Electric field gradients and bipolar electrochemistry effects on neural growth : A finite element study on immersed electroactive conducting electrode materials

Acknowledgments This work was funded by the European Commission FP6 NEST Program (Contract 028473), RTI2018-097753, MAT2011-24363 and MAT2015-65192-R from the Spanish Science Ministry, La Marató de TV3 Foundation (Identification Number 110131), and Severo Ochoa Programme for Centres of Excellence in R&D (SEV-2015-0496). LI. Abad thanks MINECO for a Ramón y Cajal Contract (RYC-2013-12640). The authors also thank A. Beardo (NanoTransport group from UAB) for useful discussions.Peer reviewedPostprin

Surface behavior of La2/3Ca1/3MnO3 epitaxial thin films

The role of the surface layers in La2/3Ca1/3MnO3 magnetic oxide epitaxialthin films is analyzed. We show that the topmost layers do play a very relevant role on the transport properties acting as an insulating barrier. Atomic force microscopy(AFM)measurements in the current sensing mode exhibit typical features of tunneling conduction. The analysis of the I-V curves by using the Simmons model give values of barrier thickness in good agreement with nonmagnetic layer thickness estimates from magnetic measurements.Ex situannealing in air at high temperature clearly improve the magnetotransport properties of the films reducing the surface insulating barrier

Evidence of thermal transport anisotropy in stable glasses of vapour deposited organic molecules

Vapour-deposited organic glasses are currently in use in many optoelectronic devices. Their operation temperature is limited by the glass transition temperature of the organic layers and thermal management strategies become increasingly important to improve the lifetime of the device. Here we report the unusual finding that molecular orientation heavily influences heat flow propagation in glassy films of small molecule organic semiconductors. The thermal conductivity of vapour-deposited thin-film semiconductor glasses is anisotropic and controlled by the deposition temperature. We compare our data with extensive molecular dynamics simulations to disentangle the role of density and molecular orientation on heat propagation. Simulations do support the view that thermal transport along the backbone of the organic molecule is strongly preferred with respect to the perpendicular direction. This is due to the anisotropy of the molecular interaction strength that limit the transport of atomic vibrations. This approach could be used in future developments to implement small molecule glassy films in thermoelectric or other organic electronic devices.Comment: main manuscript: 17 pages and 7 figures; supplementary material: 6 pages and 7 figure

Oscillatory patterns in redox gradient materials through wireless bipolar electrochemistry. The dynamic wave-like case of copper bipolar oxidation

Altres ajuts: ICN2 is funded by the CERCA program/Generalitat de Catalunya.Bipolar electrochemistry allows the development of processes in a wireless manner, with reactions occurring at the induced anodes and cathodes of an immersed conducting material in the electrolyte. As a result, a gradient oxidation state may appear along the main axis field on the surface or bulk of the material depending on the type of reaction available at each induced potential. Redox intercalation gradients have been observed, metal anodization, or deposition, and also reactions at the electrolyte in the nearby environment of the poles induced. The complex oxidation of copper and interconversion between phases formed yields in this work an oscillating redox gradient, thanks to the great resistance change when the oxidized phases are formed. Parallel stripes containing mainly CuO, CuO, and Cu(OH) with large resistance are formed perpendicular to the electric field, forming a sequence of secondary dipoles in intermediate Cu stripes, that depends on the external voltage applied, and that oscillates in time at the same spatial coordinates. With longer times, copper solubilizes at the larger induced potential zones, probably as Cu(OH) . A simple finite element electrostatic model defines the complex potential waves induced in the piece. The resulting dynamics offer an example of the complexity of order in unwired conducting materials in wet media, either in catalysis, bioelectrodes, electronics, photovoltaics, or energy storage

Wireless magneto-ionics: voltage control of magnetism by bipolar electrochemistry

Modulation of magnetic properties through voltage-driven ion motion and redox processes, i.e., magneto-ionics, is a unique approach to control magnetism with electric field for low-power memory and spintronic applications. So far, magneto-ionics has been achieved through direct electrical connections to the actuated material. Here we evidence that an alternative way to reach such control exists in a wireless manner. Induced polarization in the conducting material immersed in the electrolyte, without direct wire contact, promotes wireless bipolar electrochemistry, an alternative pathway to achieve voltage-driven control of magnetism based on the same electrochemical processes involved in direct-contact magneto-ionics. A significant tunability of magnetization is accomplished for cobalt nitride thin films, including transitions between paramagnetic and ferromagnetic states. Such effects can be either volatile or non-volatile depending on the electrochemical cell configuration. These results represent a fundamental breakthrough that may inspire future device designs for applications in bioelectronics, catalysis, neuromorphic computing, or wireless communications.Comment: 32 pages, 4 figures, Supplementary Information (9 figures

Regulating oxygen ion transport at the nanoscale to enable highly cyclable magneto-ionic control of magnetism

Altres ajuts: acords transformatius de la UABMagneto-ionics refers to the control of magnetic properties of materials through voltage-driven ion motion. To generate effective electric fields, either solid or liquid electrolytes are utilized, which also serve as ion reservoirs. Thin solid electrolytes have difficulties to (i) withstand high electric fields without electric pinholes and (ii) maintain stable ion transport during long-term actuation. In turn, the use of liquid electrolytes can result in poor cyclability, thus limiting their applicability. Here we propose a nanoscale-engineered magneto-ionic architecture (comprising a thin solid electrolyte in contact with a liquid electrolyte), that drastically enhances cyclability while preserving sufficiently high electric fields to trigger ion motion. Specifically, we show that the insertion of a highly nanostructured (amorphous-like) Ta layer (with suitable thickness and electric resistivity) between a magneto-ionic target material (i.e., Co3O4) and the liquid electrolyte, increases magneto-ionic cyclability from < 30 cycles (when no Ta is inserted) to more than 800 cycles. Transmission electron microscopy together with variable energy positron annihilation spectroscopy reveal the crucial role of the generated TaOx interlayer as a solid-electrolyte (i.e., ionic conductor) that improves magneto-ionic endurance by proper tuning of the types of voltage-driven structural defects. The Ta layer is very effective in trapping oxygen and hindering O2- ions from moving into the liquid electrolyte, thus keeping O2- motion mainly restricted between Co3O4 and Ta when voltage of alternating polarity is applied. We demonstrate that this approach provides a suitable strategy to boost magneto-ionics by combining the benefits of solid and liquid electrolytes in a synergetic manner

Magneto-ionics in single-layer transition metal nitrides

Altres ajuts: Acord transformatiu CRUE-CSICMagneto-ionics allows for tunable control of magnetism by voltage-driven transport of ions, traditionally oxygen or lithium and, more recently, hydrogen, fluorine, or nitrogen. Here, magneto-ionic effects in single-layer iron nitride films are demonstrated, and their performance is evaluated at room temperature and compared with previously studied cobalt nitrides. Iron nitrides require increased activation energy and, under high bias, exhibit more modest rates of magneto-ionic motion than cobalt nitrides. Ab initio calculations reveal that, based on the atomic bonding strength, the critical field required to induce nitrogen-ion motion is higher in iron nitrides (≈6.6 V nm -1) than in cobalt nitrides (≈5.3 V nm -1). Nonetheless, under large bias (i.e., well above the magneto-ionic onset and, thus, when magneto-ionics is fully activated), iron nitride films exhibit enhanced coercivity and larger generated saturation magnetization, surpassing many of the features of cobalt nitrides. The microstructural effects responsible for these enhanced magneto-ionic effects are discussed. These results open up the potential integration of magneto-ionics in existing nitride semiconductor materials in view of advanced memory system architectures

Boosting room-temperature magneto-ionics in a non-magnetic oxide semiconductor

Voltage control of magnetism through electric field-induced oxygen motion (magneto-ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in magnetically actuated devices. Boosting the induced changes in magnetization, magneto-ionic rates and cyclability continue to be key challenges to turn magneto-ionics into real applications. Here, it is demonstrated that room-temperature magneto-ionic effects in electrolyte-gated paramagnetic Co3O4 films can be largely increased both in terms of generated magnetization (6 times larger) and speed (35 times faster) if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer) instead of placing the electric contacts at the side of the semiconductor (electric-double-layer transistor-like configuration). This is due to the greater uniformity and strength of the electric field in the capacitor design. These results are appealing to widen the use of ion migration in technological applications such as neuromorphic computing or iontronics in general

Regulating Oxygen Ion Transport at the Nanoscale to Enable Highly Cyclable Magneto-Ionic Control of Magnetism

Magneto-ionics refers to the control of magnetic properties of materials through voltage-driven ion motion. To generate effective electric fields, either solid or liquid electrolytes are utilized, which also serve as ion reservoirs. Thin solid electrolytes have difficulties in (i) withstanding high electric fields without electric pinholes and (ii) maintaining stable ion transport during long-term actuation. In turn, the use of liquid electrolytes can result in poor cyclability, thus limiting their applicability. Here we propose a nanoscale-engineered magneto-ionic architecture (comprising a thin solid electrolyte in contact with a liquid electrolyte) that drastically enhances cyclability while preserving sufficiently high electric fields to trigger ion motion. Specifically, we show that the insertion of a highly nanostructured (amorphous-like) Ta layer (with suitable thickness and electric resistivity) between a magneto-ionic target material (i.e., Co3O4) and the liquid electrolyte increases magneto-ionic cyclability from <30 cycles (when no Ta is inserted) to more than 800 cycles. Transmission electron microscopy together with variable energy positron annihilation spectroscopy reveals the crucial role of the generated TaOx interlayer as a solid electrolyte (i.e., ionic conductor) that improves magneto-ionic endurance by proper tuning of the types of voltage-driven structural defects. The Ta layer is very effective in trapping oxygen and hindering O2- ions from moving into the liquid electrolyte, thus keeping O2- motion mainly restricted between Co3O4 and Ta when voltage of alternating polarity is applied. We demonstrate that this approach provides a suitable strategy to boost magneto-ionics by combining the benefits of solid and liquid electrolytes in a synergetic manner.Financial support by the European Union’s Horizon 2020 Research and Innovation Programme (“BeMAGIC” European Training Network, ETN/ITN Marie Skłodowska–Curie Grant No. 861145), the European Research Council (2021-ERC-Advanced “REMINDS” Grant No. 101054687), the Spanish Government (CEX2019-000917-S y PID2021-123276OB-I00, PID2020-116844RB-C21, and PDC2021-121276-C31), and the Generalitat de Catalunya (2021-SGR-00651) is acknowledged. J.S. thanks the Spanish “Fábrica Nacional de Moneda y Timbre” (FNMT) for fruitful discussions. E.M. is a Serra Húnter Fellow. Parts of this research were carried out at ELBE at the Helmholtz-Zentrum Dresden - Rossendorf e. V., a member of the Helmholtz Association. We would like to thank the facility staff for assistance. This work was partially supported by the Impulse-und Net-working fund of the Helmholtz Association (FKZ VH-VI-442 Memriox) and the Helmholtz Energy Materials Characterization Platform (03ET7015).With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe