Mechanistic and Kinetic Analysis of Perovskite Memristors with Buffer Layers: The Case of a Two-Step Set Process

With the increasing demand for artificially intelligent hardware systems for brain-inspired in-memory and neuromorphic computing, understanding the underlying mechanisms in the resistive switching of memristor devices is of paramount importance. Here, we demonstrate a two-step resistive switching set process involving a complex interplay among mobile halide ions/vacancies (I−/VI + ) and silver ions (Ag+ ) in perovskite-based memristors with thin undoped buffer layers. The resistive switching involves an initial gradual increase in current associated with a drift-related halide migration within the perovskite bulk layer followed by an abrupt resistive switching associated with diffusion of mobile Ag+ conductive filamentary formation. Furthermore, we develop a dynamical model that explains the characteristic I−V curve that helps to untangle and quantify the switching regimes consistent with the experimental memristive response. This further insight into the two-step set process provides another degree of freedom in device design for versatile applications with varying levels of complexityFunding for open access charge: CRUE-Universitat Jaume

Spectral properties of the dynamic state transition in metal halide perovskite-based memristor exhibiting negative capacitance

The evolution of device properties in memristor switching between high- and low-resistance states is critical for applications and is still highly subjected to significant ambiguity. Here, we present the dynamic state transition in a 2D Ruddlesden–Popper perovskite-based memristor device, measured via impedance spectroscopy. The spectral evolution of the transition exhibits a significant transformation of the low frequency arc to a negative capacitance arc, further decreasing the device resistance. The capacitance–frequency evolution of the device indicates that the appearance of the negative capacitance is intimately related to a slow kinetic phenomenon due to ionic migration and redistribution occurring at the perovskite/metal contact interface. In contrast, no negative capacitance arc is observed during the state transition of a memristor device where the contact is passivated by an undoped Spiro-OMeTAD interfacial layer. The switching mechanisms are entirely different, one due to interface transformation and the other due to filamentary formation

Role of Metal Contacts on Halide Perovskite Memristors

Halide perovskites are promising candidates for resistive memories (memristors) due to their mixed electronic/ionic conductivity and the real activation mechanism is currently under debate. In order to unveil the role of the metal contact and its connection with the activation process, four model systems are screened on halide perovskite memristors: Nearly inert metals (Au and Pt), low reactivity contacts (Cu), highly reactive contact (Ag and Al), and pre-oxidized metal in the form of AgI. It is revealed that the threshold voltage for activation of the memory effect is highly connected with the electrochemical activity of the metals. Redox/capacitive peaks are observed for reactive metals at positive potentials and charged ions are formed that can follow the electrical field. Activation proceeds by formation of conductive filaments, either by the direct migration of the charged metals or by an increase in the concentration of halide vacancies generated by this electrochemical reaction. Importantly, the use of pre-oxidized Ag+ ions leads to very low threshold voltages of ≈0.2 V indicating that an additional electrochemical reaction is not needed in this system to activate the memristor. Overall, the effect of the metal contact is clarified, and it is revealed that AgI is a very promising interfacial layer for low-energy applications.This study forms part of the Advanced Materials programme and was supported by MCIN with funding from European Union NextGenerationEU (PRTR-C17.I1) and by Generalitat Valenciana (CIGRIS/2022/150). The authors also thank the financial support of CONICET (External Fellowship 2020), Comunidad de Madrid (S2018/NMT-4326-SINFOTON2-CM), and of Universidad Rey Juan Carlos “Grupo DELFO de alto rendimiento”, reference M2363, under research program “Programa de fomento y desarrollo de la investigación”. C.G. would like to thank Generalitat Valenciana for a Grisolia grant (GRISOLIAP/2019/048). The authors thank the University Jaume I to allow for the use of Serveis Centrals d'Instrumentació Científica (SCIC)

Interfacial Passivation of Perovskite Solar Cells by Reactive Ion Scavengers

Lead halide perovskites suffer from uncontrolled ion migration and the interactions at the external contacts play a fundamental role in the hysteretic response and performance degradation kinetics. In this work, we passivate the external interfaces by a reaction of migrating iodide ions with a silver buffer layer placed between Spiro-OMeTAD and Au layers. In the presence of an electrical field, iodine migration occurs and ions that are close to the perovskite/contact interface irreversibly form a layer of AgI. Overall, the interfacial reaction of iodide ions totally suppresses hysteresis and leads to more stable devices. A new sample preparation method unburies the reactive interface, which is then probed by X-ray photoelectron spectroscopy measurements. The kinetics of the layer formation are monitored by impedance spectroscopy highlighting that in the presence of an electrical field and light, the reaction occurs in the order of minutes. We further identify the resistive response of AgI in operating devices. The present work provides a new approach to passivate the external interfaces in lead halide perovskites.Funding for open access charge: CRUE-Universitat Jaume

Physical Model for the Current–Voltage Hysteresis and Impedance of Halide Perovskite Memristors

An investigation of the kinetic behavior of MAPbI3 memristors shows that the onset voltage to a high conducting state depends strongly on the voltage sweep rate, and the impedance spectra generate complex capacitive and inductive patterns. We develop a dynamic model to describe these features and obtain physical insight into the coupling of ionic and electronic properties that produce the resistive switching behavior. The model separates the memristive response into distinct diffusion and transition-state-formation steps that describe well the experimental current–voltage curves at different scan rates and impedance spectra. The ac impedance analysis shows that the halide perovskite memristor response contains the composition of two inductive processes that provide a huge negative capacitance associated with inverted hysteresis. The results provide a new approach to understand some typical characteristics of halide perovskite devices, such as the inductive behavior and hysteresis effects, according to the time scales of internal processes.Funding for open access charge: CRUE-Universitat Jaume IWe acknowledge the financial support from Generalitat Valenciana for a Grisolia grant (GRISOLIAP/2019/048) and Ministerio de Ciencia y Innovación (PID2019-107348GB-100). We also acknowledge the financial support of CONICET (Extern Fellowship 2020); Comunidad de Madrid (S2018/NMT-4326-SINFOTON2-CM); and Universidad Rey Juan Carlos “Grupo DELFO de alto rendimiento”, reference M2363, under research program “Programa de fomento y desarrollo de la investigación”

Terahertz Emission of Gallium Arsenide on Textured p-type Silicon (100) Substrates Grown via Molecular Beam Epitaxy

This study presents the terahertz (THz) emission of molecular beam epitaxy (MBE)-grown Gallium Arsenide (GaAs) on surface textured p-type Silicon (p- Si) (100) substrates. Surface texturing was achieved by anisotropic wet chemical etching using 5% wt Potassium Hydroxide (KOH): Isopropyl alcohol (IPA) (50:1) solution for 15, 30, 45, and 60 minutes. Reflectivity measurements for the textured p-Si(100) substrates indicated that the overall texturing increases with longer etching times. Raman spectroscopy of the post-growth samples confirmed GaAs growth. The THz emission intensities were the same order of magnitude. The GaAs grown on p-Si(100) textured for 60 minutes exhibited the most intense THz emission attributed to the increased absorption from a larger surface-to-volume ratio due to surface texturing. All GaAs on textured p-Si(100) samples had frequency bandwidth of ~2.5 THz

Muscle cells of sporadic amyotrophic lateral sclerosis patients secrete neurotoxic vesicles

BACKGROUND: The cause of the motor neuron (MN) death that drives terminal pathology in amyotrophic lateral sclerosis (ALS) remains unknown, and it is thought that the cellular environment of the MN may play a key role in MN survival. Several lines of evidence implicate vesicles in ALS, including that extracellular vesicles may carry toxic elements from astrocytes towards MNs, and that pathological proteins have been identified in circulating extracellular vesicles of sporadic ALS patients. Because MN degeneration at the neuromuscular junction is a feature of ALS, and muscle is a vesicle-secretory tissue, we hypothesized that muscle vesicles may be involved in ALS pathology. METHODS: Sporadic ALS patients were confirmed to be ALS according to El Escorial criteria and were genotyped to test for classic gene mutations associated with ALS, and physical function was assessed using the ALSFRS-R score. Muscle biopsies of either mildly affected deltoids of ALS patients (n = 27) or deltoids of aged-matched healthy subjects (n = 30) were used for extraction of muscle stem cells, to perform immunohistology, or for electron microscopy. Muscle stem cells were characterized by immunostaining, RT-qPCR, and transcriptomic analysis. Secreted muscle vesicles were characterized by proteomic analysis, Western blot, NanoSight, and electron microscopy. The effects of muscle vesicles isolated from the culture medium of ALS and healthy myotubes were tested on healthy human-derived iPSC MNs and on healthy human myotubes, with untreated cells used as controls. RESULTS: An accumulation of multivesicular bodies was observed in muscle biopsies of sporadic ALS patients by immunostaining and electron microscopy. Study of muscle biopsies and biopsy-derived denervation-naïve differentiated muscle stem cells (myotubes) revealed a consistent disease signature in ALS myotubes, including intracellular accumulation of exosome-like vesicles and disruption of RNA-processing. Compared with vesicles from healthy control myotubes, when administered to healthy MNs the vesicles of ALS myotubes induced shortened, less branched neurites, cell death, and disrupted localization of RNA and RNA-processing proteins. The RNA-processing protein FUS and a majority of its binding partners were present in ALS muscle vesicles, and toxicity was dependent on the expression level of FUS in recipient cells. Toxicity to recipient MNs was abolished by anti-CD63 immuno-blocking of vesicle uptake. CONCLUSIONS: ALS muscle vesicles are shown to be toxic to MNs, which establishes the skeletal muscle as a potential source of vesicle-mediated toxicity in ALS

Hysteresis Control in Hybrid Perovskite-based Devices for Photovoltaic and Neuromorphic Computing Applications

Compendi d'articles, Doctorat internacionalPerovskite solar cells have been rapidly advancing in record efficiencies at an unprecedented rate already approaching the theoretical maximum efficiency in just over a decade of development. Most notably, perovskites exhibit intrinsic hysteretic effect which can be exploited for memory devices or memristors emulating the synaptic functions of the brain circumventing the physical limits of conventional computing architecture. In this thesis, we demonstrate hysteresis control in perovskite memristors by incorporating a thin Ag layer, establish the correlation of the IS response with the normal and inverted hysteresis, demonstrate a perovskite solar cell exhibiting a transformation from a normal capacitive to an inverted inductive hysteresis, and modulated the resistive switching of perovskite memristors exhibiting a two-step set process. This understanding of the dynamic ionic transport, in conjunction with the electronic transport, provides the necessary insight to control the hysteresis response of perovskite-based devices specifically suited for the targeted optoelectronic application.Programa de Doctorat en Cièncie

Transition from Capacitive to Inductive Hysteresis: A Neuron-Style Model to Correlate I–V Curves to Impedances of Metal Halide Perovskites

Metal halide perovskite (MHP) devices often show different types of hysteresis in separate voltage domains. At low voltage, the impedance response is capacitive, and the cell gives regular hysteresis. At high voltage, the hysteresis is inverted, corresponding to an inductive response that causes a negative capacitance feature. We calculate the hysteresis current due to a chemical inductor model, and we show that the current is inversely proportional to the voltage scan rate. We formulate a general dynamical model for the solar cell response in the style of neuronal models for the action potential, based on a few differential equations. The model allows us to track the transition from capacitive to inductive properties, both by impedance spectroscopy and current–voltage measurements at different voltage sweep rates. We obtain a correlation of the time constants for the capacitor and the inductor. We interpret the origin of the low-frequency features in terms of ion-controlled surface recombination. This explains the strong correlation of the low-frequency capacitance and inductor, as both originate from the same mechanism. The methodology derived in this paper provides great control over the dynamic properties of metal halide perovskite solar cells, even in cases in which there are qualitative changes of the solar cell current–voltage response over a broad voltage range

Theory of Hysteresis in Halide Perovskites by Integration of the Equivalent Circuit

Perovskite solar cells show a number of internal electronic–ionic effects that produce hysteresis in the current–voltage curves and a dependence of the temporal response on the conditions of the previous stimulus applied to the sample. There are many models and explanations in the literature, but predictive methods that may lead to an assessment of the solar cell behavior based on independent measurements are needed. Here, we develop a method to predict time domain response starting from the frequency domain response measured by impedance spectroscopy over a collection of steady states. The rationale of the method is to convert the impedance response into a set of differential equations, in which the internal state variables emerge naturally and need not be predefined in terms of a physical (drift/diffusion/interfaces) model. Then, one solves (integrates) the evolution for a required external perturbation such as voltage sweep at a constant rate (cyclic voltammetry). Using this method, we solve two elementary but relevant equivalent circuit models for perovskite solar cells and memristors, and we show the emergence of hysteresis in terms of the relevant time and energy constants that can be fully obtained from impedance spectroscopy. We demonstrate quantitatively a central insight in agreement with many observations: regular hysteresis is capacitive, and inverted hysteresis is inductive. Analysis of several types of perovskite solar cells shows excellent correlation of the type of equivalent circuit and the observed hysteresis. A new phenomenon of transformation from capacitive to inductive hysteresis in the course of the current–voltage curve is reported.We thank the financial support by Generalitat Valenciana for a Prometeo grant (PROMETEU/2020/028), Grisolia grant (GRISOLIAP/2019/048), and Ministerio de Ciencia y Innovación (PID2019-107348GB-100)