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Ionic Conductivity Increased by Two Orders of Magnitude in Micrometer-Thick Vertical Yttria-Stabilized ZrO 2 Nanocomposite Films

Shinbuhm Lee, Wenrui Zhang, Fauzia Khatkhatay, Haiyan Wang, Quanxi Jia, Judith L. MacManus-Driscoll

Nano Letters, vol. 15, no. 11, pp. 7362-7369

We design and create a unique cell geometry of templated micrometer-thick epitaxial nanocomposite films which contain ∼20 nm diameter yttria-stabilized ZrO2 (YSZ) nanocolumns, strain coupled to a SrTiO3 matrix. The ionic conductivity of these nanocolumns is enhanced by over 2 orders of magnitude compared to plain YSZ films. Concomitant with the higher ionic conduction is the finding that the YSZ nanocolumns in the films have much higher crystallinity and orientation, compared to plain YSZ films. Hence, “oxygen migration highways” are formed in the desired out-of-plane direction. This improved structure is shown to originate from the epitaxial coupling of the YSZ nanocolumns to the SrTiO3 film matrix and from nucleation of the YSZ nanocolumns on an intermediate nanocomposite base layer of highly aligned Sm-doped CeO2 nanocolumns within the SrTiO3 matrix. This intermediate layer reduces the lattice mismatch between the YSZ nanocolumns and the substrate. Vertical ionic conduction values as high as 10–2 Ω–1 cm–1 were demonstrated at 360 °C (300 °C lower than plain YSZ films), showing the strong practical potential of these nanostructured films for use in much lower operation temperature ionic devices.


Strongly enhanced oxygen ion transport through samarium-doped CeO2 nanopillars in nanocomposite films

Sang Mo Yang, Shinbuhm Lee, Jie Jian, Wenrui Zhang, Ping Lu, Quanxi Jia, Haiyan Wang, Tae Won Noh, Sergei V. Kalinin, Judith L. MacManus‐Driscoll

Nature Communications, vol. 6, p. 8588

Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500°C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO2 embedded in supporting matrices of SrTiO3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness.

Research Update: Atmospheric pressure spatial atomic layer deposition of ZnO thin films: Reactors, doping, and devices

Robert L. Z. Hoye, David Muñoz-Rojas, Shelby F. Nelson, Andrea Illiberi, Paul Poodt, Fred Roozeboom, Judith L. MacManus-Driscoll

APL Materials, vol. 3, no. 4, p. 040701

Atmospheric pressure spatial atomic layer deposition (AP-SALD) has recently emerged as an appealing technique for rapidly producing high quality oxides. Here, AP-SALD was used to deposit functional, doped ZnO thin films. We highlight how these films are advantageous for the performance of solar cells, organometal halide perovskite light emitting diodes, and thin-film transistors. Future AP-SALD technology will enable the commercial processing of thin films over large areas on a sheet-to-sheet and roll-to-roll basis, with new reactor designs emerging for flexible plastic and paper electronics.

Enhanced Performance in Fluorene-Free Organometal Halide Perovskite Light-Emitting Diodes using Tunable, Low Electron Affinity Oxide Electron Injectors

Robert L. Z. Hoye, Matthew R. Chua, Kevin P. Musselman, Guangru Li, May-Ling Lai, Zhi-Kuang Tan, Neil C. Greenham, Judith L. MacManus-Driscoll, Richard H. Friend, Dan Credgington

Advanced Materials, vol. 27, no. 8, pp. 1414-1419

Fluorene-free perovskite light-emitting diodes (LEDs) with low turn-on voltages, higher luminance and sharp, color-pure electroluminescence were obtained by replacing the F8 electron injector with ZnO, which is directly deposited onto the CH3NH3PbBr3 perovskite using spatial atmospheric atomic layer deposition. The electron injection barrier was reduced by decreasing the ZnO electron affinity through Mg incorporation, leading to lower turn-on voltages.

Novel Electroforming-Free Nanoscaffold Memristor with Very High Uniformity, Tunability, and Density

Shinbuhm Lee, Abhijeet Sangle, Ping Lu, Aiping Chen, Wenrui Zhang, Jae Sung Lee, Haiyan Wang, Quanxi Jia, Judith L. MacManus-Driscoll

Advanced Materials, vol. 26, no. 36, pp. 6284-6289

 

Dynamical tuning of the concentration of defects in oxides provides a route to controlling new functionalities. The chemical potential to capture the functionalities driven by mobile ions and defects can be one of key control parameters (as well as electric field, magnetic field, and stress) for tuning the functionality of complex oxides. Interesting signatures related to oxygen vacancies have been explicitly observed in widespread physical applications, including solid oxide fuel cells, catalysts, optoelectronics, and electronics. Here, in very simple, self-assembled nanoscaffold films containing nanocolumns with ~10-nm-radius and ~10-nm-intercolumnar-spacing, we demonstrate electroforming-free reversible electroresistance at room temperature. The nanoscaffold films are very easy-to-grow, since they self-assemble to give vertical heterointerfaces with Vo¨ channels along the interfaces. The structure has a clear advantage over conventional multilayers in multifunctional device nanoengineering

Schematic diagrams of device structures to generate Vo¨. (a) Irreversible electroforming with application of a high electrical stimulus to single-phase oxides (b) Conventional single-phase oxide film fractionally substituted with dopants. (c) Conventional multilayer film causing oxygen disorder at the lateral interfaces of dissimilar crystal structures. (d) Nanoscaffold film causing oxygen defect formation at the vertical interfaces of dissimilar crystal structures. Bottom figure shows strong retention of resistive switching behaviour of composite films. In comparison the plain films do not show switching because nanoionic channels are not present.


Caloric materials near ferroic phase transitions

X. Moya, S. Kar-Narayan, N. D. Mathur

Nature Materials, vol. 13, no. 5, pp. 439-450

This Review Article presents the first thorough comparison of research into materials that get hot and cold due to changes of magnetic field near ferromagnetic phase transitions, with the growing bodies of analogous research into materials that get hot and cold due to changes of electric field near ferroelectric phase transitions, and materials that get hot and cold due to changes of stress field near structural phase transitions. We cross-fertilize the resulting magnetocaloric (MC), electrocaloric (EC) and mechanocaloric (mC) effects by contrasting experimental methods and performance, and we present a historical perspective that dates back to the Battle of Trafalgar and includes secret developments from opposing sides in World War II.

Prospective cooling applications are discussed critically based on industrial input. The relevant materials parameters of adiabatic temperature change |ΔT| and isothermal heat |Q| are plotted here for the three different types of caloric material [mC effects are sub-divided into elastocaloric (eC) and barocaloric (BC) effects, which arise due to uniaxial and isotropic stress fields, respectively].

A Scalable Nanogenerator Based on Self-Poled Piezoelectric Polymer Nanowires with High Energy Conversion Efficiency

Richard A. Whiter, Vijay Narayan, Sohini Kar-Narayan

Advanced Energy Materials, vol. 4, no. 18, p. 1400519

Nanogenerators based on piezoelectric materials can convert ever-present mechanical vibrations into electrical power for energetically autonomous wireless and electronic devices. Nanowires of piezoelectric polymers are particularly attractive for harvesting mechanical energy in this way, as they are flexible, lightweight and sensitive to small vibrations. Previous studies have focused exclusively on nanowires grown by electrospinning, but this involves complex equipment, and high voltages of ~10 kV that electrically pole the nanowires and thus render them piezoelectric. In the present paper, nanowires of poly(vinylidene fluoride‑trifluoroethylene) [P(VDF‑TrFE)], grown using a simple, scalable, and cost-effective template-wetting technique, are shown to be successfully exploited in high-output nanogenerators without the need for electrical poling. The “self-poled” nanowires reported here exhibit high mechanical-to-electrical conversion efficiency comparable to the best previously reported values. This work therefore offers a scalable means of achieving high‑performance nanogenerators for the next generation of self-powered electronics, and also for the development of low-cost strain sensors in applications ranging from biomedicine to robotics.


Controlled Injection of Spin-Triplet Supercurrents into a Strong Ferromagnet

J. W. A. Robinson, J. D. S. Witt, M. G. Blamire

Science, vol. 329, no. 5987, pp. 59-61

The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors.

Ferroelectric Control of Spin Polarization

V. Garcia, M. Bibes, L. Bocher, S. Valencia, F. Kronast, A. Crassous, X. Moya, S. Enouz-Vedrenne, A. Gloter, D. Imhoff, C. Deranlot, N. D. Mathur, S. Fusil, K. Bouzehouane, A. Barthelemy

Science, vol. 327, no. 5969, pp. 1106-1110

A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics, which relies on "zero-current," gate-controlled operations. Efforts have been made to control the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. We used ferroelectric tunnel junctions with ferromagnetic electrodes to demonstrate local, large, and nonvolatile control of carrier spin polarization by electrically switching ferroelectric polarization. Our results represent a giant type of interfacial magnetoelectric coupling and suggest a low-power approach for spin-based information control.


Spin-filter Josephson junctions

Kartik Senapati, Mark G. Blamire, Zoe H. Barber

Nature Materials, vol. 10, no. 11, pp. 849-852

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years. Of particular interest has been the realization of so called ?-junctions with a built-in phase difference, and induced triplet pairing. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (IF) have been predicted to show a range of unique properties including ?-shifts with intrinsically low dissipation and an unconventional temperature dependence of the critical current Ic, difficulties with the few known IF materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of Ic is strongly modified by the IF. In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the IF barrier can be modified by magnetic inhomogeneity in the barrier
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