A Strategy to Design High‐Density Nanoscale Devices utilizing Vapor Deposition of Metal Halide Perovskite Materials

The demand for high memory density has increased due to increasing needs of information storage, such as big data processing and the Internet of Things. Organic–inorganic perovskite materials that show nonvolatile resistive switching memory properties have potential applications as the resistive switching layer for next‐generation memory devices, but, for practical applications, these materials should be utilized in high‐density data‐storage devices. Here, nanoscale memory devices are fabricated by sequential vapor deposition of organolead halide perovskite (OHP) CH₃NH₃PbI₃ layers on wafers perforated with 250 nm via‐holes. These devices have bipolar resistive switching properties, and show low‐voltage operation, fast switching speed (200 ns), good endurance, and data‐retention time >10⁵ s. Moreover, the use of sequential vapor deposition is extended to deposit CH₃NH₃PbI₃ as the memory element in a cross‐point array structure. This method to fabricate high‐density memory devices could be used for memory cells that occupy large areas, and to overcome the scaling limit of existing methods; it also presents a way to use OHPs to increase memory storage capacity.     

Multi‐Nonvolatile State Resistive Switching Arising from Ferroelectricity and Oxygen Vacancy Migration

Resistive switching phenomena form the basis of competing memory technologies. Among them, resistive switching, originating from oxygen vacancy migration (OVM), and ferroelectric switching offer two promising approaches. OVM in oxide films/heterostructures can exhibit high/low resistive state via conducting filament forming/deforming, while the resistive switching of ferroelectric tunnel junctions (FTJs) arises from barrier height or width variation while ferroelectric polarization reverses between asymmetric electrodes. Here the authors demonstrate a coexistence of OVM and ferroelectric induced resistive switching in a BaTiO₃ FTJ by comparing BaTiO₃ with SrTiO₃ based tunnel junctions. This coexistence results in two distinguishable loops with multi‐nonvolatile resistive states. The primary loop originates from the ferroelectric switching. The second loop emerges at a voltage close to the SrTiO₃ switching voltage, showing OVM being its origin. BaTiO₃ based devices with controlled oxygen vacancies enable us to combine the benefits of both OVM and ferroelectric tunneling to produce multistate nonvolatile memory devices.     

Pseudohalide‐Induced 2D (CH3NH3)2PbI2(SCN)2 Perovskite for Ternary Resistive Memory with High Performance

Recently, organic–inorganic hybrid perovskites (OIHP) are studied in memory devices, but ternary resistive memory with three states based on OIHP is not achieved yet. In this work, ternary resistive memory based on hybrid perovskite is achieved with a high device yield (75%), much higher than most organic ternary resistive memories. The pseudohalide‐induced 2D (CH₃NH₃)₂PbI₂(SCN)₂ perovskite thin film is prepared by using a one‐step solution method and fabricated into Al/perovskite film/indium–tin oxide (glass substrate as well as flexible polyethylene terephthalate substrate) random resistive access memory (RRAM) devices. The three states have a conductivity ratio of 1:10³:10⁷, long retention over 10 000 s, and good endurance properties. The electrode area variation, impedance test, and current–voltage plotting show that the two resistance switches are attributable to the charge trap filling due to the effect of unscreened defect in 2D nanosheets and the formation of conductive filaments, respectively. This work paves way for stable perovskite multilevel RRAMs in ambient atmosphere.     

Na‐Cation‐Assisted Exfoliation of MX2 (M = Mo,W; X = S,Se) Nanosheets in an Aqueous Medium with the Aid of a Polymeric Surfactant for Flexible Polymer‐Nanocomposite Memory Applications

2D nanosheets of transition metal dichalcogenides (TMDCs) have been attracting attention due to their sizable band gap. Facile and effective Na‐cation‐assisted exfoliation of TMDC (MX₂, M = Mo, W; X = S, Se) nanosheets in an aqueous medium and their application as a composite filler in a polyvinyl alcohol (PVA) matrix are explored in this work. The presence of Na cations is highly beneficial for exfoliating defect‐free and few‐layer MX₂ nanosheets in water in the presence of small‐sized micelles of polymeric surfactant, and significantly elevates the exfoliation yield by more than one order of magnitude compared to a conventional surfactant‐assisted exfoliation. The strategy suggested in this work is very advantageous compared to both Li cation intercalation in organic solvents and conventional low‐yield surfactant‐assisted exfoliations. As an application of the exfoliated nanosheets, the fabrication of memory devices with the configuration of Ga‐doped ZnO/MX₂–PVA/Ag is demonstrated, and they exhibit bistable and write‐once‐read‐many‐times resistive switching behavior with a high ON/OFF current ratio of 3 × 10³ at ?1.0 V (for WS₂) and 2.0 V (for MoS₂). Furthermore, MX₂–PVA nanocomposite fibrous films and mats are successfully fabricated using an electrospinning technique, which can expand the use of TMDC nanofillers in applications involving highly flexible polymer‐based MX₂ composites.     

Synergies of Electrochemical Metallization and Valance Change in All‐Inorganic Perovskite Quantum Dots for Resistive Switching

The in‐depth understanding of ions' generation and movement inside all‐inorganic perovskite quantum dots (CsPbBr₃ QDs), which may lead to a paradigm to break through the conventional von Neumann bottleneck, is strictly limited. Here, it is shown that formation and annihilation of metal conductive filaments and Br^? ion vacancy filaments driven by an external electric field and light irradiation can lead to pronounced resistive‐switching effects. Verified by field‐emission scanning electron microscopy as well as energy‐dispersive X‐ray spectroscopy analysis, the resistive switching behavior of CsPbBr₃ QD‐based photonic resistive random‐access memory (RRAM) is initiated by the electrochemical metallization and valance change. By coupling CsPbBr₃ QD‐based RRAM with a p‐channel transistor, the novel application of an RRAM–gate field‐effect transistor presenting analogous functions of flash memory is further demonstrated. These results may accelerate the technological deployment of all‐inorganic perovskite QD‐based photonic resistive memory for successful logic application.     

Photocatalytic Reduction of Graphene Oxide–TiO2 Nanocomposites for Improving Resistive‐Switching Memory Behaviors

Graphene oxide (GO)‐based resistive‐switching (RS) memories offer the promise of low‐temperature solution‐processability and high mechanical flexibility, making them ideally suited for future flexible electronic devices. The RS of GO can be recognized as electric‐field‐induced connection/disconnection of nanoscale reduced graphene oxide (RGO) conducting filaments (CFs). Instead of operating an electrical FORMING process, which generally results in high randomness of RGO CFs due to current overshoot, a TiO₂‐assisted photocatalytic reduction method is used to generate RGO‐domains locally through controlling the UV irradiation time and TiO₂ concentration. The elimination of the FORMING process successfully suppresses the RGO overgrowth and improved RS memory characteristics are achieved in graphene oxide–TiO₂ (Go‐TiO₂) nanocomposites, including reduced SET voltage, improved switching variability, and increased switching speed. Furthermore, the room‐temperature process of this method is compatible with flexible plastic substrates and the memory cells exhibit excellent flexibility. Experimental results evidence that the combined advantages of reducing the oxygen‐migration barrier and enhancing the local‐electric‐field with RGO‐manipulation are responsible for the improved RS behaviors. These results offer valuable insight into the role of RGO‐domains in GO memory devices, and also, this mild photoreduction method can be extended to the development of carbon‐based flexible electronics.     

Flexible Nonvolatile Transistor Memory with Solution‐Processed Transition Metal Dichalcogenides

Nonvolatile field‐effect transistor (FET) memories containing transition metal dichalcogenide (TMD) nanosheets have been recently developed with great interest by utilizing some of the intriguing photoelectronic properties of TMDs. The TMD nanosheets are, however, employed as semiconducting channels in most of the memories, and only a few works address their function as floating gates. Here, a floating‐gate organic‐FET memory with an all‐in‐one floating‐gate/tunneling layer of the solution‐processed TMD nanosheets is demonstrated. Molybdenum disulfide (MoS₂) is efficiently liquid‐exfoliated by amine‐terminated polystyrene with a controlled amount of MoS₂ nanosheets in an all‐in‐one floating‐gate/tunneling layer, allowing for systematic investigation of concentration‐dependent charge‐trapping and detrapping properties of MoS₂ nanosheets. At an optimized condition, the nonvolatile memory exhibits memory performances with an ON/OFF ratio greater than 10⁴, a program/erase endurance cycle over 400 times, and data retention longer than 7 × 10³ s. All‐in‐one floating‐gate/tunneling layers containing molybdenum diselenide and tungsten disulfide are also developed. Furthermore, a mechanically‐flexible TMD memory on a plastic substrate shows a performance comparable with that on a hard substrate, and the memory properties are rarely altered after outer‐bending events over 500 times at the bending radius of 4.0 mm.     

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 SrFeO_2.5 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, SrFeO_2.5, and the conductive perovskite phase, SrFeO₃, gives rise to the resistive‐switching properties of SrFeO_x memristive devices. Furthermore, it is found that the electric‐field‐induced phase transition spreads over a large area in (001) oriented SrFeO_2.5 devices, where oxygen vacancy channels are ordered along the in‐plane direction of the device. In contrast, (111)‐grown SrFeO_2.5 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 SrFeO_x‐based memristive devices within the framework of metal–insulator topotactic phase transitions.     

White-light-controlled resistive switching and photovoltaic effects in TiO2/ZnO composite nanorods array at room temperature

White-light-controlled resistance switching and photovoltaic effects in TiO₂/ZnO composite nanorods array grown on fluorine-doped tin oxide (FTO) substrate by hydrothermal process were investigated. The average length of TiO₂/ZnO nanorods is about 3 μm, and the average diameter is about 200 nm. ZnO nanoparticles with size 5–10 nm are embedded in TiO₂ base material. The current–voltage characteristics of Ag/[TiO₂/ZnO]/FTO device demonstrate an outstanding rectifying property and bipolar resistive switching behavior. Specially, the resistive switching behavior can be regulated by white-light illuminating. In addition, this structure also exhibits a substantial white-light photovoltaic effect. This study is helpful for exploring the multifunctional materials and their applications in nonvolatile multistate memory devices and solar cells.     

Resistive Switching in Nanogap Systems on SiO2 Substrates

Voltage‐controlled resistive switching in various gap systems on SiO₂ substrates is reported. The nanoscale‐sized gaps are made by several means using different materials including metals, semiconductors, and amorphous carbon. The switching site is further reduced in size by using multiwalled carbon nanotubes and single‐walled carbon nanotubes. The switching in all the gap systems shares the same characteristics. This independence of switching on the material compositions of the electrodes, accompanied by observable damage to the SiO₂ substrate at the gap region, bespeaks the intrinsic switching from post‐breakdown SiO₂. It calls for caution when studying resistive switching in nanosystems on oxide substrates, since oxide breakdown extrinsic to the nanosystem can mimic resistive switching. Meanwhile, the high ON/OFF ratio (≈10⁵), fast switching time (2 µs, tested limit), and durable cycles show promising memory properties. The observed intermediate states reveal the filamentary nature of the switching.     

Hybrid Flexible Resistive Random Access Memory‐Gated Transistor for Novel Nonvolatile Data Storage

Here, a single‐device demonstration of novel hybrid architecture is reported to achieve programmable transistor nodes which have analogies to flash memory by incorporating a resistive switching random access memory (RRAM) device as a resistive switch gate for field effect transistor (FET) on a flexible substrate. A high performance flexible RRAM with a three‐layered structure is fabricated by utilizing solution‐processed MoS₂ nanosheets sandwiched between poly(methyl methacrylate) polymer layers. Gate coupling with the pentacene‐based transistor can be controlled by the RRAM memory state to produce a nonprogrammed state (inactive) and a programmed state (active) with a well‐defined memory window. Compared to the reference flash memory device based on the MoS₂ floating gate, the hybrid device presents robust access speed and retention ability. Furthermore, the hybrid RRAM‐gated FET is used to build an integrated logic circuit and a wide logic window in inverter logic is achieved. The controllable, well‐defined memory window, long retention time, and fast access speed of this novel hybrid device may open up new possibilities of realizing fully functional nonvolatile memory for high‐performance flexible electronics.     

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2‐Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS₂) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS₂ is introduced into the 2H‐MoS₂ nanosheets by two‐step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write‐once read‐many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H‐MoS₂‐polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H‐MoS₂‐PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS₂ nanosheets plays an important role on the resistive switching behaviors of the MoS₂‐based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.     

Dielectric and impedance spectroscopy of Ni doped BiFeO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> electronic system

A nickel modified BiFeO₃–BaTiO₃ electronic system has been fabricated by using a high-temperature solid-state reaction process. Preliminary X-ray structural analysis has confirmed the formation of a single-phase material in the orthorhombic crystal system. The dielectric and impedance characteristics of the prepared material have been studied in a wide range of frequency (1 kHz-1 MHz) at different temperatures (25–500 °C) for the better understanding of the frequency-temperature dependence of its capacitive and resistive behavior respectively. A significant effect of grains and grain boundaries of the resistive characteristics of the material is observed at high temperatures. The electrical conductivity of the material increases with increase in frequency in the low-temperature region. Preliminary study of a small amount of Ni doping in the above binary system (i.e., BiFeO₃–BaTiO₃) has provided many interesting results which may be useful for the fabrication of an electronic device.     

Effects of drying temperature on preparation of pectin polysaccharide thin film for resistive switching memory

Commercially purchased pectin powder was formulated and processed into a thin film with different drying temperatures (60 °C–180 °C) and subsequently, sandwiched between Ag and ITO as top and bottom electrodes, respectively, for resistive switching memory study. Chemical functional groups, surface roughness, elemental chemical composition, and cross-sectional morphology of the dried pectin thin film were investigated via Fourier transform infrared spectroscopy, atomic force microscopy, and high-resolution transmission electron microscopy-energy dispersive X-ray spectroscopy. The higher the drying temperature, the rougher the surface with less residual moisture and more defect sites within the thin film was observed. Current–voltage measurements together with the chemical analysis results were used to establish the resistive switching mechanism. Resistive switching controlled by oxygen vacancy as conductive paths with ON/OFF ratio of 10⁴, memory window of 3.6 V, and retention time of 10 years with READ voltage as low as ? 0.01 V for pectin dried at 180 °C has been demonstrated.

     

Non‐Volatile ReRAM Devices Based on Self‐Assembled Multilayers of Modified Graphene Oxide 2D Nanosheets

2D nanomaterials have been actively utilized in non‐volatile resistive switching random access memory (ReRAM) devices due to their high flexibility, 3D‐stacking capability, simple structure, transparency, easy fabrication, and low cost. Herein, it demonstrates re‐writable, bistable, transparent, and flexible solution‐processed crossbar ReRAM devices utilizing graphene oxide (GO) based multilayers as active dielectric layers. The devices employ single‐ or multi‐component‐based multilayers composed of positively charged GO (N‐GO(+) or NS‐GO(+)) with/without negatively charged GO(‐) using layer‐by‐layer assembly method, sandwiched between Al bottom and Au top electrodes. The device based on the multi‐component active layer Au/[N‐GO(+)/GO(‐)]_n/Al/PES shows higher ON/OFF ratio of ≈10⁵ with switching voltage of ?1.9 V and higher retention stability (≈10⁴ s), whereas the device based on single component (Au/[N‐GO(+)]_n/Al/PES) shows ≈10³ ON/OFF ratio at ±3.5 V switching voltage. The superior ReRAM properties of the multi‐component‐based device are attributed to a higher coating surface roughness. The Au/[N‐GO(+)/GO(–)]_n/Al/PES device prepared from lower GO concentration (0.01%) exhibits higher ON/OFF ratio (≈10⁹) at switching voltage of ±2.0 V. However, better stability is achieved by increasing the concentration from 0.01% to 0.05% of all GO‐based solutions. It is found that the devices containing MnO₂ in the dielectric layer do not improve the ReRAM performance.     

Electrochemical Tantalum Oxide for Resistive Switching Memories

Redox‐based resistive switching memories (ReRAMs) are strongest candidates for the next‐generation nonvolatile memories fulfilling the criteria for fast, energy efficient, and scalable green IT. These types of devices can also be used for selector elements, alternative logic circuits and computing, and memristive and neuromorphic operations. ReRAMs are composed of metal/solid electrolyte/metal junctions in which the solid electrolyte is typically a metal oxide or multilayer oxides structures. Here, this study offers an effective and cheap electrochemical approach to fabricate Ta/Ta₂O₅‐based devices by anodizing. This method allows to grow high‐quality and dense oxide thin films onto a metallic substrates with precise control over morphology and thickness. Electrochemical‐oxide‐based devices demonstrate superior properties, i.e., endurance of at least 10⁶ pulse cycles and/or 10³I–V sweeps maintaining a good memory window with a low dispersion in R_OFF and R_ON values, nanosecond fast switching, and data retention of at least 10⁴ s. Multilevel programing capability is presented with both I–V sweeps and pulse measurements. Thus, it is shown that anodizing has a great prospective as a method for preparation of dense oxide films for resistive switching memories.     

The Bipolar Resistive Switching in BiFeO3 Films

Pure-phase polycrystalline BiFeO₃ films have been successfully prepared by pulsed-laser deposition on surface oxidized Si substrates using LaNiO₃ buffer layer with substrate temperature (T _s) ranging from 550?°C to 800?°C and a laser frequency of 5?Hz and 10?Hz. Bipolar resistive switching has been observed in all the films using LaNiO₃ as bottom electrodes and silver glue dots as top electrodes, the resistivity switches from a high-resistance state (HRS) to a low-resistance state (LRS) with positive voltage applied on the top Ag electrodes, and from LRS to HRS with positive voltage applied on the bottom LaNiO₃ electrodes. The mechanism of the resistive switching has been confirmed to be due to the voltage polarity dependent formation/rupture of the conducting filaments formed by the O vacancies. The highest resistive ratio of HRS to LRS, of more than 2 orders of magnitude, has been achieved in the highest resistive BiFeO₃ film prepared at T _s of 650?°C and laser frequency of 10?Hz.     

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.     

Supramolecular Framework Constructed by Dendritic Nanopolymer for Stable Flexible Perovskite Resistive Random-Access Memory

The 3D supramolecular framework (3D-SF) is constructed in this work through the hydrogen bond assisted self-assembly of spherical dendritic nanopolymer to regulate the flexibility, stability, and resistive switching (RS) performance of perovskite resistive random-access memory (RRAM). Herein, the 3D-SF network acts as the perovskite crystallization template to regulate the perovskite crystallization process due to its coordination interaction of functional groups with the perovskite grains, presenting the uniform, pinhole-free, and compact perovskite morphology for stable flexible RRAM. The 3D-SF network in situ stays at the perovskite intergranular boundaries to crosslink the perovskite grains. The RS performance of 3D-SF-modified perovskite RRAM device is evidently improved to the ON/OFF ratio of 10⁵, the cycle number of 500 times, and the data retention time of 10⁴ s. The 50-days exposure of unencapsulated RRAM device at ambient environment still makes the ON/OFF ratio to be kept at ≈10⁴, indicating the potential of long-term stable multilevel storage in the high-density data storage. The bending action under different radius also does not change the RS performance due to the excellent bending-resistant ability of 3D-SF-modified perovskite film. This work explores a novel polymer additive strategy to construct the 3D supramolecular framework for stable flexible perovskite optoelectronic devices.     

Unipolar resistive switching behavior in Dy2O3 films for nonvolatile memory applications

In this article, we report the forming-free resistive switching behavior of a Ru/Dy₂O₃/TaN memory device incorporating a Dy₂O₃ thin film fabricated entirely through processing at room temperature. We used X-ray diffraction, secondary ion mass spectrometry, and X-ray photoelectron spectroscopy to investigate the structural and chemical features of the Dy₂O₃ film. The dominant conduction mechanisms in the low- and high-resistance states were ohmic behavior and Poole-Frenkel emission, respectively. The Ru/Dy₂O₃/TaN memory device exhibited a high resistance ratio and provided nondestructive readout and reliable data retention. This memory device has a great potential for application in nonvolatile resistive switching memory.