Size Dependence of Resistive Switching at Nanoscale Metal‐Oxide Interfaces

Size dependent variations in resistive switching using a metal‐semiconducting oxide model to examine the underlying mechanisms are reported. In the range of 20 nm to 200 nm, Au nanoparticle/SrTiO₃ interface transport properties are size dependent. The size dependence is attributed to the combination of geometric scaling and size‐dependent Schottky properties. After electroforming, the observed “eight‐wise” bipolar resistive hysteresis loop is modulated by trap/detrap process. The size‐dependent high resistance state is consistent with changes in both the interfacial area and Schottky properties. The low resistance state exhibits size independent resistance through the dominant fast conductive path. Detrapping requires more work for smaller interfaces due to the associated larger built‐in electric field.     

Collective Motion of Conducting Filaments in Pt/n‐Type TiO2/p‐Type NiO/Pt Stacked Resistance Switching Memory

Filamentary resistance switching (RS) is one of the more obvious and useful phenomena in the family of RS mechanisms. In filamentary RS, the long reset switching time and substantially large power consumption are the critical obstacles for microelectronic applications. In this study, an innovative solution to overcome this reset problem is suggested by stacking n‐type TiO₂ and p‐type NiO films. Interestingly, in this stacked structure, the region where filament rupture and rejuvenation occurs could be arbitrarily controlled to be at any location between the interface with the metal electrode and the TiO₂/NiO interface by using an appropriate switching sequence. This collective motion behavior of conducting filaments can be practically used to reduce reset switching time from ～100 μs to ～150 ns, with an extremely high off/on resistance ratio of ～10⁶.     

Configurable Resistive Switching between Memory and Threshold Characteristics for Protein‐Based Devices

The employ of natural biomaterials as the basic building blocks of electronic devices is of growing interest for biocompatible and green electronics. Here, resistive switching (RS) devices based on naturally silk protein with configurable functionality are demonstrated. The RS type of the devices can be effectively and exactly controlled by controlling the compliance current in the set process. Memory RS can be triggered by a higher compliance current, while threshold RS can be triggered by a lower compliance current. Furthermore, two types of memory devices, working in random access and WORM modes, can be achieved with the RS effect. The results suggest that silk protein possesses the potential for sustainable electronics and data storage. In addition, this finding would provide important guidelines for the performance optimization of biomaterials based memory devices and the study of the underlying mechanism behind the RS effect arising from biomaterials.     

Nanoscale Ferroelectricity in Crystalline γ‐Glycine

Ferroelectrics are multifunctional materials that reversibly change their polarization under an electric field. Recently, the search for new ferroelectrics has focused on organic and bio‐organic materials, where polarization switching is used to record/retrieve information in the form of ferroelectric domains. This progress has opened a new avenue for data storage, molecular recognition, and new self‐assembly routes. Crystalline glycine is the simplest amino acid and is widely used by living organisms to build proteins. Here, it is reported for the first time that γ‐glycine, which has been known to be piezoelectric since 1954, is also a ferroelectric, as evidenced by local electromechanical measurements and by the existence of as‐grown and switchable ferroelectric domains in microcrystals grown from the solution. The experimental results are rationalized by molecular simulations that establish that the polarization vector in γ‐glycine can be switched on the nanoscale level, opening a pathway to novel classes of bioelectronic logic and memory devices.     

Nanoscale Resistive Switching in Amorphous Perovskite Oxide (a‐SrTiO3) Memristors

Memristive devices are the precursors to high density nanoscale memories and the building blocks for neuromorphic computing. In this work, a unique room temperature synthesized perovskite oxide (amorphous SrTiO₃: a‐STO) thin film platform with engineered oxygen deficiencies is shown to realize high performance and scalable metal‐oxide‐metal (MIM) memristive arrays demonstrating excellent uniformity of the key resistive switching parameters. a‐STO memristors exhibit nonvolatile bipolar resistive switching with significantly high (10³–10⁴) switching ratios, good endurance (>10⁶I–V sweep cycles), and retention with less than 1% change in resistance over repeated 10⁵ s long READ cycles. Nano‐contact studies utilizing in situ electrical nanoindentation technique reveal nanoionics driven switching processes that rely on isolatedly controllable nano‐switches uniformly distributed over the device area. Furthermore, in situ electrical nanoindentation studies on ultrathin a‐STO/metal stacks highlight the impact of mechanical stress on the modulation of non‐linear ionic transport mechanisms in perovskite oxides while confirming the ultimate scalability of these devices. These results highlight the promise of amorphous perovskite memristors for high performance CMOS/CMOL compatible memristive systems.     

Oxygen Ion Drift‐Induced Complementary Resistive Switching in Homo TiOx/TiOy/TiOx and Hetero TiOx/TiON/TiOx Triple Multilayer Frameworks

Developing a means by which to compete with commonly used Si‐based memory devices represents an important challenge for the realization of future three‐dimensionally stacked crossbar‐array memory devices with multifunctionality. Therefore, oxide‐based resistance switching memory (ReRAM), with its associated phenomena of oxygen ion drifts under a bias, is becoming increasingly important for use in nanoscalable crossbar arrays with an ideal memory cell size due to its simple metal–insulator–metal structure and low switching current of 10–100 μA. However, in a crossbar array geometry, one single memory element defined by the cross‐point of word and bit lines is highly susceptible to unintended leakage current due to parasitic paths around neighboring cells when no selective devices such as diodes or transistors are used. Therefore, the effective complementary resistive switching (CRS) features in all Ti‐oxide‐based triple layered homo Pt/TiO_x/TiO_y/TiO_x/Pt and hetero Pt/TiO_x/TiON/TiO_x/Pt geometries as alternative resistive switching matrices are reported. The possible resistive switching nature of the novel triple matrices is also discussed together with their electrical and structural properties. The ability to eliminate both an external resistor for efficient CRS operation and a metallic Pt middle electrode for further cost‐effective scalability will accelerate progress toward the realization of cross‐bar ReRAM in this framework.     

Highly Uniform,Electroforming‐Free,and Self‐Rectifying Resistive Memory in the Pt/Ta2O5/HfO2‐x/TiN Structure

The development of a resistance switching (RS) memory cell that contains rectification functionality in itself, highly reproducible RS performance, and electroforming‐free characteristics is an impending task for the development of resistance switching random access memory. In this work, a two‐layered dielectric structure consisting of HfO₂ and Ta₂O₅ layers, which are in contact with the TiN and Pt electrode, is presented for achieving these tasks simultaneously in one sample configuration. The HfO₂ layer works as the resistance switching layer by trapping or detrapping of electronic carriers, whereas the Ta₂O₅ layer remains intact during the whole switching cycle, which provides the rectification. With the optimized structure and operation conditions for the given materials, excellent RS uniformity, electroforming‐free, and self‐rectifying functionality could be simultaneously achieved from the Pt/Ta₂O₅/HfO₂/TiN structure.     

Nonvolatile Ferroelectric Memory Effect in Ultrathin α‐In2Se3

High‐density memory is integral in solid‐state electronics. 2D ferroelectrics offer a new platform for developing ultrathin electronic devices with nonvolatile functionality. Recent experiments on layered α‐In₂Se₃ confirm its room‐temperature out‐of‐plane ferroelectricity under ambient conditions. Here, a nonvolatile memory effect in a hybrid 2D ferroelectric field‐effect transistor (FeFET) made of ultrathin α‐In₂Se₃ and graphene is demonstrated. The resistance of the graphene channel in the FeFET is effectively controllable and retentive due to the electrostatic doping, which stems from the electric polarization of the ferroelectric α‐In₂Se₃. The electronic logic bit can be represented and stored with different orientations of electric dipoles in the top‐gate ferroelectric. The 2D FeFET can be randomly rewritten over more than 10⁵ cycles without losing the nonvolatility. The approach demonstrates a prototype of rewritable nonvolatile memory with ferroelectricity in van der Waals 2D materials.     

Memristor Kinetics and Diffusion Characteristics for Mixed Anionic‐Electronic SrTiO3‐δ Bits: The Memristor‐Based Cottrell Analysis Connecting Material to Device Performance

Memristors based on mixed anionic‐electronic conducting oxides are promising devices for future data storage and information technology with applications such as non‐volatile memory or neuromorphic computing. Unlike transistors solely operating on electronic carriers, these memristors rely, in their switch characteristics, on defect kinetics of both oxygen vacancies and electronic carriers through a valence change mechanism. Here, Pt|SrTiO_3‐δ|Pt structures are fabricated as a model material in terms of its mixed defects which show stable resistive switching. To date, experimental proof for memristance is characterized in hysteretic current–voltage profiles; however, the mixed anionic‐electronic defect kinetics that can describe the material characteristics in the dynamic resistive switching are still missing. It is shown that chronoamperometry and bias‐dependent resistive measurements are powerful methods to gain complimentary insights into material‐dependent diffusion characteristics of memristors. For example, capacitive, memristive and limiting currents towards the equilibrium state can successfully be separated. The memristor‐based Cottrell analysis is proposed to study diffusion kinetics for mixed conducting memristor materials. It is found that oxygen diffusion coefficients increase up to 3 × 10^–15 m²s^–1 for applied bias up to 3.8 V for SrTiO_3‐δ memristors. These newly accessible diffusion characteristics allow for improving materials and implicate field strength requirements to optimize operation towards enhanced performance metrics for valence change memristors.     

Highly Stable Cycling of Amorphous Li2CO3‐Coated α‐Fe2O3 Nanocrystallines Prepared via a New Mechanochemical Strategy for Li‐Ion Batteries

In this study, an amorphous Li₂CO₃‐coated nanocrystalline α‐Fe₂O₃ hierarchical structure is synthesized for the first time using a facile one‐step mechanochemical process at room temperature, taking advantage of the concurrence of repeated fracture‐cold welding of material's particles and a gas‐solid redox reaction. The conformal coating and hierarchical structure significantly increase the cycling durability and rate capability. Typically, a 1–3 nm thick amorphous Li₂CO₃ layer is conformally coated on Fe₂O₃ nanocrystallines (≈10 nm in size) that form hierarchically aggregated particles 400–800 nm in size by ball milling α‐Fe₂O₃ with LiH in CO₂. The prepared Li₂CO₃‐coated nanocrystalline α‐Fe₂O₃ exhibits highly stable long‐term cyclability as it delivers a reversible capacity of 975 mAh g^?1 with 99% of retention after 400 cycles at 100 mA g^?1. At a high rate of 3000 mA g^?1, its reversible capacity still remains at 537 mAh g^?1, superior to the uncoated counterpart (311 mAh g^?1). Moreover, amorphous Li₂O and Li₂CO₃ coatings are also similarly produced on Fe₂O₃ and NiO nanocrystallines, respectively, representing the general applicability of this mechanochemical approach.     

Continuous Aspect‐Ratio Tuning and Fine Shape Control of Monodisperse α‐Fe2O3 Nanocrystals by a Programmed Microwave–Hydrothermal Method

A rapid and economical route based on an efficient microwave–hydrothermal process has been developed to synthesize monodisperse α‐Fe₂O₃ nanocrystals with continuous aspect‐ratio tuning and fine shape control, which takes advantage of microwave irradiation and hydrothermal effects. This method easily programs the experimental conditions (e.g., temperature and time) and significantly shortens the synthesis time to minutes. It allows the creation of numerous recipes for optimizing and scaling up production. The effects of experimental conditions including reaction temperature and reactant concentration on the morphology of α‐Fe₂O₃ have been investigated systematically. Results reveal that the initial molar ratio of Fe³⁺ to PO plays a crucial role in the final morphology of the α‐Fe₂O₃ products. Several morphologies, which include ellipsoids/spindles with aspect ratios that range from 1.1 to 6.3, nanosheets, nanorings, and spheres can be obtained. The as‐formed α‐Fe₂O₃ exhibits shape‐dependent infrared optical properties. The growth process of colloidal α‐Fe₂O₃ crystals in the presence of phosphate ions is discussed. The products have been characterized by using X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, and infrared spectroscopy. This work presents an efficient and cost‐effective approach that is potentially competitive for scaling‐up industrial production. The as‐formed α‐Fe₂O₃ crystals with controllable morphologies not only provide flexible building blocks for advanced functional devices, but are also ideal candidates for studying their nanoarchitecture‐dependent performance in optical, catalytic, and magnetic applications.