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  1. Quantitative Measurement of Li-Ion Concentration and Diffusivity in Solid-State Electrolyte

    Here, we present a quantitative method to measure the concentration and diffusivity of Li ion in a solid-state electrolyte with nanoscale depth resolution. We designed a standard sample with different depths of trenches and used time-of-flight secondary ion mass spectroscopy, inductively coupled plasma optical emission spectroscopy, three-dimensional (3D) optical microscopy, and electrochemical strain microscopy to obtain effective Li-ion concentration and the corresponding diffusivity as a function of depth. As a result, we verified the dependence of electromechanical strain induced by ionic oscillation on mobile-ion concentration. We also showed the diffusivity change as a function of effective concentration and discussed themore » difference in diffusivity near the surface and inside the bulk. Our method can be widely used for electromechanical strain in various ionic conductors, rendering our technique universal for quantitative analysis of ionic dynamics at a multiscale.« less
  2. Reducing Time to Discovery: Materials and Molecular Modeling, Imaging, Informatics, and Integration

    Multiscale and multimodal imaging of material structures and properties provides solid ground on which materials theory and design can flourish. Recently, KAIST announced 10 flagship research fields, which include KAIST Materials Revolution: Materials and Molecular Modeling, Imaging, Informatics and Integration (M3I3). The M3I3 initiative aims to reduce the time for the discovery, design and development of materials based on elucidating multiscale processing–structure–property relationship and materials hierarchy, which are to be quantified and understood through a combination of machine learning and scientific insights. In this review, we begin by introducing recent progress on related initiatives around the globe, such as themore » Materials Genome Initiative (U.S.), Materials Informatics (U.S.), the Materials Project (U.S.), the Open Quantum Materials Database (U.S.), Materials Research by Information Integration Initiative (Japan), Novel Materials Discovery (E.U.), the NOMAD repository (E.U.), Materials Scientific Data Sharing Network (China), Vom Materials Zur Innovation (Germany), and Creative Materials Discovery (Korea), and discuss the role of multiscale materials and molecular imaging combined with machine learning in realizing the vision of M3I3. Specifically, microscopies using photons, electrons, and physical probes will be revisited with a focus on the multiscale structural hierarchy, as well as structure–property relationships. Additionally, data mining from the literature combined with machine learning will be shown to be more efficient in finding the future direction of materials structures with improved properties than the classical approach. Examples of materials for applications in energy and information will be reviewed and discussed. A case study on the development of a Ni–Co–Mn cathode materials illustrates M3I3’s approach to creating libraries of multiscale structure–property–processing relationships. We end with a future outlook toward recent developments in the field of M3I3.« less
  3. Modulation of oxygen vacancies assisted ferroelectric and photovoltaic properties of (Nd, V) co-doped BiFeO 3 thin films

    We are reporting on the improved ferroelectric and photovoltaic properties of (Nd3+, V5+) co-doped (Bi0.95Nd0.05)(Fe-1 V-x(x))O-3 (BNFVO) (x = 0.01, 0.03) thin films grown by PLD. BNFVO thin films showed reduced leakage current, lower optical bandgap, and improved ferroelectricity compared to BFO films, which can be explained by valance change and extinguishing oxygen vacancies due to the doping effect. Piezoresponse force microscopy measurements showed an improved domain back switching in doped thin films indicating that the suppression of oxygen vacancies offset the effect of polarization flipping caused by doping. Further, we found a relatively stable and enhanced photovoltaic effect inmore » BNFVO films with an order of magnitude higher photocurrent and almost doubled photovoltage in comparison to BFO films, which can be explained by less recombination between hopping electrons and oxygen vacancies. Our results demonstrate the significance of dopant selection to suppress the oxygen vacancies for improved ferroelectric and photovoltaic properties of BFO films.« less
  4. Tuning piezoelectric properties through epitaxy of La2Ti2O7 and related thin films

    Current piezoelectric sensors and actuators are limited to operating temperatures less than ~200°C due to the low Curie temperature of the piezoelectric material. High temperature piezoelectric materials such as La2Ti2O7 (LTO) would facilitate the development of high-temperature sensors if the piezoelectric coupling coefficient could be maximized. We have deposited epitaxial LTO films on SrTiO3(001), SrTiO3(110), and rutile TiO2(110) substrates by pulsed laser deposition, and show that the crystalline orientation of the LTO film, and thus its piezoelectric coupling direction, can be controlled by epitaxial matching to the substrate. The structure and phase purity of the films were investigated by x-raymore » diffraction and scanning transmission electron microscopy. To characterize the piezoelectric properties, piezoresponse force microscopy was used to measure the in-plane and out-of-plane piezoelectric coupling in the films. We find that the strength of the out-of-plane piezoelectric coupling can be increased when the piezoelectric crystalline direction is rotated partially out-of-plane via epitaxy. The strongest out-of-plane coupling is observed for LTO/STO(001). Deposition on TiO2(110) results in epitaxial La2/3TiO3, an orthorhombic perovskite of interest as a microwave dielectric material. La2/3TiO3 can be difficult to stabilize in bulk form, and epitaxial deposition has not been previously reported. These results confirm that control of the crystalline orientation of LTO-based materials can increase the out-of-plane strength of its piezoelectric coupling, which can be exploited in piezoelectric devices.« less
  5. Effect of nanopatterning on mechanical properties of Lithium anode

    One of the challenges in developing Lithium anodes for Lithium ion batteries (LIB) is controlling the formation of Li dendrites during cycling of the battery. Nanostructuring and nanopatterning of electrodes shows a promising way to suppress the growth of Li dendrites. However, in order to control this behavior, a fundamental understanding of the effect of nanopatterning on the electromechanical properties of Li metal is necessary. In this paper, we have investigated the mechanical and wear properties of Li metal using Atomic Force Microscopy (AFM) in an airtight cell. By using different load regimes, we determined the mechanical properties of Limore » metal. Here, we show that as a result of nanopatterning, Li metal surface underwent work hardening due to residual compressive stress. The presence of such stresses can help to improve cycle lifetime of LIBs with Li anodes and obtain very high energy densities.« less
  6. Long-range stripe nanodomains in epitaxial (110) BiFeO3 thin films on (100) NdGaO3 substrate

    Here, we report the observation of ferroelectric and ferroelastic nanodomains in (110)-oriented BiFeO3 (BFO) thin films epitaxially grown on low symmetric (100) NdGaO3 (NGO) substrate. We observed long range ordering of ferroelectric 109° stripe nanodomains separated by periodic vertical domain walls in as-grown 130 nm thick BFO films. The effect of La0.67Sr0.33CoO3 (LSCO) conducting interlayer on domain configurations in BFO/NGO film was also observed with relatively short range-ordering of stripe domains due to the modified electrostatic boundary conditions in BFO/LSCO/NGO film. Additional studies on B-site doping of Nb ions in BFO films showed change in the domain structures due tomore » doping induced change in lattice anisotropy while maintaining the stripe domain morphology with 109° domain wall. Finally, this long-range array of ferroelectric and ferroelastic domains can be useful for optoelectronic devices and ferroelastic templates for strain coupled artificial magnetoelectric heterostructures.« less
  7. Tuning piezoelectric properties through epitaxy of La2Ti2O7 and related thin films

    Current piezoelectric sensors and actuators are limited to operating temperatures less than ~200°C due to the low Curie temperature of the piezoelectric material. High temperature piezoelectric materials such as La2Ti2O7 (LTO) would facilitate the development of high-temperature sensors if the piezoelectric coupling coefficient could be maximized. We have deposited epitaxial LTO films on SrTiO3(001), SrTiO3(110), and rutile TiO2(110) substrates by pulsed laser deposition, and show that the crystalline orientation of the LTO film, and thus its piezoelectric coupling direction, can be controlled by epitaxial matching to the substrate. The structure and phase purity of the films were investigated by x-raymore » diffraction and scanning transmission electron microscopy. To characterize the piezoelectric properties, piezoresponse force microscopy was used to measure the in-plane and out-of-plane piezoelectric coupling in the films. We find that the strength of the out-of-plane piezoelectric coupling can be increased when the piezoelectric crystalline direction is rotated partially out-of-plane via epitaxy. The strongest out-of-plane coupling is observed for LTO/STO(001). Deposition on TiO2(110) results in epitaxial La2/3TiO3, an orthorhombic perovskite of interest as a microwave dielectric material. La2/3TiO3 can be difficult to stabilize in bulk form, and epitaxial deposition has not been previously reported. These results confirm that control of the crystalline orientation of LTO-based materials can increase the out-of-plane strength of its piezoelectric coupling, which can be exploited in piezoelectric devices.« less
  8. Ferroelectric Domain Studies of Patterned (001) BiFeO3 by Angle-Resolved Piezoresponse Force Microscopy

    We have studied the ferroelectric domains in (001) BiFeO3 (BFO) films patterned into mesas with various aspect ratios, using angle-resolved piezoresponse force microscope (AR-PFM), which can image the in-plane polarization component with an angular resolution of 30 degrees. We observed not only stable polarization variants, but also meta-stable polarization variants, which can reduce the charge accumulated at domain boundaries. We considered the number of neighboring domains that are in contact, in order to analyze the complexity of the ferroelectric domain structure. Comparison of the ferroelectric domains from the patterned and unpatterned regions showed that the elastic relaxation induced by removalmore » of the film surrounding the mesas led to a reduction of the average number of neighboring domains, indicative of a decrease in domain complexity. Finally, we also found that the rectangular BFO patterns with high aspect ratio had a simpler domain configuration and enhanced piezoelectric characteristics than square-shaped mesas. Manipulation of the ferroelectric domains by controlling the aspect ratio of the patterned BFO thin film mesas can be useful for nanoelectronic applications.« less
  9. Interfacial stability of ultrathin films of magnetite Fe3O4 (111) on Al2O3(001) grown by ozone-assisted molecular-beam epitaxy

    Thin films of iron oxides including magnetite (Fe3O4) and hematite (α-Fe2O3) have many important applications. Both forms of oxide can occur naturally during film growth by iron deposition under various oxidation environment; an important issue is to understand and control the process resulting in a single-phase film. We have performed in-situ real-time studies using x-ray diffraction of such film growth on sapphire (001) under pure ozone by monitoring the (00L) rod. Stable magnetite growth can be maintained at growth temperatures below 600° C up to a certain critical film thickness, beyond which the growth becomes hematite. The results demonstrate themore » importance of interfacial interaction in stabilizing the magnetite phase.« less
  10. Zinc Adsorption and Hydration Structures at Yttria-Stabilized Zirconia Surfaces

    Zinc adsorption and interfacial hydration on yttria-stabilized zirconia (YSZ) surfaces in contact with aqueous zinc solutions at room temperature and neutral pH have been probed, with combined specular high-resolution X-ray reflectivity and element-specific (Zn) resonant anomalous X-ray reflectivity techniques. The total and partial zinc-Specific electron density profiles in the surface normal direction show the detailed interfacial hydration structures with zinc adsorption: Strongly depending on its crystallographic orientations, the YSZ (110) surface adsorbs zinc species only within adsorbed water layers above the terminal plane, while on (111) surface, zinc further-penetrates the substrate (below the terminal plane). Considering that both surfaces aremore » enriched with oxygen vacancies and metal-depleted sites; on which chemisorbed water species are expected, the observed contrast indicates that specific zinc adsorption is controlled strongly by the intrinsic surface chemistry that results from orientation-dependent interfacial structures.« less
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