Hydrogen-Induced Reduction Improves the Photoelectrocatalytic Performance of Titania.

One of the main challenges to expand the use of titanium dioxide (titania) as a photocatalyst is related to its large band gap energy and the lack of an atomic scale description of the reduction mechanisms that may tailor the photocatalytic properties. We show that rutile TiO single crystals annealed in the presence of atomic hydrogen experience a strong reduction and structural rearrangement, yielding a material that exhibits enhanced light absorption, which extends from the ultraviolet to the near-infrared (NIR) spectral range, and improved photoelectrocatalytic performance. We demonstrate that both magnitudes behave oppositely: heavy/mild plasma reduction treatments lead to large/negligible spectral absorption changes and poor/enhanced (×10) photoelectrocatalytic performance, as judged from the higher photocurrent.

A 2D ferroelectric vortex pattern in twisted BaTiO freestanding layers.

The wealth of complex polar topologies recently found in nanoscale ferroelectrics results from a delicate balance between the intrinsic tendency of the materials to develop a homogeneous polarization and the electric and mechanical boundary conditions imposed on them. Ferroelectric-dielectric interfaces are model systems in which polarization curling originates from open circuit-like electric boundary conditions, to avoid the build-up of polarization charges through the formation of flux-closure domains that evolve into vortex-like structures at the nanoscale level. Although ferroelectricity is known to couple strongly with strain (both homogeneous and inhomogeneous), the effect of mechanical constraints on thin-film nanoscale ferroelectrics has been comparatively less explored because of the relative paucity of strain patterns that can be implemented experimentally.

Hexagonal Hybrid Bismuthene by Molecular Interface Engineering.

High-quality devices based on layered heterostructures are typically built from materials obtained by complex solid-state physical approaches or laborious mechanical exfoliation and transfer. Meanwhile, wet-chemically synthesized materials commonly suffer from surface residuals and intrinsic defects. Here, we synthesize using an unprecedented colloidal photocatalyzed, one-pot redox reaction a few-layers bismuth hybrid of "electronic grade" structural quality.

Bimodal ionic photomemristor based on a high-temperature oxide superconductor/semiconductor junction.

Memristors, a cornerstone for neuromorphic electronics, respond to the history of electrical stimuli by varying their electrical resistance across a continuum of states. Much effort has been recently devoted to developing an analogous response to optical excitation. Here we realize a novel tunnelling photo-memristor whose behaviour is bimodal: its resistance is determined by the dual electrical-optical history.

Combining Freestanding Ferroelectric Perovskite Oxides with Two-Dimensional Semiconductors for High Performance Transistors.

We demonstrate the fabrication of field-effect transistors based on single-layer MoS and a thin layer of BaTiO (BTO) dielectric, isolated from its parent epitaxial template substrate. Thin BTO provides an ultrahigh-κ gate dielectric effectively screening Coulomb scattering centers. These devices show mobilities substantially larger than those obtained with standard SiO dielectrics and comparable with values obtained with hexagonal boron nitride, a dielectric employed for fabrication of high-performance two-dimensional (2D) based devices.

Linear imaging theory for differential phase contrast and other phase imaging modes in scanning transmission electron microscopy.

We propose a linear imaging theory for differential phase contrast under the weak-phase-weak-amplitude object approximation. Contrast transfer functions are defined for thin and thick weak objects, and they successfully describe several imaging characteristics of differential phase contrast. We discuss the defocus dependence of the contrast for several examples: atomic resolution, a p-n junction, a heterointerface, and grain boundaries.

An Oxygen Vacancy Memristor Ruled by Electron Correlations.

Resistive switching effects offer new opportunities in the field of conventional memories as well as in the booming area of neuromorphic computing. Here the authors demonstrate memristive switching effects produced by a redox-driven oxygen exchange in tunnel junctions based on NdNiO , a strongly correlated electron system characterized by the presence of a metal-to-insulator transition (MIT). Strikingly, a strong interplay exists between the MIT and the redox mechanism, which on the one hand modifies the MIT itself, and on the other hand radically affects the tunnel resistance switching and the resistance states' lifetime.

Strongly Anisotropic Strain-Tunability of Excitons in Exfoliated ZrSe.

The effect of uniaxial strain on the band structure of ZrSe , a semiconducting material with a marked in-plane structural anisotropy, is studied. By using a modified three-point bending test apparatus, thin ZrSe flakes are subjected to uniaxial strain along different crystalline orientations monitoring the effect of strain on their optical properties through micro-reflectance spectroscopy. The obtained spectra show excitonic features that blueshift upon uniaxial tension.

Franckeite as an Exfoliable Naturally Occurring Topological Insulator.

Franckeite is a natural superlattice composed of two alternating layers of different composition which has shown potential for optoelectronic applications. In part, the interest in franckeite lies in its layered nature which makes it easy to exfoliate into very thin heterostructures. Not surprisingly, its chemical composition and lattice structure are so complex that franckeite has escaped screening protocols and high-throughput searches of materials with nontrivial topological properties.

Large intrinsic anomalous Hall effect in SrIrO induced by magnetic proximity effect.

The anomalous Hall effect (AHE) is an intriguing transport phenomenon occurring typically in ferromagnets as a consequence of broken time reversal symmetry and spin-orbit interaction. It can be caused by two microscopically distinct mechanisms, namely, by skew or side-jump scattering due to chiral features of the disorder scattering, or by an intrinsic contribution directly linked to the topological properties of the Bloch states. Here we show that the AHE can be artificially engineered in materials in which it is originally absent by combining the effects of symmetry breaking, spin orbit interaction and proximity-induced magnetism.

Controlled Sign Reversal of Electroresistance in Oxide Tunnel Junctions by Electrochemical-Ferroelectric Coupling.

The persistence of ferroelectricity in ultrathin layers relies critically on screening or compensation of polarization charges which otherwise destabilize the ferroelectric state. At surfaces, charged defects play a crucial role in the screening mechanism triggering novel mixed electrochemical-ferroelectric states. At interfaces, however, the coupling between ferroelectric and electrochemical states has remained unexplored.

Direct Transformation of Crystalline MoO into Few-Layers MoS.

We fabricated large-area atomically thin MoS layers through the direct transformation of crystalline molybdenum trioxide (MoO) by sulfurization at relatively low temperatures. The obtained MoS sheets are polycrystalline (~10-20 nm single-crystal domain size) with areas of up to 300 × 300 µm, 2-4 layers in thickness and show a marked p-type behavior. The synthesized films are characterized by a combination of complementary techniques: Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements.

Symmetry Breakdown in Franckeite: Spontaneous Strain, Rippling, and Interlayer Moiré.

Franckeite is a naturally occurring layered mineral with a structure composed of alternating stacks of SnS-like and PbS-like layers. Although this superlattice is composed of a sequence of isotropic two-dimensional layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic. We demonstrate that this rippling comes hand in hand with an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational, and optical properties.

Direct electric field imaging of graphene defects.

Material properties are sensitive to atomistic structure defects such as vacancies or impurities, and it is therefore important to determine not only the local atomic configuration but also their chemical bonding state. Annular dark-field scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy has been utilized to investigate the local electronic structures of such defects down to the level of single atoms. However, it is still challenging to two-dimensionally map the local bonding states, because the electronic fine-structure signal from a single atom is extremely weak.

Probing the Internal Atomic Charge Density Distributions in Real Space.

Probing the charge density distributions in materials at atomic scale remains an extremely demanding task, particularly in real space. However, recent advances in differential phase contrast-scanning transmission electron microscopy (DPC-STEM) bring this possibility closer by directly visualizing the atomic electric field. DPC-STEM at atomic resolutions measures how a sub-angstrom electron probe passing through a material is affected by the atomic electric field, the field between the nucleus and the surrounding electrons.

Direct Determination of Atomic Structure and Magnetic Coupling of Magnetite Twin Boundaries.

Clarifying how the atomic structure of interfaces/boundaries in materials affects the magnetic coupling nature across them is of significant academic value and will facilitate the development of state-of-the-art magnetic devices. Here, by combining atomic-resolution transmission electron microscopy, atomistic spin-polarized first-principles calculations, and differential phase contrast imaging, we conduct a systematic investigation of the atomic and electronic structures of individual FeO twin boundaries (TBs) and determine their concomitant magnetic couplings. We demonstrate that the magnetic coupling across the FeO TBs can be either antiferromagnetic or ferromagnetic, which directly depends on the TB atomic core structures and resultant electronic structures within a few atomic layers.

Quantitative electric field mapping in thin specimens using a segmented detector: Revisiting the transfer function for differential phase contrast.

Differential phase contrast in scanning transmission electron microscopy can visualize local electromagnetic fields inside specimens. The contrast derived from first moments, the so-called center of mass, of the diffraction patterns for each probe position can be quantitatively related to the local electromagnetic field under the phase object approximation. While only approximate first moments can be obtained with a segmented detector, in weak phase objects the fields can be accurately quantified on the basis of a phase contrast transfer function.

Direct Visualization of Local Electromagnetic Field Structures by Scanning Transmission Electron Microscopy.

The functional properties of materials and devices are critically determined by the electromagnetic field structures formed inside them, especially at nanointerface and surface regions, because such structures are strongly associated with the dynamics of electrons, holes and ions. To understand the fundamental origin of many exotic properties in modern materials and devices, it is essential to directly characterize local electromagnetic field structures at such defect regions, even down to atomic dimensions. In recent years, rapid progress in the development of high-speed area detectors for aberration-corrected scanning transmission electron microscopy (STEM) with sub-angstrom spatial resolution has opened new possibilities to directly image such electromagnetic field structures at very high-resolution.

Electric field imaging of single atoms.

In scanning transmission electron microscopy (STEM), single atoms can be imaged by detecting electrons scattered through high angles using post-specimen, annular-type detectors. Recently, it has been shown that the atomic-scale electric field of both the positive atomic nuclei and the surrounding negative electrons within crystalline materials can be probed by atomic-resolution differential phase contrast STEM. Here we demonstrate the real-space imaging of the (projected) atomic electric field distribution inside single Au atoms, using sub-Å spatial resolution STEM combined with a high-speed segmented detector.

Resonant electron tunnelling assisted by charged domain walls in multiferroic tunnel junctions.

The peculiar features of domain walls observed in ferroelectrics make them promising active elements for next-generation non-volatile memories, logic gates and energy-harvesting devices. Although extensive research activity has been devoted recently to making full use of this technological potential, concrete realizations of working nanodevices exploiting these functional properties are yet to be demonstrated. Here, we fabricate a multiferroic tunnel junction based on ferromagnetic LaSrMnO electrodes separated by an ultrathin ferroelectric BaTiO tunnel barrier, where a head-to-head domain wall is constrained.

A new method to detect and correct sample tilt in scanning transmission electron microscopy bright-field imaging.

Important properties of functional materials, such as ferroelectric shifts and octahedral distortions, are associated with displacements of the positions of lighter atoms in the unit cell. Annular bright-field scanning transmission electron microscopy is a good experimental method for investigating such phenomena due to its ability to image light and heavy atoms simultaneously. To map atomic positions at the required accuracy precise angular alignment of the sample with the microscope optical axis is necessary, since misalignment (tilt) of the specimen contributes to errors in position measurements of lighter elements in annular bright-field imaging.

An artificial photosynthesis anode electrode composed of a nanoparticulate photocatalyst film in a visible light responsive GaN-ZnO solid solution system.

The artificial photosynthesis technology known as the Honda-Fujishima effect, which produces oxygen and hydrogen or organic energy from sunlight, water, and carbon dioxide, is an effective energy and environmental technology. The key component for the higher efficiency of this reaction system is the anode electrode, generally composed of a photocatalyst formed on a glass substrate from electrically conductive fluorine-doped tin oxide (FTO). To obtain a highly efficient electrode, a dense film composed of a nanoparticulate visible light responsive photocatalyst that usually has a complicated multi-element composition needs to be deposited and adhered onto the FTO.

Hybridization-controlled charge transfer and induced magnetism at correlated oxide interfaces.

At interfaces between conventional materials, band bending and alignment are classically controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO and SrTiO. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from strong Coulomb interactions between transition metal and oxygen ions.

Paving the way to nanoionics: atomic origin of barriers for ionic transport through interfaces.

The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor.