc cation sites with half atomic density,
forming a complex ordered superstructure. Such ability to resolve
reconstructed heterojunctions to the atomic dimensions helps elucidate
atomistic mechanisms and discover novel properties with applications
in a diverse range of scientific disciplines