Space charge governs the kinetics of metal exsolution.

Nanostructured composite electrode materials play a major role in the fields of catalysis and electrochemistry. The self-assembly of metallic nanoparticles on oxide supports via metal exsolution relies on the transport of reducible dopants towards the perovskite surface to provide accessible catalytic centres at the solid-gas interface. At surfaces and interfaces, however, strong electrostatic gradients and space charges typically control the properties of oxides. Here we reveal that the nature of the surface-dopant interaction is the main determining factor for the exsolution kinetics of nickel in SrTi0.9Nb0.05Ni0.05O3-δ. The electrostatic interaction of dopants with surface space charge regions forming upon thermal oxidation results in strong surface passivation, which manifests in a retarded exsolution response. We furthermore demonstrate the controllability of the exsolution response via engineering of the perovskite surface chemistry. Our findings indicate that tailoring the electrostatic gradients at the perovskite surface is an essential step to improve exsolution-type materials in catalytic converters.

Atomistic Insights into Activation and Degradation of La0.6Sr0.4CoO3-δ Electrocatalysts under Oxygen Evolution Conditions.

The stability of perovskite oxide catalysts for the oxygen evolution reaction (OER) plays a critical role in their applicability in water splitting concepts. Decomposition of perovskite oxides under applied potential is typically linked to cation leaching and amorphization of the material. However, structural changes and phase transformations at the catalyst surface were also shown to govern the activity of several perovskite electrocatalysts under applied potential. Hence, it is crucial for the rational design of durable perovskite catalysts to understand the interplay between the formation of active surface phases and stability limitations under OER conditions. In the present study, we reveal a surface-dominated activation and deactivation mechanism of the prominent electrocatalyst La0.6Sr0.4CoO3-δ under steady-state OER conditions. Using a multiscale microscopy and spectroscopy approach, we identify the evolving Co-oxyhydroxide as catalytically active surface species and La-hydroxide as inactive species involved in the transient degradation behavior of the catalyst. While the leaching of Sr results in the formation of mixed surface phases, which can be considered as a part of the active surface, the gradual depletion of Co from a self-assembled active CoO(OH) phase and the relative enrichment of passivating La(OH)3 at the electrode surface result in the failure of the perovskite catalyst under applied potential.

Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis.

Structure-activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.

In-Gap States and Band-Like Transport in Memristive Devices.

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

Introduction to new memory paradigms: memristive phenomena and neuromorphic applications.

This article provides a brief introduction to the Faraday Discussion "New memory paradigms: memristive phenomena and neuromorphic applications" held in Aachen, Germany, 15-17 October 2018. It will cover basic definitions of memristive switching elements, their main switching modes, and their most important performance parameters as well as applications in neuromorphic computing. The article comprises parts from the following sources: General Introduction and Introduction to Part V of Nanoelectronics and Information Technology, ed. R. Waser, Wiley-VCH, 2012; Chapter 4 of Nanotechnology: Volume 3: Information Technology I, ed. R. Waser, Wiley-VCH, Weinheim, 2008; Chapters 3-9 of Emerging Nanoelectronic Devices, ed. A. Chen, J. Hutchby, V. Zhirnov and G. Bourianoff, Wiley, 2015; Chapter 1 of Resistive Switching, ed. D. Ielmini and R. Waser, Wiley-VCH, 2016 (with permission by Wiley-VCH).

Spectroscopic elucidation of ionic motion processes in tunnel oxide-based memristive devices.

Resistive switching oxides are highly attractive candidates to emulate synaptic behaviour in artificial neural networks. Whilst the most widely employed systems exhibit filamentary resistive switching, interface-type switching systems based on a tunable tunnel barrier are of increasing interest, since their gradual SET and RESET processes provide an analogue-type of switching required to take over synaptic functionality. Interface-type switching devices often consist of bilayers of one highly mixed-conductive oxide layer and one highly insulating tunnel oxide layer. However, most tunnel oxides used for interface-type switching are also prone to form conducting filaments above a certain voltage bias threshold. We investigated two different tunnel oxide devices, namely, Pr1-xCaxMnO3 (PCMO) with yttria-stabilized ZrO2 (YSZ) tunnel barrier and substoichiometric TaOx with HfO2 tunnel barrier by interface-sensitive, hard X-ray photoelectron spectroscopy (HAXPES) in order to gain insights into the chemical changes during filamentary and interface-type switching. The measurements suggest an exchange of oxygen ions between the mixed conducting oxide layer and the tunnel barrier, that causes an electrostatic modulation of the effective height of the tunnel barrier, as the underlying switching mechanism for the interface-type switching. Moreover, we observe by in operando HAXPES analysis that this field-driven ionic motion across the whole area is sustained even if a filament is formed in the tunnel barrier and the device is transformed into a filamentary-type switching mode.

Complex behaviour of vacancy point-defects in SrRuO3 thin films.

The behaviour of point defects in thin, epitaxial films of the oxide electrode SrRuO3 was probed by means of diffusion measurements. Thin-film SrRuO3 was deposited by means of pulsed laser deposition (PLD) on (100) oriented, undoped single crystal SrTiO3 substrates. (16)O/(18)O exchange anneals were employed to probe the behavior of oxygen vacancies. Anneals were performed in the temperature range 850 ≤T/K≤ 1100 at an oxygen partial pressure of pO2 = 500 mbar. Samples were subsequently analyzed by means of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The measured oxygen isotope penetration profiles comprised, surprisingly, two features. Oxygen tracer diffusion coefficients determined for thin-film SrRuO3 are amongst the lowest measured for nominally undoped perovskite-type oxides. The activation enthalpy of oxygen tracer diffusion was found to be ≈ 2 eV. Diffusion of Ti from the SrTiO3 substrates into the SrRuO3 thin films, probing the cation defects, was also observed in ToF-SIMS profiles; here, too, the diffusion profiles showed two features. The activation enthalpy of titanium diffusion was found to be ΔHDTi≈ 4 eV. We propose a model-cation sublattice equilibration-that accounts for the appearance of two features in both anion and cation diffusion profiles. We suggest that the observed complex behavior arises from the metastable defect structure of PLD thin films and the unusual defect structure of SrRuO3.

Studies of local structural distortions in strained ultrathin BaTiO3 films using scanning transmission electron microscopy.

Ultrathin ferroelectric heterostructures (SrTiO3/BaTiO3/BaRuO3/SrRuO3) were studied by scanning transmission electron microscopy (STEM) in terms of structural distortions and atomic displacements. The TiO2-termination at the top interface of the BaTiO3 layer was changed into a BaO-termination by adding an additional BaRuO3 layer. High-angle annular dark-field (HAADF) imaging by aberration-corrected STEM revealed that an artificially introduced BaO-termination can be achieved by this interface engineering. By using fast sequential imaging and frame-by-frame drift correction, the effect of the specimen drift was significantly reduced and the signal-to-noise ratio of the HAADF images was improved. Thus, a quantitative analysis of the HAADF images was feasible, and an in-plane and out-of-plane lattice spacing of the BaTiO3 layer of 3.90 and 4.22 Å were determined. A 25 pm shift of the Ti columns from the center of the unit cell of BaTiO3 along the c-axis was observed. By spatially resolved electron energy-loss spectroscopy studies, a reduction of the crystal field splitting (CFS, ΔL3=1.93 eV) and an asymmetric broadening of the eg peak were observed in the BaTiO3 film. These results verify the presence of a ferroelectric polarization in the ultrathin BaTiO3 film.

Interfacial Thermal Resistive Switching in (Pt,Cr)/SrTiO3 Devices.

The operation of oxide-based memristive devices relies on the fast accumulation and depletion of oxygen vacancies by an electric field close to the metal-oxide interface. Here, we show that the reversible change of the local concentration of oxygen vacancies at this interface also produces a change in the thermal boundary resistance (TBR), i.e., a thermal resistive switching effect. We used frequency domain thermoreflectance to monitor the interfacial metal-oxide TBR in (Pt,Cr)/SrTiO3 devices, showing a change of ≈20% under usual SET/RESET operation voltages, depending on the structure of the device. Time-dependent thermal relaxation experiments suggest ionic rearrangement along the whole area of the metal/oxide interface, apart from the ionic filament responsible for the electrical conductivity switching. The experiments presented in this work provide valuable knowledge about oxide ion dynamics in redox-based memristive devices.

Detection of Fe2+ valence states in Fe doped SrTiO3 epitaxial thin films grown by pulsed laser deposition.

We present an X-ray absorption spectroscopy study on Fe-doped SrTiO3 thin films grown by pulsed laser deposition. The Fe L2,3 edge spectra are recorded for doping concentrations from 0-5% after several annealing steps at moderate temperatures. The Fe valence state is determined by comparison with an ilmenite reference sample and calculations according to the charge transfer multiplet model. We found clear evidence of Fe(2+) and Fe(3+) oxidation states independently of the doping concentration. The Fe(2+) signal is enhanced at the surface and increases after annealing. The Fe(2+) configuration is in contrast to the mixed Fe(3+)/Fe(4+) valence state in bulk material and must be explained by the specific defect structure of the thin films due to the kinetically limited growth which induces a high concentration of oxygen vacancies.

Atomic-scale measurement of structure and chemistry of a single-unit-cell layer of LaAlO3 embedded in SrTiO3.

A single layer of LaAlO3 with a nominal thickness of one unit cell, which is sandwiched between a SrTiO3 substrate and a SrTiO3 capping layer, is quantitatively investigated by high-resolution transmission electron microscopy. By the use of an aberration-corrected electron microscope and by employing sophisticated numerical image simulation procedures, significant progress is made in two aspects. First, the structural as well as the chemical features of the interface are determined simultaneously on an atomic scale from the same specimen area. Second, the evaluation of the structural and chemical data is carried out in a fully quantitative way on the basis of the absolute image contrast, which has not been achieved so far in materials science investigations using high-resolution electron microscopy. Considering the strong influence of even subtle structural details on the electronic properties of interfaces in oxide materials, a fully quantitative interface analysis, which makes positional data available with picometer precision together with the related chemical information, can contribute to a better understanding of the functionality of such interfaces.

Separating the Effects of Band Bending and Covalency in Hybrid Perovskite Oxide Electrocatalyst Bilayers for Water Electrolysis.

The Co-O covalency in perovskite oxide cobaltites such as La1-xSrxCoO3 is believed to impact the electrocatalytic activity during electrochemical water splitting at the anode where the oxygen evolution reaction (OER) takes place. Additionally, space charge layers through band bending at the interface to the electrolyte may affect the electron transfer into the electrode, complicating the analysis and identification of true OER activity descriptors. Here, we separate the influence of covalency and band bending in hybrid epitaxial bilayer structures of highly OER-active La0.6Sr0.4CoO3 and undoped and less-active LaCoO3. Ultrathin LaCoO3 capping layers of 2-8 unit cells on La0.6Sr0.4CoO3 show intermediate OER activity between La0.6Sr0.4CoO3 and LaCoO3 evidently caused by the increased surface Co-O covalency compared to single LaCoO3 as detected by X-ray photoelectron spectroscopy. A Mott-Schottkyanalysis revealed low flat band potentials for different LaCoO3 capping layer thicknesses, indicating that no limiting extended space charge layer exists under OER conditions as all catalyst bilayer films exhibited hole accumulation at the surface. The combined X-ray photoelectron spectroscopy and Mott-Schottky analysis thus enables us to differentiate between the influence of the covalency and intrinsic space charge layers, which are indistinguishable in a single physical or electrochemical characterization. Our results emphasize the prominent role of transition metal oxygen covalency in perovskite electrocatalysts and introduce a bilayer approach to fine-tune the surface electronic structure.

Spectroscopic study of the electric field induced valence change of Fe-defect centers in SrTiO3.

The electrochemical changes induced by an electric field in Fe-doped SrTiO(3) have been investigated by X-ray absorption spectroscopy (XANES and EXAFS), electron paramagnetic resonance (EPR) and Raman spectroscopy. A detailed study of the Fe dopant in the regions around the anode and cathode reveals new insights into the local structure and valence state of Fe in SrTiO(3) single crystals. The ab initio full multiple-scattering XANES calculations give an evidence of the oxygen vacancy presence in the first coordination shell of iron. Differences in the length and disorder of the Fe-O bonds as extracted from EXAFS are correlated to the unequivocal identification of the defect type by complementary spectroscopical techniques to identify the valence state of the Fe-dopant and the presence of the Fe - V(Ö) complexes. Through this combinatorial approach, novel structural information on Fe - V(Ö) complexes is provided by X-ray absorption spectroscopy, and the relation of Fe-O bond length, doping level and oxidation state in SrTi(1-x)Fe(x)O(3) is briefly discussed.

Exploring Area-Dependent Pr0.7Ca0.3MnO3-Based Memristive Devices as Synapses in Spiking and Artificial Neural Networks.

Memristive devices are novel electronic devices, which resistance can be tuned by an external voltage in a non-volatile way. Due to their analog resistive switching behavior, they are considered to emulate the behavior of synapses in neuronal networks. In this work, we investigate memristive devices based on the field-driven redox process between the p-conducting Pr0.7Ca0.3MnO3 (PCMO) and different tunnel barriers, namely, Al2O3, Ta2O5, and WO3. In contrast to the more common filamentary-type switching devices, the resistance range of these area-dependent switching devices can be adapted to the requirements of the surrounding circuit. We investigate the impact of the tunnel barrier layer on the switching performance including area scaling of the current and variability. Best performance with respect to the resistance window and the variability is observed for PCMO with a native Al2O3 tunnel oxide. For all different layer stacks, we demonstrate a spike timing dependent plasticity like behavior of the investigated PCMO cells. Furthermore, we can also tune the resistance in an analog fashion by repeated switching the device with voltage pulses of the same amplitude and polarity. Both measurements resemble the plasticity of biological synapses. We investigate in detail the impact of different pulse heights and pulse lengths on the shape of the stepwise SET and RESET curves. We use these measurements as input for the simulation of training and inference in a multilayer perceptron for pattern recognition, to show the use of PCMO-based ReRAM devices as weights in artificial neural networks which are trained by gradient descent methods. Based on this, we identify certain trends for the impact of the applied voltages and pulse length on the resulting shape of the measured curves and on the learning rate and accuracy of the multilayer perceptron.

Utilizing the Switching Stochasticity of HfO2/TiOx-Based ReRAM Devices and the Concept of Multiple Device Synapses for the Classification of Overlapping and Noisy Patterns.

With the arrival of the Internet of Things (IoT) and the challenges arising from Big Data, neuromorphic chip concepts are seen as key solutions for coping with the massive amount of unstructured data streams by moving the computation closer to the sensors, the so-called "edge computing." Augmenting these chips with emerging memory technologies enables these edge devices with non-volatile and adaptive properties which are desirable for low power and online learning operations. However, an energy- and area-efficient realization of these systems requires disruptive hardware changes. Memristor-based solutions for these concepts are in the focus of research and industry due to their low-power and high-density online learning potential. Specifically, the filamentary-type valence change mechanism (VCM memories) have shown to be a promising candidate In consequence, physical models capturing a broad spectrum of experimentally observed features such as the pronounced cycle-to-cycle (c2c) and device-to-device (d2d) variability are required for accurate evaluation of the proposed concepts. In this study, we present an in-depth experimental analysis of d2d and c2c variability of filamentary-type bipolar switching HfO2/TiOx nano-sized crossbar devices and match the experimentally observed variabilities to our physically motivated JART VCM compact model. Based on this approach, we evaluate the concept of parallel operation of devices as a synapse both experimentally and theoretically. These parallel synapses form a synaptic array which is at the core of neuromorphic chips. We exploit the c2c variability of these devices for stochastic online learning which has shown to increase the effective bit precision of the devices. Finally, we demonstrate that stochastic switching features for a pattern classification task that can be employed in an online learning neural network.

Exsolution of Embedded Nanoparticles in Defect Engineered Perovskite Layers.

Exsolution phenomena are highly debated as efficient synthesis routes for nanostructured composite electrode materials for the application in solid oxide cells (SOCs) and the development of next-generation electrochemical devices for energy conversion. Utilizing the instability of perovskite oxides, doped with electrocatalytically active elements, highly dispersed nanoparticles can be prepared at the perovskite surface under the influence of a reducing heat treatment. For the systematic study of the mechanistic processes governing metal exsolution, epitaxial SrTi0.9Nb0.05Ni0.05O3-δ thin films of well-defined stoichiometry are synthesized and employed as model systems to investigate the interplay of defect structures and exsolution behavior. Spontaneous phase separation and the formation of dopant-rich features in the as-synthesized thin film material is revealed by high-resolution transmission electron microscopy (HR-TEM) investigations. The resulting nanostructures are enriched by nickel and serve as preformed nuclei for the subsequent exsolution process under reducing conditions, which reflects a so far unconsidered process drastically affecting the understanding of nanoparticle exsolution phenomena. Using an approach of combined morphological, chemical, and structural analysis of the exsolution response, a limitation of the exsolution dynamics for nonstoichiometric thin films is found to be correlated to a distortion of the perovskite host lattice. Consequently, the incorporation of defect structures results in a reduced particle density at the perovskite surface, presumably by trapping of nanoparticles in the oxide bulk.

Identifying Ionic and Electronic Charge Transfer at Oxide Heterointerfaces.

The ability to tailor oxide heterointerfaces has led to novel properties in low-dimensional oxide systems. A fundamental understanding of these properties is based on the concept of electronic charge transfer. However, the electronic properties of oxide heterointerfaces crucially depend on their ionic constitution and defect structure: ionic charges contribute to charge transfer and screening at oxide interfaces, triggering a thermodynamic balance of ionic and electronic structures. Quantitative understanding of the electronic and ionic roles regarding charge-transfer phenomena poses a central challenge. Here, the electronic and ionic structure is simultaneously investigated at the prototypical charge-transfer heterointerface, LaAlO3 /SrTiO3 . Applying in situ photoemission spectroscopy under oxygen ambient, ionic and electronic charge transfer is deconvoluted in response to the oxygen atmosphere at elevated temperatures. In this way, both the rich and variable chemistry of complex oxides and the associated electronic properties are equally embraced. The interfacial electron gas is depleted through an ionic rearrangement in the strontium cation sublattice when oxygen is applied, resulting in an inverse and reversible balance between cation vacancies and electrons, while the mobility of ionic species is found to be considerably enhanced as compared to the bulk. Triggered by these ionic phenomena, the electronic transport and magnetic signature of the heterointerface are significantly altered.

Electronic Inhomogeneity Influence on the Anomalous Hall Resistivity Loops of SrRuO3 Epitaxially Interfaced with 5d Perovskites.

SrRuO3, a 4d ferromagnet with multiple Weyl nodes at the Fermi level, offers a rich playground to design epitaxial heterostructures and superlattices with fascinating magnetic and magnetotransport properties. Interfacing ultrathin SrRuO3 layers with large spin-orbit coupling 5d transition-metal oxides, such as SrIrO3, results in pronounced peaklike anomalies in the magnetic field dependence of the Hall resistivity. Such anomalies have been attributed either to the formation of Néel-type skyrmions or to modifications of the Berry curvature of the topologically nontrivial conduction bands near the Fermi level of SrRuO3. Here, epitaxial multilayers based on SrRuO3 interfaced with 5d perovskite oxides, such as SrIrO3 and SrHfO3, were studied. This work focuses on the magnetotransport properties of the multilayers, aiming to unravel the role played by the interfaces with 5d perovskites in the peaklike anomalies of the Hall resistance loops of SrRuO3 layers. Interfacing with large band gap insulating SrHfO3 layers did not influence the anomalous Hall resistance loops, while interfacing with the nominally paramagnetic semimetal SrIrO3 resulted in pronounced peaklike anomalies, which have been lately attributed to a topological Hall effect contribution as a result of skyrmions. This interpretation is, however, under strong debate and lately alternative causes, such as inhomogeneity of the thickness and the electronic properties of the SrRuO3 layers, have been considered. Aligned with these latter proposals, our findings reveal the central role played in the anomalies of the Hall resistivity loops by electronic inhomogeneity of SrRuO3 layers due to the interfacing with semimetallic 5d5 SrIrO3.

Topotactic Phase Transition Driving Memristive Behavior.

Redox-based memristive devices are one of the most attractive candidates for future nonvolatile memory applications and neuromorphic circuits, and their performance is determined by redox processes and the corresponding oxygen-ion dynamics. In this regard, brownmillerite SrFeO2.5 has been recently introduced as a novel material platform due to its exceptional oxygen-ion transport properties for resistive-switching memory devices. However, the underlying redox processes that give rise to resistive switching remain poorly understood. By using X-ray absorption spectromicroscopy, it is demonstrated that the reversible redox-based topotactic phase transition between the insulating brownmillerite phase, SrFeO2.5 , and the conductive perovskite phase, SrFeO3 , gives rise to the resistive-switching properties of SrFeOx memristive devices. Furthermore, it is found that the electric-field-induced phase transition spreads over a large area in (001) oriented SrFeO2.5 devices, where oxygen vacancy channels are ordered along the in-plane direction of the device. In contrast, (111)-grown SrFeO2.5 devices with out-of-plane oriented oxygen vacancy channels, reaching from the bottom to the top electrode, show a localized phase transition. These findings provide detailed insight into the resistive-switching mechanism in SrFeOx -based memristive devices within the framework of metal-insulator topotactic phase transitions.