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.

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.

Defect chemistry of donor-doped BaTiO3 with BaO-excess for reduction resistant PTCR thermistor applications - redox-behaviour.

The electrical conductivity of donor-doped BaTiO3 thermistor ceramics with excessive BaO revealing a reduction-persistent PTCR effect has been carefully examined depending on materials' composition and oxygen partial pressure at moderate temperatures between 973 and 1273 K. This thermal regime represents the range which is relevant for the realization of insulating grain boundaries in these electrically inhomogeneous ceramic materials through reoxidation. Based on the experimental results strong evidence for a general correlation between the PTCR characteristics, DC-conductivity and the herewith associated defect chemistry significant to thermistor applications is presented for the system (Ba, La)mTiO3, where m designates the BaO-excess (m≥ 1). Nominal compositions with a relatively low (Ba + La)/Ti ratio m only show a rather poor PTCR effect and an overall donor-type response in conductivity can be observed at all levels of oxygen partial pressure considered in the present study. With increasing (Ba + La)/Ti ratio m the nonlinear resistivity jump with increasing temperature strongly improves and the acceptor-type behaviour seems to dominate the total conductivity at partial pressures of oxygen above approximately 10-6 MPa. This result for compositions with high BaO-excess can be understood by the local formation of point defect associates in the grain boundary regions that consist of both acceptor-type titanium vacancies and donor-type oxygen vacancies. Their origin is attributed to the preferential local incorporation of excessive BaO into the BaTiO3 lattice at the intergranular interfaces.

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).

On the universality of the I-V switching characteristics in non-volatile and volatile resistive switching oxides.

The I-V switching curves of bipolar switching non-volatile ReRAM devices show peculiar characteristics, such as an abrupt ON switching and the existence of a universal switching voltage. This switching behavior has been explained by the presence of a filamentary process, in which the width of a conductive filament changes during switching resulting in different resistance states. Vice versa, similar (ON) switching behavior, e.g. that of volatile switching Cr-doped V2O3 devices, has been interpreted as an indication of the presence of similar filamentary switching. In this paper, we want to review the correlation between filamentary (width) switching and the (SET) I-V characteristics by discussing the existing models. For the Cr-doped V2O3 devices, on the other hand, it is argued that a different, constant filament width switching mode may be present.

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.

Addressing Multiple Resistive States of Polyoxovanadates: Conductivity as a Function of Individual Molecular Redox States.

The sustainable development of IT-systems requires a quest for novel concepts to address further miniaturization, performance improvement, and energy efficiency of devices. The realization of these goals cannot be achieved without an appropriate functional material. Herein, we target the technologically important electron modification using single polyoxometalate (POM) molecules envisaged as smart successors of materials that are implemented in today's complementary metal-oxide-semiconductor (CMOS) technology. Lindqvist-type POMs were physisorbed on the Au(111) surface, preserving their structural and electronic characteristics. By applying an external voltage at room temperature, the valence state of the single POM molecule could be changed multiple times through the injection of up to 4 electrons. The molecular electrical conductivity is dependent on the number of vanadium 3d electrons, resulting in several discrete conduction states with increasing conductivity. This fundamentally important finding illustrates the far-reaching opportunities for POM molecules in the area of multiple-state resistive (memristive) switching.

Improvement of SET variability in TaO x based resistive RAM devices.

Improvement or at least control of variability is one of the key challenges for Redox based resistive switching memory technology. In this paper, we investigate the impact of a serial resistor as a voltage divider on the SET variability in Pt/Ta2O5/Ta/Pt nano crossbar devices. A partial RESET in a competing complementary switching (CS) mode is identified as a possible failure mechanism of bipolar switching SET in our devices. Due to a voltage divider effect, serial resistance value shows unequal impact on switching voltages of both modes which allows for a selective suppression of the CS mode. The impact of voltage divider on SET variability is demonstrated. A combination of appropriate write voltage and serial resistance allows for a significant improvement of the SET variability.

Resistive Switching of Individual, Chemically Synthesized TiO2 Nanoparticles.

Resistively switching devices are considered promising for next-generation nonvolatile random-access memories. Today, such memories are fabricated by means of "top-down approaches" applying thin films sandwiched between nanoscaled electrodes. In contrast, this work presents a "bottom-up approach" disclosing for the first time the resistive switching (RS) of individual TiO2 nanoparticles (NPs). The NPs, which have sizes of 80 and 350 nm, respectively, are obtained by wet chemical synthesis and thermally treated under oxidizing or vacuum conditions for crystallization, respectively. These NPs are deposited on a Pt/Ir bottom electrode and individual NPs are electrically characterized by means of a nanomanipulator system in situ, in a scanning electron microscope. While amorphous NPs and calcined NPs reveal no switching hysteresis, a very interesting behavior is found for the vacuum-annealed, crystalline TiO(2-x) NPs. These NPs reveal forming-free RS behavior, dominantly complementary switching (CS) and, to a small degree, bipolar switching (BS) characteristics. In contrast, similarly vacuum-annealed TiO2 thin films grown by atomic layer deposition show standard BS behavior under the same conditions. The interesting CS behavior of the TiO(2-x) NPs is attributed to the formation of a core-shell-like structure by re-oxidation of the reduced NPs as a unique feature.

Low-current operations in 4F(2)-compatible Ta2O5-based complementary resistive switches.

Complementary resistive switches (CRS), which consist of two anti-serially connected bipolar switching ReRAM cells, can reduce sneak path currents in passive crossbar arrays. However, the high operation current restrains the implementation of the CRS device. In this article, we present low current operation (<300 µA) of vertically stacked, 4F(2)-compatible Ta2O5-based CRS devices exhibiting two terminals. Two types of devices, either offering a nano- or a micrometer scale bottom cell (BC), are considered. The top cell (TC) in both configurations is designed of micrometer size. A novel three-step electroforming procedure for the vertical CRS device having no access to the middle electrode is exemplified and compared to the conventional forming procedure using three-terminal CRS devices. This three-step electroforming procedure provides adjustment of the maximum switching current in the nano-BC CRS: a low-level current compliance during forming enables low current CRS operation in subsequent switching cycles. Further, the nano-BC CRS shows the stable switching up to 10(4) cycles whereas the micro-BC CRS endures up to 10(6) cycles.

Volatile resistance states in electrochemical metallization cells enabling non-destructive readout of complementary resistive switches.

Redox-based resistive memory cells exhibit changes of OFF or intermediate resistance values over time and even ON states can be completely lost in certain cases. The stability of these resistance states and the time until resistance loss strongly depends on the materials system. On the basis of electrical measurements and chemical analysis we found a viable explanation for these volatile resistance states (VRSs) in Ag-GeSx-based electrochemical metallization memory cells and identified a technological application in the field of crossbar memories. Complementary resistive switches usually suffer from the necessity of a destructive read-out procedure increasing wear and reducing read-out speed. From our analysis we deduced a solution to use the VRS as an inherent selector mechanism without the need for additional selector devices.

Physical origins and suppression of Ag dissolution in GeS(x)-based ECM cells.

Electrochemical metallisation (ECM) memory cells potentially suffer from limited memory retention time, which slows down the future commercialisation of this type of data memory. In this work, we investigate Ag/GeSx/Pt redox-based resistive memory cells (ReRAM) with and without an additional Ta barrier layer by time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray absorption spectroscopy (XAS) and synchrotron high-energy X-ray diffractometry (XRD) to investigate the physical mechanism behind the shift and/or loss of OFF data retention. Electrical measurements demonstrate the effectiveness and high potential of the diffusion barrier layer in practical applications.

Reliability effects of lateral filament confinement by nano-scaling the oxide in memristive devices.

Write-variability and resistance instability are major reliability concerns impeding implementation of oxide-based memristive devices in neuromorphic systems. The root cause of the reliability issues is the stochastic nature of conductive filament formation and dissolution, whose impact is particularly critical in the high resistive state (HRS). Optimizing the filament stability requires mitigating diffusive processes within the oxide, but these are unaffected by conventional electrode scaling. Here we propose a device design that laterally confines the switching oxide volume and thus the filament to 10 nm, which yields reliability improvements in our measurements and simulations. We demonstrate a 50% decrease in HRS write-variability for an oxide nano-fin device in our full factorial analysis of modulated current-voltage sweeps. Furthermore, we use ionic noise measurements to quantify the HRS filament stability against diffusive processes. The laterally confined filaments exhibit a change in the signal-to-noise ratio distribution with a shift to higher values. Our complementing kinetic Monte Carlo simulation of oxygen vacancy (re-)distribution for confined filaments shows improved noise behavior and elucidates the underlying physical mechanisms. While lateral oxide volume scaling down to filament sizes is challenging, our efforts motivate further examination and awareness of filament confinement effects in regards to reliability.

Atomically controlled electrochemical nucleation at superionic solid electrolyte surfaces.

Electrochemical equilibrium and the transfer of mass and charge through interfaces at the atomic scale are of fundamental importance for the microscopic understanding of elementary physicochemical processes. Approaching atomic dimensions, phase instabilities and instrumentation limits restrict the resolution. Here we show an ultimate lateral, mass and charge resolution during electrochemical Ag phase formation at the surface of RbAg(4)I(5) superionic conductor thin films. We found that a small amount of electron donors in the solid electrolyte enables scanning tunnelling microscope measurements and atomically resolved imaging. We demonstrate that Ag critical nucleus formation is rate limiting. The Gibbs energy of this process takes discrete values and the number of atoms of the critical nucleus remains constant over a large range of applied potentials. Our approach is crucial to elucidate the mechanism of atomic switches and highlights the possibility of extending this method to a variety of other electrochemical systems.

Surface deformations as a necessary requirement for resistance switching at the surface of SrTiO3:N.

Atomic force microscopy (AFM), conductive AFM and electrochemical strain microscopy were used to study the topography change at the defect surface of SrTiO3:N, breakdown in the electrical conduction of the tip/sample/electrode system and ionic motion. The IV curves show resistance switching behavior in a voltage range ±6 V < U <± 10 V and a current of maximum ±10 nA. A series of sweeping IV curves resulted in an increase in ionically polarized states (surface charging), electrochemical volume (surface deformations) and sequential formations of stable surface protrusions. The surface deformations are reversible (U <± 5 V) without IV pinched hysteresis and remained stable during the resistance switching (U >± 6 V), revealing the additional necessity (albeit insufficient due to 50% yield of working cells) of surface protrusion formation for resistance switching memory.

Switching kinetics of electrochemical metallization memory cells.

The strongly nonlinear switching kinetics of electrochemical metallization memory (ECM) cells are investigated using an advanced 1D simulation model. It is based on the electrochemical growth and dissolution of a Ag or Cu filament within a solid thin film and accounts for nucleation effects, charge transfer, and cation drift. The model predictions are consistent with experimental switching results of a time range of 12 orders of magnitude obtained from silver iodide (AgI) based ECM cells. By analyzing the simulation results the electrochemical processes limiting the switching kinetics are revealed. This study provides new insights into the understanding of the limiting electrochemical processes determining the switching kinetics of ECM cells.