Resistive Switching of Individual, Chemically Synthesized TiO2 Nanoparticles

Memristive Behavior of Mixed Oxide Nanocrystal Assemblies

Recent advances in memristive nanocrystal assemblies leverage controllable colloidal chemistry to induce a broad range of defect-mediated electrochemical reactions, switching phenomena, and modulate active parameters. The sample geometry of virtually all resistive switching studies involves thin film layers comprising monomodal diameter nanocrystals. Here we explore the evolution of bipolar and threshold resistive switching across highly ordered, solution-processed nanoribbon assemblies and mixtures comprising BaZrO₃ (BZO) and SrZrO₃ (SZO) nanocrystals. The effects of nanocrystal size, packing density, and A-site substitution on operating voltage (V_SET and V_TH) and switching mechanism were studied through a systematic comparison of nanoribbon heterogeneity (i.e., BZO-BZO vs BZO-SZO) and monomodal vs bimodal size distributions (i.e., small-small and small-large). Analysis of the current-voltage response confirms that tip-induced, trap-mediated space-charge-limited current and trap-assisted tunneling processes drive the low- and high-resistance states, respectively. Our results demonstrate that both smaller nanocrystals and heavier alkaline earth substitution decrease the onset voltage and improve stability and state retention of monomodal assemblies and bimodal nanocrystal mixtures, thus providing a base correlation that informs fabrication of solution-processed, memristive nanocrystal assemblies.

Memristive TiO2: Synthesis, Technologies, and Applications

Titanium dioxide (TiO2) is one of the most widely used materials in resistive switching applications, including random-access memory, neuromorphic computing, biohybrid interfaces, and sensors. Most of these applications are still at an early stage of development and have technological challenges and a lack of fundamental comprehension. Furthermore, the functional memristive properties of TiO2 thin films are heavily dependent on their processing methods, including the synthesis, fabrication, and post-fabrication treatment. Here, we outline and summarize the key milestone achievements, recent advances, and challenges related to the synthesis, technology, and applications of memristive TiO2. Following a brief introduction, we provide an overview of the major areas of application of TiO2-based memristive devices and discuss their synthesis, fabrication, and post-fabrication processing, as well as their functional properties.

Dissecting complex nanoparticle heterostructures via multimodal data fusion with aberration-corrected STEM spectroscopy

With nanostructured materials such as catalytic heterostructures projected to play a critical role in applications ranging from water splitting to energy harvesting, tailoring their properties to specific tasks requires an increasingly comprehensive characterization of their local chemical and electronic landscape. Although aberration-corrected electron spectroscopy currently provides sufficient spatial resolution to study this space, an approach to concurrently dissect both the electronic structure and full composition of buried metal/oxide interfaces remains a considerable challenge. In this manuscript, we outline a statistical methodology to jointly analyze simultaneously-acquired STEM EELS and EDX datasets by fusing them along their shared spatial factors. We show how this procedure can be used to derive a rich descriptive model for estimating both transition metal valency and full chemical composition from encapsulated morphologies such as core-shell nanoparticles. We demonstrate this on a heterogeneous Co-P thin film catalyst, concluding that this system is best described as a multi-shell phosphide structure with a P-doped metallic Co core.

Understanding memristive switching via in situ characterization and device modeling

Owing to their attractive application potentials in both non-volatile memory and unconventional computing, memristive devices have drawn substantial research attention in the last decade. However, major roadblocks still remain in device performance, especially concerning relatively large parameter variability and limited cycling endurance. The response of the active region in the device within and between switching cycles plays the dominating role, yet the microscopic details remain elusive. This Review summarizes recent progress in scientific understanding of the physical origins of the non-idealities and propose a synergistic approach based on in situ characterization and device modeling to investigate switching mechanism. At last, the Review offers an outlook for commercialization viability of memristive technology.

Memristor as the fourth basic element of electric circuits has drawn substantial attention for developing future computing technologies. Sun et al. report the progress and the challenges facing researchers on understanding memristive switching, and advocate continuous studies using a synergistic approach.

Microscopic investigations of switching phenomenon in memristive systems: a mini review

Resistive switching memories have been regarded as one of the most up and coming memory systems and researchers have shown great interest in them because of their simple structure, high speed and low fabrication cost. These memory systems also have great potential for scaling, however, this has been difficult to achieve without detailed understanding of underlying switching mechanisms. Meanwhile, scaling down could also raise reliability concerns in its performance. This work provides an overview of various switching mechanisms and their investigations at nanoscale levels using high resolution microscopy techniques. In this mini review, the main focus was to understand the working mechanism derived from the so-called filament model. The high resolution conductive atomic force microscope, transmission electron microscope and scanning electron microscopes are the best tools available to investigate the dynamics of filamentary switching. Several issues with the existing techniques are also highlighted.

Electrochemical and Electronic Charge Transport Properties of Ni-Doped LiMn₂O₄ Spinel Obtained from Polyol-Mediated Synthesis

LiNi_0.5Mn_1.5O₄ (LNMO) spinel has been extensively investigated as one of the most promising high-voltage cathode candidates for lithium-ion batteries. The electrochemical performance of LNMO, especially its rate performance, seems to be governed by its crystallographic structure, which is strongly influenced by the preparation methods. Conventionally, LNMO materials are prepared via solid-state reactions, which typically lead to microscaled particles with only limited control over the particle size and morphology. In this work, we prepared Ni-doped LiMn₂O₄ (LMO) spinel via the polyol method. The cycling stability and rate capability of the synthesized material are found to be comparable to the ones reported in literature. Furthermore, its electronic charge transport properties were investigated by local electrical transport measurements on individual particles by means of a nanorobotics setup in a scanning electron microscope, as well as by performing DFT calculations. We found that the scarcity of Mn³⁺ in the LNMO leads to a significant decrease in electronic conductivity as compared to undoped LMO, which had no obvious effect on the rate capability of the two materials. Our results suggest that the rate capability of LNMO and LMO materials is not limited by the electronic conductivity of the fully lithiated materials.

How Ligands Affect Resistive Switching in Solution-Processed HfO2 Nanoparticle Assemblies

Advancement of resistive random access memory (ReRAM) requires fully understanding the various complex, defect-mediated transport mechanisms to further improve performance. Although thin-film oxide materials have been extensively studied, the switching properties of nanoparticle assemblies remain underexplored due to difficulties in fabricating ordered structures. Here, we employ a simple flow coating method for the facile deposition of highly ordered HfO₂ nanoparticle nanoribbon assemblies. The resistive switching character of nanoribbons was determined to correlate directly with the organic capping layer length of their constituting HfO₂ nanoparticles, using oleic acid, dodecanoic acid, and undecenoic acid as model nanoparticle ligands. Through a systematic comparison of the forming process, operating set/reset voltages, and resistance states, we demonstrate a tunable resistive switching response by varying the ligand type, thus providing a base correlation for solution-processed ReRAM device fabrication.

Resistive Switching of Sub-10 nm TiO₂ Nanoparticle Self-Assembled Monolayers

Resistively switching devices are promising candidates for the next generation of non-volatile data memories. Such devices are up to now fabricated mainly by means of top-down approaches that apply thin films sandwiched between electrodes. Recent works have demonstrated that resistive switching (RS) is also feasible on chemically synthesized nanoparticles (NPs) in the 50 nm range. Following this concept, we developed this approach further to the sub-10 nm range. In this work, we report RS of sub-10 nm TiO₂ NPs that were self-assembled into monolayers and transferred onto metallic substrates. We electrically characterized these monolayers in regard to their RS properties by means of a nanorobotics system in a scanning electron microscope, and found features typical of bipolar resistive switching.

Transferable Memristive Nanoribbons Comprising Solution-Processed Strontium Titanate Nanocubes

Memristors, often comprising an insulating metal oxide film between two metal electrodes (MIM), constitute a class of two-terminal devices that possesses tunable variations in resistance based on the applied bias history. Intense research remains focused on the metal-insulator interface, which serves as the crux of coupled electronic-ionic interactions and dictates the underpinning transport mechanisms at either electrode. Top-down, ultrahigh-vacuum (UVH) deposition approaches for MIM nanostructures yield highly crystalline, heteroepitaxial interfaces but limit the number of electrode configurations due to a fixed bottom electrode. Here we report on the convective self-assembly, removal, and transfer of individual nanoribbons comprising solution-processed, single-crystalline strontium titanate (STO) perovskite oxide nanocrystals to arbitrary metallized substrates. Nanoribbon transferability enables changes in transport models ranging from interfacial trap-detrap to electrochemical metallization processes. We also demonstrate the endurance of memristive behavior, including switching ratios up to 10⁴, after nanoribbon redeposition onto poly(ethylene terephthalate) (PET) flexible substrates. The combination of ambient, aerobic prepared nanocrystals and convective self-assembly deposition herein provides a pathway for facile, scalable manufacturing of high quality, functional oxide nanostructures on arbitrary surfaces and topologies.

Ultra-Lightweight Resistive Switching Memory Devices Based on Silk Fibroin

Ultra-lightweight resistive switching memory based on protein has been demonstrated. The memory foil is 0.4 mg cm(-2) , which is 320-fold lighter than silicon substrate, 20-fold lighter than office paper and can be sustained by a human hair. Additionally, high resistance OFF/ON ratio of 10(5) , retention time of 10(4) s, and excellent flexibility (bending radius of 800 μm) have been achieved.

Correlating the Integral Sensing Properties of Zeolites with Molecular Processes by Combining Broadband Impedance and DRIFT Spectroscopy--A New Approach for Bridging the Scales

Zeolites have been found to be promising sensor materials for a variety of gas molecules such as NH₃, NOx, hydrocarbons, etc. The sensing effect results from the interaction of the adsorbed gas molecules with mobile cations, which are non-covalently bound to the zeolite lattice. The mobility of the cations can be accessed by electrical low-frequency (LF; mHz to MHz) and high-frequency (HF; GHz) impedance measurements. Recent developments allow in situ monitoring of catalytic reactions on proton-conducting zeolites used as catalysts. The combination of such in situ impedance measurements with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), which was applied to monitor the selective catalytic reduction of nitrogen oxides (DeNOx-SCR), not only improves our understanding of the sensing properties of zeolite catalysts from integral electric signal to molecular processes, but also bridges the length scales being studied, from centimeters to nanometers. In this work, recent developments of zeolite-based, impedimetric sensors for automotive exhaust gases, in particular NH₃, are summarized. The electrical response to NH₃ obtained from LF impedance measurements will be compared with that from HF impedance measurements, and correlated with the infrared spectroscopic characteristics obtained from the DRIFTS studies of molecules involved in the catalytic conversion. The future perspectives, which arise from the combination of these methods, will be discussed.