Alkaline Earth Bismuth Fluorides as Fluoride-Ion Battery Electrolytes.

Fluoride-ion batteries have several potential advantages over lithium-ion batteries. Materials development is still needed, however, to realize electrolytes with sufficiently high anion conductivity and compatibility with anode and cathode layers. Fluoride compounds are difficult to synthesize directly as single crystals but can be realized from oxide film precursors via topotactic chemistry techniques.

3D oxygen vacancy distribution and defect-property relations in an oxide heterostructure.

Oxide heterostructures exhibit a vast variety of unique physical properties. Examples are unconventional superconductivity in layered nickelates and topological polar order in (PbTiO)/(SrTiO) superlattices. Although it is clear that variations in oxygen content are crucial for the electronic correlation phenomena in oxides, it remains a major challenge to quantify their impact.

Composite Spin Hall Conductivity from Non-Collinear Antiferromagnetic Order.

Non-collinear antiferromagnets (AFMs) are an exciting new platform for studying intrinsic spin Hall effects (SHEs), phenomena that arise from the materials' band structure, Berry phase curvature, and linear response to an external electric field. In contrast to conventional SHE materials, symmetry analysis of non-collinear antiferromagnets does not forbid non-zero longitudinal and out-of-plane spin currents with polarization and predicts an anisotropy with current orientation to the magnetic lattice. Here, multi-component out-of-plane spin Hall conductivities are reported in L1 -ordered antiferromagnetic PtMn thin films that are uniquely generated in the non-collinear state.

Limits to the strain engineering of layered square-planar nickelate thin films.

The layered square-planar nickelates, NdNiO, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in NdNiO thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, NdNiO, and subsequent reduction to the square-planar phase, NdNiO. We synthesize our highest quality NdNiO films under compressive strain on LaAlO (001), while NdNiO on NdGaO (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties.

Liberating a hidden antiferroelectric phase with interfacial electrostatic engineering.

Antiferroelectric materials have seen a resurgence of interest because of proposed applications in a number of energy-efficient technologies. Unfortunately, relatively few families of antiferroelectric materials have been identified, precluding many proposed applications. Here, we propose a design strategy for the construction of antiferroelectric materials using interfacial electrostatic engineering.

Superconductivity in a quintuple-layer square-planar nickelate.

Since the discovery of high-temperature superconductivity in copper oxide materials, there have been sustained efforts to both understand the origins of this phase and discover new cuprate-like superconducting materials. One prime materials platform has been the rare-earth nickelates and, indeed, superconductivity was recently discovered in the doped compound NdSrNiO (ref. ).

Dimensionality-Induced Change in Topological Order in Multiferroic Oxide Superlattices.

We construct ferroelectric (LuFeO_{3})_{m}/(LuFe_{2}O_{4}) superlattices with varying index m to study the effect of confinement on topological defects. We observe a thickness-dependent transition from neutral to charged domain walls and the emergence of fractional vortices. In thin LuFeO_{3} layers, the volume fraction of domain walls grows, lowering the symmetry from P6_{3}cm to P3c1 before reaching the nonpolar P6_{3}/mmc state, analogous to the group-subgroup sequence observed at the high-temperature ferroelectric to paraelectric transition.

Enhanced Electrical Resistivity and Properties via Ion Bombardment of Ferroelectric Thin Films.

A novel approach to on-demand improvement of electronic properties in complex-oxide ferroelectrics is demonstrated whereby ion bombardment - commonly used in classic semiconductor materials - is applied to the PbTiO system. The result is deterministic reduction in leakage currents by 5 orders of magnitude, improved ferroelectric switching, and unprecedented insights into the nature of defects and intergap state evolution in these materials.

Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic.

Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications.

High-quality EuO thin films the easy way via topotactic transformation.

Epitaxy is widely employed to create highly oriented crystalline films. A less appreciated, but nonetheless powerful means of creating such films is via topotactic transformation, in which a chemical reaction transforms a single crystal of one phase into a single crystal of a different phase, which inherits its orientation from the original crystal. Topotactic reactions may be applied to epitactic films to substitute, add or remove ions to yield epitactic films of different phases.

Magnetic structure and ordering of multiferroic hexagonal LuFeO_{3}.

We report on the magnetic structure and ordering of hexagonal LuFeO_{3} films of variable thickness grown by molecular-beam epitaxy on YSZ (111) and Al_{2}O_{3} (0001) substrates. These crystalline films exhibit long-range structural uniformity dominated by the polar P6_{3}cm phase, which is responsible for the paraelectric to ferroelectric transition that occurs above 1000 K. Using bulk magnetometry and neutron diffraction, we find that the system orders into a ferromagnetically canted antiferromagnetic state via a single transition below 155 K regardless of film thickness, which is substantially lower than that previously reported in hexagonal LuFeO_{3} films.

Controlling band alignments by artificial interface dipoles at perovskite heterointerfaces.

The concept 'the interface is the device' is embodied in a wide variety of interfacial electronic phenomena and associated applications in oxide materials, ranging from catalysts and clean energy systems to emerging multifunctional devices. Many device properties are defined by the band alignment, which is often influenced by interface dipoles. On the other hand, the ability to purposefully create and control interface dipoles is a relatively unexplored degree of freedom for perovskite oxides, which should be particularly effective for such ionic materials.

Visualizing the interfacial evolution from charge compensation to metallic screening across the manganite metal-insulator transition.

Electronic changes at polar interfaces between transition metal oxides offer the tantalizing possibility to stabilize novel ground states yet can also cause unintended reconstructions in devices. The nature of these interfacial reconstructions should be qualitatively different for metallic and insulating films as the electrostatic boundary conditions and compensation mechanisms are distinct. Here we directly quantify with atomic-resolution the charge distribution for manganite-titanate interfaces traversing the metal-insulator transition.

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics.

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects.

Tuning oxygen reduction reaction activity via controllable dealloying: a model study of ordered Cu3Pt/C intermetallic nanocatalysts.

A promising electrocatalyst prototype of low Pt mole fraction, intermetallic nanoparticles of Cu(3)Pt, has been prepared using a simple impregnation-reduction method, followed by a post heat-treatment. Two dealloying methods (electrochemical and chemical) were implemented to control the atomic-level morphology and improve performance for the oxygen reduction reaction (ORR). The morphology and elemental composition of the dealloyed nanoparticles were characterized at angstrom resolution using an aberration-corrected scanning transmission electron microscope equipped with an electron energy loss spectrometer.

Quantum many-body interactions in digital oxide superlattices.

Controlling the electronic properties of interfaces has enormous scientific and technological implications and has been recently extended from semiconductors to complex oxides that host emergent ground states not present in the parent materials. These oxide interfaces present a fundamentally new opportunity where, instead of conventional bandgap engineering, the electronic and magnetic properties can be optimized by engineering quantum many-body interactions. We use an integrated oxide molecular-beam epitaxy and angle-resolved photoemission spectroscopy system to synthesize and investigate the electronic structure of superlattices of the Mott insulator LaMnO(3) and the band insulator SrMnO(3).

Data processing for atomic resolution electron energy loss spectroscopy.

The high beam current and subangstrom resolution of aberration-corrected scanning transmission electron microscopes has enabled electron energy loss spectroscopy (EELS) mapping with atomic resolution. These spectral maps are often dose limited and spatially oversampled, leading to low counts/channel and are thus highly sensitive to errors in background estimation. However, by taking advantage of redundancy in the dataset map, one can improve background estimation and increase chemical sensitivity.

Three-dimensional tracking and visualization of hundreds of Pt-Co fuel cell nanocatalysts during electrochemical aging.

We present an electron tomography method that allows for the identification of hundreds of electrocatalyst nanoparticles with one-to-one correspondence before and after electrochemical aging. This method allows us to track, in three-dimensions, the trajectories and morphologies of each Pt-Co nanocatalyst on a fuel cell carbon support. In conjunction with the use of atomic-scale electron energy loss spectroscopic imaging, our experiment enables the correlation of performance degradation of the catalyst with changes in particle/interparticle morphologies, particle-support interactions, and the near-surface chemical composition.

Atomic-resolution spectroscopic imaging of ensembles of nanocatalyst particles across the life of a fuel cell.

The thousand-fold increase in data-collection speed enabled by aberration-corrected optics allows us to overcome an electron microscopy paradox: how to obtain atomic-resolution chemical structure in individual nanoparticles yet record a statistically significant sample from an inhomogeneous population. This allowed us to map hundreds of Pt-Co nanoparticles to show atomic-scale elemental distributions across different stages of the catalyst aging in a proton-exchange-membrane fuel cell, and relate Pt-shell thickness to treatment, particle size, surface orientation, and ordering.