Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3 This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs. The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction

Enriching the functionality of ferroelectric materials with visible-light sensitivity and multiaxial switching capability would open up new opportunities for their applications in advanced information storage with diverse signal manipulation functions. We report experimental observations of robust intralayer ferroelectricity in two-dimensional (2D) van der Waals layered α-In2Se3 ultrathin flakes at room temperature. Distinct from other 2D and conventional ferroelectrics, In2Se3 exhibits intrinsically intercorrelated out-of-plane and in-plane polarization, where the reversal of the out-of-plane polarization by a vertical electric field also induces the rotation of the in-plane polarization. On the basis of the in-plane switchable diode effect and the narrow bandgap (∼1.3 eV) of ferroelectric In2Se3, a prototypical nonvolatile memory device, which can be manipulated both by electric field and visible light illumination, is demonstrated for advancing data storage technologies.

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Intercorrelated In-Plane and Out-of-Plane Ferroelectricity in Ultrathin Two-Dimensional Layered Semiconductor In2Se3.

We report on nanoscale strain gradients in ferroelectric HoMnO3 epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

/pdf/giant-flexoelectric-effect-in-ferroelectric-epitaxial-thin-24sbqwd74x.pdf

Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films

Ferroelectric materials have remained one of the major focal points of condensed matter physics and materials science for over 50 years. In the last 20 years, the development of voltage-modulated scanning probe microscopy techniques, exemplified by Piezoresponse force microscopy (PFM) and associated time- and voltage spectroscopies, opened a pathway to explore these materials on a single-digit nanometer level. Consequently, domain structures and walls and polarization dynamics can now be imaged in real space. More generally, PFM has allowed studying electromechanical coupling in a broad variety of materials ranging from ionics to biological systems. It can also be anticipated that the recent Nobel prize [“The Nobel Prize in Chemistry 2016,” http://www.nobelprize.org/nobel_prizes/chemistry/laureates/2016/ (Nobel Media, 2016)] in molecular electromechanical machines will result in rapid growth in interest in PFM as a method to probe their behavior on single device and device assembly levels. However, the br...

Ferroelectric or non-ferroelectric: Why so many materials exhibit "ferroelectricity" on the nanoscale

Flexoelectricity in solids: Progress, challenges, and perspectives

We report that a deionized water etching and thermal annealing technique can be effective for preparing atomically-flat and singly-terminated surfaces of single crystalline SrTiO3 substrates. After a two-step thermal-annealing and deionized-water etching procedure, topography measured by atomic force microscopy shows the evolution of substrates from a rough to step-terraced surface structure. Lateral force microscopy confirms that the atomically-flat surfaces are singly-terminated. Moreover, this technique can be used to remove excessive strontium oxide or hydroxide composites segregated on the SrTiO3 surface. This acid-etchant-free technique facilitates the preparation of atomically-aligned SrTiO3 substrates, which promotes studies on two-dimensional physics of complex oxide interfaces.

Preparation of atomically-flat SrTiO3 surfaces using a deionized-water etching and thermal annealing procedure

We report that a deionized water leaching and thermal annealing technique can be effective for preparing atomically flat and singly terminated surfaces of single crystalline SrTiO3 substrates. After a two-step thermal-annealing and deionized-water leaching procedure, topography measured by atomic force microscopy shows the evolution of substrates from a rough to step-terraced surface structure. Lateral force microscopy confirms that the atomically flat surfaces are singly terminated. Moreover, this technique can be used to remove excessive strontium oxide or hydroxide composites segregated on the SrTiO3 surface. This acid-etchant-free technique facilitates the preparation of atomically aligned SrTiO3 substrates, which promotes studies on two-dimensional physics of complex oxide interfaces.

Preparation of atomically flat SrTiO3 surfaces using a deionized-water leaching and thermal annealing procedure

Indium tin oxide (ITO) is one of the most widely used transparent conductors in optoelectronic device applications. We investigated the optical properties of ITO thin films at high temperatures up to 800 °C using spectroscopic ellipsometry. As temperature increases, amorphous ITO thin films undergo a phase transition at ~ 200 °C and develop polycrystalline phases with increased optical gap energies. The optical gap energies of both polycrystalline and epitaxial ITO thin films decrease with increasing temperature due to electron–phonon interactions. Depending on the background oxygen partial pressure, however, we observed that the optical gap energies exhibit reversible changes, implying that the oxidation and reduction processes occur vigorously due to the low oxidation and reduction potential energies of the ITO thin films at high temperatures. This result suggests that the electronic structure of ITO thin films strongly depends on temperature and oxygen partial pressure while they remain optically transparent, i.e., optical gap energies > 3.6 eV.

/pdf/high-temperature-optical-properties-of-indium-tin-oxide-thin-29ngexzyau.pdf

High-temperature optical properties of indium tin oxide thin-films.

Piezoresponse force microscopy imaging in conjunction with first-principles calculations provide strong evidence for room-temperature ferroelectricity in epitaxially stabilized hexagonal TbMnO3 thin films, which in the bulk form are with orthorhombic structure. The obtained results demonstrate that new phases and functional properties of complex oxide materials can be strain-engineered using epitaxial growth.

Room‐Temperature Ferroelectricity in Hexagonal TbMnO3 Thin Films

/pdf/room-temperature-ferroelectricity-in-hexagonal-tbmno-3-thin-11zpc779o7.pdf

J. G. Connell

Papers

Preparation of atomically-flat SrTiO3 surfaces using a deionized-water etching and thermal annealing procedure

Preparation of atomically flat SrTiO3 surfaces using a deionized-water leaching and thermal annealing procedure

High-temperature optical properties of indium tin oxide thin-films.

Room‐Temperature Ferroelectricity in Hexagonal TbMnO3 Thin Films

Room-Temperature Ferroelectricity in Hexagonal TbMnO$_{3}$ Thin Films