Detecting magnetic ordering with atomic size electron probes

Layer-structured materials have been widely used in various applications due to their distinctive structures and physical/ chemical properties. As the electrode materials for sodium (Na)-ion batteries (NIB), their intercalation mechanisms remain unclear and their electrochemical performances are unsatisfied with low storage capacity and poor cycling stability. Therefore, it is essential to find new layered materials, explore their intercalation mechanisms and improve their electrochemical performance. In these regards, this thesis focuses on four layered materials in terms of atomic structural characterization, phase transition and intercalation mechanism, and improvement of their electrochemical performance, including anti-P2 Na0.5NbO2, Mxene Ti3C2X and transition-metal dichalcogenides MoS2. Most of the layered AxMO2 (A=alkali ions, M= transition metals) are composed of edge-shared MO6 octahedrons, wherein A ions occupy in the interlayers with varied coordination polyhedron such as octahedrons (O), tetrahedrons (T) or trigonal prisms (P). We synthesized a new layered oxide material anti-P2 Na0.5NbO2 with negative strain effect. Its structure is composed of NbO6 trigonal prisms and NaO6 octahedrons. Analysis by X-ray absorption spectroscopy(XAS) and density functional theory (DFT) calculations indicates that the negative volume change is mainly a result of the enhanced interlayer (Na-O) interaction and the weakened Nb-Nb and Nb-O bonding in the O-Nb-O slab upon Na intercalation. As an anode material for NIB, Na0.5NbO2 exhibits high structural stability, long cycle life and prominent rate performance for NIB. Thanks to its negative stain effect, Na0.5NbO2 can be used as a “volume buffer” for the positive-strain electrode materials to construct long-term batteries with high energy density. MXenes represent a large family of functionalized two-dimensional (2D) transition-metal carbides and carbonitrides. However, most of the understandings on their unique structures and applications are stopped at the theoretical suggestion. We clarified the surface structure and intercalation chemistry of Ti3C2X at the atomic scale by aberration-corrected scanning transmission electron microscope (STEM) and DFT calculations. The STEM studies show that the functional groups (e.g. OH-, F-, O-) and the intercalated sodium (Na) ions prefer to stay on the top sites of the centro-Ti atoms and the C atoms of the Ti3C2 monolayer, respectively. Double Na-atomic layers are found within the Ti3C2X interlayer upon extensive Na intercalation via two-phase transition and solid-solution reactions. In addition, aluminum (Al)-ion intercalation leads to horizontal sliding of the Ti3C2X monolayer. Based on these observations, the previous monolayer surface model of Ti3C2X is modified. DFT calculations using the new modeling help to understand more about their physical and chemical properties. Meanwhile, the Ti3C2X exhibits prominent rate performance and long-term cycling stability as an anode material for NIB. The distinctive electronic and chemical properties of transition-metal dichalcogenides are closely related to their structure and intercalation chemistry. By means of STEM, we clarified the phase transition from semiconductive 2H-MoS2 to metallic 1T-MoS2 and occupancy of the intercalated Na at the atomic scale. It is shown that x = 1.5 in NaxMoS2 is a critical point for the reversibility of the structural evolution. If light of the understanding of the phase transitions and intercalation chemistry of the MoS2, the cycling stability and rate performance of MoS2 are improved by controlling its reaction depth, optimizing the binder and nano-treatment. Figure 1

Probing the vectorial properties of light–matter interactions inherently requires control over the polarization state of light. The generation of extreme-ultraviolet attosecond pulses has opened new perspectives in measurements of chiral phenomena. However, limited polarization control in this regime prevents the development of advanced vectorial measurement schemes. Here, we establish an extreme-ultraviolet lock-in detection scheme, allowing the isolation and amplification of extremely weak chiral signals, by achieving dynamical polarization control. We demonstrate a time-domain approach to control and modulate the polarization state, and perform its characterization via an in situ measurement. Our approach is based on the collinear superposition of two independent, phase-locked, orthogonally polarized extreme-ultraviolet sources and the control of their relative delay with sub-cycle accuracy. We achieve lock-in detection of magnetic circular dichroism, transferring weak amplitude variations into a phase modulation. This approach holds the potential to significantly extend the scope of vectorial measurements to the attosecond and nanometre frontiers. A time-domain approach that can continuously tune the polarization state of extreme-ultraviolet attosecond pulses allows the isolation and amplification of extremely weak chiral signals, extending vectorial measurements to the attosecond and nanometre scales.

Isotopic engineering provides a powerful route to control phonon behavior in crystalline solids, enabling fundamental studies of lattice dynamics and heat transport at the atomic scale. Here, we directly visualize isotope-dependent phonon propagation in epitaxial Cr2O3 using aberration-corrected scanning transmission electron microscopy (STEM) combined with monochromated, high-energy-resolution electron energy-loss spectroscopy (EELS). Guided by ab initio phonon calculations, we demonstrate that optical phonon modes above 70 meV are predominantly oxygen-derived and exhibit measurable redshifts upon substitution of natural 16O by enriched 18O. Spatially resolved vibrational spectrum imaging reveals isotope-enriched tracer layers within Cr2O3 thin films, correlating isotope concentration with phonon intensity variations and vibrational energy shifts. At the nanometer and atomic scales, vibrational EELS mapping uncovers coherent phonon propagation across isotopic interfaces, consistent with theoretical phonon density of states and dispersion relations. These results establish vibrational EELS as a quantitative probe for isotope-dependent phonon transport in materials, opening new possibilities for studying energy dissipation and lattice dynamics.

The inert gas condensation method (IGC) produces multimetallic nanoparticles in a metastable state that may exhibit heterogeneities of size, structure, and composition. The deposition of IGC-fabricated nanoparticles on substrates allows for a detailed characterization by combination of aberration-corrected scanning transmission electron microscopy (TEM) and atom probe tomography (APT). Multiple particle monitoring and high-resolution scanning TEM give access to the size distribution of Au–Cu nanoparticles (<10 nm in diameter, bimodal distribution). TEM and APT show that the alloying between Cu and Au may stabilize the Ih structure for smaller particles (<4 nm). Combining high-resolution scanning transmission electron microscopy/energy dispersive X-ray and three-dimensional composition analysis by APT reveals that an excess of Cu may be present in a shell around the larger particles (>7 nm), while Cu is more randomly distributed in smaller particles.

ABSTRACTAchieving sub-ångström resolution has long been restricted to sophisticated aberration-corrected scanning transmission electron microscopy (AC-STEM). Recent advances in computational super-resolution techniques, such as deconvolution and electron ptychography, have enabled uncorrected STEM to achieve sub-ångström resolution without the need for delicate aberration correctors. However, these methods have strict requirements for sample thickness and thus have yet to be widely implemented. In this study, we introduce SARDiffuse—a deep-learning diffusion model designed to enhance spatial resolution and correct the noise level of uncorrected STEM images. Trained with experimental AC-STEM data, SARDiffuse has the capability to restore high-frequency information of STEM images, enabling sub-ångström resolution in an uncorrected microscope. We demonstrate the effectiveness of the model on representative materials, including silicon, strontium titanate and gallium nitride, achieving substantial improvements (<1 Å) in spatial resolution. Detailed statistical analysis confirms that SARDiffuse reliably preserves atomic positions, demonstrating that it is a powerful tool for high-precision material characterization. Furthermore, SARDiffuse effectively mitigates spherical-aberration-induced artifacts, outperforming current methods in artifact correction. Meanwhile, the background information of images, such as thickness variation or carbon contamination distribution, is also preserved. This work highlights the potential of deep learning to realize sub-ångström-resolution imaging in the uncorrected electron microscope, offering a cost-effective alternative to delicate AC-STEM when imaging conventional single crystals.

Edge structures and atomic defects are of fundamental importance since they can significantly affect the physical and chemical properties of low-dimensional materials, such as nanoribbons, and therefore merit thorough investigations at the atomic level. Recent developments of direct imaging and analytical techniques using an aberration-corrected scanning transmission electron microscope (STEM) have provided direct access to information on the local atomic structure and the chemical composition at the atomic scale. In this review, we report on the discrimination of single atoms including dopant atoms on a monolayered transition-metal dichalcogenide (TMD) nanoribbon and a single nitrogen adatom on graphene by time-resolved annular dark-field (ADF) imaging and spatially resolved electron energy loss spectroscopy (EELS). We also show that in situ scanning transmission electron microscopy can be used to monitor the structural transformation between semiconducting (2H) and metallic (1T) phases in monolayer MoS2, and can enable direct observation of in-plane graphene growth at a step edge of a bi-layer graphene and domain boundary formation during growth with atomic-resolution.

Two-dimensional (2D) materials, such as graphene and transition-metal dichalcogenide monolayers, have unique properties that are distinctly different from those of their bulk counterparts, and hopefully possess a wide range of applications in 2D semiconductor device. Structural defects are known to have profound influences on the properties of crystalline materials; thus, correlating the defect structure with local properties in 2D material is of fundamental importance. However, electron microscopy studies of 2D materials on an atomic scale have become a challenge as most of these materials are susceptible to electron beam irradiation damage under high voltage and high dose experimental conditions. The development of low voltage aberration-corrected scanning transmission electron microscopy (STEM) has made it possible to study 2D materials at a single atom level without damaging their intrinsic structures. In addition, controllable structural modification by using electron beam becomes feasible by controlling the electron beam-sample interaction. New nanostructures can be created and novel 2D materials can be fabricated in-situ by using this approach. In this article, we review some of our recent studies of graphene and transition-metal dichalcogenides to showcase the applications of low voltage aberration corrected STEM in 2D material research.

Direct observation of delithiated structures of LiCoO(2) at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. The ordered Li, Co, and O columns for LiCoO(2) nanoparticles are clearly identified in ABF micrographs. Upon the Li ions extraction from LiCoO(2), the Co-contained (003) planes distort from the bulk to the surface region and the c-axis is expanded significantly. Ordering of lithium ions and lithium vacancies has been observed directly and explained by first-principles simulation. On the basis of HAADF micrographs, it is found that the phase irreversibly changes from O3-type in pristine LiCoO(2) to O1-type Li(x)CoO(2) (x ≈ 0.50) after the first electrochemical Li extraction and back to O2-type Li(x)CoO(2) (x ≈ 0.93) rather than to O3-stacking after the first electrochemical lithiation. This is the first report of finding O2-Li(x)CoO(2) in the phase diagram of O3-LiCoO(2), through which the two previously separated LiCoO(2) phases, i.e. O2 and O3 systems, are connected. These new investigations shed new insight into the lithium storage mechanism in this important cathode material for Li-ion batteries.

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The introduction of semiconductor and metallic nanostructures in photovoltaic materials is an active and promising area of development. This is due to the existence of several characteristics of nanostructures that improve the functionality of photovoltaic materials, for example by an enhancement of efficiency in solar cells, an increase of collected sun light and by the use of up- and down-converters to change the energy of incident photons. Nanostructures for solar cells offer, in comparison with single junction solar cells, new photovoltaic mechanisms for transforming sun light energy into electrical energy. A critical issue to understand the behavior of nanostructures based solar cells is to know their structural characteristics (size, shape, strain, distribution and composition) at nanoscale. In addition to this, the knowledge of the processes of nucleation, segregation and intermixing in nanostructures at nanoscale, and in some cases to atomic scale, is another important point to improve their design. This communication reviews our contribution toward the understanding of these issues and present some prospective studies and experiments in this context, by the use and development of methods based in aberration-corrected scanning transmission electron microscopy, high resolution electron microscopy and other complementary techniques. These methods are applied to several semiconductor nanostructures for intermediate band solar cells and some prospective results are presented in relation to their potential use for nanostructured thin film solar cells. We also present a method to prepare special samples to correlate, in individual or some few nanostructures, high resolution structural properties with other functional properties for the better design of photovoltaic materials. The nanostructures studied by aberration-corrected scanning transmission electron microscopy that are reviewed in this paper consist of III-V quantum dots and wires, silicon wires and strain-compensated stacked nanostructures grown by epitaxial techniques.

X-ray analysis is one of the most robust approaches to extract quantitative information from various materials and is widely used in various fields ever since Raimond Castaing established procedures to analyze electron-induced X-ray signals for materials characterization '70 years ago'. The recent development of aberration-correction technology in a (scanning) transmission electron microscopes (S/TEMs) offers refined electron probes below the Å level, making atomic-resolution X-ray analysis possible. In addition, the latest silicon drift detectors allow complex detector arrangements and new configurational designs to maximize the collection efficiency of X-ray signals, which make it feasible to acquire X-ray signals from single atoms. In this review paper, recent progress and advantages related to S/TEM-based X-ray analysis will be discussed: (i) progress in quantification for materials characterization including the recent applications to light element analysis, (ii) progress in analytical spatial resolution for atomic-resolution analysis and (iii) progress in analytical sensitivity toward single-atom detection and analysis in materials. Both atomic-resolution analysis and single-atom analysis are evaluated theoretically through multislice-based calculation for electron propagation in oriented crystalline specimen in combination with X-ray spectrum simulation.

Atomic-resolution imaging in an aberration-corrected scanning transmission electron microscope (STEM) can enable direct correlation between atomic structure and materials functionality. The fast and precise control of the STEM probe is, however, challenging because the true beam location deviates from the assigned location depending on the properties of the deflectors. To reduce these deviations, i.e. image distortions, we use spiral scanning paths, allowing precise control of a sub-Å sized electron probe within an aberration-corrected STEM. Although spiral scanning avoids the sudden changes in the beam location (fly-back distortion) present in conventional raster scans, it is not distortion-free. “Archimedean” spirals, with a constant angular frequency within each scan, are used to determine the characteristic response at different frequencies. We then show that such characteristic functions can be used to correct image distortions present in more complicated constant linear velocity spirals, where the frequency varies within each scan. Through the combined application of constant linear velocity scanning and beam path corrections, spiral scan images are shown to exhibit less scan distortion than conventional raster scan images. The methodology presented here will be useful for in situ STEM imaging at higher temporal resolution and for imaging beam sensitive materials.

An Atomic-Scale Perspective of the Challenging Microstructure of YBa2Cu3O7−x Thin Films

A detailed knowledge of the atomic structure of magnetic semiconductors is crucial to understanding their electronic and magnetic properties, which could enable spintronic applications. Energy-dispersive X-ray spectrometry (EDX) in the scanning transmission electron microscope and atom probe tomography (APT) experiments reveal the formation of Cr-rich regions in Cd1-x Cr x Te layers grown by molecular beam epitaxy. These Cr-rich regions occur on a length scale of 6-10 nm at a nominal Cr composition of x=0.034 and evolve toward an ellipsoidal shape oriented along directions at a composition of x=0.083. Statistical analysis of the APT reconstructed volume reveals that the Cr aggregation increases with the average Cr composition. The correlation with the magnetic properties of such (Cd,Cr)Te layers is discussed within the framework of strongly inhomogeneous materials. Finally, difficulties in accurately quantifying the Cr distribution in the CdTe matrix on an atomic scale by EDX and APT are discussed.

Prospects of annular differential phase contrast applied for optical sectioning in STEM