Inherent stochasticity during insulator-metal transition in VO2.

Vanadium dioxide (VO2), which exhibits a near-room-temperature insulator-metal transition, has great potential in applications of neuromorphic computing devices. Although its volatile switching property, which could emulate neuron spiking, has been studied widely, nanoscale studies of the structural stochasticity across the phase transition are still lacking. In this study, using in situ transmission electron microscopy and ex situ resistive switching measurement, we successfully characterized the structural phase transition between monoclinic and rutile VO2 at local areas in planar VO2/TiO2 device configuration under external biasing. After each resistive switching, different VO2 monoclinic crystal orientations are observed, forming different equilibrium states. We have evaluated a statistical cycle-to-cycle variation, demonstrated a stochastic nature of the volatile resistive switching, and presented an approach to study in-plane structural anisotropy. Our microscopic studies move a big step forward toward understanding the volatile switching mechanisms and the related applications of VO2 as the key material of neuromorphic computing.

Quantification of Charge Transfer at the Interfaces of Oxide Thin Films.

The interfacial electronic distribution in transition-metal oxide thin films is crucial to their interfacial physical or chemical behaviors. Core-loss electron energy-loss spectroscopy (EELS) may potentially give valuable information of local electronic density of state at high spatial resolution. Here, we studied the electronic properties at the interface of Pb(Zr0.2Ti0.8)O3 (PZT)/4.8 nm La0.8Sr0.2MnO3 (LSMO)/SrTiO3 (STO) using valance-EELS with a scanning transmission electron microscope. Modeled with dielectric function theory, the charge transfer in the vicinity of the interfaces of PZT/LSMO and LSMO/STO was determined from the shifts of plasma peaks of valence EELS (VEELS), agreeing with theoretical prediction. Our work demonstrates that the VEELS method enables a high-efficient quantification of the charge transfer at interfaces, shedding light on the charge-transfer issues at heterogenous interfaces in physical and chemical devices.

Revealing and Rationalizing the Rich Polytypism of Todorokite MnO2.

Polytypism, or stacking disorder, in crystals is an important structural aspect that can impact materials properties and hinder our understanding of the materials. One example of a polytypic system is todorokite-MnO2, which has a unique structure among the transition-metal oxides, with large ionic conductive channels formed by the metal oxide framework that can be utilized for potential functionalization, from molecular/ion sieving to charge storage. In contrast to the perceived 3 × 3 tunneled structure, we reveal a coexistence of a diverse array of tunnel sizes in well-crystallized, chemically homogeneous one-dimensional todorokite-MnO2. We explain the formation and persistence of this distribution of tunnel sizes thermochemically, demonstrating the stabilization of a range of coherent large-tunnel environments by the intercalation of partially solvated Mg2+ cations. Based on structural behavior of the system, compared to the common well-ordered alkali-stabilized polymorphs of MnO2, we suggest generalizable principles determining the selectivity of structure selection by dopant incorporation.

Clinical characteristics and short-term prognosis of LGI1 antibody encephalitis: a retrospective case study.

BACKGROUND: Recently, most reports of Leucine-rich glioma-inactivated 1 (LGI1) antibody encephalitis are from Europe and the US, while the short term outcome and clinical characteristics of Chinese patients are rarely reported,we study the clinical manifestations, laboratory results and brain magnetic resonance images (MRI) of eight patients who were recently diagnosed with LGI1 antibody encephalitis in our hospital to improve the awareness and knowledge of this disease. METHODS: Eight patients (five males and three females; mean age, 63.4) with LGI1 antibody encephalitis who were diagnosed and treated in the Department of Neurology of Shengjing Hospital of China Medical University from September 2016 to June 2017 were recruited for the current study. Their general information, clinical manifestations, treatment regimens, and short-term prognoses were retrospectively analyzed, as were the results from MRI and laboratory findings. RESULTS: Overall, patient symptoms included cognitive impairment, which manifested primarily as memory deficits (8/8), seizures (including faciobrachial dystonic seizure, (FBDS)) (8/8), psychiatric and behavioral disorders (7/8), sleep disorders (4/8), and autonomic abnormalities (3/8). Five patients also had abnormal findings on brain MRI, mainly involving the hippocampus, basal ganglia and insula. Hyponatremia occurred in six cases. All patients tested positive for LGI1 antibodies in their serum/cerebrospinal fluid (CSF)and patients were negative for tumors. Symptoms rapidly improved after treatment with immunoglobulin and/or steroid therapy. The patients were followed up for 4-13 months after discharge, and two patients relapsed. CONCLUSION: Primary symptoms of LGI1 antibody encephalitis include memory impairments, seizures, FBDS, and mental and behavioral abnormalities. Increased titers of LGI1 antibodies are also present in the serum/CSF of patients. Patients often have hyponatremia, and MRIs show abnormalities in various brain regions. Finally, immunotherapy shows good efficacy and positive benefits, although patients may relapse in the short-term.

Nonperturbative Quantum Nature of the Dislocation-Phonon Interaction.

Despite the long history of dislocation-phonon interaction studies, there are many problems that have not been fully resolved during this development. These include an incompatibility between a perturbative approach and the long-range nature of a dislocation, the relation between static and dynamic scattering, and their capability of dealing with thermal transport phenomena for bulk material only. Here by utilizing a fully quantized dislocation field, which we called a "dislon", a phonon interacting with a dislocation is renormalized as a quasi-phonon, with shifted quasi-phonon energy, and accompanied by a finite quasi-phonon lifetime, which are reducible to classical results. A series of outstanding legacy issues including those above can be directly explained within this unified phonon renormalization approach. For instance, a renormalized phonon naturally resolves the decade-long debate between dynamic and static dislocation-phonon scattering approaches, as two limiting cases. In particular, at nanoscale, both the dynamic and static approaches break down, while the present renormalization approach remains valid by capturing the size effect, showing good agreement with lattice dynamics simulations.

Strain Coupling of Conversion-type Fe3 O4 Thin Films for Lithium Ion Batteries.

Lithiation/delithiation induces significant stresses and strains into the electrodes for lithium ion batteries, which can severely degrade their cycling performance. Moreover, this electrochemically induced strain can interact with the local strain existing at solid-solid interfaces. It is not clear how this interaction affects the lithiation mechanism. The effect of this coupling on the lithiation kinetics in epitaxial Fe3 O4 thin film on a Nb-doped SrTiO3 substrate is investigated. In situ and ex situ transmission electron microscopy (TEM) results show that the lithiation is suppressed by the compressive interfacial strain. At the interface between the film and substrate, the existence of Lix Fe3 O4 rock-salt phase during lithiation consequently restrains the film from delamination. 2D phase-field simulation verifies the effect of strain. This work provides critical insights of understanding the solid-solid interfaces of conversion-type electrodes.

Prognosis of very preterm infants with severe respiratory distress syndrome receiving mechanical ventilation.

OBJECTIVE: To evaluate the prognosis of very preterm infants with severe respiratory distress syndrome (RDS) receiving mechanical ventilation. METHODS: A total of 288 preterm infants mechanically ventilated for severe RDS and completed follow-up till 18 months of corrected age comprised these study subjects. The associations of prenatal and postnatal factors, mode and duration of conventional mechanical ventilation (CMV), medication and treatment, and complications with cerebral palsy or mental developmental index (MDI) < 70 at 18 months of age were analyzed. RESULTS: The incidences of CP among study subjects were 17, 5, and 2% in infants less than 28, 28-30, and 30-32 weeks, respectively. The incidences of MDI < 70 were 49, 24, and 13% in infants less than 28 weeks, 28-30 weeks, and 30-32 weeks, respectively. Antenatal corticosteroids, preeclampsia, fetal distress, early and late bacteremia, and decreased weight gain were associated with CP and an MDI < 70. In the CP and MDI < 70 groups, the number of infants on CMV was significantly higher than on high-frequency oscillatory ventilation (HFOV). Longer duration of mechanical ventilation and blood transfusions were associated with an increased risk of having an MDI < 70 or CP. The complications in study subjects associated with an MDI < 70 or CP were BPD, NEC, and IVH grade III-IV. CONCLUSION: The prognosis of very preterm infants with severe RDS may be influenced by several prenatal and postnatal factors. HFOV although decreased the duration of mechanical ventilation, whether it will decrease the incidence of neurodevelopmental disability, needs to be explored further.

Near-field optical effect of a core-shell nanostructure in proximity to a flat surface.

We provide an analytical solution for studying the near-field optical effect of a core-shell nanostructure in proximity to a flat surface, within quasi-static approximation. The distribution of electrostatic potential and the field enhancement in this complex geometry are obtained by solving a set of linear equations. This analytical result can be applied to a wide range of systems associated with near-field optics and surface plasmon polaritons. To illustrate the power of this technique, we study the field-attenuation effect of an oxidized shell in a silver tip in a near-field scanning microscope. The thickness of oxidized layer can be monitored by measuring the intensity of light. We also find a linear relation between resonant frequency and temperature in an Ag-Au core-shell structure, which provides insight for local temperature detection with nm scale resolution. Our results also show good agreement with recent finite element method results.

A size-dependent sodium storage mechanism in Li4Ti5O12 investigated by a novel characterization technique combining in situ X-ray diffraction and chemical sodiation.

A novel characterization technique using the combination of chemical sodiation and synchrotron based in situ X-ray diffraction (XRD) has been detailed illustrated. The power of this novel technique was demonstrated in elucidating the structure evolution of Li4Ti5O12 upon sodium insertion. The sodium insertion behavior into Li4Ti5O12 is strongly size dependent. A solid solution reaction behavior in a wide range has been revealed during sodium insertion into the nanosized Li4Ti5O12 (~44 nm), which is quite different from the well-known two-phase reaction of Li4Ti5O12/Li7Ti5O12 system during lithium insertion, and also has not been fully addressed in the literature so far. On the basis of this in situ experiment, the apparent Na(+) ion diffusion coefficient (DNa+) of Li4Ti5O12 was estimated in the magnitude of 10(-16) cm(2) s(-1), close to the values estimated by electrochemical method, but 5 order of magnitudes smaller than the Li(+) ion diffusion coefficient (D(Li+) ~10(-11) cm(2) s(-1)), indicating a sluggish Na(+) ion diffusion kinetics in Li4Ti5O12 comparing with that of Li(+) ion. Nanosizing the Li4Ti5O12 will be critical to make it a suitable anode material for sodium-ion batteries. The application of this novel in situ chemical sodiation method reported in this work provides a facile way and a new opportunity for in situ structure investigations of various sodium-ion battery materials and other systems.

Atomic Structure Evolution of Pt-Co Binary Catalysts: Single Metal Sites versus Intermetallic Nanocrystals.

Due to their exceptional catalytic properties for the oxygen reduction reaction (ORR) and other crucial electrochemical reactions, PtCo intermetallic nanoparticle (NP) and single atomic (SA) Pt metal site catalysts have received considerable attention. However, their formation mechanisms at the atomic level during high-temperature annealing processes remain elusive. Here, the thermally driven structure evolution of Pt-Co binary catalyst systems is investigated using advanced in situ electron microscopy, including PtCo intermetallic alloys and single Pt/Co metal sites. The pre-doping of CoN4 sites in carbon supports and the initial Pt NP sizes play essential roles in forming either Pt3 Co intermetallics or single Pt/Co metal sites. Importantly, the initial Pt NP loadings against the carbon support are critical to whether alloying to L12 -ordered Pt3 Co NPs or atomizing to SA Pt sites at high temperatures. High Pt NP loadings (e.g., 20%) tend to lead to the formation of highly ordered Pt3 Co intermetallic NPs with excellent activity and enhanced stability toward the ORR. In contrast, at a relatively low Pt loading (<6 wt%), the formation of single Pt sites in the form of PtC3 N is thermodynamically favorable, in which a synergy between the PtC3 N and the CoN4 sites could enhance the catalytic activity for the ORR, but showing insufficient stability.

Mapping valence electron distributions with multipole density formalism using 4D-STEM.

Recent advancement in aberration correction and detector technology opened a door to various applications using 4D-STEM, which yields a diffraction pattern for each scanning position within a crystal unit-cell in scanning transmission electron microscopy (STEM) and generates incredible amounts of data in momentum space. Currently 4D-STEM analysis relies on the center-of-mass of the diffraction patterns in electric field and charge density mapping. It only derives the total projected charge density and is limited to phase objects, e.g. extremely thin samples. Here, we propose a new analytical method to accurately map aspherical valence electron distributions with atom-centered multipolar functions formalism using the whole 4D-STEM dataset. We demonstrate that, with the full dynamical calculations for various sample thicknesses, the method is sensitive not only to the miniscule charge transfer, but also to the atomic site symmetry and aspherical electron orbitals. The process of the refinement is much more robust and reliable than quantitative convergent beam electron diffraction.

Size-dependent kinetics during non-equilibrium lithiation of nano-sized zinc ferrite.

Spinel transition metal oxides (TMOs) have emerged as promising anode materials for lithium-ion batteries. It has been shown that reducing their particle size to nanoscale dimensions benefits overall electrochemical performance. Here, we use in situ transmission electron microscopy to probe the lithiation behavior of spinel ZnFe2O4 as a function of particle size. We have found that ZnFe2O4 undergoes an intercalation-to-conversion reaction sequence, with the initial intercalation process being size dependent. Larger ZnFe2O4 particles (40 nm) follow a two-phase intercalation reaction. In contrast, a solid-solution transformation dominates the early stages of discharge when the particle size is about 6-9 nm. Using a thermodynamic analysis, we find that the size-dependent kinetics originate from the interfacial energy between the two phases. Furthermore, the conversion reaction in both large and small particles favors {111} planes and follows a core-shell reaction mode. These results elucidate the intrinsic mechanism that permits fast reaction kinetics in smaller nanoparticles.

Retrieving the energy-loss function from valence electron energy-loss spectrum: Separation of bulk-, surface-losses and Cherenkov radiation.

With recent rapid advancement in electron microscopy instrumentation, in particular, bright electron sources and monochromators, valence electron energy-loss spectroscopy (VEELS) has become attractive for retrieving band structures, optical properties, dielectric functions and phonon information of materials. However, Cherenkov radiation and surface-loss contribution significantly alter fine structures of VEELS, even in simple semiconductors and insulators. This leads to the problem that dielectric function or bandgap structure of these materials cannot be determined correctly if these effects are not removed. In this work we present a solution to this dilemma. We demonstrate that energy-loss function and real part of inverse complex dielectric function can be retrieved from raw data of VEELS. Based on the calculated example of Si, the limitation of our approach is discussed. We believe that our approach represents an improvement over previous procedures and has a broad prospect for applications.

Non-uniform Stress-free Strains in a Spherically Symmetrical Nano-sized Particle and Its Applications to Lithium-ion Batteries.

The stress-free strain originated from local chemical composition and phase transformation can significantly alter the microstructures of materials; and then affect their properties. In this paper, we developed an analytical method to calculate stress-strain field due to the non-uniform stress-free strain in a spherically symmetrical particle. Applying the method to a lithium ion (Li-ion) battery electrode, the evolution of Li-ion concentration and strain field during the lithiation process is studied. Our studies reveal that the maximum strain in the electrode generally occurs on surface of sample, and is mainly dependent on the difference of Li-ion concentration of surface and of center in sample. Decreasing the difference of Li-ion concentration can efficiently decrease the maximum strain so that cracks of electrodes can been prevented. Our analytical results provide a useful guidance for practical applications of energy storage materials.

Bimetallic Nanoparticle Oxidation in Three Dimensions by Chemically Sensitive Electron Tomography and in Situ Transmission Electron Microscopy.

The formation of hollow-structured oxide nanoparticles is primarily governed by the Kirkendall effect. However, the degree of complexity of the oxidation process multiplies in the bimetallic system because of the incorporation of more than one element. Spatially dependent oxidation kinetics controls the final morphology of the hollow nanoparticles, and the process is highly dependent on the elemental composition. Currently, a theoretical framework that can predict how different metal elements result in different oxide morphologies remains elusive. In this work, utilizing a combination of state-of-the-art in situ environmental transmission electron microscopy and three-dimensional (3D) chemically sensitive electron tomography, we provide an in situ and 3D investigation of the oxidation mechanism of the Ni-Fe nanoparticles. The direct measurements allow us to correlate the 3D elemental segregation in the particles with the oxidation morphologies, that is, single-cavity or dual-cavity hollow structure, and multicavity porous structures. Our findings in conjunction with theoretical calculations show that metal concentration, diffusivity, and particle size are important parameters that dictate the mechanical and phase stabilities of the hollow oxide shell, which in turn determine its barrier properties and the final hollow oxide morphology. It sheds light on how to use multielemental oxidation to control morphology in nanomaterials and demonstrates the power of 3D chemical imaging.

Electron transfer dynamics from single near infrared emitting lead sulfide-cadmium sulfide nanocrystals to titanium dioxide.

In this study we report the first successful demonstration of electron transfer between single near infrared emitting PbS/CdS nanocrystals and an external acceptor, titanium dioxide (TiO2). We demonstrate distance-dependent electron transfer from single nanocrystals to TiO2 and explore the effect of this process on the photoluminescence dynamics of these nanocrystals. Isolated PbS/CdS QDs are found to exhibit blinking dynamics similar to other nanocrystals like CdSe/ZnS; however, their photoluminescence follows a quasi two-state pattern with heterogeneous photoluminescence lifetimes which may be the result of their emission originating from different energy states. Electron transfer of these nanocrystals with an external acceptor inhibits their photoluminescence lifetime heterogeneity and biases their blinking dynamics in a manner similar to that observed for visible emitting CdSe/ZnS nanocrystals undergoing electron transfer with external acceptors. While the present study reconfirms the universality of quantum dot blinking among various types of nanocrystals, it also demonstrates that universality remains valid for the communication of various types of nanocrystals with the exterior world, here pictured as electron transfer with external acceptors.

Visualization of lithium-ion transport and phase evolution within and between manganese oxide nanorods.

Multiple lithium-ion transport pathways and local phase changes upon lithiation in silver hollandite are revealed via in situ microscopy including electron diffraction, imaging and spectroscopy, coupled with density functional theory and phase field calculations. We report unexpected inter-nanorod lithium-ion transport, where the reaction fronts and kinetics are maintained within the neighbouring nanorod. Notably, this is the first time-resolved visualization of lithium-ion transport within and between individual nanorods, where the impact of oxygen deficiencies is delineated. Initially, fast lithium-ion transport is observed along the long axis with small net volume change, resulting in two lithiated silver hollandite phases distinguishable by orthorhombic distortion. Subsequently, a slower reaction front is observed, with formation of polyphase lithiated silver hollandite and face-centred-cubic silver metal with substantial volume expansion. These results indicate lithium-ion transport is not confined within a single nanorod and may provide a paradigm shift for one-dimensional tunnelled materials, particularly towards achieving high-rate capability.

Kinetic Phase Evolution of Spinel Cobalt Oxide during Lithiation.

Spinel cobalt oxide has been proposed to undergo a multiple-step reaction during the electrochemical lithiation process. Understanding the kinetics of the lithiation process in this compound is crucial to optimize its performance and cyclability. In this work, we have utilized a low-angle annular dark-field scanning transmission electron microscopy method to visualize the dynamic reaction process in real time and study the reaction kinetics at different rates. We show that the particles undergo a two-step reaction at the single-particle level, which includes an initial intercalation reaction followed by a conversion reaction. At low rates, the conversion reaction starts after the intercalation reaction has fully finished, consistent with the prediction of density functional theoretical calculations. At high rates, the intercalation reaction is overwhelmed by the subsequently nucleated conversion reaction, and the reaction speeds of both the intercalation and conversion reactions are increased. Phase-field simulations show the crucial role of surface diffusion rates of lithium ions in controlling this process. This work provides microscopic insights into the reaction dynamics in non-equilibrium conditions and highlights the effect of lithium diffusion rates on the overall reaction homogeneity as well as the performance.

Interrogation of bimetallic particle oxidation in three dimensions at the nanoscale.

An understanding of bimetallic alloy oxidation is key to the design of hollow-structured binary oxides and the optimization of their catalytic performance. However, one roadblock encountered in studying these binary oxide systems is the difficulty in describing the heterogeneities that occur in both structure and chemistry as a function of reaction coordinate. This is due to the complexity of the three-dimensional mosaic patterns that occur in these heterogeneous binary systems. By combining real-time imaging and chemical-sensitive electron tomography, we show that it is possible to characterize these systems with simultaneous nanoscale and chemical detail. We find that there is oxidation-induced chemical segregation occurring on both external and internal surfaces. Additionally, there is another layer of complexity that occurs during the oxidation, namely that the morphology of the initial oxide surface can change the oxidation modality. This work characterizes the pathways that can control the morphology in binary oxide materials.